The Beneficial Role of The Co-metals Pd and Au in the Carbon Supported PtPdAu Catalyst Towards Promoting Ethanol Oxidation Kinetics in Alkaline Fuel Cells: Temperature Effect and Reaction Mechanism.

Abstract

Electrochemical investigations have been carried out to study the oxidation kinetics of ethanol in alkaline solution on carbon supported ternary alloy catalysts Pt-Pd-Au within the temperature range of 200 to 800 C. To derive a better understanding of the contribution of each of the metallic components towards the catalytic oxidation of ethanol, some of the investigations were extended to the individual noble metals for comparison, however at a single temperature (200 C). The individual metals could barely show their catalytic efficiency towards ethanol oxidations when compared to the alloyed catalyst. The ternary catalyst exhibited much lower values and larger temperature dependence of onset potential for ethanol oxidation. With the rise of potential, the apparent activation energy (Ea(app)) for ethanol oxidation on Pt/C electrode increased whereas a decreasing trend was observed with the Pt30Pd38Au32/C electrode. It was suggested that Pt30Pd38Au32/C electrode bears an excellent tolerance towards ethanolic residues, for the temperature range studied. In co-relation with the results obtained from the above study, attempts were made to elucidate the oxidation reaction mechanism and this further evoked interest in extending the work to the estimation of products formed during oxidation of ethanol within the same temperature range through ion chromatographic analysis. The pronounced increase in the quantity of oxidation products like acetate and carbonate, obtained over the ternary catalyst as compared to single Pt, substantiates the kinetic enhancement of ethanol oxidation, attributable to the co-metal partnership between Pd and Au when incorporated in the Pt matrix. In summary the multi-metallic nano crystallites can not only show their capability of extracting the best possible number of electrons from the alcohol fuel in alkaline solutions, harnessing more energy, but at the same time bring down the cost of the catalyst material by reducing the Pt content to a considerable extent.

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Published:July 20, 2011
r2011 American Chemical Society
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pubs.acs.org/JPCC
The Beneficial Role of the Cometals Pd and Au in the
Carbon-Supported PtPdAu Catalyst Toward Promoting Ethanol
Oxidation Kinetics in Alkaline Fuel Cells: Temperature Effect and
Reaction Mechanism
Jayati Datta,*,†Abhijit Dutta,†and Sanjeev Mukherjee‡
†Electrochemistry & Fuel Cell Laboratory, Department of Chemistry, Bengal Engineering and Science University, Shibpur,
Howrah?711 103, West Bengal, India
‡Department of Chemistry and Chemical Biology, 317 Egan Center, Northeastern University, 360 Huntington Avenue, Boston,
Massachusetts 02115, United States
b
S Supporting Information
1. INTRODUCTION
The Pt-based alloys and composites are being explored as the
alternatives tosinglePtfor electrocatalysis infuelcellreactions.1?8
Particularly for the anodic reaction in DEFCs operating in acidic
environments, several Pt-based alloys have superseded the single
Pt with respect to their catalytic activity toward promoting faster
kinetics and complete conversion of the alcohol fuel to the end
products. The success in electrocatalysis of these alloy materials
has been attributed to the bifunctional mechanism9and the
electronic effect10?12induced by the electronic interaction of Pt
with other metals. However, the search for efficient catalysts for
selective oxidation in alkaline solutions is scarce. Therefore, a
majorthrustmay be put to investigate the catalytic activity of the
alloy materials in alkaline media for obtaining favorable kinetics
and better selectivity toward CO2production.13?16
In the present investigation, the carbon-supported single
metal catalysts, Pt/C, Au/C, and Pd/C, and the ternary
Pt:Pd:Au/C [∼1:1:1] catalysts were synthesized through a bor-
ohydride reduction method. The identification of crystal phases,
the matrix morphology, and the surface area of the nanoparticles
have been studied by X-ray diffraction (XRD), transmission
electron microscopy (TEM-EDAX), andthe Brunauer?Emmet?
