Influence of zirconia crystal phase on the catalytic performance of

Au/ZrO2 catalysts for low-temperature water gas shift reaction

Juan Li, Junli Chen, Wei Song, Junlong Liu, Wenjie Shen *

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

Received 11 July 2007; received in revised form 10 October 2007; accepted 22 October 2007

Available online 26 October 2007

www.elsevier.com/locate/apcata

Available online at www.sciencedirect.com

Applied Catalysis A: General 334 (2008) 321–329

Abstract

The influence of crystal phase of zirconia on the performance of Au/ZrO2 catalysts for low temperature water gas shift reaction was

investigated. Au/ZrO2 catalysts with pure tetragonal and monoclinic phases of ZrO2 were prepared by the deposition-precipitation method with

similar gold loading and dispersion. It was found that the Au/m-ZrO2 catalyst showed much higher activity than that of the Au/t-ZrO2 catalyst,

which could be attributed to the higher CO adsorption capacity of the Au/m-ZrO2 catalyst. The chemical state of gold that was strongly related to

the pretreatment atmosphere also played an essential role in determining the catalytic activity for water gas shift reaction. Hydrogen and/or helium

pretreated samples only contained Au0 species and exhibited higher activity than that of the sample pretreated with oxygen-containing atmosphere

which resulted in the co-existence of Au0 and Au+ species. FTIR study further revealed that the formate species formed by the reaction of the

adsorbed CO on gold nanoparticle with the hydroxyl groups on the surface of m-ZrO2 acted as the most important intermediates for the water gas

shift reaction.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Au/ZrO2; Crystal phase; Low-temperature; Water gas shift; FTIR

1. Introduction

Recently, the water gas shift (WGS) reaction is receiving

renewed interest for CO cleanup during hydrogen production

from alcohols and/or hydrocarbons for fueling polymer

electrolyte fuel cells, which requires removing the traceable

amounts of CO in H2-rich stream for preventing the Pt electrode

catalysts from deactivation of by CO adsorption [1–3]. Due to

the thermodynamic equilibrium limit, low CO levels can only

be achieved at low temperature although it is not favorable

kinetically. The commercial low temperature WGS catalyst

(Cu/ZnO/Al2O3) is not suitable for this specific application,

because it is very sensitive to air and requires very careful

activation prior to use. Therefore, substantial efforts have been

paid to the development of novel WGS catalysts with

sufficiently high activity at low temperatures.

Au/ZrO2 catalysts have been reported to show exceptionally

high activities for WGS reaction [4,5]. In addition to the typical

* Corresponding author. Tel.: +86 411 84379085; fax: +86 411 84694447.

E-mail address: shen98@dicp.ac.cn (W. Shen).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.10.020

size-dependent effect as well as the chemical states of gold

nanoparticles [5,6], the nature of zirconia was also found to play

an essential role in determining the catalytic performance [7,8].

Au nanoparticles on well crystallized zirconia exhibited higher

WGS activity than that on amorphous one [4], and the high

activity was usually attributed to the perimeter interface between

Au nanoparticle and zirconia [5]. However, the influence of

zirconia crystal phase on the activity of Au/ZrO2 catalyst is rarely

concerned. Zirconia could present as tetragonal (t-ZrO2) and

monoclinic (m-ZrO2) phases, which have different coordination

environments of zirconium and oxygen [9]. Zr4+ cation is

octacoordinated and O2� anion is tetracoordinated in the

tetragonal phase, while Zr4+ cation is heptacoordinated and

O2� anion is either tri- or tetracoordinated in the monoclinic

phase. Because of this, the acidic/basic properties as well as the

concentrations of surface hydroxyl group might be quite different

between the two crystal phases [10–12], which would in turn

affect the interaction between gold nanoparticle and ZrO2. In

fact, the CO adsorption capacity of m-ZrO2 was found to be 5 to

10-fold higher than that of t-ZrO2 [11]. The Cu/m-ZrO2 catalyst

also showed 8-fold higher activity for methanol synthesis by CO

hydrogenation than that of the Cu/t-ZrO2 catalyst [13,14].

