Delivered by Ingenta to: Sung Kyun Kwan UniversityIP: 115.145.210.72 On: Sat, 18 Feb 2017 00:05:09

Copyright: American Scientific Publishers

Copyright © 2016 American Scientific PublishersAll rights reservedPrinted in the United States of America

ArticleJournal of

Nanoscience and NanotechnologyVol. 16, 11438–11442, 2016

www.aspbs.com/jnn

Binary Oxide Catalyst Supported on MesoporousCeO2 for Low Temperature Water-Gas Shift Reaction

Chengbin Li1, Hwa-Yong Oh1, Hye-Jin Cho1, Sivaranjani Kumarsrininasan1, Jong-Ha Choi2,Zhenghua Li1, Gyoung-Hee Hong1, Su-Bin Park1, and Ji Man Kim1�∗1Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea

2Korean Minjok Leadership Academy, Anheung-myeon, Hoengseong-gun, Gangwon-do, 225-823, Republic of Korea

Binary oxide catalyst (Cu0�15Co2�84O4� supported on mesoporous CeO2 has been prepared by anincipient wetness impregnation method. The produced catalysts with different loading of binaryoxide (0–30 wt%) have been thoroughly characterized and applied for a water-gas shift (WGS)reaction. The characterization results indicate that the binary oxide catalysts are highly dispersedon the mesoporous CeO2 support without significant loss of mesostructural properties, even thoughthe catalyst loading reaches 30 wt%. The highest catalytic activity towards WGS reaction hasbeen achieved from the 30 wt% Cu0�15Co2�84O4/CeO2 catalyst, showing 100% CO conversionand very low methane yield (nearly zero%) at relatively low temperature (∼300 �C). The presentCu0�15Co2�84O4/CeO2 catalyst can be recycled more than three times with loss of the activity. Metal-lic cobalt species are formed during the reaction, whose structural properties are maintained underthe repeated WGS reaction conditions. Excellent catalytic activity and durability originate from thehighly dispersed binary oxides on the pore surface of mesoporous CeO2, which is probably due tothe strong metal-support interaction.

Keywords: Binary Oxide, Mesoporous CeO2, Water-Gas Shift Reaction.

1. INTRODUCTIONThe water-gas shift (WGS) reaction is playing a signifi-cant role in generating pure hydrogen for fuel cells andother applications. Use of pure hydrogen is very importantfor polymer electrolyte membrane fuel cell application,where CO concentration must be below 10 ppm to preventpoisoning of the Pt catalysts for oxygen reduction.1 TheWGS reaction is exothermic. Owing to thermodynamiclimitations, a high conversion of CO can be obtainedwith a two-stage process: high temperature shift (HTS,350–500 �C) and low temperature shift reactions (LTS,200–300 �C). Currently in industries, Fe2O3–Cr2O3 andCuO/ZnO/Al2O3 catalytic systems are used for HTS andLTS, respectively. There are lots of known limitationsassociated with these catalysts. Moreover these are notappropriate for mobile applications. Hence, in recent years,pursuing a catalyst with high activity, stability, and lowertemperature operation has been an important research area.

∗Author to whom correspondence should be addressed.

In modern chemistry, a noble metal (Au, Pt, Rh etc.) sup-ported oxide system is considered the best alternate LTScatalyst with high catalytic activity. However, the rela-tively high cost limited the mobile application of partic-ular catalyst systems. Hence, developing transition metalssupported oxide catalyst with all the positive aspects ofa noble metal based system is the aim of the presentwork.In literature, CeO2 has been widely used as support for

WGS reaction due to its low cost and excellent oxygenstorage capacity (OSC), which facilitates the reversibletransition between Ce4+ and Ce3+.2�3 Cu/CeO2 is thebest known catalyst for LTS. Although the Cu/CeO2 per-formed well in LTS, it is not suitable for the HTSreaction.4 Moreover Cu based catalysts are very sensi-tive to sulfide poisoning. Introduction of second metalsuch as Ni or, Co to the Cu helps the stability of thecatalyst. Ni based catalysts are known for methanation,which is major side product in a WGS reaction. Moreover,Co based catalysts show excellent catalytic activity forCO oxidation.5�6

