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Ammonia electro-oxidation on alloyed PtIr nanoparticles ofwell-defined size

Anis Allagui a, Mohamed Oudah a, Xenia Tuaev b, Spyridon Ntais c, Fares Almomani a,Elena A. Baranova a,*aDepartment of Chemical & Biological Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, ON K1N 6N5, CanadabThe Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division,

Technical University Berlin, 10623 Berlin, Germanyc Institut national de la recherche scientifique, Energie, Materiaux et Telecommunications (INRS-EMT), Universite du Quebec, 1650 Boul.

Lionel Boulet, Varennes, QC J3X 1S2, Canada

a r t i c l e i n f o

Article history:

Received 15 May 2012

Received in revised form

26 October 2012

Accepted 16 November 2012

Available online 31 December 2012

Keywords:

Ammonia oxidation

PlatinumeIridium nanoparticles

Small-angle X-ray scattering

Alloy

Surface oxides

a b s t r a c t

Carbon-supported PtIr nanoparticles with the nominal atomic ratio of 70 to 30 % in the

3 nm scale are investigated for the ammonia oxidation reaction (AOR) in 1 M KOH. The

morphological and surface properties of alloyed PtIr electrocatalysts synthesized at pH 7

and 8.5 are characterized by TEM, XRD, SAXS and XPS. According to SAXS, nanoparticles

prepared at lower pH (PtIr (1)) are mono-dispersed with particle size of 2.9 nm, whereas

nano-sized catalyst PtIr (2) synthesized at higher pH has bi-modal size distribution with

modes at 1.8 and 3.4 nm. XPS revealed that Pt on the surface of PtIr (1) is present in metallic

state contrary to PtIr (2) where platinum surface species of higher oxidation state are

identified. Iridium on the surface of both samples is present in Ir0, Ir3þ and Ir4þ form. From

the electrochemical characterizations, the onset potential of PtIr for ammonia oxidation is

more negative (�1.07 V vs. MSE) compared to the monometallic Pt nanoparticles of 3.0 nm

in size (�0.94 V vs. MSE). Long term electrolysis (12 h) demonstrated a 33% higher degra-

dation rate of ammonia on PtIr (1) than on Pt with N2 as the main product, in addition to

some traces of NO3 and NO2. Higher catalytic activity, stability and activity recovery of PtIr

nanoparticles is attributed to the electronic effect generated between Pt and Ir atoms in

alloy. The surface of alloyed PtIr nanoparticles displays a complex of physicochemical and

electrocatalytic properties, thus the maximum electrocatalytic activity towards AOR is

highly correlated with the narrow size distribution and lower amount of surface oxygen-

ated species.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The ammonia oxidation reaction (AOR) for environmental

applications such as wastewater treatment, as well as for the

detection of ammonia in water and in air has been under

continuous development for several decades [1e3]. Currently,

AOR is receiving a growing attention from the electrochemical

community due to its potential use as a fuel in direct oxidation

fuel cells or as a hydrogen storage compound [3,4]. Indeed, the

theoretical specific charge of complete ammonia oxidation to

* Corresponding author. Tel.: þ1 613 562 5800x6302; fax: þ1 613 562 5172.E-mail address: elena.baranova@uottawa.ca (E.A. Baranova).

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N2 is 4.75 Ah g�1 that is 95% of the charge of methanol

oxidation to CO2 [5]. Because of the sluggish kinetics of AOR at

low temperatures (<100 �C), efficient catalysts are required in

order to oxidize ammonia at reasonable rates. The Pt-group

metals show promising catalytic activity towards AOR, while

acceptable catalytic rates at low overpotentials have yet to be

realized.

Studying ammonia reactions cycle on platinum in both

alkaline [5e9] and in acidic [10] media confirmed the high

surface sensitivity of this reaction, where ammonia is pref-

erentially oxidized on Pt (100) crystalline plane, and is rather

sluggish on (111) and (110) planes. Ammonia oxidation on Pt

and Pt-group metals follows the reaction mechanism (1) to (5)

below, wherein partially dehydrogenated ammonia species

NH2,ads and NHads are active species and Nads is an inactive

surface adsorbate [11]:

NH3ðaqÞ/NH3;ads (1)

NH3;ads!NH2;ads þHþ þ e� (2)

NH2;ads/NHads þHþ þ e� (3)

