Role of redox couples of Rh0/Rhd+ and Ce4+/Ce3+ in CH4/CO2

reforming over Rh–CeO2/Al2O3 catalyst

Rui Wang a, Hengyong Xu a,b,*, Xuebin Liu a, Qingjie Ge a, Wenzhao Li a

a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Chinab Department of Chemistry, Harbin Normal University, Harbin 150080, China

Received 19 October 2005; received in revised form 22 February 2006; accepted 3 March 2006

Abstract

The interaction between Rh and CeO2 over the Rh–CeO2/Al2O3 catalyst was investigated for the reaction of methane reforming with CO2. It

was shown that the activity and coke resistance of Rh/Al2O3 were enhanced by the addition of CeO2, which greatly depend on the interaction

between rhodium and ceria under the reaction atmosphere. In situ electrical conductivity results showed that the reducing agents (CH4, H2) in the

reaction system could be activated and dissociated on Rh0, releasing electrons to CeO2 in close contact with Rh0 and generating the Ce4+/Ce3+

redox couple. Meanwhile, it was found from XPS measurements that the electron transfer could also happen from Rh0 to CeO2, creating the Rh0/

Rhd+ couple. Thus, the very coexistence of Ce4+/Ce3+ and Rh0/Rhd+ redox couples facilitated the activation of CH4 and CO2 and further enhanced

the catalytic activity and coke resistance of Rh/Al2O3. For CH4 activation, the electron-deficient state of Rhd+ had higher ability to accept s

electrons of CH4 to promote CH4 adsorption and C–H bond cleavage, while CO2 activation was mainly facilitated by accepting free electrons of

Ce3+ species, which resulted in the enhancement of carbon elimination to yield CO. Finally, a cycle mechanism of redox couples over Rh–CeO2/

Al2O3 in CH4/CO2 reforming was proposed.

# 2006 Elsevier B.V. All rights reserved.

Keywords: CH4/CO2 reforming; Rh–CeO2 catalyst; Redox couples

www.elsevier.com/locate/apcata

Applied Catalysis A: General 305 (2006) 204–210

1. Introduction

Carbon dioxide reforming of methane to synthesis gas

(CO2 + CH4! 2CO + 2H2) has received a great deal of

attention during the past decade [1,2]. This reaction is of

interest because of its ability to adjust H2/CO ratio suitable for

the synthesis of a wide variety of valuable hydrocarbons and

oxygenates by combining with stream-reforming or partial

oxidation of methane. Meanwhile, CO2, a typical greenhouse

gas, is consumed in a useful manner.

One of the major disadvantages of CH4/CO2 reforming is its

high thermodynamic potential for coke formation [3]. A

number of reports have shown that noble metal catalysts [4,5]

and the rare earth oxides-promoted catalysts [6,7] exhibit better

activity and coking resistivity. Among them, the addition of

CeO2 has been studied extensively as an effective promoter.

Many authors have pointed out that ceria may promote the

* Corresponding author. Tel.: +86 84581234; fax: +86 84691570.

E-mail addresses: xuhy@dicp.ac.cn, wrviolet@163.com (H. Xu).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.03.021

metal dispersion and prevent sintering of metals [8,9]. This

promotion has also been associated with the metal–ceria

interaction, considering the unique function of ceria in oxygen

storage capacity [10], redox behaviors of Ce4+/Ce3+ and

electrical effect on metals [11]. Wang et al. [12] have inferred

that the lattice oxygen of ceria over Ni supported on yttria-

doped ceria may participate in the activation of both CH4 and

CO2, and that the formation of interfacial Ni–Ce3+ active

centers enhances the activity and coking resistivity. Damyanova

et al. [13] have reported that a low loading CeO2 (1 wt.%)

