Reversible Structure Transformation of Antimony Oxides on SiO2

Sb/SiO2 catalysts were prepared by chemical vapor deposition of Sb(OEt)3 on a SiO2 surface followed by water vapor admission and calcination. The cata...
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Langmuir 2003, 19, 2756-2762

Reversible Structure Transformation of Antimony Oxides on SiO2 Relevant to Selective Catalytic Oxidation of Ethanol K. Matsuzawa, T. Shido, and Y. Iwasawa* Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received October 7, 2002. In Final Form: December 18, 2002 Sb/SiO2 catalysts were prepared by chemical vapor deposition of Sb(OEt)3 on a SiO2 surface followed by water vapor admission and calcination. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectra, and X-ray absorption fine structure and by catalytic performance for selective oxidation of ethanol. At low loadings (2-4 wt %) Sb(OEt)3 reacted with surface OH groups on the SiO2 surface to form surface-attached Sb(III) species. At high Sb loadings (>5 wt %), in addition to the attached Sb(III) species, crystalline Sb2O3 species were formed after exposure to water vapor. The amount of crystalline Sb2O3 species increased with Sb loading, and 30-50% of Sb species existed as crystalline Sb2O3 at 7-12 wt %. Most of the Sb(III) species remained trivalent after calcination. On the other hand, the crystalline Sb2O3 was oxidized to Sb(V) and spread on the SiO2 surface. The spread antimony oxide species were active for selective catalytic oxidation of ethanol through a redox mechanism. When the spread antimony oxide species were reduced with ethanol, they were reversibly transformed to crystalline Sb2O3 species. This paper shows a dynamic structure change that occurs under the ethanol selective oxidation reaction conditions.

1. Introduction Antimony is widely used as a key element in mixed oxide catalysts for selective oxidation of hydrocarbons, such as Fe-Sb-O,1-16 Sn-Sb-O,17-24 V-Sb-O,11,25-39 Mo-Sb-O,40-43 and Re-Sb-O.44-48 Many studies on the * To whom correspondence should be addressed. FAX: +81-35800-6892. E-mail: [email protected]. (1) Teller, R. G.; Brazdil, J. F.; Grasselli, R. K.; Yelon, W. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1693. (2) Benvenutti, E. V.; Gushikem, Y.; Vasquez, A.; de Castro, S. C.; Zaidivar, G. A. P. J. Chem. Soc., Chem. Commun. 1991, 1325. (3) Davanzo, C. U.; Gushikem, Y.; de Castro, S. C.; Benvenutti, E. V.; Vasquez, A. J. Chem. Soc., Faraday Trans. 1996, 92, 1569. (4) Bowker, M.; Bicknell, C. R.; Kerwin, P. Appl. Catal. 1996, 136, 205. (5) Carraza´n, S. R. G.; Cadus, L.; Dieu, P.; Ruiz, P.; Delmon, B. Catal. Today 1996, 32, 311. (6) Allen, M. D.; Poulston, S.; Bithell, E. G.; Goringe, M. J.; Bowker, M. J. Catal. 1996, 163, 204. (7) Li, K. T.; Yen, C. S.; Shyu, N. S. Appl. Catal. 1997, 156, 117. (8) Schnoble, M.; van Steen, E. Ind. Eng. Chem. Res. 1997, 36, 3568. (9) Walczak, J.; Filipek, E.; Bosacka, M. Solid State Ionics 1997, 101-103, 1363. (10) van Steen, E.; Schnobel, M.; Walsh, R.; Riedel, T. Appl. Catal. 1997, 165, 349. (11) Poulston, S.; Price, N. J.; Weeks, C.; Allen, M. D.; Parlett, P.; Steinberg, M.; Bowker, M. J. Catal. 1998, 178, 658. (12) Benvenutti, E. V.; Zawislak, L. I. Polyhedron 1998, 17, 1627. (13) Magagula, Z.; van Steen, E. Catal. Today 1999, 49, 155. (14) Xiong, Y.-L.; Weng, L.-T.; Bertrand, P.; Ladrie`re, J.; Daza, L.; Ruiz, P.; Delmon, B. J. Mol. Catal. A: Chem. 2000, 155, 59. (15) Mitchell, P. C. H.; Bowker, M.; Price, N.; Poulston, S.; James, D.; Parker, S. F. Top. Catal. 2000, 11/12, 223. (16) Allen, M. D.; Bowker, M. Catal. Lett. 1995, 33, 269. (17) Berry, F. J.; Smith, D. J. J. Catal. 1984, 88, 107. (18) Ono, T.; Yamanaka, T.; Kubokawa, Y.; Komiyama, M. J. Catal. 1988, 109, 423. (19) Weng, L. T.; Spitaele, N.; Yasse, B.; Ladrie´re, J.; Ruiz, P.; Delmon, B. J. Catal. 1991, 132, 319. (20) Aghabozorg, H. R.; Sakakini, B. H.; Roberts, A. J.; Vickerman, J. C.; Flavell, W. R. Catal. Lett. 1996, 39, 97. (21) Dusastre, V.; Williams, D. E. J. Phys. Chem. B 1998, 102, 6732. (22) Albonetti, S.; Blanchard, G.; Burattin, P.; Cavani, F.; Masetti, S.; Trifiro`, F. Catal. Today 1998, 42, 283. (23) Slater, B.; Catlow, C. R. A.; Gay, D. H.; Williams, D. E.; Dusastre, V. J. Phys. Chem. B 1999, 103, 10644. (24) Rockenberger, J.; zum Felde, U.; Tischer, M.; Tro¨ger, L.; Haase, M.; Weller, H. J. Chem. Phys. 2000, 112, 4296.

