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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 7

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Carbon supported PtRh catalysts for ethanol oxidation inalkaline direct ethanol fuel cell

S.Y. Shen, T.S. Zhao*, J.B. Xu

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 26 May 2010

Received in revised form

20 August 2010

Accepted 25 August 2010

Available online 26 September 2010

Keywords:

Fuel cell

Ethanol oxidation reaction (EOR)

Alkaline direct ethanol fuel cell

PtRh catalyst

The CeC bond cleavage

* Corresponding author. Tel.: þ852 2358 8647E-mail address: metzhao@ust.hk (T.S. Zh

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.08.107

a b s t r a c t

Owing to the formation of an oxametallacyclic conformation, the CeC bond cleavage is the

preferential channel for the ethanol dissociation on the Rh surface, the addition of Rh to Pt

can increase the CO2 yield during the ethanol oxidation. However, in acidic media the slow

oxidation kinetics of COads to CO2 limits the overall reaction rate. In this work, we prepare

carbon supported PtRh catalysts and compare their catalytic activities with that of Pt/C in

alkaline media. Cyclic voltammetry tests demonstrate that the Pt2Rh/C catalyst exhibits

a higher activity for the ethanol oxidation than Pt/C does. Linear sweep voltammetry tests

show that the peak current density on Pt2Rh/C is about 2.4 times of that on Pt/C. The

enhanced electro-activity can be ascribed not only to the improved CeC bond cleavage in

the presence of Rh, but also to the accelerated oxidation kinetics of COads to CO2 in alkaline

media.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction break the CeC bond at low temperatures [4e6]. Up to now,

In terms of fuel, a direct ethanol fuel cell (DEFC) is more

attractive than a direct methanol fuel cell (DMFC), because

ethanol has higher energy density than methanol

(8.0 kWh kg�1 vs. 6.1 kWh kg�1), is less toxic, and can be

produced in large quantities from agricultural products or

biomass, which will not change the natural balance of carbon

dioxide in the atmosphere in contrast to the use of fossil

fuels [1e3]. However, unlike the methanol oxidation reaction

(MOR) that can almost completely go to CO2, the ethanol

oxidation reaction (EOR) undergoes both parallel and consec-

utive oxidation reactions, resulting in more complicated

adsorbed intermediates and byproducts. Most importantly,

the complete oxidation of ethanol to CO2 requires the cleavage

of the CeC bond, which is between two atoms with little

electron affinity or ionization energy, making it difficult to

.ao).ssor T. Nejat Veziroglu. P

platinum is the best-known material for the dissociative

adsorption of small organic molecules at low temperatures;

PtRu/C and PtSn/C have been widely accepted as the most

effective catalysts for the EOR in acidicmedia [4,7]. Combining

cyclic voltammetry (CV) with in-situ Fourier transform

infrared (FTIR) spectroscopy and differential electrochemical

mass spectroscopy (DEMS), the EOR on PtRu/C and PtSn/C

in acidic media was studied extensively [8e10]. The CV

results showed the addition of Ru or Sn to Pt could increase

the overall reaction rate of the EOR, both lowering the

onset potential and increasing the peak current density;

however, the FTIR and DEMS results demonstrated that as

compared to pure Pt, the PtRu or PtSn catalysts did not help

that much in improving the selectivity for CO2 formation, and

acetaldehyde and acetic acid were dominant products during

the EOR.

ublished by Elsevier Ltd. All rights reserved.

CH2

CH2

O

Rh

a

CH3

C

Pd

OH

b

Scheme 1 e An oxametallacyclic conformation formed

during ethanol adsorbed on an Rh (111) surface (a) and

h2-acetaldehyde formed during ethanol adsorbed on a Pd

(111) surface (b) [14].

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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 712912

It has recently been reported that rhodium has a great

potential to achieve the CeC bond cleavage during the EOR

[11,12]. Owing to the formation of an oxametallacyclic confor-

mation (Scheme 1a), the CeC bond cleavage is the preferential

channel for the dissociation of ethanol on Rh surface, while

h2-acetaldehyde (Scheme 1b) is preferred on Pt or Pd surfaces

[13e15]. Tacconi et al. [16] investigated the EOR on Ir and Rh

electrodes in acidicmedia by in-situ FTIR technique, and found

that themajor product with the Ir electrodewas acetic acid, but

was CO2 with the Rh electrode. Since Rh is a far less active

catalyst for the EOR, it is usually combinedwith Pt as a catalyst.

