Removal of Carbon Monoxide from Hydrogen-rich Fuels over CeO2-promoted Pt/Al2O3

doi:10.4236/aces.2013.34B002

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Advances in Chemical Engineering and Science, 2013, 3, 7-14

doi:10.4236/aces.2013.34B002 Published Online October 2013 (http://www.scirp.org/journal/aces)

Removal of Carbon Monoxide from Hydrogen-rich

Fuels over CeO2-pro mot ed P t/Al2O3

Akkarat Wongkaew1*, Pichet Limsuwan2

1Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi, Thailand

2Department of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand

Email: [email protected]

Received May, 2013

ABSTRACT

A comparative study of catalytic CO oxidation and selective CO oxidation over Pt/Al2O3 and CeO2-promoted Pt/Al2O3

catalysts has been investigated for the removal of a trace amount of CO from the reformed gas. The catalysts were pre-

pared by sol gel and incipient wetness impregnation. CO oxidation and selective CO oxidation were carried out with a

5%Pt/Al2O3 and a 5%Pt/15%CeO2/Al2O3. The presence of 15%CeO2 in the 5%Pt/Al2O3 dramatically improves the ac-

tivities to CO oxidation and selective CO oxidation at low temperature (<180℃). FTIR results indicate that CO could

react with lattice oxygen from ceria and release CO2 as a product. Low space velocity would obtain high CO conversion

at low temperatures while high space velocity would obtain high CO conversion at high temperatures. The results also

show that a 5%Pt/15%CeO2/Al2O3 can completely oxidize 1% CO at 180℃ with selectivity of 52% and space velocity

of 70,000 cm3·g-1·h-1. Under the realistic gas feed with 1%O2, this catalyst is very stable and retains its activity and se-

lectivity at 180℃ during 72 h.

Keywords: CO Oxidation; CeO2-Pt/alumina; CO Adsorption; Selective CO Oxidation; Fuel Processing

1. Introduction

The polymer-electrolyte-membrane fuel cell (PEMFC)

has been attracting significant attention in several appli-

cations including electric vehicles and residential power-

generations. This is because of its many attractive fea-

tures such as high power density, rapid start-up, and high

efficiency [1, 2]. As the PEMFCs utilize hydrogen gas as

a fuel and since hydrogen can be produced by means of a

fuel reformer followed by water gas shift reaction for

further conversion of CO to H2, carbon monoxide is al-

ways present in the hydrogen stream. Generally, catalytic

steam reforming of methanol or partial oxidation of

gasoline followed by water gas shift reaction will pro-

duce a gas stream with 40%-75% H2, 15%-20% CO2,

~10% H2O, 0-25% N2 and 0.5%-1.0% CO. This amount

of CO contained in the reformed gas is high enough to

poison the Pt anode of PEM fuel cells and in turn dra-

matically degrades the fuel cell potential and energy

conversion efficiency [3, 4]. Experimentally, it has been

found that the tolerable level of CO without harmful ef-

fects is about 10 ppm [5]. This means the 0.5%-1.0% CO

needs to be reduced to 10 ppm or less in order to increase

the use of proton exchange membrane (PEM) fuel cells

running with on-board generated hydrogen. The most

economical and straightforward technique for this pur-

pose is selective catalytic oxidation of CO in the H2-rich

reformed gas using O2 or preferential oxidation (PROX).

This method needs a suitable catalyst to enhance the CO

oxidation reaction with minimal oxidation of hydrogen

which is the desired fuel. The crucial requirement for the

PROX re- actor is a high CO conversion with high selec-

tivity. A number of catalysts have been investigated for

the PROX reaction [6-10]. Noble metals supported on

alumina such as Pt, Au, Ru and Pd, have been proposed

as ideal catalysts for PROX reaction, especially Pt/Al2O3,

Pt/ Fe2O3/ Al2O3, Pt/CeO2, Pt/CeO2_ZrO2, Pt/zeolite

[11-13]. An improvement in the selectivity at low tem-

peratures is needed. Oxidation of CO on alu-

mina-supported Pt catalysts is known to take place via

the Langmuir-Hinshel- wood mechanism. Kahlich et al.

