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SiO2/Ni and CeO2/Ni catalysts for single-stagewater gas shift reaction

Sung Ho Kim a, Ji Hye Chung a, Yun Tae Kim a, Jonghee Han b, Sung Pil Yoon b,Suk Woo Nam b, Tae-Hoon Lim b, Ho-In Lee a,*a School of Chemical and Biological Engineering & Research Center for Energy Conversion and Storage, Seoul National University, 599

Gwanangno, Gwanak-gu, Seoul 151-744, Republic of Koreab Center for Fuel Cell Research, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul 136-791,

Republic of Korea

a r t i c l e i n f o

Article history:

Received 22 July 2009

Accepted 28 September 2009

Available online 17 October 2009

Keywords:

Fuel processor

WGS

Methanation

Ni bulk catalyst

CO removal

* Corresponding author. Tel.: þ82 2 880 7072E-mail address: hilee@snu.ac.kr (H.-I. Lee

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.09.091

a b s t r a c t

SiO2- and CeO2-promoted Ni catalysts, as an alternative to traditional water gas shift (WGS)

catalysts, were studied through various characterization methods. CO removal was

enhanced by CeO2 promotion on Ni, but CeO2 affected methanation rather than the WGS

reaction. Mobile surface oxygen, a typical property of CeO2, affected the Ni surface

properties and weakened the bond strength between C and O atoms of carbon monoxide.

On the contrary, in the case of SiO2-promoted Ni catalyst, the SiO2 supplied a hydroxyl

group (–OH) to the Ni catalyst resulting in greater selectivity toward the WGS reaction than

to methanation. Selectivity to the WGS reaction on the CeO2-promoted Ni catalyst was

lower than that on Ni bulk catalyst, while the SiO2-promoted Ni catalyst showed higher

selectivity to WGS than both the Ni bulk and CeO2-promoted Ni catalysts.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction respectively. However, the conventional double-stage WGS

The water gas shift (WGS) reaction is one of the oldest and

most important heterogeneous reactions, is used in several

industries, and is represented by: COþH2O / CO2þH2.

Recently, interest has been concentrated on H2 generation

from the reformate gas produced by fuel reforming within fuel

processing systems [1]. In such systems, WGS processes have

been used for CO clean up and additional H2 production. The

WGS is a moderately exothermic reaction, which limits its

equilibrium conversion at high reaction temperatures. A WGS

process is composed of high temperature WGS (HTS) [2] and

low temperature WGS (LTS) [3]. Conventionally, iron–

chromium oxide and copper–zinc oxide catalysts have been

used for HTS at 300w450 �C and for LTS at 180w270 �C,

; fax: þ82 2 888 1604.).sor T. Nejat Veziroglu. Pu

process is disadvantageous due to the large volume required

in the fuel processing system [4].

An Ni bulk catalyst was suggested as a substitute for

conventional WGS catalysts in order to overcome the size-

related weakness of the WGS process, which can occupy ca.

70% of the reactor volume in a fuel processing system, and

previously we developed an Ni bulk catalyst for use in a single-

stage WGS reaction [5]. The Ni bulk catalyst was able to reduce

high concentrations (15%) of CO to below 1% at 380w440 �C

through a CO removal reaction. However, methanation could

not be avoided during the CO removal reaction, resulting in

the consumption of H2; thereby reducing fuel processing

efficiency. We found that the Ni bulk catalyst while active in,

is not selective for, the WGS reaction. Even though the Ni bulk

blished by Elsevier Ltd. All rights reserved.

340 360 380 400 420 440 460 480 5000

20

40

60

80

100

Sele

ctiv

ity

to W

GS

(%)

320 340 360 380 400 420 440 460 480 5000

20

40

60

80

100C

O c

onve

rsio

n (%

)

Reaction temperature (°°C)

Reaction temperature (°°C)

NiCeO2/Ni

SiO2/Ni

NiCeO2/Ni

SiO2/Ni

a

b

Fig. 1 – (a) CO conversion and (b) selectivity to WGS reaction

over Ni, CeO2/Ni, and SiO2/Ni.

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 e n e r g y 3 5 ( 2 0 1 0 ) 3 1 3 6 – 3 1 4 0 3137

catalyst affected methanation, the single-stage CO removal

reaction was still deemed advantageous to the fuel processing

system [5].