improved activity and stability of Pt/Al2O3, due to an increase

of the metal-support interface area, caused by the higher Pt

dispersion. Moreover, the nature of Pt–Ce interaction varies

with CeO2 loading. In the some studies on CeO2-promoted Rh

catalysts [14–17], the beneficial effects might have occurred

either because the cooperation between the partial CeOx and Rh

sites generated sites with higher activity [14], or because the

oxidative properties of CeO2 increased the dissociation of CO2

[15]. The detailed states of metal and ceria under the real

reaction conditions and how they influence catalytic activity

and coking resistivity are, however, still unclear. Previous work

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210 205

done by our group on Ni–CeO2/Al2O3 catalyst has shown that

the Ni–ceria interaction led to the electron-rich state of Ni,

which inhibited CH4 adsorption and decomposition and

decreased carbon deposition [18]. In this research, the

interaction between Rh and CeO2 and its effects on catalytic

activity and coke resistance were further studied on CeO2-

promoted Rh/Al2O3 catalyst, by various characterization

methods, such as transient pulse reactions, temperature-

programmed reduction (TPR), X-ray photoelectron spectro-

scopy (XPS), and in situ electronic conductivity measurements.

2. Experimental

2.1. Catalyst preparation

An Rh/Al2O3 catalyst was prepared by impregnating g-

Al2O3 (20–40 mesh, BET area 126 cm2/g) with an aqueous

solution of RhCl3�nH2O, followed by drying at 383 K for 4 h

and calcining at 773 K for 4 h for decomposition. A Ce–Rh/

Al2O3 catalyst was prepared by co-impregnation of g-Al2O3

with a mixed solution of RhCl3�nH2O and Ce(NO3)3�6H2O (the

mole ratio of Rh/Ce = 1.2), and Rh/CeO2 catalyst was prepared

by impregnation of CeO2 with a solution of RhCl3�nH2O. CeO2

support was obtained after calcinations of Ce(NO3)3�6H2O at

773 K in air for 6 h. The Rh content was 1 wt.% for all the

catalysts.

2.2. Catalytic performance studies

Catalytic reactions were performed in a tubular quartz

reactor with an inner diameter of 8 mm. Prior to reaction,

7.5 mg of catalyst was reduced in H2 (20 ml/min) at 773 K for

0.5 h and then heated to 973 K in N2 (40 ml/min). The reactions

were performed at 973 K with a CO2:CH4:N2 ratio of 5:5:90

and a flow rate of 700 ml/min. The effluents were analyzed

online with a TCD gas chromatograph with two columns of

TDX-01 and MS-8A. The conversions of reactants were

evaluated according to the carbon balance.

2.3. Catalyst characterization

Pulse experiments were performed using a micro-pulse

quartz reactor (i.d. = 4 mm). Forty milligrams of catalyst was

reduced in H2 (20 ml/min) at 773 K for 0.5 h, followed by

heating to the 973 K in Ar (25 ml/min) and then in Ar exposed

to pulses of CH4 or CO2 (305.8 ml). During each pulse, the

effluent gases were monitored with a gas chromatograph (GC-

Table 1

Catalytic activities and amounts of coke over Rh-based catalysts during CH4/CO2

Time on stream (h) CH4 conversions (%)

0.12 0.5 0.85 2

Rh/Al2O3 14.70 9.14 8.56 5.91

Rh–CeO2/Al2O3 23.57 20.31 17.80 15.50

a Reaction conditions: CO2/CH4/N2 = 5/5/90, GHSV = 56,000,000 ml h�1 g�1, Tb Total amount of coke was determined by successive pulsing O2 at 1073 K.

920), equipped with a TDX-01 column and a TCD. The area of

each pulse was converted to moles using a conversion factor

that was determined from a calibrated injection prior to the

experiment.

Temperature-programmed reduction (TPR) was performed

by heating the samples from room temperature to 1173 K at rate

of 10 K/min in a 5% H2/Ar gas flow (25 ml/min). The sample

was pretreated at 773 K in flow of Ar to remove the absorbed

gases before the TPR. The response was measured using a

thermal conducting detector.

