TixZr1 − xO2 catalysts for low-temperature CO oxidation

Catalysis Communications 15 (2011) 41–45

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Short Communication

Mesoporous CuO/TixZr1 − xO2 catalysts for low-temperature CO oxidation Jing Huang a, Yanfei Kang b, Liwei Wang b, Taili Yang b, Yao Wang b, Shurong Wang b,⁎ a b

Sinopec Research Institute of Petroleum Engineering, Beijing, 100101, China Department of Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 27 March 2011 Received in revised form 7 August 2011 Accepted 9 August 2011 Available online 18 August 2011 Keywords: Mesoporous CuO/TixZr1 − xO2 Low-temperature CO oxidation

a b s t r a c t Mesoporous CuO/TixZr1 − xO2 catalysts were prepared by a surfactant-assisted method, and characterized by N2 adsorption/desorption, TEM, XPS, in-situ FTIR and H2-TPR. The catalysts exhibited high specific surface area (SBET = 241 m2/g) and uniform pore size distribution. XPS and in-situ FTIR displayed that Cu + and Cu 2+ species coexisted in the catalysts. The CuO/TixZr1 − xO2 catalysts presented obviously higher activity in CO oxidation reaction than the CuO/TiO2 and CuO/ZrO2 catalysts. Effect of molar ratios of Ti to Zr and calcination temperature on catalytic activity was investigated. The CuO/Ti0.6Zr0.4O2 catalyst calcined at 400 °C exhibited excellent activity with 100% CO conversion at 140 °C. © 2011 Elsevier B.V. All rights reserved.

1. Introduction CO is the major toxic pollutant in urban atmosphere. Catalytic oxidation is the most effective solution to reduce the emission of CO [1]. Noble metal catalysts show high activity for CO oxidation at low temperature and are mainly used for eliminating CO [2–6]. However, the high price of noble metals has long motivated the search for substitutes. Therefore, studies of non-noble metal catalysts have attracted much attention. Catalysts comprising CuO supported on metal oxides, such as Fe2O3 [7], CeO2 [8,9] and TiO2 [10,11] have been studied in CO oxidation. It has been found that catalysts using TiO2 as carriers displayed high activity, and that the CuO supported on TiO2 catalysts have much higher activity than the corresponding MoOx, FeOx and CoOx supported on TiO2 catalysts [9–11]. However, the preferred anatase phase has poor thermal stability. Our previous studies revealed that CuO/TiO2 exhibited low catalytic activity after calcination at 500 °C due to the phase transformation from anatase to rutile [12]. Thermal stability is an important factor in catalyst selection. In this context, much attention has been paid to mixed oxide supports [13]. By comparison, ZrO2 has better thermal stability, unique surface acidity and alkalescence and redox properties. Meanwhile, ZrO2 is a semiconductor, which means it can easily produce cavity and has strong interaction with active component. So it is possible to improve the performance of catalysts by changing the composition of the carrier. Recent studies have also revealed that TiO2–ZrO2 is an effective support for MoO3-based hydro-processing catalysts [14] and Fe2O3-based catalysts for the oxidative dehydrogenation of ethylbenzene to styrene

⁎ Corresponding author. Tel.: + 86 22 23505896; fax: + 86 22 23502458. E-mail address: [email protected] (S. Wang). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.08.014

