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

The selective reduction of NO with propene was investigated on sulfated TiO2, either pure or containing 1 wt% Rh, using in situ diffuse reflectance Fourier infrared transform spectroscopy (DRIFTS). During the NO-C3H6-O2 reaction, the main surface species detected were adsorbed nitrate, a carbonylic compound, acetate, cyanide (CN), and isocyanate (NCO). The formation of nitrates at the TiO2 surface occurs by a fast disproportionation of NO, and by its oxidation 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 compound tentatively identified as acetaldehyde. This carbonylic compound reacts very fast with a mixture 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 observed 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-catalysed process, yielding cyanates, which are further hydrolysed to ammonia and CO2. N2 is then be formed by the reaction of NH3 with NO.