DRIFTS study of the nature and reactivity of the surface compounds formed by co-adsorption of NO, O2 and propene on sulfated titania-supported rhodium catalysts
Abstract
The selective redn. of NO with propene was investigated on sulfated TiO2, either pure or contg. 1 wt% Rh, using in situ diffuse reflectance Fourier IR transform spectroscopy (DRIFTS). During the NO-C3H6-O2 reaction, the main surface species detected were adsorbed nitrate, a carbonylic compd., acetate, cyanide (-CN), and isocyanate (-NCO). The formation of nitrates at the TiO2 surface occurs by a fast disproportionation of NO, and by its oxidn. on Rh. Rh promotes the formation of -CN and -NCO species. Different steps of the process were investigated using cycles of adsorption. The interaction of a nitrated surface with propene leads to the formation of a carbonylic compd. tentatively identified as acetaldehyde. This carbonylic compd. reacts very fast with a mixt. NO + O2, leading to the rapid formation of -CN and -NCO species. The appearance of Ν(NH) bands ascribed to the formation of ammonia surface complexes was obsd. simultaneously with a decrease of the -NCO band, suggesting that the -NCO species is hydrolyzed to the -NH complexes by the reaction with some traces of water. A reaction mechanism can be proposed in which nitrate species react with acetaldehyde in an acid-catalyzed process, yielding cyanates, which are further hydrolyzed to ammonia and CO2. N2 is then be formed by the reaction of NH3 with NO.