surface (electro-) chemistry on pt (111) modified by a pseudomorphic pd monolayer

Surface Science 573 (2004) 57–66

www.elsevier.com/locate/susc

Surface (electro-)chemistry on Pt(111) modifiedby a Pseudomorphic Pd monolayer

M. Arenz *, V. Stamenkovic, P.N. Ross, N.M. Markovic

Materials Science Division, Lawrence Berkeley National Laboratory, University of California, 1 Cyclotron Road,

Berkeley, CA 94720, USA

Received 3 September 2003; accepted for publication 14 May 2004

Available online 3 August 2004

Abstract

The formic acid and methanol oxidation reaction are studied on Pt(111) modified by a pseudomorphic Pd mono-

layer (denoted hereafter as the Pt(111)–Pd1ML system) in 0.1 M HClO4 solution. The results are compared to the bare

Pt(111) surface. The nature of adsorbed intermediates (COad) and the electrocatalytic properties (the onset of CO2 for-

mation) were studied by FTIR spectroscopy. The results show that Pd has a unique catalytic activity for HCOOH oxi-

dation, with Pd surface atoms being about four times more active than Pt surface atoms at 0.4 V. FTIR spectra reveal

that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be

detected although a high production rate of CO2 is observed at low potentials. This indicates that the reaction can pro-

ceed on Pd at low potentials without the typical ‘‘poison’’ formation. In contrast to its high activity for formic acid

oxidation, the Pd film is completely inactive for methanol oxidation. The FTIR spectra show that neither adsorbed

CO is formed on the Pd sites nor significant amounts of CO2 are produced during the electrooxidation of methanol.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Infrared absorption spectroscopy; Metal–electrolyte interfaces; Alcohols; Biological compounds; Carbon monoxide;

Metallic surfaces; Platinum; Palladium

1. Introduction

The electrooxidation of small organic molecules

such as carbon monoxide, formaldehyde, formic

0039-6028/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.susc.2004.05.144

* Corresponding author. Tel.: +1 510 486 4793; fax: +1 510

486 5530.

E-mail address: [email protected] (M. Arenz).

acid and methanol has been extensively studied

due to the increasing interest in fuel cell technol-

ogy (for overview see Refs. [1–3]). It is now well

established that on platinum the electrooxidationof formic acid and methanol to CO2 proceeds via

the so-called dual-path mechanism proposed origi-

nally by Capon and Parsons [4]. In this reac-

tion scheme the ‘‘direct’’ path is a fast reaction

ed.

mailto:[email protected]

58 M. Arenz et al. / Surface Science 573 (2004) 57–66

involving one or more ‘‘active intermediates’’,

while the second reaction pathway involves the

formation of COad which, depending on the elec-

trode potential, may act either as a site blocking

species or as an active intermediate [1,3]. Theself-poisoning behavior of methanol and formic

acid oxidation on Pt is well documented in FTIR

studies [5–10]. Very recently, by utilizing surface-

enhanced IR absorption spectroscopy (SEIRAS)

Osawa and co-workers [11,12] identified formate

as another reaction intermediate for methanol as

well as formic acid oxidation. Furthermore, differ-

ential electrochemical mass spectroscopy (DEMS)investigations have suggested that during metha-

nol oxidation [13] formic acid is formed as an

intermediate [14]. As a consequence, formic acid

oxidation is often regarded as a model reaction

for the oxidation of technologically more impor-

tant organic molecules such as methanol. How-

ever, there are some important differences

between these two molecules which will be dis-cussed in this paper.

Hoping that the combination of different metals

with Pt would have superior catalytic activity rela-

tive to the pure metal, the electrooxidation of

small organic molecules was extensively studied

on different platinum-bimetallic surfaces. Consid-

ering that the oxidation of methanol/formic acid

is closely related to the onset of OH adsorptionthe concept was fairly straightforward: to modify

the surface of Pt with an ad-metal which is more

oxophilic than Pt, thereby shifting the onset poten-

tial for OH formation towards the equilibrium

potential for the electrooxidation of organic mole-

cules [3]. The ultimate goal of these bimetallic sur-

faces is to optimize the adsorption of the molecule

(CH3OH or HCOOH) while oxidizing COad with aminimum surface coverage by OHad, e.g., the

nearly autocatalytic oxidation of COad [15–20].

