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Studies on Dispersion and Reactivity of Vanadium Oxide Catalysts Supported on Titania† Komandur V. R. Chary,* Gurram Kishan, Katar Sri Lakshmi, and Kanaparthi Ramesh Catalysis Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received July 7, 1999. In Final Form: May 4, 2000 Vanadium oxide catalysts with V2O5 loadings ranging from 2 to 12% w/w supported on various modifications of titania have been prepared by the wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), electron spin resonance (ESR), oxygen chemisorption, temperatureprogrammed desorption (TPD) of ammonia, and surface area measurements. The catalytic properties were evaluated for ammoxidation of 3-picoline to nicotinonitrile. Dispersion of vanadia was determined by the oxygen chemisorption at 640 K and by the static method on the samples prereduced at the same temperature. Oxygen chemisorption results suggest that vanadium oxide disperses better on TiO2 (anatase) than it is supported on TiO2 (rutile) or TiO2 containing mixture of anatase and rutile. ESR spectra obtained under ambient conditions for the catalysts reduced at 640 K show the presence of V4+ ions in axial symmetry. The results of XRD and ESR substantiate the findings of oxygen chemisorption. Ammonia TPD results suggest that V2O5 supported on anatase is more acidic than V2O5 supported on TiO2 (rutile) or TiO2 (rutile and anatase). V2O5-supported anatase TiO2 exhibited higher activities for 3-picoline ammoxidation than V2O5/TiO2 (rutile) and V2O5/TiO2 (anatase + rutile). The catalytic properties during ammoxidation are related to the oxygen chemisorption sites.
Introduction The use of titanium dioxide as a catalyst support has several advantages over other classical supported oxides such as alumina and silica. One of the unique features of TiO2 is that it interacts strongly with the active phase of metal/metal oxide and thereby increases the catalytic activity. TiO2 exists in three crystallographic polymorphs, e.g., anatase, rutile, and brookite. The anatase is a lowtemperature modification of titania, and the phase transition to rutile generally occurs above 973 K. The vanadia supported on anatase TiO2 is a classical example of support enhancement of active phase, especially if the vanadium oxide is applied to titania as a monomolecular layer. The optimal catalytic activity and selectivity is achieved when one monolayer of vanadia is dispersed on anatase phase of TiO2.1,2 The structure of highly dispersed VOx species present on the TiO2 surface in V2O5/TiO2 catalysts has been studied extensively in the recent past by various spectroscopic techniques including laser Raman spectra,2-6 51V NMR,7,8 ESR,9-14,53 EXAFS,15 and †
IICT communication number: 4300. * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: 91-40-7173387. (1) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Cherisch, C. C. Appl. Catal. 1985, 15, 339. (2) Bond, G. C.; Bruckman, K. Faraday Discuss. Chem. Soc. 1982, 72, 235. (3) Went, G. T.; Leu, L.j.; Bell, A. T. J. Catal. 1992, 134, 4. (4) Deo, G.; Turek, A.; Wachs, I. E.; Machej, T.; Haber, J.; Das, N.; Eckert, H.; Hirt, A. M. Appl. Catal. A. 1992, 91, 27. (5) Saleh, R. Y.; Wachs, I. E.; Chan, S. S.; Cherisch, C. C. J. Catal. 1986, 98, 102. (6) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94, 420. (7) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796. (8) Eckert, H.; Deo, G.; Wachs, I.E.; Hirt, A. M. Colloids Surf. 1990, 45, 347. (9) Inomata, M.; Mori, K.; Miamoto, A.; Ui, T.; Murakami, Y. J. Phys Chem. 1983, 87, 754. (10) Sanati, M.; Wallenberg, L. R.; Andersson, A.; Jansen, S.; Tu, Y. J. Catal. 1991, 132, 128. (11) Busca, G.; Tittarelli, P.; Tronconi, E.; Forzatti. P. J. Solid State Chem. 1987, 67, 91.
