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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: [email protected] (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.
i n t e r n 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 8 ( 2 0 1 3 ) 2 4 5 5e2 4 6 3 2461
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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.
r e f e r e n c e s
[1] Marin�ci�c L, Leitz FB. Electrooxidation of ammonia in wastewater. J Appl Electrochem 1978;8:333e45.
[2] Lopez de Mishima B, Lescano D, Holgado TM, Mishima H.Electrochemical oxidation of ammonia in alkaline solutions:its application to an amperometric sensor. Electrochim Acta1998;43(3e4):395e404.
[3] Rosca V, Duca M, de Groot MT, Koper MTM. Nitrogen cycleelectrocatalysis. Chem Rev 2009;109(6):2209e44.
[4] Simons EL, Cairns EJ, Surd DJ. The performance of directammonia fuel cells. J Electrochem Soc 1969;116(5):556e61.
[5] Vidal-Iglesias F, Solla-Gullon J, Montiel V, Feliu J, Aldaz A.Screening of electrocatalysts for direct ammonia fuel cell:ammonia oxidation on PtMe (Me: Ir, Rh, Pd, Ru) andpreferentially oriented Pt(1 0 0) nanoparticles. J PowerSources 2007;171(2):448e56.
[6] Vidal-Iglesias FJ, Solla-Gullon J, Montiel V, Feliu JM, Aldaz A.Ammonia selective oxidation on Pt(100) sites in an alkalinemedium. J Phys Chem B 2005;109(26):12914e9.
[7] Solla-Gullon J, Vidal-Iglesias FJ, Rodrıguez P, Herrero E,Feliu JM, Clavilier J, et al. In situ surface characterization ofpreferentially oriented platinum nanoparticles by usingelectrochemical structure sensitive adsorption reactions.J Phys Chem B 2004;108(36):13573e5.
[8] Oswin HG, Salomon M. The anodic oxidation of ammonia atplatinum black electrodes in aqueous KOH electrolyte. Can JChem 1963;41(7):1686e94.
[9] Vot SL, Reyter D, Roue L, Belanger D. Electrochemicaloxidation of NH3 on platinum electrodeposited onto graphiteelectrode. J Electrochem Soc 2012;159(4):F91e6.
Fig. 7 e The normalized concentration profile of Ntot, NH3
and NOx (x [ 2,3) during electrolysis of ammonia under
galvanostatic conditions on PtIr (1) and Pt in 1 M
KOH D 25 mM NH4OH; current density 10 mA cmL2.
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 8 ( 2 0 1 3 ) 2 4 5 5e2 4 6 32462
Author's personal copy
[10] Halseid R, Wainright JS, Savinell RF, Tunold R. Oxidation ofammonium on platinum in acidic solutions. J ElectrochemSoc 2007;154(2):B263e70.
[11] Gerischer H, Mauerer A. Untersuchungen zur anodischenoxidation von ammoniak an platin-elektroden. J ElectroanalChem Interfacial Electrochem 1970;25(3):421e33.
[12] Vitse F, Cooper M, Botte GG. On the use of ammoniaelectrolysis for hydrogen production. J Power Sources 2005;142(1e2):18e26.
[13] Endo K, Nakamura K, Katayama Y, Miura T. PteMe (Me ¼ Ir,Ru, Ni) binary alloys as an ammonia oxidation anode.Electrochim Acta 2004;49(15):2503e9.
[14] Moran E, Cattaneo C, Mishima H, Lopez de Mishima Ba,Silvetti SP, Rodriguez JL, et al. Ammonia oxidation onelectrodeposited PteIr alloys. J Solid State Electrochem 2007;12(5):583e9.
[15] Boggs BK, Botte GG. Optimization of PteIr on carbon fiberpaper for the electro-oxidation of ammonia in alkalinemedia. Electrochim Acta 2010;55(19):5287e93.
[16] de Vooys A, Koper M, van Santen R, van Veen J. The role ofadsorbates in the electrochemical oxidation of ammonia onnoble and transition metal electrodes. J Electroanal Chem2001;506(2):127e37.
[17] Lomocso TL, Baranova EA. Electrochemical oxidation ofammonia on carbon-supported bi-metallic PtM (M ¼ Ir, Pd,SnOx) nanoparticles. Electrochim Acta 2011;56(24):8551e8.
[18] Wang Y, Ren J, Deng K, Gui L, Tang Y. Preparation of tractableplatinum, rhodium, and ruthenium nanoclusters with smallparticle size in organicmedia. ChemMater 2000;12(6):1622e7.
[19] Baranova EA, Page YL, Ilin D, Bock C, MacDougall B,Mercier PH. Size and composition for 1e5 nm f PtRu alloynano-particles from Cu Ka X-ray patterns. J Alloys Compd2009;471(1e2):387e94.
[20] Tuaev X, Paraknowitsch JP, Illgen R, Thomas A, Strasser P.Nitrogen-doped coatings on carbon nanotubes and theirstabilizing effect on Pt nanoparticles. Phys Chem Chem Phys2012.
[21] Hufner S, Wertheim GK. Core-line asymmetries in the x-ray-photoemission spectra of metals. Phys Rev B (Solid State)1975;11(2):678e83.
[22] Peuckert M. XPS study on thermally and electrochemicallyprepared oxidic adlayers on iridium. Surf Sci 1984;144(2e3):451e64.
[23] Sheppard SA, Campbell SA, Smith JR, Lloyd GW, Walsh FC,Ralph TR. Electrochemical and microscopic characterisationof platinum-coated perfluorosulfonic acid (Nafion 117)materials. Analyst 1998;123:1923e9.