by CO2 [15]. However, there have been few studies of CuO supported on TiO2–ZrO2 mixed oxide for CO oxidation. The mesoporous catalysts may give rise to well-dispersed and stable catalyst particles on the surface upon calcination and reduction because of their abundant pores and large surface area, exhibiting great potential to further improve catalytic performance [16]. In this study, for the first time, the mesoporous CuO/TixZr1 − xO2 catalysts were prepared by a surfactant-assisted method and examined by N2 adsorption–desorption, TEM, XPS, in-situ FTIR and H2-TPR. Effect of the Ti/Zr ratio and calcination temperature on catalytic activity for CO oxidation was investigated. 2. Experimental 20 mol% CuO/TixZr1 − xO2 (x =0, 0.2, 0.4, 0.6, 0.8, 1) catalysts was synthesized using a surfactant-assisted method, denoted as CuZr, CuTi2Zr8, CuTi4Zr6, CuTi6Zr4, CuTi8Zr2 and CuTi, respectively. At room temperature, 6 mmol cetyltrimethylammonium bromide (CTAB) was dissolved in 200 ml distilled water under ultrasound irradiation for 15 min, then 20 mmol Cu(NO3)2·3H2O and appropriate amount of Ti (SO4)2 and Zr(NO3)4·5H2O were added under vigorous stirring. After 0.5 h, 0.2 mol/l NaOH was added gradually until the pH was 10. After further stirring for 12 h, the suspension was aged at 90 °C for 3 h, washed with distilled water, dried at 80 °C for 8 h in air and calcined at 400 °C for 4 h in air. In order to investigate the influence of calcination temperature, the CuTi6Zr4 catalysts calcined at 300, 500, 600, 700 and 800 °C for 4 h in air, respectively, were also obtained, denoted as CuTi6Zr4-300, CuTi6Zr4500, CuTi6Zr4-600, CuTi6Zr4-700 and CuTi6Zr4-800. The samples were characterized by N2 adsorption–desorption (Micromeritics TriStar 3000), transmission electron microscope (TEM, Philips FEI Tecnai 20ST, 200 kV), X-ray diffraction (XRD, Philips XPERT MPD X-ray diffractometer, Cu Kα radiation), X-ray photoelectron

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J. Huang et al. / Catalysis Communications 15 (2011) 41–45

spectroscopy (XPS, Kratos Axis Ultra DLD spectrometer, Al Kα X-ray monochromator), in-situ Fourier transform infrared spectroscopy (insitu FTIR, Shimadzu IRPrestige-21 spectrometer, 250 scans, 0.5 cm−1 resolution) and temperature-programmed reduction (TPR, 5% H2 in Ar (30 ml/min), 50 mg catalyst, 10 °C/min). Catalytic activity was evaluated in a Hiden CATLAB system, using a certified gas mixture containing 1% CO and 10% O2 in N2 balance, at a constant flow rate of 50 ml/min. 50 mg catalyst was loaded into the quartz micro-reactor with quartz wool plugs. The gas product composition was analyzed using a Hiden quadrupole mass spectrometer (HPR20). 3. Results and discussion N2 adsorption/desorption isotherm and corresponding pore size distribution for CuTi6Zr4 are shown in Fig. 1a. A distinct hysteresis loop can be observed between the adsorption and desorption curves between the relative pressure of 0.7 and 1.0, characteristic of a mesoporous solid [17,18]. The pore-size distribution curve, determined by the BJH method from the adsorption branch of the isotherm, exhibits one single narrow peak centered at 3.9 nm (insert in Fig. 1a), indicating the good homogeneity of the pores. The BET specific surface

a 0.11

500 450 400

0.10

dV/dD (cm3/g/nm)

Volume adsorbed (cm3/g)

550

0.09 0.08 0.07 0.06 0.05 0.04

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Relative pressure (P/P0)

TiO2(anatase)

TiO2(rutile)

ZrO2

CuO

f

Intensity (a.u.)