The EOR on the PtRh electrodes in acidic media was studied by

both in-situ FTIR and DEMS techniques [17e23]. It was found

that the addition of Rh to Pt could indeed enhance the CO2 yield

during the EOR, but the overall rate of the EOR on the PtRh

catalyst was lower than that on the pure Pt catalyst. There are

two possible reasons that are responsible for the lower rate of

the EOR on the PtRh catalyst in acidic media. First, Rh has less

efficientdehydrogenationability thanPtdoes, leading toa lower

rate of the CeC bond cleavage to form COads, thus lowering the

overall reaction rate. Theother reason is related to theveryhigh

barrier for the COads oxidation caused by the strong COeRh

bonding. It should be recognized that the kinetics of the COads

oxidation can be accelerated at high pH values and a change

from acidic to alkaline media may also facilitate the CeC bond

cleavage during the EOR. In line with this idea, in this work we

prepared carbon supported PtRh catalysts by the microwave-

polyolmethod [24,25], and investigated their catalytic activities

for the EOR in alkaline media. The obtained PtRh/C catalysts

withdifferentPt/RhatomicratioswerecharacterizedwithX-ray

diffraction (XRD), transmission electron microscopy (TEM) and

X-ray photoelectron spectroscopy (XPS). The EOR on the PtRh/C

catalysts in alkalinemediawere examined by the CV and linear

sweep voltammetry (LSV) methods.

2. Experimental

2.1. Synthesis of the PtRh/C catalysts

All the chemicals usedwere of analytical grade. Chloroplatinic

acid hydrate (H2PtCl6$xH2O) and rhodium chloride hydrate

(RhCl3$xH2O) were purchased from Aldrich. Ethylene glycol

(EG), potassiumhydroxide (KOH), and ethanol (CH3CH2OH) (all

from Merck KGaA) were used as received. Vulcan XC-72

carbon (particle size 20e40 nm) was purchased from E-TEK,

while 5 wt.% Polytetrafluoroethylene (PTFE) emulsion was

received from Dupont. Carbon supported PtRh catalysts were

prepared by the microwave-polyol method. The metal

precursors of H2PtCl6$xH2O and RhCl3$xH2O with different

atomic ratios were first completely dissolved in EG/water

(3/1, v/v), carbon powders were then suspended into the

resulting solution under vigorous stirring. After a homoge-

neous suspension was formed, the resulting mixtures were

heated in a household microwave oven (Output: 800 W;

Frequency: 2450 MHz) for 180 s. The so-obtained precipitate

was collected by filtration, washed several times with ethanol

and deionized (DI) water, respectively, and dried at 70 �C in an

oven. For comparison, carbon supported Pt or Rh catalysts

were also prepared with the same method, and within all the

catalysts, a 20 wt.%metal (Pt and Rh) loading was guaranteed.

2.2. Catalyst characterizations

The XRD patterns of the Pt/C, Rh/C and PtRh/C catalysts with

different Pt/Rh atomic ratios were obtained with a Philips

powder diffraction system (model PW 1830) using a Cu Ka

source operating at 40 keV at a scan rate of 0.025�s�1. The TEM

images were obtained by using a high-resolution JEOL 2010F

TEM system operating with a LaB6 filament at 200 kV. The XPS

characterization was carried out with a Physical Electronics

PHI 5600 multi-technique system using Al monochromatic X-

ray at a power of 350 W. The survey and regional spectra were

obtained by passing energy of 187.85 and 23.5 eV, respectively.

2.3. Electrochemical characterizations

Both the CV and LSV tests were carried out using a potentio-

stat (EG&G Princeton, model 273A) in a conventional three-

electrode cell, in which a glass carbon electrode (GCE) with an

area of 0.1256 cm2 was used as the underlying support of the

working electrode, a platinum foil as the counter electrode,

and Hg/HgO/KOH (1.0 mol L�1) (MMO, 0.098 V vs. SHE) as the

reference electrode, which was connected to the cell through

a Luggin capillary. The GCE was modified by depositing

a catalyst layer onto it and served as the working electrode.

The catalyst ink was prepared by ultrasonically dispersing

10 mg of 20 wt.% Pt/C, Rh/C or PtRh/C catalysts in 1.9 mL of

ethanol, to which 0.1 mL of 5 wt.% PTFE emulsion was added.

