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Electrochemical behavior of ammonia on Ni98Pd2

nano-structured catalyst

Anis Allagui 1, Saad Sarfraz, Spyridon Ntais, Fares Al momani,Elena A. Baranova*

Department of Chemical & Biological Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, ON K1N 6N5,

Canada

a r t i c l e i n f o

Article history:

Received 17 April 2013

Received in revised form

24 September 2013

Accepted 3 October 2013

Available online 8 November 2013

Keywords:

Ammonia electrooxidation

pH effect

Electrolysis

Ni hydroxides

* Corresponding author. Tel.: þ1 (613) 562 58E-mail addresses: elena.baranova@uottaw

1 Present address: Department of Sustaina27272, Sharjah, United Arab Emirates.0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.10.0

a b s t r a c t

Small amounts of Pd served as a reducing agent to produce sub-100 nm polygonally-shaped

Ni98Pd2 materials in ethylene glycol. As-synthesized particles were crystallized into fcc Ni

with a fraction of b-Ni(OH)2, and exhibited very low to no activity towards ammonia

electrooxidation. Their catalytic activity has been significantly improved by building up a

layer of Ni(OH)2 by cyclic voltammetry between �0.95 and 1.35 V vs. HgO/Hg in NaNO3 at

pH 9. XPS analysis before and after the electrochemical treatment confirmed the trans-

formation of Ni0 to higher state of oxidation. Ammonia electrooxidation on Ni(OH)2/NiPd

occurred at around 1.28 V vs. HgO/Hg and was highly pH-dependent. At concentrations less

than 100 mM, the direct electron transfer took place, whereas at higher ammonia con-

centrations it was the indirect electron transfer mechanism. A 9-h galvanostatic electrol-

ysis at 20 mA cm�2 showed that 64% of the initial ammonia was degraded at 38% average

current efficiency.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction requirements of recent electrochemical studies to oxidize

Ammonia is a major toxic pollutant in discharged waters that

leads to the eutrophication of the ecosystem [1]; thus its

removal is essential for ecological and environmental reasons.

On the other hand, anhydrous liquid ammonia is a compact

hydrogen carrier, as well as a distribution and storage me-

dium: its specific volume of hydrogen content is higher by 70%

than liquid hydrogen with 50% increase in the specific energy

density [2e7]. Ammonia can also be directly used as a fuel in

direct ammonia fuel cells, as the theoretical specific charge of

complete ammonia oxidation to N2 is 4.75 A h g�1 which is 95%

of the charge of methanol oxidation to CO2 [8]. The

00x6302; fax: þ1 (613) 562a.ca, obaranov@uottawa

ble & Renewable Energy E

2013, Hydrogen Energy P24

ammonia consist on finding high-performance electro-

catalysts with low overpotential and low production of ni-

trogen and carbon oxides.While platinum groupmetals (PGM)

and their bi-metallic alloys (e.g. PtIr, PtRu, PtPd, PtSnO2)

exhibit the highest degradation strength and stability towards

this process [7e14], their application at industrial scale is

restricted due to economical constraints, and therefore, there

is an urgent need to develop non-PGM catalysts.

To date, very few works are reported on the ammonia

electrooxidation reaction on non-PGM. Despic et al. [15] re-

ported that Raney nickel showed insignificant activity for the

anodic oxidation of ammonia in 5 M KOH because of the

5172..ca (E.A. Baranova).ngineering, College of Engineering, University of Sharjah, P.O. Box

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 842

immediate and spontaneous oxidation of the electrode sur-

face at the potential coinciding with the ammonia oxidation

process itself. This has been supported by Yao and Cheng [16]

where they assumed that the ammonia electrooxidation

mechanism of Gerischer and Mauerer [4] is applicable on Ni

electrode and that its spontaneous oxidation, once in contact

with alkaline media, prevents the adsorption of NH3 needed

for the subsequent dehydrogenation reactions to N2. The

oxidation of NHþ4 on Ni oxide electrode is also believed by Yao

and Cheng to be ineffective because, as a p-type semi-

conductor, the negatively charged cation vacancies in the

oxides playing the role of charge carriers will cause a reverse

electron generation in Gerischer and Mauerer mechanism [4]

and the fast deactivation of the Ni anode [16]. However, it

has been recently shown that Ni electrode with a surface layer

of Ni(OH)2 can be a potential candidate for efficient ammonia

oxidation in alkaline media [17]. Kapalka et al. found that the

oxidation of ammonia to N2 with traces of nitrate takes place

in the oxyhydroxide region via a direct electron transfer

mechanism. The 12-h 20 mA cm�2 bulk electrolysis with

passivated nickel wire in 1 M NaClO4 þ 50 mM NH4ClO4 at pH

11 indicated a degradation rate of 0.14 mmol cm�2 h�1, cor-

responding to the transformation of 55% of the initial con-

centration of ammonia into volatile nitrogenous species and

nitrate [17].