b

area is 241 m 2/g. TEM image of CuTi6Zr4, as shown in Fig. 1b, clearly demonstrates that CuTi6Zr4 has a disordered wormhole-like mesoporous structure, in good agreement with the N2 adsorption– desorption isotherms. The XRD patterns of all samples calcined at low-temperature (below 600 °C) reveal that they are X-ray amorphous (not shown). Fig. 2 contains the XRD patterns for CuO/TixZr1 − xO2 calcined at 600 °C. The diffractograms show that CuO/TiO2 contains both anatase and trace amounts of rutile, while CuO/ZrO2 contains tetragonal ZrO2. In all CuO/ TixZr1 − xO2 mixed oxides, the tetragonal phase of ZrO2 is observed, except in CuO/Ti0.6Zr0.4O2. Trace amounts of TiO2 are found in CuO/ Ti0.8Zr0.2O2. The CuO/Ti0.6Zr0.4O2 is X-ray amorphous, indicating that modifying titania with ZrO2 inhibits the phase transformations of the compositions in the solid [19]. The amorphous nature of mixed TiO2– ZrO2 oxides in the vicinity of Ti/Zr = 58/42 has been reported by other researchers [20]. On the other hand, a peak associated with CuO, present as the tenorite phase (PDF No. = 45–937), was found at 2θ = 38.80° in all samples except CuO/Ti0.6Zr0.4O2. The appearance of CuO diffraction peak is ascribed to the presence of bulk CuO, which may form as a result of decreased BET surface area [8]. The absence of CuO in CuO/Ti0.6Zr0.4O2 suggests a synergistic effect between CuO and Ti0.6Zr0.4O2, leading to high dispersion of CuO. Fig. 3a–c shows XPS of Ti2p, Zr3d and Cu2p for the CuTi6Zr4 catalyst and Fig. 3d and e shows XPS of Cu2p for the CuTi and CuZr catalysts, respectively. As shown in Fig. 3a, the binding energy of Ti2p3/2 and Ti2p1/2 is centered at 458.5 eV and 464.1 eV, respectively, which is in good agreement with the reported value for Ti4+ in anatase TiO2 [21]. XPS of Zr3d is shown in Fig. 3b, the peak centered at 181.7 and 184.1 eV corresponds to Zr3d5/2 and Zr3d3/2, respectively, with the difference of 2.4 eV between the two peaks, indicating the existence of Zr 4+ [22]. It has been well established that the presence of shake-up peak at about 940–945 eV and the Cu2p3/2 binding energy at 933.0–933.8 eV are two major XPS characteristics of CuO, while Cu2p3/2 peak at 932.2– 933.1 eV and the absence of the shake-up peak are the characteristics of Cu+ species [23]. In Fig. 3c–e, the shake-up peaks appear in the range from 938 to 945 eV and the Cu2p3/2 peaks present a broad range from 930 to 937 eV. The signal, centered at 934.7 eV with a shoulder on the low binding energy side, can be deconvoluted into two peaks, one locates at (934.6±0.2) eV, corresponding to Cu2+ bonded to O (i.e., CuO), and the other at (932.5 ±0.2)eV, corresponding to either reduced copper oxides (e.g., Cu2O) or small clusters of Cu0 [21,23]. The surface composition of the CuTi6Zr4 catalyst is estimated by XPS. The surface atomic contents of Cu, Ti and Zr are 6.14, 14.32 and 12.24%, respectively. The surface atomic ratio of Cu/(Cu +Ti + Zr) is 19.4%, which is approximately consistent with the nominal atomic ratio (0.20).

e d c b a 10

20

30

40

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60

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2θ (deg.) Fig. 1. Nitrogen adsorption/desorption isotherm and pore diameter distribution (a) and TEM image (b) for the CuTi6Zr4.

Fig. 2. XRD patterns of CuO/TixZr1 − xO2 catalysts with different x values, after calcination at 600 °C for (a) x = 1, (b) x = 0.8, (c) x = 0.6, (d) x = 0.4, (e) x = 0.2 and (f) x = 0.

J. Huang et al. / Catalysis Communications 15 (2011) 41–45

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Binding Energy (eV) Fig. 3. XPS of Ti 2p (a), Zr 3d (b) and Cu 2p (c) for CuTi6Zr4; XPS of Cu 2p for CuTi (d) and for CuZr (e).

In order to further investigate the copper species present, in-situ FTIR spectra were examined during a CO oxidation experiment over 25 mg CuTi6Zr4 catalyst. Fig. 4 shows the spectral region from 2000 to 2500 cm−1. At 30 °C, one strong band is observed at about 2119.6 cm−1, due to copper carbonyl species. As the temperature is increased, the intensity of CO band weakens and almost vanishes at 150 °C. Meanwhile, when the temperature increases to 50 °C, two weak bands at 2332.9 and 2362.6 cm−1, due to CO2 gas adsorption [24], can also be observed and the intensity increases with the increase of the temperature, indicating the oxidation of CO into CO2.

The characteristic linear carbonyl stretching absorption bands of CO on copper species appear in the range of 2050–2160 cm−1. The Cu 0\CO bands are usually detected below 2110 cm−1 and bands associated with Cu+\CO are usually observed in the range of 2115–2140 cm−1 [25]. It has been reported that CO can be easily desorbed from Cu2+ and Cu 0 sites, but it can be chemisorbed on Cu+ species [26], and that the Cu+ species can still exist on the surface of the CuO/CeO2 catalyst in oxygen atmosphere at 100 °C or after reduction at 500 °C [27]. In our case, the copper carbonyl band appears at about 2119.6 cm−1 and still exists at 120 °C, which supports the assumption of the occurrence of Cu+ species

J. Huang et al. / Catalysis Communications 15 (2011) 41–45

2362.6 2332.9

2119.6

Absorbance (a.u.)

