Journal of Catalysis 299 (2013) 272–283

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Journal of Catalysis

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Preferential oxidation of CO in the presence of excess of hydrogenon Ru/Al2O3 catalyst: Promoting effect of ceria–terbia mixed oxide

Xiaowei Chen ⇑, Juan J. Delgado, José M. Gatica, Samira Zerrad, José M. Cies, Serafín BernalDepartamento de Ciencia de los Materiales, Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, E-11510 PuertoReal (Cádiz), Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 December 2012Accepted 15 December 2012Available online 28 January 2013

Keywords:Preferential CO oxidation in the presence ofexcess of hydrogenRuCeria–terbia mixed oxideCO methanationDiffuse reflectance infrared fouriertransformTransient experiment

0021-9517/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcat.2012.12.025

⇑ Corresponding author. Fax: +34 956 016288.E-mail address: xiaowei.chen@uca.es (X. Chen).

The promoting effect of a Ce0.80Tb0.20O2�x mixed oxide on the catalytic behavior of a Ru supported on acommercial alumina catalyst for preferential CO oxidation in the presence of excess hydrogen reaction(PROX) has been investigated. Ru supported on this alumina and ceria–terbia mixed oxide has also beenstudied as reference systems. N2 physisorption, X-ray diffraction, high-angle annular dark field–scanningtransmission electron microscopy (HAADF–STEM), CO and H2 chemisorption, and diffuse reflectanceinfrared Fourier transform have been used to characterize these Ru catalysts. The Ru/Ce0.80Tb0.20O2�x–Al2O3 catalyst resulting from the modification of the Ru/Al2O3 sample with the ceria–terbia mixed oxideexhibits higher activity for PROX reaction and wider working temperature window than Ru/Al2O3 and Ru/Ce0.80Tb0.20O2�x samples. This remarkable behavior is due to the simultaneous occurrence of CO oxidationand CO hydrogenation to methane on the former catalyst. Moreover, Ru/Ce0.80Tb0.20O2�x–Al2O3 catalyst isthe most active catalyst under the ideal PROX feed. It also shows high activity in presence of CO2 and H2O.The CO methanation also has been performed over Ru/Ce0.80Tb0.20O2�x–Al2O3 catalyst. The hydrogen con-centration in the PROX feed has been varied in order to study the effect of hydrogen on PROX activity. ThePROX reaction and transient experiment have been carried out using in situ diffuse reflectance infraredFourier transform technique. The combination of Ru nanoparticles and the promotion effect of ceria–ter-bia are attributed to the unique performance of PROX reaction over Ru/Ce0.80Tb0.20O2�x–Al2O3 catalyst.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

The Pt catalysts for proton exchange membrane fuel cells (PEM-FCs) are well known to be prone to suffering strong deactivation byCO. Therefore, extremely low CO content (<0.2 ppm) is required inthe H2 stream feeding PEMFCs. To achieve such a high purity, thehydrocarbon reformers must include a cleaning-up process follow-ing the water–gas shift step. Preferential CO oxidation in the pres-ence of excess of hydrogen (PROX) [1–4], selective CO methanation[1–3], and selective diffusion [1] have been considered as some ofthe most promising approaches for this purpose.

Noble metals (Ru, Au, and Pt) supported on high-surface areaalumina have shown to be the active catalyst for PROX [1,5–8].However, there are still some problems for the PROX catalysts.On the one hand, the required CO conversion is difficult to reach[6] because at high temperatures, CO and H2 oxidation start to becompetitive being a critical issue in this respect [4]. On the otherhand, plenty of PROX results over noble metal catalysts have beenreported; however, the details of the reaction mechanism stillremain as a major open question.

ll rights reserved.

Ceria is one of the most investigated supports for PROX reaction[9–12]. As in many other catalytic applications [13–15], redoxproperties of ceria are considered to play a relevant role in this pro-cess. Recently, ceria-based mixed oxides have been employed as apromoter of alumina support in order to incorporate an activecomponent to the formation of catalysts and to improve the redoxthermal stability of alumina [16–18]. Previous studies from ourlaboratory have reported that the ceria–terbia mixed oxides sup-ported on lanthana-modified alumina show good stability up to700 �C under a 5% H2/Ar flow and even better in air or inert atmo-sphere [16]. Furthermore, the Pd nanosized particles and Ce0.80-

Tb0.20O2�x mixed oxide maintain over the Pd/Ce0.80Tb0.20O2�x/La2O3–Al2O3 catalyst when the temperature is up to 700 �C [17].Meanwhile, Silva et al. revealed that Pt/CeZrOx/Al2O3 catalystshowed the best catalytic performance on the partial oxidation ofmethane because the addition of ceria–zirconia to alumina leadsto form homogeneous solid solution and high oxygen storagecapacity [19]. The high activity and stability of this catalyst com-bined with higher metal dispersion, higher oxygen storage capac-ity, and larger degree of coverage of alumina by CeZrO2 improvethe effectiveness of the carbon cleaning mechanism. To summa-rize, ceria or ceria-based mixed oxide have interesting propertiesas promoters of alumina supports.

X. Chen et al. / Journal of Catalysis 299 (2013) 272–283 273

In this work, the influence of the incorporation a ceria–terbiamixed oxide (Ce0.80Tb0.20O2�x) on the PROX activity of an aluminasupported Ru catalyst has been studied. As reference systems, Ru/Al2O3 and Ru/Ce0.80Tb0.20O2�x samples have also been investigated.As will be shown, in addition to exhibiting the highest CO conver-sion, the ceria–terbia promoted sample widens significantly thetemperature range at which 100% conversion of CO is achieved.To gain more understanding of the behaviors exhibited by ceria–terbia promoted Ru/Al2O3 catalyst, additional DRIFT studies are re-ported and discussed.

2. Experimental

2.1. Catalyst preparation

The sample used as support in this work consisted of a lanthan-a-doped alumina (4.0 wt% La2O3) was supplied by Grace Davison.This sample is referred to AL. The mixture of cerium nitrate andterbium nitrate solution with a molar ratio of 4:1 was depositedon this commercial alumina, and the weight percentage of Ce0.80-

Tb0.20O2�x is 19.4 wt%. After impregnation, the sample was driedat 110 �C overnight and then calcined at 500 �C for 4 h in air. Thissupport is labeled as CT–AL. The Ru catalysts were prepared byincipient wetness impregnation from an aqueous solution ofRuNO(NO3)3 on AL and CT–AL supports. Ce0.80Tb0.20O2�x with rela-tively high-surface area (99% pure, decoded as CT), which is alsofrom Grace Davison, was also employed as a support as a referencecatalyst. The samples impregnated with the ruthenium precursorwere further dried in air at 110 �C and then reduced in a 5% H2/Ar flow at 350 �C for 1 h and finally purged with a flow of He at400 �C for 1 h. The abbreviation name and composition of the sam-ples are listed in Table 1.