Teller (BET) method, respectively. The catalytic activity toward
ethanol oxidation in alkaline solutions was investigated with the
help of several electrochemical techniques, such as cyclic voltam-
metry (CV), chronoamperometry (CA), and electrochemical
impedance spectroscopy (EIS), and the temperature effect on
ethanol oxidation was investigated with Pt/C and PtPdAu/C
electrodes in particular. Apparent activation energies (Ea(app)) for
the oxidation reaction were determined at different potentials
Received:
Revised:
January 11, 2011
April 23, 2011
ABSTRACT:Electrochemicalinvestigationshavebeencarriedouttostudytheoxidation
kinetics of ethanol in alkaline solution on carbon-supported ternary alloy catalysts
Pt?Pd?Auwithinthetemperaturerangeof20?80?C.Toderiveabetterunderstanding
of the contribution of each of the metallic components toward the catalytic oxidation of
ethanol, some of the investigations were extended to the individual noble metals for
comparison,however,atasingletemperature(20?C).Theindividualmetalscouldbarely
show their catalytic efficiency toward ethanol oxidations when compared to the alloyed
catalyst. The ternary catalyst exhibited much lower values and a larger temperature
dependence of onset potential for ethanol oxidation. With the rise of potential, the
apparent activation energy (Ea(app)) for ethanol oxidation on the Pt/C electrode
increased, whereas a decreasing trend was observed with the Pt30Pd38Au32/C electrode.
It was suggested that the Pt30Pd38Au32/C electrode bears an excellent tolerance toward
ethanolic residues, for the temperature range studied. In correlation with the results
obtainedfromtheabovestudy,attemptsweremadetoelucidatetheoxidationreactionmechanism,andthis further evoked interest
inextendingtheworktotheestimationofproductsformedduring oxidationofethanolwithinthesametemperaturerangethrough
ion chromatographic analysis. The pronounced increase in the quantity of oxidation products, such as acetate and carbonate,
obtainedover theternarycatalystas comparedtosingle Pt, substantiatesthe kinetic enhancementof ethanol oxidation,attributable
to the cometal partnership between Pd and Au when incorporated in the Pt matrix. In summary, the multimetallic nanocrystallites
can not only show their capability of extracting the best possible number of electrons from the alcohol fuel in alkaline solutions,
harnessing more energy, but also, at the same time, bring down the cost of the catalyst material by reducing the Pt content to a
considerable extent.

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within the temperature range of 20?80 ?C. The reaction products
werealsoestimatedthroughion-chromatographicanalyseswithinthe
same temperature regime. On the basis of the information obtained,
the kinetics and mechanism of the reaction were elucidated.
2. EXPERIMENTAL SECTION
2.1. Preparation of Catalysts. The nanocatalysts (40 wt %
each) ofthe single metals, Pt, Au,and Pd, and the ternary system
PtPdAu/C were prepared under the NaBH4-reduction scheme
from the chemical bath containing the corresponding precursor
salts (H2PtCl6, PdCl2, AuCl4), Arora Matthey Ltd. and their
mixtures in 0.002 M concentrations of each of the salts. An
appropriate amount of Vulcan XC-72 carbon (Cabot India) was
added to the 400 mL of milli-Q water (F = 18.2 MΩ3cm), and
themixturewasultrasonicatedfor30min,followedbystirringfor
1 h. Subsequently, an excess quantity of 0.05 M NaBH4(Merck)
solution was added, drop by drop, to the mixture with vigorous
stirring until the yellowish-brown colloidal solution became
colorless, containing dispersed black metals particles for the
complete reduction of metals from the precursor salt. The
resulting heterogeneous colloidal mixtures were allowed to stir
for1hatroomtemperatureandcentrifuged.Blackmetalcatalyst
particles were then isolated by centrifugation, and the super-
natant colorless liquid was tested by silver nitrate to detect the
presenceofanyresidualbromideorchlorideioninsolution.The
catalysts were washed with plenty of milli-Q water, and the
bromide test was done until the supernatant liquid gave the
negativetestofit.Thesynthesizedcatalystsweredriedinanoven
at 100 ?C for 3?4 h.
