Spectroscopic Study of V2O5 Supported on Zirconia and Modified with

Vanadium oxide−tungsten oxide supported on zirconia was prepared by adding Zr(OH)4 powder into a mixed aqueous solution of ammonium metavanadate ...
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Spectroscopic Study of V2O5 Supported on Zirconia and Modified with WO3 Jong Rack Sohn,*,† Im Ja Doh,‡ and Young Il Pae‡ Department of Industrial Chemistry, Engineering College, Kyungpook National University, Taegu 702-701, Korea, and Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea Received March 5, 2002. In Final Form: May 17, 2002 Vanadium oxide-tungsten oxide supported on zirconia was prepared by adding Zr(OH)4 powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR) and Raman spectroscopies, solid-state 51V NMR, X-ray diffraction (XRD), and differential scanning calorimetry (DSC). In the case of a calcination temperature of 773 K, for samples containing a low loading of V2O5, below 18 wt %, vanadium oxide was in a highly dispersed state, while for samples containing a high loading of V2O5, equal to or above 18 wt %, vanadium oxide was well crystallized because the V2O5 loading exceeded the formation of a monolayer on the surface of ZrO2. The experimental results indicate that the presence of WO3 and V2O5 retards the crystallization of the zirconia and stabilizes the tetragonal ZrO2 phase. The ZrV2O7 compound was formed through the reaction of V2O5 and ZrO2 at 873 K and the compound decomposed into V2O5 and ZrO2 at 1073 K; these results were confirmed by FTIR spectroscopy, solid-state 51V NMR, and XRD. The catalytic tests for 2-propanol dehydration have shown that the addition of WO3 to V2O5/ZrO2 enhanced both catalytic activity and acidity of V2O5-WO3/ZrO2 catalysts.

Introduction Vanadium oxides are widely used as catalysts in oxidation reactions, for example, the oxidation of sulfur dioxide, carbon monoxide, and hydrocarbons.1-4 These systems have also been found to be effective catalysts for the oxidation of methanol to methylformate5,6 and for the selective catalytic reduction of nitrogen oxide.7,8 Much research has been done to understand the nature of active sites, the surface structure of catalysts, and the role played by the promoter of the supported catalysts, using infrared (IR), X-ray diffraction (XRD), electron spin resonance (ESR), and Raman spectroscopy.6,9-11 So far, silica, titania, zirconia, and alumina12-18 have been commonly employed as the vanadium oxide supports. * To whom correspondence should be addressed. Tel: 82-53-9505585. Fax: 82-53-950-6594. E-mail: [email protected]. † Kyungpook National University. ‡ University of Ulsan. (1) Nakagawa, Y.; Ono, T.; Miyata, H.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2929-2936. (2) Miyata, H.; Kohno, M.; Ono, T.; Ohno, T.; Hatayama, F. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3663-3673. (3) Lakshmi, L. J.; Ju, Z.; Alyea, E. Langmuir 1999, 15, 3521-3528. (4) Sachtler, W. M. H. Catal. Rev. 1971, 4, 27-52. (5) Forzatti, P.; Tronoconi, E.; Busca, G.; Titarellp, P. Catal. Today 1987, 1, 209-218. (6) Busca, G.; Elmi, A. S.; Forzatti, P. J. Phys. Chem. 1987, 91, 52635269. (7) Armor, J. N. Appl. Catal., B 1992, 1, 221-256. (8) Alemany, L. J.; Lietti, L.; Ferlazzo, N.; Forzatti, P.; Busca, G.; Giamello, E.; Bregani, F. J. Catal. 1995, 155, 117-130. (9) Elmi, A. S.; Tronoconi, E.; Cristiani, C.; Martin, J. P. G.; Forzatti, P. Ind. Eng. Chem. Res. 1989, 28, 387-393. (10) Miyata, H.; Fujii, K.; Ono, T.; Kubokawa, Y.; Ohno, T.; Hatayama, F. J. Chem. Soc., Faraday Trans. 1 1987, 83, 675-685. (11) Cavani, F.; Centi, G.; Foresti, E.; Trifiro, F. J. Chem. Soc., Faraday Trans. 1 1988, 84, 237-254. (12) Hatayama, F.; Ohno, T.; Maruoka, T.; Ono, T.; Miyata, H. J. Chem. Soc., Faraday Trans. 1991, 87, 2629-2633. (13) del Arco, M.; Holgado, M. J.; Martin, C.; Rives, V. Langmuir 1990, 6, 801-806.

Recently, metal oxides modified with sulfur compounds have been studied as strong solid catalysts,19,20 especially sulfate-promoted zirconia containing iron or manganese as promoters21,22 or noble metals to inhibit deactivation.23,24 The high catalytic activity and small deactivation upon the addition of noble metals can be explained by both the elimination of coke by hydrogenation and hydrogenolysis25 and the formation of Bro¨nsted acid sites from H2 on the catalysts.24 Recently, Hino and Arata reported zirconiasupported tungsten oxide as an alternative material in reactions requiring strong acid sites.26,27 Several advantages of tungstate, over sulfate, as a dopant include that it does not suffer from dopant loss during thermal treatment and it undergoes significantly less deactivation during catalytic reactions.28 So far, however, comparatively few studies have been reported on the binary oxide (14) Centi, G.; Pinelli, D.; Trifiro, F.; Ghoussoub, D.; Guelton, M.; Gengembre, L. J. Catal. 1991, 130, 238-256. (15) Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1983, 87, 761-768. (16) Scharf, U.; Schraml-Marth, M.; Wokaun, A.; Baiker, A. J. Chem. Soc., Faraday Trans. 1991, 87, 3299-3307. (17) Miyata, H.; Kohno, M.; Ono, T.; Ohno, T.; Hatayama, F. J. Mol. Catal. 1990, 63, 181-191. (18) Sohn, J. R.; Park, M. Y.; Pae, Y. I. Bull. Korean Chem. Soc. 1996, 17, 274-279. (19) Ward, D. A.; Ko, E. I. J. Catal. 1994, 150, 18-33. (20) Kustov, L. M.; Kazansky, V. B.; Figueras, F.; Tichit, D. J. Catal. 1994, 150, 143-149. (21) Hsu, C. Y.; Heimbuch, C. R.; Armes, C. T.; Gates, B. C. J. Chem. Soc., Chem. Commun. 1992, 1645-1646. (22) Wan, K. T.; Khouw, C. B.; Davis, M. E. J. Catal. 1996, 158, 311-326. (23) Iglesia, E.; Soled, S. L.; Kramer, G. M. J. Catal. 1993, 144, 238253. (24) Ebitani, K.; Konishi, J.; Hattori, H. J. Catal. 1991, 130, 257267. (25) Vaudagna, S. R.; Comelli, R. A.; Canavese, S. A.; Figoli, N. S. J. Catal. 1997, 169, 389-393. (26) Arata, K. Adv. Catal. 1990, 37, 165-211. (27) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1987, 12591260.