CeO2 and SiO2 are well-known as catalyst supports and

promoters. The typical properties of CeO2 and SiO2 that have

been effective in catalyst reactions are mobile surface oxygen

[6] and hydroxyl groups [7] on their surfaces, respectively. In

this study, we used CeO2 and SiO2 promoters on the Ni bulk

catalyst and studied the resultant catalyst characteristics and

CO removals in a single-stage WGS reaction, using various

characterization methods. In addition, we investigated how

the activity and catalytic properties of Ni were changed by the

promoters.

Fig. 2 – SEM images of the samples (a: Ni, b: CeO2/N

2. Experimental

The Ni bulk catalyst was prepared by a tape-casting method and

wascut toadoughnut-typedisk shapefor catalyticactivity tests,

details of which are described elsewhere [5,8]. Commercial SiO2

(15 wt% SiO2 in H2O, Alfa Aesar) and CeO2 (20 wt% CeO2 in H2O,

Alfa Aesar) sols were introduced to prepare 2.9 wt% SiO2/Ni and

3 wt% CeO2/Ni catalysts by dip-coating method. The standard

reactant gas, which simulated the gas reformed from an auto-

thermal methane reforming process, was 15% CO, 40% H2, and

10% CO2 with N2 asthebalance (dry%).TheWGS activity test was

carried out at the temperature region of 330w500 �C. The reac-

tant gas flow rate was 400 cm3 min�1 and H2O was supplied with

an evaporator (steam: CO¼ 5:1, 77.5 �C). Analysis of the product

mixture of CO, CH4, CO2, and O2 gases was conducted using a CO

analyzer (Ultramat23, Siemens) and a microgas chromatograph

(Agilent 3000A, Agilent Technologies) with two thermal

conductivity detectors (TCDs) composed of a ø 0.32 mm� 10 m

column of MolSieve 5A (Agilent Technologies) to separate H2,

CO, N2, and CH4, and a ø 0.32 mm� 3 m pre-column of PLOT U

(Agilent Technologies) to separate CO2 using a 1.0 mL backflush

injector. The carrier gases for MolSieve 5A and PLOT U were Ar

and He, respectively. For all conditions, N2 was used as the

balance gas to calculate CO conversion and product amount.

The temperature-programmed reduction (TPR) with H2 and

temperature-programmed oxidation (TPO) with O2 were

performed by an automated catalyst characterization system

(BEL CAT, BEL Japan, Inc.). The experiments were carried out at

aheatingrate of 10 �C min�1.The reactive gaswasH2 (10 vol%) in

Ar and O2 (5 vol%) in He. The flow rate was fixed at 30 cm3 min�1

and the total reactive gas consumption was measured by TCD.

The surface morphology of the catalyst was observed with

scanning electron microscopy (SEM; FEI XL-30 FEG, Philips FEI

Company) and its crystal structure was measured by X-ray

diffraction (XRD; D/MAX-IIIA, Rigaku) with Cu-Ka radiation. All

catalysts were pretreated by oxidation treatment process at

600 �C for 2 h with air followed by subsequent reduction treat-

ment process at 600 �C for 1 h with simulated reactant gas and

steam before single-stage WGS reaction [5].

The CO conversion (XCO, dry%) and the selectivity to WGS

(SWGS) were calculated using the following formulae:

XCO ¼½CO�in�½CO�out

½CO�in� 100ð%Þ

SWGS ¼½CO2�out�½CO2�in

½CO2�out�½CO2�inþ½CH4�out

� 100ð%Þ

i, and c: SiO2/Ni) after an oxidation treatment.

20 40 60 802 theta

SiO2 / Ni

CeO2 / Ni

Ni

Fig. 3 – XRD patterns of the samples after a single-stage

WGS reaction.

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 e n e r g y 3 5 ( 2 0 1 0 ) 3 1 3 6 – 3 1 4 03138

We could exclude Boudouard reaction (2CO 4 CO2þC) as

a possible side reaction due to no detection of carbon in the

catalyst after activity test. Therefore, only methanation was

considered as considerable side reaction and in calculating

the SWGS.

3. Results and discussion

Fig. 1a and 1b show the XCO and SWGS during the tested single-

stageWGSreaction, respectively. Fig.1ashowsthat the XCO of the

CeO2/Ni catalyst reached ca. 94% at 350 �C and then decreased

slightly with increasing reaction temperature. Using the Ni bulk

catalyst, the XCO was ca. 84% at 400 �C with a similar trend to

lower XCO at increased reaction temperature. The CeO2/Ni cata-

lyst not only produced a higher XCO than the Ni bulk catalyst, but

also expanded the reaction temperature window to include

a lower reaction temperature, 350 �C. When SiO2 promoter was

added to the Ni bulk catalyst, the XCO decreased rapidly at 400 �C

and, overall, was lower than that of the Ni bulk catalyst (Fig. 1a).

