Changes in properties of V2O5-K2SO4-SiO2 catalysts in air, hydrogen and toluene vapors

V2O5-K2SO4-SiO2 catalysts were studied by TG, DTA, TPR and XRD methods. Dur ing activation NH4VO3 and (COOH)(2), used for impregnation of the support, decompose in air below a temperature of 350 degrees C. Over 350 degrees C o xidation of vanadium up to V5+ and changes in the catalyst reducibility occ ur. In TPR using pure hydrogen, the freshly activated catalyst is reduced i n three steps. The first peak with a maximum around 430 degrees C is a resu lt of reduction of the vanadium oxide species. The second and the third TPR peaks correspond mainly to the reduction of sulfate species to H2S. It was found that vanadium oxides noticeably enhanced the reduction of sulfate to H2S. Similar TPR profiles were found after catalytic tests (vapor phase ox idation of toluene by air) and after reduction of the same catalyst by H-2, and its reoxidation by air in some particular conditions. By increasing te mperature and prolonging time of activation, or by prolonging time-on-strea m of toluene oxidation the reducibility of vanadium oxides decreases and th e easily reducible part of sulfate species is converted into a less reducib le part. (C) 1999 Elsevier Science B.V. All rights reserved.