10.1021/la020223y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/30/2002

Modified V2O5 Supported on Zirconia

vanadium oxide-tungsten oxide supported on zirconia which has been recently reported to be active for the decomposition of 2-propanol.29 Moreover, there is no study using a solid-state 51V nuclear magnetic resonance method, although some limited work using Raman and infrared spectroscopy and X-ray diffraction was reported.8,30,31 The dispersion and the structural features of supported species can strongly depend on the support. Structure and other physicochemical properties of supported metal oxides are considered to be in different states compared with bulk metal oxides because of their interaction with the supports. Solid-state nuclear magnetic resonance (NMR) methods represent a promising approach to these systems. Since only the local environment of a nucleus under study is probed by NMR, this method is well suited for the structural analysis of disordered systems such as the two-dimensional surface of vanadium oxide phases, which is of particular interest in the present study. In addition to the structural information provided by NMR methods, the direct proportionality of the signal intensity to the number of contributing nuclei makes NMR useful for quantitative studies. This paper describes the spectroscopic study of vanadium oxide supported on zirconia and modified with WO3. The characterization of the samples was performed by means of solid-state 51V nuclear magnetic resonance (51V NMR), Fourier transform infrared (FTIR) spectroscopy, XRD, laser Raman (LR) spectroscopy, and differential scanning calorimetry (DSC). The catalytic tests for 2-propanol dehydration are discussed in the next section. Experimental Section Catalyst Preparation. Precipitate of Zr(OH)4 was obtained by adding aqueous ammonia slowly into an aqueous solution of zirconium oxychloride (Aldrich) at room temperature with stirring until the pH of the mother liquor reached about 8. The precipitate thus obtained was washed thoroughly with distilled water until chloride ion was not detected by AgNO3 solution and was dried at room temperature for 12 h. The dried precipitate was powdered below 100 mesh. The catalysts containing various vanadium oxide contents and modified with WO3 were prepared by adding Zr(OH)4 powder into a mixed aqueous solution of ammonium metavanadate (NH4VO3) (Aldrich) and ammonium metatungstate [(NH4)6(H2W12O40)‚ nH2O] (Aldrich) followed by drying and calcining at high temperatures for 1.5 h. This series of catalysts were denoted by their weight percentage of V2O5 and WO3 and calcination temperature. For example, 4V2O5-15WO3/ZrO2(773) indicated the catalyst containing 4 wt % V2O5 and 15 wt % WO3 calcined at 773 K. 2-Propanol Dehydration. 2-Propanol dehydration was carried out at 433 and 453 K in a pulse microreactor connected to a gas chromatograph. Fresh catalyst in the reactor made of 1/4 in. stainless steel was pretreated at 673 K for 1 h in the nitrogen atmosphere. Pulses of 1 µL 2-propanol were injected into a N2 gas stream which passed over 0.05 g of catalyst. The packing material for the gas chromatograph was diethyleneglycol succinate on Shimalite, and the column temperature was 423 K. Catalytic activity for 2-propanol dehydration was represented as mole of propylene converted per gram of catalyst. Conversion was taken as the average of the first to sixth pulse values. Characterization. FTIR absorption spectra of V2O5-WO3/ ZrO2 powders were measured by the KBr disk method over the (28) Larsen, G.; Lotero, E.; Parra, R. D. In Proceedings of the 11th International Congress on Catalysis; Elsevier: New York, 1996; pp 543551. (29) Toda, Y.; Ohno, T.; Hatayama, F.; Miyata, H. Phys. Chem. Chem. Phys. 1999, 1, 1615-1621. (30) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M. J. Phys. Chem. 1991, 95, 9928-9937. (31) Alemany, L. J.; Berti, F.; Busca, G.; Ramis, G.; Robba, D.; Toledo, G. P.; Trombetta, M. Appl. Catal., B 1996, 10, 299-311.

