Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B) (original) (raw)

Abstract

Different phases of titania were prepared and used to support ca. 1 wt.-% V,O,. The different tit&a phases prepared were: anatase (A22), rutile (F&28), brook& (BTllO) and B-phase (B18). Physical characterization of the various vanadia-tit&a catalysta was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and "'V solid stats nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the aiI the cataiysta contain various levele of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminai V= 0 bond at ca. 1030 cm-'. Solid state s'V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around -660 to -670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on aii the titania supports irrespective of the crystai structure of the bulk tit&a phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (9296% ) . The turnover number for methanol oxidation was essentially the same for aii the vanadia-titania cat&&s and ranged from 1.4 to 2.8 s-l. These results indicate that the type of titania phase used as the support is not criticaI for partial oxidation over vanadia-tit&a catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity Correspondence to: Prof. I.E. Wachs, ZettIemoyer Center for Surface Studies, Department of Chemical Engineering, 7 Asa Drive, Lehigh University, Bethlehem, PA 16015, USA. Tel. ( + l-215)7564274, fax. (+ l-215)7593079. 0926-3373/92/$05.00 0 1992 EIsevier Science Publishera B.V. Ail rights reserved.

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