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329322

In this work, we investigated the effect of zirconia crystal

phase on the activity of Au/ZrO2 catalysts for low temperature

WGS reaction. Au/t-ZrO2 and Au/m-ZrO2 catalysts were

prepared with similar Au loading and dispersion. The structural

features of the catalysts were characterized with X-ray power

diffraction, transmission electron spectroscopy, temperature-

programmed reduction/desorption, and X-ray photoelectron

spectroscopy. The nature of the adsorption sites and the reaction

mechanism was studied by in situ FTIR.

2. Experimental

2.1. Preparation of ZrO2 supports and Au/ZrO2 catalysts

The tetragonal zirconia (t-ZrO2) was prepared by a

homogeneous precipitation process. 32 g ZrOCl2�8H2O and

60 g urea were dissolved in 500 mL deionized H2O, and the

solution was heated to 100 8C and kept at this temperature for

5 h. The precipitation was filtered, washed with hot water, and

then dried at 110 8C overnight. The solid obtained was calcined

at 450 8C for 4 h in air.

The monoclinic zirconia (m-ZrO2) was prepared using a

reflux method similar to that described by Jung and Bell [15].

80 g ZrOCl2�8H2O was dissolved in 500 mL deionized H2O,

and the solution was boiled under refluxed at 100 8C for 240 h.

After the suspension was cooled to room temperature, 25 wt.%

ammonia aqueous solution was slowly added to agglomerate

the fine particles to facilitate the filtration. The precipitate was

then dried at 110 8C overnight and calcined at 450 8C for 4 h in

air. For comparison, the dried solid was also calcined at 500 8Cfor 4 h in air and the obtained sample was nominated as

m-ZrO2-R.

The Au/ZrO2 catalysts were prepared by a deposition–

precipitation method. 3 g ZrO2 was suspended in 250 mL H2O

and heated to 60 8C. The pH value of the mixture was adjusted

to 8 by adding 0.25 M Na2CO3 solution. 50 mL aqueous

solution containing 0.2 g HAuCl4�4H2O was also adjusted to

pH = 8 with 0.25 M Na2CO3 solution, which was then added

dropwise to the slurry containing ZrO2. The resulting mixture

was further aged at 60 8C for 2 h under stirring. The solid was

filtrated and washed thoroughly with water, and then dried at

room temperature for 24 h.

2.2. Catalyst characterization

N2 adsorption–desorption isotherm was recorded at

�196 8C on a Micrometrics ASAP 2000 instrument. Prior to

measurement, the sample was degassed at 250 8C for 5 h. The

specific surface area was calculated from a multipoint

Braunauer–Emmett–Teller (BET) analysis of the nitrogen

adsorption isotherms.

The actual loading of gold was determined by inductively

coupled plasma atomic emission spectroscopy (ICP-AES) on a

Plasma-Spec-I spectrometer.

X-ray powder diffraction (XRD) pattern was recorded using

a D/Max-2500/PC powder diffractometer (Rigaku, Japan) with

Cu Ka radiation operated at 40 kV and 100 mA. The average

crystal size of ZrO2 was calculated by the Scherer equation

using the ð1 1 1Þ diffraction (2u = 28.2) for m-ZrO2 and the

(0 1 1) diffraction (2u = 30.38) for t-ZrO2, respectively.

High-resolution transmission electron spectroscopy

(HRTEM) image was obtained with a Philips Tecnai G220

instrument. The sample powders were ultrasonically dispersed

in anhydrous ethanol and drops of the suspensions were

deposited on a holey carbon film supported copper grid.

Temperature-programmed reduction (TPR) measurement

was conducted with a conventional setup equipped with a TCD

detector. 100 mg as-prepared samples were loaded and

pretreated with He (30 mL/min) at 250 8C for 30 min to

remove the adsorbed carbonates and hydrates. After cooling

down to room temperature and introducing the reduction agent

(5% H2/N2, 30 mL/min), the sample was then heated to 800 8Cat a ramp of 10 8C/min.

Temperature-programmed desorption (TPD) of CO was

conducted with a micro-reactor equipped with a quadrupole

mass spectrometer (Omnistar, Balzers). 50 mg as-prepared

samples were initially heated to 250 8C under He flowing

(40 mL/min) and held at this temperature for 30 min to remove

the adsorbed carbonates and hydrates. After cooling down to

room temperature, adsorption of CO was then performed by

passing 1% CO/He (40 mL/min) through the sample for 1 h.