NHx;ads þNHy;ads/N2 þ ðxþ yÞHþ þ ðxþ yÞe� (4)

NHads/Nads þHþ þ e� (5)

with x, y ¼ 1 or 2. Recent studies showed that bi-metallic PtIr

bulk electrodes have improved AOR kinetics if compared to

pure Pt [12e15]. The advantage of Ir electrode is that it has

lower overpotential of ammonia oxidation (0.35 V vs. RHE)

than Pt (0.43 V vs. RHE) in 0.1 M NH3 þ 1 M KOH [16]. However,

the calculated adsorption energy of atomic Nads on Ir is higher

than on Pt, e.g., �453 and �398 kJ mol�1 [16], respectively,

indicating that Ir has a higher tendency to be poisoned byNads.

Improved AOR on PtIr electrodes prepared by thermal

decomposition was attributed to the synergetic interaction

between the two metals [13] or in the case of carbon-

supported alloyed Pt7Ir3 nanoparticles to an electronic effect

generated between Pt and Ir in the alloy [17].

For practical implementation of noble metal catalysts for

AOR, their loading must be significantly decreased, which can

be obtained by using metallic nanoparticles ( � B 5 nm) sup-

ported on conductive supports. However, a few number of

reports exist on using nanoparticles for AOR [5,7]. Recently, we

investigated AOR on a series of carbon-supported bimetallic

nanoparticles: PtPd, PtIr and PtSnOx and compared them to

monometallic Pt nanoparticles [17]. Among the catalysts

investigated PtIr (70:30 at.%) showed the best combination of

activity and long term stability for AOR.

In the present work, further detailed investigations on the

ability of PtIr nanoparticles to oxidize ammonia are reported.

Several parameters, e.g. particle size, size distribution, as well

as surface and bulk structure have significant influence on the

catalytic activity of the nanoparticles. Therefore, the aim of

this study is (i) to investigate the effect of the addition of Ir to

Pt nanomaterials, and (ii) to establish the correlation between

the surface oxidation state of PtIr nano-alloys on their cata-

lytic activity for AOR. To this end, two carbon-supported PtIr

electrocatalysts at 70:30 at. % ratio and monometallic Pt with

well-defined particle sizes were synthesized with modified

polyol method and tested for the AOR. First, structure and

surface properties of the nano-structured PtIr were studied

using transmission electron microscopy (TEM), small angle

X-ray scattering (SAXS), and X-ray photoelectron spectros-

copy (XPS) techniques. Then, electrocatalytic activity for

ammonia degradation is investigated in an alkaline solution

of 1 M KOH þ 0.5 M NH4OH using cyclic voltammetry and

chronoamperometry and compared to the performance of Pt

nanoparticles prepared with the same method. Finally, a 12-h

galvanostatic electrolysis was carried out with the nano-

catalysts in 25 mM NH4OH þ 1 M KOH and the by-products of

AOR were analyzed using standard titration techniques.

2. Experimental

2.1. Synthesis of carbon-supported PtIr and Ptnanoparticles

The synthesis of nanoparticles was done based on a modified

version of the polyol technique [18,19]. Synthesis of PtIr and Pt

nanoparticles is detailed in [17]. In short, for the PtIr nano-

particles (70:30 at. % ratio), 108.7 mg of H2IrCl6 H2O (45%

Iridium, ACROS Organics) and 165.4 mg of PtCl4 (57.75% Pt,

Alfa Aesar) were dissolved in 50 mL of ethylene glycol (anhy-

drous, 99.8% SigmaeAldrich). The pH of the synthesis solution

was adjusted with NaOH (EM Science ACS grade). Two PtIr

samples were prepared: (i) one was synthesized at lower pH

using a 50mMNaOH (final pH¼ 7) and denoted PtIr (1), and (ii)

the secondwas prepared at higher pHwith 60mMNaOH (final

pH ¼ 8.5) and denoted PtIr (2). The mix of precursor salts was

stirred for 1 h at room temperature or until salts were

completely dissolved, and then refluxed at 160 �C for another

hour, leading to the formation of colloidal dark brown solu-

tion. After the completion of the synthesis reaction, the

mixture was cooled to room temperature and carbon black

(Vulcan XC-72, Cabot Corp.) was subsequently added to result

in 20wt.%metal loading. The carbon-supported particleswere

filtered and washed excessively with deionized water

(18 MU cm) and then dried in air at 80 �C. The same procedure

is followed for the synthesis of carbon-supported Pt nano-

particles, however, the metal precursor was provided by

243.8 mg of PtCl4 [17].