After 30 min, a homogeneous solution was obtained and

a quantity of 12 mL of the ink was pipetted out on top of the

GCE and dried in air to yield a metal loading of 96 mg cm�2.

Solutions were prepared from analytical grade reagents and

DI water. All the CV and LSV experiments were done at room

temperature and in 1.0 M KOH solution containing 1.0 M

ethanol, which was deaerated by bubbling nitrogen (99.9%) for

30 min in advance. The CV tests were performed between the

potential ranges of �0.926e0.274 V at a scan rate of 50 mV s�1,

while 1 mV s�1 for the LSV tests. The potentials in this paper

all refer to theMMO, and the current densities were calculated

according to the geometric area of the GCE (0.1256 cm2).

Table 1 e Structural characteristics of the Pt/C, Rh/C andPtRh/C catalysts with different Pt/Rh ratios.

Nominalcomposition

Surfacecomposition by

XPS

d111space(�A)

Particle size(nm) by XRD

Pt/C 2.277 1.7

Pt3Rh/C Pt4.4Rh/C 2.271 1.7

Pt2Rh/C Pt2.8Rh/C 2.252 1.8

PtRh/C Pt1.5Rh/C 2.231 2.0

PtRh2/C Pt0.8Rh/C 2.223 2.4

Rh/C 2.199 2.6

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3. Results and discussion

3.1. Physicochemical characterizations

Fig. 1 shows the XRD diffraction patterns of the PtRh/C cata-

lysts, and the diffraction patterns of both Pt/C and Rh/C are

also given for comparison. For all these samples, the first

diffraction peak located at the 2q value of about 25� is referredto the graphite (002) facet of the carbon powder support, and

the other four diffraction peaks are characteristics of the face-

centered cubic (fcc) crystalline structure, corresponding to the

(111), (200), (220) and (311) planes, respectively. It can be

observed that the four diffraction peaks of the PtRh/C cata-

lysts are located at higher 2q values with respect to the same

reflection of Pt/C, while at lower 2q values compared to that of

Rh/C; and the diffraction peaks of the PtRh/C catalysts shift to

higher 2q values with an increase in Rh content, which can be

indexed to the incorporation of a lower d space crystal struc-

ture of Rh (d111 ¼ 2.20) compared to that of Pt (d111 ¼ 2.265).

Such evidence indicates a lattice constriction due to the

incorporation of smaller Rh atoms into the Pt fcc structure,

and suggests the alloy formation between Pt and Rh during

the synthesis of the PtRh/C catalysts [18,19]. The average size

of themetal particles is calculated based on the broadening of

the (220) diffraction peaks according to Scherrer’s equation

[26]:

d ¼ 0:9lB2qcos qmax

(1)

where l represents the wavelength of the X-ray (1.54056 �A), q

is the angle of the maximum peak, and B2q is the width of the

peak at the half height. The particle size and d111 space

parameters of all the samples are presented in Table 1.

The typical TEM images of the Pt/C, Rh/C and Pt2Rh/C

samples are, respectively, shown in Figs. 2aec. As can be seen,

the metal particles on all the three catalysts exhibit a spher-

ical-like shape and are well dispersed on the carbon powder

20 30 40 50 60 70 80 90

graphite (002)

Rh (200)

Rh (200)Rh (220) Rh (311)

graphite (002)Pt (311)Pt (220)

Pt (200)Pt (111)

Rh/C

PtRh2/C

PtRh/C

Pt2Rh/C

Pt3Rh/C

).u.a(ytisnetnI

2θ (degree)

Pt/C

Fig. 1 e XRD diffraction patterns of the Pt/C, Rh/C and PtRh/C

catalysts with different Pt/Rh atomic ratios.

support. The metal particles size distributions of the Pt/C,

Rh/C, and Pt2Rh/C catalystswere, respectively, evaluated from

an ensemble of 100 particles. Both the Pt/C and Pt2Rh/C cata-

lysts show the same metal particle size distribution ranging

from 1.2 nm to 4 nm, and the average metal particle sizes of

Pt/C and Pt2Rh/C are, respectively, 2.0 nm and 2.1 nm; while

Rh/C has a different particle size distribution, which is from

1.6 nm to 4.4 nm, and a larger average metal particle size of

2.5 nm. Fig. 2d shows the high-resolution TEM (HRTEM) image

of the Pt2Rh/C catalyst. It can be seen that the lattice fringes

can be observed across the entire image, indicating that the

prepared PtRh nanoparticles are entirely crystalline. The

d space of one randomly chosen particle, as denoted in Fig. 2d,

is 2.250 �A, very close to the value of 2.252 �A, which was pre-

dicted from the XRD data via Bragg law.

