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University of Southampton

Faculty of Engineering, Science and Mathematics

School of Chemistry

High-Throughput Synthesis and Screening of Binary

Alloys for Hydrogen Evolution and Oxidation

Reactions

By

Faisal A. Al Odail

Thesis for the degree of Doctor of Philosophy

June 2010

I

Abstract

High throughput methods were employed for the physical vapour deposition (PVD) and

electrochemical screening of random and equilibrated (annealed at 300 ºC for 15

minutes) binary metal alloys for the hydrogen evolution reaction (HER) and the

hydrogen oxidation reaction (HOR). Results are presented for the Pd-Au, Pd-Bi and Ru-

Au alloy systems. Thin films of each alloy system were synthesized on a series of 10x10

array electrodes with a graded composition. A variety of analytical techniques including

Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS)

and powder X-ray Diffraction (XRD) were used for the analysis of bulk composition,

surface composition and structure respectively.

The compositional analysis reveals that alloy formation and synthesis of nearly the

whole compositional range of the alloy systems were achieved. Surface segregation of

Au in the Pd-Au alloy system and Ru in the Ru-Au alloy system was observed. The

surface segregation in the equilibrated Pd-Bi alloy system did not take place.

The HER and HOR activity on all the examined alloy systems exhibit a similar

compositional dependence. This suggests that the HER activity provides a good

descriptor for the HOR activity for systems with low overpotentials. An exception of

this occurs, however, at high concentrations of Au and Bi where the HER activity

decreases monotonically towards 100 at. %, while the HOR activity decreases more

rapidly at alloy compositions of ca. 90 at. %. An enhancement in the activity was found

on a variety of alloy compositions over the constituent components. The optimum Pd-Au

alloy composition for the HER and HOR was found to be at a composition of ca.

Pd50Au50 (more active than pure Pd). A comparable activity to pure Pd was found on Bi-

rich alloys (ca. Pd25Bi75) in the Pd-Bi alloy system. The activity on ca. Ru90Au10 and 60-

80 % Au was found to be higher than pure Ru.

The CO-tolerance in the HOR along the whole compositional range of each alloy system

was also assessed in the presence of a mixture of hydrogen and 500 ppm CO. The results

suggest that the Ru-Au alloy system is more CO tolerant than the other two systems.

II

Contents

Abstract I

Contents II

Declaration of Authorship VI

Acknowledgments VII

Table of Abbreviations VIII

Chapter 1: Introduction 1

1.1- Electrocatalysis: Principles and Applications 1

1.2- Fuel Cells 3

1.2.1- Fundamentals 3

1.2.2- Polymer Electrolyte Membrane Fuel Cell (PEMFC) 8

1.3- Electrocatalysis on Alloy Surfaces 10

1.3.1- Advantages of Electrocatalysis by Alloys 11

1.3.2- Enhancement of Catalytic Processes on Metal Alloys 15

1.3.3- Bulk & Surface Composition of Alloys 16

1.3.4- High Throughput Synthesis and Screening of Metal Alloys for

Electrocatalysis

17

1.4- The Hydrogen Evolution Reaction (HER)/Hydrogen Oxidation Reaction

(HOR)

19

1.4.1- Reaction Mechanisms 19

1.4.2- Alloy Catalysts for the HER/HOR in Acids 21

1.5- Aims of the Project 24

III

Chapter 2: Experimental 25

2.1- The High-Throughput Physical Vapor Deposition (HT-PVD) System 25

2.2- Sample Preparation 30

2.2.1- Sample Calibration 30

2.2.2- Electrochemical Array 31

2.3- Analytical Tools 32

2.3.1- Powder X-ray Diffraction (XRD) 32

2.3.2- Energy Dispersive X-ray Spectroscopy (EDS) 34

2.3.3- X-ray Photoelectron Spectroscopy (XPS) 35

2.4- Electrochemical Measurements 38

2.4.1- Electrochemical Cell 38

2.4.2- Array Measurements 40

2.4.2.1- Cyclic Voltammetry Measurements 40

2.4.2.2- Potential Step Measurements 42

Chapter 3: Palladium-Gold (Pd-Au) Alloy Surfaces 43

3.1- Introduction 43

3.1.1- Industrial Applications of Pd-Au Catalysts 45

3.1.2- Electrochemical Applications of Pd-Au Catalysts 46

3.2- Composition and Structure Analysis 48

3.2.1- EDS Analysis 48

3.2.2- XRD Analysis 49

3.2.3- XPS Analysis 54

3.3- Base Voltammetry and CO Stripping Measurements 58

3.4- The Catalytic Activity for the HER 73

3.5- The Catalytic Activity for the HOR 82

IV

3.6- The Carbon Monoxide Tolerance during the HOR 88

3.7- Conclusions 92

Chapter 4: Palladium-Bismuth (Pd-Bi) Alloy Surfaces 94


4.1.1- Industrial Applications of Pd-Bi Catalysts 95

4.1.2- Applications of Pd-Bi Catalysts in Electrocatalysis 97





4.3- Base Voltammetry and CO stripping Measurements 109



4.6- Conclusions 123

Chapter 5: Ruthenium-Gold (Ru-Au) Alloy Surfaces 125






5.3- Base Voltammetry and CO stripping Measurements 135



5.6- Conclusions 150

V

Chapter 6: Conclusions and General Discussions 153

6.1- Sample Characterization 153

6.2- The HER and HOR Activity 154

6.3- Suggestions for Further Studies 157

References 158

VI

Declaration of Authorship

I, Faisal A. Al Odail, declare that the thesis entitled:

High-Throughput Synthesis and Screening of Binary Alloys for Hydrogen

Evolution and Oxidation Reactions

and the work presented in the thesis are both my own, and have been generated by me as

the result of my original research. I confirm that:

This work was done wholly while in candidature for a research degree at this

university.

Where I consulted the published work of others, this is always clearly attributed.

Where I have quoted from the work of others, the source is always given. With the

exception of such quotations, this thesis is entirely my own work.

I have acknowledge all main sources of help.

Parts of this work have been prepared for publication as:

1- Faisal A. Al Odail, Alexandros Anastasopoulos, and Brian E. Hayden "The Hydrogen

Evolution Reaction and Hydrogen Oxidation Reaction on Thin Film PdAu Alloy

Surfaces", submitted to Physical Chemistry Chemical Physics.

2- Faisal A. Al Odail, Alexandros Anastasopoulos, and Brian E. Hayden "Hydrogen

Evolution and Oxidation Reactions on Palladium-Bismuth Alloys ", to be published.

3- GB Patent Application: Alexandros Anastasopoulos and Brian E. Hayden "Alloys for

Hydrogen Oxidation and Water Reduction", March 2010.

Signed:

Date:

VII

Acknowledgments

Thanks to all people who have helped me during my PhD study at the University of

Southampton. I would like to express my sincere thanks to Professor Brian E. Hayden

for giving me the opportunity to do my PhD research in his laboratory, helpful

discussions, lessons related to my area of research and his encouragement to carry on my

work. I would also like to express my sincere thanks to Professor John M. Dyke for his

advises and helpful discussions.

Thanks to the Government of Saudi Arabia for the PhD scholarship. I also thank the

European Union (EU) for funding this project.

A special thanks to Alexandros Anastasopoulos for training me on the instruments and

helping me in the experimental work, data analysis, computing, and also for helpful

discussions. Thanks to Robert Noble for his help especially with computing and writing

a script for surface atoms population. I also thank Mehdi Mirsaneh for helpful

discussions. Thank you to all other people that I have met in the surface science and

heterogeneous catalysis group: Laura W., Jens-Peter, Ben W, Duncan S., Rafael C-M.,

John B., Talal G., Louise H., Abdulrahman H. and Daniel C. I also thank Audrey, Naruo

Y., Piers, Claire M. for their help.

This work is for my parents, wife and children. I am very grateful to them for their

support in my various endeavours. I also thank and offer my regards to the rest of my

family.

VIII

Table of Abbreviations

High throughput Physical Vapor Deposition HT-PVD

Ultrahigh Vacuum UHV

Hydrogen Evolution Reaction HER

Hydrogen Oxidation Reaction HOR

Oxygen Reduction Reaction ORR

Oxygen Evolution Reaction OER

Proton Exchange Membrane Fuel Cell PEMFC

Polymer Electrolyte Membrane Fuel Cell PEMFC

Solid Polymer Electrolyte Fuel Cell SPEFC

Direct Methanol Fuel Cell DMFC

Alkaline Fuel Cell AFC

Phosphoric Acid Fuel Cell PAFC

Molten Carbonate Fuel Cell MCFC

Solid Oxide Fuel Cell SOFC

Membrane Electrode Assembly MEA

Polytetrafluoroethylene PTFE

Catalytic Preferential Oxidation CPOX

Underpotential Deposition UPD

Energy Dispersive X-ray Spectroscopy EDS

X-ray Energy Dispersive Spectrometer XEDS

X-ray Photoelectron Spectroscopy XPS

Electron Spectroscopy for Chemical Analysis ESCA

X-ray Diffraction XRD

IX

Low Temperature K-Cells LTKS

High Temperature K-Cells HTKS

Multichannel Analyzer MCA

Working Electrode WE

Reference Electrode RE

Counter Electrode CE

Reversible Hydrogen Electrode RHE

Rotating Disk Electrode RDE

Cyclic Voltammetry CV

Initial Potential Ein

Lower Potential Elo

Upper Potential Eup

Platinum Group Metals PGM

Hydrodesulfurization HDS

Vinyl Acetate Monomer VAM

Face Centered Cubic FCC

Hexagonal Close Packed HCP

Binding Energy BE

Monolayer ML

1

Chapter 1: Introduction

1.1- Electrocatalysis: Principles and Applications

Catalysis is an important phenomenon for industrial and economical considerations. An

enormous number of chemical and biochemical processes require a catalyst to be

completed. The use of catalysts is also essential to reduce air and water pollution such as

reducing emission of nitrogen monoxide (NO) from cars [1, 2]. Therefore, catalysis is an

important tool to approach ˝Green Chemistry˝ [3].

A catalyst is a material which can be added to a chemical process to accelerate the

reaction without being consumed during the process [4]. There are two common

categories of catalysis, homogenous and heterogeneous. In a homogeneous catalytic

process, the catalyst and the reactants as well as the products are in the same phase.

Examples of this type of catalyst are acids and bases, enzymes, transition metal ions and

alkyls. On the other hand, the phase of the catalyst in heterogeneous catalysis differs

from that of the reactants. Typically, a heterogeneous catalysis refers to a process in

which the catalyst is a solid (transition metal) and the reactants are gases [3, 4]. The role

of a catalyst in a catalyzed process has been argued along the history of catalysis. It has

been suggested that the presence of the catalyst may: (i) initiate the reaction, (ii) stabilize

the intermediates of the reaction, (iii) hold the reactants in close proximity and in the

right configuration to react (reducing the entropy of activation), (iv) facilitate bond

breaking by stretching them (reducing the energy of activation), (v) transfer energy into

reactants required to activate molecules, (vi) block undesired reactions, and (vii) donate

and accept electrons [4]. Nevertheless, the overall result is that the presence of the

catalyst accelerates the approach to equilibrium by providing an alternative pathway

with a lower activation barrier (energy) compared to that of the uncatalyzed reaction [2].

Electrocatalysis can be described as a combination between the principles of

electrochemistry and catalysis whereby the presence of an electrocatalyst enhances the

rate of an electrochemical reaction at the anode or the cathode [5, 6]. The increase in the

rate of the reaction can be realized by the increase in the exchange current density (jo) at

2

a fixed potential or by the decrease in the overpotential (η) of the electrochemical

reaction at a fixed current density (j) [5, 7]. The electrocatalyst can be either the

electrode material or an adsorbed species from the solution on the electrode surface. An

example of the latter type is in the oxidation of toluenes to benzaldehydes which takes

place at high overpotentials. It was found that the addition of a number of metal ions

Mn+ (such as Ag+, Ce3+, Mn2+, and Co2+) to the electrolyte lowers the overpotential of

the process [8].

The origin of electrocatalytic studies was as early as 1920s, when Bowden and Rideal

measured the rate of hydrogen evolution reaction (HER) on a series of metals [9, 10].

However, the first use of the term ˝electrocatalysis˝ has been traced back to Kobozev

and Monblanova in 1930s [9, 11]. The main application of electrocatalysis today is in

fuel cell technology, especially those incorporating a proton exchange membrane (PEM)

[12]. However, there are other possible applications which aim to reduce environmental

pollution. Examples of some applications of electrocatalysis are outlined in Figure 1.1

[12, 13]. More detail regarding these applications and others can be found in the latter

two references.

Figure 1.1: An outline of a number of applications of heterogeneous electrocatalysis [12, 13].

Applications of Electrocatalysis

Fuel Cell Technology Research Environmental Protection

Solid Oxide Fuel Cell

Phosphoric Acid Fuel Cell

Polymer Electrolyte Membrane Fuel Cell

Alkaline Fuel Cell

Molten Carbonate Fuel Cell

Electrosynthesis of H2O2 (safer than any other oxidizing agent)

Electrosynthesis of ClO2 as an alternative

to Cl2 (less hazard)

Oxidation of SO2 to H2SO4

Conversion of H2S into H2 and S

Destruction of organic pollutants from the aquatic environment

3

The nature of electrode material controls the kinetics and mechanism of the

electrochemical reaction [5]. There are several materials which can be utilized as

electrocatalysts including single metals, alloys, two component catalysts (such as using

Sn [14] or As [15] ad-atoms to enhance the electrocatalytic performance of Pt for CO

oxidation), oxides, or transition metal complexes [5, 8]. The choice of the electrode

material ˝electrocatalyst˝ is crucial and a number of factors should be considered in this

regard. An effective electrocatalyst should:

1- Improve the rate of the reaction under investigation and, equivalently, prevent the

undesired reactions [9]. An example of that is in a chlor-alkali cell where a proposed

catalyst should catalyze Cl2 evolution and, at the same time, inhibit the O2 evolution [5].

2- Resist cracking (physical stability) and corrosion (chemical stability) in the

experimental environment [5, 9, 16].

3- Be economically viable [9].

4- Be environmentally friendly (non-pollutant) [9].

5- Achieve a degree of CO tolerance in low temperature fuel cells [17].

This work is primarily related to electrocatalysis in fuel cell technology.

1.2- Fuel Cells

1.2.1- Fundamentals

The rapid progress in mobile communications requires the development of improved

power sources to solve the performance shortfall in the lithium ion and nickel metal

hydride batteries [18]. Also, the massive day-to-day use of electricity makes it important

to convert chemical energy into electrical energy. Furthermore, the production of

electricity and the propulsion of vehicles are, currently, based on the combustion of

fossil fuels (coal, oil, and natural gas). This process releases a number of pollutants into

the air including various metals, sulfur and nitrogen. The reaction of sulfur and nitrogen

with oxygen produces SO2 (the oxidation of SO2 could cause acid rain) and NOx (toxic

gas) respectively [13, 19]. The environmental problems associated with the use of fossil

4

fuels have dictated the search of alternatives. Fuel cell technology has been suggested to

meet all the above requirements [18-20].

A fuel cell is a device that converts the chemical energy released from reactions at the

anode and the cathode into electrical energy [21]. The discovery of fuel cell technology

is attributed to Professor Sir William Grove in 1839 who carried out measurements on

the dissociation of water into hydrogen and oxygen observing that the process is

reversible and that an electric current is produced with the recombination of the

components to form water [22].

Fuel cells are analogous to batteries in that the chemical energy is converted into

electrical energy through reactions at the anode and cathode (redox reactions). However,

the process of storage and energy conversion is different. In batteries, energy storage and

conversion occur internally (closed system) where the charge is produced by the

oxidation and reduction of the anode and cathode respectively which implies that the

anode and cathode are active masses in the redox process. On the other extreme, fuel

cells are open systems where the reactants ˝active masses˝ are supplied from external

sources (such as air or a tank) to the anode and cathode which means that the anode and

cathode are not active masses in the redox process. Thus, energy storage (in the tank) is

separated from energy conversion (in the cell) [23].

Following are the main advantages of fuel cell technology [20, 23-26]:

1- It is regarded as a preferable energy source to fossil fuel. This is because the use of

fuel cells could achieve zero-emission electricity generation and, therefore, reduces

environmental pollution.

2- High-efficient energy conversion.

3- Produces high power density.

3- Reduces noise pollution.

4- Requires low maintenance.

5- Offers a degree of flexibility, since it can be applied in various applications such as

transportations, electronics, or to supply electricity to a home or factory.

5

6- It may play an important role in establishing a hydrogen fuel economy.

7- Clean power generator.

There are, on the other hand, disadvantages of fuel cell technology such as [23]:

1- Limited availability.

2- Complexity in operation.

3- The presence of impurities in gas stream could influence the performance and life of

the cell.

4- High-costs of the units.

There are various types of fuel cells under research and development. Table 1.1

summarizes the common types and a number of their features [19, 20, 22, 27]. The

classification of fuel cells is based on either the type of the electrolyte used in the cell

(excluding the DMFC) or the operating temperature of the cell. Based on the operating

temperatures, the fuel cells can be classified into two categories: low temperature fuel

cells (PEMFC, DMFC, AFC and PAFC) and high temperature fuel cells (MCFC and

SOFC) [19].

6

Table 1.1: The common types of fuel cells [19, 20, 22, 27]

Type Common

abbreviation Fuel Electrolyte

Operating temperature, ºC

Polymer Electrolyte Membrane

PEMFC H2, CH3OH proton conducting polymer

50-125

Direct Methanol DMFC CH3OH sulphuric acid or solid polymer

50-120

Alkaline AFC H2 potassium hydroxide

50-90

Phosphoric Acid PAFC H2 orthophosphoric acid

190-210

Molten Carbonate MCFC hydrocarbons, CO

lithium/potassium carbonate mixture

630-650

Solid Oxide SOFC hydrocarbons, CO

stabilized zirconia 900-1000

Although the electrolyte is varied from a fuel cell to another, the fundamental principles

of how the electrical power is produced remain the same. A simple fuel cell system

consists of an anode, a cathode and an ion conducting electrolyte (Figure 1.2 [19, 22]).

The three components together are known as the membrane electrode assembly (MEA).

The anode compartment is fed with a fuel (typically hydrogen) and the cathode

compartment is fed with an oxidant (typically oxygen). Accordingly, catalyzed chemical

reactions take place at the anode and the cathode giving rise to ions which migrate

through the electrolyte carrying electric current to the other half cell. The anodic and

cathodic reactions for the different types of fuel cells are shown in Table 1.2. Typically,

hydrogen is oxidized at the anode and oxygen is reduced at the cathode. The electrolyte

allows ions to migrate through it and, simultaneously, acts as a barrier to gas diffusion.

The electrical power is generated by the flow of electrons from the anode compartment

to the cathode compartment through an external load (electrons are always produced at

the anode and consumed at the cathode) [19, 20, 22, 27].

7

Figure 1.2: A schematic representation of the components of a fuel cell [19, 22].

Table 1.2: The chemical reactions on the anode and cathode of the common fuel cells [19, 20, 27]

Fuel Cell Anode reaction Mobile ion Cathode reaction

PEMFC H2 → 2H+ + 2e- H+ → O2 + 4H+ + 4e- → 2H2O

DMFC CH3OH + H2O → CO2 + 6H+ + 6e-

H+ → O2 + 4H+ + 4e- → 2H2O

AFC H2 + 2OH- → 2H2O + 2e- ← OH- O2 + 2H2O + 4e- → 4OH-

PAFC H2 → 2H+ + 2e- H+ → O2 + 4H+ + 4e- → 2H2O

MCFC H2 + CO3

2- → H2O + CO2 + 2e-

CO + CO3 2- → 2CO2 + 2e-

← CO3 2- O2 + 2CO2 + 4e- → CO3

2-

SOFC

H2 + O2 2- → H2O + 2e-

CO + O2 2- → CO2 + 2e-

CH4 + O2 2- → 2H2O + CO2 + 8e-

← O2 2- O2 + 4e- → 2O2

2-

External load

Cat

hod

e- e-

Catalyst layers

An

ode

Electrolyte

8

Further detail regarding the PEMFC will be given in the following section due to its

relevant to this project.

1.2.2- Polymer Electrolyte Membrane Fuel Cell (PEMFC)

The Polymer Electrolyte Membrane Fuel cell (PEMFC) is also known as Proton

Exchange Membrane Fuel Cell or as Solid Polymer Electrolyte Fuel Cell (SPEFC) [19].

It was invented in 1960s by General Electric for a spacecraft and is, currently,

considered as the most promising fuel cell for the various applications owing to its lower

cost and ability to produce higher power density compared to the other types of fuel cells

(excluding AFC) [28]. The significant advantage of the PEMFC is the use of a solid

polymer electrolyte as proton exchange membrane [19]. This type of electrolyte

eliminates the concerns associated with the liquid electrolytes such as corrosion [28].

These membranes are, however, stable in a small temperature range which consequently

imposes the low operating temperature in the PEMFC [19, 28]. Typically, Nafion

produced by DuPont is used as an electrolyte in PEMFC [20]. The structure of this

electrolyte consists of a polytetrafluoroethylene (PTFE) which is chemically inert for

reduction and oxidation processes taking place in the cell (hydrophobic region), and

sulphate ions which participate in the proton (H+) exchange process (hydrophilic region)

[19]. The hydrophobic region provides chemical stability, while the hydrophilic region

allows proton conductivity [20]. However, the Nafion polymers are expensive and the

development of cheaper materials (such as hydrocarbon-based membranes) is a matter

under consideration in the recent PEMFC research [29]. A proposed electrolyte

membrane is supposed to be: (i) fast proton transporter; (ii) low gas permeable; and (iii)

mechanically, chemically and thermally stable [29].

The PEMFC uses hydrogen as fuel at the anode and oxygen or air as oxidant at the

cathode [30]. However, employing pure hydrogen is impractical due to its high cost

[31]. Methanol, gasoline, or natural gas is alternatively reformed to produce a mixture of

gases containing 40-70 % hydrogen (reformate) because of their lower cost [30, 31]. The

following equations exemplify the sequence of methanol reforming [31]:

9

CH3OH → CO + 2H2 (1.1)

CO + H2O → CO2 + H2 (1.2)

Besides hydrogen, the reformate contains 15-25% CO2, 1-2% CO as well as other

impurities such as H2S and NH3 [30, 32]. The high level of CO in the mixture influences

the catalyst performance in the cell and, therefore, must be lowered to below 100 ppm

[30, 31]. This can be achieved by passing the reformate through a catalytic preferential

oxidation (CPOX) reactor in order to oxidize CO to CO2 prior to entering the anode

compartment [30]. In recognition of this, the catalytic conversion of CO to CO2 has been

the subject of several investigations [33-46]. The mutual objective of them was to

develop proper and economically viable catalysts that enhance this process. Au nano-

particles supported on oxides, such as Au/TiO2, are reported as promising catalysts for

low temperature CO oxidation [47-49].

The anodic reaction in PEMFC is hydrogen oxidation (1.3), and the cathodic reaction is

oxygen reduction (1.4). The overall reaction (1.5) in the cell results in water [19]:

2H2 → 4H+ + 4e- (1.3)

O2 + 4H+ + 4e- → 2H2O (1.4)

2H2 + O2 → 2H2O (1.5)

The anodic hydrogen oxidation reaction (HOR) will be discussed later in more detail due

to its relevance to this work. Both reactions are catalyzed in practical PEMFC by Pt [50],

but its high cost and low abundance provide a major hurdle to commercial

implementation. Developing low-cost, high-efficiency alternatives to Pt is therefore an

ongoing area of research. One of the most researched alternatives to pure Pt is metal

alloys.

10

1.3- Electrocatalysis on Alloy Surfaces

The term ˝Alloy˝ should be distinguished from the term ˝Intermetallic Compound˝. An

alloy can be defined as ˝a metallic system containing two or more components,

irrespective of their intimacy of mixing or, precise manner of mixing˝ [51]. It can further

be expanded to contain non-metals as follows ˝a material consisting of two or more

metals (e.g. brass is an alloy of copper and zinc) or a metal and a nonmetal (e.g. steel is

an alloy of iron and carbon, sometimes with other metals included)˝ [52]. An alloy

system can thus be either a monophasic when the metallic components are completely

miscible forming a continuous series of solid solutions, or biphasic at the critical

temperature (when the components are not in complete miscibility and the system is

segregated into distinct phases) [51, 53]. On the other hand, an intermetallic compound

is ˝a chemical compound of two or more metallic elements and adopts an – at least

partly- ordered crystal structure that differs from those of the constituent metals˝ [54].

An intermetallic compound is thus a single phase system [54]. Based on the mixing

enthalpy change, alloys can be divided into a number of categories as revealed in Table

1.3 [51, 55-57].

Table 1.3: The behavior of metallic systems based on the mixing enthalpy changes.

∆H Category of the system Example Comment

Very near to zero Nearly ideal solid solutions Au-Ag The difference in atomic radii is negligible.

Small and negative Nearly ideal or regular solid solutions

Pt-Cu The difference of atomic radii in a regular solid solution is not negligible.

Large and negative Intermetallic compounds or ordered solutions

Pt-Sn -

Small and positive Mono or biphasic alloy Ni-Cu The type depends on the temperature of equilibration.

Large and positive Surface alloy can only be formed

Ru-Cu The solubility between the elements is very limited.

11

1.3.1- Advantages of Electrocatalysis by Alloys

Electrocatalysis by alloys can be beneficial in a number of aspects.

A- Economical Benefits

One of the main challenges in fuel cell technology is how to reduce the costs of

fabrication and the materials used in fuel cells [23]. The employment of Pt as catalysts

for the anode and cathode in fuel cells is costly. The price of Pt (5th October 2009,

www.platinum.matthey.com) is compared to a number of selected metals in Figure 1.3.

The development of low-cost non-Pt catalysts is, therefore, economically more favorable

[58]. However, the development of a cheaper electrocatalyst should be associated with a

good catalytic performance [59].

Figure 1.3: A column chart representing the prices ($) per kg of a number of metals. Data of Pt, Rh, Pd, Ru, and Ir were taken from www.platinum.matthey.com on the 5th of October 2009. The prices of Au and Ag were taken from www.goldprice.net on the 5th of October. The price of Bi ($20.944, 30th of September 2009) was taken from www.minormetals.com.

0

10000

20000

30000

40000

50000

60000

Pt Rh Pd Ru Ir Au Ag Bi

$ / kg

12

In relation to the issue of alloys, mixing a precious metal with a non-precious metal

implies that the content of the precious metal in the catalyst is reduced. One of the best

examples is Pt-Bi alloy system. It has been theoretically predicted and experimentally

proved that the HER activity on a Bi-Pt surface alloy (Bi as a host element and Pt as a

solute element, the solute coverage is 1/3 ML) is better than that on pure Pt [60]. From

an economical point of view, a Pt-Bi alloy catalyst costs less than that of a pure Pt

sample. Assuming that the price of Pt is $40000/kg and of Bi is $20/kg, and 100g of

both metals is required to prepare an alloy catalyst in the ratio of 50:50. This means that

the overall cost of the preparation of this catalyst is $4002. The same principle can be

applied when two or more precious metals, for instance Pd and Au, are alloyed.

B- Thermodynamic Stability

The thermodynamic stability under the experimental conditions (temperature, gas

atmosphere, pressure, pH, and electrochemical potential) is a fundamental requirement

of any proposed catalyst for the electrochemical applications [16, 59]. This feature of a

single metal can be improved by alloying. For instance, the Pourbaix diagram for Cu-

water system (Figure 1.4 [61]) shows that Cu undergoes anodic dissolution in acids and

very strong alkaline solutions. It has, however, recently been shown that the anodic

dissolution of Cu can be prevented by alloying Cu with Pd through a co-deposition

process of both elements from a 0.6 M HClO4 electrolyte containing CuSO4 and PdSO4

[62]. The resulting Cu-Pd alloy catalyst showed a significant activity for the nitrate

reduction in alkaline solutions.

13

Figure 1.4: The Pourbaix diagram for Cu-water system showing the domains of corrosion, immunity and passivation at 25 ºC [61].