2.2. Surface Characterization and Morphology of the
Catalyst Matrix. The characteristics of the crystalline structure
of the supported catalysts were determined by using the powder
XRD technique. The data were determined by using the Seifert
2000 diffractometer operating with CuKR radiation (λ =
0.1540 nm) generated at 35 kV and 30 mA. Scans were recorded
at 1? min?1for 2θvalues between2and90?. Scherrer andBragg
formulas were employed to calculate the mean diameter and the
lattice parameter of the catalysts. The recorded XRD patterns
were analyzed following the JCPDS file. To get the morphology
andparticlesize,TEMimagesofcatalystswereobtainedbyusing
an FEI model STWIN operated at an accelerating voltage of 200
kV. Specimens for TEM analysis were prepared by ultrasonically
suspendingtheparticlesinalcohol.Adropofthesuspensionwas
deposited onto a standard carbon-coated Cu grid and allowed to
dry before being inserted into the microscope. The particle size
distribution histograms were obtained by the observation of
about 300 particles from the four different locations of the
corresponding TEM images. An electron-dispersive X-ray de-
tecting system, EDAX, coupled with the TEM instrument, was
used to analyze the chemical composition of the alloy, consider-
ing several regions of the catalyst particles throughout the
surface. The spatial resolution of the EDAX probe is 0.35 nm.
Theadsorptionisothermofthecatalystswasrecordedat77Kby
the BET method using nitrogen as the adsorbate in a Quanta-
chrome Autosorb instrument (model AS1-CT). Prior to the
measurements,samplesweredegassedat150?Cfor30min.The
surface area was evaluated according to the BET theory,17using
themultipointmethod atrelativepressuresuptoP/P0=0.2,and
the total volume (Vtot) was calculated from the volume of
nitrogen adsorbed at near to saturation pressure, P/P0= 0.99.
The volume of micropores (Vm) was derived from the so-called
t-plot dependence of the nitrogen sorption isotherm.18The pore
sizedistributionwasderivedfromtheadsorptionisothermaccord-
ing to Barrett?Joyner?Halenda (BJH) model calculations.
2.3. Fabrication of the Electrodes. A catalyst ink was
prepared by taking an appropriate amount of catalyst in 5 wt %
Nafion ionomer (Eloctrochem. Inc., USA) and 2-propanol (GR
grade, Merck), and the mixture was sonicated. A calculated
amount of this slurry was micropipetted out onto the graphite
support (0.65 cm2) to maintain a constant catalyst loading of
0.77 mg cm?2. A graphite plate (GLM grade, Graphite India
Ltd.) with a thickness of approximately 2 mm was used as the
support for the working electrode.
2.4. Electrochemical Measurements. Electrochemical mea-
surements were conducted using a computer-controlled poten-
tiostat/galvanostat with PG stat 30 and FRA modules (Eco
Chemie B.V., The Netherlands). Ethanol (AR grade, Merck)
was added to the nitrogen (XL grade, 99.999%, BOC India Ltd.)
saturated 0.5 M NaOH electrolyte to make a 1.0 M EtOH
concentration. All these solutions were prepared using Mili-Q
water. The geometrical area of the electrocatalyst exposed to the
solution was always 0.65 cm2. Voltammetry was carried out in a
glass cell using a conventional three-electrode setup incorporat-
ing a mercury/mercurios oxide (Hg?HgO) reference electrode
and a bright Pt foil (10 mm ? 10 mm) counter electrode along
with the working electrode fabricated with the synthesized
catalysts. Steady-state potantiostatic data were recorded after
polarization for 5 min. Electrochemical impedance measure-
ments were performed with an amplitude of 5 mV in the
frequency range of 30 kHz to 30 mHz at a potential of ?0.3 V.
2.5. Ion-Chromatographic Analysis. Aliquots from the
working electrolyte were analyzed by ion chromatography after
polarizationatconstantpotentialfor2hatselectedtemperatures
in the range of 20?80 ?C. Metrohm’s Advanced Modular Ion
Chromatographysystems,aMetrosepASupp5-250columnand
Table 1. Results of EDAX, XRD and Voltammetric Analysis of the Single and Ternary Catalsyts
average crystallite size (nm)ECSA (m2/g)
electrocatalystsPt/Pd/Au
(atomic ratio)
EDAX
composition (at. %)
interplanar
distance (Å)
lattice
parameter (Å)
XRD TEMH2ads/des
region
reduction of adsorbed
O2region
Pt/C
Pd/C
Au/C
PtPdAu/C
1:0:0
0:1:0
0:0:1
1:1:1
Pt: 100
Pd: 100
Au: 100
Pt: 30
Pd: 38
Au: 32
2.260
2.231
2.240
2.165
3.925
3.893
4.081
3.903
3.84
3.40
3.22
3.90
3.50
4.00
3.50
3.50
27.3
19.8
11.3
69.3
33.1
24.9
09.5
73.7

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a Metrosep A Supp 4-550 organic acid column, were used for
analyzingtheacetateandcarbonateions.A1mLsamplesolution
wasdilutedwith19mLoffilteredMilli-Qwater,and25μLofthis
sample was injected into the ion chromatograph.