[14] studied the kinetics of selective CO oxidation in

H2-rich gas on Pt/Al2O3 and observed that CO conversion

never reached 100% for a 0.5%Pt/Al2O3. The maximum

CO conversion was ~80% at temperatures as high as 250

℃. Other studies reported that CO conversion occurred

in the reaction temperature range of 200℃-250℃ [15]

or else needed high oxygen concentration for complete

elimination of CO, corresponding to lower selectivity.

We have previously [16] investigated the catalytic activ-

ity of 2%Pt/Al2O3, which was prepared by the sol-gel

method, in selective CO oxidation reaction under the

A. WONGKAEW, P. LIMSUWAN

excess H2 gas stream. We found that 2% Pt was well

dispersed in alumina supports. Therefore, it can selec-

tively oxidize CO down to ppm level with constant se-

lectivity and high space velocity. The performance of Pt

catalysts can be improved by modifying supports such as

adding alkali metals into supports [17] or by promoting

with other metal oxides such as Fe2O3 or CeO2 [18].

Serre, et al. [19] found that the presence of CeO2 in a

2%Pt/Al2O3 after a reductive pretreatment drastically

enhanced the activity of the catalyst to CO oxidation.

The promoting effect of ceria was attributed to the en-

hancement of the metal dispersion and the stabilization

of Al2O3 support toward thermal sintering. Moreover,

ceria can be a chemically active component, working as

an oxygen store that releases lattice oxygen in the pres-

ence of reductive gases and re- placement of the lattice

oxygen with oxygen gas when oxygen gas is present in

excess [20]. Parinyaswan et al. [21] investigated the per-

formance of Pt-Pd/CeO2 catalysts for selective CO oxi-

dation. This catalyst could maximally convert 83% CO to

CO2 with selectivity of 60% at 90℃with a gas feed con-

taining 1% CO, 1% O2, 25% CO2 and 10% H2O. This

means there would still be 1,700 ppm of CO left in the

gas feed and this gas could dramatically deplete the effi-

ciency of a PEM fuel cell in a very short time. The au-

thors suggest a multi-stage reactor to reduce CO to below

10 - 100 ppm for the use of this catalyst with PEM fuel

cells. Silva et al. [22] studied the effect of the presence of

ceria with Pt over alumina catalysts for the partial oxida-

tion of methane reaction. They reported that the addition

of ceria in alumina led to the formation of a homogenous

solid solution, which exhibited a high-oxygen storage

capacity. Brown et al. [23] compared the activities of Pt

over alumina with ceria-promoted Pt over alumina in the

production of hydrogen from methanol decomposition.

They reported that promoting with ceria had a positive

effect on activity and selectivity. Indeed, the use of ceria

coupled with alumina as a support for Pt might enhance

the catalytic selective CO oxidation of platinum over

alumina catalysts. Son et al. [24,25] investigated the per-

formance of Ce-Pt/-Al2O3 for selective oxidation of CO

in H2 for PEFCs. They found that the addition of 5% Ce

in the Pt over alumina dramatically enhanced CO con-

version and selectivity at low temperatures. The catalyst

completely converted 1%CO to CO2 at 200℃ with 50%

selectivity. Although the effect of other gases such as

CO2 and H2O was stud- ied, their gas compositions con-

taining 1% CO, 2.3% H2O, 10.09% CO2 and H2 as bal-

ance were far from a realistic gas composition containing

40%-75% H2, 0.5%-2.0% CO, 15%-20% CO2, 10% H2O

and 0-25% N2 by volume. Therefore, the activity of

promoted platinum over alu- mina with ceria for prefer-

ential CO oxidation still needs to be investigated.

