Changes in properties of V 2O 5–K 2SO 4–SiO 2 catalysts in air, hydrogen and toluene vapors

Abstract

V 2O 5–K 2SO 4–SiO 2 catalysts were studied by TG, DTA, TPR and XRD methods. During activation NH 4VO 3 and (COOH) 2, used for impregnation of the support, decompose in air below a temperature of 350°C. Over 350°C oxidation of vanadium up to V 5+ and changes in the catalyst reducibility occur. In TPR using pure hydrogen, the freshly activated catalyst is reduced in three steps. The first peak with a maximum around 430°C is a result of reduction of the vanadium oxide species. The second and the third TPR peaks correspond mainly to the reduction of sulfate species to H 2S. It was found that vanadium oxides noticeably enhanced the reduction of sulfate to H 2S. Similar TPR profiles were found after catalytic tests (vapor phase oxidation 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 temperature and prolonging time of activation, or by prolonging time-on-stream of toluene oxidation the reducibility of vanadium oxides decreases and the easily reducible part of sulfate species is converted into a less reducible part.