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Figure 1. Infrared spectra of catalysts calcined at 773 K: (a) V2O5, (b) 28V2O5-15WO3/ZrO2, (c) 23V2O5-15WO3/ZrO2, (d) 18V2O5-15WO3/ZrO2, (e) 12V2O5-15WO3/ZrO2, (f) 8V2O5-15WO3/ ZrO2, and (g) 4V2O5-15WO3/ZrO2. range 1200-400 cm-1. The samples for the KBr disk method were prepared by grinding a mixture of the catalyst and KBr powders in an agate mortar and pressing them in the usual way. FTIR spectra of ammonia adsorbed on the catalyst were obtained in a heatable gas cell at room temperature using a Mattson model GL 6030E spectrophotometer. The self-supporting catalyst wafers contained about 9 mg/cm2. Before the spectra were obtained, the samples were heated under vacuum at 673-773 K for 1.5 h. The FT-Raman spectra were obtained with a Bruker model FRA 106 A spectrometer equipped with an InGaAs detector and a Nd:YAG laser source with a resolution of 4 cm-1. The laser beam was focused onto an area 0.1 × 0.1 mm2 in size of the sample surface; a 180° scattering geometry was used. 51V NMR spectra were measured by a Varian Unity Inova 300 spectrometer with a static magnetic field strength of 7.05 T. The Larmor frequency was 78.89 MHz. An ordinary single pulse sequence was used, in which the pulse width was set at 2.8 µs and the acquisition time was 0.026 s. The spectral width was 500 kHz. The number of scans was varied from 400 to 4000, depending on the concentration of vanadium. The signal was acquired from the time point 4 µs after the end of the pulse. The sample was static, and its temperature was ambient (294 K). The spectra were expressed with the signal of VOCl3 being 0 ppm, and the higher frequency shift from the standard was positive. Practically, solid NH4VO3 (-571.5 ppm) was used as the second external reference.32 Catalysts were checked in order to determine the structure by means of a Joel model JDX-8030 diffractometer, employing Cu KR (Ni-filtered) radiation. DSC measurements were performed by a PL-STA model 1500H apparatus in air, and the heating rate was 5 K per minute. For each experiment, 10-15 mg of sample was used. The specific surface area was determined by applying the Brunauer-Emmett-Teller (BET) method to the adsorption of N2 at 77 K.

Results and Discussion Infrared and Raman Spectra. Figure 1 shows IR spectra of V2O5-15WO3/ZrO2(773) catalysts with various (32) Hayashi, S.; Hayamizu, K. Bull. Chem. Soc. Jpn. 1990, 63, 961963.

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Figure 2. Infrared spectra of 4V2O5-15WO3/ZrO2 calcined at (a) 1173 K, (b) 1073 K, (c) 973 K, (d) 873 K, (e) 773 K, and (f) 673 K.

V2O5 contents calcined at 773 K for 1.5 h. Although with samples having less than 18 wt % of V2O5 definite peaks were not observed, the absorption bands at 1022 and 820 cm-1 appeared for 18V2O5-15WO3/ZrO2, 23V2O5-15WO3/ ZrO2, 28V2O5-15WO3/ZrO2, and pure V2O5 containing high V2O5 content. The band at 1022 cm-1 was assigned to the VdO stretching vibration, while that at 820 cm-1 was attributable to the coupled vibration between VdO and to V-O-V.33 Generally, the IR band of VdO in crystalline V2O5 showed at 1020-1025 cm-1 and the Raman band at 995 cm-1.2,34 The intensity of the VdO absorption gradually decreased with increasing ZrO2 content, although the band position did not change. This observation suggests that vanadium oxide below 18 wt % is in a highly dispersed state. It is reported that a V2O5 loading exceeding the formation of a monolayer on the surface of ZrO2 is well crystallized and observed in the spectra of IR and solidstate 51V NMR.35 It is necessary to examine the formation of crystalline V2O5 as a function of calcination temperature. Variation of IR spectra against calcination temperature for 4V2O5-15WO3/ZrO2 is shown in Figure 2. There were no VdO stretching bands at 1022 cm-1 at calcination temperatures from 673 to 973 K, indicating no formation of crystalline V2O5. However, as shown in Figure 2, VdO stretching bands due to crystalline V2O5 at 1073 and 1173 K appeared at 1022 cm-1 together with lattice vibration bands of V2O5 and WO3 below 900 cm-1.36,37 The formation of crystalline V2O5 at above 1073 K can be explained in terms of the decomposition of the ZrV2O7 compound which was formed through the reaction of V2O5 and ZrO2 at 873-973 K. In this study, on X-ray diffraction patterns and 51V NMR spectra described later, the cubic phase of ZrV2O7 was observed in the samples calcined at 873 K and for the (33) Mori, K.; Miyamoto, A.; Murakami, Y. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3303-3315. (34) Bjorklund, R. B.; Odenbrand, C. U. I.; Brandin, J. G. M.; Anderson, L. A. H.; Liedberg, B. J. Catal. 1989, 119, 187-200. (35) Sohn, J. R.; Cho, S. G.; Pae, Y. I.; Hayashi, S. J. Catal. 1996, 159, 170-177. (36) Park, E. H.; Lee, M. H.; Sohn, J. R. Bull. Korean Chem. Soc. 2000, 21, 913-918. (37) Highfield, J. G.; Moffat, J. B. J. Catal. 1984, 88, 177-187.

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Figure 3. Infrared spectra of 28V2O5-15WO3/ZrO2 calcined at (a) 1073 K, (b) 973 K, (c) 873 K, (d) 773 K, and (e) 673 K.

sample calcined at 1173 K the ZrV2O7 phase disappeared due to the decomposition of ZrV2O7, leaving the V2O5 phase and the monoclinic phase of ZrO2. These results are in good agreement with those of 51V solid-state NMR and XRD described later. In fact, it is known that the formation of ZrV2O7 from V2O5 and ZrO2 occurs at a calcination temperature of 873 K and the ZrV2O7 decomposes into ZrO2 and V2O5 at 1073 K.35,38 In separate experiments, variation of IR spectra against calcination temperature for 12V2O5-15WO3/ZrO2 (not shown in the figure) was similar to that for 4V2O5-15WO3/ZrO2 as shown in Figure 2. Figure 3 shows IR spectra of 28V2O5-15WO3/ZrO2 catalysts calcined at 673-1073 K for 1.5 h. Unlike 4V2O5-15WO3/ZrO2 and 12V2O5-15WO3/ZrO2 catalysts, for 28V2O5-15WO3/ZrO2 crystalline V2O5 appeared at a lower calcination temperature from 673 to 873 K and consequently the VdO stretching band was observed at 1022 cm-1. This is because a V2O5 loading exceeding the formation of a monolayer on the surface of ZrO2 is well crystallized.35 However, at 973 K all V2O5 reacted with ZrO2 and changed into ZrV2O7 so that VdO stretching at 1022 cm-1 disappeared completely, as shown in Figure 3. At a calcination temperature of 1073 K, some of the ZrV2O7 decomposed into V2O5 and ZrO2 and then the VdO stretching band due to the crystalline V2O5 was again observed at 1022 cm-1. These results are in good agreement with those of 51V solid-state NMR and XRD described later. Raman spectroscopy is a valuable tool for the characterization of dispersed metal oxides and detects vibrational modes of surface and bulk structures. To analyze the nature of the surface species, laser Raman measurements of bulk WO3 and V2O5-WO3/ZrO2 samples calcined at 773 K and with different tungsten oxide and vanadium oxide loadings were made. Figure 4 shows the spectra of bulk WO3 and 4V2O5-WO3/ZrO2(773) samples with different WO3 loadings under ambient conditions. The WO3 structure is made up of distorted WO3 octahedra. Bulk WO3, obtained by calcining ammonium metatungstate at 773 K, shows the main bands in good agreement with data (38) Roozeboom, F.; Mittelmelijer-Hazeleger, M. C.; Moulijn, J. A.; Medema, J.; de Beer, U. H. J.; Gelling, P. J. J. Phys. Chem. 1980, 84, 2783-2791.