In this study formic acid and methanol electroox-

idation will be tested on pseudomorphic Pd films

supported on Pt(111) and the results will be com-

pared to the bare Pt(111) surface. The catalytic

properties of both systems are examined by

potentiodynamic measurements. The nature of ad-sorbed intermediates and the onset of oxidation of

the adsorbed molecules (CO2 formation) are ob-

tained by FTIR spectroscopy.

2. Experimental

2.1. Electrochemical measurements

In the presented work ultrathin Pd films depos-ited on Pt(111) were used as working electrodes.

For electrochemical measurements the Pt(111)

crystal was prepared by flame annealing in a

hydrogen flame and subsequent cooling in a mild

stream of a mixture of Ar and H2. For the electro-

chemical measurements after flame annealing the

Pt(111) electrode was transferred into the electro-

chemical cell protected by a drop of ultrapurewater. The preparation of the Pt(111)–Pd1ML film

has been described before [21,22]. In short, the

Pt(111) crystal was cycled in a 0.05 M

H2SO4 + 5 · 10�6 M PdO solution at a scan rate

of 50 mV/s. The amount of Pd deposited was con-

trolled by the continuous change of the voltam-

metric features, from those characteristic of

Pt(111) to those characteristic for a pseudomor-phic monolayer of palladium. The palladium cov-

erage, indicated in the text, is calculated using a

calibration curve which compared the surface cov-

erage of UHV prepared films, derived by LEIS,

with the integrated charges of the electrode in cyc-

lic voltammetry (after a transfer from UHV into

an electrochemical cell, for details see Ref. [21]).

The oxidation of methanol and formic acid wasconducted in a thermostated standard three-

compartment electrochemical cell. A circulating

constant temperature bath (Fisher Isotemp Circu-

lator) maintained the temperature of the electro-

lyte within ±0.5 �C. The reference electrode wasa saturated calomel electrode (SCE) separated by

a closed bridge to prevent chloride contamination

of the electrolyte. All potentials shown in the text,however, refer to the reversible hydrogen electrode

in the same solution, calibrated by measuring the

reversible potential for the hydrogen evolution/

oxidation reaction in the same solution using a

rotating disk electrode configuration. The experi-

mental procedure for methanol/formic acid oxida-

tion was as follows. After preparation the

electrode was immersed under potential controlat 20 mV into Ar-purged (Air Products 5N5 pur-

ity) 0.1 M HClO4 containing 50 mM CH3OH/50

mM HCOOH. Prior to recording the polarization

M. Arenz et al. / Surface Science 573 (2004) 57–66 59

curves the electrode was polarized for 30 s at a

potential of 900 mV in order to oxidize (pre)ad-

sorbed CO and to produce comparable results.

The aqueous solutions were prepared from HClO4(Aldrich, double destilled), CH3OH, HCOOH (Ald-rich, ACS grade), H2SO4 (Baker Ultrex), PdO

(Alfa Products), using pyrolitically triply destilled

water.

2.2. FTIR measurements

For the in situ FTIR measurements a Nicolet

Nexus 670 spectrometer was available equippedwith a liquid-nitrogen cooled MCT detector. All

IR measurements were performed in a spectro-

electrochemical glass cell designed for an external

reflection mode in a thin layer configuration. The

cell is coupled at its bottom with a CaF2 prism

beveled at 60� from the surface normal. The exper-

imental procedure for the IR measurements was

adapted from the measurement of the polarizationcurves. Prior to each experiment the solution was

saturated with argon in order to deoxygenate the

solution. The electrodes were immersed into the

electrolyte and pressed onto the prism under

potential control at a potential of 0.00 V and the

electrode potential was increased in steps of about

100 mV.