2.2. Catalyst characterization

The samples were characterized by means of N2 adsorption at77 K using Micromeritics ASAP 2020C device to study the texturalproperties. The Ru loadings of the catalysts were determined byinductively coupled plasma (ICP) analysis with an Iris IntrepidSpectrometer (Thermo Elemental). An Ethos 1600 microwave lab-station (Milestone) was employed to prepare the acid solution ofall the samples for ICP analysis. X-ray diffraction (XRD) analyseswere performed on a Philips PW1820 diffractometer (Cu Ka radia-tion) operating at 40 kV and 40 mA.

H2 chemisorption studies in dynamic conditions were con-ducted on the catalysts, and consecutive pulses of 1.12 lmol H2

were injected at 35 �C on the sample maintained in He flow untilreaching stable pulse signals registered in a Pfeiffer, ThermostarQMS200, quadrupole mass spectrometer. Both H2 and CO volumet-ric adsorption isotherms were registered using an automaticMicromeritics ASAP 2020C instrument. In the case of the H2 anal-ysis, different temperatures (35, 100, and 200 �C) and a wide pres-sure range (PH2 = 5–450 Torr) were studied. For the CO

Table 1Composition (ICP analysis) and textural (N2 physisorption) results of the supports and Ru

Samples Compositiona

AL Al2O3–4.0%La2O3

CT Ce0.80Tb0.20O2�x

CT–AL 19.4% Ce0.80Tb0.20O2�x/Al2O3–4.0%La2O3

Ru/AL 2.6%Ru/Al2O3–4.0%La2O3

Ru/CT 0.67%Ru/Ce0.80Tb0.20O2�x

Ru/CT–AL 2.6%Ru/19.4% Ce0.80Tb0.20O2�x/Al2O3–4.0%La2O3

Ru/CT–AL(C) 2.6%Ru/19.4%Ce0.80Tb0.20O2�x/Al2O3–4.0%La2O3 calcined at 500 �C

a Weight percentage of per gram of sample.

chemisorption, two consecutive isotherms at 35 �C with an evacu-ation step of 30 min, at 35 �C in between, were performed for theCO chemisorption experiments. Prior to all the chemisorptionexperiments, the catalysts were reduced in situ at 350 �C for 1 hin a flow of 5% H2/Ar and further evacuated at 400 �C for 1 h.

Transmission electron microscopy (TEM) measurements wereperformed on a JEOL2010F instrument, with 0.19 nm spatial reso-lution at Scherzer defocus conditions. High-angle annular darkfield scanning transmission electron microscopy (HAADF–STEM)images were recorded on the same microscope by using an elec-tron probe of 0.5 nm of diameter at a diffraction camera lengthof 10 cm. Based on the HAADF–STEM images of the Ru catalysts,the metal particle size distribution and Ru dispersion data havebeen obtained using a home-made software (GAUSS). The modelof Ru particle used for calculation is truncated bipyramid.

2.3. Activity testing

The catalytic activity was studied on a U-shape reactor in which25 mg of catalyst mixed with 50 mg of quartz powder (0.10–0.18 mm) was placed. The catalysts were re-reduced under a flowof 5% H2/Ar at 350 �C for 1 h and then purged with a flow of He at500 �C for 1 h. The standard PROX mixture was 1% CO, 1% O2, 50%H2, and 48% He. Additional activity tests were also run in which 5%CO2 and/or 5% H2O were incorporated to the feedstock. Likewise,the concentration of H2 was varied from 0, 10, 20, 50–60 vol%,while keeping constant the concentration of CO and O2. The totalflow rate was always 100 mL/min. The reaction temperature wasvaried in a stepwise manner from 35, 50, 75, 100, 125, 150, 175–200 �C. Duration of isothermal steps is 1 h. A micro gas chromato-graph (Varian CP 4900) with two channels was used as an analyt-ical device. One channel is coupled to a column of Molsieve 5 Å toseparate CO and O2; the other one being attached a column of CP-PoraPLOT Q to analyze CO2 and H2O. The CO and O2 conversionswere calculated using the changes of CO concentration betweenthe inlet and the outlet of the catalyst bed. CO and O2 conversionand O2 selectivity to CO oxidation were calculated as follows:

CO conversion ð%Þ ¼ ½CO�in � ½CO�out

½CO�in� 100

O2conversion ð%Þ ¼ ½O2�in � ½O2�out

½O2�in� 100

O2 selectivity to CO2 ð%Þ ¼ 0:5� ½CO�in � ½CO�out � ½CH4�out

½O2�in � ½O2�out� 100

2.4. In situ DRIFT studies of PROX over Ru catalysts

In situ DRIFT spectra were recorded with a Bruker VERTEX 70spectrometer. The high temperature diffuse reflectance IR cell(from Spectra-Tech) is fitted with ZnSe windows. A reference DRIFT

catalysts.

SBET (m2/g) Micropore volume (cm3/g) Total pore volume (cm3/g)

182 0.065 0.73183 0.028 0.162

164 0.058 0.594179 0.008 0.631

80 0.027 0.158166 0.059 0.510151 0.055 0.489

o

Ru/CT

Ru/CT-AL (C)

Ru/AL

Ru/CT-AL

CT-AL

dV/d

r (cm

3 /A/g

)

AL

0.025

274 X. Chen et al. / Journal of Catalysis 299 (2013) 272–283

was registered over a mirror at room temperature and used as abackground. As in the case of the catalytic assays, prior to theDRIFT measurements, the Ru catalysts were reduced at 300 �C for1 h and then purged with He at 300 �C for 1 h and cooled downto 25 �C also in the flow of He. Following the activation pretreat-ment of the catalyst, the gas flow was switched to the PROX reac-tion mixture, and the reaction temperature of the DRIFT cell wasincreased in a stepwise at 50, 75, 100, 125, 150, 175, and 200 �C.The duration of the isothermal steps was always 1 h. Transientexperiment was also performed using the same in situ DRIFT cell.After reduction in the Ru catalyst, 1% CO/He was introduced intothe DRIFT cell and maintained for 1 h. Then, 1% O2 was added tothe gas feed. After 1 h under flowing of CO and O2, H2 was addedto the feed. One more hour being kept the same feed, the changesoccurred in the spectra were recorded. Then, H2 flow was stopped,and CO oxidation feed was recovered and maintained for 1 h. Thelast step of the transient experiment was to stop the flow of O2

and only kept the flow of CO. The total flow rate was always kept100 mL/min, and the temperature of the catalyst cup was 175 �Call the time for transient experiment.

100 1000o

Average pore size (A)o

Fig. 1. BJH pore size distribution of the supports and Ru catalysts.

20 40 602θ (º)

80 100

Fig. 2. XRD profiles of the supports and Ru catalysts.