In order to obtain a better understanding of the activity

of the ceria promoted platinum alumina catalyst to the

selective CO oxidation, 5%Pt/15%CeO2/Al2O3 and 5%

Pt/Al2O3 catalysts were tested for their activities to both

CO oxidation in H2 free-stream and CO oxidation in the

presence of excess H2-containing feed stream and the

obtained results were compared with others reported in

literatures. All supports in this work were prepared by sol

gel method. The CO coverage of the catalysts was also

studied using FTIR. The FTIR results were used to ex-

plain the enhancement of catalyst activity in the presence

of ceria. It should be pointed out that all gases containing

in reformed gas affect to the selectivity to CO oxidation

of the catalysts as reported in literatures. Therefore, the

effect of space velocity on the activity of the catalyst was

investigated. These results will lead us to the proper op-

erating conditions in order to obtain high selectivity and

high CO conversion of ceria promoted Pt alumina cata-

lyst.

2. Experimental

2.1. Catalyst Preparation

A cerium aluminum oxide supported platinum (Pt) cata-

lyst with 5.0 wt% Pt loading was prepared by sol gel

technique [26] and incipient wetness impregnation. Alu-

minum is protoxide; cerium (III) acetate and hydrogen

hex anchor oplatinate (IV) hydrate were obtained from

Aldrich. Preparation of supports containing cerium alu-

minum oxide began with dissolving the desired amount

of aluminum is protoxide in hot demonized water at 80℃.

After 30 min of aging with continuous stirring, nitric acid

(HNO3) was added to start the hydrolysis reaction result-

ing in a fibrillar sol. Then, the known amount of cerium

(III) acetate was incorporated into the solution at room

temperature. The solution was stirred overnight to obtain

uniformity. The obtained solution was heated to 60℃

and kept at this temperature until gelation occurred. The

gel was dried in air at 110℃ overnight, and then cal-

cined at 500℃ for 13 hours. After calcination, the re-

sulting powder was ground and sieved to obtain a

100-140 mesh powder. Incipient wetness impregnation

was used to deposit platinum into the support. With this

method, a desired amount of solution of hydrogen hexa-

chloroplatinate (IV) hydrate was added into a cerium

alumina support and then mixed together until the mix-

ture uniformly. The obtained solid was dried in air at

110oC overnight, and then calcined at 500℃ for 13

hours. The final powder was 5% Pt/15% CeO2/Al2O3. In

this work, the activity of this catalyst was compared with

activity of a 5% Pt/ Al2O3. For a 5% Pt/Al2O3 analogous

preparation techniques were used such that the Al2O3

support was prepared by sol gel and Pt was impregnated

in the support by incipient wetness impregnation. Before

testing the activities of these catalysts, the catalysts were

purged with H2 at 400℃ for 5 hrs.

A. WONGKAEW, P. LIMSUWAN 9

2.2. Characterization

The BET surface area and average pore radius of cata-

lysts were determined with adsorption-desorption iso-

therms of N2 at 77 K using a MicroMeritics ASAP 2010

instrument. Average crystalline sizes of oxides were de-

termined by Scherrer’s equation using the X-ray line

broadening from X-ray diffraction, Bruker AXS model D

8 Discover equipped with a CuKα radiation with a nickel

filter. Diffraction intensity was measured in the 2 theta

ranges between 20° and 85°, with a step of 0.02° for 8 s

per point.

2.3. Catalytic Activity

CO oxidation and CO oxidation in the H2-rich stream

was performed in a fixed-bed reactor. The reaction tem-

peratures inside the reactor were measured with a K-type

thermocouple placed on the top of the catalyst bed and

were controlled by a temperature controller (OMEGA:

CN3251). The amount of catalyst used in each run was

68 mg. The total gas flow rates of the reaction mixture

were 40 cm3·min-1 and 80 cm3·min-1, corresponding to

the space velocity of 70,000 cm3·g-1·h-1 and of 35,000

cm3·g-1·h-1, respectively. For CO oxidation, the activity

tests were conducted with a feed mixture of 1% CO, 1%

O2 and He as balance. For selective CO oxidation, a feed

mixture contained 1% CO, 0.5%-1% O2, 0-10% H2O,

0-20% CO2, 55% H2 and He as balance. A volumetric

flow controller with an accuracy of 0.5 cm3.min-1 was

used for measuring the total gas flow rate at the bypass

(for calibration purposes) and at the outlet of the reactor.