Samples (400 mg) for electrical conductivity measurement

were compressed at 12 MPa during 1 min, and placed between

two platinum electrodes. The electrical resistance of the sample

was measured with a Fluke 8840A multimeter according to the

range investigated. The sample was first reduced in hydrogen

for 1 h by heating to 843 K at 8 K/min, and then 5% CH4/He,

N2 and 5% CO2/He were subsequently introduced after heating

to 973 K in H2. The electrical conductivity s was calculated

from the equation s = L/RS, where R is the electrical resistance

measured, L is the thickness of the sample and S is the area of

the sample.

XPS spectra of the samples were obtained on a VG

ESCALAB MK-2 spectrometer using Al Ka radiation

(1468.6 eV). Binding energies were corrected for charging

by reference to the Al (2p) peaks at 74.7 eV. For XPS analysis,

the powder samples were reduced under hydrogen flow at

773 K for 5 h and placed in an ultrahigh vacuum (UHV)

chamber at 5 � 10�9 Pa housing the analyzer. The experi-

mental data were curve fitted with a combination of Lorentzian

and Gaussian lines after subtracting the background contribu-

tion. The background contribution was obtained by a Shirley

function.

3. Results

3.1. Catalytic performance

The results of the catalytic performance over non-promoted

and ceria-promoted Rh catalyst are briefly summarized in

Table 1. We found that CH4 conversion was always lower than

CO2 conversion in all cases. This reveals the simultaneous

occurrence of the reverse water-gas shift reaction

(CO2 + H2! CO + H2O) [1,9] during the reaction. It can also

be seen that CH4 conversions over Rh–CeO2/Al2O3 were much

higher (about 9%) than that over Rh/Al2O3 catalyst. However,

such higher methane conversions are not necessarily related to

the larger amounts of coke and faster deactivation of the

reforminga

CO2 conversions (%) Total cokeb

(mmol g�1)

0.12 0.5 0.85 2

30.97 25.24 21.22 16.01 368.54

41.28 38.02 33.10 28.12 214.73

= 973 K, m = 7.5 mg.

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210206

Fig. 1. TPR profiles of (a) Rh2O3 (b) CeO2 (c) Rh/Al2O3 (d) Rh/CeO2 (e) Rh–

CeO2/Al2O3.

catalyst, because deactivation also critically depends on the rate

with which the coke can be removed by CO2 under reaction

conditions [19].

CH4 ! CHxþð2� ðx=2ÞÞH2 (1)

CO2þCHx ! 2CO þ ðx=2ÞH2 (2)

As Table 1 shows, CO2 conversions were enhanced with

addition of CeO2 by 12–13% more than those over Rh/

Al2O3, and the total amount of carbon deposits after reaction

over Rh/Al2O3 was larger than that over Rh–CeO2/Al2O3

catalyst. It is obvious that CeO2 addition promotes CO2 activa-

tion and inhibits carbon deposition on Rh–CeO2/Al2O3 cata-

lyst.

3.2. Pulse experiments

In order to further investigate the interaction of CH4 and CO2

with catalysts, we performed pulse experiments over Rh/Al2O3

and Rh/CeO2, with CeO2 as references. Table 2 shows the

results obtained by introducing four pulses of CH4, eight pulses

of CO2, and four pulses of CH4 in order. It can be seen that the

first series of CH4 pulses resulted in the production of both H2

and CO, with the yield amounts decreasing in the order Rh/

CeO2 > Rh/Al2O3 > CeO2. The amount of H2 was signifi-

cantly higher than that of CO over Rh/Al2O3, indicating that

methane activation and decomposition (Eq. (1)) on Rh should

be dominant. The only source of oxygen in the system is from

the support and the partial oxidation of methane to form CO

seems to happen too. For Rh/CeO2, the maximum amounts of

H2 and CO were produced, which were the consequences of

both the partial oxidation of methane by a large amount of

oxygen species of ceria and the methane decomposition. Only

traces of H2 and CO formation over CeO2 suggested that

methane decomposition could not occur on ceria in the absence

of rhodium.