C- Catalytic Activity

It has been shown in a number of electrocatalytic studies [63-68] that the catalytic

activity of a monocomponent catalyst was enhanced by alloying with another

component. For instance, the catalytic performance of a carbon supported Pt catalyst

(Pt/C) for the oxygen reduction reaction (ORR) was compared to that of a Pt-Pd/C

catalyst (Pt:Pd atomic ratio was 77:23) [66]. The ORR activity, in this case, was

observed to be higher on the binary catalyst than on the single component catalyst.

D- Reaction Selectivity

Electrocatalysis by alloys can be beneficial via increasing the selectivity of a desired

reaction. For instance, formic acid oxidation on Pt takes place through a direct

(dehydrogenation) or indirect (dehydration) mechanism [69]:

14

Pt + CO2 + 2H+ + 2e- (1.6)

Pt + HCOOH

Pt-COads + H2O (1.7)

Pt + H2O → Pt-OHads + H+ + e- (1.8)

Pt-COads + Pt-OHads → CO2 + 2Pt + H+ + e- (1.9)

It has been observed by Huang and colleagues [70] that formic acid oxidation activity on

a carbon nanofiber supported Pt-Au (Pt-Au/CNF) catalyst was better than on Pt/CNF

catalyst. The authors suggested that alloying Pt with Au enhances the selectivity of the

reaction through the direct pathway.

E- CO Tolerance in PEMFC

Besides its high cost, Pt exhibits low CO tolerance where a small amount of CO (>10

ppm) in the reformate stream poisons the Pt electrocatalysts at the anode compartment

[50, 71]. This is because CO adsorbs strongly on Pt surface blocking the active sites for

hydrogen adsorption and preventing, subsequently, the HOR [19, 30, 71]. Besides using

an effective catalyst to convert CO into CO2 before being fed to the anode, there are

other possible solutions to overcome the problem associated with the contamination of

the PEMFC anode catalyst by CO. For instance, an oxidizing agent (such as oxygen or

hydrogen peroxide) can be added to the reformate stream which facilitates the

conversion of CO to CO2. This, however, results in some fuel loss and an increase in

cost [29]. Increasing the operating temperatures of PEMFC has also been proposed. This

solution appears impractical as the development of high-temperature membranes is

consequently required which implies an increase in the cost of the system [19, 29].

Hence, the most appropriate way to solve the problem is possibly to develop new

electrocatalysts for the HOR that are more tolerant to CO poisoning and cheaper than Pt

[29, 58]. One of the best options in this regard is alloy catalysts. A number of Pt-based

binary alloy catalysts including Pt-Ru [72], Pt-Fe, Pt-Ni, Pt-Co and Pt-Mo [73] have

been reported to be more CO-resistant than pure Pt in the PEMFC.

dehydrogenation

dehydration

15

1.3.2- Enhancement of Catalytic Processes on Metal Alloys

The chemical and catalytic properties of a single-component catalyst can be improved

by alloying [74]. There are a number of proposed effects through which alloying

enhances the catalytic performance of a single-component catalyst.

A- Ensemble (Geometric) Effects

The "active sites" on a binary alloy catalyst consist of particular groups or ˝ensembles˝

of surface atoms arranged in a specific geometric orientation. Assume a binary alloy

catalyst consists of an active component and inactive component. The presence of the

inactive component facilitates the formation of these ensembles and enhances the overall

catalytic activity by blocking certain sites suppressing the formation of undesired

intermediates (improving selectivity) or inhibiting species (improving activity) [75-77].

For example, Ni-Cu alloyed Raney-type catalysts showed improved catalytic

performance for methane decomposition (CH4 C + 2H2) in comparison with

monometallic Raney-Ni catalysts [78]. Alloying Cu ˝inactive component˝ with Ni

˝active component˝ is believed to form ensembles of Ni surface atoms which are

responsible for the improvement in the catalytic performance on the alloy catalysts. The

addition of Cu reduces catalyst deactivation by minimizing the adsorption of carbon

species and, therefore, prevents the formation of encapsulating carbon.

B- Electronic (Ligand) Effects

The enhancement in the catalytic performance of a monocomponent catalyst by alloying

can be ascribed to a modification in its electronic properties (e.g. d-electron density)

resulting from the presence of the other component(s). This modification produces

heteronuclear metal-metal bonds (interactions) between the components of the alloy

leading to a system with improved chemical and catalytic properties [75, 77, 79]. The

electronic interaction between the components in the alloys is weak when the enthalpy

of formation (∆Hf) is positive, and is strong when ∆Hf is negative [80].

16

C- Bifunctional Effects

In this model, the promotion in the catalytic performance by alloying is achieved

by each individual component in the alloy activating a certain elementary reaction step

[75]. The catalytic behavior of various Pt-M alloys (M = Sn, Re, Mo and Ru) for

methanol oxidation [79] and Pt-Ru alloys for CO oxidation [72, 81] was reported to be

better than that of Pt. The enhancement on the binary alloy catalysts was explained by

both a modification in the electronic properties of Pt in the presence of the other

component and a bifunctional effect where Pt is believed to be active for CH3OH

adsorption (in the former process) or CO adsorption (in the later process) and the other

component on the surface is active for water or oxygen containing species (OH-)

adsorption.

1.3.3- Bulk & Surface Composition of Alloys

One of the fundamentals in electrocatalysis is that the nature and surface composition of

the catalyst dominates the electrocatalytic process including the interactions of reactants

with the surface, the strength of these interactions and the redox processes [82]. The

surface composition of an alloy catalyst can be different from the bulk composition. The

enrichment of an alloy surface by one of the component is known as ˝surface

segregation˝. This phenomenon could play a crucial role in changing the catalytic

performance of the alloy catalyst such as enhancing /inhibiting a desired reaction or an

undesired reaction [83].

The surface segregation in an alloy system can theoretically be predicted based on the

surface energies of the alloy components. An alloy component tends to segregate at the

surface if its surface energy is lower than that of the other component [83]. However,

there are a number of factors that could, in practice, result in a difference between the

surface and bulk composition and lead to surface enrichment with one of the alloy

components (sometimes different from the theoretical prediction) such as preparation

method of the alloy, chemical and thermal treatment, and applying a high

electrochemical potential [84]. Besides that, the difference could be ascribed to the

17

purity of the metallic systems as they always contain elements such as H, C, N, O and S.

The segregation of these elements with one of the alloy component could result in a co-

segregation effect changing the surface composition [83]. Furthermore, some reacting

molecules favor adsorption on a specific atom causing a segregation of this atom

towards the surface [84, 85]. For example, it was reported that Ag and Au in Pt-Ag and

Pd-Au alloy systems segregate under vacuum to the surface. In contrast, the presence of

CO on the surface acts as driving force of surface segregation of Pt and Pd in both

systems to form a strong metal carbonyl bond [86].

1.3.4- High Throughput Synthesis and Screening of Metal Alloys for Electrocatalysis

The development and examination of a catalyst material for a heterogeneous reaction

through traditional methods is inefficient and a time-consuming process. The use of high

throughput (or combinatorial) methods, in return, allows fast and parallel synthesis and

screening of arrays (libraries) of materials increasing the possibility of producing

efficient catalysts [87-89]. Advances in the discovery of pharmaceutical compounds

have been accomplished during the last two decades by employing combinatorial

methods. The success in this area has subsequently led to interest in employing these

methods for the discovery of inorganic and solid state materials [89]. There are a number

of examples where high throughput synthesis and/or screening methodology was applied

for the study of electrochemical reactions [60, 63, 90-95].

The synthesis of an alloy catalyst for electrochemical reactions can be achieved through

a number of preparation methods such as co-electrodeposition of both elements on a

substrate material [96], electroless deposition [97], co-melting followed by mixing of the

components [98], chemical synthesis using precursor salts [99], or by means of physical

vapor deposition (PVD) methodology [100, 101]. The employment of high throughput

methods in the synthesis of alloy electrocatalysts facilitates the exploration of new

catalyst compositions [102]. This ultimately improves knowledge of the composition-

activity relationship [63].

18

Among the various preparation methods of alloy catalysts, the PVD is believed to be a

promising method for high-throughput synthesis of libraries of thin film materials. The

high-throughput physical vapor deposition (HT-PVD) of thin film libraries can be

achieved either by sequential deposition of the material employing combinatorial

shadow masks, or by simultaneous deposition of the components employing multiple

evaporation sources [63, 89]. Following are a number of examples where the

employment of HT-PVD methods was shown to be beneficial in identifying new alloy

catalysts for electrochemical reactions.

Guerin and Hayden [89] have recently introduced a modified HT-PVD method through

which the fabrication of alloy catalysts is achieved by co-deposition of the elemental

components from multiple sources. This method was applied for the synthesis and

screening of a 100-electrode array of Pt-Pd-Au alloy catalysts for the oxygen reduction

reaction (ORR) showing that the ORR activity on various Pt-Pd alloy compositions are

better than on either of Pt or Pd alone [63].

Methanol oxidation activity was assessed on Pt-Ru-W and Pt-Ru-Co ternary alloy

systems in 0.5 M H2SO4 employing a combinatorial process [102]. Various

compositions of these two systems were prepared on 76 pads supported on a 2 inch

silicon wafer. The deposition of the alloy systems was achieved through a sputtering

system and computer controlled shutters. It was revealed through the electrochemical

screening at various temperatures ranging from room temperature to 60 ºC that

Pt25Ru0W75 and Pt17Ru17Co66 catalysts are superior to Pt-Ru catalyst at room

temperature. The optimum ternary composition at 60 ºC was found to be at Pt44Ru12W44

and Pt12Ru50Co38.

A combinatorial synthesis of a 64-electrode array of Pt-Co-Ru alloy catalysts was

carried out by Strasser [103] using an automated sputtering procedure with a movable

shutter to control the gradient of the deposited thin films. The high throughput

electrochemical screening of the resulting array sample for methanol oxidation in 0.5 M

19

H2SO4 has shown that the activity on a ternary Pt20Co60Ru20 catalyst is better than on

standard Pt-Ru catalysts.

A library of 63 discrete ternary Pt-Ni-Cr alloy catalysts was prepared through sequential

sputtering process on a silicon wafer employing a number of shadow masks and

examined for methanol oxidation reaction [104]. Among the examined catalysts,

Pt28Ni36Cr36 has been observed to have the highest activity for methanol oxidation and

also showed good corrosion resistance. Further examples are available in references [59,

105-108].

1.4- The Hydrogen Evolution Reaction (HER)/Hydrogen Oxidation Reaction

(HOR)

The HER is an important process in electrochemical technology. This is because of its

relevance to a wide range of electrochemical processes such as water electrolysis and

chlorine production. It is also a reaction in corrosion of metals in acid media and a

competing reaction in electroplating and organic reductions. Furthermore, it produces

hydrogen which can be employed as a fuel for a variety of industrial processes such as

fuel cell technology [5, 8, 60]. The HOR is also important in electrocatalysis as it is the

anodic reaction in PEMFC.

1.4.1- Reaction Mechanisms

The overall reaction of hydrogen evolution in acid media is expressed by the following

equation [5]:

2H+ + 2e- H2 (1.10)

, while in neutral and basic solutions is:

2H2O + 2e- H2 + 2OH- (1.11)

20

The detailed mechanism in acid media will be considered here as all the measurements

presented in this thesis were carried out in acid medium.

There are two proposed pathways through which the HER takes place on a metal surface

(M) in acid solutions: (i) Volmer-Heyrovsky mechanism or (ii) Volmer- Tafel

mechanism [5, 97, 109-111]. The first step in both cases is known as the charge transfer,

underpotential deposition (UPD) of hydrogen ions, or Volmer reaction (1.13). This step

involves the transfer of hydrogen ion (proton) from the bulk of the solution towards the

electrode surface and the formation of adsorbed hydrogen atom on the surface (M-Hads).

The discharge of hydronium atoms (H3O+) is the source of protons in acid media.

M + H+ + e- M-Hads (1.13)

This step can be followed by either an electrochemical reaction (Heyrovsky reaction

(1.14)) which involves desorption of hydrogen atom to combine with a proton producing

hydrogen gas (ion-atom recombination):

M-Hads + H+ + e- H2 + M (1.14)

or by a chemical reaction (Tafel Reaction (1.15)) which involves atom-atom

recombination:

2M-Hads H2 + 2M (1.15)

The rate determining step in both mechanisms can be either of the two steps. It is,

however, difficult to determine whether the reaction follows the first or second

mechanism when the first step is the rate determining step [5]. In the case that the

second step is the rate determining step, the HER pathway may be determined by the

catalyst activity. At low overpotentials (in the presence of a good catalyst), the reaction

follows the Volmer-Tafel mechanism and the Tafel reaction is the slowest of the two

steps (rate determining step). At higher overpotentials, on the other hand, the reaction

follows the Volmer-Heyrovsky mechanism and the Heyrovsky reaction is the rate

determining step [97].

21

The HOR is the reverse reaction of the HER. The overall reaction in acid media is

written as follows [19, 112]:

H2 → 2H+ + 2e- (1.16)

In reverse to the HER, the HOR follows either the Heyrovsky-Volmer or Tafel-Volmer

mechanism. In the former case, hydrogen adsorbs and dissociates on the catalyst surface

forming an adsorbed hydrogen atom on the surface (M-Hads) and a hydrogen ion (proton)

(1.17) followed by desorption of the adsorbed hydrogen atom to form another proton

(1.18).

Heyrovsky reaction: M + H2 → M-Hads + H+ + e- (1.17)

Volmer reaction: M-Hads → H+ + e- + M (1.18)

In the Tafel-Volmer mechanism, the HOR occurs through the adsorption and, then,

dissociation of hydrogen molecule on the surface into two adsorbed atoms (1.19)

followed by electrochemical desorption of the adsorbed atoms to give two protons

(1.20).

Tafel reaction: 2M + H2 → M-Hads + M-Hads (1.19)

Volmer reaction: 2M-Hads → 2H+ + 2e- + 2M (1.20)

1.4.2- Alloy Catalysts for the HER/HOR in Acids

The formation of the M-H bond (adsorbed hydrogen atom on the catalyst surface) is the

key process in the HER/HOR [109]. The strength of this bond can be determined from

the free energy of hydrogen adsorption (∆GH) on the catalyst surface. The ∆GH value on

a good catalyst for the HER is 0.0 eV [93]. The absolute values of ∆GH on several

binary surface alloys has been determined by Greeley and Nørskov [93]. Table 1.4

shows the measured values on the systems relevant to this work.

22

Table 1.4: Absolute free energies of hydrogen adsorption (∆GH, eV) on various surface alloys. The values were determined at T=298 K, θH = 1/3 ML. Coverage of solute elements in the surface layer of host elements is 1/3 ML [93].

Alloy system Host Solute |∆GH|

Pd-Au Pd Au 0.3 → 0.4

Au Pd 0 → 0.1

Pd-Bi Pd Bi > 0.5

Bi Pd 0 → 0.1

Ru-Au Ru Au 0.1 → 0.2

Au Ru 0.3 → 0.4

There is a relation between the free energy of hydrogen adsorption (or the strength of the

M-H bond) on a catalyst surface and its activity for the HER/HOR [31]. Figure 1.5

shows the HER exchange current densities as a function of the ∆GH values on several

pure metals and metal overlayers [60]. The relation occurs as a Volcano plot

demonstrating that the HER activity increases as the absolute value of ∆GH is closer to

0.0 eV. Similar plots are available in [31, 109, 113].

Figure 1.5: Volcano plot for the HER activity as a function of free energy of hydrogen adsorption on a series of pure metals and metal overlayers [60]. ∆GH values were calculated at 298 K , 1 bar of H2 and θH = 1/4 or 1/3 ML. α refers to the assumed transfer coefficient.

23

The Volcano plot shows that the ∆GH of an ideal catalyst for the HER is 0.0 eV. The

negative and positive values of ∆GH refer respectively to a strong (exothermic) and a

weak (endothermic) bonding. The M-H bond becomes stronger as the ∆GH is more

negative and weaker as the ∆GH is more positive. There is no HER activity when the

hydrogen adsorption is too weak or too strong. The very strong hydrogen adsorption

blocks the available sites on the catalyst surface poisoning the reaction. The intermediate

bonding is, therefore, the favorable case to permit the HER to take place on the catalyst

surface [31, 60, 113]. This is known as the Sabatier principle that suggests that an

optimum catalytic activity is achieved when the catalyst exhibits intermediate binding

energy or free energy of adsorption [60].

A variety of binary alloy catalysts have been employed for the HER [50, 96, 98, 114-

117] and HOR [99, 100, 118-120] in acid media. The selection of two elements to form

an alloy catalyst with high activity for the HER/HOR has been a matter of discussion in

a number of references [9, 97, 114, 121]. It is generally proposed that mixing a hypo-d-

electronic transition metal having empty or half-filled d orbitals with a hyper-d-

electronic transition metal having paired d electrons not available for bonding (i.e. hypo-

hyper-d-electronic combination) produces a favorable alloy catalyst for hydrogen

reactions [9, 114, 121]. This is because of a change in the electronic densities of pure

constituents upon alloying. For instance, an alloy catalyst consisting of Ni and Mo (Mo

as a hypo- and Ni as a hyper-d metal) has been proposed to be more active for the HER

than Ni or Mo alone. The enhancement in the catalytic activity by alloying was

explained by a change in the electronic densities of both elements and the strength of

proton bonding due to a spillover of electrons from Ni (3d8) to Mo (4d5) [97]. Other

examples of hypo-hyper-d-electronic alloy catalysts for the HER are Cu-Zr and Cu-Ti

[114], Mo-Co, Ni-Zr and Co-Zr [9].

The above assumption does not necessarily imply that any alloy catalyst for the

HER/HOR is supposed to consist of hypo-hyper d-metals. There are other cases where

alloying a transition metal with a non-transition metal or a non-transition metal alloy

24

system produced an active catalyst for the HER or HOR processes. Examples here are

Ni-P [96] and Pb-Bi [98] for the HER, and Pt-Pb, Pt-Sn and Pt-Sb [120] for the HOR.

1.5- Aims of the Project

The broad aim of the present work is to explore binary alloy catalysts as alternatives to

Pt for the HER and HOR through high throughput synthesis and electrochemical

screening methods. The choice of the elemental components that form the binary alloys

was based on: (i) mixing an element from Pt group metals (PGMs) with a non-Pt group

metal element, (ii) mixing an element from the right side in the Volcano plot [60] with

an element from the left side (Figure 1.5). The key objectives are to:

Employ a HT-PVD method for the synthesis of libraries of Pd-Au, Pd-Bi and Ru-Au

binary alloy systems.

Determine the bulk and surface composition of the prepared samples using Energy

Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS)

respectively.

Analyze the structure of the prepared alloys using powder X-ray diffraction (XRD)

technique.

Assess the HER and HOR activity on the whole compositional range of the studied

systems in order to identify the optimum composition of Pd-Au, Pd-Bi and Ru-Au alloy

systems for these electrochemical processes.

Investigate the effect of heat treatment on the compositions and catalytic performances

of the prepared alloy systems for the HER and HOR.

Assess the CO-tolerance in the HOR on the whole composition of the alloy systems.

25

Chapter 2: Experimental

2.1- The High-Throughput Physical Vapor Deposition (HT-PVD) System

Thin film arrays of binary catalysts composed of Pd-Au, Pd-Bi, and Ru-Au alloys were

deposited employing an ultrahigh vacuum (UHV) high-throughput physical vapor

deposition (HT-PVD) system (DCA Instruments) [89]. A schematic representation of the

system is illustrated in Figure 2.1. It consists of two physical vapor deposition chambers

(A and B). Both chambers were used for the HT-synthesis of the alloy samples. The

system is also provided with a single target sputtering chamber (this method of

deposition was not used in this work and this chamber was occasionally used for

annealing the samples instead). It is also equipped with an analytical chamber

incorporating imaging X-ray Photoelectron Spectroscopy (XPS, Resolve - 120 mm

Hemispherical Analyzer) which was used for analyzing the surface composition of the

deposited alloys. All chambers are linked by a transfer line composed of two trolleys

which were used to convey the sample holders to the desired chamber. A load lock,

which can separately be pumped down to the UHV and vented to atmospheric pressure,

was used to load/unload the samples in the system. The samples were easily transferred

into all chambers using a pick-up mechanism and transfer arms (TA). The thermal

deposition of the metal alloys was performed under an UHV environment with base

pressure between 1-5 x 10-9 mbar. Cryo (Helix Technology Corporation) and titanium

sublimation (Varian) pumps were employed in both chambers to achieve the UHV

conditions. The transfer line and the surface analysis chamber were pumped by ion

(Varian) and titanium sublimation pumps. The load lock was serviced by an oil free

rotary (Pfeiffer) and a turbo-molecular (Pfeiffer) pumps [122].

26

Figure 2.1: An over view of the High-Throughput Physical Vapor Deposition (HT-PVD) system used for depositing thin film arrays of Pd-Au, Pd-Bi and Ru-Au alloys. TA: transfer arm.

A top view of the source geometrics of chamber A and B is schematically depicted in

Figure 2.2. The substrate position in the deposition chambers is also indicated. Chamber

A consists of six off axis evaporation sources, three electron beam guns (e-guns,

Temescal) and three Knudsen cells (K-cells, DCA). Chamber B, on the other hand,

incorporates an e-gun and three K-cells. There are two types of K-cells which can be

used with the system, low and high temperature K-cells. The low-temperature K-cells

(LTKS, DCA) can be used to evaporate materials up to 1400 ºC, while the high-

temperature K-cells (HTKS, DCA) can be used to evaporate materials up to 2000 ºC.

The studied alloys were synthesized in both chambers (A or B) employing both e-guns

and K-cells. In each chamber a rotatable manipulator was used to hold the substrates

during the deposition. The E-guns were used to evaporate high melting point materials,

whereas the K-cells were used to evaporate low melting point materials. Palladium

(Unicore, 99.99 %), gold (Unicore, 99.99 %) and ruthenium (Alfa Aesar, 99.95 %) were

all deposited employing e-guns, whereas bismuth (Alfa Aesar, 99.999 %) was deposited

using a low temperature K-cell (LTKS). The deposition time of each sample was

typically about 30 minutes and film thicknesses were approximately around 100 nm.

27

Three types of substrates were used in the synthesis of the alloy samples. More detail

regarding these types is given later in this chapter.

Chamber A Chamber B

Figure 2.2: A schematic representation of the source positions in the growth chambers.

The HT-PVD methodology employed in this work for sample preparation has recently

been developed and described in detail by Guerin and Hayden [89]. In this method, a

combination between co-evaporation of pure elements from multiple finite-size sources

and movable wedge shutters (or apertures) is applied to achieve deposited materials with

controlled gradients. Employing this method, a simultaneous deposition with a

controlled gradient of up to six elements can be achieved on a substrate or an array of

pads [63, 90, 91]. The gradient of the deposited materials can be controlled by adjusting

the wedge shutters prior to the deposition as shown schematically in Figure 2.3 [89].

Using such a method for the preparation of alloy catalysts ensures mixing and alloy

formation and prevents the separation into bulk phases and surface segregation of one of

the alloy components as no heat treatment "annealing" is required to form the alloys

[63]. The bulk alloy composition is identical to the surface composition as no heat

treatment is required to form the alloy [63]. Alloys prepared without heat treatment are

K-cell 3 E-gun

Substrate

K-cell 1 K-cell 2

E-gun 1

K-cell 3

E-gun 3

K-cell 1

Buffer line

E-gun 2

K-cell 2

Buffer line

Substrate

28

described here as "random" or (non-equilibrated) alloys, and described "at equilibrium"

when the synthesis was followed by heat treatment.

Figure 2.3: A schematic representation of how the gradation of a material is achieved by using an aperture (a wedge shutter) [89].

In this scheme, (0, 0) refers to the center of the substrate face. A is the substrate size and

A1 and A2 express its two boundaries. B denotes the offset of the wedge shutter

(aperture) with respect to the axis center of the source. Bmin indicates the initial position

of the wedge shutter. B1, B2 and Bmax refer to different distances from Bmin. The source

size is given the symbol C and its two extremities are C1 and C2. D refers to the source

offset with respect to the substrate. The distance between the source and the aperture is

29

expressed by the symbol E, whereas the distance between the aperture and the substrate

is expressed by the symbol F. The flux direction of the evaporated materials from the

source C to the substrate A is shown by the lines A1C1, A1C2, A2C1 and A2C2. H refers

to the interaction point between lines A1C2 and A2C1 [89].

The wedge composition has been observed to be dependent on the offset of the wedge

shutter with respect to the axis center of the source B. At the position Bmin, a uniform

film can be obtained. The wedge composition can be accomplished by moving the

wedge shutter away from the position Bmin into the flow of evaporated materials. For

instance, moving the wedge shutter to the position B1 (which means that B is between

Bmin and B1) allows the point A1 at the substrate to be exposed to all the material from

the two extremities of the source C1 and C2. On the other hand, the point A2 is covered

from the material which comes from the region C2 and it will only be exposed to the

material near to the region C1. As a result of this, a partial gradient of the material from

point A1 to A2 can be achieved. A similar result (partial gradient) can be obtained by

moving the wedge shutter to the position between B2 and Bmax. A linear gradient across

the whole sample was accomplished at the position between B1 and B2. No deposition

was observed at the position Bmax. More detail regarding this method for the synthesis of

solid state materials is available in reference [89].

For the purpose of getting a uniform layer of a material, the wedge shutter was not used.

In this case, a motor drive associated with the sample holder (manipulator) in the

deposition chamber was employed to rotate the substrate. The wedge shutter was

employed in the synthesis of the alloy samples in order to obtain gradual concentrations

of the elemental components. Figure 2.4 shows an example of a wedge composition of a

binary alloy composed of elements A and B. It can be seen that the concentration of

element A increases gradually from 0 % on the right side to 100 % on the left side. In

contrast, element B increases gradually from 0 % on the left side to 100 % on the right

side. A unique mixture of both elements can be achieved in the middle. Thus, the

employment of the wedge shutter allowed the achievement of nearly the whole

compositional range of the studied alloy systems in this work.

30

Figure 2.4: A schematic representation of the wedge composition of an alloy consisted of two elements (A and B) which can be achieved by the HT-PVD methodology employed in the synthesis of the samples.

2.2- Sample Preparation

The alloy samples were initially calibrated on silicon or glass substrates and then

prepared on a number of electrochemical arrays. The calibration method and the

electrochemical array used in this work are described in what follows.

2.2.1- Sample Calibration

The alloy samples were calibrated prior to the synthesis on the electrochemical arrays in

order to determine an appropriate wedge composition and to observe the film thickness

as a function of time and evaporation temperature. A quartz microbalance (incorporated

in the deposition chamber) was used to determine the rate of deposition. The calibration

was carried out using either squares of silicon wafers (32 or 35 mm2 and thickness 0.5

mm, Nova Electronic Materials Ltd) or squares of glass (32 or 35 mm2 and thickness 1

mm, UQG Optics). The composition of the elemental components was, then, measured

by energy dispersive X-ray spectroscopy (EDS).

Thickness / nm

A

B

100% A 100% B

Position along the sample

31

2.2.2- Electrochemical Array

After calibration, the examined alloy systems were deposited on 100-element

electrochemical array electrodes. Figure 2.5 shows the components of an

electrochemical array (35 x 35 = 1225 mm2). It consists of a silicon nitride coated silicon

wafer as a substrate. On the top of the silicon nitride there are 100 gold pad electrodes

which are used as substrates for the deposited materials to fulfill the conductivity. The

area of each individual electrode is approximately 1.2 x 1.2 = 1.44 mm2. The electrical

conductivity between the 100 electrodes and the electrochemical screening instrument is

achieved by using gold tracks and 100 gold contact pads on the edges. The area of each

gold contact pad is approximately 0.8 x 0.8 = 0.64 mm2. The electrochemical arrays

were rinsed with ethanol and acetone before deposition to avoid the presence of

impurities on the surface [122, 123]. Pure Pd or Ru were deposited onto 10 electrodes in

the electrochemical array in order to compare their catalytic activities to that of the alloy

system for both the HER and HOR.