A Varian CP-4900 micro gas chromatograph (micro

GC) equipped with 2 channels (A and B) was used for

analysis of the outlet gas compositions from the reactor.

Channel A was used to detect H2, O2, CO and CH4 by a

Molsieve 5A PLOT column. Channel B was used to de-

tect CO2 by a PoraPLOT Q column. Along with GC,

FTIR was used to detect CO at low concentrations (ppm

level) in the outlet gas from the reactor. Because water

deteriorates the performance of these columns, an ice

cooled water condenser was used to remove water from

the gas streams before entering the GC and FTIR.

The CO conversion was obtained by comparing the

CO concentration in the feed measured at the bypass line

and the CO concentration in the outlet stream from the

reactor. Selectivity was defined as the ratio of oxygen

consumed by CO oxidation to the total oxygen consump-

tion (obtained by subtracting the O2 concentration at the

reactor outlet from the O2 concentration in the feed). The

amount of O2 not used in the CO oxidation reaction was

assumed to oxidize H2 in the H2 oxidation reaction. Im-

portantly, there was no methane formation observed un-

der reaction conditions performed in this study.

3. Results and Discussion

3.1. Characterization of the Catalysts

The alumina support prepared via the sol gel method

yielded a BET area of 227.8 m2/g with an average pore

size of 5.3 nm while the sol gel made 15%CeO2/Al2O3

had a BET area of 230.0 m2/g with an average pore size

of 5.2 nm. Pt crystalline sizes were estimated from the

line broadening of Pt (111) peaks. For both a 5%Pt/Al2O3

catalyst and a 5%Pt/15%CeO2/Al2O3 catalyst, no Pt (111)

peaks were observed indicating that Pt metal was well

impregnated and dispersed on the supports for both cata-

lysts. For ceria structure in a 5%Pt/15%CeO2/Al2O3 fresh

catalyst reduced under hydrogen, XRD results matched

with those for CeO2 with average CeO2 crystallite sizes

of ~6.8 nm. Dispersions measured by CO chemisorption

were 48% for the5%Pt/Al2O3 and 54% for the 5%Pt/

15%CeO2/Al2O3.

3.2. Activity Tests for CO Oxidation with

Free-H2 Gas Stream

The two catalysts were tested for their activities in CO

oxidation as a function of temperature under gas feed

containing 1%CO, 1%O2 and He as balance as seen in

Figure 1.

At 100℃ the integral conversions were 2% and 10%

over the 5%Pt/Al2O3 and the 5%Pt/15%CeO2/Al2O3 cat-

alysts. Increasing the temperature to 150℃ increased the

CO conversion from 2% to 27% for the 5%Pt/Al2O3 and

from 10% to 60% for the 5%Pt/15%CeO2/Al2O3. Further

increasing reaction temperature to 170℃ increased the

CO conversion to 70% for the 5%Pt/Al2O3 and to 98.9%

for the 5%Pt/15%CeO2/Al2O3. Finally, CO completely

converted to CO2 at 180℃ for both catalysts. Clearly,

the catalyst containing ceria showed better activity in CO

oxidation, especially at low temperatures. For the

5%Pt/Al2O3, it has been known that CO strongly chemi-

sorbs over Pt sites from room temperatures to 150℃

100

80100 120 140 160 180 200 220

Temperature,

CO conversion,

5%Pt/Al2O3

5%Pt/15%CeO2/Al2O3

Figure 1. Activities of platinum over alumina and promoted

platinum over alumina catalysts to CO oxidation. Gas

composition: 1%CO, 1%O2 and He as balance.