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Figure 5. Raman spectra of (a) 22V2O5-15WO3/ZrO2(773), (b) 18V2O5-15WO3/ZrO2(773), and (c) 8V2O5-15WO3/ZrO2(773). Figure 4. Raman spectra of (a) WO3, (b) 4V2O5-20WO3/ZrO2(773), (c) 4V2O5-10WO3/ZrO2(773), and (d) 4V2O5-5WO3/ZrO2(773).

previously reported.39,40 The major vibrational modes of WO3 are located at 808, 714, and 276 cm-1 and have been assigned to the W-O stretching mode, the W-O bending mode, and the W-O-W deformation mode, respectively.41 Zhao et al. have observed the formation of WO3 crystallines only when the monolayer capacity of WO3 on zirconia has been exceeded.42 In this work, the tungsten oxide exceeding the monolayer capacity of the zirconia support forms crystalline WO3 under ambient conditions at elevated temperatures. The crystalline WO3 was also observed in the XRD patterns described below. So, as shown in Figure 4d, for 4V2O5-5WO3/ZrO2(773) no bands corresponding to WO3 crystallites appear, indicating that WO3 is in a highly dispersed state. In view of Figure 4, a monolayer capacity for WO3 seems to be between 5 and 10 wt %. As described in the IR spectra, the catalysts at vanadia loadings below 18 wt % gave no absorption bands due to crystalline V2O5. As shown in Figure 4, for the samples containing 4% V2O5 no bands due to V2O5 crystalline were observed, showing good agreement with the results of IR spectra. The molecular structure of the supported tungsten oxide species depends on the loading. Several authors observed that the nature of surface tungsten species on Al2O3, TiO2, and ZrO2 depends on the amount of WO3. For a low tungsten loading, there appear tetrahedrally coordinated tungsten oxide species; for a high tungsten loading, octahedral polymeric WO3 species appear in addition to the tetrahedral ones.40,43,44 As shown in Figure 4, at a low tungsten loading tetrahedrally coordinated tungsten oxide species (at ∼935 cm-1) are predominantly formed, while with increased loading polytungstate species with an octahedral environment become predominant (at 965985 cm-1). The frequency of the Raman feature (1000-

940 cm-1), the maximum of which shifts slightly upward on increasing vanadium content, is assigned to the VdO stretching mode of vanadyl species in a hydrated form.8,45 Therefore, the broad band observed in the 930-990 cm-1 region in Figure 4 will be interpreted as an overlap of three characteristic bands (two tungsten oxide species and one vanadyl species). Tetragonal zirconia is expected to yield a spectrum consisting of six Raman bands with frequencies at about 148, 263, 325, 472, 608, and 640 cm-1, while monoclinic zirconia exhibits the characteristic features at 180, 188, 221, 380, 476, and 637 cm-1.46,47 As shown in Figure 4d, the 4V2O5-5WO3/ZrO2 sample exhibits the characteristic features of tetragonal zirconia, indicating no transformation of ZrO2 from tetragonal to monoclinic. For high tungsten loading samples calcined at 773 K in Figure 4b,c, zirconia is amorphous to X-ray diffraction described below. The Raman spectrum of amorphous zirconia is characterized by a very weak and broad band at 550-600 cm-1.48 However, the Raman band of amorphous zirconia in Figures 4 and 5 is not well observed because of overlapping of another band. We will discuss the Raman spectra of the series of V2O5-15WO3/ZrO2(773) samples containing different V2O5 loadings, which are shown in Figure 5. For both samples 22V2O5-15WO3/ZrO2 and 18V2O5-15WO3/ZrO2, the spectra displayed bands at 144, 196, 284, 304, 406, 484, 528, 702, and 996 cm-1, all of which are characteristic of crystalline V2O5.49 These results are in good agreement with those of the IR spectra mentioned above. The 996 cm-1 band is assigned to the vibration of the short vanadium-oxygen bond normally regarded as a VdO species.49 However, for 8V2O5-15WO3/ZrO2 containing a low V2O5 loading, no bands due to crystalline V2O5 are observed, indicating a high dispersion of V2O5 on the ZrO2 surface. A broad band containing a maximum at 940 cm-1 is associated with the overlap of three characteristic bands

(39) Salvati, L.; Makovsky, L. E.; Stencel, J. M.; Brown, F. R.; Hercules, D. M. J. Phys. Chem. 1981, 85, 3700-3707. (40) Sohn, J. R.; Park, M. Y. Langmuir 1998, 14, 6140-6145. (41) Chan, S. S.; Wachs, I. E.; Murrell, L. L. J. Catal. 1984, 90, 150155. (42) Zhao, B.; Xu, X.; Gao, J.; Fu, Q.; Tang, Y. J. Raman Spectrosc. 1996, 27, 549-554. (43) Engweiler, J.; Harf, J.; Baiker, A. J. Catal. 1996, 159, 259-269. (44) Vaudagna, S. R.; Canavese, S. A.; Comelli, R. A.; Figoli, N. S. Appl. Catal., A 1998, 168, 93-111.