3. Results and discussion

3.1. Composition and textural results

The physicochemical properties of the supports and Ru catalystsare summarized in Table 1. After deposition of Ce0.80Tb0.20O2�x onAL support, the BET-specific surface area of the alumina supportdecreases from 182 to 164 m2/g, while the total pore volume dropsfrom 0.731 to 0.594 cm3/g. This suggests the incorporation of theceria–terbia mixed oxide at the alumina sample. The texturalchanges occurred upon impregnation with the ruthenium precur-sor salt were found to be very slight. The Ru loadings of the Ru/AL and Ru/CT–AL catalysts were the same 2.6 wt% according toICP analysis. To investigate the effect of an additional calcinationtreatment on the catalytic behavior the Ru/CT–AL catalyst, it is fur-ther heated at 500 �C for 4 h in air. The BET-specific surface areadecreases slightly from 166 to 151 m2/g; meanwhile, the total porevolume also declines from 0.510 to 0.489 cm3/g. This indicates thatthe textural properties of the catalyst are slightly modified by thetreatment at 500 �C.

Fig. 1 shows the BJH mesopore size distribution of the supportsand Ru catalysts. The shape of pore size distributions for AL, CT–AL,and Ru/CT–AL is very similar, which means the incorporation ofCe0.80Tb0.20O2�x mixed oxide and Ru on does not significantlychange the textual properties of the AL support.

3.2. X-ray diffraction

The XRD profiles of the supports and Ru catalysts are shown inFig. 2. After deposition of ceria–terbia mixed oxide on alumina, themain peaks observed at 28.6�, 33.1�, 47.5�, and 56.3� can be attrib-uted to CT mixed oxide. The additional features at 37.3� and 66.9�are interpreted to be due to the alumina support. The crystal size ofmixed oxide Ce0.80Tb0.20O2�x has been calculated using Warren andScherrer equations in their peaks at half the maximum intensity(FWHM) in radians. The average calculated crystal size of ceria ter-bia mixed oxide is 7.7 nm. This result is in accordance with earlierresults from the laboratory for other CT/Al2O3 sample which wasprepared using the same synthesis method and characterized byXRD and TEM techniques [16].

The Ru/AL catalyst only exhibits the typical diffraction peaks ofc-alumina at 37.3�, 45.9�, and 66.9�, suggesting that Ru is well dis-persed on the alumina. The XRD profile of the Ru/CT catalyst isquite similar to that of the pure cubic fluorite-like CT support. No

Ru diffraction peak is observed on the Ru/CT and Ru/CT–AL cata-lysts. However, after calcination at 500 �C, new peaks at 28�,35.2�, and 54.5�, which are assigned to RuO2, can be clearly seen.The mean size of RuO2 nanocrystals as determined form XRD anal-ysis was found to be 27 nm. In good agreement with earlier studies[20], this indicates that under oxidizing conditions, the heating at500 �C induces a strong sintering of the Ru phase.

3.3. TEM results

Fig. 3 shows representative HAADF–STEM images for CT–ALsupport and the corresponding Ru catalysts. As deduced fromFig. 3a, Ce0.80Tb0.20O2�x aggregates on alumina support. This obser-vation is in good agreement with the XRD study and commentedon above, some earlier HRTEM studies on an alumina supportedcerium-praseodymium mixed oxide [18]. Parallel nano-analytical(EDX) studies show that the brighter parts in the HAADF–STEMimages are due to the ceria–terbia mixed oxide nanocrystals. How-ever, there is still some ceria–terbia mixed oxide on the darker

Fig. 3. HAADF–STEM images of the support and Ru catalysts: (a) CT–AL support, (b) Ru/CT, (c) Ru/AL, (d) Ru/CT–AL and (e) Ru/CT–AL (C).

X. Chen et al. / Journal of Catalysis 299 (2013) 272–283 275

parts. This means that ceria–terbia mixed oxide covers almost allthe surface of alumina. Fig. 3b is a HAADF image of 0.67 wt% Ru/CT catalyst. No Ru particle could be found on this sample, which re-veals a high dispersion of the Ru phase. As seen in Fig. 3c, Ru ishomogeneously dispersed on AL support. The Ru dispersion calcu-lated according to this study is 41%. The HAADF–STEM studies ofthe Ru/CT–AL catalyst reduced at 350 �C (Fig. 3d) indicate thatthe metal phase is well dispersed (Ru dispersion is 53%). By con-trast, the Ru/CT–AL calcined at 500 �C shows Ru as big as 100 nm(Fig. 3e). No reliable metal dispersion data could therefore be esti-mated for the latter sample. The Ru particle size distribution, metaldispersion, and average particle size for Ru/AL and Ru/CT–AL cata-lysts are summarized in Fig. 4.

3.4. Catalytic activity for PROX and CO methanation

Fig. 5a displays the evolution of CO conversion with the temper-ature over Ru catalysts under ideal PROX reaction conditions. For

the Ru/CT catalyst, the highest CO conversion, 80%, is reached at150 �C. The maximum of CO conversion is 90% at 125 �C over Ru/AL catalyst. After reaching the maximum values, the CO conversionstarts to decrease with the temperature increasing; meanwhile, theO2 conversion maintains 100% (Fig. 5c). As mentioned before, thecompetition of CO oxidation and hydrogen oxidation is the key is-sue for PROX reaction. Over the Ru/AL and Ru/CT catalysts, for tem-peratures higher than 150 �C, hydrogen oxidation becomes thefastest process. However, the catalytic behavior of Ru/CT–AL cata-lyst for PROX reaction is remarkably different from that of abovementioned two catalysts. Two major differences may be noticed:(a) the CO conversion reaches 100% and (b) this high conversionis kept for a rather wide range of temperatures (175–200 �C). COis almost fully oxidized at 150 �C over this catalyst (CO conversionis 97%); when the temperature is higher than 175 �C, CO conver-sion is up to 100%, but part of CO is actually eliminated by the vir-tue of the methanation reaction as methane could be identified inthe reaction products. Fig. 5b presents the CO conversion to CH4

0 1 2 3 4 5 6 70

5

10

15

20

25

30Ru/ALDispersion=41%Average diameter=2.0 nm

Perc

enta

ge o

f par

ticle

s (%

)

Particle size (nm)

0 1 2 3 4 5 6 70

2

4

6

8

10

12

14

16

18

20

Particle size (nm)

Perc

enta

ge o

f par

ticle

s (%

)

Ru/CT-ALDispersion=53%Average diameter=1.7 nm

Fig. 4. Particle size distribution of the Ru/AL and Ru/CT–AL catalysts.