3. RESULT AND DISCUSSION
3.1. Morphology and Structural Characterization of
PtPdAu/C Electrocatalyst. It has been found in an earlier work
that optimization of the catalyst loading with respect to the
carboncontentinthe catalyst matrix, for afuel cellreaction, isan
essential prerequisite to deriving the best catalytic performance
of the nanoparticles toward the respective red-ox processes (see
Figures S1?S3 in the Supporting Information). In this regard, a
40% catalyst loading was found to be the choice for catalyst
support combination.
The EDS measurement was recorded to derive the atomic
composition of the PtPdAu/C catalysts prepared from the
chemical bath containing the metal precursors, all having a
equimolar concentration, and the corresponding atomic ratios
are given in Table 1.
TheXRDpatternsinFigure1clearlydemonstratethatthePt/
C, Pd/C, Au/C, and Pt30Pd38Au32/C alloy catalysts have face-
centered cubic(fcc) structures. All thecatalystsexhibiteddiffrac-
tion peaks of (111), (200), (220), (311), and (222) planes
corresponding to an fcc crystal structure (JCPDS 04-0802). The
2θ values of the (111) planes were found to be shifted slightly
from the values of Pt/C at 39.763?40.101 for Pd/C, 38.184 for
Au/C,and40.069fortheternarysystemPtPdAu.Assumingalloy
formation among Pt, Pd, and Au based on a substantial solid
solution,such ashift can beattributed tothe difference inatomic
sizes of the order of Pd > Pt > Au. Seemingly, the gold catalyst
exhibited the lowest 2θ value, being significantly different from
that of the other catalysts.19The lattice parameters of these
catalysts were obtained from the XRD patterns displayed in
Table 1. It was found that the addition of Pd and Au to the Pt
matrix barely makes any effect on the crystalline structure of Pt/
C because Pt, Pd, and Au have almost the same fcc crystalline
Figure1. X-raydiffractionpatternsofPt/C,Pd/C,Au/C,andPt-based
ternary catalysts. Inset: magnified view of the (111) peak.
Figure2. (a)TEMimageofthePt30Pd38Au32/Ccatalyst.(b)Particlesizedistribution.(c)FringepatternandtheFFTimages.(d)Point-resolvedEDS
spectrum of the PtPdAu/C catalyst.

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structure. The diffraction position of the ternary catalyst is
positively shifted compared with that of the other catalysts,
resulting in the decreased lattice parameter of 0.3903 for
Pt30Pd38Au32/C compared with 0.3925 nm of pure Pt/C.
Low-resolution TEM images of Pt30Pd38Au32/C catalysts and
thecorrespondingparticlesizedistributionhistogramsareshown
in Figure 2a,b, respectively, and the data from TEM analysis are
listed in Table 1. There is clear tradeoff in particle size with the
incorporation of Au in the ternary system. The average sizes of
the particles for all the catalysts calculated by using the Debye?
Scherrer equation, agreed well with the TEM results, and the
size distribution is found to be narrower with almost 80% of
thecatalystparticlesinthe3?5nmrange.Thefringepatternsfor
the Pt30Pd38Au32/C catalyst are shown in Figure 2c, where the
crystalline plane is calculated with the help of FFT analysis
(Figure 2c, inset). The d-spacing of Pt30Pd38Au32/C is shifted to
0.2165nmfrom0.226nm,thenormal valueofPt(111)(JCPDS
04-0802), indicating alloy formation, which is also supported by
XRD data, discussed earlier. The overall impression is such that
the narrow size distribution and better dispersion are achieved
due to the existence of Au in the ternary matrix. A representative
point-resolved EDS spectrum (Figure 2d) collected from a
number of individual particles reveals that the ternary catalyst
matrix is grown with an ensemble of atoms of the individual
metals Pt, Pd, and Au forming the nanoparticles.