(45) Ramis, G.; Cristiani, C.; Forzatti, P.; Busca, G. J. Catal. 1990, 124, 574-576. (46) Mercera, P. D. L.; Van Ommen, J. G.; Doeburg, E. B. M.; Burggraaf, A. J.; Ross, J. R. H. Appl. Catal. 1990, 57, 127-148. (47) Scheithauer, M.; Grasselli, R. K.; Kno¨zinger, H. Langmuir 1998, 14, 3019-3029. (48) Schild, C. H.; Wokaun, A.; Ko¨ppel, R. A.; Baiker, A. J. Catal. 1991, 130, 657-661. (49) Dines, T. J.; Rochester, C. H.; Ward, A. M. J. Chem. Soc., Faraday Trans. 1 1991, 87, 653-656.

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Figure 6. IR spectra of 4 V2O5-15WO3/ZrO2 evacuated at (a) 298 K and (b) 773 K and of 18V2O5-15WO3/ZrO2 evacuated at (c) 298 K and (d) 773 K.

by two tungsten oxide species and one vanadyl species discussed above. The IR spectra in Figures 1-3 have been taken in contact with air and KBr pressed disks. The Raman spectra in Figures 4 and 5 have been taken in air using the pure powders. To examine the structure of metal oxides supported on ZrO2 under dehydration conditions, IR spectra of V2O5-WO3/ZrO2 samples were obtained in a heatable gas cell after evacuation at 773 K for 1.5 h. The IR spectra for 4V2O5-15WO3/ZrO2 and 18V2O5-15WO3/ ZrO2(773) are presented for the range 1200-1800 cm-1 in Figure 6. For 4V2O5-15WO3/ZrO2 and 18V2O5-15WO3/ ZrO2 samples, the IR band at 1012 cm-1 after evacuation at 773 K is due to the WdO stretching mode of the tungsten oxide complex bonded to the ZrO2 surface.40,50 As shown in Figure 6, these WdO bands due to tungstyl species appear only on evacuated samples, being undetectable on wet samples. This can be rationalized by assuming that the adsorption of water causes a strong perturbation of the corresponding tungsten oxide species, with a consequent strong broadening and shift down of these bands which become almost undetectable.51 The IR band at 1012 cm-1 matches the Raman absorption at 1015 cm-1.30 However, the band at 1015 cm-1 in Figures 4 and 5 was not observed because Raman spectra were recorded under ambient conditions. These isolated tungsten oxide species are stabilized through multiple W-O-Zr bonds between each tungsten oxide species and the zirconia surface.47,50,51 Zhao et al. observed, in the Raman spectrum of WOxZrO2, a band at 580 cm-1, assigned to a W-O-Zr species.42 Upon dehydration at an elevated temperature, the hydrated surface metal oxide species are unstable and decompose to form dehydrated surface metal oxide species by direct interaction with the surface OH groups of the support, giving the formation of a metal-oxygen-support bond.52 (50) Barton, D. G.; Shtein, M.; Wilson, R. D.; Soled, S. L.; Iglesia, E. J. Phys. Chem. B 1999, 103, 630-640. (51) Gutierrez-Alejandre, A.; Ramirez, J.; Busca, G. Langmuir 1998, 14, 630-639. (52) Kim, D. S.; Ostromecki, M.; Wachs, I. E. J. Mol. Catal. A: Chem. 1996, 106, 93-102.

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Figure 7. Solid-state 51V NMR spectra of V2O5-15WO3/ZrO2 catalysts calcined at 773 K.

For the 18V2O5-15WO3/ZrO2 sample, the band at 1022 cm-1 is due to the VdO stretching vibration of crystalline V2O5 as mentioned above, because a V2O5 loading exceeding the formation of a monolayer on the surface of ZrO2 is well crystallized.35 Therefore, with 4V2O5-15WO3/ZrO2 the definite peak due to the crystalline V2O5 is undetectable, explaining that a vanadium oxide loading below 18 wt % is in a highly dispersed state. Solid-State 51V NMR Spectra. Solid-state NMR methods represent a promising approach to vanadium oxide catalytic materials. The solid-state 51V NMR spectra of V2O5-WO3/ZrO2 catalysts calcined at 773 K followed by exposure to air are shown in Figure 7. There are three types of signals in the spectra of catalysts with varying intensities depending on V2O5 content. At low loadings up to 12 wt % V2O5, a shoulder at about -260 ppm and an intense peak at -590 to -650 ppm are observed. The former is assigned to the surface vanadium-oxygen structures surrounded by a distorted octahedron of oxygen atoms, while the latter is attributed to the tetrahedral vanadium-oxygen structures.53,54 However, the surface vanadium oxide structure is remarkably dependent on the metal oxide support material. Vanadium oxide on TiO2 (anatase) displays the highest tendency to be 6-coordinated at low surface coverages, while in the case of γ-Al2O3 a tetrahedral surface vanadium species is favored.54 As shown in Figure 7, at a low vanadium loading on zirconia a tetrahedral vanadium species is exclusively dominant compared with an octahedral species. In general, it is known that low surface coverages favor a tetrahedral coordination of vanadium oxide, while at higher surface coverages vanadium oxide becomes increasingly octahedral-coordinated. Increasing the V2O5 content on the zirconia surface changes the shape of the spectrum to a rather intense and sharp peak at about -300 ppm and a broad low-intensity peak at about -1400 ppm, which are due to the crystalline V2O5 of square pyramid coordination.53 These observations of crystalline V2O5 for samples containing high V2O5 (53) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796-6805. (54) Reddy, B. M.; Reddy, E. P.; Srinivas, S. T.; Mastikhim, V. M.; Nosov, N. V.; Lapina, O. B. J. Phys. Chem. 1992, 96, 7076-7078.

