A Simple Method for Preparing the Highly Dispersed Supported

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A Simple Method for Preparing the Highly Dispersed Supported Co3O4 on Silica Support Jianfeng Chen,† Yurui Zhang,† Li Tan,‡ and Yi Zhang*,† †

Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100029, PR China ‡ Taiyuan University of Technology, 79 West Yingze Street, Taiyuan, 030024, PR China ABSTRACT: A novel, simple, and general method for preparing a highly dispersed supported metal catalyst was developed by modification of a silica surface with ethylene glycol (EG) before the impregnation of cobalt precursors. The modified surface of the silica support significantly improved the dispersion of supported cobalt oxide and formed more Co3þ species on the surface of Co3O4 particles, resulting in very high catalytic activity in CO oxidation. The obtained catalysts were characterized by XRD, TEM, and XPS.

1. INTRODUCTION It is well-known that the dispersion and the physicochemical characteristics of the active species are the key factors determining the activity of supported catalysts. The increased dispersion of active species contributes to a high catalytic activity of the catalysts. The chemical and texture properties of the support influence the catalytic activity via their modifications of the dispersion of supported metal or the formation of well-fined phases. Generally, small pore size favored formation of small supported metal or metal oxide particles.1 However, the small pore size of supports results in poor diffusion efficiency of reactants and products, which is a disadvantage to catalytic performance of catalysts. Moreover, coating preprepared metal nanoparticles on the supports is too complicated and expensive to apply in industry. On the other hand, the concentration, distribution, and nature of hydroxyl groups (silanols) on a silica surface also play an important role on the dispersion of supported metal or metal oxide on the silica. It is found that there are two kinds of silanol groups on silica surface, H-bonded SiOH and isolated (non-H-bonded) SiOH.2 The two distinctly different types of silanol groups exhibit variable relative concentrations on silica surface at different thermal conditions.3 Thus, it will change the interaction of active species and silica support, leading to different dispersion of active species, resulting in different catalytic activities. Bao et al. reported that calcination of SiO2 at appropriate temperatures preferentially increases the proportion of isolated SiOH, which leads to the formation of relatively smaller silver particles and thus improves the catalytic performances.4 However, calcination of SiO2 at higher temperature dramatically decreases the surface area and pore structure which results in the inverse effect on catalytic performance. On the other hand, Ho reported that the smaller Co3O4 crystalline size supported on silica was obtained by using ethanol as impregnation solvent than water, even though the preparation process is complex and the solubility of cobalt precursors is limited by the organic solvent used.5 Therefore, the modification of silica surface, adjusting the hydroxyl groups or adsorbing organic groups, is an effective method to promote or control the dispersion of support metal or metal oxide. r 2011 American Chemical Society

Here, we prepared highly dispersed Co3O4/SiO2 catalyst via surface modification of silica support. The silica support was pretreated by ethylene glycol (EG) at room temperature before the impregnation of cobalt nitrate aqueous solution. The modified silica support was expected to form the favorite dispersion of cobalt oxide, contributing to highly catalytic active catalysts. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Since the Co3O4 (bulk or supported) shows very high activity in CO oxidation,6,7 the obtained catalysts were tested in this reaction to illustrate promotional effects of EG-modified silica support.

2. EXPERIMENTAL SECTION Co3O4/SiO2 catalysts were prepared by incipient-wetness impregnation of SiO2 support (ID gel, Fuji Silisya, 20-40 mesh, specific surface area of 270 m2 3 g-1, pore volume of 1.22 mL 3 g-1, average pore diameter = 8.7 nm) with Co(NO3)2 3 6H2O in aqueous solution. The Co3O4 loading of all catalysts were 10 wt %. The silica support was pretreated using ethylene glycol for 1 h at room temperature by incipient-wetness impregnation. The samples were then dried in air at 393 K for 12 h. After that, the cobalt nitrate aqueous solution was impregnated onto the modified silica support. The samples were then dried in air at 393 K for 12 h and calcined at 673 K for 2 h, with a heating rate of 2 K 3 min-1. The obtained catalyst was denoted as Co3O4-EG. The sample, Co3O4-Silica ID, which used nonpretreated silica support, is used as a reference. The oxidation of carbon monoxide by oxygen was carried out in a fixed-bed quartz tubular reactor (i.d., 10 mm) at atmospheric pressure. A 0.2 g portion of catalysts was loaded, and the gas mixture (1 vol % CO, 1 vol % O2, 1 vol % Ar, and balanced with He) was fed into the reactor with a space velocity (SV) of Received: November 10, 2010 Accepted: January 28, 2011 Revised: January 11, 2011 Published: February 28, 2011 4212

dx.doi.org/10.1021/ie1022749 | Ind. Eng. Chem. Res. 2011, 50, 4212–4215

Industrial & Engineering Chemistry Research 15000 mL 3 g-1 3 h-1. The effluent gas was periodically analyzed online with a gas chromatograph equipped with a thermal conductivity detector (TCD). The reaction temperature was measured by putting a thermo-couple in the middle of the catalyst bed. Before reaction, the catalysts were not further pretreated by oxygen. XRD patterns of Co3O4/SiO2 catalysts were recorded on a SHIMADZU XRD-6000 diffractometer with Cu KR radiation

Figure 1. The XRD patterns of various catalysts: (a) Co3O4-Silica ID; (b) Co3O4-EG.

