Cobalt Silicide Nanoparticles in Mesoporous Silica as Efficient

Feb 17, 2010 - Xiao Chen , Miao Li , Jingchao Guan , Xinkui Wang , Christopher T. Williams .... Xiao Chen , Jianhui Jin , Guangyan Sha , Chuang Li , B...
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Cobalt Silicide Nanoparticles in Mesoporous Silica as Efficient Naphthalene Hydrogenation Catalysts by Chemical Vapor Deposition Anqi Zhao, Xiaofei Zhang, Xiao Chen, Jingchao Guan, and Changhai Liang* State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, People’s Republic of China ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: January 25, 2010

Cobalt silicide nanoparticles in mesoporous silica SBA-15 were successfully prepared by metal-organic chemical vapor deposition of a single-source precursor and were characterized by nitrogen physorption, X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption, and transmission electron microscopy. The catalytic hydrogenation of naphthalene on the cobalt silicide nanoparticles in mesoporous silica was investigated in a fixed-bed reactor at 340 °C under 4.0 MPa hydrogen pressure. No cobalt silicide was observed in the mesoporous silica without calcination because of the hydrolysis of Co(SiCl3)(CO)4. Highly dispersed and evenly distributed cobalt silicide particles with diameters of 2-4 nm could be formed by adsorption and reduction of Co(SiCl3)(CO)4 in the mesoporous silica with calcination above 500 °C. The cobalt silicide loading in the mesoporous silica depended on both the amount of precursor provided and the amount of hydroxyl groups in the mesoporous silica. A 8.3% CoSi/SBA-15 sample showed lower conversion and higher selectivity to tetralin (about 100%) in the naphthalene hydrogenation, while a 16.1% CoSi/SBA-15 sample showed a higher catalytic activity up to 87% and a lower selectivity to tetralin (35%) than other CoSi/SBA15 samples, which may be due to the size effect and the strong interaction of CoSi and support. Introduction High aromatic content in distillate fuels such as diesel fuel, gasoline, and jet fuel has been recognized both to lower the fuel quality and to produce undesired emissions in the exhaust gases. Meanwhile, these aromatics and the resulting emissions have potentially hazardous and carcinogenic effects.1-3 As a result, the more and more stringent environmental regulations and fuel specifications are demanding a reduction of aromatics in fuels. The conventional industrial hydrotreating process, in which sulfur and nitrogen atoms are simultaneously removed by hydrodesulfurization and hydrodenitrogenation reactions, is currently adapted for aromatic reduction. It is also important to produce solvents and advanced fuels using hydrogenation and selective hydrogenation of aromatic compounds. Thermodynamic equilibrium limitations on aromatic hydrogenation led to a strong interest in catalytic materials different from the classic supported metal sulfide catalysts. Supported noble metal catalysts had showed high catalytic activity for aromatic hydrogenation at low temperature. However, the presence of a small amount of sulfur strongly influences their catalytic activity because of the poisoning of active sites. Metal carbides and nitrides are also initially highly active but are easily poisoned by a small amount of sulfur-containing compounds.4 Thermochemical calculations have indicated that transition-metal silicides can tolerate much higher H2S concentration than the corresponding carbides and nitrides.5 This means that transitionmetal silicides are potentially stable and sulfur-resistant in the catalytic reaction with sulfur-containing compounds. Because of good electrical conductivity, high chemical inertness, and thermal stability, transition-metal silicides are more and more attractive and important in microelectronics as ohmic contacts and interconnections in Complementary Metal * To whom correspondence should be addressed. Fax: +86-41139893991; Tel: +86-411-39608806; E-mail: [email protected].