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Figure 8. Solid-state 51V NMR spectra of 4V2O5-15WO3/ZrO2 calcined at different temperatures.

content, above 12 wt %, are in good agreement with the results of the IR spectra in Figure 1. Namely, this is because a V2O5 loading exceeding the formation of a monolayer on the surface of zirconia is well crystallized.35 Moreover, the increase in V2O5 content resulted in the appearance of an additional signal with a peak in the range from -810 to -840 ppm. The intensity of the signal increased with an increase in V2O5 loading. Different peak positions normally indicate the differences of the spectral parameters and are observed due to different local environments of vanadium nuclei.53-57 Thus, species at -590 to -650 ppm and -810 to -840 ppm can be attributed to two types of tetrahedral vanadium complexes with different oxygen environments. Namely, the signals at -590 to -650 ppm can be attributed to the surface vanadium complexes containing OH groups or water molecules in their coordination sphere,54 because the evacuation treatment decreases the intensities remarkably. On the other hand, the signals at -810 to -840 ppm are due to the surface tetrahedral vanadium complexes which do not contain OH groups or adsorbed water molecules. It is necessary to examine the effect of calcination temperature on the surface of the vanadium oxide structure. The spectra of 4V2O5-15WO3/ZrO2 and 12V2O5-15WO3/ZrO2 containing lower vanadium oxide contents and calcined at various temperatures are shown in Figures 8 and 9, respectively. The shape of the spectrum is very different depending on the calcination temperature. For both samples calcined at lower temperatures (673-773 K), two peaks at about -260 ppm and -590 to -650 ppm due to the octahedral and tetrahedral vanadium-oxygen structures are shown, indicating the monolayer dispersion of V2O5 on the ZrO2-WO3 surface. These results are in good agreement with the results of the IR spectra in Figure 2. However, for samples calcined at 873 K, in addition to (55) Le Costumer, L. R.; Taouk, B.; Le Meur, M.; Payen, E.; Guelton, M.; Grimblot, J. J. Phys. Chem. 1988, 92, 1230-1235. (56) Larsen, G.; Lotero, E.; Petkovic, L. M.; Shobe, D. S. J. Catal. 1997, 169, 67-75. (57) Afanasiev, P.; Geantet, C.; Breysse, M.; Coudurier, G.; Vedrine, J. C. J. Chem. Soc., Faraday Trans. 1994, 90, 193-202.

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Figure 9. Solid-state 51V NMR spectra of 12V2O5-15WO3/ZrO2 calcined at different temperatures.

the above two peaks, a peak at -800 ppm due to crystalline ZrV2O7 appeared, indicating the formation of a new compound from V2O5 and ZrO2 at a high calcination temperature. For samples calcined at 873-1073 K, X-ray diffraction patterns of ZrV2O7 were observed. Roozeboom et al. reported the formation of ZrV2O7 from V2O5 and ZrO2 at a calcination temperature of 873 K.38 At a calcination temperature of 973 K, only a peak at -800 ppm due to the ZrV2O7 phase appeared, saying that most of the V2O5 on the surface of zirconia was consumed to form the ZrV2O7 compound. However, at a calcination temperature of 1073-1173 K we can observe only a sharp peak of crystalline V2O5 at -300 and about -1300 ppm, indicating the decomposition of ZrV2O7. These results are in good agreement with those of the IR spectra in Figure 2. The spectra of 28V2O5-15WO3/ZrO2 containing a higher vanadium oxide content than monolayer loading and calcined at various temperatures are shown in Figure 10. Unlike the 4V2O5-15WO3/ZrO2 and 12V2O5-15WO3/ZrO2 catalysts, for 28V2O5-15WO3/ZrO2 calcined even at lower temperatures (673-773 K) a sharp peak due to crystalline V2O5 appeared at -300 ppm together with peaks at -670 and -870 ppm due to the tetrahedral surface species. However, for samples calcined at 873 K, in addition to a peak at -300 ppm due to crystalline V2O5, a sharp peak at -800 ppm due to the ZrV2O7 compound appeared. As shown in Figure 10, the peak intensity of ZrV2O7 increased with an increase in calcination temperature, consuming the content of crystalline V2O5. Consequently, at a calcination temperature of 973 K only a peak due to the ZrV2O7 phase appeared at -800 ppm. At a calcination temperature of 1073 K, a sharp peak of crystalline V2O5 at -300 ppm due to the decomposition of ZrV2O7 was again observed. However, unlike 4V2O5-15WO3/ZrO2 and 12V2O5-15WO3/ZrO2, for 28V2O5-15WO3/ZrO2 the ZrV2O7 compound was not decomposed completely at 1073 K, leaving some ZrV2O7. It seems likely that it is very difficult for all ZrV2O7 to decompose in 1.5 h because a large amount of ZrV2O7 was formed in the case of 28V2O5-15WO3/ZrO2. Crystalline Structure of the Catalyst. The crystalline structures of V2O5-WO3/ZrO2 calcined in air at

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Figure 10. Solid-state 51V NMR spectra of 28V2O5-15WO3/ ZrO2 calcined at different temperatures.