The spectra were recorded with a resolution of8 cm�1. All measurements were performed using

p-polarized light. In order to obtain a single beam

spectrum 100 scans were collected at each poten-

tial resulting in a recording time of �1 min. SinceCOad production during methanol oxidation is in

comparison to formic acid oxidation a slow proc-

ess, for methanol oxidation before recording a

spectrum the potential was hold for an additionaltime of 1 min. Absorbance spectra were calculated

as the ratio �log(R/R0) where R and R0 are the

reflectance values corresponding to the sample

and reference spectra, respectively. For obtaining

the COad spectra (right-hand side of Figs. 2 and

3) the reference spectrum was recorded at 1.0 V.

In order to follow the production of CO2, (left

hand side of Figs. 2 and 3), the spectrum recordedat 0.00 V was referenced to the spectra recorded at

the potentials indicated (0.25, 0.35 V, etc.). The

potential in the spectroelectrochemical cell was

controlled by a reversible hydrogen electrode

(RHE).

3. Results

It is now well-established that the (elec-

tro)chemical rate of reaction (the turn over fre-

quency, TOF) is determined by an exponential

term (energy of adsorption, activation energy

etc.) and the pre-exponential term which is, in fact,

the availability of active surface sites [3]. While in

gas-phase catalysis the latter term is mainly deter-mined by the surface coverage of active intermedi-

ates and products, in an electrochemical

experiments (with the exception of oxidation of

small organic molecules, where CO and formate

production may act as intermediate poisons) the

availability of active metal sites is mainly deter-

mined by the adsorption of spectator species,

which are produced from the dissociation of water,the adsorption of anions and the adsorption of

hydrogen. In other words, the electrocatalysis of

small organic molecules takes place not on bare

metals but rather on a metal which is modified

with adsorbates such as Hupd, oxide(s), and anions.

Keeping that in mind, we first present the results

for electrochemical behavior of the Pt(111)–

Pd1ML system in solution free of organic mole-cules. Fig. 1a shows typical cyclic voltammograms

of Pt(111) (dashed line) and Pt(111)–Pd�1 ML(solid line) electrodes in perchloric acid solution.

Briefly summarizing (for more details see Ref.

[22]) there are two characteristics of Pd surface

atoms in the cyclic voltammetry discernible. First,

the charge in the hydrogen underpotential region

(Hupd) on Pt(111) increases from 0.66 ML (0.6Hupd per Pt surface atom) to about 1 ML Hupd(i.e., 1 Hupd per Pd surface atom) upon deposition

of the Pd film [19]. We assigned this finding previ-

ously to a stronger Pd–Hupd interaction compared

to the Pt–Hupd interaction [21]. Secondly, although

the ‘‘butterfly’’ feature (pseudocapacitance at

E � 0.5–0.8 V) shifts to more negative potentialson the Pd film, suggesting that Pd is indeed moreoxophilic than Pt, the charge under the peak

decreases from �80 lC/cm2 on Pt(111) to �60lC/cm2 on the pseudomorphic Pd monolayer.

-0.10

-0.05

0.00

0.05

0.10I

[mA

/cm

2 ]

0

1

2

3

4

E [V] vs RHE

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

Pt(111)-Pd~1ML

Pt(111)

(a)

(b)

(c)

Fig. 1. (a) Cyclic voltammograms of bare Pt(111) (dashed line)

and Pt(111)–Pd�1MLPd (solid line) in 0.1 M HClO4 recorded

with a sweep rate of 50 mV/s at room temperature; Polarization

curves of bare Pt(111) (dashed line) and Pt(111)–Pd�1MLPd(solid line) recorded with a sweep rate of 10 mV/s at 20 �C in 0.1M HClO4 containing (b) 50 mM HCOOH and (c) 50 mM

CH3OH.