In the subsequent CO2 pulses, no H2 but CO formation was

again observed and the CO yield amount decreased in the order

Rh/CeO2 > CeO2 > Rh/Al2O3. For Rh/Al2O3, one found the

reforming of CHx species deposited on rhodium with CO2

(Eq. (2)). The amount of CO over pure CeO2 was larger than

that over Rh/Al2O3 in spite of no carbon deposition over CeO2

during the CH4 pulses, which provided the evidence that the

oxygen vacancies of ceria were generated under H2 treatment.

The maximum CO production over Rh/CeO2 demonstrated

that, apart from the reforming reaction, CO2 could replenish

oxygen vacancies of ceria to form CO.

Table 2

Amounts of product over Rh-based catalysts in CH4 and CO2 pulses at 973 K

Sample CH4 pulses CO2 pul

H2 (mmol) CO (mmol) H2 (mmo

Rh/Al2O3 22.89 1.6 –

Rh/CeO2 33.86 21.15 –

CeO2 0.12 0.01 –

During the last CH4 pulses, the activities of catalysts were

found to have recovered. The H2 and CO yields over Rh/Al2O3

in this series were slightly lower than those in the initial CH4

series, due to the partial removal of carbonaeous species

deposited on metal surface by CO2. On the contrary, for Rh/

CeO2, the catalytic activities were recovered completely and

even slightly higher than those in the initial CH4 series.

3.3. Characterization

3.3.1. H2-TPR studies

The TPR profiles of all samples are illustrated in Fig. 1. Part

a (Rh2O3) shows that only one peak of hydrogen uptake

appeared at 420 K, corresponding to a complete reduction of

Rh2O3.

For CeO2 (Fig. 1b) primarily two peaks were detected at

approximately 760 and 1130 K, these peaks could be assigned

to the partial reduction of surface CeO2 and bulk CeO2 to

Ce2O3, respectively.

For Rh/Al2O3 catalyst (Fig. 1c), two peaks were observed at

about 410 and 730 K. The former peak could be assigned to a

reduction of isolated RhOx species. The latter one at 730 K

corresponded to the reduction of RhOx species in interaction

with Al2O3 support. A very similar observation has been

ses CH4 pulses

l) CO (mmol) H2 (mmol) CO (mmol)

13.21 19.05 0.67

41.29 39.28 26.06

17.41 0.15 0.01

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210 207

reported by Burch et al. [20], suggesting that some rhodium

oxide supported on the Al2O3 when calcined at 773 K may

spread over the support and diffuse into defect sites of alumina,

becoming strongly bound and non-reducible.

The TPR profile of Rh/CeO2 is shown in Fig. 1d. The first

very sharp signal was due to the reduction of free Rh2O3.

Instead of the surface CeO2 reduction peak that previously

appeared at 760 K in Fig. 1b, a multiplicity of peaks were

detected in the temperature region of 500–700 K. The amount

of H2 consumed in this process largely exceeded the amount

needed for total reduction of bare Rh2O3, which means that the

reduction of surface CeO2 in the presence of Rh0 could occur at

much lower temperature. This indicates that Rh0 can effectively

activate H2 molecules, which subsequently spillover to the

surface CeO2 and promote its reduction [21,22]. The peak for

the reduction of bulk ceria seemed not to be shifted and still

appeared at 1100 K, suggesting that the presence of Rh0 does

not affect the reduction of bulk ceria. This observation is

typically interpreted in terms of a kinetic model [23], which

assumes that the high temperature reduction process is

controlled by the slow bulk diffusion of the oxygen vacancies

created at the oxide surface.

For CeO2-modified Rh/Al2O3 catalyst (Fig. 1e), the reduction

peak of Rh2O3 shifted upward to around 510 K, and a

remarkable shift to a lower temperature (about 650 K) was also

observed for the reduction peak of RhOx–Al2O3 species at

730 K. In addition, the reduction peak of surface ceria seemed

to overlap with that of the rhodium oxide. These observations

imply that a strong interaction exists between Rh and CeO2,

which suppresses the reduction of isolated RhOx and aids the

reduction of RhOx–Al2O3 interactive species. Furthermore, the

reduction of surface ceria is simultaneously promoted.