Figure 2.5: A schematic representation of the 100-element working electrode electrochemical array.

35 mm

35 mm

Silicon Nitride

Passivation

100-gold electrical

contact pads

Gold track

100 gold electrodes

The deposition of the alloys and pure constituents (Pd or Ru)

arrays was carried out using two types of contrary matrix contact masks (

The size of the contact masks was similar to that of the electrochemical array. These

specially designed contact masks were employed in order to match the number of

electrodes and to obtain the same compositions when another substrate (silicon) was

used for the analysis of the sample. A 10 x 10 contact mask with 10 blank squares

(Figure 2.6A) was initially employed to achieve the deposition of the alloy samples.

This contact mask was removed after the deposition and replaced by a blank contact

mask with only 10 squares (

Annealing of the array samples w

15 minutes.

Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample, (A) a contact mask with 10 blank squares which was used duringsamples, (B) a blank contact mask with only 10 squares which was used during the deposition of the active component (Pd or Ru) in the alloy sample.

2.3- Analytical Tools

2.3.1- Powder X-ray Diffraction (XRD)

XRD method is widely used in crystallography to identify the phase of a material

consisting of many crystals and to determine lattice type (structure) and parameters. X

rays are produced by bombarding a metal target with high energy electron beams (1

A

32

the alloys and pure constituents (Pd or Ru) on the electrochemical

arrays was carried out using two types of contrary matrix contact masks (



rodes and to obtain the same compositions when another substrate (silicon) was


) was initially employed to achieve the deposition of the alloy samples.

ct mask was removed after the deposition and replaced by a blank contact

mask with only 10 squares (Figure 2.6B) available for the deposition of

Annealing of the array samples was carried out in the deposition chamber at 300 ºC for

Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample, (A) a contact mask with 10 blank squares which was used during the deposition of the alloy samples, (B) a blank contact mask with only 10 squares which was used during the deposition of the active component (Pd or Ru) in the alloy sample.

ray Diffraction (XRD)


consisting of many crystals and to determine lattice type (structure) and parameters. X

rays are produced by bombarding a metal target with high energy electron beams (1

B

on the electrochemical

arrays was carried out using two types of contrary matrix contact masks (Figure 2.6).



rodes and to obtain the same compositions when another substrate (silicon) was


) was initially employed to achieve the deposition of the alloy samples.

ct mask was removed after the deposition and replaced by a blank contact

) available for the deposition of pure Pd or Ru.

carried out in the deposition chamber at 300 ºC for

Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample,

the deposition of the alloy samples, (B) a blank contact mask with only 10 squares which was used during the deposition of


consisting of many crystals and to determine lattice type (structure) and parameters. X-

rays are produced by bombarding a metal target with high energy electron beams (1-100

33

keV). Their wavelengths are in the order of 10-10 m. Owing to their high energy, X-ray

photons are able to penetrate through the metal target in all directions causing multiple

collisions with the electrons of the atoms. The reflection of X-rays from the crystals

results in a diffraction pattern that reflects the structure of the target material. Atoms in a

crystal are aligned in planes and almost all planes participate in the diffraction of X-rays

[124-126].

The XRD measurements (Bruker D8 diffractometer with a 2 dimensional (C2) detector,

Cu Kα source with λ = 1.54184 Å) of the Pd-Au, Pd-Bi, and Ru-Au alloys were carried

out ex-situ. For these measurements, the alloy films were deposited on silicon wafers

and 10x10 matrix contact masks (Figure 2.6A-B) were employed to give 10x10 squares

with compositions similar to that obtained with the electrochemical arrays. The XRD

measurements were performed by Alexandros Anastasopoulos.

The lattice parameters of the alloys were calculated in some cases in order to assess the

lattice parameter-composition relationship and the extent of the obedience to Vegard’s

law in the alloy system. According to this law, it is proposed that the lattice parameters

change linearly with the composition of an alloy system consisting of two elements of

similar crystal structure and form solid solutions [127].

In a cubic crystal system, the lattice parameter (a) can be, simply, calculated using the

following equations [125, 128]:

a = d * (2.1)

d = λ / 2 sinθ (2.2)

where (d) is the distance between planes of a lattice which can be calculated according

to Bragg’s law as depicted in equation 2.2. (hkl) refer to miller indices of the studied

structure, whereas (λ) is the wavelength of X-ray photon (λ = 1.5418 Å).

34

2.3.2- Energy Dispersive X-ray Spectroscopy (EDS)

EDS, also known as X-ray Energy Dispersive Spectroscopy (XEDS), measurements

provide qualitative and quantitative analysis of chemical elements presented in a

specimen. The bombardment of a solid material with a high energy electron beam results

in ionization of an electron from an inner shell (K shell) in the atom and an electron

from a higher energy shell (L shell) falls into the vacancy emitting its excess energy as

an X-ray photon. The emitted X-rays are characteristic of atoms present in the specimen.

The measurement of the wavelength (or energy) of X-ray spectrum emitted by the

specimen provides information regarding the elements present in the specimen

(qualitative analysis), while the number of emitted X-rays per second allows the

measurement of the concentrations of the elements in the specimen (quantitative

analysis). A detector of incoming X-rays is used in the EDS technique to produce charge

pulses (signals) proportional to the detected energies of X-ray photons. The pulses are,

then, amplified and transferred to a multichannel analyzer (MCA) which collects all the

X-ray energies and displays them on a screen [125, 129, 130]. A schematic

representation of the process is shown in Figure 2.7 [129, 130].

Figure 2.7: A schematic representation of the EDS analysis of a target material [129, 130]. MCA is a multichannel analyzer.

35

The EDS (JEOL JSM5910 and Oxford Instruments INCA 300) analysis of the deposited

alloys was performed using silicon substrates to determine the bulk composition of the

100-element alloy samples. In this case, 10 x 10 matrix contact masks (Figure 2.6A-B)

were used to give 10 x 10 squares with compositions similar to that obtained with the

electrochemical arrays.

2.3.3- X-ray Photoelectron Spectroscopy (XPS)

XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a surface

characterization method that is used in chemical analysis to determine the composition

of elements present in a solid surface. It provides both qualitative and quantitative

analysis of all elements (except H and He) by measuring electrons ejected from the

surface atoms. The main feature in XPS is the escape depth of the measured electrons,

since these electrons come from layers which have thicknesses in the outermost 10 nm.

Hence, the contribution only comes from the surface species [125, 131].

In an XPS measurement, the target sample is placed in a vacuum environment and

irradiated with X-ray photons. The energies of X-ray photons are, then, transferred to

core-level electrons of the atoms present in the surface. As a result of that, the core-level

electrons will be ejected from these atoms as schematically illustrated in Figure 2.8.

These electrons are, then, separated according to their core ionization energies and

counted. The energy of the photoelectrons provides information regarding the nature of

the atoms in the sample (qualitative analysis) and the number of electrons provides

information regarding the concentration of these atoms in the surface (quantitative

analysis) [125, 131].

36

Figure 2.8: Ejection of a core-level electron by an X-ray photon [131].

The XPS (Resolve - 120 mm Hemispherical Analyzer) measurements were carried out

in situ in an ultrahigh vacuum environment with a base pressure of 1 x 10-9 mbar. The

source of X-rays was Al Kα with hν = 1486.6 eV. As in the cases of the XRD and EDS

measurements, the alloy films were deposited on silicon substrates. A 10 x 10 matrix

contact mask (Figure 2.9) was employed to give squares and compositions identical to

the 100-electrodes in the electrochemical arrays. This contact mask was removed inside

the masking station shown in Figure 2.1 prior to the XPS measurements. The XPS

measurements of an alloy system were performed on a number of alloy thin films along

the growth direction of the elemental components in the array sample to represent the

whole compositional range of the alloy system.

e-

photoejected electron

X-ray photon

2P

2S

1S

37

Figure 2.9: The 10 x 10 matrix contact mask used during the deposition of the alloy samples on silicon wafers for XPS measurements to give the same number of electrodes and compositions similar to that obtained with the electrodes in the electrochemical arrays.

The XPS spectra of the alloy systems were calibrated to: (i) the Pd (3d5/2) and Au (4f7/2)

peaks in the Pd-Au alloy system, (ii) the Pd (3d5/2) and Bi (4f7/2) peaks in the Pd-Bi alloy

system, and (iii) the Ru (3d5/2) and Au (4f7/2) in the Ru-Au alloy system. The binding

energies of these elements in XPS spectra are shown in Table 2.1 [132-134].

Table 2.1: The binding energies of the constituents of the alloy systems in XPS spectra [132-134].

Peak Pd (3d5/2) Au (4f7/2) Bi (4f7/2) Ru (3d5/2)

Binding Energy / eV 335 84 159 280

The surface compositions of the elements in the examined alloys were calculated using

the following equation [133]:

XA = (IA / I∞A) / ((IA / I∞

A) + (IB / I∞B)) (2.3)

Where A and B are the elemental components of an alloy AB, XA refers to the atomic

percentage of the element A in the alloy surface, I is the area under the peak (or the

intensity of the signal), and I∞ refers to the atomic sensitivity factor of a pure element

(Pd = 2.7, Au = 2.8, Bi = 4.25, and Ru = 2.15) [133]. The quantitative analysis of the

elemental surface composition by XPS has an experimental error of < ±10 % [131].

38

2.4- Electrochemical Measurements

2.4.1- Electrochemical Cell

The electrochemical measurements were performed at room temperature in a specially

designed electrochemical cell [90]. A schematic representation of this cell and its size

are illustrated in Figure 2.9. The design of this cell allows a precise position of the array

and the electrical contacts. It consists of working electrode (WE), reference electrode

(RE) and counter electrode (CE) compartments. A Polytetrafluorethylene (PTFE) plate,

also called Teflon, (10 x 7 cm2) was used to make the body of the working electrode

compartment. The electrochemical array was held on a clamp socket (Figure 2.10) in

order to achieve a good solution seal. The clamp socket was provided with electrical

connections to make connectivity between the array and the electrochemical instrument.

Also, gaskets were positioned between the array and the clamp socket from one side,

and between the clamp socket and the PTFE plate on the other side. Pt gauze was

utilized as a counter electrode which is separated from the working electrode by a glass

sinter. The reference electrode was a commercial mercury/mercuric-sulfate electrode

(Hg/Hg2SO4, Sentec). The tip of the reference electrode is located close to the

electrochemical array (WE) in order to minimize IR drop effect [135]. The cell is also

provided with a water jacket, which is used for circulation of water from a water bath, to

keep the temperature during the measurements at 25 ºC. A gas inlet (GI) with a glass

sinter (GS) is utilized for purging the electrolyte with gases. The connection between the

electrical pads of the array wafer and the electrochemical instrument is achieved by an

integrated circuit (IC) socket. Before the electrochemical measurements, the cell, PTFE

plate and gasket were boiled in pure water, for about two hours, to remove contaminants.

Figure 2.9: The compartments of thethe alloy samples.

Figure 2.10: The clamp socket used with the electrochemical array to achieve a good solution sealand prevent electrolyte leaking.

3 cm

Front

The electrochemical array

Bolts used to attach PTFE plate and glass cell

39

compartments of the electrochemical cell used in the electrochemical screening of

2.10: The clamp socket used with the electrochemical array to achieve a good solution sealand prevent electrolyte leaking.

8 cm

17 cm

2 cm

Electrical connections

Front Back

The electrochemical array

Bolts used to attach PTFE plate and glass cell

Electrical connections to the electrochemical instrument

e electrochemical screening of

2.10: The clamp socket used with the electrochemical array to achieve a good solution seal

Electrical connections

Electrical connections to the electrochemical instrument

40

2.4.2- Array Measurements

The electrocatalytic assessment of the Pd-Au, Pd-Bi and Ru-Au alloy systems for the

HER and HOR were carried out at room temperature in a 0.5 M HClO4 electrolyte

synthesized using concentrated HClO4 (GFS Chemicals, 70% double distilled) and pure

water (Elga Option-Q 7BP, 18.2 mΩ cm). The reason of using HClO4 instead of H2SO4

is to avoid the inhibition of the studied reactions by the adsorption of sulfate or bisulfate

anions on the surface of the alloy catalyst [63].

An electrochemical instrument composed of a three-electrode potentiostat, two 64-

channel current followers (the current conversion sensitivity was 10 µA V-1) and data

acquisition cards (PCI-DAS6402/16, Talisman Electronic) monitored by a PC and a

software was employed to measure the electrochemical responses of the 100 electrodes

in the array sample [90]. The full description of this electrochemical instrument is

available elsewhere [92].

Both cyclic voltammetry and potential step methods were employed in the

electrochemical screening of the three alloy systems. All potentials stated here were

calibrated against the Reversible Hydrogen Electrode (RHE) in 0.5 M HClO4. A digital

voltmeter (Fluke, Model 83 Multimeter) was used for this calibration.

2.4.2.1- Cyclic Voltammetry Measurements

Cyclic voltammetry (CV) is a potential sweep technique (electrochemical spectroscopy)

that provides valuable information about processes taking place at the electrode /

electrolyte interface of a studied system (such as adsorption / desorption of species and

surface redox behavior) [8, 136].

The cyclic voltammetry measurements were carried out at room temperature with scan

rate either 50 or 20 mV s-1. The initial assessments of the three alloy systems by cyclic

voltammetry were performed using a sweep rate of 50 mV s-1 under the following

potential limits: cathodic limit (lower potential, Elo) = - 0.03 VRHE, anodic limit (upper

41

potential, Eup) = 0.5 VRHE, and initial potential (Ein) = 0.2 VRHE. In the case of the Pd-Au

system, the anodic limit was sometimes increased to 1.5 VRHE in order to observe the

surface redox processes of the elemental components on the alloy films. Prior to the

cyclic voltammetry measurements, the electrolyte was purged with Ar (Air Products,

99.997%) for 20 minutes to avoid the presence of the air. The voltammograms are

presented here with respect to current density values as a function of the alloy

composition. The current density values were determined by dividing the current values

obtained on each electrode in the electrochemical array by the geometric surface area of

the electrode (ca. 0.0144 cm2).

The CO stripping measurements were carried out using a scan rate of 20 mV s-1 and

under potential limits of Elo = - 0.03 VRHE, Eup = 1.1 VRHE and Ein = 0.2 VRHE. Before

these measurements, the electrolyte was saturated with CO (BOC, CP grade) for 20

minutes followed by purging with Ar for at least 20 minutes in order to remove the CO

molecules from the electrolyte. Figure 2.11 shows an example of a typical cyclic

voltammogram of Pd, recorded in a 0.5 M HClO4 electrolyte bubbled with 500 ppm CO,

and the potential region of various surface processes.

Figure 2.11: A typical cyclic voltammogram of Pd showing the potential region of various surface processes. The CV in this figure was recorded in 0.5 M HClO4 bubbled with 500 ppm CO, scan rate = 50 mV/s.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

-2

-1

0

1

2

j / m

A c

m-2

(g

eo

)

Potential / VRHE

Oxidation region

Reduction region

HOR

HER

Double layer region

Removal of surface oxide

CO oxidation Surface oxide

formation

42

The anodic and cathodic features in Figure 2.11 are well-established in the literature

[84, 137-143] and the potentials at which they occur vary with the electrode type. A

good electrocatalyst provides a high rate of a reaction at low overpotential (i.e. closer to

the equilibrium potential) [5].

2.4.2.2- Potential Step Measurements

The idea behind the potential step method is that the current-time response of the

working electrode is recorded at a potential (E0) where no reaction of species takes

place. At time t, the applied potential to the working electrode is rapidly stepped to a

value (E1) where the reaction of interest occurs [5, 8, 136].

The electrocatalytic activities of the Pd-Au, Pd-Bi, and Ru-Au alloy systems for both the

HER and HOR were assessed employing the potential step measurements. In each

experiment, the current was initially recorded at 0 VRHE and then stepped to either

negative potentials (in the case of the HER) or positive potentials (in the case of the

HOR). The applied potential to the working electrode was held in each step for 90

second. The electrocatalytic assessment of the HER and HOR on the alloy systems by

the step potential method was carried out prior to characterizing the catalyst surface by

cyclic voltammetry at high potentials.

The HOR measurements were carried out in a H2 atmosphere with either pure H2 (BOC,

99.995 %) or a mixture of H2 and 500 ppm CO (BOC, 500 ppm CO / Hydrogen)

bubbling through the electrolyte. The aim of carrying out the HOR in the latter case was

to assess the CO tolerance of the examined alloy systems.

43

Chapter 3: Palladium-Gold (Pd-Au) Alloy Surfaces

3.1- Introduction

For a long time, gold was regarded to be catalytically inert because of its poor ability to

chemisorb reactants [144, 145]. The historical perception of gold as an inactive material

has been changed in the recent time due to the fact that nanocrystalline structures of gold

have exhibited unique catalytic properties in a variety of redox reactions [146]. The

unique catalytic properties of Au were initially realized in 1980s when supported Au

nanoparticles were proved to be active for the CO oxidation at low temperature [47].

Also, Au was predicted to be the best catalyst for ethyne hydrochlorination [48]. Since

then, catalysis by gold has become an attractive area for many researchers.

Au-based catalysts are used now in chemical industries, environmental protection and

maintenance, chemical sensors and electrochemical processes such as fuel cells,

batteries, and electrochemical sensors [146, 147]. One of the current widespread

applications of Au-based catalysts is in the low temperature oxidation of CO in

reformate gas for fuel cell technology, since CO poisons the catalyst surface [146, 148,

149]. The reason of the extensive interest in using Au-based catalysts is ascribed to their

abilities to catalyze reactions at low temperatures compared to Pt- or Pd-based catalysts

which are inactive below 200ºC [148, 150]. The particle size and support are important

influencing factors on the activity of Au for the CO electrochemical oxidation. It was

shown, in this respect, that reaction activity on titania supported gold nanoparticles (<

6.5nm) is higher than on carbon substrate. A maximum activity was observed with

particle size of ca. 3 nm [151].

The success of gold as a catalyst has led to an extensive interest in bimetallic catalysts

containing gold as one of the components. A particular concern has been directed

towards alloying Au with platinum group metals (pgm), since the Au-(pgm) alloy

catalysts have been observed to be superior to either Au or a pgm alone [152]. Examples

of the applications of the Au-(pgm) catalysts are in the synthesis of hydrogen peroxide

44

(H2O2), the methanol decomposition process, the hydrodesulfurisation of

dibenzothiophene, CO oxidation, toluene hydrogenation and the reduction of NO by

propene [152].

In particular, Pd-Au alloy system is used widely to catalyze a number of chemical

reactions important to industry, hydrogen fuel cells and pollution control. There are

many advantages of using a Pd-Au alloy as a catalyst instead of pure Pd. One of them is

to reduce the cost of the catalysts, since a less amount of the more precious metal is

contained in the alloy. Also, the catalytic activity, selectivity and stability of Pd can be

enhanced by alloying with Au [76, 148, 149].

The enhancement in the catalytic activity of Pd in the presence of Au has been attributed

to the formation of several active sites on the alloy surface that accelerate the reaction.

These sites are composed of ensembles of single Pd atoms (Pd monomers) surrounded

by Au atoms. The suggested role of Au in this case is to promote the formation of these

ensembles and, hence, enhances the catalytic activity of Pd [75, 76, 142, 153].

As pointed out by Goodman and colleagues [74], two model systems of the Pd-Au alloys

appear in the literature. The first is stable bulk alloys and the second involves the

deposition of one metal on a single crystal of another metal (such as the deposition of Pd

on Au (111) surface). The synthesis and the control of the compositions of the alloy

surfaces were difficult to achieve in these studies. Therefore, Goodman’s group has

proposed a system whereby stable alloy films of the Pd-Au system can be prepared by

the deposition of both metals on a substrate of a third metal, such as Mo (110). The

stabilization of the prepared films in this case was achieved by annealing. This method

of alloy synthesis is similar to the method applied here where the synthesis of alloys is

achieved through co-deposition of the elements on a substrate [89].

45

3.1.1- Industrial Applications of Pd-Au Catalysts

Pure noble metals, such as Pd, have a central role as catalysts for various reactions

which have environmental or industrial applications. The catalytic performance of Pd in

many catalyzed reactions was enhanced by alloying with Au. Hydrodesulfurization

(HDS) of petroleum feedstocks is an example of these reactions. The presence of sulfur

in this process results in catalyst poisoning. To overcome this problem, a number of

Aux-Pdy catalysts supported on silica were employed to examine the influence of

alloying Pd with Au on the resistance of the catalysts to poisoning by sulfur during the

HDS process. Au-Pd alloys were found to be more resistant to sulfur poisoning

compared to pure Pd due to the ensemble effect between the two elements in the alloy

[154].

One of the most important industrial processes catalyzed by Au-(pgm) catalysts, in

particular Au-Pd catalysts, is the production of vinyl acetate monomer (VAM) [152].

This process takes place according to the following chemical equation:

CH3COOH + C2H4 + 0.5 O2 → CH3COOCHCH2 + H2O (3.1)

Au was observed to have a promotional effect on the performance of Pd in this process

as alloying Au with Pd has lead to a better catalytic performance, selectivity and stability

compared to Pd alone. It was proposed that critical ensembles of several Pd atoms on the

alloy surface are the active sites in this reaction. The role of Au in the Pd-Au alloy

catalyst is to facilitate the formation of these critical ensembles by isolating single Pd

sites and, at the same time, it hinders any undesired reaction between products [76]. The

current production of vinyl acetate is based on the use of supported Pd-Au alloys with

low concentration of Au [146].

Hydrogen peroxide (H2O2) is used as a bleach and as a disinfectant. The production of

H2O2 by Pd-Au alloy catalysts has been found to produce higher yields than catalysts of

pure Pd or Au due to the ensemble effect in the alloy [146]. In most cases, supported Pd-

Au catalysts were used to enhance the rate of H2O2 production. For instance, the

46

catalytic activities of a range of supported Pd, Au, Pd-Au catalysts for the synthesis of

H2O2 from H2 and O2 have been discussed by a number of researchers [146, 155-157].

The general conclusion from these studies is that the supported Pd-Au catalysts, such as

Pd-Au/TiO2, were observed to perform better than the supported Pd or Au catalysts in

the formation of H2O2.

Au, Pd and Pd-Au alloy catalysts have also displayed remarkable activities in other

selective oxidation reactions such as alkene epoxidation, alcohol oxidation, sorbitol

oxidation and reducing sugars [48, 146, 158, 159]. For example, the catalytic activity

and selectivity of the Pd-Au / C catalyst in the selective oxidation of D-sorbitol was

greater than Pd/C or Au/C catalysts. The better performance of the bimetallic catalyst in

this case was again attributed to the ensemble effect between the components [158].

3.1.2- Electrochemical Applications of Pd-Au Alloy Catalysts

A major potential use of the Au-(pgm) catalysts is in fuel cell technology [152]. A

number of investigations have been concerned with the study of the electrocatalytic

behavior of Pd-Au alloys or Pd overlayers on a single crystal of Au in reactions

important to fuel cells such as the hydrogen oxidation reaction (HOR) and CO tolerance

[101, 139], the oxygen reduction reaction (ORR) [160, 161], carbon monoxide oxidation

[162, 163] and formic acid oxidation [29-31]. The hydrogen evolution reaction (HER)

has also been studied on Pd overlayers on Au single crystals [164, 165] and Au/Pd (111)

surface alloys [142]. Following are various examples where Pd-Au catalysts were

employed for studying electrochemical reactions.

The HER activity has been examined on Au/Pd (111) alloy films supported on a

Ru(0001) substrate in 0.1 M H2SO4 [142]. The Au/Pd (111) alloy catalysts were

synthesized by electrochemical deposition followed by heating up to 700 °C. The active

centers for the HER on Au-Pd alloys were suggested to consist of ensembles of Pd

atoms (a number of single Pd atoms, also called Pd monomers) surrounded by Au atoms.

The Au-Pd alloys containing 0.1-0.3 Pd surface fractions exhibited the maximum

47

activity for the HER and found to be catalytically superior to Pd(111) by a factor of ca.

20. The reason of this was ascribed to the presence of a high number of the Pd monomer

ensembles.

The electrooxidation of H2 has been examined by Schmidt et al [101] over two well

defined Au(111)-Pd surface alloys with Pd surface concentrations of ca. 38 and 65 %.

The catalysts were prepared by Pd vapor deposition in a UHV chamber. The authors

suggest that: (i) the HOR over Au(111)-Pd is slower by one order of magnitude in

comparison with Pt(111); (ii) the HOR kinetics on the Pd-rich surface is faster than the

other surface; (iii) the adsorption/dissociation of molecular H2 on the Pd sites of the

Au(111)-Pd surfaces may be the chemical rate-determining step in the HOR.

A comparative investigation of Pd (111), (100) and (110) overlayers of various

thicknesses on Au (hkl) and on Pt (hkl) [166] showed that formic acid activity on Pd / Pt

(hkl) catalysts is higher than on Pd / Au (hkl) catalysts. Kibler and colleagues [167]

examined the reaction kinetics on massive Pd (111) and Pd adlayers on Au (111). It was

concluded in their study that Pd film thickness on the metal overlayer influences the

adsorption behavior and catalyst reactivity due to electronic modifications. Formic acid

oxidation was also tested on both unsupported and carbon supported Pd-based catalysts

[168]. The reaction activity was found to be higher on carbon supported catalysts and

alloying Au with Pd on C (Pd-Au/C) has exhibited further improvement for the process.

A barrier in the electrocatalytic studies of the HER and HOR on alloy catalysts

containing Pd as one of the constituents is the absorption of hydrogen into the bulk of

the alloy forming bulk hydrides. This is because this process takes place in the under

potential deposition of hydrogen (UPD H) region influencing hydrogen

adsorption/desorption as well as the HER and HOR on the catalyst surface [169]. α-Pd/H

is formed when the hydrogen concentration is low, while the high concentration of

hydrogen forms β-Pd/H [170]. Hydrogen absorption/desorption into/from bulk Pd

electrodes may proceed through a dual mechanism as follows [171, 172]:

48

H+ +e- (3.2)

Habs = [Hβ ↔ Hα ↔ Hsubs] ↔ Hads

1/2 H2 (3.3)

where: Hα and Hβ denote hydrogen absorbed in the α- and β- phases respectively, while

Hsubs signifies the subsurface hydrogen (hydrogen atoms presented directly under the

catalyst surface in a layer of thickness between 20-50 nm). According to this scheme,

the removal of absorbed hydrogen from the bulk of a Pd electrode takes place through an

electrochemical process (the Volmer reaction, 3.2) or/and non-electrochemical

recombination process (the Tafel reaction, 3.3). The former process takes place under an

electrochemical potential and the latter process occur through the diffusion of hydrogen

from the bulk to the electrode surface to combine with an adsorbed hydrogen atom on

the surface producing a hydrogen molecule.

3.2- Composition and Structure Analysis

Several 100-element Pd-Au array samples were synthesized through simultaneous

deposition of the elements employing the HT-PVD method described in the previous

chapter. After the synthesis, a number of analytical tools were employed to determine

the composition and structure of the prepared samples. The bulk and surface

composition were determined by EDS and XPS respectively. The structural analysis of

the samples was achieved by XRD.

3.2.1- EDS Analysis

The use of the wedge shutter in the HT-PVD system facilitated the synthesis of non-

equilibrium (random) Pd-Au alloys with a compositional gradient through simultaneous

deposition of the elements. The atomic percentage of Pd and Au in a Pd-Au sample with

respect to the position (x, y) in the electrochemical array is illustrated by false color

maps in Figure 3.1. It can be clearly seen that Pd concentration in the alloy increases

gradually from the bottom left side to the top right side, whereas Au concentration

increases gradually from the bottom right side to the top left side. The latter Figure

49

shows that deposition of nearly the whole compositional range of the Pd-Au alloy

system was achieved. For instance, the atomic percentages of the elements in an Au-rich

position (electrode: 1, 1) is ca. Pd7Au93, while at a Pd-rich position (electrode: 1, 10) is

ca. Pd89Au11. The electrode (6, 5) has a bulk composition of ca. Pd49Au51. The

compositional gradient in this array sample is in agreement with the prediction of the

HT-PVD deposition method employed here for the synthesis of alloy systems [89].