It was shown in our recent reports [18,19] that Ni-rich

NixPd1�x nanostructured materials are promising catalysts

for ammonia electrooxidation when activated by a long pre-

treatment using cyclic voltammetry (200 cycles) in alkaline

NaNO3 solution. This electrochemical treatment was neces-

sary to build the Ni(OH)2 phase, the active species towards

ammonia electrooxidation [17]. In this study we present a

detailed investigation on the Ni98Pd2 type of nanoparticles

includingmainly the XPS surface analysis before and after the

electrochemical treatment. The physicochemical character-

izations were carried out by TEM-EDS, ICP-AES and XRD an-

alyses. The detailed electrochemical study at various

ammonia concentrations and solution pH is presented and

discussed. Galvanostatic electrolysis at optimized conditions,

accompanied with preliminary reaction kinetics results are

demonstrated.

2. Experimental

2.1. Synthesis of nanostructured NiPd materials

All chemicals used in this work were reagent grade products

and were used without further purifications. The detailed

description of the modified polyol method used for the syn-

thesis of NiPd materials of this study is reported elsewhere

[20e22]. In brief, it consists consisted on mixing 0.4 g of

Ni(OH)2 salt (Acros Organics) with 150 mL of ethylene glycol

(Sigma Aldrich) for 1 h in a three-neck flask and refluxing for

10 min. On the other hand, 8.7 mg of PdCl2 (Alfa Aesar) was

dissolved in ethylene glycol and adjusted to pH 5 with HCl. At

198 �C, the boiling point of the polyol, the Pd-containing so-

lutionwas quickly injected into the former. The small amount

of Pd salt was used to activate the heterogeneous nucleation

and acted as a reducing agent and a catalyst for the growth of

the Ni-rich materials. The NiPd powder was separated from

the synthesis solution by a neodymium (NdFeB) magnet, fol-

lowed by several cycles of centrifugation and washing with

deionized water, and then stored in isopropanol.

2.2. Physicochemical characterization

Nanostructured NiPd sample preparation for transmission

electron microscopy (TEM) that is coupled to an energy

dispersive X-ray detector (EDS) consisted of depositing and

air-drying a droplet from the NiPd-in-isopropanol solution,

onto a carbon-coated copper micro-grid. TEM micrographs

were carried out with a JEOL JEM 2100F FETEM at 200 kV

accelerating voltage, and EDS elemental analysis was per-

formed during 100 s live time at the same operating voltage.

The powder X-ray diffraction (XRD) patterns were recorded

with Rigaku Ultima IV multi-purpose diffractometer with Cu

Ka radiation (l¼ 1.5418�A) at 40 kV and 44mA in the 2q range of

35e90� 2qwith 0.02� 2q s�1 scanning rate. Surface composition

of NiPd catalysts was analyzed by X-ray photoelectron spec-

troscopy (XPS) using a Kratos Axis Ultra DLD with a Hybrid

lens mode. XPS measurements were conducted at 140 W and

20 eV pass energy using a monochromatic Al Ka. The binding

energy scale was assigned by adjusting the C 1s peak at

284.6 eV.

2.3. Electrochemical measurements

The electrochemical experiments were carried out with a

BioLogic VSP potentiostat together with the EC-Lab software

at ambient temperature. For the potentiodynamic measure-

ments, a 75mL three-compartment Pyrex electrochemical cell

was used with HgO/Hg�KOH as a reference electrode (Koslow

Scientific Company) separated from the working electrode

compartment. All potentials are reported vs. HgO/Hg elec-

trode. A large surface area Ptmesh served as counter electrode

and was situated in a separate compartment. Thoroughly

polished and washed glassy carbon (GC) disc (Pine Research

Instrumentation) of 19.62 mm2-exposed surface served as the

current collector. The working electrode was prepared by

depositing onto the GC disk and air drying 5 mL of NiPd ink

prepared by mixing the following: (i) 0.6 g of NiPd particles, (ii)

45mg of carbon black VulcanXC-72 (Cabot Corp.), (iii) 0.5mL of

a 5%Nafion solution (SigmaeAldrich), (iv) 1 mL of isopropanol,

and (v) 10 mL of deionized water. The cyclic voltammetry

experiments at 100 mV s�1 scan rate were performed in a

supporting solution of NaNO3 (SigmaeAldrich, ACS reagent,

�99.0%) at unit molarity with varying concentrations of

NH4NO3 (SigmaeAldrich, �99.0%) under controlled pH,

adjusted with a 3 M NaOH solution.