2.2. Morphological characterizations

Transmission electronmicroscopy (TEM) of carbon-supported

nanoparticles was carried out using a JEOL JEM 2100F FETEM

microscope operating at 200 kV. Samples were prepared by

depositing and air-drying onto a carbon-coated copper grid

a droplet from the nanocatalysts ink after intensive sonication

for 10 min.

Small angle X-ray scattering (SAXS) data were collected

with a Bruker AXS NanoStar lab system, operated with

a rotating Cu-target anode at 45 kV and 110 mA. The scattered

X-rays of a Cu-Ka wavelength (0.154 nm) were detected with

a 2D HiStar photon counting detector. Scattering patterns of

pristine powders were collected for 300 s. For background

correction, SAXS patterns from a support powder was used.

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Raw SAXS 2D patterns were integrated in the Chi range of

�180� to 180� and a 2q range of 0.1�e5.0� with a step size of

0.01� [20].Surface composition of the PtIr samples was analyzed by

X-ray photoelectron spectroscopy (XPS) using a KRATOS Axis

Ultra DLD with a Hybrid lens mode. XPS measurements were

conducted at 140 W and pass energy 20 eV using a mono-

chromatic Al-Ka. The XPS core level spectra were analyzed

using the XPS Peak program with a fitting routine which

decomposes each spectrum into individual mixed Gaus-

sianeLorentzian peaks using a Shirley background subtrac-

tion over the energy rate of the fit. Error estimation for binding

energies (BE) is �0.1 eV. The deconvolution of the Ir4f peak

was performed by constraining the spin orbit splitting of the Ir

peaks at 2.9 eV and the intensity ratio Ir4f7/2:Ir4f5/2¼ 4/3, while

in the case of the Pt4f peaks the analysis was performed using

a peak asymmetry for the metallic state based on the work of

Hufner andWertheim [21] and Peuckert [22]. The BE scale was

assigned by adjusting the C1s peak at 284.6 eV with

FWHM ¼ 1.0 eV.

2.3. Electrochemical characterizations

The electrochemical measurements were performed with

a BioLogic VSP potentiostat together with the EC-Lab software.

All experiments were conducted in a three-compartment

Pyrex electrochemical cell at room temperature and in

continuously de-aerated solutions by N2 gas (99.9% Linde). All

potentials were measured with respect to the mercury

sulphate electrode (MSE). A large surface area gauze-Pt served

as counter electrode and was situated in a separate

compartment. A glassy carbon (GC) electrode of 19.62 mm2

geometric area was used for deposition of carbon-supported

nanoparticles. The GC-electrode was polished prior to each

experiment using a solution of 30 and 6 mmground alumina on

polishing paper. The catalyst powder solutions or inks were

prepared by dissolving 6 mg of the carbon-supported PtIr or Pt

nanoparticles in 2 ml of deionized water, 200 mL isopropanol,

and 100 mL of Nafion solution (Aldrich, 5 wt.%, 15e20 wt.% of

water). The inkwas sonicated for 10min. Subsequently, 3 mL of

the solution was deposited onto the GC-electrode surface and

dried in oven at 60 �C for 15min, which constitute the working

electrode. Electrolytes are aqueous solutions of KOH (EMD)

and NH4OH (14.8 N, Fisher Scientific).

Cyclic voltammograms (CVs)were carried out at a scan rate

of 20 mV s�1. Chronoamperograms (CAs) were conducted at

�0.83, �0.88, �0.93, �0.98, and �1.03 V vs. MSE. The electrode

was first preconditioned at�1.46 V for 5min and then stepped

to each of the respective voltages. CVs were recorded before

and after each CA measurement to verify the stability of the

electrode. The electrochemical results are normalized per

electrochemical surface area (ECSA) of the catalyst, whichwas

determined from the specific charge of hydrogen desorption

with the reference surface charge of 210 mC cm�2 for Pt [23].