Figure 3.1: A contour plot of the component elements in a sample of the Pd-Au alloy catalysts with respect to the position (x, y) in the array sample. The arrows refer to the growth direction of the elements in the array sample.

3.2.2- XRD Analysis

Pd and Au exhibit complete miscibility in the solid phase [77]. A continuous series of

solid solutions across the alloy compositional range is formed [140, 159, 173]. A Pd-Au

alloy has a face-centered cubic (fcc) crystal structure with Pd and Au atoms arbitrarily

located at all the faces of the cube [174, 175]. The formation of solid solutions in the Pd-

Au alloys means that Pd and Au atoms have almost the same size and structure. It also

denotes that Pd and Au are able to be completely soluble in each other and that atoms of

one component can be replaced by atoms of the other component. Thus, Pd and Au can

form substitutional alloys [53, 124].

Y

X

50

The phase diagram of the Pd-Au alloy system [53] is shown in Figure 3.2. The first

observation from this diagram is that the melting points of Pd-Au alloys increase

gradually from ca. 1064 to 1550 °C with the decrease of the Au concentration in the

alloy. It also confirms that Pd and Au are able to form solid solutions and completely

miscible in the solid phase at a wide range of temperatures (below 1064 °C in the Au-

rich alloys and below 1550 °C in the Pd-rich alloys). However, the phase diagram shows

that a miscibility gap arises at the Au rich side and disappears at alloy compositions ≤ 40

at. % Au. At concentrations of Au lower than ca. 40 at. %, Au and Pd are fully miscible

irrespective of the phase of the alloy. The Pd-Au alloys with concentrations of Au > 50

% are stable in the solid form below between ca. 1070-1500 °C and stable above this

range in the liquid form. The Pd-Au alloys with concentrations of Au < 50 % are stable

in the solid form below between ca. 1500-1555 °C and stable above this range in the

liquid form.

Figure 3.2: The phase diagram of the Pd-Au system [53]. L: refers to the liquid phase.

The structure of an array sample of Pd-Au alloys was characterized using thin film

powder XRD. Figure 3.3 illustrates typical X-ray diffractograms for a number of Pd-Au

alloys. The Au (111) Bragg peak appears at about 2θ = 38° and Au (200) Bragg peak

51

occurs at about 2θ = 44° [174]. Two peaks at about 2θ = 40° and 47° correspond to Pd

(111) and Pd (200) Bragg peaks respectively [176, 177]. The shift of 2θ values from that

of Au (111) peak to that of Pd (111) peak is an indication of the formation of solid

solutions of the alloys. Therefore, the peaks lie between 2θ = 38° and 40° are attributed

to the formation of a Pd-Au (111) alloy in each case. The peaks in the region between 2θ

= 44.5° - 47° similarly correspond to the Pd-Au (200) alloys. The results correspond

closely to the structure of Pd-Au alloys reported in the literature [174, 177, 178]

confirming the spontaneous formation of the alloy phase at ambient temperature.

Figure 3.3: XRD patterns for a number of Pd-Au alloys. The dashed lines indicate the 2θ values of the pure elements.

The Pd-Au array sample used in the latter measurement was also annealed at 300 °C for

15 minutes in order to assess the effect of annealing on the crystal structure or the

intensities of the peaks appearing in the latter result. Similar X-ray diffractograms and

intensities were obtained (Figure 3.4) suggesting that annealing Pd-Au alloys under

these conditions exhibits little impact on the bulk crystal structure (a similar phase

before and after annealing is obtained).

0

2

4

6

8

10

12

14

16

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Intn

en

sity

/ A

rbitra

ry U

nit

s

2θ Scattering Angle /

96 % Au

88 % Au

79 % Au

65 % Au

48 % Au

25 % Au

8 % Au

Pd (200) Au (200)

Au (111)

Pd (111)

52

Figure 3.4: XRD patterns for a number of Pd-Au alloys annealed at 300° C for 15 minutes. The dashed lines indicate the 2θ values of the pure elements.

Additional analysis of the (111) Bragg peaks in the region between 2θ = 38°-41° was

performed in order to investigate the variation of the position and the intensities of the

peaks as a function of the compositions of the Pd-Au alloy (Figure 3.5A-B). Increase in

the Au concentration decreases 2θ of the (111) diffraction peak in three stages (Figure

3.5A): (i) a steady decrease with concentrations of ca. at. % Au ≤ 20, (ii) an inflection of

the curve with concentrations of ca. 20-50 at. % Au and (iii) a steep decrease with

concentrations at. % Au ≥ 50. A plot of the peak intensity as a function of Au

composition (Figure 3.5 B) also shows three characteristic compositional regions: Au at.

% < 20, 20 < Au at. % < 75, and Au at. % > 75. The most intense peaks correspond to

the limiting pure compositions (alloys rich in Pd or Au) and the least intense peaks

correspond to the Pd-Au alloys with intermediate compositions.

0

2

4

6

8

10

12

14

16

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Inte

nsit

y /

Arb

itra

ry U

nit

s

2θ Scattering Angle /

96 % Au

88 % Au

79 % Au

65 % Au

48 % Au

25 % Au

8 % Au

Au (111)

Pd (111)

Au (200) Pd (200)

53

Figure 3.5: (A) A plot of the position of the (111) Bragg peaks in the region 2θ = 38° to 41° as a function of Au concentration, (B) The intensity of the peaks.

The lattice parameters (a) of pure crystalline Au and Pd are (4.08 Å) and (3.89 Å)

respectively [174]. According to Vegard’s law in crystallography, the lattice parameter

of a Pd-Au alloy should lie between these two values and follows a linear relation as a

function of the alloy composition [127, 140, 174]. Considering this, the XRD data were

used to calculate the lattice parameters of a number of Pd-Au alloys. The calculations

were performed on the (111) peak data. Figure 3.6 shows the lattice parameters of a

number of Pd-Au alloys as a function of Au composition. The dependency of the lattice

parameter on the alloy composition clearly obeys Vegard’s law, since a linear relation is

observed. This finding is consistent with the behavior one may expect upon alloying Pd

with Au [127, 140, 174].

0 10 20 30 40 50 60 70 80 90 10038.6

38.8

39

39.2

39.4

39.6

39.8

40

40.2

40.4

Atomic percent Au

FOMPeakPosition

111

0 10 20 30 40 50 60 70 80 90 1003

4

5

6

7

8

9

10

Atomic percent Au

FOMPeakMaxInt

111 B A

54

Figure 3.6: The lattice parameters (a) of a number of Pd-Au alloy catalysts as a function of Au composition. The red circles are the lattice parameters of pure elements (aAu = 4.08 Å, aPd = 3.89 Å) [174]. The dashed line represents the ideal relation according to Vegard’s law [127, 140, 174].

3.2.3- XPS Analysis

Annealing temperature and length influence surface morphology and composition of

alloys, since annealing at high temperatures smoothes the surface and makes it

dominated by the more stable surface component [74]. The surface composition and

surface segregation phenomenon in the Pd-Au alloys have been assessed in the literature

by a number of techniques [74, 77, 179-181]. It appears to be agreed that a strong

surface segregation of Au indeed takes place in this system at equilibrium (after

annealing). The surface free energy of Pd and Au are 2.043 Jm-2 and 1.626 Jm-2

respectively. The surface free energy of Au is lower than that of Pd. Therefore, Au tends

to segregate at equilibrium to the surface in order to minimize the surface free energy

[77].

A compositional analysis of a number of Pd-Au alloys was carried out by XPS in order

to determine the surface composition and compare it to the bulk composition measured

by EDS. The XPS measurements were performed on various fields in the 100-field Pd-

Au array. The selected fields were chosen to provide a range of alloy compositions

3.8

3.85

3.9

3.95

4

4.05

4.1

3.8

3.85

3.9

3.95

4

4.05

4.1

0 20 40 60 80 100

latt

ice p

ara

mete

r /

Å

latt

ice p

ara

mete

r /

Å

Au at. %

55

appearing in Figure 3.1. The main peak binding energies expected for metallic Au and

Pd are summarized in Table 3.1 [132].

Table 3.1: Binding energies of metallic Au and Pd [132].

Element Au(4f 7/2) Au(4f 5/2) Pd(3d 5/2) Pd(3d 3/2)

Binding Energy / eV 84 88 335 340

The Au (4f) XPS region of a number of unannealed and annealed (300 ºC for 15

minutes) Pd-Au alloys are shown in Figures 3.7 and 3.8 respectively. A shift, in both

cases, in the Au (4f) peak positions towards lower binding energies is observed with

decreasing Au concentration in the alloy. The Au (4f7/2) binding energy is shown in

Figure 3.9 as a function of bulk Au composition for various unannealed Pd-Au alloys.

There is clearly a chemical perturbation in the Au environment reflecting an interaction

between the Au and Pd components. The shift in the Au (4f) peak position upon mixing

with Pd can be considered as an indication of alloy formation [182].

Figure 3.7: The Au (4f) XPS region of various random Pd-Au alloys.

-50

0

50

100

150

200

80 82 84 86 88 90 92

Inte

nsit

y /

CP

S

Binding Energy (BE) / eV

91 % Au

82 % Au

58 % Au

32 % Au

12 % Au

Au (4f7/2)

Au (4f5/2)

56

Figure 3.8: The Au (4f) XPS region after annealing Pd-Au alloys at 300 °C for 15 minutes.

Figure 3.9: The Au (4f 7/2) peak positions as a function of Au composition for various unannealed Pd-Au alloys.

The surface Au composition (before and after annealing at 300 °C for 15 minutes) in a

number of Pd-Au alloys has been compared to the Au bulk composition (Figure 3.10).

The surface Au and surface Pd compositions were calculated using the area under the

Au (4f7/2) and the Pd (3d5/2) peaks respectively. As one may expect, similar

-50

0

50

100

150

200

80 82 84 86 88 90 92

Inte

nsit

y /

CP

S


91 % Au

82 % Au

58 % Au

40 % Au

12 % Au

83.3

83.5

83.7

83.9

84.1

84.3

0 20 40 60 80 100

Bin

din

g E

nerg

y (B

E)

/ eV

Au at. %

Au (4f5/2) Au (4f7/2)

57

compositions are observed before heat treatment indicating that the Au composition in

the surface is similar to that in the bulk of the alloy (no surface segregation). The surface

Au composition after annealing is higher than that in the bulk indicating a surface

segregation of Au upon annealing under these conditions. The dotted red line in Figure

3.10 represents a fitting to the data after annealing with a polynomial equation (y=1.85x-

0.0085x2). This equation was subsequently employed for the calculation of the surface

composition along the whole compositional range of the alloy system in order to assess

the HER and HOR activity as a function of surface composition. This will be shown

later in this chapter.

The surface segregation of Au observed after annealing is consistent with the behavior

reported in the literature as well as the theoretical prediction based on the surface free

energies of the elemental components [74, 77, 179-181]. It is also in accordance with the

observation by Yi and colleagues [77] that the surface composition of a 1:1 Pd-Au alloy

was changed to Au0.8Pd0.2 after annealing at 800 K.

Figure 3.10: Au surface composition as a function of bulk Au composition before heat treatment (blue diamonds) and after heat treatment at 300 °C for 15 minutes (red diamonds). The dotted blue line represents the relation one may expect before annealing based on that the bulk composition is similar to that in the surface (no surface segregation). The red dotted line represents a fitting to the data obtained after annealing with a polynomial equation: y=1.85x-0.0085x2.

0

20

40

60

80

100

0 20 40 60 80 100

Su

rfa

ce

Au

at.

%

bulk Au at. %

before heat treatment

after heat treatment

58

3.3- Base Voltammetry and CO Stripping Measurements

A preliminary understanding of an alloy system can be achieved by running a cyclic

voltammetry experiment due to its ability to provide a rapid qualitative analysis of the

system [5]. Therefore, the initial assessment of the Pd-Au system was carried out in a

0.5 M HClO4 electrolyte using this technique. An example of the cyclic voltammetry

responses of a 100 electrode array of the Pd-Au system with various compositions of Pd

and Au is shown in Figure 3.12. The direction of the arrow refers to the growth of Au

concentration in the array. The upper potential (Eup) in this case was restricted to 0.5

VRHE in order to assess the hydrogen underpotential deposition (Hupd), hydride formation

and hydrogen evolution/oxidation regions with minimal perturbation by surface

oxidation.

Figure 3.12: Cyclic voltammograms of a 100-electrode of Pd-Au alloys. The arrow refers to the growth direction of Au in the sample. X: refers to a dead electrode.

001

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

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4e -5

060

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

061

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

0 62

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

063

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

064

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

065

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

066

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

067

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

068

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

069

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

070

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

071

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

0 72

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

073

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

074

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-2e -5

0

2e -5

4e -5

075

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

076

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

077

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

078

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

079

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

080

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

081

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

0 82

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

083

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

084

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

085

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

086

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

087

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

088

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

089

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

090

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

091

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

0 92

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

093

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

094

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

095

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

096

0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6

-4 e-5

-3 e-5

-2 e-5

-1 e-5

0

1e-5

2e-5

3e-5

4e-5

097

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

098

0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

099

0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6

-4e -5

-3e -5

-2e -5

-1e -5

0

1e -5

2e -5

3e -5

4e -5

100

0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6

-4e-5

-3e-5

-2e-5

-1e-5

0

1e-5

2e-5

3e-5

4e-5

Cu

rren

t (A

)

Potential / VR H E

X X X

X

X X

X

X

59

Cyclic voltammetry responses of various electrodes are expanded in Figure 3.13A-F.

There are two characteristic features which appear in some of these figures and refer to:

(1) the HER in the cathodic sweep at approximately -0.05 VRHE, (2) the HOR in the

anodic sweep at about 0.25 VRHE. Clearly, the addition of Au to Pd influences the extent

of the HER and HOR. The catalytic activity with respect to the HER is also reflected in

the subsequent HOR. A higher concentration of evolved hydrogen will subsequently

result in more HOR in the anodic sweep. The Pd-Au alloy with a small concentration of

Pd (ca. 16 at. % Pd, Figure 3.13A) exhibits no HER activity and it behaves almost as

pure Au. As the concentration of Pd increases, the geometric current densities and the

peaks intensities increased. The maximum activity for the HER in Figure 3.13 is

displayed by the alloy containing Pd76Au24 (higher than pure Pd sample). This result

suggests that alloying Pd with Au improves the HER activity.

60

Figure 3.13: Cyclic voltammograms of six electrodes with different compositions of Pd and Au (Pd:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4. CVs are from the 2nd Cycle.

Further voltammetric assessment of the Pd-Au system was carried out by cycling to an

upper limiting potential of 1.5 VRHE in order to observe the surface redox features which

occur at high electrode potentials. A number of features in the anodic and cathodic

sweeps could be identified as shown in Figures 3.14A-F. The voltammograms are a

revision of the HER (1) and HOR (2) observed at a lower potential (Figure 3.13). A

maximum HER activity is again observed to be higher on some of the alloy surfaces

than on pure Pd. No significant influence on the HER and HOR features is observed

suggesting a little or no perturbation of the surface for these processes upon cycling to

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4

-2

0

2

4

27 : 73

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4

-2

0

2

4

16 : 84

59 : 41

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4

-2

0

2

4

76 : 24

Potential (E ) / V

-0 .1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-4

-2

0

2

4

100 : 0

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4

-2

0

2

4

41 : 59

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

j / m

A c

m-2

(g

eo

)

-4

-2

0

2

4

A

C

E F

D

B

2

1

61

this potential. The peak (3) in the anodic cycle above ca. 0.8 VRHE corresponds to the

oxidation of the surface and three peaks in the subsequent cathodic cycle correspond to

reduction of the surface oxide (4-6). The cathodic peaks at ca. 1.2 VRHE (4) and at ca.

0.65 VRHE can be ascribed to reduction of Au oxide and Pd oxide respectively [63, 110,

169, 183], while the cathodic peak at ca. 0.8 VRHE can be associated with the reduction

of Pd-Au ensemble oxide. As one may expect, the intensities of the peaks (4-6) vary

with the surface composition of the alloy. Similar compositional dependences of the

surface redox behavior on Pd-Au alloys were observed elsewhere [63, 183]. In

particular, the reduction peak of the Pd-Au oxide (peak 6) can be considered as further

evidence of the formation of the alloy. This is because it locates between the reduction

peaks of the pure Pd at ca. 0.65 VRHE and Au at ca. 1.2 VRHE. The broadness and position

of the alloy peak (6) vary with the alloy composition. It shifts towards more positive

potentials (above 0.8 VRHE) with increasing Au concentration and to more negative

potentials (below 0.8 VRHE) with decreasing Au concentration in the alloy. This variation

indicates a variation in the ensemble size on the alloy surface [75].

The key role in the HER and HOR is the adsorption of hydrogen on the catalyst surface.

Distinctively, Pd is a hydrogen absorbing metal which means that hydrogen can also

absorb into the bulk of both Pd and to perhaps a lesser extent its alloys. As a result of

this behavior, bulk Pd hydrides (Pd-H) can be formed in acidic electrolyte. Pd and

hydrogen can form two phases of hydrides (α-Pd/H with a low concentration of

hydrogen and β-Pd/H with a high concentration of hydrogen). The amount of hydrogen

that can absorb into an alloy of Pd-Au is governed by the bulk Pd content in the alloy.

Hence, there are also other peaks which may occur at the HER and HOR potential range

in the presence of the high Pd content alloys. These peaks could be ascribed to hydrogen

adsorption or hydride formation (7), and hydrogen desorption from the surface or

hydride decomposition (8) [84, 170]. In recognition of this, it is difficult sometimes to

distinguish the individual process due to the small range of potential where the responses

of all these processes may appear. A voltammetric examination on a number of Pd-Au

alloys [140] has shown that the absorption ratio of hydrogen into the bulk decreases

gradually with the increase of the Au content in the alloy composition and it becomes

62

zero at approximately 70 % Au. This indicates that alloying Pd with a high content of

Au suppresses the formation of Pd/H phases. Therefore, the peaks (7 and 8) disappeared

with the increase of Au concentrations in the alloys.

Figure 3.14: Cyclic voltammograms of various Pd-Au alloys (Pd:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4 with scan rate (γ) = 50 mVs-1. CVs are from the second cycle. The limiting potentials are: Ein = 0 VRHE, Elo = 0 VRHE and Eup = 1.5 VRHE.

16 : 84

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

23 : 77

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

70 : 30

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

j /

mA

cm

-2 (

ge

o)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

50 : 50

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

86 : 14

Potential / VRHE

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

100 : 0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

A

C D

E F

B

8

7

8

7

1

2 3

4 5 6

63

The potential region where the surface oxide reduction peaks of Pd, Au and Pd-Au alloy

occur is enlarged in Figure 3.15. The position of the alloy reduction peak is plotted in

the inset as a function of Au atomic composition. A linear increase is observed, i.e. shift

in the peak position towards higher potentials (away from that of pure Pd) with the

increase in the Au concentration in the alloy.

Figure 3.15: Enlarged surface reduction peaks on various Pd-Au alloys. The inset is the position of the surface reduction peak of the Pd-Au alloy as a function of Au composition in the alloy.

In order to assess the effect of heat treatment at 300° C for 15 minutes on the surface

redox processes on Pd-Au surfaces, voltammetric experiments were carried out at an

upper limiting potential of 1.5 VRHE using the same electrodes before and after heat

treatment under these conditions. Figure 3.16 shows window opening cyclic

voltammograms of a number of Pd-Au compositions before annealing (the solid lines)

and after annealing (the dashed lines). No remarkable difference in the features ascribed

to the HER and HOR (1-2) or the oxidation and reduction of the surfaces (3-6) in the

anodic and cathodic sweeps can be observed. The features ascribed to hydride formation

and decomposition (7-8) disappeared after heat treatment. This could be correlated with

the surface segregation of Au upon annealing at these experimental conditions.

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

j / m

A c

m-2

(geo

)

Potential / VRHE

16 : 84

31 : 69

50 : 50

69 : 31

86 : 14

100 : 0

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0 20 40 60 80 100

Po

ten

tia

l / V

RH

E

Au at. %

64

Figure 3.16: Cyclic voltammograms of different Pd-Au compositions (Pd:Au ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 (γ = 50 mVs-1). The solid lines are before heat treatment and the dashed lines are after heat treatment. CVs are from the second cycles. The limiting potentials are: Ein = - 0.03 VRHE, Elo = 0.2 VRHE and Eup = 1.5 VRHE.

The surface composition of a metal alloy and its components can be determined from the

charges under surface oxide reduction peaks [84]. The charge associated with the surface

reduction peaks (4-6) on a single array of random Pd-Au alloys was determined by

integrating the peaks and are plotted as a function of Au composition in Figure 3.17.

The total charge associated with the three peaks is shown in Figure 3.18. As anticipated,

the charge associated with the reduction peak of Au increases steadily with the increase

in Au content in the alloy. It is also understandable that the charge associated with the

5 : 95

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

23 : 77

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

46 : 54

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

j /

mA

cm

-2 (

ge

o)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

57 :43

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

76 : 24

Po tentia l / V RH E

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

A

E F

DC

B

1

2 3

4 6

5 7

8

65

reduction peak of Pd decreases with the increase in Au atomic concentration in the alloy.

This behavior could be clearly observed with compositions (≥ 10 % Au). A steep

decrease in the Pd peak charge occurs in the compositional range ≤ 10 at. % Au

followed by a steady decrease with the increase in the Au content in the alloy. This

suggests that a very strong electronic perturbation of Pd occurs by alloying with low

concentrations of Au (ca. 5-10 at. % Au). The charge associated with the Pd-Au

ensemble peak increases and decreases smoothly and symmetrically with a maximum at

ca. Pd50Au50. The total charge is not a linear function of composition, but exhibits a

minimum between (5-10 % Au) and a maximum at ca. 50:50. This behavior is an

indication of the electronic interaction between the components, since a linear decrease

in charge is expected for a non interacting system (the dotted black line in Figure 3.18).

Figure 3.17: Charge associated with reduction peaks of Au, Pd and the Pd-Au alloy as a function of Au composition. The dotted lines are guides to the eye.

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0 20 40 60 80 100

Ch

arg

e /

µC

Au at. %

Au

Pd

Pd-Au

66

Figure 3.18: The total charge (dashed line) associated with the reduction peaks of Au, Pd and Pd-Au alloy as a function of Au concentration. The dotted black line indicates the behavior for a non interacting system.

The surface redox behavior of Pd, Au and Pd-Au alloy can be interpreted based on the

ensembles of atoms as follows: (i) the surface redox behavior of Au ensembles is not

influenced by the neighbouring Pd atoms, (ii) the surface redox behavior of Pd

ensembles is strongly influenced by the neighbouring Au atoms, and (iii) the surface

redox behavior of the alloy is composition-dependent.

The adsorption and stripping of CO on and from the Pd-Au alloys could provide further

insights into the properties of the surfaces and the electronic behavior. Figure 3.19A-F

shows typical CO stripping voltammograms of a number of Pd-Au alloys. A sharp

anodic peak at around 0.95 VRHE was detected in the first scan (red lines). This peak can

be ascribed to the CO stripping from the surfaces [75, 184]. No peaks ascribed to the

HER or hydride formation could be observed in the first scan (red lines) due to a

blocking effect by adsorbed CO on the surface [162]. In the second cycle (black lines),

the CO stripping peaks are not observed which means that CO was completely removed

from the surface in the first scan. This allowed hydrogen to be adsorbed on the alloy

0.00

4.00

8.00

12.00

16.00

20.00

24.00

0 20 40 60 80 100

Ch

arg

e /

µC

Au at. %

Pd Au

Pd-Au

Total Charge

67

surfaces. The two features ascribed to the HER and hydride formation were, then,

detected in the second cycle.

Figure 3.19: Typical CO stripping voltammograms of a number of Pd-Au compositions (Pd:Au ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with 500 ppm CO. The CVs are from the first cycle (red lines) and the second cycle (black lines). The limiting potentials are: Ein = 0.2 VRHE, Elo = 0 VRHE and Eup = 1.1 VRHE. The scan rate = 50 mV/s.

As one may expect, the voltammograms indicate that CO adsorption on the alloy surface

increases with increasing Pd composition. This is consistent with the proposition that

CO adsorption on the Pd-Au surfaces takes place on Pd surface sites and not on Au

surface sites [75, 101]. The relative positions of the CO stripping peaks on the Pd-Au

alloys in Figure 3.19A-F are enlarged in Figure 3.20.

9 : 91

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2

-1

0

1

2 21 : 79

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2

-1

0

1

2

40 : 60

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

j /

mA

cm

-2 (

ge

o)

-2

-1

0

1

2 62 : 38

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2

-1

0

1

2

88 : 12

Potential / VRHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2

-1

0

1

2 100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-2

-1

0

1

2

A

E F

DC

B

68

Figure 3.20: The relative positions of the CO stripping peaks on various compositions of Pd-Au alloys. Pd:Au ratio, atomic %.

Clearly, the position and intensity of the CO stripping peak on a Pd-Au alloy vary with

the alloy composition. In order to understand the trend of this variation, the position of

the CO stripping peak is plotted as a function of bulk Au composition in Figure 3.21. A

symmetrical decrease and increase is observed with increasing the Au composition in

the alloy with a minimum potential at an alloy composition of Pd50Au50.

Figure 3.21: The positions of the CO stripping peaks as a function of Au composition in the alloy.

-0.2

0.2

0.6

1

1.4

1.8

0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02

j / m

A c

m-2

(geo

)

Potential / VRHE

9 : 91

21 : 79

40 : 60

62 : 38

88 : 12

100 : 0

0.92

0.925

0.93

0.935

0.94

0.945

0.95

0.955

0.96

0 20 40 60 80 100

Po

ten

tia

l / V

RH

E

Au at. %

69

The position of CO oxidation peak on a catalyst surface primarily depends on the

overpotential for water activation to produce adsorbed OH- species on the surface that is

required to react with adsorbed CO on the surface producing CO2 according to the

Langmuir-Hinshelwood mechanism [185]:

CO → COads (3.4)

H2O → OHads + H+ + e- (3.5)

COads + OHads → CO2 + H+ + e- (3.6)

The lowest peak potential in Figure 3.21 occurs on an alloy composition of ca. Pd50Au50

indicating that this alloy composition is possibly the optimum Pd-Au composition for

CO oxidation. Of relevance to this is the CO oxidation activity on Pt-Ru alloy catalysts

as the overpotential for CO oxidation on the alloy catalysts is lower than on pure Pt [81,

186]. A Pt-Ru alloy catalyst with a composition of Pt50Ru50 has been found to be the

optimum Pt-Ru composition for CO oxidation and is considered as one of the most

promising catalysts for this process [72, 187]. It is interesting to observe that the

optimum Pd-Au composition for CO oxidation (Pd50Au50, as indicated from Figure

3.21) is similar to the optimum Pt-Ru alloy composition.

The CO adsorption and stripping were also assessed on a number of annealed Pd-Au

alloy surfaces (300 °C for 15 minutes). Figure 3.22A-F shows the effect of annealing on

the CO stripping voltammograms of a number of Pd-Au alloys. It appears that annealing

the Pd-Au alloys under these conditions alters the properties of the surfaces towards the

adsorption of CO. The CO stripping peaks from the annealed samples (dashed lines) are

not observed with alloy compositions below 50 % Pd and can be (sometimes hardly)

seen with Pd concentrations > 50 %. Also, the intensities of the CO stripping peaks are

less after annealing which indicate that CO adsorbs weakly on the alloy surface at

equilibrium. This behavior suggests that annealing a Pd-Au alloy hinders the adsorption

of CO. The reason for that can be attributed to the concentration of Pd at the alloy

surface (the active sites for CO adsorption) being lower after annealing compared to the

case before annealing.

70

Figure 3.22: The CO stripping peaks of a number of Pd-Au compositions (Pd:Au ratio, atomic %) before annealing (the solid lines) and after annealing (the dashed lines).