0 200 400 600 800

Inte

nsit

y (a

.u.)

380°C

395°C

430°C

Temperature (°°C)

Ni

Ni

SiO2/Ni

SiO2/Ni

CeO2/Ni

CeO2/Ni

Fig. 4 – TPR profiles of the samples after an oxidation

treatment.

In Fig. 1b, the SWGS values among the catalysts show which

reaction is dominant during CO removal: i.e., WGS producing

H2 or methanation consuming H2. Fig. 1b shows that the

methanation reaction as a side reaction for WGS was consid-

erably accompanied over the Ni and CeO2/Ni catalysts. The Ni

bulk catalyst produced different SWGS values at different

temperatures, ca. 97% at 400 �C and ca. 73% at 500 �C, because

the WGS reaction is a slightly exothermic and thermodynam-

ically preferential reaction at lower temperatures [9]. Using the

CeO2/Ni catalyst, the SWGS did not change markedly under all

reaction temperatures. That result suggests that the lower and

more consistent values are due to increased methanation

resulting from CeO2 promotion. On the other hand, the SiO2/Ni

catalyst produced the highest SWGS value among the catalysts

and maintained the higher SWGS values constantly, even at

high reaction temperature (500 �C).

The SEM images in Fig. 2 show the surface morphologies of

the Ni, CeO2/Ni, and SiO2/Ni catalysts after oxidation treatment

at 600 �C for 2 h with air. While the Ni bulk catalyst had large

intermolecular pores, the CeO2/Ni catalyst pores were smaller

ones, presumably due to blockage by CeO2 particles. In the SiO2/

Ni catalyst examined, the Ni surface was covered by SiO2 in

a film-like coating. This coating by SiO2 might interrupt the

surface reorganization of Ni, resulting in a catalyticactivation [5].

The XRD experiment was carried out to ascertain changes in

the properties of the catalysts that might provide evidence

supporting the differences in SWGS shown in Fig. 1b. Fig. 3

shows the typical diffraction peaks of Ni at 44.50, 51.84, and

76.36o in 2q. In the case of CeO2, peaks could be observed at

28.55, 33.08, 47.48, 56.34, 59.08, 69.40, 76.70, and 79.07o in 2q,

while for SiO2, peaks appeared at 20.51, 36.34, and 49.45o. The

amounts of CeO2 and SiO2 loaded to Ni were 3 wt% and 2.9 wt%,

respectively. Because 3 wt% CeO2 was too small to present

a characteristic peak in XRD and the intensity of Ni’s charac-

teristic peak was very high, no distinct CeO2 peak appeared. In

the SiO2/Ni catalyst, no characteristic SiO2 peak was observed

because SiO2 was amorphous (Fig. 3). The XRD results of Fig. 3,

along with the SEM observation that CeO2 and SiO2 were only

on the surface of Ni (Fig. 2), suggest that CeO2 and SiO2 did not

affect the lattice parameters of Ni. This indicates that only the

surface properties of CeO2 and SiO2 affected WGS selectivity

(Fig. 1b) in the single-stage WGS reaction.

The reduction behaviors of the SiO2/Ni and CeO2/Ni cata-

lysts were investigated by H2-TPR and the profiles are shown

in Fig. 4. The reduction of NiO to Ni has been reported to follow

NiO / Nidþ/ Ni0 [10]. In Fig. 4, there are two peaks for NiO

reduction, corresponding to the stepwise reduction at ca. 438

and 550 �C. The CeO2 promotion changed the reducibility of

the Ni bulk catalyst, as has been reported previously in

Ref. [11]. In Fig. 4, using the CeO2-promoted Ni catalyst, the

reduction temperature of NiO shifted from ca. 438 to ca. 400 �C.

This shift was presumed to be the result of a spillover of

surface oxygen of CeO2 as previous research indicated that

such a reduction peak shift was related to the mobile and

active surface oxygen on the CeO2 surface affecting the

surface oxygen of NiO [12]. The second reduction peak of NiO

in the CeO2-promoted Ni catalyst was assumed to be related to

the strong interaction between CeO2 and NiO [11]. In the SiO2-

promoted Ni catalyst, the first NiO reduction occurred at

380 �C, presumably related to the surface –OH group of SiO2

0 200 400 600 8000 200 400 600 800

Inte

nsit

y (a

.u.)