ESCA.16-18 The V2O5/TiO2 catalyst system has been employed in selective oxidation and ammoxidation of aromatic hydrocarbons and alcohols.19-36 In environmental catalytic applications V2O5/TiO2 catalysts in combination with WO3 have been successfully employed for selective catalytic reduction of NOx reactions.37-44 In the present investigation we report the characterization of vanadium oxide catalysts supported on various modifications of TiO2 (anatase, rutile, and titania containing anatase and rutile) by XRD, ESR, and oxygen chemisorption methods. We also report the differences in catalytic properties exhibited by these catalysts during vapor-phase ammoxidation of 3-picoline to nicotinonitrile. The purpose of this work is to estimate the dispersion of (12) Rusiecka, M.; Gryzybowska, B.; Gasior, M. Appl. Catal. 1984, 10, 101. (13) Busca, G.; Centi, G.; Marchetti, L.; Triffiro, F. Langmuir 1986, 2, 568. (14) Busca, G. Langmuir 1986, 2, 577. (15) Kozlowski, R.; Pettfer, R. F.; Thomas, J. M. J. Phys. Chem. 1983, 87, 5176. (16) Bond, G. C.; Zurita, J. R.; Flamerz, S. Appl. Catal. 1986, 27, 353. (17) Chiarello, G.; Robba, D.; De Michele, G.; Parmigini, F. Appl. Surf. Sci. 1993, 64, 91. (18) Haber, J.; Machej, T.; Serwicka, E. M.; Wachs, I. E. Catal. Lett. 1995, 27, 353. (19) Chary, K.V. R. J. Chem. Soc., Chem. Commun. 1989, 104. (20) Bond, G. C.; Sarkany, J.; Parfitt, G. D. J. Catal. 1979, 57, 476. (21) Dias, C. R.; Portela, M. F.; Bond, G. C. J. Catal. 1995, 157, 344. (22) Wang, C.; Deo, G.; Wachs, I. E. J. Catal. 1998, 178, 670. (23) Van Hengstum, A. J.; van Ommen, J. G.; Bosch. H.; Gellings, P. J. Appl. Catal. 1983, 8, 369. (24) Gryzybowska, B.; Gasior, A.; Gasior, I. Appl. Catal. 1984, 10, 87. (25) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (26) Andersson, A.; Lundin, S. T. J. Catal. 1979, 58, 383. (27) Chary, K. V. R.; Kishan, G.; Bhaskar, T.; Sivaraj, CH. J. Phys. Chem. 1998, 102, 6792. (28) Andersson, A.; Bovin, J. O.; Walter, P. J. Catal. 1996, 98, 204. (29) Deo, G.; Wachs, I. E.; Haber, J. J. Crit. Rev. Surf. Chem. 1994, 4, 1. (30) Wachs, I. E.; Weckhuysen, B. M. Appl. Catal. 1997, 157, 67. (31) Bond, G. C. Appl. Catal. A. 1997, 157, 91. (32) Andersson, A.; Lundin, S. T. J. Catal. 1980, 65, 9. (33) Andersson, A. J. Catal. 1982, 76, 144.
10.1021/la9908900 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
Vanadium Oxide Catalysts Supported on Titania
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Table 1. Results of Oxygen Uptake, Dispersion, Oxygen Atom Site Density, and Surface Area of Various V2O5/ TiO2 (Anatase) Catalysts catalyst composition, wt % V2O5 on TiO2
surface area m2 g-1
O2 uptakea, µmol g-1
oxygen atom site density, 1018 m-2
dispersion (O/V)b
0c 2 4 6 8 10 12
92 65 60 58 62 58 53
107.4 187.0 267.7 315.3 363.1 362.3
1.99 3.75 5.56 6.13 7.54 8.23
0.98 0.85 0.81 0.72 0.66 0.55
a T(reduction) ) T(adsorption) ) 640 K. b Dispersion ) fraction of vanadium atoms at the surface assuming O ads/V surf ) 1. c Impurity content in TiO anatase (CLDD 1932/1) ) Cl% ) 0.08; 2 SO3% ) < 0.01; P2O5% )
