Activity and sulfur resistance of CuO/SnO₂/PdO catalysts supported on γ-Al₂O₃ for the catalytic combustion of benzene

Abstract

In this paper, five types of catalysts, (i.e. SnO₂/PdO/γ-Al₂O₃, CuO/PdO/γ-Al₂O₃, PdO/γ-Al₂O₃, CuO/SnO₂/γ-Al₂O₃ and CuO/SnO₂/PdO/γ-Al₂O₃) were prepared by multiple step impregnation for the catalytic combustion of benzene. The catalysts were characterized by XRD, BET, H₂-TPR and IR to investigate the internal structural and textural changes. The results indicated that the addition of copper to Pd-containing catalyst could promote the catalytic activity, and the addition of tin was beneficial for promoting the sulfur resistance of catalysts but did not bring any benefit to activity. In addition, the CuO/SnO₂/PdO catalyst exhibited better sulfur resistance and catalytic activity than the other prepared catalysts which was attributed to the addition of both copper and tin.

---

Volatile organic compounds (VOCs) that are emitted from material industries are considered to be an important class of air pollutants. Benzene is one of the most common types of VOCs, and is stable and extensively applied in various fields, such as the petrochemical industry and the manufacture of motor fuels, paints, plastics, medications and detergents.^1–3 In addition, catalytic combustion has been extensively investigated in the past few decades due to its practical applications in pollutant abatement, and this technology has been determined to be more environmentally friendly than conventional flame combustion due to lower NO_x, CO, and unburned hydrocarbon emissions as well as a higher energy efficiency.^4–6

Currently, noble metal catalysts are being extensively used for the complete oxidation of VOCs.^7–13 In regards to the activity and selectivity of the catalytic combustion catalysts, noble metals are typically regarded as the most desirable catalysts. In particular, Pd-based catalysts offer several advantages such as a higher activity, thermal stability, better performance and a lower cost compared to other noble catalysts (Pt, Ru).^7–9 The PdO catalysts, which are the most active, exhibit a strong sensitivity to sulfur containing compounds, which is a serious drawback to their use in VOCs exhaust after treatment. Sulfur-containing compounds can be readily converted to SOx and strongly adsorb on the surface of the active ingredient as a stable sulfate species, which decreases the number of active sites until saturation of the active sites surface by the sulfate species results in a complete loss of catalytic activity for benzene oxidation.^14,15

Researchers have reported that the addition of transition metals can enhance the sulfur resistance of PdO catalyst.^16–19 In addition, the addition of transition metals with good thermal resistance (e.g. CuO) can act as a promoter, which is beneficial for a higher specific surface area and better catalytic activity.¹⁶ Ferrandon¹⁷ and Reyes¹⁹ both have reported that the addition of copper greatly improves the catalytic activity of the PdO catalyst. Tin oxide which exhibits superior performance and thermal stability, has been widely used for the catalytic oxidation of methane.^20,21 However, tin oxide has not been extensively investigated for the catalytic combustion of VOCs in the presence of SO₂. H. Meng²² has determined that tin oxide improved the sulfur resistance of catalysts, because tin oxide is a type of acidic oxide. This chemical property can reduce SO₂ adsorption on the surface of a catalyst. Therefore, we investigated a series of catalysts supported on γ-Al₂O₃, including the SnO₂/PdO, CuO/PdO, PdO, CuO/SnO₂ and CuO/SnO₂/PdO catalysts for the catalytic combustion of benzene. In addition, the catalytic activity in presence of SO₂ was also studied. BET, XRD, H₂-TPR and IR characterizations were used to investigate the structural, morphological and catalytic properties changes.

1 Experimental

1.1 Catalyst preparation

The SnO₂/PdO, CuO/PdO, PdO, CuO/SnO₂ and CuO/SnO₂/PdO catalysts were manufactured using a multiple step impregnation method with Cu(NO₃)₂·6H₂O, Pd(NO₃)₂·3H₂O and SnCl₄·5H₂O as precursors. 4.0 g of a commercial γ-Al₂O₃ supporter was dipped into a solution mixture containing adequate amounts of an active component and distilled water for 2 h, dried at 80 °C for 2 hours and calcined in air up to 500 °C for 5 h to acquire desired catalysts. The catalysts contained 0.06 wt% PdO; 10 wt% SnO₂ and 10 wt% CuO based on the active ingredients. In order to describe the preparation process clearer, the total scheme of catalyst preparation was exhibited in Fig. 1.

Fig. 1 The total scheme of the catalysts preparation process.