180020002400

wave number [cm-1]

180020002400

2050 cm-1

2049 cm-1

2054 cm-1

2063 cm-1

2048 cm-1

0.45 V

0.55 V

0.65 V

0.24 V

0.35 V

0.13 V

0.1 M HClO4 + 50mM HCOOH

(b)CO2 Pt-COad

(a)

Pt(111)-Pd~1MLPt(111)

CO2

0.45 V

0.55 V

0.65 V

0.24 V

0.35 V

0.13 V

1 10-3

x 0.5

Fig. 2. FTIR spectra in 0.1 M HClO4 solution containing 50

mM HCOOH (a) Pt(111) and (b) Pt(111)–Pd�1MLPd; the

applied potentials for each spectra are indicated in the figure;

The reference spectrum the right part for COad were calculated

as the ratio �log(R/R0) where R and R0 are the reflectance

values corresponding to the sample and reference spectra,

respectively. The reference spectra was recorded at 1.0 V. In

order to follow the production of CO2, the spectrum recorded

at 0.00 V (before the onset of CO2 production) was referenced

to the spectra at the potentials indicated (0.25, 0.35 V, etc.).

recorded at 0.00 V for the spectral range of 2150–2450 cm�1

(CO2 band) and at 1.0 V for the spectral range of 1850–2150

cm�1 (CO band); CO2 bands intensities are divided by the

factor of 20.


Previously, we have proposed that this surprising

decrease in the charge under the ‘‘butterfly’’ peak

on Pd relative to Pt is a consequence of the strong

competition between the adsorption of oxygenated

species and anions from the supporting electrolyte(ClO4 and Cl�). In solution free of specifically

adsorbing anions, however, viz. alkaline solution,

the charge which corresponds to the formation

of OH layer is much higher, i.e., �150 lC/cm2.For further details see Ref. [22].

Having established the electrochemical behav-

ior of the Pt(111)–Pd electrode in the basic elec-trolyte we turn our attention to the catalytic

activity of bare and Pd modified Pt(111)

(Pt(111)–Pd�1ML) for formic acid oxidation. In

Figs. 1b and 2 the polarization curves and the

FTIR results, respectively, of both surfaces in 0.1

M HClO4 + 50 mM HCOOH solution are pre-

sented. Note, that while the observed current den-

sity in the polarization curves serves as a measure

180020002400

inte

nsit

y a.

u.

Pt(111)

wave number [cm-1]

180020002400

Pt(111)~ 1MLPd

2050 cm-1

2049 cm-1

2054 cm-1

2063 cm-1

0.1 M HClO4 + 50mM CH3OH

(a) (b)

0.45 V

0.55 V

0.60 V

0.25 V

0.35 V

x 0.05

x 0.05

x 0.5

0.5×10-3

Fig. 3. FTIR spectra in 0.1 M HClO4 solution containing 50

mM CH3OH (a) Pt(111) and (b) Pt(111)–Pd�1MLPd; the

applied potentials for each spectra are indicated in the figure;

reference spectra are recorded at 0.00 V for the spectral range of

2150–2450 cm�1 (CO2 band) and at 1.0 V for the spectral range

of 1800–2150 cm�1 (CO band); note that some CO2 bands

intensities are divided by a constant factor as indicated.


of the activity, the FTIR spectra may serve to

establish both the catalytic activity (the formation

of CO2 band at �2343 cm�1) as well as the ‘‘poi-

son formation’’ (i.e., the COad band �2050 cm�1

for a-top Pt–CO [23] and �1920 cm�1 for bridgebonded Pd–CO [24], respectively). Fig. 2 clearly

shows that the Pt(111)–Pd�1 ML surface is more

active than bare Pt(111). While the onset of CO2formation on Pt(111) is between 0.24 and 0.35 V

on Pt(111)–Pd�1 ML the onset is around 0.13 V

and at 0.24 V already a large CO2 band is seen.

