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Structure of WOx/TiO2 Catalysts Prepared from Hydrous Titanium Oxide Hydroxide: Influence of Preparation Parameters S. Eibl,† B. C. Gates,‡ and H. Kno¨zinger* Department Chemie, Physikalische Chemie, Universita¨ t Mu¨ nchen, Butenandtstrasse 5-13 Haus E, D-81377 Munich, Germany Received July 11, 2000. In Final Form: October 17, 2000 Tungstated titania catalysts (WOx/TiO2) were prepared by wet impregnation of hydrous titanium oxide hydroxide. The influences on the catalyst structure of tungsten loading (in the range of 0-30 wt % WO3 supported on TiO2), calcination temperature (varied from 473 to 973 K), and the form of the applied tungstate precursor (ammonium metatungstate or ammonium monotungstate) were investigated by surface area measurements, X-ray diffraction, thermal analysis, temperature-programmed reduction, vibrational and UV/vis spectroscopy, and X-ray absorption spectroscopy. The data show that tungsten loadings giving higher than monolayer coverage of the TiO2 and the application of a high-surface-area titania precursor lead to new structural properties of the surface tungstate phase. A tungstate overlayer is formed that is stable at loadings up to ca. two monolayers (20 wt % WO3/TiO2) at a calcination temperature of 923 K. Two tungstate species are characterized by two WdO bands in the vibrational spectra. One tungstate species shows a strong dependence of its domain size and degree of condensation on calcination temperature and tungsten loading, but the other does not. The first is attributed to accessible outer segments of a three-dimensional tungstate structure and the latter to the interface providing the linkage to the TiO2 support. A three-dimensional structure is formed even at low tungsten coverages. This tungstate overlayer retards the sintering of the TiO2 support and its phase transformation from anatase to rutile. With increasing tungsten loading, the surface area increases and the TiO2 particle sizes and pore diameters decrease. When the tungsten loading exceeds 20 wt % WO3 and the calcination temperature exceeds 923 K, WO3 is formed. These results are supposed to help to explain the properties of these materials including acidity, reactivity in reduction, and isotope exchange.
Introduction Tungstated titania catalysts are applied in industry for processes such as the selective reduction of NOx in exhaust gases1 and various olefin conversions.2-4 They act as strong acids and as redox catalysts and show photocatalytic activity.5,6 Structural analysis of the tungsten-containing phases in these catalysts has been a target of research, but only for catalysts prepared with commercially available crystalline TiO2 supports; the focus has been on catalysts containing tungsten at approximately monolayer coverages (10% WO3/TiO2). On the basis of EXAFS,7 Raman,1,8 and TEM9 data, researchers have proposed twodimensional overlayers consisting of networks of WO4 and WO5 units in the dehydrated state1,9 and pseudooctahedral coordination of the tungsten in the hydrated state. On † Present address: WIWEB, Landshuter Str. 70, 85435 Erding, Germany. ‡ Permanent address: Department of Chemical Engineering and Material Science, University of California, Davis, California 95616.
(1) Hilbrig, F.; Schmelz, H.; Kno¨zinger, H. Stud. Surf. Sci. Catal. 1993, 75, 1351. Hilbrig, F. Dissertation, Universita¨t Mu¨nchen, Germany, 1989. (2) Meijers, S.; Gielgens, L. H.; Ponec, V. J. Catal. 1995, 156, 147. (3) Patrono, P.; Ginestra, A. L.; Ramis, G.; Busca, G. Appl. Catal. A 1994, 107, 249. (4) Yamaguchi, T.; Nakamura, S.; Tanabe, K. J. Chem. Soc., Chem. Commun. 1982, 621. (5) Papp, J.; Soled, S.; Dwight, K.; Wold, A. Chem. Mater. 1994, 6, 496. (6) Do, Y. R.; Lee, W.; Dwight, K.; Wold, A. J. Solid State Chem. 1994, 108, 198. (7) Hilbrig, F.; Go¨bel, H. E.; Kno¨zinger, H.; Schmelz, H.; Lengeler, B. J. Phys. Chem. 1991, 95, 6973. (8) Hardcastle, F. D.; Wachs, I. E. J. Raman Spectrosc. 1995, 26, 397. (9) Burrows, A.; Kiely, C. J.; Joyner, R. W.; Kno¨zinger, H.; Lange, F. Catal. Lett. 1996, 39, 219.