11438 J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 11 1533-4880/2016/16/11438/005 doi:10.1166/jnn.2016.13525

Delivered by Ingenta to: Sung Kyun Kwan UniversityIP: 115.145.210.72 On: Sat, 18 Feb 2017 00:05:09

Copyright: American Scientific Publishers

Li et al. Binary Oxide Catalyst Supported on Mesoporous CeO2 for Low Temperature Water-Gas Shift Reaction

Hence, in order to overcome the drawback of lowerthermal stability of Cu/CeO2 catalyst, a new binary oxideCu0�15Co2�84O4 was dispersed on the cerium oxide as theactive phase, employing the mesoporous CeO2 (meso-CeO2� as a support for enhanced the catalytic activity andstability due to the high specific surface area and interac-tion with the loading oxides. The effect of the loading ratiowas compared by carrying out the water-gas shift reaction.

2. EXPERIMENTAL DETAILS2.1. Synthesis of Mesoporous CeO2

A mesoporous silica template (KIT-6) with cubic Ia3dmesostructure was synthesized following the methoddescribed elsewhere.7 Cerium (III) nitrate hexahydrate(Ce(NO3�3 ·6H2O, Aldrich, Melting point 65 �C) was usedas the CeO2 precursor. Typically, 5.0 g of the calcinedKIT-6 template was heated at 100 �C for 1 h. The pre-heated silica template was poured into a polypropylenebottle containing 3.2 g of Ce(NO3�3 · 6H2O that was pre-melted to liquid phase at 80 �C. The bottle containingthe mixture was closed and shaken vigorously to mix theCe(NO3�3 · 6H2O and silica template. Subsequently, thebottle was put in an oven at 80 �C overnight to achievethe spontaneous infiltration of the CeO2 precursor withinthe mesopores of silica templates. The composite mate-rials were then heated to 450 �C under ambient con-ditions for 3 h. Subsequently, the silica template wascompletely removed by treating the composite materialwith 2 M sodium hydroxide solution two times. Finally,the meso-CeO2 (m-CeO2� material thus obtained waswashed with distilled water and acetone several times anddried at 80 �C.8�9

2.2. Synthesis of Cu0�15Co2�84O4/Meso-CeO2 MaterialCu0�15Co2�84O4/meso-CeO2 (CC/m-CeO2� catalysts wereprepared using an incipient wetness impregnation methodat ambient temperature. For synthesizing the CC/m-CeO2

with various loading catalysts, a mixed aqueous consistingof Cu(NO3�2 · 2.5H2O and Co(NO3�2 · 6H2O dissolved in1.6 ml deionized water was impregnated on 1.0 g of meso-CeO2 by an incipient wetness method. The impregnatedCC/m-CeO2 catalysts were dried at 80 �C overnight, andcalcined at 450 �C in air for 3 h.

2.3. CharacterizationX-ray diffraction (XRD) patterns were obtained in reflec-tion mode using a Rigaku D/MAX-2200 Ultima equippedwith Cu K� radiation at 30 kV and 40 mA. Nitrogen sorp-tion isotherms were collected on a Micromeritics Tristarsystem at −196 �C. All the samples were completelydried under vacuum at 80 �C for 8 h before the measure-ment. The specific BET (Brunauer–Emmett–Teller) sur-face areas were calculated in the relative pressure (p/p0)range from 0.05 to 0.20. The pore size distribution curves

were obtained from the N2 adsorption branches by theBJH (Barrett–Joyner–Halenda) method. Hydrogen temper-ature programmed reduction (H2-TPR) was performed ina fixed-bed reactor system connected with TCD (thermalconductivity detector).