mixed oxides except for Re-Sb-O have demonstrated that excess antimony at surfaces is a clue to high selectivity to partially oxidized products. Allen and Bowker investigated propene oxidation on FeSbO4 catalysts and reported that the catalyst surfaces were covered by antimony rich thin layers under the selective oxidation (25) Zanthoff, H. W.; Buchholz, S. A.; Ovsitser, O. Y. Catal. Today 1996, 32, 291. (26) Agahabozorg, H. R.; Flavell, W. R.; Sakakini, B. H. J. Catal. 1997, 167, 164. (27) Zanthoff, H. W.; Schaefer, S.; Wolf, G.-U. Appl. Catal. 1997, 164, 105. (28) Carrazan, S. R. G.; Peres, C.; Ruwet, M.; Ruiz, P.; Delmon, B. Solid State Ionics 1997, 101-103, 737. (29) Centi, G.; Mazzoli, P.; Perathoner, S. Appl. Catal., A 1997, 165, 273. (30) Brazdil, J. F.; Ebner, A. M.; Cavalcanti, F. A. P. Appl. Catal., A 1997, 165, 51. (31) Zanthoff, H. W.; Buchholz, S. A. Catal. Lett. 1997, 49, 213. (32) Vaarkamp, M.; Ushikubo, T. Appl. Catal., A 1998, 174, 99. (33) Sanati, M.; Akbari, R.; Masetti, S.; Trifiro`, F. Catal. Today 1998, 42, 325. (34) Nilsson, J.; Landa-Ca`novas, A. R.; Hansen, S.; Andersson, A. J. Catal. 1999, 186, 442. (35) Bru¨ckner, A.; Zanthoff, H.-W. Colloids Surf. A 1999, 158, 107. (36) Hinz, A.; Andersson, A. Chem. Eng. Sci. 1999, 54, 4407. (37) Zanthoff, H. W.; Buchholz, S. A.; Pantazidis, A.; Mirodatos, C. Chem. Eng. Sci. 1999, 54, 4397. (38) Li, K.-T.; Chien, T.-Y. Catal. Lett. 1999, 57, 77. (39) Vislovskiy, V. P.; Bychkov, V. Y.; Sinev, M. Y.; Shamilov, N. T.; Ruiz, P.; Schay, Z. Catal. Today 2000, 61, 325. (40) Faus, F. M.; Zhou, B.; Matralis, H.; Delmon, B. J. Catal. 1991, 132, 157. (41) Faus, F. M.; Zhou, B.; Matralis, H.; Delmon, B. J. Catal. 1991, 132, 183. (42) Faus, F. M.; Zhou, B.; Matralis, H.; Delmon, B. J. Catal. 1991, 132, 200. (43) Gaigneaux, E. M.; Dieterle, M.; Ruiz, P.; Mestl, G.; Delmon, B. J. Phys. Chem. 1998, 102, 10542. (44) Yuan, Y.; Liu, H.; Imoto, H.; Shido, T.; Iwasawa, Y. Chem. Lett. 2000, 674. (45) Gaigneaux, E. M.; Liu, H.; Imoto, H.; Shido, T.; Iwasawa, Y. Topics Catal. 2000, 11/12, 185. (46) Liu, H.; Gaigneaux, E. M.; Imoto, H.; Shido, T.; Iwasawa, Y. J. Phys. Chem. B 2000, 104, 2033. (47) Liu, H.; Imoto, H.; Shido, T.; Iwasawa, Y. J. Catal. 2001, 200, 69. (48) Liu, H.; Iwasawa, Y. J. Phys. Chem. B 2002, 106, 2319.

10.1021/la020833y CCC: $25.00 © 2003 American Chemical Society Published on Web 02/25/2003

Transformation of Antimony Oxides on SiO2

conditions.16 Carbucicchio et al. claimed that surface SbVdO double bonds were the key issue for the high selectivity and conversion,49 while Straguzzi et al. claimed that excess antimony oxides restricted complete oxidation of hydrocarbons.50 On the other hand, Faus et al. reported that antimony species provided adsorption sites to activate oxygen.40-42 It is indispensable to know the structure and dynamic property of antimony oxide species under oxidation reaction atmospheres to understand the key issue for the selective catalysis of antimony based binary oxides. Few studies have been done to elucidate the intrinsic nature of supported antimony oxides. Benvenutti et al. prepared a Sb2O5 thin layer on a SiO2 surface by reaction of SbCl5 with surface OH groups on SiO2 (Sb(V)/SiO2)51 and also prepared Fe/Sb(V)/SiO2 model catalysts by impregnation of FeCl3 on Sb(V)/SiO2.2,52 Pillep et al. studied the oxidation state and morphology of antimony oxide species prepared by a physical mixing of Sb2O3 with TiO2.53 They found that Sb2O3 was easy to spread on the TiO2 surface by heating at elevated temperatures after milling and that the spread antimony species were oxidized up to Sb6O13. The relation between the structure and catalytic property of supported antimony oxides, however, has not been studied yet. In previous studies, we have prepared thin layers of niobium, titanium, and bismuth oxides by using Ti(OiPr)4,54 Nb(OEt)5,55 and Bi(OEt)456 on SiO2 surfaces and characterized them by extended X-ray adsorption fine structure (EXAFS) and several spectroscopic techniques. Such thin layer catalysts have an advantage for detailed characterization of active structures under the catalytic reaction conditions by physical techniques because the thin layers are located on the support surface, all of them may be associated with the catalytic performance, and hence their structural change can be directly related to the key issue of fundamentals of catalysis. In this paper we have prepared antimony species chemically attached on a SiO2 surface by chemical vapor deposition (CVD) of Sb(OEt)3 and subsequent water exposure and calcination. The structures of the obtained antimony oxide species were characterized by Sb K edge EXAFS and X-ray diffraction (XRD), and the oxidation states were also characterized by X-ray photoelectron spectroscopy (XPS) and Sb LI edge X-ray absorption near edge structure (XANES). The supported SbOx species were active for ethanol selective oxidation. It was found that the SbOx species changed dynamically in structure depending on the reaction atmospheres. 2. Experimental Section 2.1. Catalyst Preparation. SiO2-supported antimony oxides were prepared by CVD of Sb(OEt)3 (Strem, 99%) on SiO2 (Aerosil 200; Japan Aerosil Co.) at 353 K. SiO2 was preevacuated at 473 K for 2 h before use as a support. The obtained sample was evacuated at room temperature for 1 h, and then exposed to water vapor (2.7 kPa) at 300 K to complete the hydrolysis of the (49) Carbucicchio, M.; Centi, G.; Trifiro`, F. J. Catal. 1985, 91, 85. (50) Straguzzi, G. I.; Bischoff, K. B.; Koch, T. A.; Schuit, G. C. A. J. Catal. 1987, 104, 47. (51) Benvenutti, E. V.; Gushikem, Y.; Davanzo, C. U.; de Castro, S. C.; Torriani, I. L. J. Chem. Soc., Faraday Trans. 1992, 88, 3193. (52) Benvenutti, E. V.; Gushikem, Y. J. Braz. Chem. Soc. 1998, 9, 469. (53) Pillep, B.; Behrens, P.; Schubert, U.-A.; Spengler, J.; Kno¨zinger, H. J. Phys. Chem. B 1999, 103, 9595. (54) Asakura, K.; Inukai, J.; Iwasawa, Y. J. Phys. Chem. 1992, 96, 829. (55) Shirai, M.; Asakura, K.; Iwasawa, Y. J. Phys. Chem. 1991, 95, 9999. (56) Shido, T.; Okita, G.; Asakura, K.; Iwasawa, Y. J. Phys. Chem. B 2000, 104, 12263.