A. WONGKAEW, P. LIMSUWAN

[14,27]. At these temperatures, the competition between

CO molecules and O2 molecules over the active sites is

crucial. The reaction occurs when O2 molecules adsorb

and dissociate next to CO molecules. At tem- peratures

less than 150℃, Pt sites are occupied by CO molecules.

This leads to low activity of platinum over alumina cata-

lysts in the CO oxidation reaction. Increas- ing reaction

temperature above 150℃ dramatically in- creases the

rate of reaction due to desorption of CO molecules from

the active sites and leaving available sites for O2 mole-

cules to be adsorbed. The addition of ceria enhances the

rate of reaction of the catalyst. This is due to the oxygen

storage property proposed as following [28]:

22 2

23 2

CO2CeOCO*Ce O

1O*O

OCeO2CeO*

ads







where “*” stands for an adsorption site on platinum and

“ads” for an adsorbed species. From this model, the key

point of high activity of the 5%Pt/15%CeO2/Al2O3 cata-

lyst at low reaction temperatures (<150℃) results from a

capacity of ceria to switch between the two-oxidation

states Ce4+ and Ce3+. Ce3+ returns back to Ce4+ by gase-

ous oxygen molecules. This phenomenon occurs even at

room temperature. However, 170℃ is high enough for

CO molecules to desorbs from active sites and leave the

active sites available for other reactant molecules to be

adsorbed and reacted [29]. Therefore, both catalysts per-

formed comparably above 170℃. FTIR was used to

check the chemisorbed CO species. The results are shown

in Figure 2. Before the test, the catalyst pellet diluted

with KBr was purged with helium at 200℃ until catalyst

surface was clean. Then, the sample was cooled to 100℃

under a helium purge. After the temperature of the sample

reached 100℃, the sample was purged with gas stream

containing 1% CO balance with helium. The adsorption

of CO over the catalyst sample was recorded.

Figure 2(a) shows the CO adsorption over the 5%Pt/

15%CeO2/Al2O3 pellet. A strong peak at 2062 cm-1 was

observed. This peak corresponds to CO adsorbed over

Pt/CeO2/Al2O3 [30]. Other peaks at 2395-2312 cm-1 were

also observed and these peaks indicate the presence of

gas phase CO2. The intensity of the CO2 band slowly

decreased with time and finally disappeared. The forma-

tion of CO2 was due to CO oxidation reaction and the O2

reactant must have come from lattice of CeO2. This result

was in agreement with a model of oxygen transport in

Pt/ceria catalyst [31]. Next, the same experiment was

carried out with the 5%Pt/Al2O3. The results are shown

in Figure 2(b). A strong peak at 2085 cm-1 was observed.

This peak corresponds to CO adsorbed on Pt with

neighboring oxidized Pt [32]. Unlike platinum over ceria-

promoted alumina catalyst, no peak of CO2 gas phase

was observed. This means that in the absence of gas

phase O2 no CO oxidation reaction occurs with this cata-

lyst. The CO adsorbed on Pt peak for promoted Pt cata-

lyst appeared at 2062 cm-1 while that of Pt/Al2O3 ap-

peared at 2085 cm-1 [29,32]. The downward shift of CO

adsorbed wave number may be due to inducing of C-O

bond weakening for CO adsorbed on Pt by Ce [19]. The

reduction of the bond strength of adsorbed CO makes it

more reactive with an oxygen atom from ceria lattice into

CO adsorbed near Pt-CeO2 interface followed by desorp-

tion of CO2 leaving Pt site available for other gas mole-

cules. Therefore, the presence of a small amount of ceria

could enhance the activity of platinum over alumina due

to its oxygen storage property.

3.3. Effect of Ceria on the Activity of Catalysts

The effect of ceria on the activity of catalysts was shown

in Figure 3.