50 75 100 125 150 175 200

0

20

40

60

80

100(a)

CO

con

vers

ion

(%)

Temperature (oC)

Ru/CTRu/ALRu/CT-ALRu/CT-AL (C)

50 75 100 125 150 175 200

0

10

20

30

CO

con

vers

ion

to C

H4 (%

)

Temperature (oC)

Ru/CTRu/ALRu/CT-ALRu/CT-AL (C)

(b)

50 75 100 125 150 175 200

0

20

40

60

80

100

0

20

40

60

80

100

O2 selectivity (%

)

Ru/CTRu/ALRu/CT-ALRu/CT-AL (C)Ru/CTRu/ALRu/CT-ALRu/CT-AL (C)

(c)

Temperature (oC)

O2

conv

ersi

on (%

)

Fig. 5. (a) Total conversion of CO; (b) CO conversion to methane and (c) O2

conversion and selectivity for ideal PROX reaction over the Ru catalysts. Reactionmixture: 1% CO, 1% O2, 50% H2 and 48% He. The total flow rate is 100 mL/min.

276 X. Chen et al. / Journal of Catalysis 299 (2013) 272–283

over the Ru catalysts. Below 150 �C, no CH4 was detected over theRu/CT–AL catalyst. Upon the increasing temperature, CO conver-sion to CH4 increases from 0% to 13.4% at 175 �C and 17.8% at200 �C. This result is in good agreement with the results from Xuand Zhang [21], who found that the CO removal was due to thepreferential oxidation of CO until 110 �C on Ru/Al2O3 catalyst; overthis temperature, CO methanation starts to make a measurablecontribution, which compensates the decrease in CO oxidationrate. The PROX reaction feed in this work is 70.1% H2, 29.1% CO2,0.5% CO, and 0.3% O2 [21]. The activity in the methanation reactionhas been recently reported for a very similar ruthenium catalystunder water–gas shift reaction conditions [22]. No methane pro-duction was observed over the Ru/AL and Ru/CT catalysts(Fig. 5b). According to the literatures, the activity of alumina sup-ported Ru catalyst for CO methanation decreases with decreasingRu particle size [23–24]. This effect probably explains why theRu/CT catalyst with much better Ru dispersion is not active forCO methanation. The Ru/CT–AL catalyst exhibits higher activityfor PROX reaction in a wider temperature range than Ru/CT andRu/AL catalysts. In this sense, the modification of ceria–terbiamixed oxide on alumina improves the catalytic activity for PROXreaction and modifies the product distribution. Han et al. demon-strated that 5% Ru/Al2O3 catalyst exhibited a significantly higheractivity and slightly higher selectivity for the selective oxidationof CO, over a wide range of CO concentration, than commonly usedPt/Al2O3 catalyst [25]. When the temperature is lower than 200 �C,only CO2 is detected. Methane was found increase from 0.1 kPa at225 �C to 0.6 kPa at 300 �C.

The catalytic activity of the calcined Ru/CT–AL catalyst for PROXreaction also was evaluated. The CO conversion of the calcined Ru/CT–AL catalyst is much lower than the uncalcined Ru/CT–AL andother two Ru catalysts in all the temperature range, and the high-est CO conversion is only 20% at 200 �C. As in the case of the pre-reduced Ru/CT–AL catalyst, CO methanation occurs over Ru/CT–AL (C) catalyst even though CO conversion to methane is verylow (less than 2%). This result certifies that CO methanation onlyoccurs on the ceria–terbia promoted alumina catalyst. As men-tioned in XRD and TEM result parts, there is severe sintering ofRu over this calcined Ru/CT–AL catalyst. The lower activity of PROX

X. Chen et al. / Journal of Catalysis 299 (2013) 272–283 277

for the Ru/CT–AL (C) catalyst is due to the loss of Ru active phase bysintering. This also confirms that well-dispersed Ru on ceria–terbiamodified alumina is essential to this unique catalytic behavior forPROX reaction.

Fig. 5c presents the O2 conversion and O2 selectivity to CO2 overthe Ru catalysts. The O2 conversion is almost 100% at 150 �C overall the Ru catalysts except the Ru/CT–AL (C) catalyst. The Ru/ALcatalyst exhibits the highest O2 selectivity among these Ru cata-lysts. The Ru/CT–AL catalyst shows lower O2 selectivity becausepart of CO reacts with H2 even though the CO conversion is quitehigh over this catalyst. H2 oxidation starts to be dominant on thiscatalyst when the temperature reaches 150 �C. The modification ofceria–terbia promotes the PROX reaction and CO methanation. TheRu/CT–AL (C) catalyst shows much lower O2 selectivity for PROXreaction. This also indicates that the ceria–terbia mixed oxide pro-motes PROX reaction, and Ru is the active phase for PROX reaction.Silva et al. reported that the incorporation of ceria or ceria zirconiamixed oxide to alumina leads to higher activity and stability formethane partial oxidation reaction because of good distributionof ceria-based oxide on alumina, good reducibility, and high oxy-gen storage capacity [26]. The ceria–terbia mixed oxide on the alu-mina support plays an important role in the PROX reaction and COmethanation over Ru/CT–AL catalyst.

Table 2 lists the reaction rates and the turnover frequencies ofPROX reaction on different supported Ru catalysts in this workand some results from the literature. Since the experimental condi-tions, such as the composition of the reaction mixture, the spacevelocity and reaction temperature are not same, the specificparameters applied in each case are also be reported in Table 2. Un-der the same reaction mixture, the reaction rates of the Ru/CT, Ru/AL, and Ru/CT–AL catalysts are higher than those of Ru/a-Al2O3 [8]and Ru/YSZ [27] catalysts. The turnover frequencies of our Ru cat-alysts are higher or similar to the Ru/c-Al2O3 catalysts [3]. Only inthe case of the work from Chin et al [28], in which the concentra-tion of CO and O2 is lower than ours, and the calculation of turn-over frequency is based the adsorption of O2, the turnoverfrequency of our Ru catalysts is lower than the value of this article.This comparison indicates that our Ru catalysts exhibit excellentcatalytic performance for PROX reaction.