The coverage of the adsorbed CO (θCO) on the Pd-Au alloys surfaces can be determined

by integrating the area under the CO stripping peaks. Figure 3.23 shows the charges

associated with the CO stripping peaks on a random (blue squares) and annealed (300° C

for 15 minutes, red circles) array samples as a function of the bulk Au composition in

the alloy. The dashed line represents the behavior one may predict based on that CO

adsorption occurs only on Pd sites [75, 101] and assuming that Au acts as an inert

diluent (no electronic interaction).

57 : 43

0.80 0.84 0.88 0.92 0.96 1.00 1.04

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

62 : 38

0.80 0.84 0.88 0.92 0.96 1.00 1.04

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

73 : 27

0.80 0.84 0.88 0.92 0.96 1.00 1.04

j /

mA

cm

-2 (

ge

o)

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

85 : 15

0.80 0.84 0.88 0.92 0.96 1.00 1.04

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

93 : 7

Potential / VRHE

0.80 0.84 0.88 0.92 0.96 1.00 1.04

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

100 : 0

0.80 0.84 0.88 0.92 0.96 1.00 1.04

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

C

FE

D

BA

71

The non-linear relationship demonstrates that alloying Pd with Au hinders the adsorption

of CO on the alloy surfaces. This indicates that the chemical and electronic properties of

Pd atoms in Pd-Au alloys are strongly perturbed by alloying with Au [75]. The charge

associated with the CO stripping peaks of the alloy surfaces in the annealed sample is

lower than that of the surfaces in the unannealed sample which implies that annealing

Pd-Au alloys with Au at. % < 80 % reduces CO adsorption on the surface. The reason

for that can be ascribed to that annealing the Pd-Au alloys under these conditions

enriches the surfaces with Au leading to less Pd atoms (the active sites for the CO

adsorption) on the alloy surface [75]. The effect of segregation phenomenon can be

clearly observed with alloy compositions (< 80 % Au).

Figure 3.23: Charges associated with the CO stripping peaks from the Pd-Au alloy surfaces before annealing (blue squares) and after annealing at 300° C for 15 minutes (red circles) as a function of the alloy composition. The dotted lines are guides to the eye. The dashed line reveals the behavior one may expect assuming a random dilution of surface Pd atoms by Au acting as an inert diluent.

The charge associated with the stripping peak of one CO monolayer (ML) on Pd (111) is

approximately 424 µC cm-2 [184]. This estimation was made assuming that each Pd

atom on the surface adsorbs one molecule of CO. Considering that, the real surface area

of the Pd-Au electrodes used in this study can be estimated by dividing the value of the

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

0 20 40 60 80 100

Ch

arg

e /

µC

Au at. %

72

charge associated with the CO stripping peak of pure Pd (ca. 16 µC) by the estimated

charge of 1 ML of Pd (111). Hence, the real surface area is approximately equal to 0.038

cm-2. Thus, the roughness factor of the studied catalysts is ca. 2.71 considering the

geometric surface area of each electrode is equal to ca. 0.014 cm-2.

The population of Pd atoms as a function of Au composition is represented by a

statistical model in Figure 3.24 (with thanks to Robert Noble, the writer of the script).

This model was designed assuming a hexagonal surface layer surrounded by randomly

distributed alloy components. A representation of this assumption is shown in Figure

3.25 for a Pd atom surrounded by three active components (Pd atoms) and three inactive

components (Au atoms). The various ensembles on the Pd-Au alloy surface are shown in

this model (Figure 3.24) where the population of the Pd atoms varies with the alloy

composition. A Pd atom on the alloy surface can be surrounded by six Pd atoms (black),

five (grey), four (yellow), trimers (blue), dimers (green) or monomers of Pd atoms (red).

The second layer interaction has been ignored in this model.

Figure 3.24: A representative model of the population of Pd atoms on the alloy surface as a function of Au composition in the alloy assuming a hexagonal surface layer where the Pd atom on the surface can be surrounded by six (black), five (grey), four (yellow), three (blue), two (green) or one (red) neighbouring Pd atom.

Au at. %

0 20 40 60 80 100

Po

pu

lati

on

/ a

.u.

0

200

400

600

800

1000

1200

1400

1600

1 neighbour 2 neighbours 3 neighbours 4 neighbours 5 neighbours 6 neighbours

73

Figure 3.25: A Pd atom surrounded by hexagonally oriented three active components (Pd atoms) and three inactive components on the alloy surface.

The CO adsorption (coverage) on the Pd-Au alloy catalysts (Figure 3.23) can be

described according to the model shown above assuming that a CO molecule adsorbs on

a single Pd atom as follows: (i) CO adsorbs preferentially on the ensembles very rich

with Pd, (ii) the intermediate Pd-Au alloy ensembles incorporating even one or two Au

atoms exhibit low affinity towards CO adsorption as a steep decrease occurs in the

intermediate composition, (iii) CO adsorption is also preferential on the ensembles

having Pd monomers and dimers as Au-rich alloys remain active for CO adsorption.

These observations are consistent with the proposal that CO adsorption is favored on the

large Pd ensembles (when the alloy rich with Pd), and that CO adsorbs on Pd monomers

at poor-Pd alloys [75].

3.4- The Catalytic Activity for the HER

In order to assess the HER catalytic activity on the Pd-Au alloys, a series of potential

step experiments were performed at room temperature in 0.5 M HClO4 using a number

of 100-electrode electrochemical arrays with various compositions of both elements. In

each experiment, the potential was stepped from 0 VRHE where the HER does not occur

to more negative potentials where the HER takes place. The current was recorded at the

following potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007 → 0 VRHE.

At each step, the potential was held for 90s. The backward steps were performed in

Pd

Pd

Au

Au Pd

Au Pd

Active component

Inactive component

74

order to examine the reproducibility of the resulting currents from the forward steps.

Examples of the current densities (j) of various Pd-Au compositions at these potentials

as a function of time are shown in Figure 3.26. The non-zero current densities at E = 0

VRHE is attributed to the accuracy of the reference electrode used in the electrochemical

measurements so that there is a small difference between the precise potential in the

potentiostat and the real applied potential on the working electrode. The difference is

about 0.003 VRHE. Clearly, the responses are varied with the compositions of the alloys.

The Pd-Au alloy with high content of Au (Pd8Au92) shows the lowest activity among

these catalysts and a better activity is shown with decreasing the Au concentration in the

alloy to ca. 80 at. %. The highest activity is exhibited by the alloy with a composition of

Pd45Au55.

75

Figure 3.26: The current densities in the HER as a function of time recorded at room temperature on a number of random Pd-Au catalysts (Pd:Au ratio, atomic %) in 0.5 M HClO4.

The HER activity on a series of Pd-Au alloy catalysts were measured with the purpose

of providing data over the whole compositional range of the Pd-Au alloy system. Figure

3.27 shows the geometric HER activity at - 17.51 mVRHE as a function of the bulk alloy

composition obtained on a single array of Pd-Au alloy catalysts. This curve can be

divided into three regions. In the compositional range between ca. 1-25 at. % Au, the Pd-

Au alloys are observed to have almost similar activities to pure Pd. An interesting

catalytic behavior for the HER is exhibited in the range of Pd-Au alloys 30-70 at. % Au.

A maximum for the HER activity (ca. 0.35 mA cm-2) occurs at ca. Pd50Au50 and is ca.

three times larger than pure Pd. A sharp decrease in the catalytic activity is observed

8 : 92

0 100 200 300 400 500 600 700

-0.4

-0.3

-0.2

-0.1

0.020 : 80

0 100 200 300 400 500 600 700

-0.4

-0.3

-0.2

-0.1

0.0

45 : 55

0 100 200 300 400 500 600 700

j /

mA

cm

-2 (g

eo

)

-0.4

-0.3

-0.2

-0.1

0.0

71 : 29

0 100 200 300 400 500 600 700

-0.4

-0.3

-0.2

-0.1

0.0

86 : 14

0 100 200 300 400 500 600 700

-0.4

-0.3

-0.2

-0.1

0.0

100 : 0

Tim e / s

0 100 200 300 400 500 600 700

-0.4

-0.3

-0.2

-0.1

0.0

C

BA

E F

D

0

- 0 .00 7

0

- 0 .00 7

- 0 .0 14 - 0 .0 14

- 0 .0 21

76

with the increase in Au concentration on the alloys having compositions > 75 % Au. A

similar compositional dependence for the HER was obtained at - 10.38 mVRHE as shown

in Figure 3.28.

Figure 3.27: The geometric HER activity at - 17.51 mV as a function of the bulk composition of a number of Pd-Au catalysts measured at room temperature in a 0.5 M HClO4 electrolyte. The geometric current densities in this plot are the average of three experiments. The dotted line is a guide to the eye.

Figure 3.28: The geometric HER activity of a number of Pd-Au catalysts (the same alloys used in the latter plot) at - 10.38 mV measured at room temperature in 0.5 M HClO4.

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

77

In order to determine the HER specific activity, the current values obtained on the Pd-

Au array sample used in the latter measurement were normalized to Pd composition in

the alloy assuming that the adsorption of hydrogen only takes place on Pd. Figure 3.29

illustrates the specific current densities at -17.51 mV as a function of the bulk alloy

composition. The maximum activity appears at a surface composition of ca. Pd30Au70

shifting towards higher compositions of Au compared to the case of the geometric

activity where the maximum activity was observed at ca. Pd50Au50 (Figure 3.27).

Figure 3.29: The specific HER activity at -17.51 mV of the Pd-Au catalysts measured at room temperature in 0.5 M HClO4.

The enhanced HER activity on the alloy surface compared to pure Pd could be

associated with the formation of low co-ordinated Pd ensembles (trimers, dimers and

monomers) as shown from the statistical model in Figure 3.24. It clearly indicates that

the optimum Pd-Au compositional range (at ca. Pd50Au50) for the HER is mainly

dominated by Pd trimers, dimers and monomers. The population of Pd monomers

increases as the Au concentration in the alloy increase. This is consistent with the

observations on Au/Pd(111) surface alloys [142] as a maximum activity was found on

the alloy with surface composition of ca. 80 % Au. The proposed role of Au at this range

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 20 40 60 80 100

j / m

A c

m-2(specific)

Au at. %

78

of composition is to isolate Pd into monomers resulting in active sites for the HER

[142].

The HER on Pd monomers has been speculated to follow the Volmer-Heyrovsky

mechanism where the adsorption of a single hydrogen atom on a single Pd atom is more

likely than the adsorption of two atoms (Tafel step) [142]. This is however in contrast

with the proposal that Pd dimers are the smallest ensemble that is required for hydrogen

adsorption to take place [75].

The enhanced HER activity on the alloy surface can also be discussed in terms of

changes in the electronic structure upon alloying. The Au d-band is fully occupied and

contains no unpaired electrons. This electronic structure of Au is probably responsible

for the very low activity of Au on its own [188]. Upon alloying with Au, the position of

the Pd d-band centre is changed leading to altered adsorption properties of an adsorbate

[21]. According to the Sabatier principle, an optimum catalyst should exhibit an

intermediate hydrogen adsorption binding energy (intermediate interaction with the

catalyst surface) [60]. A continuous downward shift of the Pd d-band has been observed

with increasing Au concentration in the alloy [189]. The deep shift in the d-band center

indicates a small number of valence d electrons [21]. This may explain the decrease in

the activity on the Au-rich alloys as the hydrogen adsorption becomes weaker.

The HER activity was also assessed at room temperature on a series of annealed (300 °C

for 15 minutes) Pd-Au catalysts. Figure 3.30 shows the geometric current densities at

- 17.39 mVRHE on a single array of annealed Pd-Au alloys as a function of the bulk

composition. The catalytic activity curve differs from that obtained on the unannealed

sample (Figure 3.27) as the maximum activity occurs at a bulk composition of ca.

Pd35Au65. In other words, the optimum Pd-Au composition on the catalytic activity

curve has shifted to lower compositions of Au compared with the case before annealing.

This can be directly linked to XPS results which have shown that annealing Pd-Au

alloys results in surface segregation of Au atoms (Figure 3.10). Therefore, the

concentrations of Au in the Au-poor content alloy surfaces were increased by annealing.

79

This is consistent with the reported literature regarding the effect of annealing on the

surface composition of Pd-Au alloys [74, 77, 179-181].

Figure 3.30: the HER activity at -17.39 mV as a function of the Au bulk composition on several annealed Pd-Au catalysts (300º C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye.

The specific activity of the same catalysts is shown in Figure 3.31. The maximum

activity appears again at higher concentrations of Au (ca. Pd60Au40) compared to the

geometric activity (Figure 3.30). The difference between the two plots can also be

associated with the formation of low co-ordinated Pd ensembles in the presence of high

concentration of Au.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

80

Figure 3.31: The specific HER activity at - 17.39 mV on a number of annealed Pd-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4.

It should be emphasized here that the atomic percentages of Au in Figures 3.27 and 3.30

refer to the bulk composition measured by EDS assuming that the bulk composition is

identical to the surface composition before heat treatment. At equilibrium, the surface

concentration of the alloys differs from that of the bulk. Therefore, the HER activity at -

17.39 mV on the equilibrated Pd-Au alloys is plotted in Figure 3.32 as a function of the

surface composition of the alloys. The surface composition on the alloy surface was

determined from XPS data (Figure 3.10). A maximum activity is observed in the

compositional range of ca. Pd50Au50. This result is consistent with the behavior observed

with unannealed catalysts (Figure 3.27) where the optimum composition for the HER

was identified to be ca. Pd50Au50.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 20 40 60 80 100

j / m

A c

m-2

(specific

)

Au at. %

81

Figure 3.32: the HER activity at - 17.39 mV as a function of the surface composition (red diamonds) of several annealed Pd-Au catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye. The blue curve is the HER activity as a function of bulk Au composition shown in Figure 3.27 for comparison.

The increase in the HER activity on Pd-Au alloys could also be linked to a decrease in

the content of Pd hydride phase in the bulk with increasing Au concentration. This

would be in accordance with the conclusion by Kibler [190] that the formation of bulk

hydride reduces the HER activity on Pd/Au(100). It is also in agreement with the

observation by others [140] that increasing Au content decreases the Pd hydride

formation and completely suppresses it at ca. 70 at. % Au.

Another observation from the HER activity is that the geometric current densities

obtained on the annealed Pd-Au alloy samples were greater than the geometric current

densities obtained with the unannealed (random) samples. Similar behavior was

observed on Pd-Bi array samples. This point will be discussed later in this thesis.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

82

3.5- The Catalytic Activity for the HOR

A series of potential step experiments were performed on a number of Pd-Au

electrochemical arrays with different compositions of Pd and Au in order to assess their

catalytic activities for the HOR. The experiments were carried out at room temperature

in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution. The potential

was stepped from 0 VRHE where the HOR does not occur to more positive potentials

where this reaction takes place. The current was recorded at the following potentials: 0

→ + 0.007 → + 0.014 → + 0.021 → + 0.014 → + 0.007 → 0 VRHE and the potential was

held for 90s at each step. The direction of the steps was reversed in order to examine the

reproducibility of the currents. Figure 3.33 shows the geometric current densities

measured at various Pd-Au alloy surfaces. Again, the non-zero current densities at E = 0

VRHE is attributed to the accuracy of the reference electrode used in the electrochemical

measurements. The Pd-Au alloys with high and low Au concentrations (Pd11Au89 and

Pd81Au19) are observed to have activities similar to pure Pd. Alloys with compositions of

Pd23Au77 and Pd64Au33 resulted in similar responses. The highest current density is

observed on the Pd48Au52 (Pd:Au ratio of 1:1) alloy surface.

83

Figure 3.33: The current densities in the HOR as a function of time recorded at room temperature on a number of Pd-Au catalysts (Pd:Au ratio, atomic %) in 0.5 M HClO4.

The responses of the 100-electrodes were quantitatively analyzed to assess the catalytic

activity of a wide compositional range of Pd-Au alloy catalysts for the HOR. Figure

3.34 shows the geometric current densities measured on a number of Pd-Au alloy

catalysts as a function of the bulk alloy composition. Interestingly, the catalytic activity

curve for the HOR is similar to that for the HER (Figure 3.27). Alloys with low and

high concentrations of Au show the lowest activities for the HOR. Pure Pd displays a

low catalytic activity in this reaction and it behaves similarly to the Au-poor / rich

100 : 0

0 100 200 300 400 500 600 700

-0.2

-0.1

0.0

0.1

0.2

11 : 89

0 100 200 300 400 500 600 700

-0.2

-0.1

0.0

0.1

0.2

48 : 52

0 100 200 300 400 500 600 700

j /

mA

cm

-2 (

ge

o)

-0.2

-0.1

0.0

0.1

0.2

23 : 77

0 100 200 300 400 500 600 700

-0.2

-0.1

0.0

0.1

0.2

64 : 36

0 100 200 300 400 500 600 700

-0.2

-0.1

0.0

0.1

0.2

81: 19

Time / s

0 100 200 300 400 500 600 700

-0.2

-0.1

0.0

0.1

0.2

A

E F

DC

B

0 0

0.007

0.0070.014

0.014

0.021

84

alloys. The highest catalytic activity is observed in a wide compositional range of the

Pd-Au alloys with a maximum at around 50 % Au. The HOR specific activity as a

function of the bulk alloy composition is also shown in Figure 3.35. Similarly to the

HER activity curve (Figure 3.29), the highest activity appears in the compositional

range between ca. 60-80 at. % Au with a maximum at Pd30Au70.

Figure 3.34: The geometric HOR activity at 17.93 mV of a wide range of Pd-Au alloy catalysts measured at room temperature in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution. The j values are the average of two experiments. The j values at Pd electrodes are the average of two samples. The dotted line is a guide to the eye.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

85

Figure 3.35: The HOR specific activity at 17.93 mV on various Pd-Au alloys measured at room temperature in 0.5 M HClO4.

The HOR activity was also examined over a number of annealed Pd-Au alloys (300 °C

for 15 minutes). The result at 17.95 mV is plotted as a function of the bulk alloy

composition in Figure 3.36. A higher activity on the alloys than pure Pd is observed in

the compositional range of ca. ≤ 50 at. % Au with a maximum at a bulk composition of

ca. Pd35Au65. A steady decrease in the catalytic activity is observed with alloy

compositions above 50 at. % Au. This behavior was also observed in the HER (Figure

3.30) and could be due to the surface segregation of Au upon annealing.

Considering the surface composition (Figure 3.37), the results are also comparable to

those in the HER (Figure 3.32) as a maximum in the activity occurs in the surface

composition of ca. Pd50Au50. The HOR specific activity on an annealed array of Pd-Au

alloys (Figure 3.38) is slightly different as a small increase in the catalytic activity

appears at ca. Pd35Au65. This is also comparable to the behavior observed for the HER

(Figure 3.31) and can be again correlated with the formation of more low co-ordinated

Pd ensembles at compositions above 50 at. % Au.

0

0.0005

0.001

0.0015

0.002

0.0025

0 20 40 60 80 100

j / m

A c

m-2

(specific

)

Au at. %

86

Figure 3.36: the HOR activity at 17.95 mV on several annealed Pd-Au catalysts (300 °C for 15 minutes) as a function of bulk composition measured at room temperature in 0.5 M HClO4. The value on pure Pd is the average of ten Pd samples. The dotted line is a guide to the eye.

Figure 3.37: the HOR activity at 17. 95 mV as a function of the surface composition of a number of annealed Pd-Au catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The value on pure Pd is the average of ten Pd samples. The dotted line is a guide to the eye.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Bulk Au at. %

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

87

Figure 3.38: The specific HOR activity at 17.95 mV on a number of annealed Pd-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in a 0.5 M HClO4.

The HER and HOR activities on random Pd-Au alloys (Figures 3.29 and 3.35) show a

similar compositional dependence as a composition that is active for the HER would

also be active for the HOR. This behavior, however, appears to be slightly different at

alloy compositions very rich with Au as the HER activity decreases monotonically

towards 100 at. % Au (Figure 3.29), while the HOR activity decreases more rapidly at

alloy composition of ca. 90 at. % Au (Figure 3.35). The HER and HOR activities at this

compositional range of the alloy is mainly dominated by Pd monomers. Thus, the

different behavior could be attributed to a higher selectivity towards the Tafel-Volmer

mechanism in the HOR that is suppressed on Pd monomers. This is consistent with the

proposition that the HER on Pd monomers proceeds according to the Volmer-Heyrovsky

mechanism and that a single hydrogen atom adsorbs on a Pd monomer [142].

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0 20 40 60 80 100

j / m

A c

m-2

(specific

)

Au at. %

88

3.6- The Carbon Monoxide Tolerance during the HOR

Another examination of the catalytic performance of Pd-Au alloys for the HOR was

carried out in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm carbon monoxide

bubbling through the solution during the experiment. The aim of this examination was to

investigate the influence of CO as a catalyst poison of Pd-Au alloys. To achieve that, a

series of potential step experiments were carried out. These experiments were performed

several times (after 1, 11, 22, 33 and 44 min. of bubbling with the H2/CO mixture) on

the same sample in order to examine the effect of CO with time on the catalytic activity

of the alloy system. The potentials used in these experiments were similar to the

potentials mentioned in the previous section. Figure 3.39 shows the geometric current

densities at 17.95 mVRHE on a 100-electrode array of Pd-Au alloys with various

compositions starting from ca. 8 % Au to approximately 96 % Au. Broadly, the HOR

activity curve in this Figure appears similar to the curve appearing in Figure 3.34 (with

only H2 bubbling through the solution) and it can be clearly observed even after

bubbling the solution with the mixture for 44 minutes. However, the HOR activity along

the whole Au compositional axis is reduced progressively with time. The HOR activity

on the optimum Pd-Au composition (Pd50Au50) is reduced for about 50% after 44

minutes of bubbling the solution with the H2/CO mixture.

89

Figure 3.39: The HOR activity at 17.95 mV on various Pd-Au alloys measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution. The values on pure Pd are the average of ten Pd samples. The black dashed curve is the HOR activity with only H2 bubbling through the solution (Figure 3.34) for comparison.

The HOR activity on a number of annealed Pd-Au alloys was also assessed in the

presence of the H2/CO mixture (Figure 3.40). Once more, a wide range of annealed Pd-

Au alloy catalysts (≤ 40 % Au) show similar activities to pure Pd and a steady decrease

in the catalytic activity is observed above this concentration. Similar result (with wider

compositional range, ≤ 60 % Au) is observed considering the surface composition

(Figure 3.41). Similar to the case before annealing, a progressive decrease in the Pd-Au

alloy activity for the HOR is observed with increase of bubbling time of the H2/CO

mixture.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

1 min.

22 min.

44 min.

90

Figure 3.40: : The HOR activity at 17.95 mV on various annealed Pd-Au alloys (300 °C for 15 minutes) as a function of the bulk composition. The measurement was performed at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution. The black dashed curve is the HOR activity on an annealed array sample with only H2 bubbling through the solution (Figure 3.36) for comparison.

Figure 3.41: : The HOR activity at 17.95 mV as a function of the surface composition of various annealed Pd-Au alloys (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Bulk Au at. %

1 min.

22 min.

44 min.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

j/ m

A c

m-2

(geo

)

Surface Au at. %

1 min

22 min.

44 min.

91

Pd-Au alloy catalysts are less active for the HOR than pure Pt [143]. However, Pd and

Pd-Au alloys appear to be superior to Pt and Pt-Ru alloys with respect to the CO

tolerance. It was shown in this regard that the presence of small concentrations of CO (≤

10 ppm) in the solution poisons the catalytic performance of Pt for the HOR [71], while

the HOR kinetic rates drop significantly on Pt-Ru alloys in the presence of CO with

concentrations above 100 ppm [191].

The results in Figures 3.39-3.41 indicate that the catalytic performance of Pd and Pd-Au

alloys for the HOR is not strongly poisoned by CO under these experimental conditions.

This is consistent with the strong reduction in CO adsorption with increasing Au

concentration on the alloy surface (Figure 3.23). It is also consistent with the high

resistivity observed by others for Pd overlayers on Au towards CO poisoning [166]. The

results also suggest that Pd-Au alloys offer a higher degree of CO tolerance than both Pt

and Pt-Ru alloys. This could be ascribed to a lower CO adsorption energy on the Pd-Au

alloy system [139].

The HOR activity on Pd-Au alloys in the presence of CO concentrations above 100 ppm

has been discussed in various studies in terms of their CO tolerance. It has been shown

that the HOR activity on various Pd-Au/C (Pd80Au20, Pd70Au30 and Pd50Au50) electrodes

is higher than on Pt50Ru50/C alloy in the presence of 1000 and 250 ppm of CO [139]. It

has also been observed that Pd38Au62 surface alloy is more active for 1000 ppm CO/H2

oxidation than Pd65Au35 above ca. 0.25 VRHE [101]. The higher HOR activity in both

cases has been attributed to a higher degree of CO tolerance.

The observations in Figures 3.39-3.41 eventually indicate that Pd-Au alloy catalysts

could offer alternatives to Pt for fuel cell reactions. A similar proposal has been put

forward by others [166].

92

3.7- Conclusions

The structure, bulk composition and surface composition of Pd-Au alloys were analyzed

by XRD, EDS and XPS respectively. The XRD data show a shift of 2θ values from that

of Au (111) peak to that of Pd (111) peak which indicate the formation of solid solutions

in the alloys. The quantitative analysis of the unannealed Pd-Au alloys by EDS and XPS

illustrated that the bulk composition is similar to the surface composition. The

comparison between the surface composition before and after annealing at 300 °C for 15

minutes showed a surface segregation of Au in agreement with the reported literature.

Cyclic voltammetry experiments have identified a number of features in the anodic and

cathodic sweeps correspond to the HER, HOR, formation of surface oxides, reduction of

Pd, Au and Pd-Au ensembles as well as hydrogen adsorption and desorption. The CO

stripping voltammograms has indicated that CO adsorption on the alloy surface

increases with increasing Pd concentration in the alloy. The analysis of charges

associated with the CO stripping peaks has shown that alloying Pd with Au strongly

hinders CO adsorption on the alloy surfaces. Further analysis of CO stripping data has

revealed that the real surface area of the studied catalysts is about 0.038 cm-2 and the

roughness factor is ca. 2.71.

The catalytic assessment of the HER has suggested that the optimum Pd-Au bulk and

surface composition for this reaction is in the compositional range of ca. Pd50Au50 which

was found to be about three times more active than pure Pd. The enhanced HER activity

on the alloy surface compared to Pd was associated with the formation of Pd ensembles

having more trimers, dimers and/or monomers. The HER activity on the alloys very rich

with Au is most likely dominated by Pd monomers (single Pd atoms surrounded by Au

atoms). The enhanced activity could also be ascribed to a modification in the electronic

structure upon alloying that results in higher activity. It could also be linked to a

decrease in the hydride formation in the bulk with increasing Au content.

93

The HER and HOR activities on random Pd-Au alloys have shown similar activity-

composition relationships as an alloy catalyst that is active for the HER would also be

active for the HOR. However, a slight deviation from this relation occurs on the very

rich Au alloys where a monotonic decrease in the HER activity towards 100 at. % Au

and a more rapid decrease in the HOR activity at alloy composition of ca. 90 at. % Au

was observed. This behavior is attributed to that the Tafel-Volmer mechanism being

favoured in the HOR that is suppressed on Pd monomers (the most available form of Pd

ensembles at this compositional range of the alloy). The HER activity on the annealed

Pd-Au array samples were greater than on the unannealed (random) samples. This point

will be discussed later in this thesis.

The catalytic performance of Pd and Pd-Au alloys for the HOR has been observed not to

be strongly poisoned in the presence of a mixture of 500 ppm CO and H2. This indicates

that Pd-Au alloys offer a higher degree of CO tolerance in comparison with both Pt (≤

10 ppm) and Pt-Ru alloys (≤ 100 ppm). A possible interpretation of this is that CO

adsorption energy on Pd-Au alloys is lower than on Pt or Pt-Ru alloys. This behavior

would suggest that Pd-Au alloys could offer alternative to Pt for fuel cell reactions.