Inte

nsit

y (a

.u.)

Inte

nsit

y (a

.u.)

Temperature (°°C) Temperature (°°C)0 200 400 600 800

Temperature (°°C)

Ni

SiO2/Ni

CeO2/Ni

Ni

SiO2/Ni

CeO2/Ni

Ni

SiO2/Ni

CeO2/Ni

a b c

Fig. 5 – (a) TPO, (b) TPR, and (c) TPO profiles of the samples. The experiment was conducted consecutively in the order of (a)

TPO / (b) TPR / (c) TPO for each sample.

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 e n e r g y 3 5 ( 2 0 1 0 ) 3 1 3 6 – 3 1 4 0 3139

[7]. Two important points in the H2-TPR results for the SiO2/Ni

catalyst are the decrease in the reduction amount and the

absence of the second reduction peak (Fig. 4). We suggest that

these differences resulted from a decrease in the oxidation of

Ni, even though the SiO2/Ni catalyst is oxidized under the

same conditions as the other catalysts.

H2-TPR experiment was performed to investigate the

reason why WGS selectivity changed when promoting SiO2 and

CeO2 onto Ni (see Fig. 1b) and to evaluate the interaction

between Ni and the two promoters. Based on the mechanism of

the methanation reaction [13], methanation could be improved

by weakening the bond strength between C and O of the CO

over the catalysts. This is because the C atom in a weakened

C–O bond reacts better with H2 than with H2O. It has been

reported that mobile surface oxygen on the surface of a catalyst

might attenuate the C–O bond strength [12]. On the other hand,

based on the mechanism of WGS reaction [14], the formation of

–OH groups is a rate-determining step. This suggests that an

abundance of –OH groups on SiO2 could promote the WGS

reaction resulting in the increase in the WGS selectivity shown

in Fig. 1b. As reported in our previous study [5], if NiO was

completely reduced to Ni, and if the promoters (SiO2 and CeO2)

have no effect on the catalytic activity of Ni, then the reaction

temperature windows for SiO2/Ni and CeO2/Ni catalysts should

cover the same range as that of the Ni bulk catalyst. However,

the promoted Ni catalysts showed different XCO and reaction

temperature windows (Fig. 1). Therefore, the present results

confirm that SiO2 and CeO2 changed both the surface property

and the catalytic activity of the Ni bulk catalyst. Furthermore,

our H2-TPR and XRD results suggest that the mobile surface

oxygen of CeO2 and the –OH group of SiO2 enhanced the

methanation and WGS reactions, respectively.

Fig. 5 shows consecutively conducted results for each

catalyst in the order of TPO / H2-TPR / TPO and contains

results that support the above SEM, XRD, and H2-TPR data. The

figure shows marked differences among the three catalysts:

first, the catalysts exhibited different oxidation or reduction

amounts; second, they presented different oxidation or

reduction patterns; and third, they exhibit distinct oxidation

behavior. For example, in Fig. 5c, new oxidation peaks in each

catalyst appeared at ca. 600 �C and the oxidation amounts were

different. The sequence of reduction (Fig. 5b) or oxidation

(Fig. 5c) amounts among the catalysts was CeO2/Ni>Ni> SiO2/

Ni, which agrees with the sequence observed in the XCO data

(Fig. 1a). Taken as a whole, the results suggest that catalytic

activation occurred by surface reorganization of the Ni bulk

catalyst, and these results support our previous findings [5].

4. Conclusions

The use of promoters (SiO2 and CeO2) on Ni bulk catalyst

changed the surface properties of the Ni bulk catalyst and

affected the catalytic activity in a single-stage WGS reaction.

Even though promotion by CeO2 positively affected the CO

conversion, the WGS reaction was inhibited and the metha-

nation reaction was enhanced. On the other hand, SiO2, which

supplied –OH groups to the Ni bulk catalyst, resulted in

promotion of the WGS reaction rather than methanation.

Although the CO conversion using the SiO2/Ni catalyst was

decreased slightly, the selectivity toward WGS over SiO2/Ni

catalyst was much improved from ca. 70% to ca. 100% at 500 �C

during a single-stage WGS reaction.

Acknowledgements

This work was financially supported by Center for Fuel Cell

Research of Korea Institute of Science and Technology, and by

the ERC program of MOST/KOSEF (Grant No. R11-2002-102-

00000-0).

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