2.4. Catalytic Activity MeasurementsThe water-gas shift reaction was performed in a packed-bed quartz reactor at atmospheric pressure. 0.06 g ofprepared catalyst was reduced at 300 �C under H2 flow(5 vol% H2 in Helium, a flow of 30 cm3 ·min−1� for 1 hand then cooled to room temperature before the reaction.A feed-gas mixture with a total flow rate was 64 cm3 ·min−1 containing 3.0 vol% CO balanced with He was pre-pared using mass flow controllers, and the steam was pre-pared by micro-pump with a heating stainless steel line.The space velocity (GHSV) is 64,000 cm3 gcat−1 h−1

and the reaction temperature ranged from 200 �C to450 �C. The effluent gas stream from the reactor was ana-lyzed online by both thermal conductivity detector (TCD)and fire ionization detector (FID) of parallel gas chro-matography (GC; Younglin) with a carboxen 1000 column.

3. RESULTS AND DISCUSSIONFigure 1 shows powder X-ray diffraction patterns forCu0�15Co2�84O4 loaded mesoporous CeO2 catalysts. In low-angle XRD patterns, mesoporous CeO2 exhibits two peaksat 0.4� and 1� corresponds to (110) and (211) planesrespectively.10�11 This observation confirms the presenceof mesoporosity in pure CeO2. All loaded catalysts alsoshow the presence of the same peaks. Nevertheless, com-pared with m-CeO2, the intensity of the peaks decreasedwith increasing loading amount. All diffraction featuresin the wide-angle XRD patterns (Fig. 1) of m-CeO2 andCC/m-CeO2 materials can be indexed to the cubic fluo-rite crystal structure of CeO2 (JCPDS 34–0394). Alongwith the CeO2 peaks, two more peaks at 37� and 65� werealso observed when the loading amount exceeded 20 wt%.

Figure 1. XRD patterns of (a) m-CeO2, (b) 10 wt% CC/m-CeO2,(c) 20 wt% CC/m-CeO2, and (d) 30 wt% CC/m-CeO2.

J. Nanosci. Nanotechnol. 16, 11438–11442, 2016 11439

Delivered by Ingenta to: Sung Kyun Kwan UniversityIP: 115.145.210.72 On: Sat, 18 Feb 2017 00:05:09

Copyright: American Scientific Publishers

Binary Oxide Catalyst Supported on Mesoporous CeO2 for Low Temperature Water-Gas Shift Reaction Li et al.

Figure 2. Nitrogen sorption isotherms and pore size distribution of(a) m-CeO2, (b) 10 wt% CC/m-CeO2, (c) 20 wt% CC/m-CeO2, and(d) 30 wt% CC/m-CeO2.

These new peaks are due to the presence of Cu0�15Co2�84O4

phase on the surface of m-CeO2. However, the peaks ofCu0�15Co2�84O4 are not very sharp, suggesting that thesespecies are very small and highly dispersed on the m-CeO2

surface.Porosity of the composite catalysts was measured by

nitrogen sorption isotherms. In Figure 2, all catalysts showtype IV absorption behavior with H2 hysteresis loop,which is a characteristic of all mesoporous materials.12

The BET surface area of all catalysts were calculated andlisted in Table I. The m-CeO2 showed high surface areaof 133 m2/g, and the surface area, pore size and total porevolume decreased with increasing the loading amount.Pore blockage by loaded species such as Cu and Co couldbe the reason for decreasing surface area and other param-eters. These results are in good agreement with the slightlymesostructure loss in low angle XRD patterns.Figure 3 shows the H2-TPR profiles of CC/ m-CeO2 cat-

alysts and mesoporous CeO2. In general, two main reduc-tion peaks can be discerned. The first reduction peak,centered at about 150 �C, can be attributed to the reductionof CuO on the surface of the catalysts. The second peak,

Table I. Physical properties of the materials.

Sample aSBET (m2/g) bVt (cm3/g) cDp (nm)

Meso-CeO2 133 0�7 2010 wt % CC/m-CeO2 130 0�47 2020 wt % CC/m-CeO2 93 0�29 1830 wt % CC/m-CeO2 80 0�28 10

Notes: aSurface area were calculated by BET method; bTotal pore volumes wereestimated at p/p0 = 0�99; cPore sizes were calculated by using the BJH methodwith the adsorption branch.