210°C 170°C 150°C 120°C 90°C 70°C 50°C 30°C 2500

2400

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2200

2100

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Wavenumber (cm-1) Fig. 4. In-situ FTIR spectra of CO adsorption on CuTi6Zr4 catalyst obtained at different temperatures.

in the CuTi6Zr4 catalyst. However, CO can be easily desorbed from Cu2+ sites. Therefore, the information from FT-IR cannot be used to support the existence of Cu2+ species. XPS results can well support the existence of Cu2+ species. Contribution of the Cu+ species to the spectrum in Fig. 3c seems to be much smaller than that of CuO, which indicates that copper is present as mainly Cu 2+ species and a small amount of Cu +. Combined with the XPS results, it is suggested that copper presents as both Cu 2+ and Cu+ in the CuTi6Zr4 catalyst. The typical TPR profiles of CuTi, CuTi6Zr4 and CuZr are shown in Fig. 5. The TPR profile of CuTi shows two overlapping peaks at about 204.8 (α1) and 226.6 °C (β1). The TPR profile of CuTi6Zr4 presents mainly one strong reduction peak at about 157.6 °C (α2) along with a weak broad reduction peak centered at 196.3 °C (β2). Similarly, the TPR profile of CuZr shows one strong reduction peak at about 165.1 °C (α3) along with a broad reduction peak centered at 203.0 °C (β3). According to general interpretation [28,29], the peaks α1, α2 and α3 can be attributed to the reduction of highly dispersed CuO interacting with the support strongly. The β1, β2 and β3 peaks should correspond to the reduction of larger CuO particles less interacting with the support. It can be clearly observed that the reduction temperature of the CuO supported on Ti0.6Zr0.4O2 is lower than that of CuO supported on ZrO2 or TiO2, and the intensity of α2 is much higher than that of α1 and α3, indicating that the mixed oxide support more effectively promotes CuO dispersion due to the synergistic effect between ZrO2 and TiO2.

Boon et al. [30] have demonstrated the importance of the oxygen vacancies in copper oxide for the oxidation of CO. Severino et al. [31] have studied CO oxidation over alumina-supported copper oxide catalysts and found that Cu + and/or Cu 0 are the active sites for CO oxidation. Jernigan et al. [32] have showed that the rate of CO oxidation at 573 K decreased with increasing copper oxidation state. Huang et al. [33] have found that the activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu2O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enable Cu2O to exhibit higher activities than the other two copper species. Based on the previous works, the high activity of the CuTi4Zr6 catalyst can be attributed to the quick reversible re-dox process of superficial Cu(I)/(II) couples in a strong synergistic interaction with the Ti–Zr–O support [34]. Fig. 6a, b shows the CO conversion over the CuO/TixZr1 − xO2 catalysts for different x values and the CuTi6Zr4 catalysts calcined at different temperature along with the Ti6Zr4 sample calcined at 400 °C as a function of reaction temperature, respectively. Notably, all the catalysts present a similar behavior that the CO oxidation activity increases with the increase of the catalytic reaction temperature. The T100 (temperature of 100% CO conversion) of the catalysts is listed in Table 1, along with the surface areas. Compared with the CuO/TiO2 and CuO/ZrO2 catalysts, the CuO/TixZr1 − xO2 catalysts exhibit lower T100. The significantly improved activity illuminates that there is a stronger synergistic effect between CuO and the Ti–Zr mixed oxide support, which is supported by the above XRD and TPR results (Fig. 5). In the CuO/

90 80 70 CuTi

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Temperature (°C) Fig. 5. H2-TPR profiles of CuTi (a), CuTi6Zr4 (b) and CuZr (c).

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Temperature (°C) Fig. 6. CO conversion as a function of temperature over the CuO/TixZr1 − xO2 catalysts and Ti6Zr4 sample calcined at 400 °C (a) and the CuTi6Zr4 catalysts calcined at various temperatures (b).