3.3.2. In situ electrical conductivity

The electrical conductivity measurement results for Rh/

CeO2 and CeO2 under H2 and N2 are given in Fig. 2. It can be

seen that the electrical conductivity of CeO2 under N2 was

always close to zero in the region of 400–850 K. The

contribution of the increase in temperature to the electrical

Fig. 2. Changes of electrical conductivity of various catalysts under different

gase with temperature.

conductivity is negligible. When CeO2 was exposed to H2, the

sCeO2was still maintained at zero level before 650 K, but a

remarkable increase was then observed above 650 K, which

indicates that ceria has been partially reduced and oxygen

vacancies VO2� with two electrons trapped have been created.

Such a vacancy is a neutral entity with respect to the surface

lattice of ceria. It can easily lose an electron by spontaneous

ionization and become singly positively charged with respect to

the solid [24]:

O2�ðlatticeÞ þ H2!H2Oþ VO2� (3)

VO2� !VþO2� þ e� (4)

In the case of Rh/CeO2, the electrical conductivity started to

increase at 550 K, and reached a much higher value after

treatment at 843 K for 1 h than the value in the case of pure

CeO2. The increase of sRh=CeO2can be ascribed to a hydrogen

spillover effect, which again confirms the existence of the Rh–

CeO2 interaction. The spillover hydrogen atoms adsorb on

anionic sites of the support, thus yielding hydroxyl groups

and releasing free electrons to the support [24,25].

H2ðgÞ þ 2Rhs ! 2Rhs�H (5)

Rhs�H þ O2� ! RhsþOH� þ e� (6)

Fig. 3 shows in situ electrical conductivity data of reduced Rh/

CeO2 and CeO2 when CH4, N2 and CO2 are introduced alter-

natively at 973 K. During introduction of CH4 (Panel A), the

electrical conductivity of reduced Rh/CeO2 (Fig. 3a) had a

significant increase. This indicates that methane adsorption on

Rh could release free electrons to contribute to the electrical

conductivity and then decompose into CHx species and atomic

hydrogen.

In the presence of N2, sRh=CeO2was almost constant. During

the subsequent shift to CO2, an obvious decrease in electrical

Fig. 3. Electrical conductivity of reduced catalysts under different sequential

gaseous atmospheres (a) Rh/CeO2 (b) CeO2 (T = 973 K).

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210208

Table 3

XPS parameters of Rh–CeO2/Al2O3 catalyst

Sample O 1s Ce 3d5/2 Rh 3d5/2 Rh 3d3/2

Rh/Al2O3 531.2 – 307.4 312.1

Rh–CeO2/Al2O3 531.1 880.2,882.9,885.9,898.1 307.7 312.4

Rh [26] – – 307.2 311.8

Rh2O [27] – – 308.0 312.8

conductivity was detected, which was because

CH4�!Rh

CHþ4 þ e��!RhCHx þ ð4� xÞH (7)

CO2 þ CeO2�x þ VþO2� þ e�!CO�2 þ CeO2�xþVþ

O2�

!CeO2�y þ COðy< xÞ (8)

that CO2 adsorbed in the oxygen vacancies of ceria and

accepted free electrons to replenish the oxygen vacancies.

In Panel B, in the case of repeated alternating exposure of

CH4, N2 and CO2 after N2 treatment, the results obtained

demonstrated the variation trend similar to that in Panel A. On

introducing CH4, sRh=CeO2was increased again. This demon-

strated that the reoxidized ceria was reduced again by methane,

regenerating the oxygen vacancies and releasing free electrons.

As suggested by the results of pulse experiments (Table 2), H2

and CO were reproduced over Rh/CeO2 during the last CH4

pulses.