94

Chapter 4: Palladium-Bismuth (Pd-Bi) Alloy Surfaces

4.1- Introduction

Bi is an inactive material for heterogeneous catalysis, but it can be an effective promoter

for the overall catalytic performances of Pd or Pt based catalysts. Incorporation of Bi

with Pd or Pt may improve the activity, selectivity and/or the lifetime of the catalyst

[192, 193]. There are various interpretations in the literature of the promoting role of Bi

on the catalytic performances of Pd or Pt. A number of them are summarized in the

following:

(i) The presence of Bi adatoms with Pt or Pd atoms on the surface may improve the

adsorption properties of the precious metal by forming new active sites for the reactant

species [192, 193].

(ii) The promoter may cause a geometric blocking of active sites on the noble metal

surface leading to a controlled surface orientation of the reactants and consequently

improve the selectivity [192, 193].

(iii) The promotion could possibly be achieved by minimizing the size of the active site

ensembles on the catalyst surface which suppress the formation and strong adsorption of

poisoning intermediates [193].

(iv) The Bi adatoms could improve the CO tolerance of the precious metal catalyst

through a so called "third-body effect" in which the presence of Bi as a third-body (in

addition to the precious metal and the adsorbed species) blocks a number of active sites

available for CO adsorption resulting in a decrease in the amount of adsorbed CO on the

surface (geometrical hindrance) [194-196].

(v) It has also been proposed that the role of Bi as a promoter is to improve the precious

metal catalyst resistance towards over-oxidation and hence deactivation during the

catalytic process [193, 197].

95

(vi) The role of the promoter is to inhibit the corrosion of the precious metal in acidic

media [192, 193].

(vii) The electrocatalytic activity of Pt or Pd in the presence of Bi as promoter can also

be enhanced through an electronic effect in which Bi influences the nearest Pt or Pd

neighbor atoms leading to a better catalytic performance [198].

4.1.1- Industrial Applications of Pd-Bi Catalysts

Pd-Bi catalysts are mostly used in the industrial synthesis of a number of organic

compounds. For instance, Bi promoted Pd supported on C (Bi-Pd/C) catalysts has been

used to catalyze the oxidation of glucose to gluconic acid at 313 K [199]. The catalyst

was synthesized at room temperature by the deposition of Bi using a surface redox

reaction on Pd supported on carbon. The presence of Bi adatoms in this process has led

to better activity, selectivity, and stability of Pd. High yields of gluconate (99.3 %) was

achieved. Oxygen acts as a poison of the metal catalysts during the catalytic oxidation of

carbohydrates or alcohol. Bi adatoms have been suggested to prevent the deactivation,

caused by oxygen poisoning, by acting as a co-catalyst in the process.

The catalytic oxidation of glucose to gluconic acid was also examined using a number of

Pd catalysts supported on SiO2 [200]. The activity and selectivity of those catalysts in

this reaction were greatly improved by the addition of Bi as a catalyst modifier. The

reason for that was attributed to the formation of intermetallic compounds BiPd and

Bi2Pd in the bimetallic catalyst due to a strong interaction between Pd and Bi (ligands

effect).

Similar investigation on the selective oxidation of glucose to gluconic acid was carried

out using bimetallic Pd-Bi, Pd-Tl, Pd-Sn, and Pd-Co catalysts supported on C and SiO2

[201]. The best activity and selectivity were obtained using catalysts modified with Bi.

XRD analysis of the Pd-Bi/SO2 catalyst proved the formation of intermetallic

compounds BiPd and Bi2Pd responsible for the increase in the selectivity of the catalyst.

96

Bi was also used as a promoter of Pd/C catalyst in the selective oxidation of glucose to

gluconate [192]. The addition of adequate amounts of Bi enhanced the overall catalytic

performance of the catalyst by forming Bi-glucose or Bi-gluconate complexes in

solution or at the catalyst surface which facilitates glucose dehydrogenation and the

subsequent steps in the proposed mechanism. The active sites on the catalyst surface

were suggested to compose of an assortment of one Bi atom and two or three Pd atoms.

Another industrial application of Pd/C catalyst promoted by Bi is for the selective

oxidation of lactose by molecular oxygen to sodium lactobionate in a pH range 7-10 and

at temperatures up to 333 K [202]. The Bi-Pd/C catalyst has uniquely enhanced the

selectivity for sodium lactobionate to 100 % resulting in yields of 95 % in about one

hour.

The synthesis of sodium gluconate from glucose oxidation has also been examined on Bi

promoted Pd/C catalyst prepared from a chemical solution containing PdCl2 and

Bi(NO)3.5H2O [203]. The ratio of Bi and Pd in the catalyst was observed to influence

the reaction rate. The best promotion effect was observed when the PdCl2 to

Bi(NO)3.5H2O weight ratio was 1 to 3 in the solution for catalyst preparation. The Bi-

Pd/C catalyst has exhibited a better catalytic performance for the synthesis of sodium

gluconate compared to Pd/C catalyst alone as the incorporation of Bi prevents Pd/C

deactivation in this process. The lifetime of the Bi-Pd/C catalyst was also found to be

longer than that of Pd/C catalyst.

Biella and colleagues have examined the selective oxidation of D-glucose to D-gluconic

acid using a number of Au, Pd and Pt on carbon catalysts [204]. The best result was

given by the Au/C catalyst. The authors suggested that adding Bi to Pt or Pd on C

catalysts may enhance their performance for the oxidation process.

The catalytic efficiency of Bi-Pd/C catalyst was also observed to be superior to that of

Pd/C catalyst (prepared by the same procedure) in the selective oxidation of glyoxal into

glyoxalic acid (OHC-COOH) [197]. The catalyst composition was observed to influence

97

the catalytic activities of Bi-Pd/C catalysts. The best results were obtained using

catalysts with molar ratios Bi/Pd between 0.5 and 1. The catalytic activity of Bi-Pd/C

catalyst was also compared to that of a commercial trimetallic Pd-Pt-Bi/C catalyst in the

same reaction. The bimetallic catalyst exhibited a constant catalytic activity with time,

while the trimetallic catalyst showed deactivation after 10 h operation.

The benzylacetate synthesis by oxyacetoxylation of toluene has been assessed by

Miyake and colleagues [205] using pure Pd, and two Pd-Bi binary catalysts (molar ratios

Pd/Bi = 3 and 1). One of the concerns in such a process is the deactivation of the catalyst

resulting from the dissolution of Pd. Incorporation of Bi with Pd (especially Pd/Bi = 3)

led to better activity, selectivity and stability. The reason of this behavior was ascribed

to the formation of an intermetallic compound in the binary catalyst. The authors

suggested that the Pd-Bi (Pd/Bi = 3) catalyst is likely to be a promising catalyst for the

industrial synthesis of benzylacetate.

The catalytic activity of Pd/Al2O3 in the hydrogenation of 1-hexyne and 2-hexyne has

been improved by modification with Bi or Pb [206]. Better selectivity in the process was

achieved using either Bi or Pb as a catalyst modifier. However, Bi has been suggested to

be a better catalyst modifier than Pb. The improvement of the selectivity in this process

was attributed to that the addition of the catalyst modifiers (Bi or Pb) suppresses

undesired isomerisation reactions of the primary products.

4.1.2- Applications of Pd-Bi Catalysts in Electrocatalysis

Pd-Bi alloys are not commonly used in electrocatalysts. To the extent of the author’s

knowledge, there is no reported data for the anodic reactions in fuel cells on Pd-Bi alloy

catalysts. Most of the research efforts in this field were directed towards the employment

of Bi to promote the performance of Pt-based catalysts. It has been theoretically

predicted that the HER activity on a surface alloy of Pt-Bi is comparable or even higher

than pure Pt [60]. The experimental data has proved this theoretical prediction [60]. A

98

further theoretical prediction has proposed that Pd3Bi bulk alloy may offer a promising

activity for the HER [93].

Another application of Pt-Bi catalysts is for the electrochemical oxidation of formic acid

where the catalytic activity of the binary catalyst was found to be better than pure Pt

[141, 198, 207-209]. The superior catalytic performance of Pt-Bi catalyst for this process

was ascribed to that the presence of Bi in the catalyst surface: (i) improves formic acid

adsorption and produces surface oxides at low potential, and (ii) minimizes the CO

poisoning effect, since pure Pt can be easily affected by poisoning species [141].

CO electro-oxidation has also been assessed on Bi modified Pt(110)-(1x2) and Pt (111)

surfaces [194, 196]. The authors concluded that the adsorption of Bi on the Pt surfaces

reduces the active sites for CO adsorption and, therefore, improves their CO tolerance. It

has also been concluded that the incorporation of Bi results in an increase and a decrease

in the CO oxidation potential on Pt(110)-(1x2) and Pt (111) respectively.


Thin film libraries of random Pd-Bi alloys have been synthesized on a number of 100-

field Pd-Bi arrays employing the HT-PVD method described previously. The

preparation method allowed the deposition of a wide compositional range of Pd-Bi alloy

catalysts. The bulk and surface compositions of the alloys have quantitatively been

determined using EDS and XPS respectively.

4.2.1- EDS Analysis

Figure 4.1 shows false color maps of the bulk Pd and Bi compositions in a single array

of Pd-Bi alloy thin films with respect to the (x, y) position in the array. A diagonal

increase in the Pd concentration from the bottom left side to the top right side and an

opposite increase in the Bi concentration are observed. The EDS analysis of the bulk

alloy composition clearly shows that the deposition method employed for synthesis of

99

alloys allowed the achievement of approximately the whole compositional range of Pd-

Bi alloy system.

Figure 4.1: False color maps of Pd and Bi concentrations in a single array of Pd-Bi alloys with respect to the position in the array sample. The dashed arrows denote the growth direction of the elemental components in the array.

4.2.2- XRD Analysis

Pure Pd has a fcc crystal structure with a lattice parameter = 3.89 Å [174], while pure Bi

crystallizes in the rhombohedral structure with lattice parameters a = 4.55 Å and c =

11.86 Å [210, 211]. The powder XRD patterns of a number of random and annealed

(300 °C for 15 minutes) Pd-Bi thin films are shown in Figures 4.2 and 4.3 respectively.

The Bragg peaks appearing in these figures could be ascribed to a number of Bi or Pd

planes [176, 177, 210, 212]. The intensities of some of these peaks are observed to be

less after heat treatment which possibly signifies that annealing a Pd-Bi alloy catalyst

produces preferential orientation of the crystallites. A Bragg peak can be identified at ca.

2θ = 32.28° indicating the formation of an intermetallic compound Bi2Pd with a

monoclinic structure [Froodite, card number: 11-251]. This peak is more pronounced

X

Y

100

with alloy composition of ca. Pd25Bi75. The formation of the intermetallic compound

Bi2Pd in this range of composition has an effective influence on the catalytic activity of

the Pd-Bi alloys for the HER and HOR. This influence will be shown in detail later in

this chapter.

The XRD measurements of a number of PdBi/SiO2 surfaces containing 2% Bi 5% Pd,

4% Bi 5% Pd and 5% Bi 5% Pd have also proved the presence of intermetallics with

compositions Bi2Pd and BiPd [200]. The presence of two intermetallic compounds

including BiPd and BiPd3 has also been reported by Alardin and colleagues [197]. A

thermodynamic assessment of the Pd-Bi system [213] showed that the energy of

formation of PdBi ( ) and PdBi2 (

) are -33.6 and -25.1 kJ mol-1

respectively, while no calculations were performed of the composition Pd3Bi due to its

complexity. The formation of an intermetallic compound in the Pd-Bi alloys could be

attributed to a strong interaction between the elemental components. This chemical

property may play an essential role in the enhancement of the catalytic performance of

Pd-Bi binary catalysts [200].

Figure 4.2: Typical XRD patterns of various Pd-Bi alloys.

0

3

6

9

12

15

20 24 28 32 36 40 44 48

Inte

ns

ity

/ A

rbit

rary

Un

its

2θ Scattering Angle / °

6 at. % Bi

26 at. % Bi

48 at. % Bi

73 at. % Bi

88 at. % Bi

98 at. % Bi

Bi

(003)

Bi

(012)

Pd

(111)

Bi

(104)

Bi

(015)

Bi

(006)

Bi2Pd

101

Figure 4.3: Typical XRD patterns of various Pd-Bi alloys annealed at 300 °C for 15 min.

The phase diagram of the Bi-Pd system (Figure 4.4) [53] shows that the formation of

the intermetallic compound α(β)-Bi2Pd takes place in a narrow compositional range (ca.

≥ 70 at. % Bi). The phases α(β) BiPd and α(β) Bi2Pd5 could be formed in the

compositional ranges between 50-70 and 30-50 at. % Bi respectively, while the

formation of the composition α(β) BiPd3 occurs in a very narrow region (ca. 25-30 at. %

Bi). The α-Bi2Pd could be formed at temperatures below 271 ºC and the β-Bi2Pd could

be formed after annealing at a temperature higher than the melting point of pure Bi

(above 271 ºC).

The phase diagram of this system also shows that the formation of solid solution alloys

between the elemental components may occur in a narrow compositional region (ca. ≤

25 at. % Bi). Considering this, the lattice parameter of a number of Pd-Bi alloys in the

compositional region between 0-25 at. % Bi was calculated. The obtained values are

plotted in Figure 4.5 as a function of Bi bulk composition. A linear relation is observed

which is in accordance with the relation one may predict based on Vegard’s low [127].

Sakamoto and colleagues [214] have shown that alloying Pd with up to 10 at. % Bi

forms fcc solid solutions. The lattice parameter (a), in this case, has also been observed

to expand with the increase in the Bi concentration in the alloy from 0.3890 nm of pure

0

2

4

6

8

10

12

14

20 24 28 32 36 40 44 48

Inte

ns

ity

/ A

rbit

rary

Un

its


7 at. % Bi

31 at. % Bi

52 at. % Bi

67 at. % Bi

82 at. % Bi

92 at. % Bi

Bi2Pd

102

Pd to 0.3947 nm of Pd-Bi alloy containing 10 at. % Bi. A similar composition

dependence of the lattice parameter (a) was observed for the bulk Pd-Bi alloys with

composition range of 0 < xBi ≤ 0.2 [215].

Figure 4.4: The phase diagram of the Pd-Bi system [53]

Figure 4.5: The calculated lattice parameter of pure Pd and Pd-Bi alloys as a function of bulk Bi composition (up to 25 at. %). The dotted line is a guide to the eye. The Pure Pd (3.89 Å) value was taken from [174].

3.85

3.9

3.95

4

4.05

0 5 10 15 20 25

latt

ice p

ara

mete

r /

Å

Bi at. %

103

Further analysis of the Bi2Pd peak appearing in Figures 4.2 and 4.3 was carried out in

order to assess the intensity-composition relation for the random and annealed Pd-Bi

array samples. Figure 4.6 shows the intensity of this peak (before and after annealing)

as a function of Bi composition. A similar trend is observed in both cases. The peak

intensity remains unchanged over a wide compositional range (up to 60 at. % Bi). The

maximum intensity occurs in the compositional range of ca. Pd25Bi75. A steady decrease

in the peak intensity is observed with alloy compositions above 80 at. % Bi.

Figure 4.6: The intensity of the Bi2Pd peak as a function of Bi at. %, (a): before annealing and (b) after annealing at 300 °C for 15 minutes.

4.2.3- XPS Analysis

The surface composition of 10-fields in an array of Pd-Bi alloys has been determined by

XPS. The measurements have been performed diagonally along the growth direction of

Bi composition in the array in order to represent nearly the whole compositional range

of Pd-Bi alloys. In the Bi (4f) XPS region, two doublets ascribed to 4f7/2 and 4f5/2 can be

detected at about 159 and 164 eV respectively [133]. In the Pd (3d) XPS region, on the

other hand, two doublets correspond to 3d5/2 and 3d3/2 can be detected at 335 and 340 eV

respectively [132]. Figure 4.7 show the Bi (4f) spectra as a function of the bulk Bi

B A

104

composition in various Pd-Bi alloys. A steady shift towards lower binding energies

(away from that of pure Bi) is observed with the decrease in the Bi concentration in the

alloy. Similar behavior is observed upon annealing at 300 ºC for 15 minutes (Figure

4.8).

Figure 4.7: The Bi (4f) XPS spectra as a function of the bulk composition of various Pd-Bi alloys.

Figure 4.8: The Bi 4f XPS spectra as a function of the bulk composition of the Pd-Bi alloys after annealing at 300 °C for 15 minutes.

-100

0

100

200

300

400

500

152 154 156 158 160 162 164 166 168 170

Inte

nsit

y /

CP

S


11 at. % Bi

26 at. % Bi

48 at. % Bi

62 at. % Bi

85 at. % Bi

96 at. % Bi

-100

0

100

200

300

400

500

150 152 154 156 158 160 162 164 166 168 170

Inte

nsit

y /

CP

S

Binding Energy /eV

2 at. % Bi

16 at. % Bi

34 at. % Bi

67 at. % Bi

85 at. % Bi

98 at. % Bi

Bi4f7/2 Bi4f5/2

Bi4f7/2

Bi4f5/2

105

The Bi (4f7/2) peak position of the 10 Pd-Bi alloys before and after annealing is plotted

as function of the bulk Bi composition in Figure 4.9. A shift in the Bi (4f) peak position

towards lower binding energy with the decrease in the bulk Bi concentration in the alloy

is observed. The shift in the peak position is not linear, since the alloys having bulk

composition of ca. 80-100 at. % Bi show similar peak positions.

Figure 4.9: The Bi (4f7/2) peak position as a function of the bulk Bi composition.

In the Pd (3d) region, a shift towards higher binding energies (away from that of pure

Pd) is observed with the increase in the bulk Bi composition before and after annealing

at 300 ºC for 15 minutes (Figurers 4.10 and 4.11). The Pd (3d5/2) peak position is

plotted as a function of bulk Bi composition in Figure 4.12 showing similar behavior to

that observed in the Bi (4f7/2) region (Figure 4.9).

156.5

157

157.5

158

158.5

159

159.5

160

0 20 40 60 80 100

Bin

din

g E

ne

rgy

(BE

) /

eV

Bulk Bi at. %

Before annealing

After annealing

106

Figure 4.10: The Pd (3d) XPS spectra as a function of the bulk composition of various Pd-Bi alloys.

Figure 4.11: The Pd (3d) XPS spectra as a function of the bulk composition of the Pd-Bi alloys after annealing at 300 ºC for 15 minutes.

-50

0

50

100

150

200

328 330 332 334 336 338 340 342 344 346 348

Inte

nsit

y /

CP

S


6 at. % Bi

26 at. % Bi

48 at. % Bi

62 at. % Bi

85 at. % Bi

-50

0

50

100

150

200

250

300

350

400

450

328 330 332 334 336 338 340 342 344 346 348

Inte

nsit

y /

CP

S


2 at. % Bi

16 at. % Bi

34 at. % Bi

67 at. % Bi

98 at. % Bi

Pd3d5/2

Pd3d3/2

Pd3d5/2

Pd3d3/2

107

Figure 4.12: The Pd (3d5/2) peak position as a function of the bulk Bi composition.

The shift in the peak positions of Pd and Bi is a clear evidence of alloy formation [77].

Commonly, shifts in the binding energy of elements forming an alloy system are

discussed in terms of electronegativity differences where a charge flow from the less

electronegative constituent towards the more electronegative one is predicted [216].

According to this, a charge flow from Bi to Pd is predicted in the Pd-Bi alloy system due

to its relatively lower electronegativity (about 2.2 eV of Pd and 2.0 eV of Bi according

to Pauling electronegativity scale). This means that the Pd (3d5/2) peak would be

predicted to shift to lower binding energy (as a Pd atom in the alloy acquire a negative

charge) and the Bi (4f7/2) peak would be predicted to shift to higher binding energy (as a

Bi atom acquires a positive charge) upon alloying [182].

The shift in the Bi (4f7/2) peak position towards lower binding energy (Figure 4.9) and

Pd (3d5/2) peak position towards higher binding energy (Figure 4.12) upon alloying is

not consistent with the prediction based on the electronegativity differences. A shift

towards higher binding energy (compared to the values of pure metals) has been

observed for Pd in the Pd3Bi alloy [205] and for Bi in Pt-Bi alloys [141, 217].

334.5

335

335.5

336

336.5

337

337.5

338

338.5

0 20 40 60 80 100

Bin

din

g E

nerg

y (B

E)

/ eV

Bulk Bi at. %

Before annealing

After annealing

108

The shift in the Pd (3d5/2) peak position to higher binding energy could be due to a

modification in the electronic density of Pd caused by the interaction with Bi to form the

alloy (intra-atomic charge transfer could result in a shift to larger binding energy) [182].

The surface compositions of the measured alloys have been compared to that in the bulk

before and after annealing at 300 °C for 15 minutes. Similar compositions have been

obtained in both cases as shown in Figure 4.13. The similarity between the bulk and

surface compositions observed at equilibrium indicates no surface segregation in the Pd-

Bi alloys (under these conditions, 300 ° for 15 minutes). No reported data for surface

segregation phenomenon in the Pd-Bi alloy system could be found. The only available

reference has referred to that a surface segregation of Bi is predicted to occur in the Pd-

Bi alloys due to its lower surface energy [197] (the surface free energies of Pd and Bi are

2.043 Jm-2 [77] and 0.55 Jm-2 [218] respectively).

There are a number of possible reasons for not observing any surface segregation of Bi

in this study. It is likely that the concentration of surface Bi is slightly higher at

equilibrium compared to that before heat treatment, but could not be measured due to the

accuracy of XPS (error < ±10 % [131]). The purity of the sample could also influence

the measurements of the surface composition. For instance, the surface composition of

an alloy may be changed due to the so-called "co-segregation effects" caused by the

surface segregation of H, O, N, C, and S present in the metallic system as well as the

segregation of the predicted metal (Bi in the case of the Pd-Bi alloy) [83]. Another

possibility is that the annealing conditions employed here are not sufficient to observe

this phenomenon in the Pd-Bi alloys. Therefore, annealing at higher temperatures than

300 °C or for time longer than 15 minutes may be required in order to observe a clear

surface segregation of Bi in this system. Nevertheless, the similarity between the bulk

and surface compositions before and after annealing in the measured Pd-Bi alloys

correlates well with the electrocatalytic activity results which will be shown later in this

chapter.

109

Figure 4.13: The Bulk and surface Bi compositions in a number of Pd-Bi alloys before annealing (blue squares) and after annealing at 300 °C for 15 minutes (red squares).


A first voltammetric assessment of the Pd-Bi alloy catalysts was carried out at room

temperature in 0.5 M HClO4 and at 50 mV/s under the following limiting potentials: Ein

= 0.2, Elo = - 0.03, Eup = 0.5 VRHE. Figure 4.14 shows voltammetric profiles of a number

of Pd-Bi alloys. Two features in the cathodic sweep can be ascribed to the adsorption /

hydride formation (1) and the HER (2) [219], while a feature due to the oxidation of

adsorbed / absorbed hydrogen (3) can be identified in the anodic sweep [84]. Clearly, the

hydrogen adsorption and desorption (hydrogen under potential deposition, Hupd) region

is dominated by the concentration of Pd in the alloy. The latter surface processes are

more pronounced in the presence of high concentration of Pd in the alloy.

0

20

40

60

80

100

0 20 40 60 80 100

Su

rfa

ce

Bi a

t. %

(fr

om

XP

S)

Bulk Bi at. % (from EDS)

Before Annealing

After Annealing

110

Figure 4.14: Cyclic voltammograms of a number of Pd-Bi alloys (Pd : Bi ratio, atomic %) recorded at room temperature in 0.5 M HClO4, scan rate of 50 mV/s, and limiting potentials: Elo = -0.03 VRHE, Ein = 0.2 VRHE and Eup = 0.5 VRHE. CVs are from the second cycle.

Further assessment of the surface has been achieved by running CO stripping

voltammetry at room temperature in 0.5 M HClO4 and at scan rate of 20 mV/s. The

electrolyte was initially saturated with CO for 20 minutes by bubbling, and then the CO

was removed from the electrolyte by purging with Ar for at least 20 minutes. The

measurements were performed under the following limiting potentials: Ein = 0.4, Elo = -

0.03, Eup = 1.1 VRHE. The cyclic voltammograms of a number of Pd-Bi alloys are shown

in Figure 4.15. The voltammetric profiles can be defined by a number of potential

10 : 90

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

27 : 73

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

49 : 51

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

j /

mA

cm

-2 (

ge

o)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

71 : 29

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

89 : 11

Potentia l / V RHE

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

100 : 0

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

A

E

DC

B

F

1 2

3

111

regions: (i) hydrogen evolution at E < 0.0 VRHE (ii) the reversible hydrogen adsorption

and desorption (Hupd region) at E < 0.4 VRHE, (iii) a large double layer in the potential

range 0.4-0.6 VRHE, and (iv) the CO stripping peak as well as the irreversible surface

oxidation / reduction "surface redox" at E > 0.6 VRHE [137, 138, 141].

Figure 4.15: The voltammetric profiles of a number of random Pd-Bi alloys (Pd : Bi ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with CO for 20 minutes, scan rate of 20 mV /s, and limiting potentials: Elo = -0.03 VRHE, Ein = 0.4 VRHE and Eup = 1.1 VRHE. CVs are from the 1st (red line) and 2nd (black line) cycles.

13 : 87

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

27 : 73

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

48 : 52

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

j /

mA

cm

-2 (

ge

o)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

71 : 29

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

86 : 14

Potential / V RHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

A

E

DC

B

F

112

Features in the Hupd region are very sensitive to the surface structure of the catalyst

[196]. This possibly explains the variety in these features on the alloy surface (Figures

4.15A-E) compared to that of pure Pd (Figure 4.15F). The magnitude of the cathodic

feature below 0.0 VRHE is significantly increased at around 75 at. % Bi (Figure 4.15B)

indicating a better hydrogen evolution in this compositional range compared to the other

alloy compositions. This could be correlated to the formation of the intermetallic

compound Bi2Pd in the compositional range 75 at. % Bi discussed early in this chapter.

The CO stripping from the surface is represented by a broad anodic feature in the first

cycle (red lines in Figures 4.15A-F) at around 0.9 VRHE. The CO stripping feature is

more pronounced with increasing the Pd concentration in the alloy (Figure 4.16)

indicating that alloying Pd with Bi reduces CO adsorption and perhaps hinders it at high

concentrations of Bi. This is consistent with the behavior one may predict as CO

adsorption only takes place on Pd [75, 101]. It is also in accordance with the conclusion

that the deposition of Bi layers on Pt(110)-(1x2) reduces the amount of adsorbed CO

[196]. The CO stripping peak on the Bi-rich alloys occurs at potentials higher than on

pure Pd, while alloys having low and intermediate concentrations of Bi exhibit CO

stripping peaks at potentials relatively lower than that on pure Pd. This indicates a better

activity for CO oxidation on the latter alloy composition than pure Pd. The deposition of

a sub-monolayer of Bi (0.1ML) has been shown to improve CO oxidation on Pt(111)

surface [194].

113

Figure 4.16: The position of the CO stripping peak exhibited by Pd and a various compositions of Pd-Bi alloy catalysts. Pd:Bi ratio, atomic %.

The removal of CO from the surface of the catalyst allows the surface redox processes to

take place. The identification of the individual process in the surface redox region on the

Pd-Bi alloy catalysts appears to be difficult due to the fact that the oxidation / reduction

processes of Pd and Bi take place in the same potential region (0.739 - 0.789 VRHE on Pd

[219] and 0.77 - 0.88 VRHE on Bi [196]). The anodic feature at around 0.95 - 1.0 VRHE

(in Figures 4.15A-F) could therefore be ascribed to the oxidation of both Pd and Bi as

well as Pd-Bi intermetallic surface [217], while the feature in the cathodic sweep at

around 0.8 VRHE is possibly due to the reduction of the formed oxides. The feature

ascribed to the reduction of surface oxides appears in a very narrow potential region

(between ca. 0.75 - 0.8 VRHE) as shown in Figure 4.17. The relative position of this peak

is plotted as a function of the Bi concentration in the alloy (Figure 4.18) showing a

steady and smooth decrease and increase with a minimum in the compositional range

between ca. 30-50 at. % Bi. The variation in the reduction peak position is a clear

evidence of alloy formation and an electronic interaction between Pd and Bi atoms.