J. Huang et al. / Catalysis Communications 15 (2011) 41–45 Table 1 Surface areas, average pore sizes and T100 of the CuO/TixZr1 − xO2 catalysts. Catalyst

SBET (m2/g)

Average pore size (nm)

T100 (°C)

CuTi CuTi8Zr2 CuTi6Zr4 CuTi4Zr6 CuTi2Zr8 CuZr CuTi6Zr4-300 CuTi6Zr4-400 CuTi6Zr4-500 CuTi6Zr4-600 CuTi6Zr4-700 CuTi6Zr4-800

131 177 241 169 168 165 203 241 172 62 22 11

3.1 2.9 3.4 3.2 3.6 3.0 3.1 3.4 3.7 4.5 8.1 13.1

230 170 140 180 190 240 170 140 180 240 300 470

TiO2–ZrO2 catalyst, the reduction temperature of copper species is lower and the intensity of α peak, due to highly dispersed CuO interacting with the support strongly, is much higher than in the CuZrO2 and CuO/TiO2 catalysts. This indicates that the mixed oxide support more effectively promotes CuO dispersion compared with the single support, which is responsible for the enhancement of the catalytic activity. The decease of the surface area, in the same order: CuTi6Zr4 N CuTi8Zr2 N CuTi4Zr6 N CuTi2Zr8, may be also corresponding to the decrease of the catalytic activity since the higher specific surface area of catalysts could promote the higher dispersion of metal or oxide catalyst particles, which can enhance the catalytic performance. It can be noted that the catalytic activity of the Ti6Zr4 support is much lower than that of the CuTi6Zr4 catalyst, illuminating that there is a synergistic effect between CuO and the Ti6Zr4 support, which strongly affect the catalytic activity in lowtemperature CO oxidation. From Fig. 6b, it can be seen clearly that the catalytic activity of the CuTi6Zr4 catalyst increases by increasing the calcination temperature from 300 to 400 °C and decreases from 400 to 800 °C. Compared with other catalysts, the CuTi6Zr4 catalyst calcined at 400 °C has higher surface area of 241 m 2/g, exhibiting the highest catalytic activity in CO oxidation, with the T100 at 140 °C. Calcination at 300 °C results in only partial surfactant decomposition and removal of organics blocking some of the active sites, which induces lower activity of the CuTi6Zr4-300 catalyst than the catalyst calcined at 400 °C. The high temperature pretreatment results in the agglomeration of the catalysts and the sharp decrease in the surface areas (see Table 1), which is responsible for the decrease in catalytic activity. 4. Conclusions A series of mesoporous CuO/TixZr1 − xO2 catalysts with high surface area and narrow mesopore size distribution has been prepared by a surfactant-assisted method. XPS and in-situ FTIR displayed that Cu+ and Cu2+ species coexisted in the catalysts. Compared with the CuO/TiO2 and CuO/ZrO2 catalysts, the CuO/TixZr1 − xO2 catalysts exhibited much higher activity. The stronger synergistic effect between CuO and the Ti–Zr mixed oxide support could more effectively promote CuO dispersion, which is responsible for the enhancement of the catalytic activity. The catalytic performance depended on the molar ratio of Ti/Zr and calcination temperature. The CuO/Ti0.6Zr0.4O2 calcined at 400 °C with high specific surface area (SBET = 241 m2/g) presented the highest

