Evaluation of carbon monoxide oxidation over CeO2/Co3O4 catalysts: Effect of ceria loading

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Evaluation of carbon monoxide oxidation over CeO

2

/Co

3

O

4

catalysts:

Effect of ceria loading

Chih-Wei Tang

a,b

, Ming-Chih Kuo

b

, Chin-Jung Lin

b

,

Chen-Bin Wang

a,

*

, Shu-Hua Chien

b,c,

**

a

Department of Applied Chemistry and Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 33509, Taiwan, ROC

b_{Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC} c_{Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan, ROC}

Available online 26 November 2007

Abstract

Modification of cobaltic oxide (obtained from the reduction of high-valence cobalt oxide and assigned as R230, SBET= 100 m2g1) with

different loading of ceria was proceeded using the impregnation method (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). The CeX/R230 catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption at 196 8C, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). Their catalytic activities towards the CO oxidation were studied in a continuous flow micro-reactor. The results revealed that the optimal modification, i.e., Ce20/R230, can increase the surface area (SBET= 109 m2g1) of cobaltic oxide, further weaken

the bond strength of Co–O and lower the activation of CO oxidation among CeX/R230 catalysts due to the combined effect of cobaltic oxide and ceria. The Ce20/R230 catalyst exhibited the best catalytic activity in CO oxidation with T50(temperature for 50% CO conversion) at 88 8C.

Keywords: Ceria; Cobaltic oxide; CO oxidation

1. Introduction

Highly active catalysts for CO oxidation have several applications including vehicle exhausts [1], gas sensor [2], catalytic combustion [3] and magnetic material [4]. Many catalysts have been designed and tested for CO oxidation under low temperature. For instance, noble metal catalysts, such as Au/Co3O4, Au/a-Fe2O3, Au/MnOx, Pt/SnO2, Pd/SnO2 have been demonstrated to be very effective in CO oxidation[5–7]. The high cost of precious metals and their sensitivity to sulfur poisoning have long motivated the search for substitute catalysts. As a single component, base metal catalysts cannot compete with precious metal catalysts, and attempts have been made to improve their activity by combining various elements. The availability of metal oxides is such that attention has been paid on

especially cerium oxide[8–13], cobalt oxide[14–19]and Co3O4/ CeO2 binary oxide [20,21]. Ceria has a high oxygen storage capacity and has well-known catalytic and redox properties with coupled of (Ce4+/Ce3+), making more oxygen available for the oxidation process. It is well known that CeO2is a promoter additive. The most important property comes from the oxygen storage and releasing capacity on CeO2 that promotes CO oxidation and NOxreduction. Both Co3O4and CoO are stable oxides in the cobalt oxide system[15,22,23]. The high activity of Co3O4 is likely to be related to the relatively low DH of vaporization of O2[22]. Therefore, the Co–O bond strength of Co3O4can affect the desorption of lattice oxygen. The oxidation of carbon monoxide on various composites of metal oxides was investigated. Under the conditions of ambient temperature, GHSV = 15,000 h1and 1% CO/21% O2/78% He, Shen et al. [20,21] found that the prepared mixed oxides by the coprecipitation–oxidation method with large surface area exhibited the high activity (complete conversion) and stability (maintained 2400 min) for CO oxidation. Also, our previous research [23] finds that the CeO2/Co3O4 catalyst possesses superior activity than other catalysts.

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Catalysis Today 131 (2008) 520–525

* Corresponding author.

** Corresponding author at: Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan, ROC.

E-mail addresses:[email protected](C.-B. Wang), [email protected](S.-H. Chien).

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This study is concerned with the effect of ceria loading for the oxidation of carbon monoxide over CeO2/Co3O4catalysts. The purpose is to find the optimized loading of ceria for this composite catalyst that abates the CO under low temperature. 2. Experimental

2.1. Catalyst preparation

The detailed high-valence cobalt oxide (marked as CoOx) prepared procedure has been described in a previous investigation [24]. Moreover, the pure cobaltic oxide (Co3O4) was refined from CoOx by a controlled hydrogen reduction in a temperature-programmed reduction system to 230 8C (assigned as R230). Modification of cobaltic oxide was further proceeded by impregnating the R230 with a series of cerium nitrate solutions (assigned as CeX/R230, X = 4, 12, 20, 35 and 50 wt%). Then, prepared samples were dried overnight in an oven at 110 8C, and then reduced in 10% H2/Ar for 1 h at 200 8C.