Figure 3. H2-TPR profiles of (a) m-CeO2, (b) 10 wt% CC/m-CeO2,(c) 20 wt% CC/m-CeO2, and (d) 30 wt% CC/m-CeO2.

at approximately 300 �C, was due the reduction of bulkCuO and/or reduction of Co2+ to metallic Cobalt.6�13 Thepure mesoporous CeO2 shows two well-resolved peaksat 147 and 548 �C. Reduction of cerium is generallyaccepted to occur stepwise; the Ce4+ on the surface willbe reduced at lower temperature, and then the Ce4+ inthe inner layers reduced at high temperature.14 The highlydispersed Cu0�15Co2�84O4 enhances the reducibility of theCeO2 as the loading of Cu0�15Co2�84O4 was increased from10 to 30 wt%, the H2 consumption was increased and themain reduction peak was shifted to lower temperature.15�16

Among all the samples, 30 wt% CC/m-CeO2 has the low-est reduction temperature. This observation confirms that30 wt% CC/m-CeO2 has the highest reducing ability of allthe prepared catalysts.The WGS activity of as-prepared catalyst is expressed

in terms of percentage of CO conversion. Figure 4 shows

Figure 4. Percentage of CO conversion of (a) m-CeO2, (b) 10 wt%CC/m-CeO2, (c) 20 wt% CC/m-CeO2, and (d) 30 wt% CC/m-CeO2.

11440 J. Nanosci. Nanotechnol. 16, 11438–11442, 2016

Delivered by Ingenta to: Sung Kyun Kwan UniversityIP: 115.145.210.72 On: Sat, 18 Feb 2017 00:05:09

Copyright: American Scientific Publishers

Li et al. Binary Oxide Catalyst Supported on Mesoporous CeO2 for Low Temperature Water-Gas Shift Reaction

Figure 5. Percentage of CO conversion of (a) m-CeO2, (b) 10 wt%CC/m-CeO2, (c) 20 wt% CC/m-CeO2, and (d) 30 wt% CC/m-CeO2.

the steady-state CO conversion over the series of loadedCC/m-CeO2 catalysts. The WGS reaction temperaturerange is from 200 �C to 450 �C. The pure mesoporousCeO2 showed just a little activity even in the high temper-ature range. However, almost 100% CO conversion wasobserved at 400 �C with 10 wt% Cu0�15Co2�84O4 loadingon the mesoporous CeO2. The Cu0�15Co2�84O4 as an activesite showed excellent WGS reaction activity. In addition,the catalytic activity increased with increasing loadingamount. Interestingly, complete CO conversion is observedat much lower temperatures such as 300 �C with 20 and30 wt% loaded catalysts. Of them all, 30 wt% CC/m-CeO2

catalyst displays the highest CO conversion and nearlyzero% CH4 formation, mainly because of the presence ofmore catalytic active sites.

Figure 6. Recyclability of the 30 wt% Cu0�15Co2�84O4/m-CeO2 catalystsat 400 �C.

XRD analysis of the spent catalyst is shown in Figure 5.In the spent catalysts, Cu0�15Co2�84O4 peaks were absent,and a new peak at 2� = 44� is observed, mainly becauseof the metallic cobalt. From the high angle XRD patterns,the small size metallic cobalt species were formed duringthe WGS reaction. The metallic cobalt species were stilldispersed on the surface of the support. In conclusion, theCo2+ was reduced to metallic cobalt, which is essential forthe reaction.The stability and recyclability was measured by testing

the 30 wt% CC/m-CeO2 catalyst for cycles and the resultswere exhibited in Figure 6. The recyclability of the 30 wt%CC/m-CeO2 catalyst was tested for three cycles by simplycooling down the reactor temperature to room temperatureand repeating the WGS reaction. Though the active phasegot segregated, the catalyst still had high activity. Therewas no decrease in catalytic activity, even after the criticalreaction conditions.