The XPS test was employed to analyze the surface

composition and the oxidation state of themetals on the PtRh/

C catalysts. The surface composition analyses based on the

intensities of XPS peaks for the PtRh/C catalysts are summa-

rized in Table 1. The Pt/Rh atomic ratios obtained by XPS show

some deviation from the nominal ratios in the precursors,

which can be ascribed to the fact that the reduction potential

of Rh3þ/Rh (E0 e 0.43 V) is much lower than that of Pt4þ/Pt(E0e0.74 V) in the presence of Cl� ions, and then the reduction

efficiency of Pt4þ to Pt is higher than that of Rh3þ to Rh during

the simultaneous reduction process, just like the case of the

PtRu/C catalyst [18,27,28]. The XPS spectra of all the Pt-con-

taining samples in the Pt4f region are shown in Fig. 3a, and the

normalized spectra are shown in Fig. 3b. According to Fig. 3,

the shape of the Pt4f XPS spectra are the same for the Pt/C and

PtRh/C catalysts, demonstrating the same distribution of

different Pt chemical states on them. For all the Pt-containing

samples, the Pt4f spectra show a doublet consisting of a high

energy band (Pt4f5/2) at about 74.8 eV and a low energy band

(Pt4f7/2) at about 71.5 eV, respectively, and this unambigously

indicates the existence of metallic state Pt.

3.2. Electrochemical properties

Fig. 4 compares the stabilized CVs of the EOR on the Pt/C, Rh/C

and PtRh/C catalysts in 1.0 M KOH containing 1.0 M ethanol.

For comparison, four parameters, including the onset poten-

tial of ethanol oxidation (Eonset), the anodic peak current

density in the forward scan ( jpeak), the potential correspond-

ing to jpeak (Epeak), and the ratio of the forward anodic peak

Fig. 2 e TEM images of Pt/C (a), Rh/C (b), Pt2Rh/C (c) and HRTEM image of Pt2Rh/C (d).

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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 712914

current density ( jf) to the backward anodic peak current

density ( jb) ( jf/jb), were extracted from the CVs and are shown

in Table 2. The following observations to the PtRh/C samples

can be made with an increase in Rh content: the Eonset first

decreases and then increases; the jpeak first increases and then

decreases; the Epeak monotonously decreases; and the ratio of

jf/jb monotonously increases. Among all the PtRh/C catalysts,

66 68 70 72 74 76 78 80 82

0

1000

2000

3000

4000

5000

6000

Pt4f5/2 Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/Cs/c

Binding Energy (eV)

Pt4f7/2

a

Fig. 3 e Pt4f XPS spectra of the Pt/C and PtRh/C catalyst with diffe

the Pt2Rh/C catalyst shows the lowest Eonset, the highest jpeak,

a relative lower Epeak, and a relative higher jf/jb ratio toward

the EOR in alkalinemedia. The Eonset on the Pt2Rh/C catalyst is

�0.55 V, which is 50 mV lower than that on Pt/C; the Epeak on

Pt2Rh/C is �0.08 V, 20 mV lower than that on Pt/C; the jpeak on

Pt2Rh/C is 0.172 A cm�2, 0.027 A cm�2 higher than that on Pt/C.

Most attractively, the ratio of jf/jb on Pt2Rh/C is 1.9, twice as

70 72 74 760.0

0.2

0.4

0.6

0.8

1.0 Pt4f5/2

Pt4f7/2

ytisn etnidezila

m roN

Binding Energy (eV)

Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C

b

rent Pt/Rh atomic ratios (a) and their normalized spectra (b).

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20ytisnedtnerru

C(

mcA

2-)

Potential (V) vs. MMO

Pt/C Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C

Fig. 4 e CVs of the EOR on the Pt/C, Rh/C and PtRh/C

catalysts in 1.0 M KOH D 1.0 M ethanol.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24 5mV/s 10mV/s 25mV/s 50mV/s 100mV/s 200mV/s

ytisnedtnerruC

(mc

A2-)

Potential (V) vs. MMO

Fig. 5 e CVs of the EOR on the Pt2Rh/C catalyst in 1.0 M

KOH D 1.0 M ethanol at different scan rates, and with the

insert: peak current density vs. square root of scan rate.