For the bulk electrolysis experiment, the used electro-

chemical cell is a closed two-compartment cell with 1 M

NaNO3 þ 0.2 M NH4NO3 at pH 10.5 and under continuous

stirring. The counter electrode was a large stainless steel coil

of grade 304. The applied current was set to 20 mA per geo-

metric cm2 during 9 h. Total nitrogen was determined using

Appollo total nitrogen analyzer (Teledyne Tekmar) pre-

calibrated with a standard solution of NH4OH. Ammonia

was determined according to the procedure stipulated in

Standard Methods [23]. 2 mL samples were collected from the

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 8 43

electrolysis solution at 0, 20, 40 and 60min and then every one

hour, and have been analyzed three times with a reproduc-

ibility of �0.1 mg-N L�1. The reported concentration values in

this work were averaged over the three measurements. Con-

trol samples were also taken and analyzed to correct the real

ammonia removal values with the stripping amounts which

did not surpass the 1%.

Fig. 2 e X-ray diffraction patterns of as-synthesized NiPd

particles.

3. Results and discussion

3.1. TEM/EDS and XRD

Fig. 1 shows a typical TEM micrograph of the NiPd particles.

Elemental composition by EDS (Ni98.8Pd1.2) and inductively

coupled plasma atomic emission spectroscopy (ICP-AES,

Ni98.2Pd1.8) analyses are in close agreement with the initial

nominal atomic content of metals used to prepare the nano-

catalyst. The difference between the nominal 1 at. % of Pd and

the 1.8 measured by ICP-AES indicates that not all of the Ni

ions have effectively taken part in the heterogeneous nucle-

ation process on Pd nuclei. From now on we will refer to the

investigated nanocatalyst of this study to either NiPd or

Ni98Pd2. Two types of particles are seen in the TEM micro-

graph. Some are large in the 50e100 nm range with three

dimensional polygonal shapes and straight sharp features

that show some surface defects due to their size range, and

are assigned to Ni materials. Others are spherically shaped

within the 5e10 nm size range, decorating the surface of the

latter, and are assigned to the Pd nanoparticles.

Fig. 2 shows the powder XRD patterns of NiPd catalyst that

crystallized into a face-centered cubic (fcc) structure of Ni

(JCPDS card # 04-0850) with diffraction peaks at 44.51�, 51.85�,76.46� corresponding to hkl planes (111), (200) and (220)

respectively. b-Ni(OH)2 (101), (110) and (111) (JCPDS card # 01-

Fig. 1 e Typical TEM micrograph of NiPd particles.

1047) are also detected. Pd (111) with the diffraction peak

centered at 40.36� (JCPDS card 05-0681) is identified in the

patterns but it represents a negligible contribution. The XRD

reflections are characterized by sharp and well-defined peaks

with strong intensity indicating the high crystallinity of the

powder. The mean crystallite size estimated using Scherrer’s

formula [24] using the most intense Ni (111) peak parameters

and the shape factor 0.89 is 22.4 nm.

3.2. Electrochemical activation & XPS of NiPd particles

An electrochemical pre-treatment was applied to the NiPd

particles to form Ni hydroxide on the surface, being the

electro-active species for ammonia oxidation [17,18]. This was

performed by carrying out 200 cyclic voltammograms in 1 M

NaNO3 at pH 9 in the voltage range �0.95 to þ1.35 V at

100 mV s�1. Under alkaline conditions, in the hydroxide for-

mation region, the adsorption of OH� on Ni leads to its

oxidation and the formation of the a-phase of Ni(OH)2 such

that [25e28]:

Niþ 2OH�!aNiðOHÞ2 þ 2e� (1)

At higher potentials, the hydrous a-Ni(OH)2 is irreversibly

transformed into the anhydrous b-form of Ni(OH)2 [27]. The

latter leads to the formation of nickel oxyhydroxide NiOOH

according to [29]:

b�NiðOHÞ2 þOH�!ðb;gÞNiOOHþH2Oþ e� (2)

The continuous electrode cycling (not shown here) led to

significant currents increase associated with the process of

reaction 2 indicating the transformation of Ni surface mate-

rials into hydroxides.

a b

Fig. 3 e High resolution XPS spectra of (a) Ni 2p3/2 and (b) Pd 3p3/2 before and after electrochemical pre-treatment.