The bulk electrolysis of 25mM of NH4OH in 1MKOH on the

PtIr and the Pt nanocatalysts was carried out in a two-

compartment cell (V z 200 mL) during 12 h at 10 mA cm�2

under vigorous stirring. The working electrode was prepared

by depositing and drying overnight 100 mL of the catalyst ink

onto a 1.27 cm2 diameter carbon fibre paper (SpectraCarb

2050-A, Fuel cell store). A large surface stainless steel spring of

grade 304 served as a counter electrode and was kept in

a separate compartment. Analysis of the by-products of

ammonia electro-oxidation was done by collecting 2 mL

samples every 2 h. Prior to the electrolysis experiment, in

a 12 h control experiment without current application, the

approximate ammonia degradation by volatilization was

estimated to 5e6 % of the initial ammonia concentration.

Total nitrogen (Ntot) 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

1985. The determination of nitrite and nitrate concentrations

was performed using Hach analytical kit. Each sample was

analyzed three times with a reproducibility of �0.1 mg N L�1.

The reported concentration values in this work are averaged

over the three measurements. All materials used for the

electrochemical characterizationswere confirmed to be stable

in 1 M KOH.

3. Results and discussion

3.1. Size, structure and surface properties ofnanocatalysts

Representative TEM micrograph of carbon-supported PtIr (1)

nanoparticles is shown in Fig. 1. The large amorphous features

correspond to the grains of carbon black while the highly

contrasted circular dots represent the PtIr nanoparticles. The

metallic nanoparticles in both PtIr samples are well dispersed

on carbon support and the mean particle size was found

2.9 � 0.5 and 2.6 � 0.5 nm for PtIr (1) and PtIr (2), respectively.

SAXS results are shown on logelog scale in Fig. 2 and

Table 1 summarizes the particle size found from this tech-

nique. Contrary to TEM, which probes a limited number of

nanoparticles, SAXS has the advantage to analyze the whole

ensemble of materials and provides accurate information on

the particles size and size distribution of the fine particles.

According to SAXS analysis, PtIr (1) catalysts has a particle size

of 2.9 nm (2.9 nm¼ 2R1 in insert of Fig. 2(a)) with a very narrow

size distribution close to monodispersion (s ¼ 0.1). Sample

prepared at higher pH shows, on the other hand, a bi-modal

size distribution with modes at 1.8 and 3.4 nm being 2R1 and

2R2 respectively, as shown in the insert of Fig. 2(b). Variation in

the particle size and size distribution between the two

samplesmay be related to the pH of the synthesis solution. As

was shown earlier formonometallic Pt nanoparticles, increase

in pH of the synthesis solution leads to the formation of Pt

nanoparticles with smaller sizes [19,24]. However, for bime-

tallic PtIr, bimodality of sizes is observed and requires further

investigation of the effect of pH on the size of monometallic Ir

nanoparticles, which is not the scope of this work.

XRD measurements of the colloidal (see supporting

information) and carbon-supported (not shown here) PtIr

samples revealed that the structure of both PtIr samples is

face-centred cubic typical for the bulk Pt and Ir metals. XRD

patterns of colloidal PtIr nanoparticles at (111) crystalline

plane (Fig. 1S and Table 1S) were used to determine the Ir

content in Pt using the Debye formula for scattering by

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randomly oriented molecules [25] and the procedure devel-

oped earlier for PtRu clusters [19]. Pt and Ir are forming an

alloy with the Ir content in Pt phase of 35 and 45% for PtIr (1)

and PtIr (2) nanoparticles, respectively. The difference from

the nominal composition of the nano-alloys indicates that

single phase Pt nanoparticles might also be present. The

crystallite size of the carbon-supported PtIr are calculated

using the Debye formula and (220) peak at w 67.7� 2q, becauseof the strong interference from the graphite reflection atw 40�

2q with the main (111) and (200) fcc peaks [26], and are

summarized in Table 1.

XPS analysis of PtIr nanoparticles was carried out so as to

obtain a better insight on the surface oxidation state of the

prepared catalysts. XPS data of nanostructures should be

carefully treated and the effect of the size of the particles

should be considered. According to Baer and Engelhard, the

nanostructure can affect both the peak intensities and

moreover the binding energies of the detected peaks [27]. The

Ir4f and Pt4f spectra for both samples are presented in Fig. 3

and the exact BEs and the percentage ratio of each compo-

nent are summarized in Table 2. For both iridium and plat-

inum the obtained components are detected at higher BEs

while they exhibit slightly increased FWHMs compared to the

values reported in the literature for bulk materials [28e30],

a change that could be attributed to the particle size. Though,

the reported BE values are in accordance to XPS results on

similar systems. In both samples, the deconvolution of the Ir4f

peak shows the existence of iridiumatoms in 3 different states

(Fig. 3). The peak at 61.1e61.4 eV (IrI) is characteristic of the

metallic iridium (Ir0), while the peaks at higher BEs, IrII ¼ 63 eV

and at IrIII ¼ 63e63.4 eV, are characteristic of Ir3þ and Ir4þ,respectively [31,32]. The analysis of the Pt4f in the case of PtIr