Oxide (CMOS) transistors.6-10 However, to the best of our knowledge, few reports have been published on metal silicides as novel catalytic materials,11-19 although they are active phases in the hydrodehalogenation of silicon tetrachloride.20,21 Palladium silicide was reported to exhibit a much higher selectivity than Pd on SiO2 in the hydrogenation of phenylacetylene with the same specific activity.14 The amorphous Pd81Si29 alloy prepared by melt spinning exhibited a more than 50 times higher Turnover Frequency (TOF) than Pd/SiO2 for the semihydrogenation of propargylic alcohol.17-19 Unfortunately, metal silicides inherited from the microelectronic industry had low surface area and were nonporous, which restricted the application of metal silicides in catalysis. It is still an important challenge to synthesize welldispersed metal silicides for catalytic applications under controllable conditions. Metal-organic chemical vapor deposition (MOCVD) is an attractive and successful method for highly dispersed catalysts in a controlled and reproducible manner.22,23 CoSi films24 and nanowires25 had been successfully synthesized by MOCVD of Co(SiCl3)(CO)4 as single-source precursor. Recently, we showed that highly dispersed CoSi nanoparticles supported on silica could be synthesized by MOCVD of Co(SiCl3)(CO)4 at atmospheric pressure and moderate temperature and could be used as catalysts for naphthalene hydrogenation.26 SBA-15, an ordered mesoporous silica with large surface area and uniform hexagonal channels, has attracted considerable attention because of its potential applications in heterogeneous catalysis.27-32 The large surface area favors a high dispersion of active species and is often conducive to high catalytic activity, and the uniform hexagonal channels with mesoporous size may reduce pore diffusion limitation and allow the effective use of the active sites on the surface of the pore wall in reaction. The SBA-15-supported catalysts have been developed for many reactions,30-33 including hydrogenation of aromatics.34,35 Herein, we report the preparation and characterization of highly dispersed CoSi nanoparticles supported on SBA-15 by MOCVD

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of Co(SiCl3)(CO)4 as single-source precursor. The CoSi/SBA15 nanocomposite catalysts exhibited excellent activity and selectivity to tetralin in hydrogenation of naphthalene and therefore might find applications in selective hydrogenation reactions. Experimental Section Co(SiCl3)(CO)4 precursor was synthesized under an atmosphere of dry argon using Schenk-line techniques according to a modified procedure from the literature.25,36 Typically, about 10 mL of SiHCl3 was added to 1.0 g of Co2(CO)8 placed in a Carius tube and was cooled to -40 °C in an acetonitrile/dry ice slurry. The reaction tube was pumped down to 0.5 Torr, was sealed, and was stirred at -40 °C for 2 h before warming to room temperature. The remaining SiHCl3 was removed with dynamic vacuum before sublimation at 40 °C and 0.1 Torr and was carried out for 2 h. The final product of yellow crystal was collected with a yield of about 60%. Co(SiCl3)(CO)4 was chosen as precursor because it allows precise control of stoichiometry, it has high vapor pressure, and it has stability in dry air at low temperature. The Fourier transform infrared (FT-IR) spectrum of the precursor dissolved in CDCl3 solution was recorded using a JASCO FT-IR 4300 Plus spectrometer, and 13C and 29Si NMR spectra were recorded on a Varian INOVA-400 (400 MHz) spectrometer. The SBA-15 support was prepared by using the triblock copolymer surfactant P123 from BASF as the template and tetramethoxysilane as the silica source in HCl solution in addition to a small amount of NH4F according to the literature.37 The CoSi deposition onto the SBA-15 support (452 m2/g) was achieved by a two-step metal-organic chemical vapor deposition of Co(SiCl3)(CO)4. Prior to CoSi deposition, the SBA-15 was calcined in 150 mL/min of air at 500 °C for 120 min to remove adsorbed water, which promotes hydrolysis of the precursor. The precursor was sublimed and adsorbed on the SBA-15 support at 50 °C in vacuum. The SBA-15 adsorbed precursor was treated in 200 mL/min of 10% H2/Ar at 300 °C for 120 min under atmospheric pressure, which led to a stable black CoSi/SBA-15. The surface area, pore volume, and pore size distribution of the support and catalysts were determined from nitrogen adsorption-desorption isotherms at -196 °C using a Micromeritics 2020 apparatus. Prior to the measurements, all samples were degassed at 300 °C at 10-3 Torr for at least 3 h. The surface areas were calculated from the linear part of the BrumauerEmmett-Teller (BET) plots. The pore volume and the pore size distribution were derived from the desorption profiles of the isotherms using the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) analysis was carried out using a D/Max 2400 diffractometer with Cu KR monochromatized radiation source (λ ) 1.5418 Å) operated at 40 kV and 100 mA. The particle size and distribution were analyzed by transmission electron microscopy (Tecnai G2 20s-Twin, 200KV). Powder samples were ultrasonicated in ethanol and were dispersed on holey carbon films on copper grids. Energy-dispersive X-ray spectroscopy (EDX) was also performed in the same microscopy. H2-temperature-programmed reduction (H2-TPR) analysis was carried out using Micromeritics Autochem 2720 equipment. About 30 mg of catalyst was placed in a quartz reactor and was reduced in a stream of 10% H2/Ar gas mixture with a flow rate of 50 mL/min. The sample was heated from room temperature to 950 at 10 °C/min. The amount of the hydrogen consumption during the reduction was estimated with the thermal conductivity detector. H2-temperature-programmed de-