Since CO2 is the product of PROX reaction, and Ru has been rec-ognized as an active catalyst for CO and CO2 methanation [29–30];is it possible that product CO2 is also involved the methanationreaction to form methane over Ru/CT–AL catalyst when the tem-

Table 2Reaction rates of PROX reaction on different supported Ru catalysts and the respective rea

Catalyst Temperature(�C)

Reaction gas mixture Space velocity

(mL min�1 g�1cat)

0.67%Ru/CT 150 1% CO, 1% O2, 50% H2, 48%He

2000

2.6%Ru/AL 150 1% CO, 1% O2, 50% H2, 48%He

2000

2.6%Ru/CT–AL 150 1% CO, 1% O2, 50% H2, 48%He

2000

0.696% Ru/a-Al2O3

120 1% CO, 1% O2, 50% H2, 48%He

1666

0.64% Ru/YSZc 130 1% CO, 1% O2, 50% H2, 48%He

3333

0.5%Ru/c-Al2O3 60 1% CO, 1% O2, 50% H2, 48%He

1000

5% Ru/c-Al2O3 60 1% CO, 1% O2, 50% H2, 48%He

10,000

1% Ru/Al2O3 120 0.5% CO, 0.5% O2, 45% H2,54% N2

2000

a TOF based on Ru dispersion calculated from CO chemisoption results.b TOF based on Ru dispersion calculated from O2 chemisoption results.c YSZ: yttria-stabilized zirconia.

perature is higher than 175 �C? Furthermore, the occurrence ofCO methanation has been reported for this kind of catalyst duringWGS reaction [22]. In order to answer this question, parallel cata-lytic tests (Fig. 6) were carried out on this catalyst, under a reactionmixture in which O2 was removed (1% CO, 50% H2, and 49% He),showing that CH4 could only be detected at temperatures higherthan 200 �C. At 275 �C, CO is converted into CH4 completely overRu/CT–AL catalyst. This result indicates that CO methanation oc-curs at higher temperature when O2 does not exist in the feed.Spencer reported that the adsorbed oxygen is not only a promoterbut also a reaction intermediate for CO methanation reaction overRu, Fe, and Co catalysts, which is in good agreement with our result[31]. CO2 (1%) was also added to the reaction feed in order to studythe catalytic activity of CO2 methanation over Ru/CT–AL catalyst. Itcan be seen that CO2 methanation is much slower than CO hydro-genation reaction over Ru/CT–AL catalyst when CO and CO2 coexistin the feed. The methanation temperature of CO and CO2 withoutO2 is higher than that in presence of O2 in the feed. Furthermore,it was also confirmed by Spencer that with presence of CO, CO2

methanation is much slower than CO methanation, possibly be-cause high coverage of adsorbed CO and adsorbed carbon hindersadsorption of CO2 [31]. Behm et al. also state that CO2 methanationover Ru/Al2O3 catalyst is not inherently inhibited, but limited byblocking the active surface by CO adsorption for simultaneous COand CO2 methanation [30]. The CO and CO2 methanation reactiondata and the results in the literature indicate that CO methanationinstead of CO2 methanation leads to form methane over Ru/CT–ALcatalyst during the PROX reaction.

The hydrogen concentration was also varied from 0%, 10%, 20%,50%, and 60% in PROX reaction feed over Ru/CT–AL. The CO and O2

concentration are fixed, and both of them are 1%. The total flowrate is 100 mL/min all the time, and He is kept in balance. Fig. 7ashows the tendency of CO conversion against the reaction temper-ature with different H2 concentration. Without any H2 in the feed,the CO conversion is much lower than that in presence of H2. TheCO conversion achieves 100% at 200 �C; however, in the presenceof hydrogen in the reaction feed, CO conversion is almost 100%when the temperature is only 150 �C. Higher H2 concentration inthe reaction feed, higher CO conversion over Ru/CT–AL catalyst.When the H2 concentration is 60%, the CO conversion is about98% at 125 �C. The presence of H2 promotes CO oxidation reactionover Ru/CT–AL catalyst. Similar results also have been reportedover Au/TiO2 catalyst by Piccolo et al. [32]. The authors propose

ction conditions.

CO conversion(%)

CO reaction rate(lmol CO s�1 g�1

cat)TOFa

(s�1)Reference

77 11.5 0.119 Thisarticle

87 12.9 0.049 Thisarticle

96 14.3 0.068 Thisarticle

15.4 1.9 n.d. [8]

11.1 2.8 n.d. [27]

2.7 n.d. 0.021 [3]

8.3 n.d. 0.061 [3]

15 n.d. 1.57b [28]

100 150 200 250 300

0

20

40

60

80

100

CO

or C

O2 c

onve

rsio

n (%

)

Temperature (oC)

CO methanationCO (CO and CO2 methanation)CO2 (CO and CO2 methantion)

Fig. 6. CO and CO2 methanation reaction over Ru/CT–AL catalyst: (j) CO conver-sion for CO methanation, reaction mixture: 1% CO, 50% H2 and 49% He; (d) COconversion for simultaneous CO and CO2 methanation, reaction mixture: 1% CO, 1%CO2, 50% H2, and 48% He; (N) CO2 conversion for simultaneous CO and CO2

methanation, reaction mixture: 1% CO, 1% CO2, 50% H2, and 48% He. The total flowrate is 100 mL/min.

50 100 150 200

0

20

40

60

80

100

CO

con

vers

ion

(%)

Temperature ( oC)

0% H2

10% H2

20% H2

50% H2

60% H2

(a)

50 100 150 200

0

20

40

60

80

100(b)

0% H2

10% H2

20% H2

50% H2

60% H2

O2 c

onve

rsio

n (%

)

Temperature ( oC)

20

40

60

80

100(c)

0% H2

10% H2

20% H2

50% H2

60% H2O2 s

elec

tivity

(%)

278 X. Chen et al. / Journal of Catalysis 299 (2013) 272–283

that PROX may proceed via the reaction of Au-bonded CO withhydroperoxy (OOH) intermediates that originate from H2 and O2

co adsorption on metallic gold. Further discussion related the reac-tion mechanism of PROX over Ru/CT–AL catalyst will be made inthe section of DRIFT results.

PROX reaction has been investigated over different noble metalcatalysts; however, realistic reformate mixture has not been stud-ied for ceria–terbia promoted alumina supported Ru catalyst. Theincorporation of 5% H2O or 5% H2O + 5% CO2 to the standard PROXreaction mixture increases the CO conversion at 100 �C signifi-cantly (Fig. 8). In contrast, in the absence of H2O, the addition of5% CO2 to the ideal PROX reaction mixture inhibits CO oxidationat 100 �C. This result could be attributed to water–gas shift reac-tion occurred over Ru/CT–AL catalyst when H2O is added to thefeed. When both CO2 and H2O coexist in the reaction feed, the ef-fect of water–gas shift is dominant, which increase the CO conver-sion at low temperature. With independence of the composition ofthe reaction mixture, however, 100% CO conversion is achieved at125 �C over the ceria–terbia promoted catalyst, thus indicating anexcellent response under all the reaction mixtures. Our results arein good agreement with that of Amiridis et al., which the presenceof H2O or both of H2O and CO2, H2O effect is predominant, whiledry CO2 suppresses the CO oxidation over Ru/Al2O3 catalyst [33].

50 100 150 200

0

Temperature (oC)

Fig. 7. The effect of different concentration of H2 for PROX reaction over Ru/CT–ALcatalyst: (a) CO conversion, (b) O2 conversion and (c) O2 selectivity. Reactionmixture: 1% CO, 1% O2, the H2 concentration is varied as the numbers in the figure,He in balance. The total flow rate is 100 mL/min.