(1), reveals the existence only of metallic platinum (Pt0),

whereas for PtIr (2), the deconvolution shows the existence of

Fig. 1 e Transmission electron micrographs of carbon supported PtIr (1) nanoparticles prepared with modified polyol

technique and corresponding frequency histogram of particle size.

PtIr (1) PtIr (2)

a b

Fig. 2 e Small-angle X-ray scattering curve of carbon-supported PtIr nanoparticles; experimental data (symbols) and

theoretical fit (solid line). Ri denotes the nanoparticles radius for mode i.

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3 components and more specifically at 71.6, 72.9 and at

74.4 eV. The peak at 71.6 eV is characteristic of Pt0 while those

at higher BEs are characteristic of Pt2þ (e.g. in PtO and/or

Pt(OH)2) and Pt4þ (in PtO2 or Pt(OH)4), respectively [33e35].

Therefore, for the PtIr (1) sample prepared at low pH, its

surface consists of metallic Pt and Ir as well as IrOx. The

surface of the PtIr (2) catalyst synthesized at higher pH shows

the existence of Pt and Ir atoms mostly in oxidized states,

indicating that excess of NaOH influences both the particle

size distribution and surface chemistry of the catalyst. The

excess of NaOH in the synthesis solution resulted in the

formation of catalyst surface with more oxygenated species.

According to previous studies on the formation of Pt alloys,

the Pt4f7/2 and the Ir4f7/2 peaks are expected to shift to higher

and lower BEs, respectively compared to the peak positions of

the pure metals [36e38]. Weinert and Watson [39] has attrib-

uted the shift of the Pt4f7/2 peak to higher BEs by the difference

in work function between pure Pt and the alloy, accompanied

by re-hybridization of the d-band as well as the sp-band. An

alternative explanation for the above mentioned shift can be

the increased vacancy of the 5d state of platinum [36]. In the

present study, XPS of the monometallic Pt nanoparticles (not

shown here), synthesized at high pH, showed the presence of

the metallic Pt at similar BEs as in the case of the PtIr samples

and Pt in oxidized states (Pt2þ and Pt4þ). XPS of the mono-

metallic Ir (not shown here) prepared by the modified polyol

method, revealed the absence of Ir nanoparticles in the

metallic form. Thus, it was not possible to compare the shift of

the Ir4f7/2 in the pure metal and PtIr nanoparticles and make

a direct conclusion about alloy formation in support of the

XRD results. Though, the existence of Ir in themetallic form in

the case of PtIr nanoparticles can be considered as an indirect

indication of an electronic effect between platinum and

iridium. In the two metal system, Ir can prevent the oxidation

of Pt atoms because it ismore electronegative than Pt, and this

phenomenon seems to be strongly affected by the experi-

mental conditions [38].

We should alsomention that nanoparticles can be oxidized

due to their exposure to air. Though, especially in the case of

the second sample of PtIr, the FWHM of the Pt4f peak in

combination with the expected energy shift due to the small

particle size, strongly indicate the existence of the Pt atoms

only in the metallic state. Moreover the Ultra High Vacuum

conditions can be considered as strongly reductive and can

cause the breakage of M-X bonds, where X ¼ halogen, though

it is more difficult to break MeO bonds. It is well known that

the study of metallic NPs with the use of Surface Sensitive

Techniques like XPS can be rather risky due to these effects.

For that reason more detailed studies are on the way so as to

have a better insight of the surface properties of this catalytic

system.