Figure 3(a) shows the activities of the two catalysts as

function of reaction temperature for a dry gas composi-

tion of 1%CO, 1%O2, 55%H2 and He as balance. At 110

℃, CO conversions were 46% and 60% over the 5%

Pt/Al2O3 and the 5%Pt/15%CeO2/Al2O3 catalysts. In-

creasing reaction temperatures further to 180C led to

dramatic increases in CO conversion from 46% to 90%

for the 5%Pt/Al2O3 and from 60% to 99.3% for the

5%Pt/15%CeO2/Al2O3. At 190℃, CO conversion reached

a maximum of approximately 99.97% for the 5%Pt/15%

CeO2/Al2O3 while CO conversion reached a maximum of

approximately 93.70% for the 5%Pt/Al2O3. Further in-

creasing reaction temperature to 230℃ resulted in de-

creases in CO conversion to 57% for the 5%Pt/Al2O3 and

89% for the 5%Pt/15%CeO2/Al2O3. Figure 3(b) shows

oxygen consumption as a function of reaction tempera-

ture. For the 5%Pt/Al2O3, O2 was quickly consumed by

the reactions. O2 conversion increased from 79% to

100% when reaction temperature increased from 110℃

Figure 2. FTIR study for CO adsorbe d over catalysts under

1%CO in He at 100C: (a) 5%Pt/15CeO2/Al2O3, (b) 15%

CeO2/Al2O3 and (c) 5%Pt/Al2O3.

A. WONGKAEW, P. LIMSUWAN 11

100

90110 130150 170190 210230 250

Temperature,

C O conversion, %

5Pt/Al2O3

5Pt/CeO2/Al2O3

(a)

100

90110 130150 170 190 210230 250

Temperature,

consumption, %

5Pt/Al2O3

5Pt/CeO2/Al2O3

(b)

100

90110130 150 170 190210 230 250

Temper ature,

S electivity, %

5Pt/Al2O3

5 Pt/CeO2/Al2O3

(c)

Figure 3. Comparison of activities to selective CO oxida-

tion of the 5%Pt/Al2O3 and the 5%Pt/ 15%CeO2/Al2O3 cat-

alysts as a function of temperature. Gas composition:

1%CO, 1%O2, 55%H2 and He as balance.

to 150℃. Unlike the 5%Pt/Al2O3, O2conversion for the

5%Pt/15%CeO2/Al2O3 slowly increased from 46% to

56%, 81%, 86% and 100% when reaction temperature

increased from 110℃ to 130℃, 150℃, 170℃, and 190

℃, respectively. These results led to differences in selec-

tivity for CO oxidation as shown in Figure 3(c). As

mentioned previously, no methane formation was ob-

served under these operating conditions. Consequently,

selectivity was defined as the ratio of oxygen used for

CO oxidation to total oxygen consumed by the reactions.

As shown in Figure 3(c), the selectivity’s of the two

catalysts are different. At 110℃, selectivity was 29% for

the 5%Pt/Al2O3. This means that oxygen consumed by

the reactions mostly goes to H2 oxidation. At the same

temperature, selectivity was 65% for the 5%Pt/15%

CeO2/Al2O3. This means that the presence of ceria in the

catalyst enhanced the rate of CO oxidation. Further in-

creases reaction temperature to 180℃ increased selec-

tiveity to 44% for the 5%Pt/Al2O3 but decreased selectiv-

ity to 50% for the 5%Pt/15%CeO2/Al2O3. At 230℃, se-

lectivity of both catalysts dropped to 28% for the 5%Pt/

Al2O3 and 45% for the 5%Pt/15%CeO2/Al2O3. The de-

crease of selectivity for both catalysts at high tempera-

tures is due to the competition between CO oxidation and

H2 oxidation reactions. At high temperatures, the H2

oxidation reaction occurs faster than the CO oxidation

reaction [32]. Further investigation of the 5%Pt/15%

CeO2/Al2O3 catalyst was conducted to understand its

behavior under different reaction conditions.