4. CONCLUSIONThe binary oxide mesoporous CeO2 supported catalystswere synthesized by the wet impregnation method invarious loading amounts and examined for the water-gas shift reaction. The catalyst was thoroughly character-ized by various physic-chemical methods. The 30 wt%Cu0�15Co2�84O4/m-CeO2 catalyst showed a higher WGSreaction activity than the other catalysts. The excellent cat-alytic activity and durability are mainly attributed to thehighly dispersed binary oxides on the surface of meso-porous CeO2, which is probably due to strong metal-support interaction.

Acknowledgments: This work was supported by theDegree and Research Center (DRC) Program (2014)through the National Research Council of Science andTechnology (NST) from the Ministry of Science, ICT andFuture Planning.

References and Notes1. G. Jacobs, P. M. Patterson, L. Williams, E. Chenu, D. Sparks,

G. Thomas, and B. H. Davis, Appl. Catal. A 262, 177(2004).

2. Y. Li, Q. Fu, and M. Flytzani-Stephanopulos, Appl. Catal. B Environ.27, 179 (2000).

3. V. M. Shinde and G. Madras, Appl. Catal. B Environ. 123, 367(2012).

4. A. A. Gokhale, J. A. Dumesic, and M. Mavrikakis, J. Am. Chem.Soc. 130, 1402 (2008).

5. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta, and W. J. Shen, Nature Lett.9, 458 (2009).

6. S. A. Singh and G. Madras, Appl. Catal. A Gen. 504, 463 (2015).7. T. W. Kim, F. Kleitz, B. Paul, and R. Ryoo, J. Am. Chem. Soc.

127, 7601 (2005).8. G. L. Xiao, C. N. Xian, H. Li, and L. Q. Chen, J. Nanosci. Nano-

technol. 11, 1923 (2011).9. Z.H. Li, H. J. Jeong, K. Sivaranjani, B. J. Song, S. B. Park,

D. H. Li, C. W. Lee, M. S. Jin, and J. M. Kim, Nano 10, 155075(2015).

J. Nanosci. Nanotechnol. 16, 11438–11442, 2016 11441

Delivered by Ingenta to: Sung Kyun Kwan UniversityIP: 115.145.210.72 On: Sat, 18 Feb 2017 00:05:09

Copyright: American Scientific Publishers

Binary Oxide Catalyst Supported on Mesoporous CeO2 for Low Temperature Water-Gas Shift Reaction Li et al.

10. Y. Y. Lee, S. Y. Bae, and J. M. Kim, J. Nanosci. Nanotechnol.12, 9033 (2014).

11. G. O. Park, J. K. Shon, Y. H. Kim, and J. M. Kim, J. Nanosci.Nanotechnol. 15, 2441 (2015).

12. J. K. Shon, S. S. Kong, S. S. Kim, M. S. Kang, and J. M. Kim,Funct. Mater. Lett. 2, 151 (2008).

13. M. S. Jin, J. N. Park, J. K. Shon, J. H. Kim, Z. H. Li, Y. K. Park,and J. M. Kim, Catal. Today 185, 183 (2012).

14. D. H. Prasad, S. Y. Park, H.-I. Kim, J.-W. Son, B.-K. Kim, H.-W.Lee, and J.-H. Lee, J. Phy. Chem. C 116, 3467 (2012).

15. S. Pradhan, A. S. Reddy, R. N. Devi, and S. Chilukuri, Catal. Today141, 72 (2009).

16. F. Z. Zhao, Z. Y. Liu, W. Q. Xu, S. Y. Yao, A. Kubacka, A. C.Johnston-Peck, S. D. Senanayake, A. Q. Zhang, E. A. Stach,M. Fernández-García, and J. A. Rodriguez, J. Phy. Chem. 118, 2528(2014).

Received: 20 August 2015. Accepted: 18 March 2016.

11442 J. Nanosci. Nanotechnol. 16, 11438–11442, 2016