3.2. Electro-oxidation of ammonia on PtIr and Ptnanocatalysts

The fifth cycles of cyclic voltammograms in 1 M KOH of PtIr

and the reference Pt nanocatalysts, in the potential range

�1.46 to �0.4 V vs. MSE are shown in Fig. 4(a). The current

density is set per unit geometric area of the electrode. The

ECSA of the nanocatalysts are calculated from the CVs by

integrating the charge specific response in the hydrogen

desorption region (�1.44 to �1.02 V) and is used to normalize

the currents for AOR, unless otherwise stated (Table 1). The Pt

response (dashed line) is typical for polycrystalline platinum

materials and clearly shows two cathodic pairs of peaks cen-

tred at �1.42 and �1.17 V associated with the hydrogen

adsorption/desorption at Pt. At more negative potentials, the

Table 1 e Characteristics of the carbon-supported PtIrnanoparticles: particle size from TEM, XRD and SAXSresults and electrochemical surface area (ECSA) from thehydrogen desorption region of CVs (Fig. 4(a)).

Catalyst [NaOH]/M Particle size/nm Dispersion ECSA/m2/gPt

TEM XRDa SAXS

PtIr (1) 0.05 2.9 2.3 2.9 Monodisperse 68.1

PtIr (2) 0.06 2.6 1.4 1.8 & 3.4 Bimodal 65.7

Pt (ref)b 0.08 2.5 3.0 N/A N/A 86.0

a From Debye formula for scattering by randomly oriented mole-

cules [19].

b [17].

a b

Fig. 3 e X-ray photoelectron spectroscopy of the (a) Ir 4f and (b) Pt 4f levels of carbon-supported Pt7Ir3 nanoparticles.

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H2 evolution reaction starts to take place. The peak at �0.64 V

is characteristic of PtOx formation and the correspondingwide

reduction peak is seen in the cathodic scan at w �0.71 V. The

pair of peaks for hydrogen adsorption/desorption on PtIr

samples is shifted to more cathodic potentials if compared to

Pt. The surface redox couple seen as a small broad peaks

centred at around �0.85 V can be attributed to Ir/Ir(OH)3electrochemical couple and formation of Ir(OH)3 or IrOOH

layer [40e42]. A small oxidation peak at potentials more

positive than �0.6 V and corresponding reduction peak at

�0.6 V (Fig. 4(a)) is due to Ir(IV)/Ir(III) surface redox couple

according to the reaction [43,44]:

IrðOHÞ3!IrOðOHÞ2 þHþ þ e� (6)

The observed potential for Ir(IV)/Ir(III) redox couple is more

negative than the one observed recently by Kapalka et al.

[43,44] in 1 M NaClO4 electrolyte of pH 9 and might be related

to the higher solution pH used in the present work (pH¼ 14). It

was shown that iridium oxide electrodes exhibit the pH-

sensitivity ranging from 57 to 90 mV/pH [45]. Moreover,

Juodkazyte et al. [40] showed that the Ir(IV)/Ir(III) peaks shift to

negative 500 mV in alkaline solution as compared to acidic

ones.

CVs in 1MKOHþ 0.5MNH4OH are shown in Fig. 4(b) for the

two PtIr samples and for the reference Pt nanocatalysts. The

shape of CVs in ammonia containing solution is similar to

ones reported for bulk PtIr and Pt electrodes prepared by

electrodeposition or thermal decomposition of the precursor

salts [13e15]. The ammonia oxidation peak is clearly seen for

the three samples at approximately�0.78 V vs. MSE. At higher

potentials, currents decrease due to the inhibition of AOR by

the oxygenated species on the surface of catalysts and/or Nads.

Peak current densities are lower for both PtIr materials when

compared to carbon-supported Pt nanoparticles. At �0.78 V,

the peak current for Pt is as high as 170 mA cm�2, whereas for

PtIr, it is 107 mA cm�2 and 96 mA cm�2 for PtIr (1) and PtIr (2),

respectively. However, the onset potential for ammonia

oxidation is lower in the presence of iridium:�1.07 V on PtIr (1)

compared to �0.94 V on Pt nanoparticles. In other words,

while the current density is some 18 mA cm�2 for Pt at �0.94 V

it is almost two and a half times higher for PtIr (1) with

44 mA cm�2. PtIr (2) catalysts with Pt oxides on the surface has

the same onset potential as monometallic Pt, indicating that

oxides hinder ammonia electro-oxidation at lower potentials.