1.2 Catalyst characterization

Nitrogen adsorption and desorption isotherms were determined at −196 °C using an ASAP-2020 analyzer (Micromeritics Inc.). Using these isotherms, the BET surface areas were calculated, and the pore volumes were determined using the procedure propose by BJH. XRD data were recorded on a Panalytical X'Pert Pro diffractometer at 40 kV and 40 mA with a step size of 0.0167°, and a scanning rate of 10° min⁻¹ using Co K_α radiation, which was then revised to Cu K_α. TPR experiments were performed with a TPR2900 Micromeritics system equipped with a thermal conductivity detector. 50 mg samples were placed in a U-shape quartz tube and purged with a synthetic air (5% O₂/He) steam at 50 ml min⁻¹ at 773 K for1 h, followed by cooling to ambient temperature. Then, the reduction profiles were measured by passing a 5% H₂/Ar flow at a rate of 25 ml min⁻¹ over the sample while heating at a rate of 5 °C min⁻¹ from ambient temperature to 800 °C. IR spectra using KBr pellets of the samples were recorded on a Nicolet 410 FT-IR spectrometer at a resolution of 4 cm⁻¹, and the amount of samples was 1.5 mg in 500 mg of KBr.

1.3 Catalytic combustion measurement

Catalytic activity tests were carried out in a fixed-bed flow reactor under atmospheric pressure. Approximately 2.0 g of catalysts were loaded in a quartz reactor and placed in the middle of the reactor. The temperature was measured and controlled with a thermocouple in the range of 150–450 °C, at a benzene concentration of 1500 ppm and a GHSV of 20 000 h⁻¹. The inlet and outlet gas compositions were analyzed after stepwise changes in the reaction temperature using an on-line gas chromatograph (GC-2014, Shimadzu Corp) equipped with an FID detector and a Restek Rtx-1 column. The catalytic efficiency was evaluated based on the benzene consumption. The catalytic efficiency is determined as:where η₁ was the conversion efficiency, C₁ and C₂ were the inlet and outlet concentration of benzene, respectively, and Q_sn1 and Q_sn2 were the inlet and outlet flux of air, respectively.

1.4 The sulfur treatment of the catalysts

The sulfur treatment of the catalysts was also carried out in the fixed-bed flow reactor. At the reaction temperature of 350 °C, 100 ppm of SO₂ was continuously added to the feed gas for 30 hours to investigate the sulfur resistance of the prepared catalysts. Reactants and products were also analyzed by the on-line gas chromatograph (GC-2014, Shimadzu Corp).

2 Results and discussion

2.1 BET analysis

The surface area, pore volume and average pore diameter of the catalysts are summarized in Table 1. The catalysts exhibit high specific surface areas, due to the use of the γ-Al₂O₃ as support, which has a surface area of 288 m² g⁻¹. Because the pore diameter is in the range of 2–50 nm, the catalysts are regarded as mesoporous materials. As expected, the surface area decreased, as the active phase loading increased. (i.e. surface area is minimized when 10 wt% of SnO₂ and 10 wt% of CuO are deposited) and the very small amounts of PdO slightly affect the surface area. The results from the physisorption studies indicate that in some cases, the sulfur treatment have an effect on the specific surface area of the catalyst samples. According to the results, the average pore diameter of the catalysts slightly increases after the sulfur treatment. However, the surface area decreases after the sulfur treatment. The most pronounced decrease after the sulfur treatment is detected for the PdO catalyst. Because the amount of PdO is too low to be measured by BET, and γ-Al₂O₃ is a well-known sulfatable support, this dramatic variation in the surface area of the PdO catalyst is due to an interaction between the γ-Al₂O₃ support and the PdO active phase.²³

Table 1 The textural properties of non-sulfated and sulfated catalysts

Materials	Surface area (m^2 g^−1)	Mesopore volume (cm^3 g^−1)	Pore diameter^a (nm)
a BJH adsorption average pore diameter.
γ-Al2O3	288	0.75	10.49
CuO/SnO2	213	0.59	11.05
CuO/SnO2–S	202	0.55	11.09
PdO	275	0.76	11.08
PdO–S	226	0.68	11.15
CuO/PdO	236	0.62	11.10
CuO/PdO–S	201	0.57	11.16
SnO2/PdO	226	0.59	11.11
SnO2/PdO–S	210	0.53	11.15
CuO/SnO2/PdO	199	0.54	11.13
CuO/SnO2/PdO–S	172	0.51	11.19