2.2. Catalyst characterization

X-ray diffraction (XRD) measurements were made using a Siemens D5000 diffractometer with Cu Ka1 radiation (l = 1.5405 A˚ ) at 40 kV and 30 mA with a scanning speed in 2u of 28 min1. Diffraction peaks of the crystalline phase were compared with those of standard compound reported in the JCPDS data files (Co3O4: 34-0394; CeO2: 09-0418). The crystallite sizes of cobaltic oxide and ceria were calculated using the Scherrer equation.

Nitrogen adsorption isotherms at196 8C were determined volumetrically with Micrometritics ASAP 2010. The catalysts were pre-outgassed under 5 105Torr and 110 8C for 3 h. The surface area of samples was determined from the nitrogen adsorption isotherm.

Reduction behavior of CeX/R230 catalysts was investigated by temperature-programmed reduction (TPR). About 25 mg of the sample was heated in a flow of 10% H2/He gas mixture at a flow rate of 10 ml min1. During TPR, the temperature was programmed to rise with 10 8C min1to 550 8C.

Cobalt oxides microstructures were characterized using TEM (Hitachi H600-3). The samples for the electron microscopy were prepared by making an ethanol suspension and deposited onto an undercoat of a holey carbon film. 2.3. Catalytic activity measurement

The catalytic activities of CeX/R230 catalysts towards CO oxidation were measured in a continuous flow micro-reactor. The reaction gas, a mixture of 10% O2/He with 4% CO/He, was fed to a 0.5 g catalyst at a rate of 20 ml min1. Steady-state catalytic activity was measured at each temperature with the reaction temperature raised from room temperature to 200 8C in steps of 25 8C. The effluent gas was analyzed on-line using a Varian 3700 gas chromatograph with a carbosphere column. Before the reaction, the catalyst was pretreated in flowing 10%

O2/He at 110 8C for 1 h to drive away molecules that had been pre-adsorbed from the atmosphere.

3. Results and discussion

Fig. 1displays the XRD patterns of the ceria, cobaltic oxide and the CeX/R230 catalysts. All of the peaks ofFig. 1(a) can be indexed to the cubic phase of ceria(IV) with fluorite structure. After reduction, the CeX/R230 catalysts exhibit three crystal-line structures: CeO2, Co3O4 and CoO, as demonstrated by XRD [Fig. 1(c)–(g)]. The diffraction peaks of ceria become stronger with the increase of loading. Observed from the width of diffraction peaks reveal that the crystallite sizes of ceria did not noticeably change.

The TPR profiles of ceria, cobaltic oxide and CeX/R230 catalysts are shown inFig. 2. The reduction of pure CeO2initiates at 503 8C [Fig. 2(a)], and the signal is much weaker than those of Co3O4and CeO2/Co3O4catalysts[23,25]. The peak is attributed to the removal of surface capping oxygen ions during the reduction reaction [26,27]. The reduction profile of the pure Co3O4 can be characterized by the two consequent peaks between 200 and 400 8C[23,28]. All of the samples, except for CeO2 [Fig. 2(a)] exhibits two well-resolved reduction peaks

Fig. 1. XRD characterization for the ceria, cobaltic oxide and the CeX/R230 catalysts: (a) CeO2, (b) R230, (c) Ce4/R230, (d) Ce12/R230, (e) Ce20/R230, (f)

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(assigned as a and b peaks). These profiles indicate a two-step reduction process: the first peak (a peak) of low intensity initiates at low temperature and overlaps with the more intense second peak (b peak). According to our previous reports[19,23,24], the apeak is attributed to the reduction of Co3+ ions, which are present in the spinel structure, into Co2+[Eq.(1)]. Subsequently, the b peak observes that comes from the reduction of CoO to metallic cobalt [Eq.(2)].

