THE INFLUENCE OF THE CHROMIUM CONTENT ON THE PHYSICOCHEMICAL PROPERTIES AND THE CATALYTIC BEHAVIOR OF CROX TIO2 CATALYSTS FOR THE SELECTIVECATALYTIC REDUCTION OF NITRIC-OXIDE BY AMMONIA AT RELATIVELY HIGH-TEMPERATURES/

Chromia on titania catalysts containing chromium loading ranged from 0 up to 8 mol% Cr have been prepared and calcined at high temperature ( 500 degrees C). These catalysts have been tested for the selective cat alytic reduction of NO by NH3 in excess of O-2 at relatively high reac tion temperatures (250-450 degrees C). The physicochemical properties of the prepared samples have been investigated by specific surface are a and pore volume measurements, X-ray powder analysis, UV-visible diff use reflectance spectroscopy, laser-Raman microscopy, X-ray photoelect ron spectroscopy, analytical electron microscopy, and temperature prog rammed reduction experiments. The best catalytic performance has been found for the catalyst containing the lowest amount of active phase (2 mol% Cr). The relatively high catalytic performance of this catalyst has been attributed to the isolated Cr-(vi) surface species strongly i nteracting with the anatase surface. In fact the physicochemical chara cterization mentioned above showed that these species are predominant at this low Cr-loading. Increase in the Cr-loading has been found to f avor the formation of badly dispersed chromia phases with lower oxidat ion states. Besides the title reaction, the direct oxidation of NH3 by O-2 and the reaction of NH3 with NO producing N2O take also place at intermediate reaction temperatures (250-350 degrees C). At elevated re action temperatures (400-450 degrees C) the direct oxidation of NH3 wa s found to be significant. (C) 1997 Academic Press.