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.75 0.8 0.85 0.9 0.95 1 1.05 1.1

j / m

Acm

-2(g

eo

)

Potential / VRHE

13:87

27:73

48:52

71:29

86:14

100:0

114

Figure 4.17: The oxide reduction peak on various random Pd-Bi alloys (Pd:Bi ratio, atomic %).

Figure 4.18: The relative position of the oxide reduction peak as a function of Bi concentration in the Pd-Bi alloys.

The effect of annealing an array of Pd-Bi alloy catalysts at 300 °C for 15 minutes on the

voltammetric features observed with random alloys (Figure 4.15A-F) has also been

assessed. Figure 4.19A-F shows the voltammetric profiles of a number of equilibrated

Pd-Bi alloys. Broadly, the anodic and cathodic processes on the annealed Pd-Bi alloys

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.5 0.6 0.7 0.8 0.9 1 1.1

j / m

A c

m-2

(geo

)

Potential / VRHE

15:85

29:71

52:48

76:24

89:11

100:0

0.758

0.762

0.766

0.77

0.774

0.778

0.782

0 20 40 60 80 100

Po

ten

tia

l / V

RH

E

Bi at. %

115

take place at potentials similar to the case before annealing. A remarkable difference

appears in the HER, Hupd and hydride formation regions in the cathodic sweep, since that

the magnitude of the features ascribed to these processes on Pd and Pd-rich alloys

(Figures 4.19D-F) is extremely enhanced by annealing under these conditions (the

geometric current density is higher by more than twofold after annealing). Similar

behavior was observed on Pd-Au alloys where higher geometric current densities were

obtained upon annealing under the same conditions. This point will be discussed in

chapter six.

Figure 4.19: The voltammetric profiles of a number of annealed Pd-Bi alloys (300 °C for 15 minutes, Pd : Bi ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with CO for 20 minutes, scan rate = 20 mV /s, CVs are from the 1st (red line) and 2nd (black line) cycles. Limiting potentials are: Elo = -0.03 VRHE, Ein = 0.4 VRHE and Eup = 1.1 VRHE.

22 : 78

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

35 : 65

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

52 : 48

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

j /

mA

cm

-2 (

ge

o)

-1.5

-1.0

-0.5

0.0

0.5

1.0

76 : 24

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

89 : 11

Poten tia l / V RH E

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

1.0

A

D

FE

C

B

116


The catalytic activity of nearly the whole compositional range of the Pd-Bi alloy

catalysts for the HER and HOR has been assessed at room temperature in 0.5 M HClO4

using a number of 100-field Pd-Bi arrays. The HER measurements have been carried out

using the potential step experiment. The current was recorded at the following

potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007 → 0 VRHE. The

potential was held for 90s at each step. Figure 4.20 shows the geometric HER activity at

- 16.44 mVRHE of a number of random Pd-Bi alloy catalysts. The catalytic activity curve

could be divided in a number of regions: (i) a similar activity to pure Pd in the

compositional range below ca. 10 at. % Bi, (ii) a sharp decrease in the activity between

10 - 20 at. % Bi, (iii) a minimum activity at around 25 at. % Bi, (iv) a steady increase in

the catalytic activity in the compositional range between about 30 - 70 at. % Bi, (v) a

catalytic activity at around Pd25Bi75 comparable to that of pure Pd, and (vi) a sharp

decrease in the catalytic activity above 80 at. % Bi.

Figure 4.20: The HER activity at - 16.44 mVRHE on a wide range of random Pd-Bi alloy catalysts measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.

0

0.04

0.08

0.12

0.16

0.2

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bi at. %

117

The current values obtained on the array sample shown in the latter measurement were

normalized to Pd composition in the Pd-Bi alloys in order to present the specific HER

activity (Figure 4.21). The behavior in this case appears similar to the geometric activity

in terms of that a maximum is observed at ca. Pd25Bi75 alloy composition. The activity in

this compositional region is superior to pure Pd.

Figure 4.21: The specific HER activity normalized to Pd composition in the Pd-Bi alloys.

A similar HER catalytic behavior is observed using an annealed array (300 °C for 15

minutes) of Pd-Bi alloy catalysts (Figure 4.22). One can distinguish that the current

density value on pure Pd is much higher after annealing (ca. 0.5 mA cm-2) than before

annealing (ca. 0.14 mA cm-2), while on the Pd-Bi alloy catalysts (10 at. % Bi and above)

is almost the same (in the range 0.1 – 0.2 mA cm-2) before and after annealing.

0

0.002

0.004

0.006

0.008

0 20 40 60 80 100

j / m

A c

m-2

(sp

ec

ific

)

Bi at. %

118

Figure 4.22: The HER activity at - 17.48 mVRHE on a wide range of annealed Pd-Bi alloy catalysts (300 ºC for 15 minutes) measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.


The catalytic activity of the Pd-Bi alloy catalysts for the HOR has been assessed using

the potential step experiment with hydrogen bubbling through the electrolyte. The

current has been recorded at the following potentials: 0 → 0.007 → 0.014 → 0.021 →

0.014 → 0.007 → 0 VRHE. The potential was held for 90s at each step. The HOR activity

at 18.95 mVRHE on a number of random Pd-Bi alloy catalysts is shown in Figure 4.23.

Clearly, the HOR activity curve is similar to that of the HER (Figure 4.20) with lower

current density values under the same compositions. An interesting catalytic behavior

could also be observed in the compositional range of Pd25Bi75. The HOR specific

activity is shown in Figure 4.24 demonstrating a similar compositional dependence to

the HER specific activity (Figure 4.21). A similar result was also obtained after

annealing at 300 °C for 15 minutes (Figure 4.25). Once more, annealing influences the

properties of pure Pd giving a rise to the current density values, while no obvious change

take place on the alloys after annealing under these conditions.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bi at. %

119

Figure 4.23: The HOR activity at 18.95 mVRHE on a wide range of random Pd-Bi alloy catalysts measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.

Figure 4.24: The Specific HOR activity normalized to Pd composition in the Pd-Bi alloys.

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bi at. %

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0 20 40 60 80 100

j / m

A c

m-2

(sp

ec

ific

)

Bi at. %

120

Figure 4.25: The HOR activity at 17.75 mVRHE on a wide range of annealed Pd-Bi alloy catalysts (300 ºC for 15 minutes) measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.

The HOR catalytic activity on Pd-Bi alloy catalysts has also been assessed with a

mixture of hydrogen and 500 ppm CO bubbling through the electrolyte. The

measurements have been carried out after 1 minute of bubbling with the mixture, and

then repeated after bubbling with the mixture for 11 minutes. Figure 4.26 shows the

HOR activity at 18.67 mVRHE on a number of random Pd-Bi alloy catalysts (the same

array sample used in the former assessment for the HOR). Apparently, the presence of

CO in the electrolyte influences the catalytic performance of the Pd-Bi alloy catalysts for

the HOR. However, poisoning effect is strongly dependent on the alloy composition. A

small effect could be observed on the catalytic performance of Pd and Pd-rich alloys (≤

20 at. % Bi) even after bubbling with the H2/CO mixture for 11 minutes. In contrast, the

HOR activity on the Pd-Bi alloys with compositions above 20 at. % Bi is significantly

reduced after 1 minute of bubbling with the H2/CO mixture and completely suppressed

after 11 minutes indicating a strong poisoning of the alloy catalysts by CO. This implies

a low-level of CO tolerance in this compositional range of the Pd-Bi alloy catalysts. It

has been shown that the deposition of Bi on a carbon supported Pt electrode reduces the

CO tolerance of the Pt catalyst [194]. This has been ascribed to Bi offering low

effectiveness towards facilitating oxygen transfer during CO oxidation [194].

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bi at. %

121

Figure 4.26: The HOR activity at 18.67 mV on various random Pd-Bi alloys measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the electrolyte. The j values on pure Pd are the average on ten Pd electrodes.

The variation in the HER and HOR catalytic activity along the alloy composition

suggests a composition-activity relation which may be correlated to the formation of a

number of intermetallic compounds shown in the phase diagram of this system (Figure

4.4). The catalytic behavior in the compositional range between 20-80 at. % Bi

(particularly at ca. Pd25Bi75) is extremely interesting and valuable. This is because of the

contrast between the catalytic activity of Pd (highly active) and Bi (totally inactive) as

shown from the volcano plot for the HER [220]. In view of its lower cost, this alloy

system provides an alternative to Pt for the HER and HOR.

The surface processes in the Hupd region and the kinetics of a reaction are sensitive to the

catalyst surface structure and morphology [196, 221]. The enhancement of the HER and

HOR activities on the Pd-Bi alloy catalysts with compositions around Pd25Bi75 could be

ascribed to the formation of a large number of isolated Pd atoms (monomers) which

results in improved activity for HER at this alloy composition. This would be in

accordance with the conclusion that Pd monomers on Au/Pd (111) surface alloys

prepared on Ru(0001) are more active for the HER than Pd atoms in a Pd(111) overlayer

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bi at. %

Pure H2

1 min.

11 min.

122

[142]. Those monomers may enhance the HER through: (i) improving the interaction

with incoming hydrogen atoms or molecules, and (ii) providing selectivity towards the

Heyrovsky step in the HER [142].

It is also possible that the formation of the intermetallic compound Bi2Pd with a

maximum at ca. Pd25Bi75 (Figure 4.6) is responsible for the improvement of the overall

catalytic performance of the Pd-Bi alloy catalysts for the HER and HOR "electronic

effect". This would be in accordance with the conclusion that formation of intermetallic

Bi2Pd results in higher selectivity and activity than pure Pd for glucose oxidation [200].

The XPS data has shown a shift in the Pd (3d5/2) peak position upon alloying with Bi

(Figure 4.12) indicating an electronic perturbation of Pd atoms. It is likely that the inter-

metallic interaction produces a change in the d-band centre [21]. This change could

directly influence the catalytic activity of a transition metal surface [222]. A negative

shift in the d-band centre of Pd would probably be accompanied with a reduction in the

hydrogen adsorption energy that results in higher activity for the HOR [21].

Similarly to the Pd-Au alloy system, the increase in the activity on the Bi-rich alloys

could be associated with a decrease in the formation of Pd hydride phase in the bulk

with increasing Bi content. It has been deduced by Kibler [190] that increasing Pd

coverage on Au(100) reduces the HER activity.

The HER activity and HOR activity on Pd-Bi alloy surfaces show a similar

compositional dependence. An exception of this dependence occurs, however, at high

concentration of Bi (≥ 75 at. % Bi) where the HER activity decreases monotonically

towards 100 at. % Bi (Figure 4.20), while the HOR activity decreases more rapidly at

ca. 90 at. % Bi (Figure 4.23). This behavior is similar to that observed in the Pd-Au

alloy system (Figures 3.29 and 3.35) and was ascribed to a higher selectivity in the

HOR towards the Tafel-Volmer mechanism that is hindered on Pd monomers (most

popular ensembles in this compositional region).

123

4.6- Conclusions

A series of Pd-Bi alloys were synthesized employing a HT-PVD methodology on 10x10

arrays. The bulk and surface compositions of the Pd-Bi alloy samples were determined

by the EDS and XPS respectively. The EDS analysis showed that the synthesis of nearly

the whole compositional range of the Pd-Bi alloy system was achieved. The XPS

measurements were performed on 10-fields in an array of Pd-Bi alloys showing that the

surface composition is similar to the bulk composition before and after annealing at 300

°C for 15 minutes. This similarity suggests no surface segregation of one of the

constituents in the alloy.

The XRD analysis of the Pd-Bi alloy catalysts (unannealed and annealed at 300 °C for

15 minutes) showed a peak at ca. 2θ = 32.28º which was more pronounced with alloy

composition of ca. Pd25Bi75. This peak was attributed to the formation of intermetallic

compound Bi2Pd with a monoclinic structure. The phase diagram of the Pd-Bi system

indicates that α-Bi2Pd is formed at temperatures < 271 °C, while β-Bi2Pd is formed at

temperatures > 271 °C in this compositional range of the alloy. The phase diagram of

this alloy system also shows that solid solutions are formed in a narrow compositional

region (< 25 at. % Bi). The calculation of lattice parameter in a number of alloys in this

compositional range showed a consistency with Vegard’s law.

The voltammetric measurements on Pd-Bi alloys in 0.5 M HClO4 at room temperature,

an upper potential limit of 0.5 VRHE, and scan rate of 50 mV/s showed that the Hupd

region is mainly dominated by the Pd composition in the alloy (the high concentration of

Pd produces more pronounced surface processes). The voltammetric scan (room

temperature and scan rate of 20 mV/s, and an upper potential of 1.1 VRHE) with CO

bubbling through the solution showed a number of features correspond to: (i) the HER at

E < 0.0 VRHE (ii) the reversible hydrogen adsorption and desorption at E < 0.4 VRHE, (iii)

a large double layer in the potential range 0.4 - 0.6 VRHE, and (iv) the CO stripping peak

as well as the irreversible surface oxidation and reduction at E > 0.6 VRHE. The

voltammetric profiles of a number of equilibrated Pd-Bi alloys exhibited features at

124

potentials similar to the unannealed alloys with a remarkable effect on the Hupd region on

Pd and the Pd-rich alloys.

The catalytic assessment of the Pd-Bi alloy system for the HER and HOR has shown

very interesting and valuable results. A compositional dependence was observed with an

optimum alloy composition at ca. Pd25Bi75 (comparable to pure Pd). A similar

compositional dependence was found after annealing Pd-Bi alloys at 300 °C for 15

minutes. The compositional dependence of the HOR is similar to that of the HER except

at Bi-rich alloys where the HER activity decreases monotonically towards 100 at. % Bi,

while the HOR activity decreases more rapidly at ca. 90 at. % Bi. This is possibly due to

that the Tafel-Volmer mechanism is favoured in the HOR that is suppressed on Pd

monomers (most popular ensembles in this compositional region).

The CO tolerance of Pd-Bi alloy system in the HOR was assessed in 0.5 M HClO4 with

a mixture of hydrogen and 500 ppm CO bubbling through the solution. The strength of

CO poisoning was found to be strongly composition-dependent. Bi-poor alloys have

shown a higher level of CO tolerance than Bi-rich alloys. This is possibly due to that Bi

provides low effectiveness towards facilitating oxygen transfer in CO oxidation [194].

The enhancement of the HER and HOR activities on the Pd-Bi alloy catalysts with

compositions around Pd25Bi75 could be ascribed to the formation of a large number of Pd

monomers at this compositional range "ensemble effect" that enhances the interaction

with hydrogen atoms and molecules, and/or provides selectivity towards the Heyrovsky

step in the HER [142]. Another possibility is that the formation of intermetallic Bi2Pd

with a maximum at ca. Pd25Bi75 results in higher activity for the HER and HOR

"electronic effect". The increase in the activity could also be linked to a decrease in the

hydride phase content in bulk with increasing Bi concentration.

In view of its lower cost, this alloy system may provide a valuable alternative to Pt for

the HER and HOR.

125

Chapter 5: Ruthenium-Gold (Ru-Au) Alloy Surfaces

5.1- Introduction

Ru is used as a catalyst to promote the production of hydrocarbons from CO and

hydrogen [223]. The employment of bimetallic catalysts containing Ru as one of the

components in the industrial catalysis could lead to better activities. For instance, Ba-

Ru/MgO catalyst (the molar ratio Ba:Ru is 1:1) has been utilized for the ammonia

synthesis yielding very high activity and stability [224]. The components of this material

are recyclable with high purity. Therefore, the authors suggested that this catalyst could

be an alternative to Ru/C and iron-based catalysts in the industrial synthesis of ammonia.

Various studies have considered either deposition of Au films on a single crystal of Ru

[225, 226] or deposition of Ru films on a single crystal of Au [227-229] suggesting that

the substrate morphology and temperature dominate the behavior of the coverage. The

deposition process results in a redistribution of charge and localization of electrons in

the interface region as a result of electronic interactions between Au and Ru. This

behavior has been suggested to be responsible for producing a largely stable Au-Ru

bond [225]. Another research effort has been directed towards the deposition of Au, Pd

and co-deposited Pd-Au ultrathin films on Ru (001) [230, 231]. The bimetallic (Pdx-Auy)

monolayer films with x = 0.4 exhibited higher activity than Pd/Ru (001) film in the

catalytic conversion of acetylene to benzene due to ensemble effects in the bimetallic

catalyst [231].

CO poisoning anode catalysts remains a challenge in fuel cell technology [232]. Ru is

considered as a constituent element of the anode in fuel cell technology due to its ability

to prevent CO poisoning [233, 234]. A number of Pt-based alloy catalysts have been

investigated in order to develop catalysts with a satisfactory degree of CO tolerance

[232]. Among those catalysts, Pt-Ru alloy has exhibited distinctive properties towards

CO tolerance and found to be the most active catalyst for the CO oxidation [72, 187] as

well as for the electrochemical oxidation of methanol or formic acid [232, 235-237].

126

Ru/Pd and Ru/Au heterobimetallic complexes have also been observed to promote the

electrochemical oxidation of methanol [236].

The catalytic performance of the Pt-Ru alloy catalyst in the latter processes has been

reported to be superior to that of pure Pt [186, 187]. A Pt-Ru alloy with a 50:50 surface

composition has been suggested to be the best catalyst for the CO oxidation, while a

90:10 Pt/Ru ratio is the best alloy composition for the methanol oxidation [72, 187]. A

study by Iwasita and colleagues [238] has shown that the preparation method of the Pt-

Ru catalysts influences their catalytic performance for the methanol oxidation. The total

number of Pt-Ru pair sites has also been reported to dominate this catalytic process

[239].

The enhancement of the CO oxidation on the Pt-Ru alloy could be ascribed to the fact

that Ru is more active than Pt for the adsorption process of dissociative water and the

subsequent production of OHads (5.1) which promotes the conversion of CO adsorbed on

Pt into CO2 (5.2) [81].

Ru + H2O → Ru-OHads + H+ + e- (5.1)

Pt-COads + Ru-OHads → Pt + Ru + CO2 + H+ + e- (5.2)

Similar mechanistic interpretation has been proposed for the electrocatalytic oxidation of

methanol [134, 239] or formic acid [232] on Pt-Ru alloy surfaces.

A further application of Ru and Pt-Ru alloy catalysts in electrocatalysis is in the HOR. A

comparative study of this reaction on Ru (0001) and on Ru (10 – 10) surfaces employing

a rotating disk electrode (RDE) in H2SO4 and HClO4 solutions showed that the rates on

the Ru (10 – 10) surface is higher than that on the Ru (0001) surface [240]. It was also

concluded from this investigation that the formation of Ru oxide inhibits the HOR. The

authors suggested that the formation of Ru oxide in H2SO4 is slower than in HClO4

solution which, subsequently, leads to faster reaction kinetics in the former acid. The

HOR has also been measured on Pt, Ru and Pt-Ru alloy (Ru surface composition of ≈ 50

127

atomic % and ≈ 90 atomic %) surfaces using RDE technique in a 0.5 M H2SO4 solution

saturated with H2 [241] and mixtures of CO/H2 [242]. The HOR activity on both Pt and

Pt-rich alloy surfaces has been observed, in the former case, to be better than on pure Ru

at room temperature. The Ru-rich alloy exhibited lower HOR activity in the presence of

CO/H2 mixtures, while pure Ru observed to be totally inactive for the oxidation of

CO/H2 mixtures.

Another interest has been paid to the employment of ruthenium oxide catalysts in fuel

cell technology due to the fact that Ru possesses the characteristic of oxophilicity

resulting in oxides with specific catalytic properties [240]. Ruthenium oxides are also

stable over a wide range of operating potentials which make them applicable in a

number of electrochemical applications such as energy storage and capacitors [243].

One of the electrocatalytic applications of ruthenium dioxide (RuO2) catalyst is in

hydrogen oxidation [244]. It is also used as a catalyst for the oxygen evolution reaction

(OER) [223]. A reduction in the electrochemical activity of RuO2 for the OER has been

observed with the increase of calcining temperature from 350 to 550 °C during the

preparation of the catalyst [245].


A series of 100-field arrays of Ru-Au alloys have been synthesized employing the HT-

PVD methodology. As in the case with the Pd-Au and Pd-Bi alloy systems, the bulk and

surface compositions of Ru-Au alloy samples have been measured by EDS and XPS

respectively.

5.2.1- EDS Analysis

The bulk Ru and Au composition in an array of the Ru-Au alloy system with respect of

(x, y) position in the array is shown by false color maps in Figure 5.1. The dashed

arrows in this Figure refer to the growth direction of the elemental components in the

array sample proving the achievement of the deposition of nearly the whole

compositional range of the Ru-Au alloy system.

128

Figure 5.1: False color maps of the bulk atomic percentage of Ru and Au in an array of Ru-Au alloy system. The dashed arrows refer to the growth direction of the constituents in the array sample.

5.2.2- XRD Analysis

Figure 5.2 shows the XRD patterns of a number of Ru-Au samples. The dotted lines

belong to 2θ values of various Au and Ru planes from the literature [174, 234, 246]. A

steady shift towards higher 2θ values from that of Au (111) is observed with decreasing

Au concentration in the alloy indicative of the formation of solid solutions between Ru

and Au atoms. However, the diffraction peak ascribed to the highest composition of Ru

(Ru74Au26) is almost flat (no strong peak) which may indicate that the Ru-rich alloys are

amorphous. It appears from those two observations that the formation of solid solutions

in the Ru-Au alloy system is composition-dependent as no formation of solid solutions

occurs at alloys very rich with Ru. This is consistent with the behavior predicted of the

Ru-Au alloy system due to the fact that its elemental components have different crystal

structures (the crystal structure of pure Ru is hexagonal close packed (h.c.p.) with lattice

constants a = 2.71 Å and c = 4.28 Å, while that of pure Au is face centered cubic (f.c.c.)

with a = 4.08 Å) [247].

X

y

129

Figure 5.2: Typical XRD patterns of a number of Ru-Au alloys.

The phase diagram of the Ru-Au system (Figure 5.3) [248] is a monotectic type [249]. It

broadly shows that Ru and Au can dissolve in each other forming solid solutions. The

very large difference in the melting points between Ru (2334 °C) and Au (1064.43 °C)

prevents the formation of intermetallic compounds between Ru and Au [249]. The Ru-

Au alloys with high concentrations of Au have been suggested to form solid solutions in

the fcc structure (a replacement of a number of Au atoms on the lattice points by Ru

atoms), while the Ru-rich alloys may form solid solutions in the hcp structure (a

replacement of a number of Ru atoms on the lattice points by Au atoms) [248]. In a Pt-

Ru catalyst, a solid solution in the fcc structure could be formed if the Ru atomic

fraction is about 0.7 or less, while above 0.7 solid solutions in the hcp structure could be

formed [250]. The absence of the diffraction peak ascribed to hexagonal Ru in Figure

5.2 suggests that the Ru-Au alloys are in the fcc structure. Parallel XRD findings and

interpretations have been reported of a number of Pt-Ru catalysts (Pt:Ru atomic ratio 1:1

and 1:3) [237, 250], since no hcp Pt-Ru reflections could be observed indicative of the

formation of fcc structures only.

0

2

4

6

8

10

32 34 36 38 40 42 44 46 48

Inte

ns

ity

/ A

rbitra

ry U

nit

s


95 at. % Au

77 at. % Au

65 at. % Au

47 at. % Au

31 at. % Au

26 at. % Au

Ru (100)

Au (111)

Ru (101)

Au (200)

Ru (002)

130

Figure 5.3: The phase diagram of the Ru-Au system [248].

5.2.3- XPS Analysis

The surface composition of a number of Ru-Au alloy catalysts have been measured by

XPS. Figures 5.4-5.5 show the Au (4f) XPS spectra of various alloys before and after

annealing at 300 ºC for 15 minutes respectively. The dotted lines represent the binding

energy values of metallic Au (84 and 88 eV [132]). The Au (4f7/2) peak position is

shown in both cases as a function of bulk Au composition in Figure 5.6. A shift in the

peak position from that of metallic Au towards lower binding energy is observed with

decreasing the Au concentration in the alloy.

131

Figure 5.4: The Au (4f) XPS spectra of various random Ru-Au alloys. The dotted lines represent the binding energies of metallic Au in the (4f) region [132].

Figure 5.5: The Au (4f) XPS spectra after annealing the Ru-Au alloys at 300 ºC for 15 minutes. The dotted lines represent the binding energies of metallic Au in the (4f) region [132].

-20

0

20

40

60

80

100

120

140

160

76 78 80 82 84 86 88 90 92

Inte

ns

ity

/ C

PS

Binding Energy / eV

17 at. % Au

28 at. % Au

41 at. % Au

61 at. % Au

86 at. % Au

91 at. % Au

-40

-20

0

20

40

60

80

100

120

140

160

78 80 82 84 86 88 90 92

Inte

nsit

y /

CP

S

Binding Energy /eV

17 at. % Au

41 at. % Au

61 at. % Au

72 at. % Au

91 at. % Au

Au (4f 7/2)

Au (4f 5/2)

Au (4f 7/2)

Au (4f 5/2)

132

Figure 5.6: The Au (4f7/2) peak position as a function of bulk Au composition in the alloy.

The Ru (3d) XPS spectra of various alloys before and after annealing are also shown in

Figures 5.7-5.8 respectively. The dotted lines at 280 and 284 eV correspond to metallic

Ru [134]. The Ru (3d5/2) peak position shifts towards higher binding energy from that of

metallic Ru with increasing Au concentration in the alloy (Figure 5.9).

The shift in the peak positions upon alloying indicates an electronic perturbation of

metallic constituents. The shift towards lower binding energy (in the case of Au) and

towards higher binding energy (in the case of Ru) is consistent with the simple charge

transfer idea based on electronegativity differences. It suggests that Au atom acquire a

negative charge and Ru atom acquire a positive charge because Au has relatively higher

Pauling electronegativity value (2.4) than Ru (2.2) [182].

81

81.5

82

82.5

83

83.5

84

0 20 40 60 80 100

Bin

din

g E

nerg

y /

eV

Au at. %

Before annealing

After annealing

133

Figure 5.6: The Ru (3d) XPS spectra of various random Ru-Au alloys. The dotted lines represent the binding energy of metallic Ru in the (3d) region [134].

Figure 5.7: The Ru (3d) XPS spectra after annealing the Ru-Au alloys at 300 ºC for 15 minutes. The dotted lines represent the binding energy of metallic Ru in the (3d) region [134].

-20

0

20

40

60

80

276 278 280 282 284 286 288 290

Inte

ns

ity /

CP

S

Binding Energy / eV

17 at. % Au

41 at. % Au

61 at. % Au

80 at. % Au

90 at. % Au

-20

-10

0

10

20

30

40

50

60

70

274 276 278 280 282 284 286 288 290

Inte

nsit

y /

CP

S

Binding Energy / eV

17 at. % Au

41 at. % Au

61 at. % Au

72 at. % Au

91 at. % Au

Ru (3d 5/2)

Ru (3d 3/2)

Ru (3d 3/2) Ru (3d 5/2)

134

Figure 5.9: The XPS Ru (3d5/2) peak position as a function of bulk Au composition in the alloy.

The surface composition of the measured Ru-Au alloy catalysts have been compared to

their bulk composition before and after annealing at 300 °C for 15 minutes (Figure

5.10). A surface segregation of Ru is observed in both cases (i.e. random and annealed

alloys). This would be in contrast with the theoretical prediction that a surface

segregation of Au takes place in this alloy system at equilibrium due to its lower surface

energy (1.626 Jm-2 [77]) compared to that of Ru (3.4 Jm-2 [231]).