After purging with He, the sample was heated to 250 8C at a

ramp of 10 8C/min under He flowing (40 mL/min), and the

effluents from the reactor were measured by MS.

X-ray photoelectron spectroscopy (XPS) measurement was

performed with an ESCALAB MK-II spectrometer (VG

Scientific Ltd., UK) using Al Ka radiation (1486.6 eV)

operated at an acceleration voltage of 10 kV. The powder

samples were pressed into thin discs and mounted on a sample

rod placed in an analysis chamber, where the XP spectra of C

1s, O 1s, Zr 3d and Au 4f levels were recorded. Charge effects

were corrected by adjusting the binding energy of C 1s to

284.6 eV. The surface atomic ratio of Au/Zr was calculated

according to the proposed procedure [16,17].

2.3. Activity test

The WGS reaction was conducted with a continuous flow

fixed-bed quartz reactor under atmospheric pressure. 200 mg

samples (40–60 mesh) were placed between two layers of

quartz wool inside a quartz tube (i.d. = 6 mm). Prior to test, the

catalyst was heated to 250 8C under He flow (50 mL/min) for

2 h. The water was supplied by flowing He over a glass bubbler

containing distilled water kept at 51 8C. The H2O/He stream

was then mixed with the CO/He stream coming from a mass

flow controller. The typical feed gas composition was 3% CO/

9% H2O balanced with He. Effluents from the reactor were

analyzed by an on-line gas chromatograph equipped with TCD

and FID detectors.

2.4. FTIR

FTIR spectra were collected over a Biorad Vector 22

spectrometer equipped with a DTGS detector, using 32 scans

Fig. 1. XRD patterns of the zirconium oxides and the Au/ZrO2 catalysts.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329 323

per spectrum in the region of 4000–1000 cm�1 with a

resolution of 4 cm�1. Au/ZrO2 sample was pressed into a thin

self-supporting pellet and introduced into a cell allowing

thermal treatments under controlled atmospheres. The pellet

was treated in a flow of He (40 mL/min) at 250 8C for 30 min to

remove the adsorbed H2O and carbonates. After cooling down

to given temperature under He flow, IR spectra were recorded

by exposing the sample to 1% CO/He or 1% CO/3% H2O/He

stream (40 mL/min). The reference of spectrum of the Au/ZrO2

wafer in He taken at the measurement temperature was

subtracted from each spectrum.

3. Results and discussion

3.1. Structural properties of the catalysts

Table 1 summarizes the structural and textural properties of

the ZrO2 supports and the Au/ZrO2 catalysts. The surface areas

of t-ZrO2 and m-ZrO2 were 96 and 136 m2/g, respectively.

Fig. 1 shows the XRD patterns of the zirconium oxides and the

Au/ZrO2 catalysts. The t-ZrO2 was indexed to pure tetragonal

phase with characteristic diffraction peak at 2u = 30.38 (JCPDS

50-1089) and crystalline size of 13 nm. The m-ZrO2 was well

described as monoclinic phase with characteristic diffraction

peaks at 2u= 28.28 and 31.38 (JCPDS 37-1484) and crystalline

size of 5 nm. The deposition of gold only slightly decreased the

surface areas of the supports, which were 94 m2/g for Au/t-

ZrO2 and 122 m2/g for Au/m-ZrO2. The actual loadings of gold

were 1.26 wt.% for Au/t-ZrO2 and 1.20 wt.% for Au/m-ZrO2,

respectively. Except a extra small peak at 2u = 28.28 attributed

to monoclinic phase appeared in the Au/t-ZrO2 after the

deposition of gold, the XRD patterns of the Au/ZrO2 catalysts

were similar to those of the corresponding ZrO2 supports, and

no distinct diffraction peaks of metallic gold could be observed,

indicating that the gold particles were too small to be detected

and/or were highly dispersed on ZrO2.

Fig. 2 shows the HRTEM images of the Au/ZrO2 catalysts. It

was observed that t-ZrO2 had a flocculent appearance and m-

ZrO2 had a discrete distribution with a large number of

wormhole-like channels. The particle sizes were about

15–20 nm for t-ZrO2 and 5–15 nm for m-ZrO2, respectively.