---

2.2 XRD analysis

In order to determine the phase of catalysts, Fig. 2 shows the XRD patterns of the catalysts before and after sulfur treatment. A considering phenomenon is that none of the Pd-containing (A, A–S, B, B–S, D, D–S, C, C–S) samples exhibit a Bragg diffractive peak for Pd oxide. This result may be due to the low loading of PdO loading (0.06 wt%) and PdO being well dispersed on the catalyst surface, which is consistent with the conclusion reported by F. L. Zhong et al.²⁴ In samples of A, C and D, intense and sharp peaks, which appears near θ = 26.8°, 34.1°, 38.3°, 52.3°, 72.0° and 79.6° corresponding to crystalline SnO₂ is registered over Sn-containing catalysts.^25,26 Noteworthily, the monoclinic CuO phase in sample B, C and D is observed at low and weak diffraction angles. It is indicated that some of the copper (apparently a very small amount) is incorporated in the catalysts, which is in agreement with the work of V. R. Choudhary.²⁷ In addition, except for metal oxides are accumulated, no new crystal structure is formed as revealed in catalyst by XRD analysis, these results suggest that the catalysts prepared by stepwise impregnation method are composed of mixed phases.

After sulfur treatment, no significant crystallographic changes in the materials induced by the presence of SO₂ are observed in X-ray diffraction patterns. This phenomenon may indicate that the resulting sulfate species are well dispersed on the catalyst. In addition, the small amount of sulfate that is formed may have been below the detection limit.²²

2.3 H₂-TPR profilesof the catalysts

The H₂-TPR profiles of sulfated and non-sulfated catalysts supported on γ-Al₂O₃ are shown in Fig. 3. A discernible and sharp peak at the high temperature hydrogen consumption temperature between 600 °C and 650 °C is due to the PdO reduction peak, which is attributed to a strong interaction with γ-Al₂O₃ support.²⁸ Researchers^20,28,29 have reported that the reduction of copper oxide species resulted in a single peak centered at approximately 300 °C, which is due to the reduction of Cu²⁺ to Cu⁰. This conclusion confirmed that the reduction peaks at 269 °C, 296 °C, 280 °C, 295 °C, 301 °C and 320 °C in Cu-containing catalysts are associated with the consumption of H₂ by surface and bulk CuO, respectively. In addition, the reduction peaks of the tin oxide species are observed at 492 °C, 503 °C, 504 °C, 514 °C, 557 °C and 563 °C, respectively.³⁰ In comparison to the PdO catalyst and CuO/PdO catalyst, the palladium oxide reduction peak of the CuO/PdO catalyst became smoother and shifted to a lower temperature region than PdO catalyst, which indicates that the addition of copper could enhance the redox ability and promote the lattice oxygen activation in the PdO catalyst.

Fig. 2 XRD patterns of the non-sulfated and sulfated catalysts (A: SnO₂/PdO/Al₂O₃ catalyst, A–S: sulfated SnO₂/PdO/Al₂O₃ catalyst, B: CuO/PdO/Al₂O₃ catalyst, B–S: sulfated CuO/PdO/Al₂O₃ catalyst, C: CuO/SnO₂/Al₂O₃ catalyst, C–S: sulfated CuO/SnO₂/Al₂O₃ catalyst, D: CuO/SnO₂/PdO/Al₂O₃ catalyst, D–S: sulfated CuO/SnO₂/PdO/Al₂O₃ catalyst, E: PdO/Al₂O₃ catalyst, E–S: sulfated PdO/Al₂O₃ catalyst).

After sulfur treatment, it can be seen that the reduction peaks of sulfated catalysts systematically shift to higher temperature regions, indicating that SO₂ have a negative effect on the reduction ability of catalysts. In contrast of the PdO catalyst and the SnO₂/PdO catalyst, the temperature of PdO reduction peak in sulfated PdO catalyst have decreased 31 °C, and the temperature of PdO reduction peak in sulfuted SnO₂/PdO catalyst have only decreased 15 °C. We can conclude that SO₂ have a greater effect on PdO catalyst than SnO₂/PdO catalyst.