The surface segregation of Ru could be directly correlated with the well known behavior

of Ru as an oxophilic metal, since it could easily form a number of oxide phases at the

solid/gas interface in the presence of oxygen [58, 233, 240]. Thus, the high

concentration of Ru at the surface before and after annealing could be ascribed to the

presence of both metallic Ru as well as Ru oxides (formed by the residual oxygen

species in the deposition chamber). This would be consistent with the proposition that an

alloy component tends to segregate to the surface in the presence of an adsorbate to form

a chemical bond with this adsorbate [84, 86]. It is also in accordance with the behavior

predicted in a Pt-Ru alloy in the presence of adsorbed oxygen as Ru tends to segregate to

the surface to form a strong bond with oxygen [251].

279.8

280

280.2

280.4

280.6

280.8

281

281.2

281.4

281.6

0 20 40 60 80 100

Bin

din

g E

nerg

y /

eV

Au at. %

Before annealing

After annealing

135

Figure 5.10: The bulk and surface Au compositions before (blue diamonds) and after annealing (red diamonds) at 300 °C for 15 minutes. The dotted line represents the relation one may expect for no surface segregation of one of the alloy constituents. The dashed line is a fitting to the data with a polynomial equation: y = 0.35x + 0.0065x2, where y is the surface composition and x is the bulk composition determined by EDS.


A series of voltammetric measurements were carried out on a number of random and

annealed Ru-Au alloys at room temperature in 0.5 M HClO4. Figure 5.11A-F shows the

voltammetric curves of various Ru-Au alloys cycled to an upper limiting potential of 0.5

VRHE. A main observation in this restricted range of potential is the increase in the

double layer current (1) with increasing Ru content in the alloy. This has also been

observed on Ru/Au(111) surface and associated with the presence of Ru [186]. The

voltammograms also show a variety in the hydrogen adsorption and evolution features

(2) between ca. - 0.05 to 0.05 VRHE with varying the alloy composition.

0

20

40

60

80

100

0 20 40 60 80 100

Su

rface A

u a

t. %

(fr

om

XP

S)

Bulk Au at. % (from EDS)

Before Annealing

After Annealing

136

Figure 5.11: The cyclic voltammograms of various random Ru-Au alloys recorded at room temperature in 0.5 M HClO4 using a scan rate of 50 mV/s. The limiting potentials are: Ein = 0.2 VRHE, Elo = 0.0 VRHE and Eup = 0.5 VRHE. The CVs are from the third cycle.

Further voltammetric assessment of the Ru-Au alloy surfaces were carried out by

increasing the limiting upper potential to 1.6 VRHE in order to observe the surface

reduction peak and to assess the effect of the alloy composition on its position. Figure

5.12A-F shows window opening cyclic voltammograms of a number of random (solid

lines) and annealed (300 ºC for 15 minutes, dashed lines) Ru-Au alloy surfaces.

15 : 85

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

30 : 70

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

46 : 54

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

j /

mA

cm

-2 (

ge

o)

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

63 : 37

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

79 : 21

Potential / VRHE

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

100 : 0

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

A

E F

DC

B

1

2

137

Figure 5.12: Window opening cyclic voltammograms of various Ru-Au electrodes (ratio, Ru:Au) before (solid lines) and after (dashed lines) annealing at 300 ºC for 15 minutes measured at room temperature in 0.5 M HClO4 with scan rate of 50 mV/s. Limiting potentials are: Ein = 0.2 VRHE, Elo = - 0.03 VRHE and Eup = 1.6 VRHE. The CVs are from the third cycles.

There is no significant effect on the voltammograms can be observed after annealing

under these conditions. The voltammograms clearly show two features (1 and 2)

ascribed to the surface redox processes. It also shows a shoulder at around 0.2 VRHE that

possibly corresponds to reduction of oxidized Ru [252]. The redox processes on pure Ru

occur at a large potential range starting from ca. 0.2 VRHE (Figure 5.12F). This could

produce overlap with other processes preventing the observation of their features [243].

The position of the reduction feature in the potential range between 1.0-1.2 VRHE varies

7 : 93

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-4

-2

0

2

4

19 : 81

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-4

-2

0

2

4

34 : 66

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

j /

mA

cm

-2 (

ge

o)

-4

-2

0

2

4

54 : 46

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-4

-2

0

2

4

77 : 23

Potential / VRHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-4

-2

0

2

4

100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

-4

-2

0

2

4

A

FE

C D

B

1

2 3

138

with varying the alloy composition (Figure 5.13). This variation is a clear evidence of

alloy formation. The oxide reduction peak becomes broader with increasing Ru content

in the alloy. This could be associated with the redox behavior of Ru.

Figure 5.13: The relative position of the oxide reduction peak on various random Ru-Au alloys (Ru:Au ratio, atomic %).

The CO stripping was subsequently assessed on a series of Ru-Au alloy surfaces at an

upper limiting potential of 1.1 VRHE (Figure 5.14). Prior to these measurements, the

electrolyte was saturated with CO for 20 minutes, and then CO was removed from the

bulk by bubbling with Ar. In the first cycle, a peak arises at around 0.7 VRHE (1) in the

anodic sweep which corresponds to the stripping of adsorbed CO from the surface. The

presence of the CO molecules on the surface prevents the observation of other surface

processes in the first cycle. The removal of CO allows the observation of the features in

the Hupd as well as the HER and HOR (2, 3) features, and surface redox features (4, 5) in

the second cycle. The voltammograms are also a revision to the significant current in the

double layer region.

-2

-1.5

-1

-0.5

0

0.5

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

j / m

A c

m-2

(g

eo

)

Potential / VRHE

0 at. % Au

23 at. % Au

46 at. % Au

66 at. % Au

81 at. % Au

93 at. % Au

139

Figure 5.14: The Cyclic voltammograms of a number of random Ru-Au alloy catalysts (Ru:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4 saturated with CO for 20 minutes, and a scan rate of 20 mV/s. The limiting potentials are: Ein = 0.2 VRHE, Elo = 0 VRHE and Eup = 1.1 VRHE. The dotted lines are from the first cycle and the solid lines are from the second cycle.

The CO stripping peak overlaps in most cases with the surface oxide formation peak (4).

This prevents the analysis of the charge associated with the CO stripping peak. The latter

peak is more pronounced on pure Ru (Figure 5.14F) than on the alloy surfaces. This is

consistent with the behavior one may predict for the CO adsorption on an alloy of Ru-

Au, since Au is inactive for this process and it only takes place on Ru sites [186, 253-

255]. This would also be in accordance with the observations by Jänsch et al. [254]

using temperature programmed desorption (TPD) where the intensity of CO desorption

13 :87

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

30 : 70

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

48 : 52

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

j / m

A c

m-2

(g

eo

)

-1.5

-1.0

-0.5

0.0

0.5

66 : 34

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-1.5

-1.0

-0.5

0.0

0.5

81 : 19

Potential / VRHE

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.5

-1.0

-0.5

0.0

0.5

100 : 0

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.5

-1.0

-0.5

0.0

0.5

A

FE

DC

B

1 3

2

4

5

140

peak has been found to decrease simultaneously with increasing Au precoverage on Ru

(001).

The CO stripping peak on Ru and Ru-Au alloy catalysts (Figure 5.14) is shifted towards

lower (more negative) potentials compared to the case on the Pd-Au (Figure 3.19) or

Pd-Bi (Figure 4.15) alloy catalysts. The enhancement of CO oxidation on Ru alloy

catalysts can be directly correlated with the oxophilicity of Ru [58, 233, 240]. A number

of publications have revealed that the CO stripping and oxidation on an alloy of Pt-Ru

catalyst occurs at overpotentials lower than that on pure Pt [72, 186, 187, 241, 252, 256].

The reason for this behavior has been attributed to the incorporation of Ru with Pt

catalysts which facilitates the adsorption of oxygen containing species (OHads) at lower

electrode potentials compared to the case on pure Pt [72, 186, 241, 252]. This is known

as a bifunctional effect where CO adsorb on Pt sites, while Ru provides a site for water

discharge reaction producing OHads at lower potential [72, 81].


The HER activity was examined on a number of unannealed and annealed (300 °C for

15 minutes) electrochemical arrays of Ru-Au alloy catalysts at room temperature in 0.5

M HClO4 using the potential step measurements. The current was recorded at the

following potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007→ 0 VRHE.

The potential was held for 90s at each step. Figure 5.15 shows the geometric current

density on pure Ru and a series of random Ru-Au alloy catalysts. An increase in the

catalytic activity for the HOR, in comparison to Ru, appears on alloy compositions < 20

at. % Au with a maximum at ca. Ru90Au10. Similar catalytic activity to that of pure Ru is

exhibited by a broad range of Ru-Au alloys (ca. 20-75 at. % Au). A steady decrease in

the activity is observed with concentrations of Au above ca. 75 at. %.

A similar HER compositional dependence was obtained after annealing the Ru-Au alloy

catalysts at 300 ºC for 15 minutes (Figure 5.16). The HER activity on Ru-Au alloys

after annealing under these conditions is slightly higher than before annealing (Figure

141

5.15). A similar behavior was observed with the Pd-Au and Pd-Bi alloy systems. A

discussion regarding this point is given latter in this thesis.

Figure 5.15: The HER activity at - 17.33 mV on several random Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4. The j values are the average of three experiments. The dotted line is a guide to the eye.

Figure 5.16: The HER activity at - 17.30 mV on a series of annealed Ru-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye.

0

0.1

0.2

0.3

0.4

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bulk Au at. %

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Bulk Au at. %

142

Figures 5.17-5.18 show the HER activity on the random and annealed Ru-Au alloy

surfaces as a function of Au surface composition. The surface composition along the

whole compositional range of the Ru-Au alloy system was determined using the

polynomial equation (y = 0.35x + 0.0065x2 where: y is the surface composition and x is

the bulk composition determined by EDS) used for the fitting of XPS data (Figure

5.10). The HER specific activity is also plotted as a function of Au surface composition

before and after annealing in Figures 5.19 and 5.20 respectively. The latter was

determined by normalizing the current values to surface Ru composition in the alloy.

Figure 5.17: The HER at - 17.33 mV on a series of random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.

0

0.1

0.2

0.3

0.4

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

143

Figure 5.18: The HER activity at -17.30 mV on a series of annealed Ru-Au alloys (300 ºC for 15 minutes) as a function of surface composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.

Figure 5.19: The HER specific activity at -17.33 mV on several random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

0

0.001

0.002

0.003

0.004

0.005

0 20 40 60 80 100

j / m

A c

m-2

(sp

eci

fic)

Surface Au at. %

144

Figure 5.20: The HER specific activity at -17.30 mV on a series of annealed (300 ºC for 15 minutes) Ru-Au alloys measured at room temperature in 0.5 M HClO4.


The HOR catalytic measurements on a number of unannealed and annealed (300 °C for

15 minutes) electrochemical arrays of Ru-Au alloy catalysts were carried out at room

temperature in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution

using the potential step technique. The current was recorded at: 0 → 0.007 → 0.014 →

0.021 → 0.014 → 0.007→ 0 VRHE. The potential was held for 90s at each step. Figures

5.21-5.22 show the HOR catalytic activity on various random and heat-treated (300 ºC

for 15 minutes) Ru-Au alloy catalysts as a function of bulk Au composition. The HOR

activity shows a similar compositional dependence to the HER activity on Ru-Au alloys

(Figures 5.15-5.16) with a maximum at around Ru90Au10.

0

0.003

0.006

0.009

0.012

0.015

0 20 40 60 80 100

j / m

A c

m-2

(sp

ecif

ic)

Surface Au at. %

145

Figure 5.21: The HOR at 18.00 mV on a several random Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4 with hydrogen bubbling through the solution. The j values are the average of two experiments. The dotted line is a guide to the eye.

Figure 5.22: The HOR activity at 18.11 mV on a number of annealed (300 ºC for 15 minutes) Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4 with hydrogen bubbling through the solution. The line is a guide to the eye.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Au at. %

0

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100

j / m

A c

m-2

(ge

o)

Au at. %

146

The HOR activity on the random and annealed Ru-Au alloys is plotted in Figures 5.23-

5.24 respectively as a function of Au surface composition. The HOR specific activity is

also plotted before and after annealing as a function of surface composition in Figures

5.25 and 5.26 respectively. In all cases, the HOR activity shows a similar compositional

dependence to the HER activity.

Figure 5.23: The HOR activity at 18 mV on a series of random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

147

Figure 5.24: The HOR activity at 18.11 mV on a series of annealed Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.

Figure 5.25: The HOR specific activity at 18.00 mV on various Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Surface Au at. %

0

0.00001

0.00002

0.00003

0.00004

0.00005

0.00006

0 20 40 60 80 100

j / m

A c

m-2

(sp

ec

ific

)

Surface Au at. %

148

Figure 5.26: The HOR specific activity at 18.11 mV on various annealed Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.

The HER and HOR data on the random and annealed Ru-Au alloys (Figures 5.15-5.20

and 5.21-5.26) indicate two compositional regions where the activity of the alloy is

higher than pure Ru, at ca. Ru90Au10 and 60-80 at. % Au. The increase in the activity for

the HER and HOR in these compositional regions could be due to a modification in the

electronic properties of Ru upon alloying with Au. It has been theoretically predicted

that a Ru overlayer on Au produces a positive shift in the d-band centre of Ru, while a

Au overlayer on Ru gives a negative shift in the d-band centre of Au [21]. A negative

shift in the d-band centre of Ru may lower the hydrogen adsorption energy giving a

better HOR activity [21]. The application of Sabatier principle to the HER [60] suggests

that |∆GH| value on the alloy is closer to zero than pure Ru.

The HER and HOR on Ru-Au alloys show a similar compositional dependence. This

behavior, however, is different at high concentrations of Au where the HER activity

decreases monotonically towards 100 at. % Au (Figure 5.15), while the HOR activity

decreases more rapidly at ca. 90 at. % Au (Figure 5.21). This observation is in

agreement with the data obtained on Pd-Au (Figures 3.29 and 3.35) and Pd-Bi (Figures

4.20 and 4.23) alloys.

0

0.0003

0.0006

0.0009

0.0012

0.0015

0 20 40 60 80 100

j / m

A c

m-2

(sp

ec

ific

)

Surface Au at. %

149

The oxophilicity of Ru [58, 233, 240] could provide a barrier to the HER and HOR

activity on Ru-containing surfaces. This is because hydrogen adsorption can be blocked

by a preferential adsorption of a hydroxide anion [257]. The increase in the HER and

HOR activity at high concentration of Au (ca. 60-80 % Au) could be associated with a

decrease in the oxophilicity of Ru that results in a suppression of hydroxide adsorption

and improves hydrogen adsorption. This is consistent with the observation by Jansch

et.al. [254] that increasing Au precoverage on Ru(001) results in a simultaneous

decrease in CO adsorption.

The HOR activity on Ru-Au alloy surfaces was also examined in 0.5 M HClO4 with a

mixture of hydrogen and 500 ppm CO bubbling through the electrolyte in order to assess

CO tolerance of the alloy system. The results after bubbling the electrolyte with the

mixture for 1 minute and 44 minutes are presented in Figure 5.27. It appears that this

alloy system offers a high degree of CO tolerance under these conditions, since the

presence of CO in the electrolyte does not significantly affect the activity for the HOR

even after 44 minutes of bubbling. This behavior is not surprising as Ru is well-

documented [72, 81, 258-260] to effectively enhance CO tolerance of Pt through the

bifunctional mechanism. Au, on the other hand, has been reported to prevent CO

adsorption on Ru(001) surface [254]. Au was also shown here to hinder CO adsorption

on Pd (Figure 3.23).

150

Figure 5.27: The HOR activity at 18.12 mV on several random Ru-Au alloy surfaces measured at room temperature in 0.5 M HClO4 in the presence of a mixture of hydrogen and 500 ppm CO. The line is a guide to the eye.

5.6- Conclusions

It was shown from the EDS analysis that the deposition of nearly the whole

compositional range of Ru-Au alloy system was achieved. Upon alloying, a shift in the

Ru (3d) XPS spectra towards higher binding energy compared with metallic Ru

accompanied by a shift in the Au (4f) XPS spectra towards lower binding energy

compared to metallic Au was observed. This result was attributed to electronegativity

differences between the constituents which suggest a charge transfer from Ru to Au due

to the higher electronegativity value of Au.

Comparing the bulk composition (measured by EDS) to the surface composition

(measured by XPS) has shown a surface segregation of Ru before and after annealing at

300 ºC for 15 minutes. This behavior was attributed to that Ru tends to segregate at the

surface to form oxides with residual oxygen species in the deposition chamber due to its

oxophilicity.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 20 40 60 80 100

j / m

A c

m-2

(geo

)

Au at. %

1 min.

44 min.

151

The XRD analysis of a number of the Ru-Au alloy catalysts has shown a consistent shift

from the 2θ value of pure Au towards higher values accompanied by a decrease in the

peak intensity with decreasing Au concentration in the alloy. The shift in the peak

position indicates the formation of solid solutions between the constituents. The

decrease in the peak intensity was observed to be associated with the increase in the Ru

concentration in the alloy suggesting that Ru-rich alloys are amorphous. The

observations from XRD analysis suggest that the formation of solid solution in the Ru-

Au alloy system is composition-dependent. The latter behavior is predicted for this alloy

system due to the fact that its elemental components have different crystal structure.

The voltammetric assessment of various random and annealed Ru-Au alloy surfaces has

shown a significant current in the double layer region which was associated with the Ru

content in the alloy. A surface reduction feature was observed in the potential range

between 1.0-1.2 VRHE upon cycling to a limiting potential of 1.6 VRHE. The position of

this feature varies with the alloy composition confirming the alloy formation. There was

no significant effect of annealing Ru-Au alloy catalyst at 300 ºC for 15 minutes on the

voltammetric features.

The CO stripping voltammetry on Ru and Ru-Au alloy surfaces has shown a peak at ca.

0.7 VRHE that corresponds to CO stripping process. In most cases, this peak was

overlapped with the surface oxide formation peak preventing the determination of the

charge associated with it. The CO stripping peak on Ru-Au alloy catalysts occurs at

potentials lower than on Pd-Au or Pd-Bi alloy catalysts. This behavior was attributed to

the presence of Ru facilitating the adsorption of OHads species at lower potentials due to

its oxophilicity leading to improved CO oxidation.

The HER activity on Ru-Au alloy surfaces was found to be composition-dependent. The

HOR activity shows a similar compositional dependence to the HER activity on the Ru-

Au alloy surfaces. An exception of this occurs at high compositions of Au where the

HER decreases monotonically towards 100 at. % Au, while the HOR activity decreases

more rapidly at ca. 90 at. % Au. A higher reaction activity than pure Ru was found at ca.

152

Ru90Au10 and 60-80 at. % Au. The increase in the activity was ascribed to a modification

in the electronic properties upon alloying that results in a better activity for the HER and

HOR. It was also suggested that the increase in the activity at ca. 60-80 at. % Au could

be due to a decrease in the oxophilicity of Ru that enhances hydrogen adsorption and

reduces hydroxide adsorption. The presence of a mixture of hydrogen and 500 ppm CO

in the electrolyte does not significantly affect the HOR activity on the Ru-Au alloy

surfaces which indicates that this alloy system offers a high degree of CO tolerance

under these experimental conditions.

153

Chapter 6: Conclusions and General Discussion

The broad aim of this project was to assess the catalytic behaviour of various binary

alloy systems in order to identify alternatives to Pt for electrochemical reactions. A high

throughput physical vapour deposition method [89] was employed for the synthesis of

libraries of thin films of Pd-Au, Pd-Bi and Ru-Au alloys. A high throughput screening

method was subsequently employed for the electrocatalytic assessment of these

materials for the HER and HOR. It is the first time that high throughput methods have

been applied for the assessment of the HER and HOR activity on these alloy systems.

The high throughput methods employed here allowed the identification of the optimum

alloy composition (in each alloy system) for the HER and HOR.

6.1- Sample Characterization

Various analytical tools were employed for the characterization of Pd-Au, Pd-Bi and Ru-

Au alloys including EDS for bulk composition analysis, XPS for surface composition

analysis and XRD for structure analysis. The key findings are:

The EDS analysis has shown that the deposition of nearly the whole compositional

range of the alloy systems was achieved.

The XPS analysis has shown shifts in the peak positions correspond to metallic

constituents of alloy systems proving the formation of the alloy in each case.

Comparing bulk composition with surface composition has shown a surface

segregation of Au in the Pd-Au alloys (Figure 3.10), no measurable surface segregation

in the Pd-Bi alloys (Figure 4.13) and a surface segregation of Ru in the Ru-Au alloys

(Figure 5.10).

The XRD analysis has shown that a continuous series of solid solutions is formed in

the Pd-Au alloy system, while the formation of solid solutions in both the Pd-Bi and Ru-

Au alloy systems is composition-dependent.

154

6.2- The HER and HOR Activity

The electrocatalytic assessment of the Pd-Au [261], Pd-Bi [262] and Ru-Au alloy

surfaces for the HER and HOR suggests that:

The HER and HOR activity on these alloy systems are composition-dependant.

There appears to be a high correlation between the HER and HOR activity on an alloy

catalyst. A similar compositional dependence was observed on Pd-Au (Figures 3.27 and

3.35), Pd-Bi (Figures 4.20 and 4.23) and Ru-Au (Figures 5.15 and 5.21) alloy surfaces.

This suggests that the HER activity provides a good descriptor for the HOR activity for

systems with low overpotentials. An exception of this occurs, however, at high

concentrations of Au and Bi where the HER activity decreases monotonically towards

100 at. %, while the HOR activity decreases more rapidly at alloy compositions of ca. 90

at. %. The activity in this compositional range would be expected to be dominated by Pd

or Ru monomers. Therefore, this behaviour could be associated with a higher selectivity

towards the Tafel-Volmer mechanism in the HOR that is suppressed on monomers. This

would be in accordance with the proposition that the Volmer-Heyrovsky mechanism is

favoured in the HER on Pd monomers, since the adsorption of two hydrogen atoms on a

single Pd atom is less likely to occur [142] .

A number of Pd-Au alloys are superior to pure Pd with respect to activity for the HER

(Figure 3.27) and HOR (Figure 3.34). The optimum Pd-Au alloy composition for the

HER and HOR is in the range of ca. Pd50Au50. The Pd-Au alloys offer a higher degree of

CO tolerance in the HOR than Pt, since they were less strongly poisoned in the presence

of a mixture of hydrogen and 500 ppm CO (Figure 3.39) than one would expect for Pt.

This suggests that the Pd-Au alloys may provide an alternative to Pt for fuel cell

reactions which could only tolerate a concentration of ca. ≤ 10 ppm CO [71]. The

improved CO tolerance of the Pd-Au alloys could be attributed to a weaker interaction

of CO with the surface resulting from the ligand effect [21, 263]. This is consistent with

the data obtained here from CO stripping measurements (Figure 3.23) which indicates

that alloying Pd with Au results in a decrease of CO adsorption.

155

The HER (Figure 4.20) and HOR (Figure 4.23) activity on ca. Pd25Bi75 is comparable

to pure Pd. The CO tolerance of Pd-Bi alloys is composition-dependent. The HOR

activity on the Pd-Bi alloys with concentrations of Bi above 20 at. % is strongly

poisoned by CO in the presence of a mixture of hydrogen and 500 ppm CO, since that

the activity was found to be significantly decreased after 1 minute of bubbling the

electrolyte with the mixture and completely suppressed after 11 minutes (Figure 4.26).

This is in accordance with the conclusion that the deposition of Bi on carbon supported

Pt lowers CO tolerance of Pt, since Bi offers low effectiveness towards facilitating

oxygen transfer in CO oxidation [194]. The Pd-Bi alloys with compositions below 20 at.

% Bi, however, offer a higher degree of CO tolerance.

Ru-Au alloys with compositions of Ru90Au10 and 60-80 at. % Au are more active than

Ru for the HER (Figure 5.19) and HOR (Figure 5.25). The Ru-Au alloy system exhibits

a high degree of CO tolerance during the HOR, since no significant influence on the

activity was found even after bubbling the electrolyte with a mixture of hydrogen and

500 ppm CO for 44 minutes (Figure 5.27).

The Ru-Au alloy system appears to be superior to the other two systems with respect

to CO tolerance. This is because the presence of CO does not significantly influence the

HOR activity on Ru-Au alloys (Figure 5.27), while a decay in the activity was found on

both Pd-Au (Figure 3.39) and Pd-Bi (Figure 4.26) alloys. This could be correlated with

well-documented role of Ru as a promoter of CO tolerance of Pt through the

bifunctional mechanism [72, 81, 258-260]. It could also be correlated with the

conclusion that the presence of Au on Ru(001) surface prevents CO adsorption [254].

Au was also found through this study to hinder CO adsorption on Pd (Figure 3.23).

Based on the results obtained on Pd-Au and Pd-Bi alloy systems, the Pd-Au and Pd-Bi

optimum compositions could be incorporated in the Volcano plot for the HER (Figure

6.1 [60]) assuming that a superior catalyst would have a |∆G| value closer to zero [60].

With respect to their lower cost, these alloys may provide alternatives to Pt for the HER

and HOR. The absence of Ru from the Volcano plot prevents prediction of the position

of the optimum Ru-Au alloy composition. Nevertheless, the application of Sabatier

156

principle [60] suggests that the |∆G| on the Ru-Au alloys that offer a higher activity for

the HER is closer to zero than Ru.

Figure 6.1: The Volcano plot for the HER on several pure metals and metal overlayers [60]. The colored circels represents the predicted position of the Pd50Au50 (red) and Pd25Bi75 (green) based on their activity for this reaction obseved throughout this work.

The increase in the HER and HOR activities observed here on the Pd-Au and Pd-Bi

alloys compared to pure Pd could be linked to a decrease in the bulk hydride content as

the formation of Pd hydrides becomes suppressed by adding higher concentrations of the

other constituent (Au or Bi). This is consistent with the conclusion that hydride

formation in the bulk lowers the activity for the HER [190]. It is also in accordance with

the observation by others that hydrogen absorption capacity decreases monotonically as

a function of bulk Au content producing zero (no hydrogen absorption) at ca. 70 at. %

Au [140]. Hydrogen absorption into bulk Pd-M (M = Cr, Mo and W) alloy and hydride

formation have also been shown to be monotonically suppressed by increasing the

content of the other constituent [264].

-8

-7

-6

-5

-4

-3

-2

-1

0

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Lo

g(i

0(A

cm

-2))

∆GH (eV)

Pd

Pd25Bi75

Au

Bi

Pd50Au50

157

The HER and HOR activity on the annealed array samples were observed to be greater

than on unannealed samples. It is generally proposed that annealing could improve the

performance of a catalyst material through: (i) removing impurities formed on the

catalyst surface during the preparation of the catalyst, (ii) forming more active sites on

the catalyst surface, and/or (iii) producing a uniform dispersion of metal particles in the

case of supported catalysts [58]. The presence of impurities on the array samples used in

this study before annealing is to a considerable extent unlikely as the synthesis of the

alloy catalysts was performed under UHV conditions. It is therefore likely that there was

a contribution from the substrate (Au) after annealing as an alloy can be formed

resulting in more activity. Another possibility is that the surface is reconstructed upon

annealing resulting in more active crystal faces for the HER and HOR [265, 266].

6.3- Suggestions for Further Studies

While more interest is given for research on Pd-Au alloy system, there is a clear lack of

information regarding surface chemistry and electrocatalytic studies on Pd-Bi and Ru-

Au alloys. The data obtained here provide valuable insights about those alloy systems.

There are many prospects for widening research on the Pd-Au, Pd-Bi and Ru-Au alloy

catalysts. Following are suggestions for further studies:

1. The high throughput methods employed for the synthesis and screening of alloy

systems examined here are powerful in terms of the study of support and particle size

effects on the electrocatalytic activity [90]. Therefore, it is interesting to investigate

these effects on the catalytic performance for the HER and HOR.

2. Valuable kinetic data of an electrochemical reaction such as transfer coefficient (α)

and limiting current density (jL) can be obtained from rotating disc electrode (RDE)

measurements [5]. Therefore, a further work may involve deposition of the optimum

alloy compositions on rotating disk electrodes (RDE) in order to determine kinetic

parameters for the HER and HOR on these catalysts.

158

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