3.5. H2 and CO chemisorption

Since both CO and H2 are main reactants for PROX and metha-nation reactions, H2 and CO volumetric chemisorption studieswere performed on all the supported Ru catalysts. For this purpose,conventional experimental conditions for H2 chemisorption havebeen employed in the volumetric studies or under dynamic pulsemeasurements. Additionally, isotherms at temperatures up to200 �C and the data corresponding to relatively high H2 equilib-rium pressures have been registered in order to get evidences ofthe H2 activation degree reached in the catalysts under the PROXand methanation reaction conditions.

In the case of the Ru/CT and Ru/CT–AL catalysts under dynamicpulse measurement, continuous hydrogen consumption after pro-longed pulse time at 35 �C shows the existence of a slow process ofhydrogen spillover to the support. This effect makes unreliable theamounts of adsorbed H2 obtained using this method in order to

know metal dispersion data (Table 3). The volumetric adsorptiondata summarized in Table 3 from the registered H2 isotherm(Fig. 9) allow to corroborating the existence of intense spilloverphenomena on the Ru/CT catalysts even at 35 �C in which the H/Ru ratio is much higher than 1.00 (H/Ru = 4.64) and increase upto H/Ru = 9.26 at 100 �C. When the temperature is 200 �C, the H/Ru ratio over Ru/CT catalyst decreases to 7.37. For the Ru/CT–ALcatalyst, the isotherms are more similar to those for Ru/CT

50 100 150 200

0

20

40

60

80

100

CO

con

vers

ion

(%)

Temperature (oC)

Fig. 8. The effect of CO2 and H2O for PROX reaction over Ru/CT–AL catalyst.Reaction conditions: 1% CO, 1% O2, 50% H2, He in balance (j); 1% CO, 1% O2, 50% H2,5% CO2, He in balance (d); 1% CO, 1% O2, 50% H2, 5% H2O, He in balance (N) and 1%CO, 1% O2, 50% H2, 5% H2O, 5%CO2, He in balance (h). The total flow rate is 100 mL/min.

Table 3H2 chemisorption results on the Ru catalysts. Data in micromol H2/g cat and apparentratio H/Ru in brackets.

Catalysts Dynamicexperimentsa

Volumetric adsorption experimentsb,c

35 �C 100 �C 200 �C

Ru/AL 112.2(0.86)

95.4 (0.73) 79.8 (0.61)

Ru/CT >23.4 (>0.71) 153.6(4.64)

306.9(9.26)

244.4(7.37)

Ru/CT–AL >121.0 (>0.93) 129.1(1.00)

282.0(2.18)

265.8(2.05)

Ru/CT–AL(C)

9.3 (0.07) – – –

a As determined by hydrogen chemisorptions pulse technique at 35 �C.b Prior to the adsorption experiments, the samples were reduced at 350 �C for 1 h

in a flow of 5% H2/Ar, and further evacuated under high vacuum (residual pressure<1 � 10�6 Torr) at 400 �C for 1 h.

c Data determined by back-extrapolation at zero pressure of the correspondingisotherms (PH2 range: 5–20).

X. Chen et al. / Journal of Catalysis 299 (2013) 272–283 279

(Fig. 9). The H/Ru data obtained besides the effect of the pressure,particularly in the analysis at 35 �C and the significant increase inthe amount adsorbed from 35 �C up to 100 �C, are consistent withhydrogen spillover Ru occurrence of strong effect on the CT-con-taining catalysts. These results indicate that our ruthenium onCT-containing supports has an interaction with H2 similar to othernoble metals (Rh, Pt, and Pd) supported on ceria [34] or ceria–zir-conia [35]. Significant amounts of hydrogen are located onto thesupport via spillover in the Ru/CT and Ru/CT–AL catalyst in the100–200 �C temperature and in the 5–400 Torr pressure range.The existence of huge amount of H over these Ru catalysts makeshydrogen oxidation occur easily in the temperature range of100–200 �C. The amounts of hydrogen chemisorbed on the Ru/ALmay be related to the metal phase, but chemisorption on the sup-port cannot be excluded. Also relevant, the increase in the 35–200 �C range of the analysis temperature induces a decrease inthe H2 consumption. Metal dispersion from H2 chemisorption mea-surements can be estimated in the case of the Ru/AL catalyst (86%).Compared with the metal dispersion over Ru/AL catalyst obtainedby HAADF–STEM (41%), it is almost double which suggests thatthere is also H2 spillover over this catalyst, but not so strong likeRu/CT and Ru/CT–AL catalysts. The metal dispersion of the calcined

Ru/CT–AL catalyst is 7% according to dynamic H2 chemisorptiondata. This result agrees with the HAADF–STEM image and theXRD result which reveals that there are big RuO2 particles up to27 nm formed over this calcined catalyst. The low dispersion ofRu over calcined Ru/CT–AL catalyst results in the low catalyticactivity for PROX reaction.

Table 4 summarizes the results of the CO volumetric chemi-sorption on the Ru catalysts. According to the literature, the inter-pretation of CO adsorption data over Ru/Al2O3 is quite complicatedbecause CO can be adsorbed over Ru with different CO/Rusurface

stoichiometry including linear, bridged, and multicarbonyl on bothreduced and partially oxidized Ru sites [33]. Furthermore, COadsorption on the support with the formation of carbonate speciesdoes also occur. The presence of a ceria-based mixed oxide in theRu catalysts makes even more complicated because the CO adsorp-tion on this kind of support depends on its redox state and ceriaalso chemisorbs large amount of CO [34,36]. Nevertheless, theamounts of CO adsorbed data provide a quantitative reference forreversible and irreversible CO adsorption over these Ru catalysts.The apparent COtotal/Ru ratios measured on the Ru/AL, Ru/CT, andRu/CT–AL catalysts are high, ranging from 0.9 up to 1.5 in this freshsample (Table 4). This result suggests that CO is adsorbed not onlyon the metal phase but also on the support for these three cata-lysts. On the other hand, very low COreversible/Ru data obtained bymeans of the second isotherm indicate the strong nature of theCO chemisorption which may generate carbonate species on thesupport (which will be confirmed by DRIFT data in the followingsection). Also of interest, there is a noticeable difference betweenthe amounts of CO adsorbed on the Ru/CT–AL fresh and calcinedsamples. This suggests that these two samples having a very differ-ent metal dispersion may result in totally distinct catalytic behav-iors for PROX reaction as shown in Section 3.4.