2.4 IR spectroscopy

IR spectroscopy provides a tool for studying the sulfate species deposited on the catalysts upon reaction with the SO₂-containing feed. It is well-known that sulfate species are characterized by IR bands in the 1040–1210 cm⁻¹ range. Al₂(SO₄)₃ exhibit a broad band at ca. 1190 cm⁻¹, and the band near 1380 cm⁻¹, which is observed in some catalyst samples, is used as evidence of surface aluminum sulfate.³¹ As shown in Fig. 4, sulfated catalysts exhibit an absorption band at 1180 cm⁻¹ and 1375 cm⁻¹, indicating the existence of aluminum sulfate. In addition to aluminum sulfate, other sulfate substances, such as SnSO₄ and CuSO₄, are also formed in the sulfated catalyst. The PdO catalyst exhibits a deep sulfate adsorption peak, which suggests that the presence of SO₂ have a substantial impact on the PdO catalyst. In the Sn-containing catalyst samples, the sulfate adsorption peak is weak, and the peak corresponding to the SnO₂/PdO catalyst is even weaker than that for the other catalysts. This phenomenon demonstrates that the addition of SnO₂ could prevent catalyst poisoning by forming a sulfate substance.

Fig. 3 H₂-TPR profiles of the non-sulfated and sulfated catalysts (A: SnO₂/PdO/Al₂O₃ catalyst, A–S: sulfated SnO₂/PdO/Al₂O₃ catalyst, B: CuO/PdO/Al₂O₃ catalyst, B–S: sulfated CuO/PdO/Al₂O₃ catalyst, C: PdO/Al₂O₃ catalyst, C–S: sulfated PdO/Al₂O₃ catalyst, D: CuO/SnO₂/Al₂O₃ catalyst, D–S: sulfated CuO/SnO₂/Al₂O₃ catalyst, E: CuO/SnO₂/PdO/Al₂O₃ catalyst, E–S: sulfated CuO/SnO₂/PdO/Al₂O₃ catalyst).

2.5 Catalyst activity

Fig. 5 shows the conversion for benzene oxidation over the catalysts supported on γ-Al₂O₃. From the catalyst activity, the CuO/SnO₂/PdO and CuO/PdO catalyst possesses better activity for benzene oxidation compared to the other catalysts. In addition, the activities are ranked in order as follows: CuO/SnO₂ < SnO₂/PdO < PdO < CuO/PdO ≈ CuO/SnO₂/PdO catalyst.

Fig. 4 IR spectra of non-sulfated and sulfated catalysts IR spectra of non-sulfated and sulfated catalysts (A: CuO/PdO/Al₂O₃ catalyst, A–S: sulfated CuO/PdO/Al₂O₃ catalyst, B: SnO₂/PdO/Al₂O₃ catalyst, B–S: sulfated SnO₂/PdO/Al₂O₃ catalyst, C: CuO/SnO₂/Al₂O₃ catalyst, C–S: sulfated CuO/SnO₂/Al₂O₃ catalyst, D: PdO/Al₂O₃ catalyst, D–S: sulfated PdO/Al₂O₃ catalyst, E: CuO/SnO₂/PdO/Al₂O₃ catalyst, E–S: sulfated CuO/SnO₂/PdO/Al₂O₃ catalyst).

Therefore, the catalysts containing a noble metal (PdO catalyst; CuO/PdO catalyst; SnO₂/PdO catalyst; CuO/SnO₂/PdO catalyst) exhibit better catalytic activity than the non-noble metal containing catalyst (CuO/SnO₂ catalyst), which confirms the excellent catalytic ability of Pd-containing catalysts. The addition of CuO slightly improves the catalytic activity. The results present herein provide evidence that the presence of CuO affects the catalytic activity because the catalytic efficiency of the CuO/PdO catalyst improves compared to the PdO catalyst. Such an effect has also been reported in the literature.^17,19 Although the CuO/PdO catalyst exhibit good catalytic performance, its sulfur resistance is slightly inferior. According to the analysis of catalytic activity and the stability test, the incorporation of SnO₂ have no obvious effect on promoting the catalytic activity. However, SnO₂ incorporation is beneficial for promoting sulfur resistance of the PdO catalyst.

2.6 Stability of the catalysts in the presence of SO₂

To simulate a long-term exposure of the catalysts to sulfur compounds at very low concentrations in benzene, the catalysts are exposed to a dry reaction mixture containing 100 ppm of SO₂. The temperature for the stability test in the presence of SO₂ is confirmed to be 350 °C to maintain consistent temperature conditions.

As shown in Fig. 6, during the sulfur treatment, the PdO and CuO/PdO catalysts exhibit a downward trend, and the PdO catalyst, which is severely deactivated, decreased from 92.66% to 78.26%. The performance of the SnO₂/PdO, CuO/SnO₂ and CuO/SnO₂/PdO catalysts is not affected by the presence of SO₂. In addition, the SnO₂/PdO catalyst exhibit a very stable performance, without appreciable deactivation during 30 h on stream. Therefore, the addition of SnO₂ improves the sulfur resistance of the PdO catalyst, which is consistent with the IR analysis. The addition of CuO does not promote the sulfur resistance of PdO catalyst.