Langmuir, Vol. 19, No. 7, 2003 2757 remaining ethoxyl groups on Sb. After that, the sample was calcined at 723 K for 2 h. The Sb loadings were determined by X-ray fluorescence (XRF), which was measured on a SEIKO SEA2010 spectrometer. The Sb/SiO2 samples with x wt % Sb are denoted as Sb(x)/SiO2, hereafter. The samples after the CVD, the CVD-H2O, and the CVD-H2O-calcination are denoted as CVDSb/SiO2, hydr-Sb/SiO2 and calcin-Sb/ SiO2, respectively. 2.2. Characterization. XPS spectra were measured using XPS-7000 (Shimadzu). Al KR was used as the X-ray source. Binding energies were referred to 103.4 eV of Si 2p. XRD patterns were recorded on Rigaku Miniflex with Cu KR as the X-ray source. Operation voltage and current were 35 keV and 15 mA, respectively. Sb L edge XANES spectra were measured at BL-7C of the Photon Factory in the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-IMSSPF) at room temperature in a transmission mode. The Si(111) double crystal was used to monochromatize X-rays stemming from the synchrotron radiation. Ion chambers filled with He/N2 (75/25) and N2 were used to monitor the X-rays before and after the sample, respectively. The synchrotron ring was operated at 2.5 GeV and 300-400 mA. Sb K edge EXAFS spectra were recorded in a transmission mode at 30 K at BL-10B of KEK-IMSS-PF. The electron storage ring was operated at 3.0 GeV, 200 mA. The X-rays were monochromatized by a Si(311) channel cut monochromator. X-ray intensities before and after the sample were monitored by ionization chambers filled with Ar and Kr, respectively. EXAFS spectra were analyzed by using the UWXAFS package.57 The detailed analysis procedure was described elsewhere.56 Briefly, k3 weighted EXAFS functions were Fourier transformed into R-space after background subtraction, and a curve fitting analysis was conducted in the R-space. The k and R ranges for Fourier transformation and curve fitting were 30-160 nm-1 and 0.10.42 nm, respectively. The backscattering amplitude and phase shift functions for Sb species were calculated by the FEFF8 code.58 The coefficients for multiphoton effects for Sb(III) and Sb(V) were estimated to be 1.0 by comparing the first shell Sb-O contribution between observed and simulated EXAFS data for Sb(OEt)3 and NaSb(OH)6. Coordination number (CN), interatomic distance (R), and Debye-Waller factor (σ2) were taken as free parameters for each shell, and the same value for the correction of edge energy (∆E0) was used for all shells. The number of fitting parameters used in this study was smaller than the number of independent parameters calculated by the Nyquist law.59 2.3. Catalytic Ethanol Oxidation. Ethanol selective oxidation was carried out in a fixed-bed flow reactor. About 100 mg of the sample was mounted in a Pyrex-glass tube combined in the flow reactor. Flow rates of He and O2 were controlled by digital mass flow controllers. Ethanol was added to the reactant gas by using a saturater kept at 298 K. The total flow rate was 30 mL min-1, and the concentrations of O2 and EtOH were 20.0 and 6.2%, respectively, which were balanced with He at 1 atm. A typical reaction temperature was 563 K. Ethanol oxidation without gas-phase oxygen was also carried out to estimate the amount of reactive lattice oxygen, where the reaction temperature was 563 K and the EtOH concentration was 7.8% (He balance). The products of ethanol oxidation reactions were analyzed by a gas chromatograph using Unibeads C (2 m) and Porapak QS (2 m) columns at 413 K.