Most of the gold particles were much smaller and hardly

observed. Only few gold particles with lattice fringes of

0.241 nm and particle size of about 3 nm were observed in the

Au/m-ZrO2 catalyst. Therefore, it can be assumed that there

should present almost same size distribution of gold

nanoparticles for the Au/ZrO2 catalysts due to the same

preparation route and gold loading. As a matter of fact, Zhang

Table 1

Structural and textural properties of the samples

Sample SBET (m2/g) XRD size (nm) TEM siz

t-ZrO2 96 13

m-ZrO2 136 5

1.26% Au/t-ZrO2 94 13 15–20 nm

1.20% Au/m-ZrO2 122 5 5–15 nm

et al. [7] also pointed out that gold particle size was independent

on the surface area for zirconia between 20 and 162 m2/g.

3.2. Temperature-programmed reduction/desorption

Fig. 3 shows the TPR profiles of the ZrO2 supports and the

Au/ZrO2 catalysts. There was a broad reduction peak at 600–

800 8C for t-ZrO2, which was probably associated with the

surface shell reduction [5]. Au addition shifted the major

surface reduction to about 650 8C, with a shoulder at about

530 8C. For m-ZrO2, no reduction took place even up to 800 8C,

but the loading of Au resulted in significant hydrogen

consumption at 540 8, indicating the partial reduction of

surface Zr4+ to Zr3+. Obviously, Au addition enhanced the

removal of surface oxygen, leading to oxygen deficiencies on

the surface of ZrO2. Pt was found to facilitate the generation of

bridging OH groups on the surface of zirconia by removing

oxygen from the OH groups or dissociating hydrogen directly

during hydrogen reduction [22]. This seems true for the Au/

ZrO2 catalysts. The easier surface reduction of the Au/m-ZrO2

catalyst indicated that the active OH groups formed in the

defect sites had higher concentrations than that of the Au/t-

ZrO2 catalyst. For both cases, however, there were no reduction

peaks which could be assigned to the reduction of gold species,

implying that the gold was mainly in metallic state, as

previously proposed [18].

Fig. 4 shows the CO-TPD profiles of the Au/ZrO2 catalysts.

Only desorption of CO2 was detected, which could be attributed

to the decomposition of formate and/or carbonate species on the

surface of ZrO2 [11]. CO2 desorption occurred at almost the

e (nm) TPR peak (8C) BE of Au 4f7/2 (eV) Au0 (%)

685 – –

– – –

650 83.8 100

540 83.5 100

Fig. 2. HRTEM images of the Au/ZrO2 catalysts: (a–b) Au/m-ZrO2 and (c–d) Au/t-ZrO2.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329324

same temperature (about 80 8) for both catalysts, indicating a

similar bonding of CO on the surface of ZrO2. But the relatively

higher intensity of CO2 desorption over the Au/m-ZrO2 catalyst

indicated a higher CO adsorption capacity and reactivity with

the surface than that of the Au/t-ZrO2 catalyst. This

phenomenon further proved that higher concentration of anion

vacancies was presented on the surface of m-ZrO2, mainly due

to the surface atomic configuration [9]. A second CO-TPD

measurement following the initial experiment further con-

firmed the higher capacity of the Au/m-ZrO2 catalyst for CO

adsorption and activation.

3.3. XPS surface analysis

Fig. 5 shows the XPS spectra of Au4f in the Au/ZrO2

catalysts. The binding energy of Zr 3d5/2 in both catalysts was

182.2 eV, which is characteristic of Zr4+. The binding energies

of Au 4f7/2 and Au 4f5/2 are 83.9 and 87.7 eV for Au0, 84.7 and

88.2 eV for Au+, 86.3 and 89.6 eV for Au3+, respectively

[19,31]. The samples pretreated with hydrogen and helium

showed binding energies of 83.8 and 87.5 eV for Au/t-ZrO2,

and 83.5 and 87.2 eV for Au/m-ZrO2, which were character-

istics of metallic Au. As compared to bulk Au, the slightly

lower binding energies of Au 4f in these catalysts indicated a

modification of the electronic structure of Au nanoparticles due

to its interaction with zirconia. The XPS results were well

agreement with the TPR measurements, showing that gold

species were in metallic state after pretreatment with He.