A typical reversible curve representative of a type III nitrogen
adsorption and desorption isotherm and the Vulcan XC-72
[C(V72)] carbon reversible curve representative of a type IV/
III are exhibited in Figure 3a. The single and ternary catalysts
were studied according to the Brunauer?Deming?Deming?
Teller (BDDT) classification,20and the types of hysteresis were
attributed to cylindrical pores. Pore volume and pore size
distributions (PSDs) were calculated on the basis of the BJH
modelperformed fromthe adsorption branch oftheisotherm, as
shown in the inset of Figure 3a.
The adsorption isotherms associated with the structure of the
singleandternarycatalystsareessentiallythoseofthecombination
ofmicroporesandmesoporesandasfurtherclarifiedfromthepore
size distribution (PSD) curves. Small micropores (<2 nm) in the
carbon-supported catalyst decreases the catalyst utilization be-
cause the mass transport of reactants and products become
insignificant for these tiny micropores. When the macropore size
is larger than 50 nm, the surface area becomes smaller and the
electricalresistanceincreases.21TernarycatalystsonVulcanXC72
with optimum pore sizes in the range of 2?50 nm (as shown in
Figure 3a, inset) become useful as they can render a better ability
toenhanceboththedispersionandtheutilizationofmetalsinthis
range. The relevant structural parameters of all the catalysts
determined on the basis of the isotherms18are revealed in
Figure 3b, where it is observed that, with the incorporation of
Pd and Au into the Pt matrix, the total BET surface area increases
along with the external surface area and the micropore area of the
catalyst surface. This indicates that Pt30Pd38Au32/C catalysts not
onlyintercalateintothemicroporeofthegraphitesupportbutalso
are grown on the external surface area of the support. The large
quantity of micropores can enhance the adsorbability of the
catalyst and extend the active sites. On the other hand, higher
surface areas and larger pore volumes allow a higher degree of
catalyst dispersion as well as lead to highly integrated intercon-
nected pore systems with periodic order and thus enable better
utilization of catalyst particles by efficient transport of reactants
and products through the catalyst matrix.
3.2. Electrochemical Investigation and the Temperature
EffectontheElectrodeKinetics.TypicalvoltammogramsofPt,
Pd, Au, and PtPdAu catalysts in 0.5 M NaOH are shown in
Figure 4. The well-established features of hydrogen adsorption
and desorption, double layer charging, oxide formation, and
oxide reduction are evident from the voltammograms of all the
Figure 3. (a) Nitrogen adsorption (filled) and desorption (unfilled)
isotherms for single and ternary catalysts. Inset: pore size distributions
calculated from the nitrogen adsorption?desorption isotherms. (b)
Different surface areas for Pt/C, Pd/C, Au/C, and Pt30Pd38Au32/C.
Figure 4. Cyclic voltammograms of Pt/C, Pd/C, Au/C, and
Pt30Pd38Au32/C in 0.5 M NaOH. Scan rate = 50 mV s?1. Inset:
magnified view of the oxide reduction peak region and the hydrogen
adsorption?desorption region.

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catalysts studied. A measure of the real electrochemical surface
area Ar(cm2Pt) can be obtained from the voltammogram using
the following equation
Ar¼ Qh=Qm
ð1Þ
where Qhis the saturated hydrogen coverage on the electrode
(μC)andQmisthechargeassociatedwithmonolayeradsorption
of hydrogen (μC cm?2Pt). It is assumed that each surface
platinum atom is associated with one chemisorbed hydrogen
atom, allowing the charge corresponding to the area under the
hydrogen adsorption peaks, Qh, to be converted to the real
electrochemical surface area. The charge associated with a
monolayer of hydrogen is commonly taken as 210 μC cm?2
for Pt22,23after double layer correction. However, for Pd-con-
taining catalysts, the method based on the hydrogen adsorption
charge seems to be less reliable because Pd can absorb large
quantities of hydrogen (up to 900 times its own volume). The
realECSAofthePd-containingPtPdAu/Ccatalystisdetermined
byconsidering theColumbicchargescorrespondingtotheoxide
reductionpeak of thealloyed catalyst.However, itis notpossible
to measure the ECSA value accurately by using this method
because the reduction peaks may be ascribed to the reduction of
the oxides of Pd, Pt, and Au formed on the surface of the
PtPdAu/C catalyst during the positive scan.24The charges
required for the reduction of PdO (qPdO-red), PtO (qPtO-
red), and Au2O3(qAu2O3-red) monolayers were taken to be
405, 420, and 400 μC cm?2, respectively, as reported in the
literature.25?27The mean value of charges required for the oxide
reduction of the alloyed catalyst with the atomic composition of
Pt30Pd38Au32was calculated to be 409.1 μC cm?2and used for
evaluating the ECSA of the alloyed catalyst, summarized in
Table 1. The value of ECSA for Pt30Pd38Au32/C was found to
be far higher than the value obtained for pure metal catalysts,
reflecting the higher catalytic surface area for the ternary alloyed
catalyst.