3. Results 3.1. Amount of Ethanol Produced by the CVD of Sb(OEt)3 on a SiO2 Surface. Only ethanol was formed at the CVD process of Sb(OEt)3 on SiO2 at 353 K. When the CVD sample was exposed to water vapor (2.7 kPa) at 300 K, ethanol was further produced. The amounts of the produced ethanol and consumed water were determined to elucidate the composition of surface species obtained by the CVD of Sb(OEt)3 and subsequent water admission. (57) Stern, E. A.; Newville, M.; Ravel, B.; Yacoby, Y.; Haskel, D. Physica B 1995, 208, 117. (58) Ankudinov, A. L.; Ravel, B.; Rehr, J.; Conradson, S. D. Phys. Rev. B 1998, 58, 7565. (59) Stern, E. A. Phys. Rev. B 1993, 48, 9825.

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Figure 1. XRD patterns for supported Sb/SiO2 catalysts with various Sb loadings before and after calcination: (a) 2.0, (b) 3.8, (c) 8.0, and (d) 15.7 wt % before calcination and (e) 2.0, (f) 5.8, and (g) 12.4 wt % calcined at 723 K.

When 1.4 × 10-4 mol of Sb(OEt)3 was admitted to 1.0 g of SiO2, which was preevacuated at 473 K, 2.5 × 10-4 mol of ethanol was produced. When the CVD sample was exposed to 2.7 kPa of water vapor, 1.8 × 10-4 mol of ethanol was produced and 1.2 × 10-4 mol of water was consumed. A part of water vapor admitted to the system was condensed on the sample surfaces. The amount of condensed water was estimated by repeated measurements of water adsorption on silica under a condition identical to that for the hydrolysis of the CVD sample. The amounts of consumed water in the surface reactions were calculated by subtraction of the amount of condensed water from the decrease of gas-phase water. The amounts of produced ethanol relative to that of Sb after the Sb(OEt)3 CVD and the subsequent water exposure are 1.8 and 1.3, respectively. The relative amount of consumed water to the Sb quantity is 0.86. At higher Sb loadings, the relative amount of ethanol formed by CVD of Sb(OEt)3 was lower than that in low loadings. 3.2. XRD. Figure 1 shows XRD patterns for hydr- and calcin-Sb/SiO2 catalysts with various Sb loadings before and after calcination at 723 K. At low loadings (2.0 and 3.8 wt %), no peak was observed before and after calcination. At higher loadings (8.0 and 15.7 wt %), on the other hand, peaks corresponding to Sb2O3 were observed at 27.70, 32.04, and 35.04/2θ for the hydr-Sb/SiO2 samples before calcination. The intensity of these peaks for the high-loading samples was about one-forth of that for a physical mixture of crystalline Sb2O3 (Soekawa Co.) and SiO2. On the other hand, the width (fwhm) of the XRD peaks for these samples were comparable to each other. These peaks disappeared by calcination at 723 K. The results of Figure 1 indicate that the Sb species after the calcination spread on the SiO2 surface as amorphous or dispersed SbOx species. For the Sb species in the hydrSb/SiO2 sample before the calcination their structures depended on the Sb loading. The low-loading samples showed no crystalline species detected by XRD, whereas a part of Sb species existed as crystalline Sb2O3 at the high loadings. Figure 2 shows XRD patterns for Sb(7.0)/SiO2 after various treatments. The characteristic three peaks due to Sb2O3 crystalline particles were observed in Figure 2a, and they became weak by calcination at 573 K and

Matsuzawa et al.

Figure 2. XRD patterns for Sb(7.0)/SiO2 after various treatments: (a) before calcination, calcined at 573 (b), 623 (c), and 723 K (d); (e) exposed to ethanol at 563 K after d; (f) calcined at 723 K after e.

Figure 3. Fourier transformed k3 weighted EXAFS functions (k3χ(k)) for Sb(2.0)/SiO2 and Sb(7.0)/SiO2 after various treatments: (a) 2 wt % before calcination, (b) 2 wt % calcined at 723 K, (c) 7 wt % before calcination, (d) 7 wt % calcined at 723 K, (e) exposed to ethanol at 563 K after d, and (f) calcined at 723 K after e.

disappeared at 623 K (Figure 2c). No XRD peaks appeared by calcination at 723 K (Figure 2d). When ethanol was admitted to the sample at 563 K (after Figure 2d), the peaks of Sb2O3 appeared again (Figure 2e). The peak intensities after exposure to ethanol were comparable to those for the hydr-Sb/SiO2 sample (Figure 2a). These peaks disappeared again by calcination at 723 K, as shown in Figure 2f. The results indicate that the crystalline phase and the amorphous phase are transformed reversibly by ethanol exposure and calcination. 3.3. EXAFS. Figure 3 shows Fourier transformed k3 weighted EXAFS functions (k3χ(k)) for Sb(2.0)/SiO2 and Sb(7.0)/SiO2 after various treatments. Phase shifts were not corrected in these figures. At the low loading (2.0 wt % Sb), EXAFS spectra for the samples before and after calcination were similar to each other, as shown in Figure 3a,b. A large peak was observed at ca. 0.15 nm, and no peak was observed above 0.2 nm. At the high loading (7.0 wt % Sb) the spectra changed substantially by calcination. Peaks for the higher shell contributions were observed in 0.3-0.45 nm in addition to a peak for the first shell at ca. 0.15 nm before calcination. The higher shell peaks disappeared by calcination at 723 K. These peaks appeared

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Table 1. Structural Parameters Delivered by Curve Fitting Analysis for the EXAFS Spectra of Sb(2)/SiO2 and Sb(7)/SiO2 after Various Treatments shell

CN

R/(0.1 nm) σ2/(10-5 nm2)

∆E0/eV

R-factor/%

(a) 2.0 wt % before Calcination SbO 3.3 ( 0.3 1.95 ( 0.01 1.2 ( 0.7 14 ( 1

1.1

(b) 2.0 wt % after Calcination at 723 K SbO 3.3 ( 0.2 1.97 ( 0.01 2.8 ( 0.4 12 ( 1