Fig. 5 Benzene conversion over the SnO₂/PdO, CuO/PdO, CuO/SnO₂, PdO and CuO/SnO₂/PdO catalysts supported on γ-Al₂O₃.

During sulfur poisoning, when SO₂ is added into the reaction, SO₂ is converted to SO₃ at a certain temperature and reacts with the active sites to form sulfate species, which results in catalyst poisoning. In previous studies,^17,32,33 the formation of sulfate on the catalyst surface is the primary source of the loss of catalytic activity. In addition, the sulfur poisoning mechanism of PdO/γ-Al₂O₃ is more complex, because γ-Al₂O₃ is a sulfatable support. PdO oxidises SO₂ to SO₃, SO₃ is trapped by PdO and spills to γ-Al₂O₃ sites surrounding the PdO particles by surface diffusion. At saturation of these sites, SO₃ poisons the PdO due to palladium sulfate formation. Therefore, as shown in Fig. 5, the PdO/γ-Al₂O₃ catalyst exhibit sulfur poisoning trend, because surface sulfate is formed on the sulfated catalysts.

M. A. Fraga have concluded that the effect of the addition of SnO₂ on the PdO catalyst improves the thermal stability of the catalyst but does not bring any benefit to combustion activity.²¹ This conclusion can also explain why SnO₂ acquires good stability in the presence of SO₂. The thermal stability of the sulfate substance have the decisive function upon the variation of the catalyst during the SO₂ treatment. In the study by C. J. Zhou,³⁴ Cr₂(SO₄)₃ is unstable and easily decomposed at certain temperature, which decreases the SO₂ adsorption on the catalyst surface. In the same manner, SnO₂ is known to form a stable sulfate substance which decomposes at 350 °C, and SnO₂ is a type of acidic oxide than that can also reduce SO₂ adsorption on the surface of the catalyst. These properties may explain why the addition of SnO₂ results in favorable sulfur stability (Fig. 6).

Fig. 6 Influence of 100 vol ppm SO₂ addition on methane conversion over SnO₂/PdO, CuO/PdO, CuO/SnO₂, PdO and CuO/SnO₂/PdO catalysts supported on γ-Al₂O₃.

3 Conclusion

In this study, SnO₂/PdO, CuO/PdO, CuO/SnO₂, PdO and CuO/SnO₂/PdO catalysts supported on γ-Al₂O₃ are investigated and characterized using BET, H₂-TPR, XRD and IR. The results reveals that the addition of copper and tin to the PdO catalyst promotes catalytic ability and sulfur resistance. The CuO/SnO₂/PdO and CuO/PdO catalysts both exhibit better catalytic combustion than the other catalysts, which indicates that CuO addition improves the catalytic activity of the PdO catalyst. During the sulfur treatment, the CuO/SnO₂, CuO/SnO₂/PdO and SnO₂/PdO catalysts exhibit good sulfur stability, indicating the favorable stability performance of the Sn-containing catalyst. The surface area and pore diameter decreases after SO₂ treatment, which indicates the influence of SO₂ on the physicochemical structural variation in the catalysts. In addition, SO₂ also have a negative effect on the reduction performance according to the H₂-TPR analysis. In the IR analysis, the presence of a sulfate substance is confirmed in the sulfated catalysts, and more of the sulfate substance is formed in the PdO catalyst.

In conclusion, the Pd-containing catalysts exhibit good catalytic activity, and the addition of CuO improved the catalytic ability of the PdO catalyst. The enhancement of the sulfur resistance of the PdO catalyst is due to the addition of SnO₂, because the chemical properties of SnO₂ result in a reduction in SO₂ adsorption on the surface of catalyst. Among the prepared catalysts, the CuO/SnO₂/PdO catalyst exhibit the highest activity and excellent resistance to sulfur compounds, and the incorporation of copper and tin promotes catalytic activity and sulfur resistance of the PdO catalyst.

Acknowledgements

This work is financially supported by the Natural Science Foundation of China (no. 51172107 and no. 51272105), the National Key Technology R&D Program of China (no. 2012BAE01B03-3), the Natural science research project in the University of Jiangsu Province (no. 14KJB430014) and the the Program for Postgraduate Research Innovation in the University of Jiangsu Province (no. CXLX13_441 and no. CXZZ13_0453).

