Chromia supported on titania .6. Properties of different chromium oxide phases in the catalytic reduction of NO by NH3 studied by in situ diffuse reflectance FTIR spectroscopy

Chromia on titania catalysts containing different chromium oxide phases, i.e., CrO2, CrOOH, and Cr2O3, have been prepared and tested for the low-temperature selective catalytic reduction of nitric oxide by ammonia (SCR), The different chromium oxide phases (10 wt% total chromium oxide content) differ in their activity and selectivity to N-2, Supported CrOOH is oxidized to CrO2 under SCR conditions above 570 K, The highest activity paired with a high selectivity to N-2 (> 90%) is found for amorphous Cr2O3 supported on titania prepared by decomposition of CrOOH/TiO2 under mild conditions, Crystallization of Cr2O3 induced at higher temperatures reduces the activity significantly, The surface adsorption sites for the components of the SCR feed gas were investigated using temperature-programmed desorption combined with in situ diffuse reflectance FTIR spectroscopy, NO, which is adsorbed as a nitrate species, is most abundant on CrO2, and seems to be responsible for the formation of undesired N2O at temperatures below 400 K over catalysis containing this chromium oxide. (C) 1995 Academic Press, Inc.