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-O-2 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 + O-2), leading to the rapid formation of -CN and -NCO species. The appearance of v(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. N-2 is then be formed by the reaction of NH3 with NO. (c) 2005 Elsevier Inc. All rights reserved.