Moreover, the peak maximum of AOR is shifted to lower

potentials for both Ir-containing samples. These results are in

excellent agreement with ammonia electro-oxidation on bulk

PtIr electrodes reported in recent works [13e15,46], where the

addition of Ir lowered the overpotential for ammonia oxida-

tion. Comparison of overpotential values reported in the

literature and obtained at the carbon-supported PtIr (1)

nanoparticles are equivalent: h vs. SHE is 0.39 V in the present

work, 0.39 V in Boggs and Botte [15] in the same solution, and

0.4 in Moran et al. [14] and Endo et al. [13].

3.3. Chronoamperometry

Fig. 5 shows the current density after 100 s of chro-

noamperometric measurements at various applied potentials

from�0.83 to�1.03 V by steps of�0.05 V, corresponding to the

Table 2 e Deconvoluted XPS data for carbon supported PtIr catalysts.

Peak/Sample PtIr (1) PtIr (2) Pt:Ir ratio

Peak/eV at. % FWHM/eV Peak/eV at. % FWHM/eV

Ir4f 61.1 38.9 1.1 61.3 29.8 1.1 2.30

61.9 44.3 1.3 62.0 53.8 1.3

63.0 16.8 1.2 63.4 16.4 1.2

Pt4f 71.5 100 1.2 71.6 77.7 1.2 2.46

72.9 12.4 1.2

74.4 9.9 1.3

a b

Fig. 4 e Cyclic voltammograms of carbon-supported PtIr and Pt nanoparticles in (a) 1 M KOH and in (b) 1 M KOH D 0.5 M

NH4OH, at 20 mV.s-1 scan rate.

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potential region of ammonia oxidation (Fig. 4). The results

show superior activity of carbon-supported PtIr (1) prepared at

low solution pH when compared to PtIr (2) and Pt. The two

later catalysts behave similarly up to �0.93 V, however,

current densities on Pt become higher at potentials greater

than �0.88 V and surpass the activity of PtIr (2) for ammonia

oxidation, but still remain inferior than PtIr (1). Very low to

negligible variations between the CVs recorded before and

after each of these CA measurements witness of the stability

of the electrode materials. More details are given in Fig. 3S

under section 2 of the supporting information.

The stability study of the PtIr- and Pt-based nanocatalysts

was analyzed by carriyng out five successive CA measure-

ments at �0.8 V vs. MSE, and the results of the third runs are

illustrated in Fig. 6(a). All five CAs for the three investigated

electrocatalysts are shown in Fig. 4S under section 2 of the

supporting information. The triggering event to pass from one

CA to the next one is when the measured current density at

PtIr or Pt falls to zero i.e., the catalyst is deactivated by Nads or

oxygenated species and is no more active for AOR. Between

each two successive CAs, the electrode was subject to an

activation step at �1.46 V for 5 min. At this potential H2

evolution results in the removal of Nads and partial reduction

of catalyst surface. From these measurements, PtIr (1)

nanocatalyst exhibits better stability performance than both

PtIr (2) and Pt. Fig. 6(b) summarizes the current density after

30 s from the beginning of each of the five CAs vs. the number

of CA runs, for the three catalysts. Monometallic Pt catalyst

shows high initial activity and activity retention, however the

subsequent catalyst recovery is poor. Both Ir-containing

nanoparticles have moderate initial current densities but

they show better stability retention with normalized variation

in i-value of 11 and 18% for PtIr (1) and PtIr (2), respectively. Pt

showed current density dispersion of 64% over the 5 CAs

steps. Therefore, PtIr alloyed nanoparticles showed higher

recovery of the catalytic activity and stability retention, as

well as ability to degrade ammonia at lower overpotentials.

3.4. Electrolysis

Oxidation products of AOR over PtIr and Pt nanomaterials,

were investigated during a 12-h galvanostatic electrolysis in

25 mM NH4OH þ 1 M KOH at 10 mA cm�2 Fig. 7 shows the

normalized concentration profiles of total nitrogen, total

ammonia, nitrate and nitrite on the best performing PtIr (1)

and Pt nanocatalysts by the initial concentration of NH3.

Aqueous NH3 and Ntot concentrations in the bulk solutions

were continuously decreasing with time. Production of NO3

and NO2 was detected for both electro-catalysts and their

concentration increased with time. Overall, PtIr (1) shows

better performance in the degradation of ammonia than Pt,

e.g., a remaining NH3 concentration was 38.6% for PtIr (1) and

51.4% for Pt nanoparticles after 2 h of electrolysis. This indi-

cates 33% higher degradation rate for the former catalyst.