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catalytic activity with 100% CO conversion at 140 °C. The high dispersion CuO species and the high surface area contributed to the high catalytic activities of the catalysts in low temperature CO oxidation. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 20871071) and the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (nos. 09JCYBJC03600 and 10JCYBJC03900). References [1] M.V. Twigg, Appl. Catal., B 70 (2007) 2–15. [2] C.R. Jung, J. Han, S.W. Nam, T.H. Lim, S.A. Hong, H.I. Lee, Catal. Today 93–95 (2004) 183–190. [3] S.R. Wang, J. Huang, Y.Q. Zhao, S.P. Wang, X.Y. Wang, T.Y. Zhang, S.H. Wu, S.M. Zhang, W.P. Huang, J. Mol. Catal. A 259 (2006) 245–252. [4] A.V. Grigorieva, E.A. Goodilin, L.E. Derlyukova, E. Lyudmila, T.A. Anufrieva, A. Tatyana, A.B. Tarasov, Y.A. Dobrovolskii, Y.D. Tretyakov, Appl. Catal., A 362 (2009) 20–25. [5] M. Mendez-Cruz, J. Ramirez-Solis, R. Zanella, Catal. Today 166 (2011) 172–179. [6] Z.W. Wang, X.V. Wang, D.Y. Zeng, M.S. Chen, H. Wan, Catal. Today 160 (2011) 144–152. [7] J.L. Cao, Y. Wang, X.L. Yu, S.R. Wang, S.H. Wu, Z.Y. Yuan, Appl. Catal., B 79 (2008) 26–34. [8] M.F. Luo, J.M. Ma, J.Q. Lu, Y.P. Song, Y.J. Wang, J. Catal. 246 (2007) 52–59. [9] K.N. Rao, P. Bharali, G. Thrimurthulu, B.M. Reddy, Catal. Commun. 11 (2010) 863–866. [10] O.V. Komova, A.V. Simakov, V.A. Rogov, D.I. Kochubei, G.V. Odegova, V.V. Kriventsov, E.A. Paukshtis, V.A. Ushakov, N.N. Sazonova, T.A. Nikoro, J. Mol. Catal. A. 161 (2000) 191–204. [11] P.O. Larsson, A. Andersson, L.R. Wallenberg, B. Svensson, J. Catal. 163 (1996) 279–293. [12] J. Huang, S.R. Wang, Y.Q. Zhao, X.Y. Wang, S.P. Wang, S.H. Wu, S.M. Zhang, W.P. Huang, Catal. Commun. 7 (2006) 1029–1034. [13] K. Tanabe, T. Sumiyosh, K. Shibata, T. Kiyoura, J. Kitagawa, Bull. Chem. Soc. Jpn. 47 (1974) 1064–1066. [14] J. Miciukiewicz, T. Mang, H. Knozinger, Appl. Catal., A. 122 (1995) 151–159. [15] B.M. Reddy, H. Jin, D.S. Han, S.E. Park, Catal. Lett. 124 (2008) 357–363. [16] V. Idakiev, T. Tabakova, Z.Y. Yuan, B.L. Su, Appl. Catal., A 270 (2004) 135–141. [17] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed, Academic Press, London, 1982, p. 131. [18] H. Yang, Q. Shi, B. Tian, S. Xie, F. Zhang, Y. Yan, B. Tu, D. Zhao, Chem. Mater. 15 (2003) 536–541. [19] X.Z. Fu, L.A. Clark, Q. Yang, M.A. Anderson, Environ. Sci. Technol. 30 (1996) 647–653. [20] M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno, H. Ohuchi, Appl. Catal., B 12 (1997) 263–276. [21] G. Avgouropoulos, T. Ioannides, Appl. Catal., A. 244 (2003) 155–167. [22] A.E. Nelson, K.H. Schulz, Appl. Surf. Sci. 210 (2003) 206–221. [23] M.S.P. Francisco, V.R. Mastelaro, P.A.P. Nascente, A.O. Florentino, J. Phys. Chem. B 105 (2001) 10515–10522. [24] A. Martínez-Arias, M. Fernández-García, A.B. Hungría, A. Iglesias-Juez, O. Gálvez, J. A. Anderson, J.C. Conesa, J. Soria, G. Munuera, J. Catal. 214 (2003) 261–272. [25] A. Gómez-Cortés, Y. Márquez, J. Arenas-Alatorre, G. Díaz, Catal. Today 133–135 (2008) 743–749. [26] H.C. Lee, D.H. Kim, Catal. Today 132 (2008) 109–116. [27] A. Martínez-Arias, A.B. Hungría, M. Fernández-García, J.C. Conesa, G. Munuera, J. Phys. Chem. B 108 (2004) 17983–17991. [28] A. George, I. Theophilos, Appl. Catal., A 244 (2003) 155–167. [29] B.L. Zhu, X.X. Zhang, S.R. Wang, S.M. Zhang, S.H. Wu, W.P. Huang, Microporous Mesoporous Mater. 102 (2007) 333–336. [30] A.Q.M. Boon, F. van Looij, J.W. Geus, J. Mol. Catal. 75 (1992) 277–291. [31] F. Severino, J.L. Brito, J. Laine, J.L.G. Fierro, A.L. Agudo, J. Catal. 177 (1998) 82–95. [32] G.G. Jernigan, G.A. Somorjai, J. Catal. 147 (1994) 567–577. [33] T.J. Huang, D.H. Tsai, Catal. Lett. 87 (2003) 173–178. [34] B. Skårman, D. Grandjean, R.E. Benfield, A. Hinz, A. Andersson, L.R. Wallenberg, J. Catal. 211 (2002) 119.