Interestingly, the production of CO2 on the

Pt(111)–Pd�1 ML surface is not accompanied by‘‘COad poison formation’’ (see Section 4) because

no COad band can be detected in the whole poten-

tial region. In contrast, on Pt(111) COad is ob-

served at �0.13 V with a maximum coverage

observed around 0.45 V. These results are in good

agreement with the polarization curves presented

in Fig. 1b. Namely, on Pt(111)–Pd�1ML a large

oxidation current starts around 0.2 V, reachingits maximum at 0.3 V, but sharply decreases at

more positive potentials. By comparison the onset

of the oxidation current on Pt(111) is shifted

about 150 mV to more positive values. At 0.4 V

the observed current densities on Pt(111)–Pd�1MLare about 4 times higher than on bare Pt(111) and

the maximum current density for Pt(111) (ob-

tained at �0.7 V) is only about 1/5th of that ob-served for the palladium film (at �0.3 V) underthe same conditions.

As mentioned above, formic acid was identified

by DEMS as a reaction intermediate during meth-

anol oxidation [13,14] and hence formic acid oxi-

dation is often regarded in the literature as a

model system for the electrooxidation of small or-

ganic molecules. In what follows we therefore pre-sent our results for methanol oxidation on bare

and palladium covered Pt(111), and subsequently

compare formic acid and methanol oxidation on

both surfaces. Fig. 1c shows that in contrast to

Pt(111) on Pt(111)–Pd1 ML the methanol oxida-

tion reaction is highly inhibited. These data are

agreement with data obtained by cyclic voltamme-

try in sulfuric acid solution containing methanol[26]. In addition, in Fig. 3a the well known FTIR

spectra for methanol oxidation on bare Pt(111)

are observed [25]. In particular, the formation of

CO adlayer during methanol oxidation is estab-

lished based on two bands, assigned to linear

bonded CO (�2050 cm�1) and bridge bonded

CO (�1810 cm�1), respectively. The appearance

of the CO-band at �0.25 V is followed by an in-crease in the amplitude up to 0.45 V and then,

due to CO oxidation, by a monotonic decrease in

CO surface coverage. Surprisingly, the polariza-

tion curves as well as the FTIR spectra unambigu-

ously show that the Pt(111)–Pd�1ML electrode is

completely inactive for the methanol oxidation.

For example, by comparing the CO2 bands in the

FTIR spectra of Pt(111) (Fig. 3a) and thePt(111)–Pd�1ML film (Fig. 3b) it is obvious that

while on bare Pt(111) a significant CO2 band

can be observed at 0.45 V on Pt(111)–Pd�1MLthe first very weak CO2 band is detected at a

potential of 0.55 V. Furthermore, the band inten-

sities at each potential (note that the CO2 band

intensities for Pt(111) at 0.55 and 0.6 V are multi-

plied by the factor of 0.05!) for Pt(111) are much


higher than for the Pd film and it is obvious that

only a very small amount of CO2 is formed on

the Pt(111)–Pd�1ML surface. The polarization

curves depicted in Fig. 1c lead to the same conclu-

sion. Although on both surfaces methanol oxida-tion starts at �0.5 V, a considerably higher

potential than for formic acid oxidation, on the

palladium film the observed maximum in the

current density is very low compared to bare

Pt(111). One might suspect that this low activity

is due to a high rate of CO poisoning of the Pd sur-

face and thus blocking the active sites for metha-

nol adsorption. The results of Fig. 3b reveal,however, that this is not the case since no CO-band

is observed in the FTIR spectra. It is obvious that

some other ‘‘spectator’’ species are blocking the

active sites, as we will discuss in the section below.

Based on CV/FTIR results we can conclude that

no methanol oxidation at all occurs on Pd atoms.

The weak currents in the polarization curve and

the small CO2 bands in the FTIR spectra are infact due to some small free Pt-sites on a not com-

pletely Pd covered Pt(111) electrode (since the for-

mation of 3-D Pd islands on Pt(111) starts slightly

before the formation of a complete Pd monolayer

it is difficult to cover all Pt atoms with Pd atoms

[27]).