After 12 h of electrolysis, 2 and 3.1% of the initial ammonia

concentration was detected in the electrolyte solution with

PtIr (1) and Pt electrocatalysts, respectively. The same trend

was found for the concentration profile of total nitrogen in the

solution on PtIr (1) and Pt nanocatalysts: after 2 h, the former

is performing better than the later by 31% and by the end of

the 12 h, 96.6 and 94.2% of nitrogen has been degraded by PtIr

(1) and Pt, respectively. The trend of production of nitrate and

nitrite are similar on both catalysts. The amount of NO2 is

quite low reaching around 2% of the initial concentration of

ammonia by the end of the experiment corresponding to the

current efficiency of 0.36% on Pt and 0.31% on PtIr, while NO3

Fig. 5 e Current density after 100 s vs. applied potential of

carbon-supported PtIr and Pt nanoparticles in 1 M

KOH D 0.5 M NH4OH.

a b

Fig. 6 e (a) Third run of chronoamperometric measurements at L0.8 V vs. MSE and (b) current density after 30 s vs. number

of CA runs of carbon-supported PtIr and Pt nanoparticles in 1 M KOH D 0.5 M NH4OH.

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production is less than 12% (current efficiency 1.51% on Pt and

1.47% on PtIr), indicating that the main product of ammonia

oxidation on PtIr and Pt in alkaline solutions is nitrogen.

From the data described it follows that in comparison to

monometallic Pt, PtIr exhibits somewhat higher catalytic

activity characterized by a lower overpotential of AOR, satis-

factory catalyst recovery and stability, as well as faster NH3

degradation in a long-term electrolysis. The higher catalytic

activity of alloyed PtIr surfaces is related to the changes in the

electron state of Pt surface when alloyed with Ir causing the

variation in the adsorption properties of PtIr in regard to active

or inactive, e.g., Nads, intermediates.

Between the two Ir-containing catalysts, the one prepared

at lower synthesis pH has monodispersed particle size distri-

bution and low amounts of oxides on the surface, i.e. metallic

Pt and 30% of Ir0. Indeed, it was reported that the oxidation

state of PtIr is a parameter of great importance in AOR and

may be advantageous for the kinetics of the reaction e.g. Ir

tends to form hydroxides in alkaline environments, and the

presence of co-adsorbed OH- lowers the surface coverage by

poisonous Nads through repulsive interaction [16]. In the same

time, excess of the surface oxygenated species can block the

active sites for NH3 adsorption and the subsequent dehydro-

genation steps [47]. Moreover, the formation of nitrogen

oxides such as NxO and NOx instead of N2 could take place.

Therefore, the best performing PtIr nanoparticles for AOR

oxidation are mono-disperse 2.9 nm diamater particles,

with low amount of Ir oxides on the surface, and metallic

surface Pt.

4. Conclusion

Alloyed PtIr nanoparticles at 70:30 at. % ratio are synthesized

with a modified polyol route. It was found that the size

distribution and surface chemistry is pH-dependent, where

PtIr nano-sized clusters show average size of 2.9 nmwhen the

final synthesis solution pH was neutral with the formation of

Pt0, Ir0 and Ir oxides/hydroxides on the surface. At higher final

solution pH, the resulting PtIr nanoparticles show bi-modal

distribution of particle sizes with maxima at 1.8 and 3.4 nm

and the presence of Pt and Ir in several oxidation states.

Ir-containing alloys show superior results with 0.1 V lower

onset potential and a better stability and durability of the

catalysts if compared to the monometallic Pt nanoparticles,

due to the changes in the electron state of Pt surface when

alloyed with Ir. The analysis of the by-products generated

during the ammonia oxidation reaction on PtIr and Pt nano-

catalysts show that the former outpace the later for the

degradation of ammonia and total nitrogen with the produc-

tion of comparable fraction of nitrate and nitrite. Overall, the

PtIr nanoparticles that were prepared at lower pH, with no Pt

oxides at its surface and narrow size distribution, perform

better for AOR than the PtIr sample prepared at high pH or

monometallic Pt nanoparticles.

Acknowledgments

The authors would like to thank the Natural Science and

Engineering Research Council (NSERC) for financial support.

Anis Allagui acknowledges the support from Fonds quebecois

de la recherche sur la nature et les technologies (FQRNT).

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2012.11.079.

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