Figure 1. 13C and 29Si NMR spectra of the organometallic precursor Co(SiCl3)(CO)4.

sorption of hydrogen (H2-TPD) experiments were performed by America Micromeritics Autochem 2720 equipment. First, 50 mg of the sample was reduced in a 10% H2/Ar gas mixture at 500 °C (heating rate of 10 °C/min) for 3 h under a flow rate of 50 mL/min. Once the catalysts were cooled down to room temperature, the surface was purged in an argon flow for 1 h. Subsequently, temperature was linearly increased from room temperature to 800 at 10 °C/min following the hydrogen desorption process with a thermal conductivity detector. The catalytic properties of CoSi/SBA-15 catalysts were tested in the hydrogenation of naphthalene at 340 °C and 4.0 MPa. The passivated catalysts (0.1 g, diluted with 2.0 g SiC) were activated in situ with H2 at 400 °C and 0.1 MPa for 4 h. The reaction was carried out in a continuous fixed-bed reactor. The liquid reactant was composed of 1 mol % undecane (as internal standard for gas chromatography (GC) analysis), 5 mol % naphthalene reactant, and 94 mol % decane (as solvent). The reaction product was analyzed by off-line gas chromatography with an OV-101 capillary column and a flame ionization detector. Product identification was performed with an Agilent 6890 gas chromatograph equipped with an HP-5MS capillary column and an Agilent 5973 mass selective detector. Results and Discussion The 13C NMR spectrum of Co(SiCl3)(CO)4 in CDCl3 solution showed one peak at 193.9 ppm besides three peaks at 77.58, 77.26, and 76.95 ppm because of CDCl3 (Figure 1). The signal at 193.9 ppm was attributed to the axial carbonyl group. The FT-IR spectrum of the organometallic precursor in CDCl3 solution showed five peaks located 2124, 2072, 2045, 2005, and 1992 cm-1, which were assigned to stretching vibrations of linearly bonded CO on metallic cobalt. These results are in good agreement with the literature,36 which had reported peaks at 2118, 2063, and 2036 cm-1 in the IR spectrum and at δ ) 192.5 in the 13C NMR spectrum. The 29Si NMR spectrum of

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TABLE 1: Surface Area and Texture Parameters of SBA-15 and CoSi/SBA-15 sample

SBET/m2/g

average pore size/nm

pore volume/cm3/g

SBA-15 CoSi/SBA-15(8.3%) CoSi/SBA-15(12.2%) CoSi/SBA-15(16.1%)