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(λ = 0.154056 nm). The samples were scanned in the 2θ range of 20-80° with a speed of 3° min-1. The average crystalline size of cobalt oxide was calculated by the Scherrer equation: L = Kλ/ β cos θ, where L is the crystalline size, K is a constant (K = 0.91.1), λ is the wavelength of X-ray (CuKR = 0.154 nm), and β is the width of the peak at half height. TEM images of the samples were obtained on a Hitachi H-800 microscope. The specimen was prepared by ultrasonically suspending the catalyst powder in ethanol. A drop of the suspension was deposited on a carbon-enhanced copper grid and dried in air. The XPS analysis was performed on a ThermoFisher Scientific ESCALAB 250 spectrometer. The spectra were excited by the monochromatized Al KR source (1486.6 eV). The analyzer operated in the constant analyzer energy (CAE) mode. Survey spectra were measured at 30 eV pass energy.

3. RESULTS AND DISCUSSION The supported Co3O4 particle size of various catalysts is characterized by XRD and TEM. As shown in Figure 1, the diffraction peak of Co3O4-Silica ID is sharp and higher than that of Co3O4-EG catalyst, and the cobalt oxide crystalline size are 21.2 nm for Co3O4-Silica ID and 5.9 nm for Co3O4-EG catalyst. It is proved that the pretreatment of silica supports by EG remarkably modified the properties of the silica surface, resulting in higher dispersion of supported cobalt oxide.

Figure 2. TEM images and Co3O4 particle size distribution of different catalysts: (a) Co3O4-Silica ID; (b) Co3O4-EG. 4213

dx.doi.org/10.1021/ie1022749 |Ind. Eng. Chem. Res. 2011, 50, 4212–4215

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Table 1. Characterization of Different Catalysts Co3O4 particle size (nm)

a:

Co3þ/(Co2þ þ Co3þ)a: T100b: (K)

catalyst

XRD

TEM

Co3O4-Silica ID

21.2

19.1

0.47

553

Co3O4-EG

5.9

3.8

0.63

413

Calculated by XPS. b: Temperature of 100% CO conversion.

Figure 2 shows the TEM images of various supported catalysts. Significant differences in Co3O4 particle size have been observed among the samples. The average crystal size of Co3O4 on the pretreated silica support was 3.8 nm, while in the case of the unpretreated silica support it was 19.1 nm, which are in good agreement with the XRD data, as tabulated in Table 1. For Co3O4-Silica ID, larger support cobalt oxide particle aggregated into even larger clusters on the silica surface (Figure 2a). However, Co3O4-EG catalyst exhibited remarkably smaller particle size and distributed homogeneously (Figure 2b). It is indicated that modified surface properties of silica support, such as adsorbed EG and nature of hydroxyl groups, prevented the sintering of cobalt oxide, contributing to formation of fine dispersed cobalt oxide particles. It was reported that the concentration, distribution, and nature of hydroxyl groups (silanols) on the SiO2 surface play an important role in the particle size of metal oxide on the SiO2. The supported metal species interacting with H-bonded SiOH results in large particle size. However, those interacting with isolated SiOH lead to formation of smaller supported metal particles.4,8 It was considered that the pretreatment of silica by EG increased the isolated SiOH ratio on the silica surface, leading to the formation of smaller Co3O4 particles. On the other hand, the dispersion of cobalt oxide depends primarily on the conditions of cobalt precursor decomposition and catalyst calcinations.9 The fast rolling motion of vibrationally excited Co2þ molecules complexed with the silica hot substrate surface increased the probability of highly energetic intermolecular collisions on top of the substrate, which resulted in the activation and decomposition of Co2þ ions.10 Besides, the organic groups on the modified silica surface formed by pretreatment of silica support hindered the sintering of Co3O4 by physically interfering and the accompanying energy influence during the thermal decomposition of nitrates, resulting in smaller Co3O4 particle size. The XPS study was carried out to determine the chemical composition and valence state of the elements on the surface of supported cobalt oxide. The spectra recorded are shown in Figure 3. For Co3O4-Silica ID and Co3O4-EG catalysts, the binding energy (BE) of the Co2p3/2 showed peaks at 781.7 and 782.3, respectively. The Co2p3/2 peaks of Co3O4-EG catalyst appeared at higher position than that of catalyst Co3O4-Silica ID, indicating a stronger interaction between silica support and cobalt oxide species, which contributed to a smaller particle size of cobalt oxide,11,12 according to the results of XRD and TEM. On the other hand, for Co3O4-EG catalyst, the line shape of Co2p3/2 was broader, and the shakeup satellite peak, which locates at ca. 6 eV higher energy side from the Co2p3/2 peak, was stronger than that of Co3O4-Silica ID. These characteristics proved that the modified silica surface, which strongly interacted with cobalt precursor, was advantageous to form higher valence state cobalt ion.13,14 Therefore, from the decomposed XPS

Figure 3. X-ray photoelectron spectra of Co2p: (a) Co3O4-Silica ID; (b) Co3O4-EG.