4. Discussion

As mentioned in the introduction section, for

the discussion of formic acid and methanol oxida-

tion, the so-called ‘‘dual-pathway’’ mechanism,

which was originally suggested by Capon and Par-

sons [4], is instructive. Our simplified version of

this scheme, which can be used for formic acid aswell as for methanol oxidation [3], is as follows:

HCOOH(CH3OH)

active intermedH2O, A -

HCOOH ad(CH3OHad)

COad + OHa

H2O

HCOOH(CH3OH)

active intermedH2O, A -

HCOOH ad(CH OH )

COad + OHa

H2O

Without going into the details of every reaction

step, the scheme displays the most important fea-

tures of both reactions (if not mentioned otherwise

in the following the discussion is applicable to

both molecules). The first step, the adsorption ofHCOOH/CH3OH on the surface, is in competition

with the adsorption of spectator species, viz. Hupdand OHad from the dissociation of water and

anions from the supporting electrolyte [28]. It

was found that HCOOH/CH3OH can easily be

displaced from the surface by these spectator

species [1,3], and hence, in an electrochemical

environment the adsorption of HCOOH/CH3OHmolecules may occur only on an ensemble of bare

(water modified) metal sites. In the simplest terms,

the notation ‘‘spectator species’’ thereby defines a

species that impedes the electrooxidation of formic

acid by blocking the active sites required for

the adsorption of HCOOH/CH3OH molecules,

whereas a ‘‘poisoning intermediate’’ refers to an

adsorbed species, formed directly from the reac-tant, which impedes the reaction via other interme-

diates by hindering their formation and/or their

oxidative conversion to CO2. In the present case,

the spectator species can be formed from H+

(Hupd), ClO�4 (ClO4, ad), Cl

� (Clad) and H2O

(OHad). While Hupd, ClO4, ad and Clad behave as

blocking species in the entire potential region,

OHad may have two modes of action: at higheroverpotentials OHad is a blocking species, whereas

at low overpotentials OHad is a reactive intermedi-

ate, which can react with adsorbed CO (see below)

formed from the dehydration reaction of HCOOH

(dehydrogenation reaction in the case of metha-

nol) [3].

The main aspect of the presented reaction

scheme, however, is that adsorbed HCOOH canbe either ‘‘directly’’ oxidized to CO2, or it can form

CO2 + 2H+ + 2e-

(CO2 + 6H+ + 6e-)iate

CO2 + H+ + e-d + H+ + e-

, A-

CO2 + 2H+ + 2e(CO2 + 6H+ + 6e-)

iate

CO2 + H+ + e-d + H+ + e-

, A-


a ‘‘poisoning reaction intermediate’’. For metha-

nol oxidation the direct path is less pronounced

than for formic acid oxidation and its existence

has been the subject of some discussion (see for

example Refs. [29–31]). For both reactions on Pt,the major ‘‘poisoning reaction intermediate’’ was

identified as being adsorbed carbon monoxide

[5,8,32–37]. Very recently, Osawa and co-workers

[11,12] investigated the oxidation of formic acid

and methanol on Pt using surface-enhanced IR

absorption spectroscopy. For both molecules the

authors reported in addition to the commonly ob-

served CO bands, a small band at �1320 cm�1

appearing at potentials E > 0.5 V. Based on gas

phase measurements [38] the adsorbed species

was assigned to formate (HCOO). In our IR exper-

iments, neither in formic acid nor in methanol con-

taining solution a band corresponding to formate

was observed, although great effort and great care

were given for its observation. Considering that in

SEIRAS the absorption from molecules adsorbedon metal nanoparticles is significantly enhanced

(factor of �100) relative to IR intensities in the

external reflection mode, it is perhaps not surpris-

ing that only CO adsorption bands were observed

in FTIR studies using a external reflection

configuration.