0.8

(c) 7.0 wt % before Calcination SbO 3.1 ( 0.3 1.97 ( 0.01 2.0 ( 0.8 14 ( 1 SbSb 1.2 ( 0.4 3.61 ( 0.01 0.3 SbSb 1.3 ( 4.7 3.94 ( 0.01 0.3 (d) 7.0 wt % after Calcination at 723 K SbO 3.7 ( 0.3 1.95 ( 0.01 3.7 ( 0.6 8(1 (e) 7.0 wt % after Exposure to EtOH after d SbO 3.1 ( 0.2 1.96 ( 0.01 2.2 ( 0.4 16 ( 1 SbSb 1.1 ( 0.2 3.62 ( 0.01 1.0 SbSb 0.9 ( 0.2 3.95 ( 0.01 1.0 (f) 7.0 wt % after Calcination at 723 K after e SbO 3.6 ( 0.1 1.95 ( 0.01 3.3 ( 0.3 10.9 ( 0.5

1.7

1.6 2.2

0.15

and disappeared reversibly by exposure to ethanol at 563 K and exposure to oxygen at 723 K, as shown in Figure 3d-f. Table 1 shows structural parameters determined by curve fitting analysis for Sb(2.0)/SiO2 and Sb(7.0)/SiO2 after various treatments. For Sb(2.0)/SiO2 before and after calcination, only the first shell (the nearest Sb-O bonding) was analyzed because no clear higher-shell peak was observed. The local structure of the Sb species in Sb(2.0)/ SiO2 was affected by calcination significantly, as suggested by the curve fitting analysis. The Sb-O distance increased slightly from 0.195 nm before calcination to 0.197 nm after calcination, as shown in Table 1. The coordination number for Sb-O bond remained unchanged by the calcination. The structural parameters of the nearest Sb-O contribution after calcination depended on the Sb loading. The Sb-O distance in the high-loading Sb(7.0)/SiO2 sample (0.195 nm) was shorter than that in the low-loading Sb(2.0)/SiO2 sample (0.197 nm), as shown in Table 1. The difference in the Sb-O distance indicates a slight difference in Sb species after calcination between the 2.0 and 7.0 wt % samples. The Sb(7.0)/SiO2 samples before the calcination and after exposure to ethanol exhibited longer distance contributions in the Fourier transformed EXAFS functions (Figure 2). As these peaks were suggested to be due to senarmontite Sb2O3 from the XRD results in Figure 2, the senarmontite crystal structure was assumed to fit the higher shells. The backscattering amplitudes and phase shifts for the first to seventh shells of Sb2O3 were calculated by the FEFF8 code, and curve fitting was performed under the following conditions: 1. The coordination number (CN), interatomic distance (R), and Debye-Waller factor (σ2) for the first shell were treated as free parameters. 2. Relative CNs ((observed CN)/(degeneracy of the shell)) for the second to sixth and the eighth shells and those of the seventh and ninth shells were set to be equal. The second to sixth and the eighth shells are contributions from the nearest Sb atoms and surrounding O atoms around the nearest shell Sb atoms, and the seventh and ninth shells are contributions from the second nearest Sb atoms and surrounding O atoms around the second shell Sb atoms. 3. Rs for the second to ninth shells were changed proportionally to the interatomic distances. 4. σ2’s for the second to ninth shells were represented by the Debye model.

Figure 4. Sb LI XANES spectra for (a) Sb2O3, (b) R-Sb2O4 (Sb3+: Sb5+ ) 1:1), (c) Na[Sb(OH)6], and calcin-Sb/SiO2 catalysts at 2.0 (d), 3.8 (e), 9.1 (f), and 15.7 wt % (g).

Thus, free parameters are CN, R, and σ2 for the first shell, two relative CNs for the higher shells, coefficient for thermal expansion, Debye temperature, and correction for edge energy. Hence, the number of free parameters (8) is much smaller than that allowed from the Nyquist law (28 in this case). As shown in Figure 3 and Table 1, the EXAFS functions for the Sb(7.0)/SiO2 samples before calcination and after ethanol exposure were fitted properly, as suggested by small R factors. As CNs, Rs, and σ2s for Sb-O and multiple scattering contributions were combined with those for Sb-Sb contributions, structural parameters for only Sb-Sb bonds are listed in Table 1. The ratio of Sb2O3 particle to the total Sb species and the particle size can be estimated from the CNs of Sb-Sb bonds, assuming that the CN for the first Sb shell is always 3 regardless of the particle size. The ratios of the crystalline Sb2O3 in Sb(7.0)/SiO2 before calcination and after exposure to ethanol were calculated to be 39 and 35% from the EXAFS results. The CN for the second Sb-Sb was about half of that of a large crystal, which indicates that the Sb2O3 crystalline particles are small. However, the CN of the second Sb-Sb of the crystalline Sb2O3 was also smaller than that expected from the crystal structure. The calculated CN was 4.0 ( 0.6 instead of 6 expected from the crystal structure. This underestimation may be due to a complicated bond length distribution for the second Sb-Sb contribution or defects in the crystal. Thus, it is difficult to estimate the exact particle size from the CNs of the Sb-Sb bonds. 3.4. XANES and XPS. Figure 4 shows Sb LI edge XANES spectra for calcin-Sb/SiO2 catalysts with different Sb loadings after calcination at 723 K, together with those of Sb2O3 and Sb2O4 (Sb3+:Sb5+ ) 1:1). White lines were observed at 4702 and 4707 eV, which corresponded to Sb(III) and Sb(V) species.53 Sb2O3 (Sb3+) and Na[Sb(OH)6] (Sb5+) exhibit a peak at 4702 eV and a peak at 4707 eV, respectively. As shown in Figure 4, the ratio of Sb(V)/ Sb(III) increased with Sb loading. At 2.0 wt %, a peak at 4702 eV was observed with a shoulder at 4707 eV, which suggests that most of the Sb species in Sb(2.0)/SiO2 exist as Sb(III) and a fractional amount (5-10%) of them exists as Sb(V) after calcination. On the other hand, at 15.7 wt %, ca. 30-40% of Sb species in Sb(15.7)/SiO2 exist as Sb(V). Figure 5 shows Sb LI edge XANES spectra for Sb(9.1)/ SiO2 after various treatments. For the hydr-Sb/SiO2 sample the white line of only Sb(III) was observed (Figure