The oxidation of Pt30Pd38Au32/C surfaces commences at
more negative potential than those observed for other catalysts,
and the double layer charging region and beyond is far more
exuberated, indicating dipolar interactions, followed by oxide
formation, in this region.
With the incorporation of Pd and Au into the Pt matrix, the
oxidation current rises in the potential region of ?750 to +250
mV, as shown in Figure 5, almost 4 times that of Pt and in the
order of PtPdAu > Pd > Pt > Au.
Theforwardpeakcurrentintheanodicsweepandthereversed
peakcurrentinthecathodicsweeparecomparedinFigure6,and
a significant increase in both the current densities are observed
with the ternary catalyst compared with the single metals. Thus,
the possibility of using the single metal as an effective catalyst for
ethanol oxidation becomes less important. Hence, for the rest of
the investigations involving the reaction kinetics at different
temperatures, the studies on Au and Pd were abandoned and
the results of electrode kinetics were compared for the ternary
catalyst and the single Pt only.
Figure 7a,b represents the cyclic voltammograms for the EOR
on Pt/C and Pt30Pd38Au32/C in 0.5 M NaOH containing 1.0 M
ethanol at the temperatures of 20, 40, 60, and 80 ?C. The Pt/C
and Pt30Pd38Au32/C catalysts show significantly different CV
profiles, particularly at higher temperatures. The ethanol oxida-
tion peak current densities obtained with Pt/C in the forward
sweep were raised by a factor of 3 as the temperature increased
from 20 to 80 ?C (shown in Figure 7a), whereas the negative
going sweeps display a sharp vertical increase of the reverse
oxidation peak, which are more prominent at higher tempera-
tures. This sharp rise in current on Pt/C is indicative of the fact
that the ethanolic residues remaining unoxidized on the surface
during the forward sweep can only be oxidized by the Pt oxide
formed at higher potentials. As the temperature was increased,
thepotentialdifferencesbetweenforwardandreversepeakswere
reduced, suggesting that surface activation of Pt/C by OH
species is facilitated at higher temperatures. The influence of
temperature on Pt30Pd38Au32/C toward the ethanol oxidation is
far more significant. Considering the voltammograms of the
ternary system, as shown in Figure 7b obtained in the same
temperature range, the oxidation peak current densities was
found to increase by almost 4 times, when the temperature was
increased from 20? to 80 ?C. In contrast to the Pt catalyst, the
diminution in the reverse oxidation current with the ternary
systemisnotedandaconsiderableriseinanodicpeakintensityis
observed, particularly at high temperature. This points toward a
fair catalytic performance of the Pt30Pd38Au32/C system com-
pared to other catalysts.
Figure 5. (a) Cyclic voltammograms of Pt/C, Pd/C, Au/C, and
Pt30Pd38Au32/C in 0.5 M NaOH containing 1 M ethanol. Scan rate =
50mVs?1.(b)Magnifiedviewofonsetregionofcyclicvoltammograms.
(c) Bar diagram of respective oxidation potential for all the catalysts at
3.00 mA cm?2.
Figure6. Variationoftheforwardandbackwardpeakcurrentratio(IF/
IB) and forward peak current (IF) vs backward peak current (IB) plots
(inset) for the electrooxidation of ethanol on Pt/C, Pd/C, Au/C, and
Pt30Pd38Au32/C.