On the basis of the reaction scheme above and

the FTIR analysis, the well known oxidation reac-tion of HCOOH on Pt(111) is discussed first. In

the FTIR spectra (Fig. 2a) COad formation starts

at �0.13 V. The fact that below 0.35 V COad is

produced without the formation of CO2 implies

that HCOOH oxidation proceeds at low overpot-

entials exclusively through the dehydration reac-

tion pathway,

HCOOH �!�H2O

COad þH2O ð1Þ

At 0.35 V a significant CO2 band appears in the

spectra. This is well below CO2 formation due to

the oxidation of adsorbed CO, which is in CO sat-urated solutions observed only at potentials posi-

tive of 0.4 V [39,40]. Concomitantly, between

0.35 and 0.45 V only a rather small increase in

the mCO intensity is observed, suggesting that theelectrooxidation of HCOOH proceeds mainly

through a ‘‘direct’’ dehydrogenation reaction

pathway, i.e. based on Osawa and co-workers [11],

HCOOH�!�H

HCOOad�!�H CO2 þ 2Hþ þ 2e� ð2Þ

Above �0.45 V, mCO intensities decrease and at 0.6V no visible mCO is observed in the spectra indicat-ing that for E > 0.45 V CO2 can be produced

simultaneously from the dehydrogenation reaction

(Eq. (2)) and from the oxidative removal of COad(which is formed below 0.45 V),

COad þOHad�!CO2 þHþ þ e� ð3Þ

Fig. 1b reveals that the maximum rate of HCOOH

oxidation on Pt(111) is obtained at �0.7 V, i.e., atthe potential where the oxidative removal of COadis optimized by a minimum surface coverage of

OHad.

A markedly different behavior was obtained for

formic acid electrooxidation on Pt(111) modified

with 1 ML of Pd. No COad is formed on the

Pt(111)–Pd�1ML surface in solution containing

HCOOH (Fig. 2b), which is consistent with previ-

ous conclusions, inferred from classical electro-chemical measurements on both Pt single crystals

modified with Pd films [41] as well as on a poly-

crystalline Pt/Pd electrode [42] that ‘‘CO poison

formation’’ does not occur or occurs at a very slow

rate. Our results show that the reaction pathway

on Pd is completely different than on Pt, i.e., the

oxidation of HCOOH on the on the Pt(111)–

Pd�1ML surface proceeds exclusively through thedirect (dehydrogenation) pathway described by

the Eq. 2. Unfortunately, the high catalytic activity

is observed only in a narrow potential region. As

shown in the FTIR spectra (Fig. 2) this deactiva-

tion is not due to CO poison formation. Recently,

we have suggested that the observed decrease in

activity arises due to blocking of active Pd sites

required for the adsorption of HCOOH with spec-tator species such as OHad and anions from sup-

porting electrolyte (ClO4 and Cl�) [43]. In order

to clarify the active intermediate in the ‘‘direct’’

oxidation pathway, SEIRAS measurements on a

Pd film would be desirable. If, as Osawa and

co-workers suggested, adsorbed formate (HCOOad)

is the intermediate it should be visible at much

lower potentials than on Pt.


Surprisingly, and contrary to both the adsorp-

tion/oxidation of methanol on either Pd single

crystals [44] and on thin Pd films deposited on

various substrates in UHV [45–47] as well as the

oxidation of HCOOH in an electrochemical envi-ronment (Fig. 2) the palladium film deposited on

Pt(111) is completely inactive for methanol oxida-

tion, as shown in Figs. 1c and 3b. The question

arises, on what basis this opposite behavior can

be rationalized. Two relevant factors should be

considered. (i) The ensemble effect, where the dilu-

tion of metal sites with the catalytically inert com-

pound changes the distribution (availability) ofactive sites. For example, if the adsorption/desorp-

tion step is rate limiting (see the reaction scheme)

one might expect that there is a critical ensemble

of Pd surface atoms needed for the extraction of

the hydrogen from the adsorbed organic mole-

cules. While in gas-phase catalysis the availability

of active metal sites is mainly determined by the

surface coverage of active intermediates and prod-ucts, in an electrochemical experiment an ensemble

of free-metal sites required for the adsorption of

CH3OH is, in addition to fractional coverage of

reaction intermediates (CO and COOH), also

determined by the fractional coverage of spectator

species. In acid solution these spectator species are

produced from the dissociation of water, the

adsorption of anions and the adsorption of hydro-gen. Therefore, the number of active sites for the