3.4. Space Velocity Effect

The dependence of CO conversion and selectivity on

flow rate is shown in Figures 4(a)-(c).

Space velocity was increased from 35,000 to 70,000

cm3·g-1·h-1. Considering at the same temperatures, in-

creasing the space velocity decreased the CO conversion

and decreased the O2 consumption. With 1% CO, 1% O2,

20% CO2, 10% H2O, 55% H2 and He as balance, maxi-

mum CO conversion for the low space velocity run was

100% at 150C while maximum CO conversion for the

high space velocity run was 100% at 180-190℃. Selec-

tivity did not change with space velocity at temperatures

less than 150℃. At higher temperatures, selectivity for

the low space velocity run was lower than that for the

high space velocity run. This result indicates that the use

of this catalyst depends on the reaction condition. Low

space velocity would obtain high CO conversion at low

temperatures while high space velocity would obtain

high CO conversion at high temperatures.

Activities of 1%Pt-1%CeO2 over activated carbon [33]

were compared with those of 5%Pt/15%CeO2/Al2O3 in

selective CO oxidation under realistic gas compositions.

Although the authors reported that their catalyst is very

active to CO oxidation in H2-excess stream and CO con-

version drastically increased with the presence of CO2,

combining both CO2 and H2O in the gas feed stream re-

sulted in CO conversion of 100% and selectivity to CO

oxidation of 50% at 150℃ with space velocity of 24,000

cm3·g-1·h-1. Unlike this catalyst, CO conversion of the

5% Pt/15%CeO2/Al2O3 decreased with the presence of

CO2 in the gas stream. This is due to the carbonate for-

mation blocking the available active sites. However,

combining both CO2 and H2O in the gas feed enhanced

A. WONGKAEW, P. LIMSUWAN

the activity of the catalyst especially at low temperatures

(<160℃). Under the realistic gas composition, our cata-

lyst obtained 100% CO conversion at 140-150℃ with

selectivity in CO oxidation of 50%-52% and space ve-

locity of 35,000 cm3·g-1·h-1.

100

90110130150 170 190 210230 250

Temperature,

CO conversion, %

70,000cc/g/h

35,000cc/g/h

(a)

100

90110130 150170190 210230250

Temperature,

consumption, %

70,000cc/g/h

35,000cc/g/h

(b)

100

90110130 150170 190210 230 250

Temperature,

S elec t ivit y, %

70,000cc/g

35,000cc/g

(c)

Figure 4. Dependence of CO conversion and selectivity of

CO oxidation on space velocity and temperature for a 5%Pt

/15%CeO2/Al2O3. Gas composition: 1%CO, 1%O2,

10%H2O, 20%CO2, 55%H2 and He as balance.

In our work, 5%Pt/15%CeO2/Al2O3 demonstrated ex-

cellent performance in preferential CO oxidation. This

catalyst is very stable under the realistic gas conditions

during 72 hr of use. The higher selectivity in CO oxida-

tion could obtain under the low O2 concentration.

4. Conclusions

The CO poisoning of PEMFCs is a major problem to

deplete the efficiency and energy conversion of PEMFCs.

To remove a trace amount of CO in the reformed gas is

essential. In this work, the catalytic performance of pro-

moted platinum over alumina with ceria in selective CO

oxidation in the presence of excess hydrogen has been

studied. The addition of ceria improved the catalytic ac-

tivity in oxidation reactions. It resulted from the oxygen

storage property of ceria. Space velocity also affected the

CO conversion. High CO conversion at low temperature

was obtained when low space velocity was chosen. Fi-

nally, the 5%Pt/15%CeO2/A l2O3 is an excellent catalyst

for removal of trace CO in reformed gas. It reduces 1%

CO in the realistic reformed gas to less than 10 ppm with

selectivity of 52% at 180℃ with space velocity of

70,000 cm3·g-1·h-1.

5. Acknowledgements

The authors gratefully acknowledge the financial support

from Thailand Research Fund (TRF) under the contract #

MRG 4680140.

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