other supports, such as alumina, tetrahedral tungstates were found, but no detectable tungsten phase other than WO3 forms on silica.10 Sulfated and tungstated zirconia are regarded as promising catalysts for alkane isomerization. Preparations with high-area support precursors such as zirconium hydroxide for WOx/ZrO2 catalysts11 yield materials with higher activities for alkane isomerization12-14 than such catalysts with crystalline supports. The tungsten overlayer influences the support with respect to the ZrO2 modification resulting from calcination, and new structures have been proposed for the tungsten-containing phase11 for WOx/ ZrO2. Here we report new catalysts prepared from a nonstoichiometric titanium oxide hydroxide. The tungsten content was varied up to a coverage of about three monolayers, and the influence of loading and calcination temperature on catalyst structure were investigated. The major goal was to characterize the structure of the tungsten phase and its dependence on the preparation conditions. Experimental Methods Catalyst Preparation. Tungstated titania was prepared by suspension of hydrous titanium oxide (precipitated from titanium (10) Mestl, G.; Kno¨zinger, H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; p 539. (11) Scheithauer, M.; Cheung, T.-K.; Jentoft, R. E.; Grasselli, R. K.; Gates, B. C.; Kno¨zinger, H. J. Catal. 1998, 180, 1. (12) Santiesteban, J. G.; Vartuli, J. C.; Han, S.; Bastian, R. D.; Chang, C. D. J. J. Catal. 1997, 168, 431. (13) Boyse, R. A.; Ko, E. I. J. Catal. 1997, 171, 191. (14) Tanabe, K.; Hattori, H.; Yamaguchi, T. Crit. Rev. Surf. Chem. 1990, 1, 1.
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isopropoxide (Heraeus, 99%) by addition of water) in aqueous solutions containing amounts of ammonium metatungstate (AMT) (Fluka, >85% WO3) sufficient to yield calcined materials with loadings in the range 0-30 wt % WO3. Water was removed from the suspensions by evaporation. Each sample was dried at 383 K for 12 h and calcined at a temperature between 473 and 973 K for 2 h in stagnant air. Surface Area Measurements. The N2-BET surface area of each sample was measured according to the BET method with a Sorptomatic 1800 instrument (Carlo Erba) after the sample had been dried at 473 K in dynamic vacuum for 1 h. Pore size distributions were obtained from the N2-adsorption isotherms using the Horvath-Kawazoe method. X-ray Diffraction. X-ray diffraction patterns were measured with a Siemens Guinier diffractometer using Cu KR radiation. Crystallite dimensions were estimated from line width using the Scherrer equation. Other influences on line width, such as defects and lattice strain, were not taken into account. Differential Thermal Analysis and Thermal Gravimetric Analysis (DTA-TGA). DTA-TGA measurements were carried out with samples in a flow of dry air (50 mL/min) in a Netzsch STA 409 thermoanalytical system with R-Al2O3 as reference. The temperature was ramped from 298 to 1273 K at 10 K/min; the sample mass was 0.100 g. TGA curves are depicted as first derivatives (DTG) of the direct weight loss traces. Temperature-Programmed Reduction. Temperatureprogrammed reduction was carried out with a mixture of 5 vol % H2 in N2 at a flow rate of 12 mL/min in a purpose-made apparatus. The temperature was ramped from 298 to 1073 K at 10 K/min; the sample mass was 0.250 g. H2 consumption was measured with a thermal conductivity detector. Experimental parameters were chosen to fulfill the criteria formulated by Monti and Baiker.15 Signal intensities were determined by curve fitting and integration without baseline correction. Raman Spectroscopy. Raman spectra were recorded in retro geometry by using the scanning multichannel technique16,17 with an OMARS 89 triple monochromator spectrometer (Dilor) equipped with a thermoelectrically cooled CCD camera (Princeton Instruments). The 488-nm line of an Ar+-ion laser (Spectra Physics, Type 2020) at a laser power of approximately 30 mW measured at the sample position was used for excitation. The spectral resolution was 5 cm-1. Dehydration and in-situ calcination were carried out with a sample in a quartz cell with an inner diameter of 1 cm in flowing O2 (30 mL(NTP)/min). Spectra were recorded at temperatures between 298 and 1073 K. 18O exchange experiments with H 18O (Fluka, >99% isotopic 2 purity) were carried out by dehydration of the catalyst at 673 K in flowing O2 followed by evacuation (