3.6. DRIFT results

Fig. 10 shows the DRIFT spectra recorded for the reduced Ru/CT–AL catalyst under ideal PROX condition at different tempera-tures. The CO adsorption spectrum at 50 �C consists of three bandsat 2122, 2063, and 1992 cm�1 (Fig. 10b). In the literature [33], it isproposed that the existence of at least three kinds of adsorbed COspecies: type I carbonyls (Ru0–CO) at 2000–2060 cm�1, type II car-bonyls Run+(CO)x at 2120–2156 cm�1 and at 2060–2110 cm�1, andtype III carbonyls Run+(CO)2, which are in the range of 2110–2000 cm�1. Chin et al. assigned the band at 2126 cm�1 to the tri-carbonyl CO species on Run+ on the reduced Ru/Al2O3 catalyst un-der a mixture of 0.5% CO and 0.5% O2 in He. In our case, the band at2122 cm�1 can be assigned to Run+(CO)3; meanwhile, the bands at2063 and 1992 cm�1 can be attributed to dicarbonyl CO species onRu0. There is no big change of the IR spectra when the temperatureincreases from 50 to 125 �C. However, the band at 2122 cm�1 dis-appears when the temperature reaches 150 �C at which the PROXreaction starts to occur, at the same time the band at 2063 cm�1

shifts to 2038 cm�1, which can be assigned to linearly adsorbedCO on Ru0. The band at 1992 cm�1 moves to lower wavenumber1968 cm�1, which is the bridge-bonded CO [Ru2–(CO)], and theintensity of this band increases. This result implies that PROX reac-tion occurs on all the types of CO adsorption sites over Ru, and allthe CO adsorption species turn into more stable adsorption speciesunder PROX reaction condition. The CO adsorption bands move tolower frequency, indicating the oxidized Ru is reduced to Ru0.Teschner et al. also obtained similar result, increasing temperatureunder PROX conditions over Pt/CeO2 and Pd/CeO2 catalysts leads toa general shift of the CO adsorption to lower wavenumber [9,37].The gaseous CO2 started to be observed at 150 �C in the region of2300–2400 cm�1 (Fig. 10b), which confirms that CO oxidationreaction occurs in presence of excess hydrogen over Ru/CT–AL cat-

Pressure (Torr)0 100 200 300 400

H2 a

dsor

bed

(cm

3 /g S

TP)

0

2

4

6

8

35 ºC 100 ºC200º C

Pressure (Torr)0 100 200 300 400

Pressure (Torr)0 100 200 300 400

Ru/AL Ru/CT-AL Ru/CT

35 ºC 100 ºC200º C

35 ºC 100 ºC200º C

Fig. 9. H2 adsorption isotherms over the Ru catalysts at different temperatures.

Table 4Volumetric studies of CO chemisorption on the Ru catalysts. Data in micromol H2/gcat and apparent ratio H/Ru in brackets.

Catalysta Amount of adsorbed CO

Totalb Reversiblec Irreversible

Ru/AL 291.8 (1.12) 30.4 (0.11) 261.4 (1.01)Ru/CT 102.2 (1.54) 5.7 (0.09) 96.4 (1.45)Ru/CT–AL 233.0 (0.89) 21.8 (0.08) 211.2 (0.81)Ru/CT–AL (C) 22.2 (0.09) 7.0 (0.03) 15.2 (0.06)

a Prior to the adsorption experiments the samples were always reduced at 350 �Cfor 1 h under flowing 5% H2/Ar, and further evacuated under high vacuum.

b As determined by extrapolation at P(CO) = 0 Torr from the first isotherm (P(CO)range: 5–40 Torr).

c As determined by extrapolation at P(CO) = 0 Torr from the second isotherm(P(CO) range: 5–40 Torr).

280 X. Chen et al. / Journal of Catalysis 299 (2013) 272–283

alyst at 150 �C and this result agrees with our reaction results (COconversion is 97% in Fig. 5). Furthermore, when the temperature ishigher than 150 �C, the CH vibration of a surface formate speciescan be observed at 2857 cm�1 (Fig. 10a), which might be the inter-mediate species for PROX reaction. Meanwhile, another band at2903 cm�1 arising from asymmetric CH2,ad also can be detected.The formation of the CH2,ad species combined with the adsorbedH atoms on the surface might produce methane over this Ru cata-lyst. The region below 1600 cm�1 is attributed to different carbon-ate, carboxylate, and formate species. When the temperature islower than 150 �C, a band at 1648 cm�1 which responds to carbon-ate-type species associated with the support [38] can be observed(Fig. 10c). When the temperature is 150 �C, three bands at 1592,1393, and 1374 cm�1 appear and the band at 1648 cm�1 vanishes.The new bands at 1592 and 1374 can be assigned to bridged for-mate species, while the band at 1393 is ascribed to bridged carbon-ate or bicarbonate. Teschner et al. also have reported a similarobservation on Pt/CeO2 catalyst under PROX reaction condition[9]. As discussed in the H2 and CO chemisorption section, CO canbe adsorbed not only on Ru metal but also on the support to formcarbonate species over Ru/CT–AL catalyst. The appearance of car-bonate species (1648 cm�1) confirms that irreversible adsorptionof CO on the support occurs. Moreover, there is a strong H2 spill-over over Ru/CT–AL catalyst even at 200 �C. With the temperatureincreasing, the adsorbed H reacts with the carbonate to form for-mate species on the surface of the support. In this work, the

appearance of gaseous CO2, CH2,ad, and formate species simulta-neously was observed. Meanwhile, a general shift to lower fre-quencies for CO adsorption over Ru was found, suggesting thereduction in Ru under PROX reaction condition.

In the Figs. S1 and S2, the evolution of DRIFT spectra underPROX condition over Ru/AL and Ru/CT catalysts is plotted. Sametendency for all the regions was found on Ru/AL catalyst and Ru/CT–AL catalysts. However, the Ru/CT catalyst exhibits a differentbehavior. In the CO adsorption over Ru region, two bands at2059 and 1995 cm�1 can be found. This indicates that Ru metalis reduced and cannot be oxidized even though O2 exists underPROX condition. These results are in good agreement with thoseof reported by Gatica et al. for a similar catalyst for WGS reaction[22]. Increasing temperature leads to higher intensity of band at2059 cm�1, a shift of band at 1995–1975 cm�1 and appearance ofa shoulder band at 1937 cm�1, revealing the further reduction inthe Ru. This might be due to the better reducibility of the pure cer-ia–terbia support than alumina and ceria–terbia modified alumina.This result indicates that the PROX mechanism over Ru/AL and Ru/CT–AL catalysts is quite similar but differs from that over Ru/CTcatalyst due to the redox property of CT support and highly dis-persed Ru over this support [31].