0.05

0.06

0.07

)2

Pt/C

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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 7 12915

large as that on Pt/C. Usually, the anodic peak in the backward

scan represents the removal of the incompletely oxidized

species formed in the forward scan, and a high ratio of jf/jb can

be an indication of excellent oxidation of ethanol to CO2 and

less accumulation of the carbonaceous residues on the cata-

lyst [29,30]. The CVs of the EOR on the Pt2Rh/C catalyst in 1.0 M

KOH containing 1.0 M ethanol at different scan rates is shown

in Fig. 5, and the insert shows the relationship between the

peak current density and the square root of scan rate. As can

be seen, the peak current densities are linearly proportional to

the square root of the scan rates, suggesting that the EOR on

the Pt2Rh/C catalyst in alkaline media may be controlled by

a diffusion process [31].

The ethanol oxidation kinetics on the Pt/C, Rh/C and PtRh/

C catalysts in alkaline media was examined under the quasi-

steady-state conditions. Fig. 6 shows the LSVs of the EOR on

the Pt/C, Rh/C and PtRh/C catalysts in 1.0 M KOH containing

1.0 M ethanol. The sweep rate is 1mV s�1. As can be seen from

Fig. 6, compared to pure Pt, the addition of Rh to Pt can

significantly improve the ethanol oxidation kinetics in alka-

line media. Four parameters, including the onset potential of

ethanol oxidation (E�onset), the peak current density ( j�peak),

the current density at �0.4 V ( j at �0.4 V), and the current

density at �0.2 V ( j at �0.2 V) were extracted from the LSVs

and are shown in Table 3. For all the PtRh/C catalysts, the

Table 2 e Onset potentials, peak potentials, peak currentdensities and jf/jb ratios of the Pt/C, Rh/C and PtRh/Ccatalysts with different Pt/Rh ratios during the CV tests.

Nominalcomposition

Eonset (V) Epeak (V) jpeak(A cm�2)

jf/jb ratio

Pt/C �0.50 �0.060 0.145 0.9

Pt3Rh/C �0.54 �0.075 0.154 1.6

Pt2Rh/C �0.55 �0.080 0.172 1.9

PtRh/C �0.54 �0.130 0.145 2.5

PtRh2/C �0.50 �0.160 0.105 3.2

Rh/C �0.52 �0.280 0.019 1.0

Pt2Rh/C catalyst shows the highest ethanol oxidation kinetics

in alkalinemedia; the E�onset on the Pt2Rh/C catalyst is�0.53 V,

which is about 40 mV lower than that on Pt/C; the j�peak on

Pt2Rh/C is 0.068 A cm�2, about 2.4 times of that on Pt/C. Fig. 7

shows the Tafel plots of the EOR on the Pt/C, Rh/C and Pt2Rh/C

catalysts at lower overpotentials, calculated from the quasi-

steady-state curves in Fig. 6. Being determined from the linear

regions, the Tafel slopes at lower overpotentials for the the Pt/

C, Rh/C and Pt2Rh/C catalysts are 112 mV dec�1, 77 mV dec�1

and 102 mV dec�1, respectively. The slope for the Pt/C catalyst

is close to 120 mV dec�1 as reported elsewhere [32,33], and the

different slope values for Pt2Rh/C and Rh/C may indicate

a different reaction mechanism caused by the different

adsorption types of ethanol on Pt and Rh [13e15]. By extrap-

olating the linear regions of the Tafel plots, the exchange

current density on these catalysts can be obtained. The

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

0.00

0.01

0.02

0.03

0.04

ytisnedtnerruC

(mc

A-

Potential (V) vs. MMO

Pt3Rh/C Pt2Rh/C PtRh/C PtRh2/C Rh/C

Fig. 6 e LSVs of the EOR on the Pt/C, Rh/C and PtRh/C

catalysts in 1.0 M KOH D 1.0 M ethanol.

Table 3 e Onset potentials, peak current densities andcurrent densities at L0.4 V and L0.2 V of the Pt/C, Rh/Cand PtRh/C catalystswith different Pt/Rh ratios during theLSV tests.