Fig. 4 e Effect of electrochemical pre-treatment on the

ammonia electrooxidation reaction on NiPd

nanostructured catalysts; cyclic voltammograms at

100 mV sL1 scan rate are conducted in 1 M NaNO3 D 0.2 M

of NH4NO3 at pH 9.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 844

Surface analysis using XPS was carried out to elucidate the

chemical state of Ni and Pd atoms in the as-synthesized NiPd

particles after being subjected to the electrochemical pre-

treatment. The Ni 2p3/2 spectrum demonstrates a complex

structure with intense and wide satellite peaks at higher

binding energies adjacent to the primary photolines (Fig. 3(a)).

These peaks are called shake-up and are due to a charge

transfer ligand-metal process by which an excitation of a

valence ligand electron to a previously unoccupied metal

orbital takes place simultaneously with the primary core

photoelectron process [30]. The satellite band is located at

higher binding energy and its position depends on the

chemical environment of nickel atoms. The recorded Ni 2p3/2

XPS spectrum before and after treatment are presented in

Fig. 3(a). Before treatment it shows the existence of a shoulder

at around 852.3 � 0.2 eV and an intense peak at around

855.3� 0.2 eV. The former peak is attributed to nickel atoms in

themetallic state [31,32], while the latter is characteristic of Ni

atoms in the 2þ oxidation state due to Ni(OH)2 [33e35]. XPS

analysis reveals the existence of an ill-defined peak in the

characteristic energy window of Pd 3p3/2 as shown in Fig. 3(b).

The signal of this peak implies the small amount of Pd on the

surface while the centre of the peak is at 335.1 eV that is

characteristic of metallic Pd [33,36]. Thus, the existence of an

O 1s peak (not shown here) observed at around 529.2 eV can be

attributed to a small amount of nickel atoms in NiO form.

After the electrochemical treatment no Pd is detected

which suggests its complete dissolution during the treatment

process. On the other hand, the Ni 2p spectrum exhibits a

rather complicated structure. Thus, no peak at 852.2 eV is

detected implying that the treatment has caused the complete

oxidation of metallic Ni atoms. A Ni 2p3/2 peak is detected at

about 857.2 eVwhich is a position characteristic of NiOOH [31].

The corresponding satellite peak should be detected at about

863 eV and should normally have lower intensity than the

main line. Though, as it can be seen, its existence is over-

lapped by another peak at about 860.8 eV. In the literature a Ni

2p3/2 peak around 861 eV is reported for K2NiF6 [37]. At this

point we should mention that fluorine and sodium are also

detected on the surface that originate from the Nafion and the

sodium nitrate solutions. Thus, we strongly believe that this

new peak is due to the formation of a similar nickel compound

e.g. Na2NiF6 or NaNiF3. More studies are necessary so as to

further elucidate this claim. The fact that the O 1s peak does

not show any component at about 529.2 eV strongly indicates

the transformation of NiO to NiOOH.

3.3. Ammonia electrooxidation reaction

The effect of the pre-treatment of NiPd catalyst on the

ammonia electrooxidation reaction is shown in Fig. 4 which

illustrates the 100 mV s�1 cyclic voltammograms of both non-

treated and treated electrode materials in 1 M NaNO3 þ 0.2 M

of NH4NO3 at pH 9. With the presence of ammonia, the couple

Fig. 5 e Cyclic voltammograms (100mVsL1) of Ni(OH)2/NiPd

particles in 1MNaNO3D 0.2MNH4NO3 at pH varying from8

to 12. Insert shows the evolution of the current densities of

ammonia oxidation peaks with the solution pH.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 8 45