However, for the pre-oxidized samples, in addition to Au0, the

XPS spectra also showed bonding energies of 84.8 and 88.5 eV

for Au/t-ZrO2, and 84.7 and 88.3 eV for Au/m-ZrO2,

representing Au+ species. As a result, the pre-oxidized Au/t-

ZrO2 catalyst contained 37% Au+ and 63% Au0 and the pre-

oxidized Au/m-ZrO2 sample contained 59% Au+ and 41% Au0.

For all the samples pretreated with He, H2 and O2, the surface

Au/Zr ratios were 0.012–0.013 for Au/t-ZrO2 and 0.012–0.015

for Au/m-ZrO2, respectively. This suggested that the surface

concentrations of Au and Zr were similar and almost

independent on the nature of the support and the pretreatment

atmosphere.

3.4. Water-gas shift reaction

Fig. 6 shows the CO conversion as a function of time-on-

stream (TOS) at 120 8C over the Au/ZrO2 catalysts. Clearly, the

Au/m-ZrO2 catalyst showed higher CO conversion than that of

the Au/t-ZrO2 catalyst. The conversion of CO reached 52%

after 10 h TOS over the Au/m-ZrO2 catalyst, however, the

corresponding conversion of CO was only 41% over the Au/t-

ZrO2 catalyst. To exclude the influence of the surface area of

zirconia in the catalytic activity, the Au/m-ZrO2-R which had

similar surface area (91 m2/g) to the Au/t-ZrO2 catalyst was

Fig. 3. H2-TPR profiles of the ZrO2 supports and the Au/ZrO2 catalysts.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329 325

tested for WGS reaction under the same reaction conditions. It

revealed that this reference catalyst could show almost the same

activity as that of the Au/m-ZrO2 catalyst, although the later

had higher surface area. This indicated that the higher catalytic

activity of Au/m-ZrO2 catalyst than that of the Au/t-ZrO2

catalyst could not be simply attributed to the surface area of

zirconia. Fig. 7 further shows the effect of space velocity on CO

Fig. 4. CO-TPD profiles of the Au/ZrO2 catalysts:

conversion at 200 8C over the Au/ZrO2 catalysts. The Au/m-

ZrO2 catalyst exhibited only a slight decrease in CO conversion

(from 96 to 90%), whereas the Au/t-ZrO2 catalyst showed a

remarkable decrease in CO conversion (from 92 to 78%) when

the space velocity was increased from 20; 000 mLg�1cat h�1 to

40; 000 mL g�1cat h�1.

For supported gold catalysts, the activity is usually

controlled by the contact structure of Au nanoparticles with

the support, that is, the size of Au particles and the nature of the

support [20,21]. Since the dispersion and the chemical state of

gold were almost the same for the Au/ZrO2 catalysts, the higher

activity of the Au/m-ZrO2 catalyst was mainly related to the

monoclinic crystal phase of zirconia. As demonstrated by the

CO-TPD measurement, the Au/m-ZrO2 catalyst had a higher

CO adsorption capacity than that of the Au/t-ZrO2 catalyst,

indicating that higher concentration and more nucleophilic

hydroxyl groups were present on the surface of m-ZrO2 [11].

This will contribute to the strong adsorption of CO as HCOO-Zr

groups, which would act as the important intermediates for the

WGS reaction. Chenu et al. [22] also noted that the higher

activity of the Pt/m-ZrO2 catalyst for WGS reaction than that of

the Pt/t-ZrO2 catalyst could be well correlated with the

intensities of the adsorbed formate formed by CO adsorption.

Significantly higher surface concentration of formate was

observed on the Pt/m-ZrO2 catalyst due to the strong interaction

of CO and the bridging OH groups. Therefore, the CO

adsorption capacity gave an important indication of the number

of active sites, and consequently, the WGS reaction activity.

Another possibility might be that the m-ZrO2 was composed of

very small particles/crystallites which could make much more

Au-ZrO2 contact boundary [7]. This kind of nanocomposite

structure in the Au/m-ZrO2 catalyst would lead to easier CO

migration from Au to ZrO2 surface, facilitating the WGS

reaction.

Fig. 8 shows the effect of the pretreatment atmosphere of the

Au/ZrO2 catalysts on CO conversion at 200 8C. H2 and He-

treated catalysts gave similar CO conversion, whereas O2-

(a) the first and (b) the second measurement.