Nominalcomposition

E�onset

(V)j�peak

(A cm�2)j at �0.4 V(A cm�2)

j at �0.2 V(A cm�2)

Pt/C �0.49 0.029 0.006 0.025

Pt3Rh/C �0.52 0.060 0.024 0.057

Pt2Rh/C �0.53 0.068 0.026 0.065

PtRh/C �0.52 0.064 0.024 0.058

PtRh2/C �0.51 0.062 0.019 0.057

Rh/C �0.48 0.023 0.010 0.008

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 5 ( 2 0 1 0 ) 1 2 9 1 1e1 2 9 1 712916

exchange current density on the Pt2Rh/C catalyst is

1.5 � 10�6 A cm�2, which is higher than that on both Pt/C and

Rh/C (8.0 � 10�7 A cm�2 for Pt/C and 2.0 � 10�8 A cm�2 for Rh/

C), further indicating that the Pt2Rh/C catalyst has a higher

catalytic activity towards the EOR in alkaline media than both

Pt/C and Rh/C.

According to XRD and TEM, the Pt/C and Pt2Rh/C catalysts

have the same metal particle size distribution and the differ-

ence between their average metal particle sizes is rather

small. Hence, the catalytic activity difference between Pt/C

and Pt2Rh/C due to the particle size contribution can be

neglected. In Fig. 3b, it can be observed that the shift in the Pt4f

binding energies for all the PtRh/C samples relative to that of

Pt/C is less than 0.1 eV, negligible small; this fact suggests that

the change in the electronic structure of Pt due to the addition

of Rh contributes little to the higher catalytic activity of the

Pt2Rh/C catalyst. As shown in Table 3, the onset potential of

ethanol oxidation on the Pt2Rh/C catalyst is �0.53 V, only

40 mV lower than that on Pt/C; besides, extended investiga-

tions indicated that Rh is more difficult for water dissociation

than Pt [34]. It can be assumed that the bi-functional mecha-

nism role of the Pt2Rh/C catalyst plays only a small part for its

higher catalytic activity toward the EOR in alkalinemedia [18].

It is confessedly proved that the addition of Rh to Pt will

increase the CO2 yield during the EOR; however, in an acidic

-3.2 -3.0 -2.8 -2.6 -2.4 -2.2 -2.0 -1.8 -1.6 -1.40.34

0.36

0.38

0.40

0.42

0.44

0.46

112 mV dec-1

77 mV dec-1

102 mV dec-1

log id (A cm-2)

)V(laitnetoprevO

Pt/C Pt2Rh/C Rh/C

Fig. 7 e Tafel plots of the EOR on the Pt/C, Rh/C and Pt2Rh/C

catalysts in 1.0 M KOH D 1.0 M ethanol.

medium the oxidation kinetics of COads to CO2 is a rate-limit

factor, still limiting the overall reaction rate [17e23]. In our

work, the EOR on the PtRh/C catalysts were studied in an

alkaline medium, and the overall reaction rate was indeed

increased due to the addition of Rh; it is suggested that not

only the CeC bond cleavage rate can be improved in alkaline

media but also the poisoning effect of both carbonyl species

and COads will be much weaker in alkaline media than in

acidic media [35]. Hence, we conclude that the enhanced

electro-catalytic activity of the Pt2Rh/C catalyst can be

ascribed not only to the improvement of the CeC bond

cleavage in the presence of Rh, but also to the accelerated

oxidation kinetics of COads to CO2 in alkaline media.

4. Conclusions

In this work, carbon supported PtRh catalysts were synthe-

sized by the microwave-polyol method and investigated for

the EOR in alkaline media. The CV results demonstrated that

in alkaline media the Pt2Rh/C catalyst had a higher catalytic

activity, in terms of both the onset potential and the peak

current density, for the EOR than Pt/C did. The LSV results

showed that the peak current density of the EOR on Pt2Rh/C

was 0.068 A cm�2, about 2.4 times of that on Pt/C and 3 time on

Rh/C. According to the Tafel plots analyses, the exchange

current density on Pt2Rh/Cwas 1.5� 10�6 A cm�2 and the Tafel

slope on Pt2Rh/C was 102 mV dec�1. The enhanced electro-

catalytic activity of the Pt2Rh/C catalyst can be ascribed not

only to the improvement of the CeC bond cleavage in the

presence of Rh, but also to the accelerated oxidation kinetics

of COads to CO2 in an alkaline medium.

Acknowledgements

The work described in this paper was fully supported by

a grant from the Research Grants Council of the Hong Kong

Special Administrative Region, China (Project No. 623008).

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