Figure 11. X-ray diffraction patterns of 4V2O5-15WO3/ZrO2 calcined at different temperatures: O, tetragonal phase ZrO2; b, monoclinic phase ZrO2; 2, V2O5; ×, WO3.

different temperatures for 1.5 h were examined. For the 4V2O5-15WO3/ZrO2, as shown in Figure 11, ZrO2 is amorphous to X-ray diffraction up to 673 K, with a tetragonal phase of ZrO2 at 773-873 K, a two-phase mixture of the tetragonal and monoclinic ZrO2 forms at 973 K, and a monoclinic phase of ZrO2 at 1073-1173 K. Three crystal structures of ZrO2, tetragonal, monoclinic, and cubic phases, have been reported.56,57 On the other hand, V2O5 for 4V2O5-15WO3/ZrO2 and 12V2O5-15WO3/ZrO2 is amorphous to X-ray diffraction up to 773 K, indicating that vanadium oxide is in a highly dispersed state and showing a good agreement with the results of IR and 51V NMR in Figures 1 and 7. In separate XRD experiments, the cubic phase of ZrV2O7 began to be observed in the sample of 12V2O5-15WO3/ZrO2 calcined at 873 K and the peak intensities of ZrV2O7 increased to some extent at 973 K (not shown in the figure). However,

Sohn et al.

Figure 12. X-ray diffraction patterns of 28V2O5-15WO3/ZrO2 calcined at different temperatures: O, tetragonal phase ZrO2; b, monoclinic phase ZrO2; 2, V2O5; 9, ZrV2O7; ×, WO3.

for samples calcined at 1073-1173 K the ZrV2O7 phase disappeared due to the complete decomposition of ZrV2O7,38 leaving only the crystalline V2O5 phase and the monoclinic phase of ZrO2. The triclinic phase of crystalline WO3 due to the decomposition of ammonium metatungstate was observed in the samples calcined at 873-1173 K. However, as shown in Figure 11, for the sample of 4V2O5-15WO3/ZrO2 the crystalline ZrV2O7 on the X-ray diffraction pattern was not observed at a calcination temperature of 873 K, although a peak at -800 ppm due to crystalline ZrV2O7 appeared in the 51V NMR spectra (Figure 8). This indicates that for 4V2O5-15WO3/ZrO2, the ZrV2O7 crystallites formed are less than 4 nm in size, that is, beyond the detection capability of the XRD technique. As shown in Figure 12, in the case of 28V2O5-15WO3/ ZrO2 containing a high content of V2O5 the crystalline V2O5 phase was observed even at a low calcination temperature of 673 K, indicating that a V2O5 loading exceeding the formation of a monolayer on the surface of ZrO2 is well crystallized.35 These results are in good agreement with those of IR and 51V NMR in Figures 3 and 10 described above. From a calcination temperature of 873 K, V2O5 began to react with ZrO2 to form the ZrV2O7 compound and at 973 K the crystalline V2O5 phase disappeared completely, due to the consumption of V2O5 for the formation of the ZrV2O7 compound. However, at 1073 K, the crystalline V2O5 phase was observed again through the decomposition of the ZrV2O7 compound,35 as shown in Figure 12. For 28V2O5-15WO3/ZrO2, ZrO2 was amorphous to X-ray diffraction up to 773 K, with a tetragonal phase of ZrO2 at 873 K and a two-phase mixture of the tetragonal and monoclinic forms at 973-1073 K. From a calcination temperature of 673 K, the crystalline WO3 phase was observed due to the decomposition of ammonium metatungstate. Thermal Analysis. In X-ray diffraction patterns, it was shown that the structure of V2O5-WO3/ZrO2 was different depending on the calcination temperature. To examine the thermal properties of precursors of samples more clearly, their thermal analysis was carried out and illustrated in Figure 13. For pure ZrO2, the DSC curve

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Figure 13. DSC curve of precursors of catalysts: (a) ZrO2, (b) 9-V2O5/ZrO2, (c) 12-V2O5/ZrO2, (d) 15-V2O5/ZrO2, (e) 12V2O5-15WO3/ZrO2, and (f) NH4VO3. Table 1. Specific Surface Areas of Some V2O5-WO3/ZrO2 Samples Calcined at 673 and 773 K catalysts

surface area (m2/g, 673 K)

surface area (m2/g, 773 K)

ZrO2 0.4V2O5-15WO3/ZrO2 1V2O5-15WO3/ZrO2 4V2O5-5WO3/ZrO2 4V2O5-10WO3/ZrO2 4V2O5-15WO3/ZrO2 4V2O5-20WO3/ZrO2 8V2O5-15WO3/ZrO2 12V2O5-15WO3/ZrO2 18V2O5-15WO3/ZrO2 20V2O5-15WO3/ZrO2 28V2O5-15WO3/ZrO2

185 235 239 238 246 224 218 201 217 206 171 144

122 207 211 198 212 201 196 198 188 176 149 106

shows a broad endothermic peak below 453 K due to water elimination and a sharp and exothermic peak at 702 K due to the ZrO2 crystallization.35,58 In the case of 12V2O5-15WO3/ZrO2, two additional endothermic peaks appear at about 454 and 563 K due to the evolution of NH3 and H2O decomposed from NH4VO3. Also, it is considered that an endothermic peak at 947 K is responsible for the melting of V2O5. However, it is of interest to see the influence of vanadium oxide and tungsten oxide on the crystallization of ZrO2 from the amorphous to the tetragonal phase. As Figure 15 shows, the exothermic peak due to crystallization appears at 702 K for pure ZrO2, while for V2O5/ZrO2 and V2O5-WO3/ZrO2 samples it is shifted to higher temperatures. The shift increases with vanadium oxide and tungsten oxide contents. Consequently, the exothermic peaks appear at 793 K for 9V2O5/ZrO2, 841 K for 12V2O5/ZrO2, and 903 K for 12V2O5-15WO3/ZrO2. Surface Properties and Catalytic Activity. The specific surface areas of some samples calcined at 673 and 773 K for 1.5 h are listed in Table 1. The presence of vanadium oxide and tungsten oxide influences the surface area in comparison with the pure ZrO2. Specific surface (58) Sohn, J. R.; Kim, T. G.; Kwon, T. D.; Park, E. H. Langmuir 2002, 18, 1666-1673.