of redox peaks assigned to Ni (II)/Ni (III) oxidation and initially

centered at þ0.75 V are shifted towards more positive poten-

tials and are accompanied by a new anodic peak that corre-

sponds to the electrooxidation of ammonia [19]. The latter

takes place at around 1.28 V, after the completion of the

oxidation of Ni(OH)2 to NiOOH, and is muchmore pronounced

for the case of NiPd electrode with a built-on Ni(OH)2 layer. In

earlier works, similar observations were reported for the

oxidation of small organic compounds such as methanol

[38,39] and other organic alcohols and amines [40] on nickel

hydroxide. Note that in the negative potentials region, the

cathodic peak at w�0.3 V corresponds to PdO reduction,

a

Fig. 6 e Cyclic voltammograms (100 mV sL1) of Ni(OH)2/NiPd pa

NH4NO3 from (a) 25e100 mM and (b) 100e350 mM, by steps of 2

which is formed at positive potentials in anodic scans [38]. The

rest of the study is on the treated materials, Ni(OH)2/NiPd.

To further elucidate the role of Ni(OH)2 in the ammonia

oxidation reaction, the effect of OH� ions concentration was

investigated. The adjustment of the pH was performed by

adding some mL-droplets from the 3 M NaOH solution so that

the dilution of the ammonia content remains negligible. From

Fig. 5, that shows the steady-state cyclic voltammograms of

Ni(OH)2/NiPd particles in 1 M NaNO3 þ 0.2 M NH4NO3 at pHs

varying from 8 to 12, it can be seen that the electrooxidation of

ammonia is strongly pH-dependent. At lower pH i.e. 6.0, 7.0

and 7.5 (not shown here), there is no measurable activity of

Ni(OH)2/NiPd for ammonia oxidation suggesting that the

alkaline environment is necessary for the completion of the

reaction. Starting from pH 8, there is a quasi-linear increase of

the current density of the ammonia oxidation peak up to pH

10.5. Because ammonia in aqueous solution at pH 8 accounts

for 7.5% in the form of free NH3, 45% at pH 9, and that goes up

to almost 90% at pH 10 [41], we conclude that it is the un-

ionized form of ammonia that is being electrooxidized on

Ni(OH)2/NiPd. The oxidation peak seems to completely over-

lap with the oxygen evolution reaction response and is not

clearly identified at pH 12 and higher.

Fig. 6 shows the 10th cycles of CVs recorded on Ni(OH)2/

NiPd particles with different concentrations of NH4NO3

ranging from 25 to 100 (Fig. 6(a)) and from 100 to 350 mM

(Fig. 6(a)) in 1 M NaNO3. The pH was set to 9 with 3 M NaOH

solution prior to each experiment. At low ammonia concen-

trations (25, 50 and 75 mM), the response of the Ni (II)/Ni (III)

redox process increases significantly with modest increase of

the current density in the ammonia electrooxidation region

that is seen as a shoulder at w1.25 V. As the ammonia con-

centration increases (� 150 mM), the current densities of the

Ni(OH)2/NiOOH pair of peaks significantly decreases, whereas

the ammonia oxidation peak maxima augment with the

concentration up to 300 mM and then reach a plateau at

2.45 mA cm�2 (Fig. 6(b)). The ratio of the current of the anodic

b

rticles in 1 M NaNO3 with increasing concentrations of

5 mM. The pH is set to 9 prior to each measurement.

Fig. 7 e Normalized concentration profile of NH4LN during

the electrolysis of ammonia at 20 mA cmL2 under

galvanostatic conditions on Ni(OH)2/NiPd in 1 M

NaNO3 D 0.2 M NH4NO3 at pH 10.5.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 846

Ni(OH)2/NiOOH peak to the one of ammonia oxidation peak

decreases exponentially with the increase of NH4NO3 con-

centration from 0.94 at 100 mM to 0.05 at 400 mM.

In the recent work of Kapalka et al. [17] on ammonia

electrooxidation on bulk Ni wire, the authors have observed a

linear increase in the maximum current density of ammonia

oxidation with the increase of NH4ClO4 concentration from 10

to 150 mM. This was accompanied by continuous increase in

the current density corresponding to Ni(OH)2/NiOOH redox

process. Based on these observations, Kapalka et al. suggested

that ammonia oxidation on Ni(OH)2/Ni bulk electrode takes

place via a direct electron transfer mechanism from ammonia

to the anode [17]:

NiOOHþNH3/NiOOHðNH3Þads/NiOOHþ 12N2 þ 3Hþ þ 3e�

(3)