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Figure 5. Sb LI XANES spectra for Sb(9.1)/SiO2 after various treatments: (a) before calcination, calcined at 573 (b), 723 (c), and 773 K (d); (e) exposed to ethanol at 563 K after d; (f) after ethanol selective oxidation reaction at 563 K.

Figure 6. Relative peak intensity of Sb 3d3/2 to Si 2p in XPS signals for Sb(11.3)/SiO2 as a function of calcination temperature.

5a). The intensity of the white line of Sb(V) increased with calcination temperature and became the ratio Sb(V)/Sb(III) reached ca. 0.7 at 723 K. The white line for Sb(V) disappeared by reduction with ethanol (Figure 5e). The Sb(V) recovered to a level between Figure 5b and c after the catalytic ethanol oxidation reaction at 563 K as shown in Figure 5f. Figure 6 shows the relative peak intensity of Sb 3d to Si 2p in XPS spectra for Sb(11.3)/SiO2 as a function of calcination temperature. The relative intensity increased with temperature, which suggests that the Sb species spread over the SiO2 surface during calcination. The binding energy of Sb 3d3/2 shifted from 537 to 540 eV by calcination, suggesting that Sb was oxidized. 3.5. Catalytic Ethanol Oxidation. Figure 7 shows the conversion of ethanol and the selectivity to acetaldehyde as a function of Sb loading for the Sb/SiO2 catalysts before and after calcination. The conversion on the catalysts before calcination were higher than those on the catalysts after calcination in all Sb loadings. The conversions for the calcined catalysts decreased a little monotonically with Sb loading and noncalcined catalysts increased up to 8 wt % of Sb loading and decreased above 8 wt %. On the other hand, the selectivity to acetaldehyde before calcination was lower than that after calcination except 2 wt %. The selectivity increased monotonically with Sb loading for both calcined and those for the noncalcined catalysts and reached 89-93% selectivities at 15.7 wt % Sb loading. The yield of acetaldehyde

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Figure 7. Conversion, yield and selectivity for acetaldehyde formation in the ethanol oxidation on Sb/SiO2 catalysts against Sb loading: (a) ethanol conversion and acetaldehyde yield and (b) selectivity to acetaldehyde; (b, 2) without calcination pretreatment and (O, 4) after calcination pretreatment.

Figure 8. Rate of acetaldehyde formation against time-onstream under ethanol/He ) 7.8/92.2 at 563 K on Sb/SiO2 catalysts at 2.0 (a), 3.8 (b), 8.1 (c), and 12.4 wt %. Catalyst weight ) 150 mg.

formation on the noncalcinated catalysts increased with Sb loading and reached a maximum at 8 wt % Sb loading. The calcinated catalyst with 8 wt % Sb also showed a maximum yield as shown in Figure 7. At the initial stage of the catalytic oxidation up to 50 min of time-on-stream, the conversion decreased and the selectivity increased with reaction time for the catalysts before calcination, while the conversion increased and the selectivity decreased a little with time-on-stream for the calcined catalysts. The conversion and selectivity reached the steady-state values after 1 h from initiating the reaction for all the catalysts employed in this study. Figure 8 shows the rates of acetaldehyde formation against the period of exposure to EtOH on the calcin-Sb/ SiO2 catalysts with different Sb loadings. As shown in Figure 8, the formation rate decreased exponentially against the exposure period. The formation rate and duration of acetaldehyde formation increased with Sb loading. As a consequence, the amount of acetaldehyde formed increased with the loading. The amounts were 0.05, 0.21, 0.28, and 0.41 molecules/(Sb atom) at 2.0, 3.8, 8.1, and 12.4 wt %, respectively. It seems that there are two steps with fast decay and slow decay in the acetaldehyde formation. The slow decay in the rate is obvious for the catalyst with 12.4 wt % Sb loading. The results indicate that Sb species in low-loading samples were hard to be oxidized by calcination at 723 K and that Sb species in high-loading samples were oxidized to form active oxygen atoms of ca. 30-40% per Sb, which agrees with the results of LI XANES characterization.

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Scheme 1. Structural Transformations of Antimony Oxide Species on Low Loaded, 2.0 wt % Sb/SiO2 (a) and High Loaded, 12.4 wt % Sb/SiO2 (b)