CH4 þ Rh�CeO2!Rh�CeO2�x þ VþO2� þ e� þ xCOþ xH2

(9)

Nevertheless, the initial sRh=CeO2in Panel B was lower than that

in Panel A. This is because that CO2 replenishes oxygen

vacancies of ceria, and the quantity of conductive free electrons

is accordingly reduced. Note that the variation intensity of

sRh=CeO2in the two CH4 Panels was almost the same, so the

creation of oxygen vacancies of ceria is a reversible process in

the redox step.

For CeO2 in Panel A (Fig. 3b), the slight decrease in sCeO2

exposed to CH4 was due to the decreased H2 adsorption on the

catalyst surface. This is consistent with prior pulse reaction

results (Table 2) that methane decomposition could not occur

over pure CeO2 in the absence of Rh.

3.3.3. XPS studies

3.3.3.1. Rh 3d XPS. Rh 3d XPS spectra of the reduced Rh-

based catalysts are shown in Fig. 4. The binding energies values

of Rh 3d are summarized in Table 3, and the reported BE values

Fig. 4. Rh 3d XPS spectra of reduced catalysts (a) Rh/Al2O3, (b) Rh–CeO2/

Al2O3 at 773 K.

of some rhodium compounds with Rh ions in different valence

states [26,27] are given for comparison. For Rh/Al2O3 catalyst

(Fig. 4a), two broad photoemission peaks were observed,

located at 307.4 and 312.1 eV, respectively. Compared with the

BE values of Rh 3d in pure rhodium metal foil (Table 3), the

doublet may be attributed to Rh0 species, indicating an almost

complete reduction of rhodium oxide and no electrical

interaction between Rh and Al2O3. In the case of CeO2-

promoted Rh catalyst (Fig. 4b), Rh (3d5/2,3/2) peaks moved to

307.7 and 312.4 eV, respectively. The small but definite

electropositive shifts detected for two peaks with the CeO2

addition should be attributed to the formation of Rhd+

(0 < d < 1) species, meaning the coexistence of both Rh0

and Rhd+ oxidation states. We conclude that this shift

corresponds to an electronic transfer from Rh to CeO2, due

to the interaction between Rh and CeO2.

3.3.3.2. Ce 3d XPS. Ce 3d XPS spectra of the reduced Rh–

CeO2/Al2O3 catalyst are shown in Fig. 5. According to the

resolution to the complexity of spectra by Burroughs et al. [28],

peaks denoted as ‘‘u’’ and ‘‘v’’ correspond to Ce 3d3/2 and 3d5/2

contribution, respectively, where uðvÞ and u00ðv00Þ peaks are the

representative of Ce4+. In Fig. 5, new u0ðv0Þ and u0ðv0Þ peaks

were also observed, which were assigned to Ce3+ state.

Moreover, the peak intensity of Ce4+ was lower than that of

Ce3+, suggesting that CeO2�x (0 < x < 0.5) was formed due to

the Rh–CeO2 interaction under reducing conditions. The

average valence should be between 3 and 4. By calculation of

the relative intensity of the u0ðv0Þ and u0ðv0Þ peaks in the total

Fig. 5. Ce 3d XPS spectra of reduced Rh–CeO2/Al2O3 catalyst at 773 K.

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210 209

Ce 3d region [29,30], the estimated reduction percentage of

ceria was about 63%. Thus we conclude that both Rh0/Rhd+ and

Ce4+/Ce3+ redox couples really exist over Rh–CeO2/Al2O3

under the reduction atmosphere.

4. Discussion

A SMSI (strong metal-support interaction) state has been

intensively investigated in ceria-supported noble metal

catalysts by many authors [10,31,32]. In this study, a strong

Rh–CeO2 interaction over Rh–CeO2/Al2O3 was induced as

well. Our TPR results showed that the strong Rh–CeO2

interaction greatly favored both the reduction of interactive

RhOx–Al2O3 species and the reduction of ceria near the Rh. The

electrical conductivity measurements (Fig. 2) showed that

oxygen vacancies of ceria were generated with the aid of H2 and

Rh, accompanied by the creation of a Ce4+/Ce3+ redox couple.