In order to understand more about the mechanism of PROXreaction over the Ru catalysts, transient experiment over Ru/CT–AL catalyst at 175 �C was carried out (Fig. 11). The adsorption ofCO on Ru/CT–AL catalyst produces three bands at 2120, 2064,and 1993 cm�1. When O2 is introduced into the DRIFT cell, thebands of 2120 and 2064 cm�1 become smaller, and the band at1993 cm�1 moves to 1999 cm�1. This small redshift can be ex-plained by the slight oxidation of the Ru particles under CO oxida-tion condition [9]. At the same time, CO2 gaseous phase appears inthe 2300–2400 cm�1 region, which means that CO oxidation reac-tion takes place at this atmosphere at this temperature. The intro-duction of H2 makes the band at 2120 cm�1 disappear completelyand both of bands at 2064 and 1999 cm�1 shift to lower frequency.This indicates that Run+ is reduced to Ru0; for this reason, the COadsorption bands move to lower wavenumber. This result is accor-dance with what was reported by Teschner et al. [9]. Gaseous CO2

also can be detected under PROX reaction mixture. There are threebands at 1648, 1436, and 1227 cm�1, which are carbonate species,when CO and CO + O2 are in the DRIFT cell. However, when H2 wasintroduced into the cell, the three bands mentioned before vanishand three new bands at 1594, 1393, and 1374 cm�1 which are cor-

4000 3800 3600 3400 3200 3000 2800

3650

2857

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

75 oC

50 oC

100 oC

125 oC

150 oC

200 oC

175 oC

0.05

2903

(a)

2400 2300 2200 2100 2000 1900 1800

2039

1992

1968

2063

200 oC

175 oC

150 oC

125 oC

100 oC

75 oC

Wave number (cm-1)

50 oC

0.2

Abs

orba

nce

(a.u

.)

2122

(b)

1800 1600 1400 1200

133713

9313

74

1592

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

200 oC

175 oC

150 oC

125 oC

100 oC

75 oC

50 oC

1648

0.25

(c)

Fig. 10. DRIFT spectra of PROX reaction over Ru/CT–AL catalyst at different temperatures. PROX reaction mixture: 1% CO, 1% O2, 50% H2 and 48% He. The total flow rate is100 mL/min.

3800 3600 3400 3200 3000 2800

cut O2

CO+O2+H2

CO+O2

2849

Wave number (cm-1)

Abs

orba

nce

(a.u

.)

2899

0.05

CO

cut H2

(a)

2400 2300 2200 2100 2000 1900

2340

2360

196220

40

1993

2064

cut O2

CO+O2+H2

CO+O2

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

0.1

CO

cut H2

2120

(b)

1800 1600 1400 1200

1393

1337

1227

1374

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

0.2

cut O2

CO+O2+H2

CO+O2

CO

cut H2

1648

1594

1436

(c)

Fig. 11. Transient DRIFT spectra of PROX reaction over Ru/CT–AL catalyst. The total flow rate is 100 mL/min.

X. Chen et al. / Journal of Catalysis 299 (2013) 272–283 281

responding to bridged formates appear. The appearance of theband 2899 and 2847 cm�1 also confirms the existence of CH2,ad

and bridged formate species under PROX condition. The introduc-tion of H2 to the feed also makes the intensity of OH region (3900–3300 cm�1) higher and broader than that in the CO and CO oxida-tion feed, which reveals that water is produced by H2 oxidationreaction. In order to know the changes of these bands are revers-ible or not, H2 flow was stopped and only CO and O2 left in theDRIFT cell, and it can be seen that the CO adsorption bands underCO and O2 atmosphere are completely recovered in the region of2400–1800 cm�1, which means that the changes are reversiblefor CO adsorption over Ru species. However, the bands at 2899,

2849, 1594, 1393, and 1374 cm�1 do not change, which indicatesthat the formation of bridged formate is not reversible. The cutof O2 flow also make the CO adsorption bands recover like onlyCO was in the DRIFT cell, and the formate species remain on thesurface of the catalyst.

Fig. 12 proposes the possible reaction route for PROX reactionover Ru/CT–AL catalyst at low and high temperatures. When thetemperature is lower or equal to 150 �C, the dicarbonyl species re-acts with the adsorbed O on the Ru surface to produce CO2. Mean-while, the formation of H2O is due to the combination of theadsorbed H and O. On the surface of support, the H2 spilloverand CO strong chemisorptions lead to the formation of OH groups

(a)

(b)Fig. 12. Possible mechanism of PROX reaction over Ru/CT–AL catalyst: (a) low temperature (6150 �C) and (b) high temperature (P175 �C).

282 X. Chen et al. / Journal of Catalysis 299 (2013) 272–283

and carbonate group. As described before, when the temperature ishigher than 150 �C, methane can be detected in the products onlyover Ru/CT–AL catalyst. This might be attributed to the reactionbetween the adsorbed CH2 species and the adsorbed hydrogen.The Ru is reduced to Ru0 under PROX atmosphere, and the mainCO adsorbed species are linearly adsorbed CO and bridged CO,which generate CO2 with the adsorbed O. The production of H2Ois still from the adsorbed H and O; at the same time, bridged for-mate can be found on the surface of the support, and it isirreversible.

4. Conclusions

The addition of a ceria–terbia mixed oxide to a Ru/Al2O3 cata-lyst significantly improves the catalytic performance of againstthe PROX reaction. Under the ideal PROX reaction condition, thepromoted catalyst shows the highest activity over the whole rangeof temperatures and CO conversion reaches 97% at 150 �C. Becauseof the overlapping of CO oxidation and methanation processes, thiscatalyst also maintains 100% conversion in the widest range oftemperatures (175-200 �C). Moreover, upon adding to the idealreaction mixture 5% H2O, 5% CO2, or (5% H2O + 5% CO2), the cer-ia–terbia promoted catalyst still reaches 100% CO conversion at125 �C. Furthermore, higher hydrogen concentration in the PROXreaction feedstock, higher catalytic activity for PROX reaction. Thisis the first time that this promoting effect has been reported overthis catalyst. However, the Ru/Ce0.80Tb0.20O2�x–Al2O3 catalyst cal-cined at 500 �C which Ru suffers severe sintering is not active forPROX reaction, indicating that the small particle size of Ru maxi-mizing the interaction with the support plays a very important rolein PROX reaction. The unique catalytic behavior of this ternary cat-alyst could be attributed to the promoting effect of ceria terbiamixed oxide and Ru nanoparticles.

Further H2 chemisorption data demonstrate that there is H2

spillover phenomena over all the Ru catalysts, being especiallymore intense in those containing ceria–terbia catalysts. CO chemi-sorption by means of volumetric method and DRIFT results indi-cate that it occurs over different oxidation state of Ru, andcarbonate species are generated on the support. DRIFT resultsshow that PROX reaction takes place on different kinds of COadsorption sites over Run+ and Ru0. During the reaction process,Run+ can be reduced to Ru0 and bridged formate appears, and it

is irreversible. Very likely, the formation of CH2, ad combined withthe adsorbed H produce methane.

Acknowledgments

This work is supported by the MCIN/FEDER (Project MAT2008-00889NAN) and Junta de Andalucía (reference number: FQM3994).Xiaowei Chen thanks ‘‘Ramón y Cajal’’ program from Spanish Min-istry of Science and Innovation. TEM, SEM, ICP, and XRD data wereobtained at SCCYT of University of Cadiz.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2012.12.025.

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