4. Discussion 4.1. Characterization of Sb/SiO2. As mentioned in section 3.1, about 2 mol of ethanol were produced when 1 mol of Sb(OEt)3 was admitted to a SiO2 surface at room temperature and about 1 mol of ethanol/(1 mol of Sb(OEt)3) was formed by the subsequent water admission. About 0.6 mol of water/(1 mol of Sb(OEt)3) was consumed by the second process. These results indicate that one Sb(OEt)3 molecule reacted with two surface OH groups to form (Si-O-)2Sb(OEt) in a bidentate form, as shown in Scheme 1. Then the (Si-O-)2Sb(OEt) was converted to (Si-O-)2SbOH by water admission. As the amount of water consumed in this step was about 60% of that of produced ethanol, 40% of the (Si-O-)2SbOH species seem to have further reacted with surface OH groups to form a tridentate species (Si-O-)3Sb, as shown in Scheme 1. No Sb-Sb bonding was observed by EXAFS. The XRD (Figure 1) and EXAFS (Figure 3 and Table 1) data suggest that Sb species in the hydr-Sb/SiO2 samples exist as dispersed or amorphous species at low Sb loadings (2.0 and 3.8 wt %). Since the Sb-O distance of these samples (0.195 nm) is different from that of crystalline Sb2O3 (0.199 nm), it can be concluded that the Sb species exist as fine particles of crystalline Sb2O3 undetectable with XRD. It is possible that Sb2O3 is produced by hydrolysis-condensation of Sb(OEt)3 when it is admitted to the SiO2 surface, and this occurred indeed at high Sb loadings. At high Sb loadings (7-15.7 wt %), the XRD results suggest that crystalline senarmontite Sb2O3 particles exist in the hydr-Sb/SiO2 samples before calcination, as shown in Figures 1 and 2. This is supported by the presence of longer bonds of Sb-Sb at 0.361 and 0.394 nm and Sb-O at 0.197 nm corresponding to the Sb2O3 in the EXAFS analysis (Figure 3 and Table 1). The amount of crystalline Sb2O3 can be estimated from the XRD pattern and the CN of Sb-Sb bond in EXAFS. The peak height in the XRD pattern of Sb(8.1)/SiO2 was about one-forth that of physically mixed Sb2O3(8.1)/SiO2. The peak width of the physical mixture can be overestimated because the observed peak width is the convolution of the peak widths due to crystal size and the resolution of the diffractometer. In this case, the amount of crystalline Sb2O3 in Sb/SiO2 can be underestimated. Thus the amount of the crystalline Sb2O3 is somehow larger than that estimated from XRD. Since XRD is sensitive to large crystals, the average crystal size determined by XRD is apt to be overestimated if the size of crystals is distributed widely. The EXAFS results suggest that the population of crystalline Sb2O3 is 0.39 ( 0.12. From the XRD and EXAFS results, the ratio of crystalline Sb2O3 to total Sb species may be in the range

of 0.3-0.5. Thus, we can conclude that the noncrystalline, dispersed Sb species, probably in monolayer, is the major species even at the high loadings. The surface density of Sb species in Sb(7)/SiO2 is 1.7 Sb nm-2 on the SiO2, while the density of surface OH groups is 5 nm-2.60-62 As one Sb(OEt)3 molecule reacts with two OH groups, 68-85% of surface OH groups were consumed at 7 wt % loading, where crystalline Sb2O3 began to appear even below monolayer coverage. This behavior is entirely different from the case of Ti and Nb thin layer catalysts. In preparation of Nb monolayer catalysts, Nb(OC2H5)5 was used as precursor. Its reactivity with surface OH groups was low, and addition of NH3(aq) was required to complete the hydrolysis-condensation reaction.55 Crystalline niobium oxides were not formed. In the case of preparation of TiOx monolayer on SiO2, Ti(i-OC3H7)4 was used.54 The preparation procedure is similar to that for the Sb/SiO2 catalysts. Ti(i-OC3H7)4 reacted with the surface OH groups at 353 K for 2 h, followed by exposure to water vapor at room temperature for hydrolysis of remaining i-OC3H7 ligands. The reaction temperature of Ti(i-OC3H7)4 and surface OH groups was higher than the case of Sb(OEt)3. Thus, Sb(OEt)3 is more reactive than Nb(OC2H5)5 and Ti(i-OC3H7)4 to surface OH groups on SiO2. Sb(OEt)3 also reacts with water vapor readily to form Sb2O3. When the dispersed Sb(III) species in the low-Sb-loading samples were oxidized by calcination, the Sb-O distance slightly increased from 0.195 to 0.197 nm, which suggests that the local structure around Sb changed by calcination. About 5-10% of the Sb species in Sb(2.0)/SiO2 were oxidized to Sb(V), as shown in the XANES spectra in Figure 4. The change in the Sb-O bond distance is small, and the CNs are similar before and after calcination. So further discussion would not be valid. When highly loaded samples were calcined, XRD peaks due to Sb2O3 and longer distance contributions in EXAFS disappeared, as shown in Figures 2 and 3 and Table 1, respectively. XPS peak intensity of Sb 3d3/2 relative to Si 2p increased with an increase in calcination temperature, as shown in Figure 6, which shows that the number of Sb atoms at the surface increased. Thus, XPS results, together with XRD and EXAFS data, demonstrate that the crystalline Sb2O3 species spread on the SiO2 surface when they were oxidized. The spread antimony oxide species and crystalline Sb2O3 species were transformed reversibly as suggested by the (60) Iwasawa, Y., Ed. Tailored Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1986. (61) Zhuravlev L. T. Colloid Surf. A 1993, 74, 71. (62) Legrand, A. P., Ed. The Surface Properties of Silicas; Wiley: New York, 1998.