[24] Baranova E, Bock C, Ilin D, Wang D, MacDougall B. Infraredspectroscopy on size-controlled synthesized Pt-based nano-catalysts. Surf Sci 2006;600(17):3502e11.
[25] Debye P. Zerstreuung von rontgenstrahlen. scattering fromnon-crystalline substances. Ann Phys 1915;46:809e23.
[26] Baranova EA, Miles N, Mercier PH, Page YL, Patarachao B.Formic acid electro-oxidation on carbon supported PdxPt1�x
0 � x� 1 nanoparticles synthesized via modified polyolmethod. Electrochim Acta 2010;55(27):8182e8.
[27] Baer D, Engelhard M. Xps analysis of nanostructuredmaterials and biological surfaces. J Electron Spectrosc RelatPhenom 2010;178-179(0):415e32.
[28] Radnik J, Mohr C, Claus P. On the origin of binding energyshifts of core levels of supported gold nanoparticles anddependence of pretreatment and material synthesis. PhysChem Chem Phys 2003;5:172e7.
[29] Lopez-Salido I, Lim DC, Dietsche R, Bertram N, Kim YD.Electronic and geometric properties of au nanoparticles on
highly ordered pyrolytic graphite (HOPG) studied using X-rayphotoelectron spectroscopy (XPS) and scanning tunnelingmicroscopy (STM). J Phys Chem B 2006;110(3):1128e36.
[30] Garbarino S, Pereira A, Hamel C, Irissou E, Chaker M, Guay D.Effect of size on the electrochemical stability of ptnanoparticles deposited on gold substrate. J Phys Chem C2010;114(7):2980e8.
[31] Shen S, Zhao T, Xu J. Carbon-supported bimetallic PdIrcatalysts for ethanol oxidation in alkaline media.Electrochim Acta 2010;55(28):9179e84.
[32] da Silva L, Alves V, de Castro S, Boodts J. XPS study of thestate of iridium, platinum, titanium and oxygen in thermallyformed IrO2 þ TiO2 þ PtOx films. Colloids Surf A 2000;170(2e3):119e26.
[33] Hu S, Xiong L, Ren X, Wang C, Luo Y. Pt-Ir binaryhydrophobic catalysts: effects of Ir content and particle sizeon catalytic performance for liquid phase catalytic exchange.Int J Hydrogen Energy 2009;34(20):8723e32.
[34] Liang Y, Zhang H, Zhong H, Zhu X, Tian Z, Xu D, et al.Preparation and characterization of carbon-supported PtRuIrcatalyst with excellent CO-tolerant performance for proton-exchange membrane fuel cells. J Catal 2006;238(2):468e76.
[35] Briggs D, Seah M, editors. Practical surface analysis by augerand x-ray photoelectron spectroscopy. 2 ed., vol. 1. JohnWiley & Sons; 1996.
[36] Toda T, Igarashi H, Uchida H, Watanabe M. Enhancement ofthe electroreduction of oxygen on Pt alloys with Fe, Ni, andCo. J Electrochem Soc 1999;146(10):3750e6.
[37] Wakisaka M, Mitsui S, Hirose Y, Kawashima K, Uchida H,Watanabe M. Electronic structures of ptco and ptru alloys forco-tolerant anode catalysts in polymer electrolyte fuel cellsstudied by ecxps. J Phys Chem B 2006;110(46):23489e96.
[38] Radev I, Topalov G, Lefterova E, Ganske G, Schnakenberg U,Tsotridis G, et al. Optimization of platinum/iridium ratio inthin sputtered films for pemfc cathodes. Int J HydrogenEnergy 2012;37(9):7730e5.
[39] Weinert M, Watson RE. Core-level shifts in bulk alloys andsurface adlayers. Phys Rev B 1995;51:17168e80.
[40] Juodkazyte J, Sebeka B, Stalnionis G, Juodkazis K. Eqcm studyof iridium anodic oxidation in H2SO4 and KOH solutions.Electroanalysis 2005;17(19):1734e9.
[41] Kotz R, Neff H, Stucki S. Anodic iridium oxide films.J Electrochem Soc 1984;131(1):72e7.
[42] Kotz R. Photoelectron spectroscopy of practical electrodematerials; chap. 2. Advances in electrochemical science andengineering. Heidelberg: Verlag Chemie; 1990. p. 75e126.
[43] Kapalka A, Fierro S, Frontistis Z, Katsaounis A, Frey O,Koudelka M, et al. Electrochemical behaviour of ammoniaNH4
þ/NH3 on electrochemically grown anodic iridium oxidefilm (airof) electrode. Electrochem Commun 2009;11(8):1590e2.
[44] Kapalka A, Fierro S, Frontistis Z, Katsaounis A, Neodo S,Frey O, et al. Electrochemical oxidation of ammonia NH4
þ/NH3 on thermally and electrochemically prepared iro2electrodes. Electrochim Acta 2011;56(3):1361e5.
[45] Hendrikse J, Olthuis W, Bergveld P. A drift free nernstianiridium oxide ph sensor. In: International conference onsolid state sensors and actuators. TRANSDUCERS 97 Chicago,vol. 2. 1997. p. 1367e370.
[46] Bonnin EP, Biddinger EJ, Botte GG. Effect of catalyst onelectrolysis of ammonia effluents. J Power Sources 2008;182(1):284e90.
[47] Peng W, Xiao L, Huang B, Zhuang L, Lu J. Inhibition effect ofsurface oxygenated species on ammonia oxidation reaction.J Phys Chem C 2011;115(46):23050e6.
i n t e r n 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 8 ( 2 0 1 3 ) 2 4 5 5e2 4 6 3 2463
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