adsorption of organic molecules is significantly re-

duced at the metal–liquid interface with the respect

to the metal–gas interface. For more reactive met-

als than Pt, such as Pd, this effect is increased and

as a consequence the number of active sites/ensem-

bles for the adsorption of methanol on the

Pt(111)–Pd1 ML electrode is considerably reduced.Keeping in mind that the number of metal sites re-

quired for the adsorption of methanol is most

likely higher than for the adsorption of formic acid

[1,3], it is reasonable to suggest that the latter mol-

ecules can be adsorbed on the Pd surface even if it

is covered by the spectator species. Notice, that

this, in fact is valid only for a very narrow poten-

tial range, i.e. between 0.3 and 0.5 V (Fig. 1b). (ii)The electronic effect where adsorbates (metal ada-

toms, anions, Hupd, reaction intermediates, etc.)

alter the electronic properties of the catalytically

active metal. Unfortunately, the relationship be-

tween the change in electronic properties (e.g.,

the position of d-band center, the work function,

the emptiness of the d-band, etc.) of the host me-

tal, induced by adsorption of spectator speciesand reaction intermediates, and the energy of

adsorption of organic molecules is very difficult

to establish. Nevertheless, based on UHV and

electrochemical measurements [48,49] it appears

that the energy of adsorption of formic acid on

Pt and Pd substrates is higher than the energy of

adsorption of methanol. As a consequence, the

affinity of available Pd sites for a dehydrogenationreaction is higher for formic acid molecules than

for methanol molecules. Certainly, electronic and

ensemble effects may in general operate simultane-

ously, so that separating these effects and assessing

there relative importance in the reaction mecha-

nism is very difficult [3]. As we discussed above,

it appears that the catalytic activity of the

Pt(111)–Pd1 ML system for methanol oxidationand formic acid oxidation is indeed controlled by

the synergism of these two effects.

5. Conclusion

The formic acid and methanol oxidation reac-

tion are studied on bare Pt(111) and Pt(111)modified by a pseudomorphic Pd monolayer.

The results show that the Pd film has a unique

catalytic activity for HCOOH oxidation, with

Pd surface atoms about four times more active

than Pt surface atoms at 0.4 V. This difference

in activity can be attributed to the fact that the

reaction pathway for formic acid electrooxida-

tion is different on the Pd film and the barePt(111) surface. At low potentials on Pt(111)

formic acid electrooxidation results always in ad-

sorbed CO, which is produced from the HCOOH

dehydration pathway. Thus on Pt the reaction is

self-poisoning in the low potential region, where

COad cannot be oxidized further. In contrast, for

the Pd film in HCOOH containing electrolyte, no

vibrational features for Pd–CO can be observed,although high CO2 production rates are observed

at potentials as low as �0.15 V. These resultsindicate that the interaction of the formic acid


molecule with Pt and Pd atoms is completely

different. While Pd has in the entire potential

region a propensity to oxidize the HCOOH mol-

ecule via the direct path, Pt seems to have a pro-

pensity to oxidize formic acid both via the‘‘indirect’’ (at low potentials) as well as direct

path (at higher potential). In contrast to its high

activity for formic acid oxidation the Pd film is

completely inactive for methanol oxidation and

in the FTIR spectra neither adsorbed CO nor

significant amounts of CO2 were observed. By

discussing the difference properties of formic acid

and methanol the low catalytic for the Pd film isexplained by its strong interaction with the sup-

porting electrolyte which leads to a strong

ensemble effect for the weakly interacting metha-

nol molecule.

Acknowledgments

This work was supported by the Director, Of-

fice of Science, Office of Basic Energy Sciences,

Division of Materials Sciences, US Department

of Energy under Contract no. DE-AC03-

76SF00098. M.A. is grateful for a Feodor Lynen

fellowship from the Alexander von Humboldt

foundation.

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