452 462 451 383

7.9 7.8 7.6 7.4

1.01 0.98 0.92 0.76

Co(SiCl3)(CO)4 in CDCl3 solution (Figure 1) showed one peak at 34.9 ppm confirming tetracoordination of the silicon atom. In comparison with the spectrum of HSiCl3, the resonance of Co(SiCl3)(CO)4 is significantly shifted to higher field (by about 25 ppm). This can be attributed to the substitution of Si-bound hydrogen atom by a Co substituent. The result is in agreement with the data reported by Novak et al.36 The C content in the precursor was 15.70% measured by Vario EL III elemental analyzer, which was consistent with the theoretical value of 15.73% in Co(SiCl3)(CO)4. Meanwhile, the sublimation temperature (40 °C, 0.1 Torr) is the same as the value reported in the literature.36 Taking the FT-IR and NMR results, C content, and sublimation temperature into consideration, we conclude that Co(SiCl3)(CO)4 had been successfully synthesized. The CoSi deposition onto the SBA-15 support was achieved by a two-step process at atmospheric pressure. The CoSi loadings were controlled by changing the amount of Co(SiCl3)(CO)4 in the sublimator. Table 1 lists the CoSi loadings in the CoSi/SBA-15 samples as obtained from Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The CoSi loadings are between 8.3 and 16.1 wt. %. The level of CoSi loading depended on both the surface properties of the SBA-15 and the amount of precursor provided in the sublimation chamber. When the SBA-15 support was not calcined in air above 500 °C, blue CoCl2/SBA-15 was formed by hydrolysis of the precursor and was further confirmed by the fact that the sample turned brownish red with water. Figure 2a shows the N2 adsorption-desorption isotherms of the pure SBA-15 support and CoSi/SBA-15 samples with different CoSi loadings. The corresponding structural parameters are summarized in Table 1. All adsorption-desorption isotherms show type IV behavior with an H1 hysteresis loop according to IUPAC classification38 meaning that the structure of the SBA15 had not been destroyed during the formation of the CoSi particles. The hysteresis loops showed sharp adsorption and desorption branches in the P/P0 range from 0.6 to 0.85 indicating a narrow mesopore size distribution. The adsorbed volume of nitrogen on the SBA-15 showed a sharp increase at about P/P0 ) 0.75 arising from capillary condensation of nitrogen within the uniform mesoporous structure. The increase became less sharp with increasing CoSi loading from 8.3 to 16.1 wt %. Mesoporous size of SBA-15 was observed at about 9.3 nm (Figure 2b). The corresponding values of CoSi/SBA-15 samples showed a dramatic decrease from 9.3 to 8.5 nm further indicating the formation of CoSi in the channel of SBA-15. As shown in Table 1, the surface area decreased from 452 to 383 m2/g, the pore volume decreased from 1.01 to 0.76 cm3/g, and the average pore size decreased from 7.9 to 7.4 nm after the MOCVD of Co(SiCl3)(CO)4 in the SBA-15. The surface area, pore volume, and average pore size of the CoSi/SBA-15 samples showed a decreasing trend with increasing CoSi loading. It can be attributed to that the formed CoSi particles filled and blocked a fraction of the pores of the SBA-15. The XRD of the SBA-15 (Figure 3a) clearly illustrates three well-resolved diffraction peaks in the small-angle region, which

Figure 2. (a) N2 adsorption/desorption isotherms at -196 °C of the pure SBA-15 support and SBA-15 supported cobalt silicide samples. (b) Pore size distribution of the samples calculated from the desorption branch of the isotherms using the BJH algorithm.

can be indexed as the characteristic (100), (110), and (200) reflections of a two-dimensional hexagonal (p6 mm) mesoporous structure. The XRD also indicated that the SBA-15 sample consisted of well-ordered packed channels. With increasing CoSi loading, the (110) and (200) reflections became weak and then disappeared, which could be explained by a decrease of the degree of ordering. The intensity of the (100) reflection for CoSi/ SBA-15 showed a dramatic decrease when the CoSi loading increased. In addition, the diffraction peaks for CoSi/SBA-15 slightly shifted to smaller angles in comparison with the pure SBA-15, which is explained by a contraction of the host framework during the formation of CoSi inside the channels. In the wide-angle region of the XRD patterns, only a broad SiO2 diffraction peak at about 23.4° was observed indicating that the CoSi particles were too small to be detected (Figure 3b). The same phenomenon was also observed in the CoSi/ SiO2 sample with a similar CoSi loading.26 The transmission electron microscopy (TEM) image in Figure 4a shows uniform and highly dispersed CoSi nanoparticles in the hexagonal channels of the SBA-15. The CoSi nanoparticles almost exclusively stayed inside the channels of the host SBA15. The uniform hexagonal channels were clearly visible in SBA-15 (Figure 4b). Some of the hexagonal channels of SBA15 appeared black in the CoSi/SBA-15 indicating confinement of CoSi in the channels of SBA-15. This was consistent with

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Figure 3. (a) Small-angle XRD of SBA-15 and CoSi/SBA-15 samples with different CoSi loadings. (b) Wide-angle XRD of SBA-15 and CoSi/ SBA-15 samples with different CoSi loadings.

Figure 5. H2-TPR profiles (a) of the CoSi/SBA-15 samples with different CoSi loadings and H2-TPD profiles (b) of the CoSi/SBA-15 samples with different CoSi loadings.

Figure 4. TEM images of CoSi/SBA-15 viewing normal to the pore axis (a) and along the pore axis (b) of host SBA-15, (c) TEM image of CoSi/SBA-15 calcined at 700 °C, and (d) HRTEM image of CoSi with SBA-15 dissolved in 2 mol/L NaOH solution; the inset in d is the corresponding SAED pattern.