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XRD and EXAFS data. When the calcined Sb(7.0)/SiO2 catalyst was exposed to EtOH at 563 K, the XRD peaks due to Sb2O3 and the long Sb-Sb bonds for Sb2O3 in EXAFS were observed as shown in Figure 2 and Figure 3, and Table 1. These peaks disappeared again by oxidation at 723 K. The ratio of Sb(III)/Sb(V) can be estimated from the XANES spectra and the amount of acetaldehyde produced in the ethanol oxidation in the absence of gas-phase oxygen. The Sb LI XANES spectra in Figure 4 indicate that the ratio Sb(V)/Sb(III) increases with Sb loading. At 2.0 wt %, about 5-10% of Sb species exist as Sb(V) and at 15.7 wt %, about 30-40% exist as Sb(V). The amount of Sb(V) can be estimated from the amount of acetaldehyde produced, assuming that Sb(V) species are relevant to acetaldehyde formation. The results of Figure 8 showed that 0.05 and 0.41 mols of acetaldehyde were produced/(1 mol of Sb) at 2.0 and 12.4 wt %, respectively. As an oxygen atom is consumed for formation of an acetaldehyde molecule from an ethanol molecule, the amount of acetaldehyde formed is directly related to the amount of active lattice oxygen atoms, that is, the amount of Sb(V) species. Thus, the reduction experiments in Figure 8 suggest that about 5 and 40% of Sb species in the 2.0 and 12.4 wt % catalysts exist as Sb(V). The values agree with the results of Sb LI XANES. Thus, the composition of the spread antimony oxide species in the highly loaded samples after calcination is estimated to be SbO1.7. The reason of the reversible structure transformation is not clear at the moment, but it may be relevant to the surface/interface energy for wetting of anitmony oxides at the SiO2 surface which depends on the oxidation state of the antimony oxides. From the whole data, we propose structural transformations of the Sb species on SiO2 in Scheme 1. At low Sb loadings, as shown in Scheme 1a, Sb(OEt)3 reacts with surface OH groups on SiO2 and subsequent H2O vapor to form (SiO)2Sb(OH) and (SiO)3Sb. Most of the Sb species remain as Sb(III) after calcination at 723 K, and 5-10% of them are oxidized to Sb(V). These mixture species may form an attached SbOx monolayer without Sb-Sb bonding characterized by EXAFS. On the other hand, as shown in Scheme 1b, Sb species in Sb/SiO2 with high Sb loadings before calcination is a mixture of the SbOx monolayer species and crystalline Sb2O3 species. The amount of the crystalline Sb2O3 is estimated to be about 30-50% from the EXAFS and XRD data. The crystalline Sb2O3 species spread on the SiO2 surface by calcination at 723 K to form amorphous antimony oxide layers. The spread antimony oxide species converts to crystalline Sb2O3 reversibly by exposure to ethanol at 563 K. 4.2. Reactivity of the Sb Species. Comparing the results of characterization by XRD, EXAFS, XANES, and XPS with the catalytic performances in Figure 7, the catalytic property of the SbOx monolayer species and spread amorphous anitmony oxide species can be estimated. On the low-Sb-loading catalysts or the samples before calcination, where dispersed Sb(III) species and crystalline Sb2O3 are involved more, the conversion of ethanol is relatively high, while the selectivity to acetaldehyde is low. On the contrary, when the spread antimony oxide species is major, the conversion is low but the selectivity is high. Thus, the spread amorphous antimony

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oxide species may be an active species for the selective catalytic oxidation of ethanol to acetaldehyde. The difference in activity and selectivity can be explained by the type of oxygen species involved in the reaction. The spread anitmony oxide species are reduced to Sb2O3 by ethanol accompanied with acetaldehyde formation, as shown in Figures 3 and 8. These results suggest that the catalytic oxidation reaction on the spread antimony oxides proceeds through a redox mechanism. On the other hand, the Sb(III) monolayer species is hardly oxidized, where lattice oxygen atoms may not be involved in the catalytic oxidation. The present study revealed that antimony species at high loading store and release oxygen atoms in conjunction with the structural transformation between crystalline Sb2O3 and spread antimony oxide species. Our study agrees with previous studies on mixed oxide catalysts which claim that a SbOy thin layer is formed under the reaction conditions and that the SbOy thin layer improves the selectivity in selective oxidation catalysis.50 5. Conclusion 1. Antimony oxides on SiO2 surfaces were prepared by a chemical vapor deposition technique using Sb(OEt)3, followed by water vapor admission and calcination. 2. The obtained SbOx/SiO2 samples were characterized by means of XRD, XPS, XAFS, and gas-phase analysis. 3. At low Sb loadings (2.0 and 3.9 wt %), Sb(OEt)3 reacted with surface OH groups stoichiometrically to form (SiO-)2Sb(OEt) in a bidentate form, and the (Si-O-)2Sb(OEt) was transformed to (Si-O-)2Sb(OH) and (Si-O-)3Sb by the hydrolysis treatment. 4. The (Si-O-)2Sb(OH) and (Si-O-)3Sb species were converted to antimony(III) oxide monolayers attached on SiO2 after calcination at 773 K. 5. At high Sb loadings (7.0-15.7 wt %), crystalline senarmontite Sb2O3 (Sb-O, 0.197 nm; Sb-Sb, 0.361 and 0.394 nm by EXAFS) particles in addition to the attached Sb(III) species existed in hydr-Sb/SiO2 samples. 6. The amount of crystalline Sb2O3 species increased with Sb loading, and 30-50% of Sb species existed as crystalline Sb2O3 at 7-15.7 wt %. 7. Most of the Sb(III) species remained trivalent after calcination at 773 K, while the crystalline Sb2O3 was oxidized to Sb(V) and spread on the SiO2 surface by calcination at 773 K. 8. The spread antimony oxides were active for selective catalytic oxidation of ethanol to acetaldehyde, where the catalytic reaction proceeded by a redox mechanism. 9. The spread antimony oxides were reduced with ethanol to be transformed to crystalline Sb2O3 species reversibly. 10. The antimony oxide species on the SiO2 surface showed a dynamic structural change under the ethanol selective oxidation conditions. Acknowledgment. This study was supported by a Grant-in-aid for The 21st Century COE Program for Frontiers in Fundamental Chemistry from Monkasho. The XAFS measurements were performed by the approval of the PAC committee (Proposal No. 99G233). LA020833Y