Co3O4þ H2 ! 3CoO þ H2O (1)

CoO þ H2 ! Co þ H2O (2)

As shown inFig. 2(b)–(f) and both the 3rd and 4th columns inTable 1, an apparent shift in both a and b peaks appears which is related to the surface area of catalysts. Compared to the SBET(the 2nd column ofTable 1), both a and b peaks shift to lower temperatures with the increase of SBET, i.e., both a and b peaks of Ce20/R230 sample (SBET= 109 m2g1) are found at 260 and 360 8C [Fig. 2(e)], respectively. Both a and b peaks of Ce50/R230 sample (SBET= 52 m2g1) occurred at 272 and 370 8C [Fig. 2(g)], respectively. These results reveal that the increasing of the SBETof Ce20/R230 catalysts can weaken the

bond strength of Co–O bond and can promote more lattice oxygen desorption from Co3O4 to decrease the reduction temperature.

In order to understand whether the optimal addition of ceria can increase the surface area and provide oxygen storage capacity, the ratio of a and b peaks for each CeX/R230 sample is quantitatively determined from the consumption of hydrogen in TPR traces. The amounts of hydrogen consumption with the same temperature of CeX/R230 catalysts are shown inFig. 3. The relative area of a and b (NH2/NCoratio) are determined to be 0.47 and 1.7; 0.43 and 1.7; 0.39 and 1.4; 0.39 and 1.4; 0.30 and 1.38, respectively, for Ce20/R230, Ce12/R230, Ce4/R230,

Fig. 2. TPR profiles for the ceria, cobaltic oxide and the CeX/R230 catalysts: (a) CeO2, (b) R230, (c) Ce4/R230, (d) Ce12/R230, (e) Ce20/R230, (f) Ce35/

R230 and (g) Ce50/R230.

Table 1

Characterization and catalytic activities for CO oxidation over CeX/R230 catalysts Catalysts SBET (m2_g1₎ TPR (8C) CO oxidation a b Ea a (kJ mol1) T50 b (8C) Ce4/R230 71 263 390 26 108 Ce12/R230 81 260 390 18 93 Ce20/R230 109 260 360 10 88 Ce35/R230 65 266 365 31 111 Ce50/R230 52 270 370 55 127

a _{Calculated according to the Arrhenius equation.} b _{Temperature for 50% CO conversion.}

Fig. 3. The amounts of hydrogen consumption with the same temperature of CeX/R230 catalysts.

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Ce35/R230 and Ce50/R230. Comparison with the theoretical values of Co3O4(0.33 and 1, respectively) that are based on Eqs.(1)and(2)also proved that the ceria can be reduced under lower temperature (T < 400 8C) over CeX/R230 oxides. The higher dispersion of CeX/R230 appears higher a and b ratio that demonstrates the storing oxygen on ceria.

The specific feature of Co3O4 is the self-assembly of nanoparticles to form a hollow spheroidal overlayers as observed inFig. 4(a). The crystallite of a hollow spheroidal crystallite with an internal diameter of approximately 30–50 nm is mainly the Co-containing phase[29]. They are similar to the particles that had been observed earlier by Potoczna-Petru et al.[30]. When the cobaltic oxide has been modified with ceria, we can find that the surface of spheroidal crystallite is covered very uniform with highly ordered array as shown in Fig. 4(b) and (c). Not only increasing the dispersion of cobaltic oxide but decreasing the internal diameter (approached 15–25 nm) is observed in the Ce4/ R230 and Ce20/R230 samples. While, the more ceria loading (i.e., Ce50/R230) encapsulates the cobaltic oxide that appears the characterization of ceria [seeFig. 4(d)]. Comparison of these results with the XRD and SBETanalysis suggest that the optimal ceria loading can increase both dispersion and surface area of catalyst and promote the catalytic activity.

Catalytic oxidation of carbon monoxide has attracted great interest in the recent decade because carbon monoxide is a well-known pollutant from automobile exhaust. Therefore, we pay

attention to the abatement of carbon monoxide under low temperature. In order to test the catalytic activities of CeX/R230 catalysts, light-off curves for CO oxidation are performed in a continuous flow micro-reactor to understand the effect of ceria loading. The catalytic activities for CO oxidation over CeX/ R230 catalysts are displayed inFig. 5. The CO conversion over each sample generally increases with the reaction temperature.

Fig. 5. Conversion profiles for CO oxidation over the CeX/R230 catalysts: (a) Ce4/R230, (b) Ce12/R230, (c) Ce20/R230, (d) Ce35/R230 and (e) Ce50/R230.