Fig. 5. XPS spectra of Au 4f in the Au/ZrO2 catalysts pretreated with H2, He and O2 atmospheres at 250 8C.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329326

treated catalysts led to lower CO conversion. This is in good

accordance with the XPS observation. The Au/ZrO2 catalysts

pretreated with H2 and He only contained Au0, however, the

catalysts pretreated with O2 contained Au0 and Au+ species

simultaneously. The positively charged gold sites have stronger

Au–CO bonding [23], and thus resulted in a decline in CO

conversion. This phenomenon further suggests that metallic

gold nanoparticles are much more active for WGS reaction than

cationic gold species. Kim and Thompson [24] treated the Au/

CeO2 catalyst with NaCN aqueous solution and found that the

reaction rates of WGS for the unleached catalyst were

significantly higher than those for the leached catalyst, which

only contained highly dispersed cationic gold species. The

perimeter interface between Au nanoparticles and the oxide

support was regarded as a unique reaction site, where CO was

adsorbed on Au nanoparticles and H2O was activated on the

surfaces of oxide [21].

Fig. 6. CO conversion as a function of time-on-stream over the Au/ZrO2

catalysts. Reaction conditions: 120 8C; 3% CO/9% H2O/He; 20; 000 mL g�1cat h�1.

3.5. Reaction mechanism

Fig. 9 shows the FTIR spectra of CO adsorption on the m-

ZrO2 support and the Au/m-ZrO2 catalyst, which were recorded

after exposure to 1% CO/He stream for 15 min at 25 8C. On the

Au/m-ZrO2 catalyst, the characteristic band representing CO

adsorption on metallic gold nanoparticles appeared at

2117 cm�1. This typical band was previously attributed CO

co-adsorbed with oxygen species on Au nanoparticles [25,26].

For the current Au/m-ZrO2 catalyst, the co-adsorbed oxygen

could originate only from the spillover of oxygen in the OH

group on m-ZrO2 to gold nanoparticles. More interestingly, this

band displayed a broadening towards lower frequencies,

suggesting that CO was adsorbed on very small gold clusters

which strongly interacted with ZrO2 [27,28]. This result further

evidenced the presence of highly dispersed gold clusters,

although they were not detected by XRD and HRTEM. The

weak band at 2177 cm�1 was characteristic for CO adsorption

on m-ZrO2 surface [29].

Fig. 7. Effect of space velocity on CO conversion over the Au/ZrO2 catalysts.

Reaction conditions: 200 8C; 3% CO/9% H2O/He; (&) 20; 000 mL g�1cat h�1;

(~) 40; 000 mL g�1cat h�1.

Fig. 8. Effect of pretreatment atmosphere on CO conversion over the Au/ZrO2

catalysts. Reaction conditions: 200 8C; 3% CO/9% H2O/He; 40; 000 mL g�1cat h�1

Fig. 9. FTIR spectra of CO adsorption on the m-ZrO2 support and the Au/m-

ZrO2 catalyst recorded after exposure to 1% CO/He stream for 15 min at 25 8C.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329 327

The bands at 1630, 1427 and 1221 cm�1 were attributed to

bicarbonate species, and the weak bands at 1555 and 1328 cm�1

could be assigned to bidentate carbonate species [11,27]. The

band at 3613 cm�1 was likely corresponding to the m-OH

vibration in the hydrogen-carbonate (i.e. bicarbonate) species.

For m-ZrO2 alone, after CO adsorption, the formation of surface

bicarbonate and bidentate carbonate species with COO vibra-

tions in 1200–1700 cm�1 region were also observed, but with

much lower intensities compared to the Au/m–ZrO2 catalyst.