Figure 4. Conversion as a function of temperature for CO oxidation over different catalysts: (9) Co3O4-Silica ID; (2) Co3O4-EG. Reaction conditions: catalyst, 0.2 g; space velocity, 15000 mL 3 g-1 3 h-1; feed gas, 1 vol % CO, 1 vol % O2, 1 vol % Ar, and balanced with He.

spectrum, the Co3þ components of Co3O4-EG significantly increased, as illustrated in Figure 3. The atomic ratio of Co3þ/(Co2þ þ Co3þ) on the subsurface region of catalysts was increased from 0.47 of Co3O4-Silica ID to 0.63 of Co3O4EG, as compared in Table 1. It is considered that stronger interaction between the supported cobalt oxide and modified silica surface would result in a charge shift from cobalt oxide to the silica support, which contributes to form more trivalent cobalt ion on the surface of cobalt oxide.15 On the other hand, since Shen et al. reported that using EG pretreated cobalt precursor formed more Co3þ species on the surface of Co3O4 nanorods,16 it is considered that the adsorbed EG on silica surface would promote to forming more Co3þ species on the surface of formed Co3O4 particles. On the basis of what was mentioned above, the EG modified silica formed highly dispersed Co3O4 particles and more Co3þ species on the surface of Co3O4, which is the active site of CO oxidation.17 To investigate the promotional effects of EG pretreatment on the catalytic activity, the obtained catalysts were tested in CO oxidation, without any further pretreatment by oxygen, and the reaction results were shown in Figure 4. Co3O4-Silica ID catalyst gave 100% CO conversion to CO2 at 553 K. On the other hand, Co3O4-EG catalyst showed 100% CO conversion at 4214

dx.doi.org/10.1021/ie1022749 |Ind. Eng. Chem. Res. 2011, 50, 4212–4215

Industrial & Engineering Chemistry Research 413 K, which is 140 K lower than that of Co3O4-Silica ID catalyst, as compared in Table 1. It was considered that for the Co3O4-EG catalyst, prepared from EG modified silica supports, the different surface properties of silica support contributed to formation of smaller supported cobalt oxide particles, which is advantageous in the formation of a more catalytic active site, thus, giving high CO oxidation activity. Furthermore, Co3þ is regarded as the active site for CO oxidation, whereas Co2þ is almost inactive. The redox cycle connecting the two stages is largely responsible for the successful CO oxidation.16 Therefore, the Co3O4-EG catalyst, which contains more Co3þ species on the surface of supported cobalt oxide as determined by XPS, realized significantly higher catalytic activity than Co3O4-Silica ID catalyst. Based on these results, the modification of silica support by EG resulted in highly dispersed supported cobalt oxide and favored to forming more Co3þ species on the surface of Co3O4, contributing to higher catalytic activity in CO oxidation.

4. CONCLUSION A novel, simple, and general method for preparing highly dispersed supported metal catalyst was developed by modification of a support surface using a simple organic solvent, ethylene glycol. For the catalyst prepared by modified silica support, the Co3O4 particle size is 4 times smaller and the temperature of 100% CO conversion is 140 K lower than those of the catalyst from unmodified silica support. Thus, by selecting a suitable organic solvent to modify the catalyst support, the supported metal or metal oxide would be adjusted or controlled to form designed particle size and surface properties. It will be highly attractive for potential application in optimizing the catalytic performance of designed catalysts.

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: (86)-10-64447274. Fax: (86)-10-64423474. E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (No. 20821004 and No. 20990221), National “863” program of China (No. 2009AA033301), and the Foundation of State Key Laboratory of Coal Conversion (No. 10-11902-1) is greatly appreciated ’ REFERENCES (1) Zhang, Y.; Yoneyama, Y.; Tsubaki, N. Simultaneous introduction of chemical effect and spatial effect via a new bimodal catalyst support preparation method. Chem. Commun. 2002, 11, 1216. (2) Bronnimann, C. E.; Chuang, I. S.; Hawkins, B. L.; Maciel, G. E. Dehydration of silica-aluminas monitored by high-resolution solid-state proton NMR. J. Am. Chem. Soc. 1987, 109, 1562. (3) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. Proton NMR study of dehydration of the silica gel surface. J. Am. Chem. Soc. 1988, 110, 2023. (4) Qu, Z.; Huang, W.; Zhou, S.; Zheng, H.; Liu, X.; Cheng, M.; Bao, X. Enhancement of the catalytic performance of supported-metal catalysts by pretreatment of the support. J. Catal. 2005, 234, 33. (5) Ho, S. W.; Su, Y. S. Effects of ethanol impregnation on the properties of silica-supported cobalt catalysts. J. Catal. 1997, 168, 51. 4215

dx.doi.org/10.1021/ie1022749 |Ind. Eng. Chem. Res. 2011, 50, 4212–4215