XPS results (Table 3) further indicated that the electrons could

transfer from Rh to CeO2 under reducing conditions. As a

consequence, the redox couples of Ce4+/Ce3+ and Rh0/Rhd+

coexist in Rh–CeO2/Al2O3 catalyst. Taking these findings into

account, we proposed a general mechanism for the Rh–CeO2

system as shown in Fig. 6, based on the activation of both CH4

and CO2 as two independent paths. Our mechanism success-

fully correlates the enhancements of catalytic activity and

coking resistivity.

It is generally accepted that methane is mainly activated on

metallic surfaces. The electron donation from the HOMO of

CH4 to the lowest unfilled molecular orbitals of metal surface

should dominate dissociative CH4 adsorption (path (i)). For

Rh–CeO2/Al2O3, the addition of CeO2 causes the presence of

an Rh0/Rhd+ redox couple. The electron-deficient state (Rhd+)

of Rh particles is apt to activate CH4 molecules by accepting s

electrons of C–H bond to promote their cleavage, which

enhances the CH4 conversion. This result appears to contradict

to a recent study of Damyanova et al. [13] in which the catalytic

activities of Pt/Al2O3 decreased a little with higher CeO2

loading (�6 wt.%). The authors attributed it to the increased

oxidation state of surface Pt sites, which decreased the quantity

of active Pt0 on the surface. However, this interpretation may

Fig. 6. Mechanistic cycle proposed over Rh–CeO2/Al2O3 in CH4/CO2

reforming.

deserve further investigation to get more explicit explanation

(for example, the coverage of Pt by ceria at high CeO2 loading

should be first excluded).

On the other hand, some researchers of Ni catalysts [33,34]

have reported that lattice oxygen may participate in CH4

activation to promote decomposition of CHxO intermediates,

one of the proposed slow kinetic steps in CH4–CO2 reforming

over supported Ni catalyst. We observed (Table 2) the

production of H2 and CO over reduced CeO2, indicating that

the Ce4+/Ce3+ redox couple could play some positive role in

CH4 activation for partial oxidation of methane to form CO.

Note that the amounts of H2 and CO produced during the

CH4 pulse series over Rh/CeO2 were significantly higher than

those over CeO2. Therefore, the existence of the Rh0/Rhd+

redox couple also favors the generation of oxygen vacancies

and the Ce4+/Ce3+ redox couple by promoting partial

oxidation of methane over ceria. Similar results over Pt/

CeO2 were reported by Otsuka et al. [35], who found that

the oxidation of CH4 by CeO2 was thermodynamically viable

at above 873 K. The reduction degree of CeO2 was 3.5% after

the reaction with CH4 at 973 K, but the reduction degree of

CeO2 was significantly improved to 17.1% in the presence of

Pt.

In contrast with methane, carbon dioxide activation on

rhodium is much slower. Aparicio et al. [36] observed that the

major part of rhodium remained unmodified as Rh0 state after

15 min under CO2 atmosphere. They proposed that carbon

dioxide activation on rhodium was a rather slow process on Rh/

Al2O3 catalyst. Trovarelli et al. [37] have pointed out that the

reduction treatment strongly affects the interaction of CO2 with

the Rh/CeO2 system. When ceria has been reduced at high

temperature (T � 773 K), oxygen vacancies, particularly those

present in the bulk, are the driving force for CO2 activation.

Therefore, CO2 activation (path (ii)) should be mainly favored

by availability of Ce3+ species of Ce4+/Ce3+ redox couple in

Rh–CeO2/Al2O3 catalyst. Pulse reaction data (Table 2) and in

situ electrical conductivity results (Fig. 2) support the

conclusion that CO2 is readily dissociated to CO and adsorbed

oxygen over reduced CeO2 by filling up the oxygen vacancies in

Ce3+ species. The occurrence of Ce4+/Ce3+ redox couples

generates oxygen vacancies and releases free electrons. Free

electrons transfer readily from Ce3+ to p* orbit of CO2 to

activate CO2. The increased adsorbed CO2 then decomposes to

CO and active surface oxygen, which reacts with the CHx

species and enhances the catalytic activity of CH4/CO2

reforming. It is clear that the role of the Ce4+/Ce3+ redox

couple in CO2 activation is dominant, in comparison with CH4

activation. As the catalytic test results showed (Table 1), the

promotional effect of CeO2 on CO2 conversion was much

higher than that on CH4 conversion.