In the present work, similar behavior is observed at concen-

trations of NH4NO3 lower than 100 mM, suggesting that direct

electron transfer mechanism might be applicable. However,

as the concentration of ammonia increases (�150 mM), the

current density of Ni(OH)2/NiOOH redox couple start to

decrease significantly, possibly indicating a change in the re-

action mechanism from a direct electron transfer to an indi-

rect oxidation of ammonia which was earlier proposed for

organic compounds [42]:

NiOOHþNH3/NiðOHÞ2 þ Products (4)

The observed decrease in the cathodic peak with the increase

of ammonia concentration indicates that some NiOOH is

transformed to Ni(OH)2 (reaction 4) and ammonia oxidation

proceeds via this reduction process. This transition concen-

tration leading to the direct to indirect electron transfer

mechanism depends most probably on the ratio between the

mass of Ni(OH)2/NiPd materials vs. the ammonia concentra-

tion [19].

Ni(OH)2/NiPd electrode was subjected to a 9-h electrolysis

at 20 mA cm�2 in 1 M NaNO3 þ 0.2 M NH4NO3 at pH 10.5. Fig. 7

shows the recorded concentration profiles of NH4�N during

the electrooxidation process, normalized to the initial con-

centration. The concentration of total ammonia in the solu-

tion decreased bymore than 64% by the end of the experiment

with an average current efficiency of 38.7%. The ammonia

degradation for for the same quasi experimental conditions

with bulk Ni electrode (i ¼ 20 mA cm�2 during 9 h in 1 M

NaClO4 þ 50 mM NH4ClO4, pH 11) is reported to be 34% by

Kapalka et al. [17]. The initial kinetics of ammonia degradation

was studied based on the initial ratemethod in the first 40min

of reaction time. The concentration of nitrite in the solution

was negligible and the one of nitrate remained constant,

therefore, one can write:

�ddt

ðCNH4�NÞ��t¼0

¼ kðCNH4�NÞn (5)

with CNH4�N being the concentration of ammonia, k the reac-

tion rate constant and n the reaction order with respect to

ammonia degradation. In Fig. 7, the fit of the first three data

points results in a straight line with a slope approximatively

equal to one (n ¼ 1) indicating that the reaction follows first-

order kinetics. The initial rate constant was found to be

0.53 � 0.03 h�1. The change of slope after one hour of elec-

trolysis at which 32% of ammonia has been removed from the

initial 0.2 M NH4NO3 may be attributed to the change of

composition of the solution and also to the transition

(125e150 mM) from indirect (high concentrations) to direct

(low concentrations) mechanism pathways.

4. Conclusion

Ammonia electrooxidation on sub-100 nm NiPd nano-

structured catalyst with 98:2 at. % ratio is investigated in

NaNO3 solutions from pH 6e12. Ni(OH)2, being the active

phase for the studied reaction, was formed on NiPd materials

by 200 voltammetric cycles in alkaline pH. From the XPS

analysis results, the electrochemical treatment led to the

transformation of the metallic nickel in the as-synthesized

NiPd particles into higher oxidation state i.e. NiOOH. In the

presence of NH4NO3, the voltammograms showed the

appearance of an adjacent peak to the Ni (II)/Ni (III) redox

process at more positive potentials at w1.28 V vs. HgO/Hg,

which was assigned to the ammonia electrooxidation reac-

tion. The pH effect on the reaction showed that the current

density of ammonia oxidation peak increased significantly

with the increase of the solution pH reaching its optimal value

at pH 10.5. From the NH4NO3 concentration effect on the Ni

(II)/Ni (III) pair of peaks and the behavior of ammonia oxida-

tion peak we concluded with the following: (1) at concentra-

tions lower than 100 mM, the direct electron transfer is

supposed to be taking place, whereas (2) at higher concen-

trations it is the indirect electron transfer pathway that is

believed to occur. 20 mA cm�2 bulk electrolysis on Ni(OH)2/

NiPd nanocatalyst in 0.2 M NH4NO3 at pH 10.5 showed that

more than 64% of ammonia has been degraded at 38% average

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 1e4 8 47

current efficiency. We concluded that Ni-rich Ni(OH)2/NiPd is

an effective catalyst for ammonia electrooxidation in alkaline

media.

Acknowledgments

The authors would like to thank the Natural Science and En-

gineering Research Council (NSERC) for financial support. Anis

Allagui acknowledges the support from Fonds quebecois de la

recherche sur la nature et les technologies (FQRNT) for the

Postdoctoral Fellowship.

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