the XRD and N2 adsorption results discussed above. The stability of the CoSi/SBA-15 was also investigated after the calcination at 700 °C. Figure 4c shows that the structure of SBA-15 was

destroyed after calcination of CoSi/SBA-15 at 700 °C. The structure of CoSi/SBA-15 had some disfigurements and was more easily destroyed at higher temperature than the pure SBA15. High-resolution transmission electron microscopy (HRTEM) could be used to probe the distribution of CoSi after SBA-15 was dissolved by 2 mol/L NaOH solution to further confirm the formation of a CoSi phase. About 2-4 nm CoSi particles were observed (Figure 4d). The observed lattice spacing of 0.221 nm matched well with the reported value of the (200) plane in the CoSi structure (JCPDS PDF72-1328). The Selected Area Electron Diffraction (SAED) pattern (inset in Figure 4d) indicates that the observed nanoparticles were CoSi crystals with cubic structure. CoSi shows the cubic structure according to the literature.25,26,36 The inserted silicon atoms could expand the crystal lattice of cobalt atoms, which induced a constriction of the d band of the transition metal and an increase of the electronic density of states near the Fermi level. The effects caused a transformation of the exterior property and the adsorption character of the solids, which made the silicides resemble those of noble metals.39,40 A similar phenomenon has been observed for transition-metal nitrides and carbides.41-43 Temperature-programmed reduction and desorption (TPR and TPD) are powerful tools for characterizing supported catalysts. Figure 5a shows H2-TPR profiles of CoSi/SBA-15 samples with different loadings. For the passivated 8.3% CoSi/SBA-15

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SCHEME 1: Reaction Network of the Hydrogenation of Naphthalene

sample, two low temperature peaks were observed at 180 and 328 °C, which can be tentatively assigned to the reaction of oxygen species adsorbed on the CoSi with H2.44,45 The results are similar to those of the TPR of nitrides.46 With increasing CoSi loading, the low temperature peak shifted to higher temperature and the high temperature peak showed the reverse trend. The two peaks were attributed to the exterior adsorption and pore interior adsorption or different oxidation degrees of CoSi. The reduction peak of CoSi could not be observed in Figure 5a probably because of the destruction of the SBA-15 structure with increasing temperature. The CoSi particles were encased in the channels of the SBA-15 and cannot be reduced by H2. This was confirmed by the TEM image (Figure 4c), which showed that the pore structure of SBA-15 had collapsed at 700 °C. The CoSi/SBA-15 sample presented a reductive behavior that was completely different from that of CoSi/SiO2,26 which might be due to the different pore structure of the support. Figure 5b showed the H2-TPD profiles of CoSi/SBA-15 samples with different loadings. The samples with different loadings showed similar profiles with two desorption peaks at 64 and 654 °C. The peak at low temperature could be due to the desorption of weakly chemisorbed H2 because of the different H-Co surface sites on the SBA-15.44,45 The one observed at high temperature could arise from strongly chemisorbed H2 related to the presence of active hydrogenation sites in the surface of CoSi. The result was similar to that of CoSi/SiO2.26 The naphthalene hydrogenation occurred in two steps as shown in Scheme 1: conversion to tetralin followed by formation of decalin.47 The conversion of naphthalene increased with the increase of contact time with different loading samples (Figure 6a). When the contact time increased from 0.5 to 3.3 min, the conversion of naphthalene increased from 0 to 17% over the 8.3% CoSi/SBA-15 sample and from 2.3 to 34% over the 12.2% CoSi/SBA-15 sample. The 16.1% CoSi/SBA-15 catalyst gave a conversion up to 87% and was much more active than the other two samples, which may be due to the big particles and the weak interaction between support and CoSi particles. The selectivity to tetralin over the 8.3% CoSi/SBA-15 catalyst (Figure 6b) showed that naphthalene converted to tetralin with a selectivity of 100%, which may be due to formation of small CoSi particles. This indicated that the naphthalene hydrogenation to tetralin was a fast step. The result showed that 8.3% CoSi/ SBA-15 is a promising catalyst for semihydrogenation, which could be very important in the synthesis of fine chemicals. Under the same reaction conditions, decalin could be detected over the 12.2% CoSi/SBA-15 catalyst. The selectivity to tetralin stayed constant at about 85%. The selectivity to tetralin over the 16.1% CoSi/SBA-15 catalyst increased first, began to decrease at 1.3 min, and then decreased to the final selectivity of 35% indicating that tetralin was an intermediate to decalin and that the rate constant of further hydrogenation of tetralin was larger than its rate constant of formation. Obviously, the CoSi/SBA-15 catalyst with high CoSi loading was very active in the naphthalene hydrogenation in good agreement with the

Figure 6. (a) Conversion of naphthalene and (b) selectivity to tetralin on the CoSi/SBA-15 samples with different CoSi loadings.