References

1. S. M. Yang, D. M. Liu and S. Y. Liu, Top. Catal., 2008, 47, 101–108.
2. Q. Huang, X. K. Yan and B. Li, J. Rare Earths, 2013, 31, 124–129.
3. J. N. Armor, Appl. Catal., B, 1992, 1, 221–256.
4. S. C. Kim and W. G. Shim, Appl. Catal., B, 2008, 79, 149–156.
5. J. G. Deng, L. Zhang, H. X. Dai, H. He and C. T. Au, Ind. Eng. Chem. Res., 2008, 47, 8175–8183.
6. W. B. Li, W. B. Chu, M. Zhuang and J. Hua, Catal. Today, 2004, 93, 205–209.
7. T. Garcia, B. Solsona, D. M. Murphy, K. L. Antcliff and S. H. Taylor, J. Catal., 2005, 229, 1–11.
8. J. Tsou, P. Magnoux, M. Guisnet, J. J. M. Orfao and J. L. Figueiredo, Appl. Catal., B, 2005, 57, 117–123.
9. B. Grbic, N. Radic and A. Terlecki-Baricevic, Appl. Catal., B, 2004, 50, 161–166.
10. S. C. Kim, J. Hazard. Mater., 2002, 91, 285–290.
11. C. H. Wang, S. S. Lin, C. L. Chen and H. S. Weng, Chemosphere, 2006, 64, 503–509.
12. Y. Li, X. Zhang, H. He, Y. Yu, T. Yuan, Z. Tian, J. Wang and Y. Li, Appl. Catal., B, 2009, 89, 659–664.
13. B. Gric, N. Radic, Z. Arsenijevic, R. Garic-Grulovic and Z. Grbavcic, Appl. Catal., B, 2009, 90, 478–489.
14. P. Hurtado, S. Ordóñez and H. Sastre, Appl. Catal., B, 2004, 47, 85–93.
15. D. L. Mowery, M. S. Graboski and T. R. Ohno, Appl. Catal., B, 1999, 21, 157–169.
16. C. K. Ryu, M. W. Ryoo and I. S. Ryu, Catal. Today, 1999, 47, 141–151.
17. M. Ferrandon, J. Camo and S. Jaras, Appl. Catal., A, 1999, 180, 153–164.
18. L. S. Escandon, S. Ordonez and F. Diez, Catal. Today, 2003, 78, 191–196.
19. P. Reyes, A. Figueroa and G. Pecchi, Catal. Today, 2000, 62, 209–217.
20. X. Wang and Y. C. Xie, React. Kinet. Catal. Lett., 2001, 216, 919–925.
21. M. A. Fraga, E. S. de Souza and F. Villain, Appl. Catal., A, 2004, 259, 57–63.
22. H. Meng and X. Sun, Environ. Monit. Assess., 1997, 13, 45–48.
23. S. C. Kim and W. G. Shim, Appl. Catal., B, 2009, 92, 429–436.
24. F. L. Zhong, Y. J. Zhong and Y. H. Xiao, Environ. Monit. Assess., 1997, 13, 45–48.
25. S. Specchia, E. Finocchio and G. Saracco, Catal. Today, 2009, 143, 86–93.
26. S. Park, H. J. Hwang and J. Moon, Catal. Today, 2003, 87, 219–223.
27. V. R. Choudhary, B. S. Uphade and S. G. Pataskar, Angew. Chem., Int. Ed., 1996, 35, 2393–2395.
28. S. G. ómez-Quero, F. Cárdenas-Lizana and M. A. Keane, Ind. Eng. Chem. Res., 2008, 47, 6841–6853.
29. S. Mosconi, I. D. Lick and A. Carrascull, Catal. Commun., 2007, 8, 1755–1758.
30. M. Iamarino, R. Chirone and L. Lisi, Catal. Today, 2002, 75, 317–324.
31. X. Zhang, R. Hu and G. Gao, Acta Chim. Sin., 2007, 23, 659–662.
32. A. Pieplu, O. Saur, J. C. Lavalley, M. Pijolat and O. Legendre, J. Catal., 1996, 159, 394–400.
33. P. Gélin, L. Urfels and M. Primet, Catal. Today, 2003, 83, 45–57.
34. C. J. Zhou, L. Wei, Y. X. Zhu and Y. C. Xie, Acta Phys.-Chim. Sin., 2003, 19, 246–250.

---

Footnote

† The authors contributed equally to this work.

---