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Evaluation of catalytic activity of each sample bases on the T50 (temperature for 50% CO conversion) and shows in the last column in Table 1. Apparently, the activity toward CO oxidation is promoted significantly by increasing the SBETof catalyst. The correlation of surface area with the T50for CO oxidation over the CeX/R230 catalysts is shown in Fig. 6. Comparison with the TPR analysis, the slowly dropped T50with increasing the SBET of CeX/R230 attributes mainly to the weakening of Co–O bonds that accelerates the desorption of oxygen from Co3O4[19]. The T50 decreases substantially by increasing the SBET, i.e., Ce20/R230 (T50= 88 8C and SBET= 109 m2g1) > Ce12/R230 (T50= 93 8C and SBET= 81 m2g1) > Ce4/R230 (T50= 108 8C and SBET= 71 m2 g1) > Ce35/R230 (T50= 111 8C and SBET= 65 m2g1) > Ce50/R230 (T50= 127 8C and SBET= 52 m2g1). The T50 of Ce20/R230 catalyst is lower than 90 8C and converts completely around 150 8C. While, the T50 of Ce50/R230 catalyst approaches 125 8C and converts completely over 175 8C. The stability of the catalytic activity under 150 8C over Ce20/R230 catalyst maintains 100% conversion of CO after 48 h time-on-stream.

In the same, the correlation of ceria loading with the surface area of the CeX/R230 catalysts is shown inFig. 7. The less ceria loading cannot disperse effectively the cobaltic oxide whereas the more ceria loading encapsulates the cobaltic oxide that decreases both active sites and the SBET of catalyst. The combined effect between cobaltic oxide and ceria can also be found from the tailing reduction of b peak as the ceria loading exceeds 20% [Fig. 2(e)–(g)][23]. Compared the SBET(the 2nd column ofTable 1) of CeX/R230 catalysts, the optimal ceria loading is 20%. Maybe some specific defect sites have been formed on the Ce20/R230 catalyst that increases the SBETand weaken the Co–O bond strength to promote the CO oxidation activity.

The rate of catalytic CO oxidation is also studied as a function of temperature. It is found that the rate of catalytic oxidation increases with the reaction temperature. The plots of ln(dC/dt) versus 1/T are found to be straight line for all samples

(Fig. 8). Activation energies (Ea) of the reaction over CeX/R230 catalysts are calculated according to the Arrhenius equation and shown in the 5th column ofTable 1. Smit et al.[31]reported that the T50 around 100–170 8C for CO oxidation over Au/ Fe2O3catalysts gave 13–25 kJ mol1activation energies. For the Pt/CeO2–Al2O3catalysts, Oran and Uner[32]found that the T50 around 120–220 8C for CO oxidation gave 42– 132 kJ mol1 activation energies. According to our results, the activation energies vary with the ceria loading that follow the order: Ce20/R230 (10 kJ mol1) < Ce12/R230 (18 kJ mol1) < Ce4/R230 (26 kJ mol1) < Ce35/R230 (30 kJ mol1) < Ce50/R230 (55 kJ mol1). Especially for the Ce20/R230 sample is better than the noble catalysts. Comparison of these results with the TEM analysis, suggest that the change of surface morphology can affect the rate of catalytic CO oxidation. The optimal ceria loading can increase both dispersion and surface area of catalyst and reduces the activation energies.

Fig. 6. The correlation of surface area with the T50for CO oxidation over the

CeX/R230 catalysts.

Fig. 7. The correlation of ceria loading with the surface area of the CeX/R230 catalysts.

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4. Conclusion

A series of CeX/R230 catalysts have been characterized and evaluated for CO oxidation. According to the results we find that the optimum loading of ceria is 20% (Ce20/R230). The Ce20/ R230 sample exhibits highest surface area and best catalytic activity on CO oxidation (T50= 88 8C) among these catalysts. We propose that the optimal ceria loading can increase both dispersion and surface area of catalyst and weaken the Co–O bond strength to promote the CO oxidation activity.

Acknowledgements

We are pleased to acknowledge financial supports for this study from Academia Sinica and the National Science Council of the Republic of China.

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