H2-TPD data. Simultaneously, the differences in selectivity to tetralin could be due to the size effect and the strong interaction of CoSi and support. Further experiments are being carried out to explore the interaction of CoSi and support and to elucidate the effect of the interaction to the naphthalene hydrogenation. Conclusion CoSi particles on mesoporous silica SBA-15 support were successfully synthesized by MOCVD of Co(SiCl3)(CO)4 as precursor at atmospheric pressure and moderate temperature. The results showed that CoSi particles with a size of about 2-4 nm were highly dispersed in the SBA-15 channels. The CoSi/ SBA-15 exhibited unique H2 reduction and desorption properties that were completely different from that of CoSi/SiO2. The 8.3% CoSi/SBA-15 sample showed a lower conversion and higher selectivity to tetralin (about 100%) in the naphthalene hydrogenation, while the 16.1% CoSi/SBA-15 sample showed a higher catalytic activity up to 87% and a lower selectivity to tetralin (35%), which may be due to the size effect and the strong interaction of CoSi and support. The findings also indicated that transition-metal silicides were novel promising catalysts for the hydrogenation of aromatics and that they might find other applications in the synthesis of fine chemicals. Acknowledgment. We gratefully acknowledge the financial support provided by the National Natural Science Foundation

Cobalt Silicide Nanoparticles in Mesoporous Silica of China (No. 20973029), the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133), and the Doctoral Fund of Ministry of Education of China (No. 20070141048). References and Notes (1) Tang, T. D.; Yin, C. Y.; Wang, L. F.; Ji, Y. Y.; Xiao, F. S. J. Catal. 2008, 257, 125. (2) Huang, T. C.; Kang, B. C. Ind. Eng. Chem. Res. 1995, 34, 1140. (3) Stanislaus, A.; Cooper, B. H. Catal. ReV. Sci. Eng. 1994, 36, 7. (4) Song, C. S.; Ma, X. L. Appl. Catal., B 2003, 41, 207. (5) Prins, R. AdV. Catal. 2001, 46, 399. (6) Reader, A. H.; van Ommen, A. H.; Weijs, P. J. W.; Wolters, R. A. M.; Oostra, D. J. Rep. Prog. Phys. 1992, 56, 1397. (7) Mangelinck, D.; Gas, P.; Grob, A.; Pichaud, B.; Thomas, O. J. Appl. Phys. 1996, 79, 4078. (8) Schmitt, A. L.; Bierman, M. J.; Schmeisser, D.; Himpsel, F. J.; Jin, S. Nano Lett. 2006, 6, 1617. (9) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (10) Szczech, J. R.; Schmitt, A. L.; Bierman, M. J.; Jin, S. Chem. Mater. 2007, 19, 3238. (11) Frank, T. C.; Kester, K. B.; Falconer, J. L. J. Catal. 1985, 95, 396. (12) Yilmaz, S.; Floquet, N.; Falconer, J. L. J. Catal. 1996, 159, 31. (13) Wessel, T. J.; Rethwisch, D. G. React. Kinet. Catal. Lett. 1996, 58, 7. (14) Panpranot, J.; Phandinthong, K.; Sirikajorn, T.; Arai, M.; Praserthdam, P. J. Mol. Catal. A 2007, 261, 29. (15) Juszczyk, W.; Karpin’ski, Z. J. Catal. 1989, 117, 519. (16) Sheu, L. L.; Karpin’ski, Z.; Sachtler, W. M. H. J. Phys. Chem. 1989, 93, 4890. (17) Tschan, R.; Wandeler, R.; Schneider, M. S.; Schubert, M. M.; Baiker, A. J. Catal. 2001, 204, 219. (18) Tschan, R.; Wandeler, R.; Schneider, M. S.; Burgener, M.; Schubert, M. M.; Baiker, A. Appl. Catal., A 2002, 223, 173. (19) Tschan, R.; Schubert, M. M.; Baiker, A.; Bonrath, W.; LansinkRotgerink, H. Catal. Lett. 2001, 75, 31. (20) Walter, H.; Roewer, G.; Bohmhammel, K. J. Chem. Soc., Faraday Trans. 1996, 92, 4605. (21) Bohmhammel, K.; Roewer, G.; Walter, H. J. Chem. Soc., Faraday Trans. 1995, 91, 3879. (22) Serp, P.; Kalck, P. Chem. ReV. 2002, 102, 3085.

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