Next comes the continuance of cycle processes with Ce4+/

Ce3+ and Rh0/Rhd+ redox couples. In situ electrical conductivity

measurement results (Figs. 2 and 3) indicated that the reduction

of ceria led to the generation of oxygen vacancies and that the

reoxidation of ceria by CO2 replenished the oxygen vacancies.

In CH4/CO2 reforming, the existence of Rh0/Rhd+ (cycle 1)

favors CH4 decomposition to CHx and H2 formation. The

R. Wang et al. / Applied Catalysis A: General 305 (2006) 204–210210

partial oxidiation of CHx species over ceria will continuously

result in the creation of oxygen vacancies and Ce4+/Ce3+ redox

couples. CH4 decomposition acts as the supplier of a hydrogen

pool, while Ce4+/Ce3+ redox couple (cycle 2) promotes CO2

activation by replenishing the oxygen vacancies, and CO2

provides a constant oxygen resource. Therefore, the redox

atmosphere is a driving force for the continuity of the cycle

process, and it is through the two redox couples of Rh0/Rhd+ and

Ce4+/Ce3+ in Rh–CeO2/Al2O3 catalyst that the electrons

transfer successfully between reactants (CH4 and CO2). The

continuous cycle of redox couples guarantees the maintenance

of catalytic activity in CH4/CO2 reforming.

Finally, carbon suppression on Rh–CeO2/Al2O3 catalyst

may also be associated with the coexistence of redox couples.

Rh0/Rhd+ redox couples promote CH4 decomposition into

surface CHx species and hydrogen. However, Ce4+/Ce3+ redox

couples facilitate the elimination of CHx species by partial

oxidation, resulting in higher methane conversion and lower

amounts of coke (Table 1). In the case of rhodium-based

catalysts, CO2 activation has always been proposed as the rate-

determining step for dry reforming of methane [38,39]. With

the aid of Ce4+/Ce3+ redox couples, CO2 is more readily

activated to release more surface oxygen, and the rate of carbon

elimination therefore has been accelerated.

5. Conclusions

An Rh–CeO2 interaction was induced by high temperature

reduction, which resulted in the creation of oxygen vacancies in

ceria. The electrons could transfer from Rh to CeO2 on the Rh–

Ceria perimeter. The Ce4+/Ce3+ and Rh0/Rhd+ redox couples

were found to coexist in Rh–CeO2/Al2O3 catalyst. CH4

decomposition on Rh released electrons, whereas CO2

replenished oxygen vacancies by accepting electrons. The

presence of Ce4+/Ce3+ and Rh0/Rhd+ redox couples is a

reversible cycle process in the reaction atmosphere.

The higher catalytic activity and coke resistance of Rh–

CeO2/Al2O3 are associated with the presence of two redox

couples, which favors the activation of both CH4 and CO2. For

CH4 activation, an Rh0/Rhd+ redox couple is apt to accept s

electrons of C–H bond to favor its cleavage. The Ce4+/Ce3+

redox couple also plays some role in the activation of CH4 for

partial oxidation of methane. Moreover, the presence of Rh0/

Rhd+ redox couples facilitate the generation of Ce4+/Ce3+ redox

couples. On the other hand, CO2 is mainly activated by the

Ce4+/Ce3+ redox couple. The Ce3+ species readily promote CO2

dissociation into CO and surface oxygen, which reacts with

CHx to form CO. This enhances the catalytic performance and

eliminates carbonaceous species.

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