Fig. 10. FTIR spectra of the Au/m-ZrO2 catalyst recorded after 1% CO/He w

Fig. 10 shows the FTIR spectra in carbonyl and carbonate

regions by switching the gas stream from CO/He to He at 25 8Cfor the Au/m-ZrO2 catalyst. The intensity of the band at

2177 cm�1 representing CO adsorption on Zr4+ rapidly

decreased and almost disappeared after 3 min. But the band

at 2117 cm�1 corresponding to CO adsorption on Au

nanoparticles decreased slowly and practically vanished after

10 min. However, the bands related to carbonate species with

COO vibrations in 1200–1700 cm�1 region only decreased

slightly, indicating that these species were strongly and

irreversibly adsorbed. Coupled with the results in Fig. 9, it

can be assumed that CO was initially adsorbed on gold

nanoparticles, and then spilled over to the surface of m-ZrO2,

where it further reacted with the OH groups on ZrO2 surface to

form stable bicarbonate and/or carbonate species.

as switched to He at 25 8C: (a) carbonyl region and (b) carbonate region.

Fig. 12. FTIR spectra of the Au/m-ZrO2 catalyst recorded after 1% CO/3%

H2O/He was switched to He at 200 8C.

J. Li et al. / Applied Catalysis A: General 334 (2008) 321–329328

Fig. 11 shows the FTIR spectra of the Au/m-ZrO2 catalyst by

switching the stream of 1% CO/He to 1% CO/3% H2O/He. At

25 8C, the inlet of water caused a broad absorption band at

3600–2800 cm�1, representing the OH stretching modes of

water molecules interacting with the OH groups on the surface

of m-ZrO2 through hydrogen bonds. Simultaneously, the

intensities of the bands at 3737 and 3637 cm�1 attributed to the

surface OH groups decreased, as evidenced by the appearance

of negative peaks in this region. Moreover, a band at

1636 cm�1, typical for non-dissociated water molecule bending

mode [30], appeared. Meanwhile, the band at 2117 cm�1 for

CO adsorption on Au nanoparticles reduced its intensity. The

weak bands at 2360 and 2335 cm�1 due to the formation of CO2

were also observed. The bands at 1630, 1427 and 1221 cm�1

belonging to bicarbonate species disappeared, and the bands at

1555 and 1327 cm�1 for bidentate carbonate species increased.

This indicates that the addition of water promoted the

decomposition of the bicarbonate species, and the produced

CO2 was re-adsorbed on the surface of m-ZrO2, leading to the

formation of surface bidentate carbonate species.

With elevating of the temperature to 150 8C, the broad band

at 3600–2800 cm�1 related to adsorbed water molecular

reduced its intensity gradually. The bidentate carbonate species

became unstable and decomposed slowly. Whereas the bands at

Fig. 11. FTIR spectra of the Au/m-ZrO2 catalyst. (1) 15 min after 1% CO/He

flow at 25 8C; (2) 15 min after 1% CO/3% H2O/He flow 25 8C; and 5 min at (3)

50 8C; (4) 75 8C; (5) 100 8C; (6) 150 8C; (7) 200 8C; and (8) 250 8C.

1573, 1383, 1357, 2967 and 2870 cm�1 associated with formate

species gradually built up (curves 2–6) due to the interaction

between the adsorbed CO and the surface OH groups of m-

ZrO2. Further increase in the intensity of formate species was

observed when the temperature was raised up to 200 8C (curve

7). Meanwhile, the bands related to surface carbonate species

disappeared completely and the bands related to CO2 increased

remarkably. Hence, it is most likely that the formate species

acted as the important intermediates in the WGS reaction over

the Au/m-ZrO2 catalyst.

Fig. 12 also shows FTIR spectra of the Au/m-ZrO2 catalyst

recorded after CO/H2O/He was switched to He at 200 8C. The

bands at 1573, 1383, 1357, 2967 and 2870 cm�1 associated

with formate species gradually reduced their intensities. After

10 min of desorption, formate species have decomposed

completely and a small quantity of carbonate species still

retained on the surface. Therefore, it can be assumed that the

buildup of carbonate species could be one reason for the slow

deactivation of the Au/ZrO2 catalyst with time-on-stream.

4. Conclusion

Au nanoparticles supported on m-ZrO2 showed significantly

higher activity for WGS reaction than that on t-ZrO2. The

chemical state of gold was also proven to be important for the

WGS reaction. The active sites associated with metallic Au

nanoparticles are more active than the ones associated with

cationic Au+ species. FTIR measurements further identified

that the interaction between CO adsorbed on gold nanoparticles

and OH groups on the surface of m-ZrO2 produced formate

species, which acted as the most important intermediates and

finally decomposed into CO2 and H2 in the presence of water.

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