Figure 14. Infrared spectra of NH3 adsorbed on 4V2O5-15WO3/ ZrO2 calcined at 973 K: (a) background of 4V2O5-15WO3/ZrO2 evacuated at 673 K for 1 h, (b) NH3 (20 Torr) adsorbed on sample a, (c) sample b evacuated at 298 K for 5 min, and (d) sample c evacuated at 503 K for 0.5 h.

areas of V2O5-WO3/ZrO2 samples are larger than that of pure ZrO2 calcined at the same temperature. It seems likely that the interaction between vanadium oxide (or tungsten oxide) and ZrO2 protects catalysts from sintering.35 Infrared spectroscopic studies of ammonia adsorbed on solid surfaces have made it possible to distinguish between Bro¨nsted and Lewis acid sites.59,60 Figure 14 shows the IR spectra of ammonia adsorbed on 4V2O5-15WO3/ZrO2 calcined at 973 K and evacuated at 673 K for 1 h. For 4V2O5-15WO3/ZrO2, the bands at 1443 cm-1 are the characteristic peaks of ammonium ions, which are formed on the Bro¨nsted acid sites, and the other set of adsorption peaks at 1621-1606 cm-1 is contributed by ammonia coordinately bonded to Lewis acid sites,59,60 indicating the presence of both Bro¨nsted and Lewis acid sites. Other samples having different vanadium contents also showed the presence of both Lewis and Bro¨nsted acids. Therefore, these V2O5-WO3/ZrO2 samples can be used as catalysts for Lewis or Bro¨nsted acid catalysis. It is also interesting to examine how the catalytic activity of an acid catalyst depends on its acidic property. The 2-propanol dehydration has been examined at 433 and 453 K with a pulse technique. The 2-propanol dehydration has been used as a test reaction for acid catalysis.29,61 The catalytic activities for the 2-propanol dehydration relative to the WO3 content are presented in Figure 15. The addition of WO3 to 4V2O5/ZrO2 up to 10 wt % caused an increase in the catalytic activity for 2-propanol dehydration. The increase of catalytic activity is due to the increase of acidic sites. In separate experiments, the acidity of the catalysts, as determined by the amount of NH3 irreversibly adsorbed at 503 K,35,40 increased with the addition of WO3 (59) Larrubia, M. A.; Ramis, G.; Busca, G. Appl. Catal., B 2000, 27, L145-L151. (60) Satsuma, A.; Hattori, A.; Mizutani, K.; Furuta, A.; Miyamoto, A.; Hattori, T.; Murakami, Y. J. Phys. Chem. 1988, 92, 6052-6058. (61) Sohn, J. R.; Jang, H. J. J. Catal. 1991, 132, 563-565.

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be attributable to electronic and structural interaction between vanadium oxide and tungsten oxide species on the ZrO2 surface. Evidence for the existence of interaction between vanadium oxide and tungsten oxide species has also been provided by acidic catalytic tests for 2-propanol dehydration. The catalytic tests for 2-propanol dehydration have shown that the addition of WO3 to V2O5/ZrO2 enhanced both catalytic activity (Figure 15) and acidity of the V2O5-WO3/ZrO2 catalysts. Conclusions

Figure 15. Variations of catalytic activity for 2-propanol dehydration with WO3 content at 433 (9) and 453 (b) K.

to 4V2O5/ZrO2. Recently, Toda et al.62 reported that the addition of WO3 to V2O5 series catalysts led to an increase of the number of both Lewis and Bro¨nsted acidic sites. From the IR results of VdO bands, they also reported that the majority of WO3 incorporated onto the V2O5/ZrO2 catalysts seems to be intercalated between vanadia and zirconia.29 It has been also reported that for the V2O5-WO3/TiO2 catalyst the electronic interaction between V oxide and W oxide surface species may occur through oxygen bridging of the polyhedra and/or through the conduction band of TiO2.8 The former possibility is consistent with the formation of mixed WxVyOz species and with the detection of relatively isolated VO2+ ions that are slightly different from those monitored over V2O5/ TiO2 by EPR. Considering the correlation between acidity and catalytic activity for 2-propanol dehydration and the experimental results of other investigators,8,29,63 it is suggested that the increase of acidic sites and their strength may (62) Toda, Y.; Ohno, T.; Hatayama, F.; Miyata, H. React. Kinet. Catal. Lett. 1998, 65, 213-218.

This paper has shown that a combination of FTIR and Raman spectroscopies, 51V solid-state NMR, XRD, and DSC can be used to perform the characterization of V2O5-WO3 catalysts supported on zirconia. On the basis of the results of FTIR and Raman spectroscopies, solidstate 51V NMR, and XRD, at a low calcination temperature of 773 K vanadium oxide up to 12 wt % was well dispersed on the surface of zirconia. However, a high V2O5 loading (equal to or above 18 wt %) exceeding the formation of a monolayer on the surface of zirconia was well crystallized. The ZrV2O7 compound was formed through the reaction of V2O5 and ZrO2 at 873 K, and the compound decomposed into V2O5 and ZrO2 at 1073 K; these results were observed in FTIR spectra, solid-state 51V NMR, and XRD. The Wd O bands (1012 cm-1) due to tungstyl species strong suggest that they interact with the zirconia via W-O-Zr bonds. It is suggested that the increase of acidic sites and their acid strength may be attributable to electronic and structural interaction between vanadium oxide and tungsten oxide species on the ZrO2 surface. Infrared spectroscopic studies of ammonia adsorbed on V2O5-WO3/ ZrO2 catalysts showed the presence of both Lewis and Bro¨nsted acids. The catalytic tests for 2-propanol dehydration have shown that the addition of WO3 to V2O5/ ZrO2 enhanced both catalytic activity and acidity of V2O5-WO3/ZrO2 catalysts. Acknowledgment. This work was supported by a Korea Research Foundation Grant (KRF-2001-041E00312). LA020223Y (63) Paganini, M. C.; Dall’Acqua, L.; Giamello, E.; Lietti, L.; Forzatti, P.; Busca, G. J. Catal. 1997, 166, 195-205.