Mechanism for the reduction of NOx on Rh/sulfated titanias
Résumé
Titanias of different surface areas have been sulfated and used as supports of Rh oxide for the selective catalytic reduction of nitrogen oxides. Only the sulfation of TiO2 of large surface areas gives strong Brönsted acid sites, retaining pyridine up to 773 K. Low surface area anatase is unable to retain sulfates. The catalytic activities measured at 523 K, increase with the number of acid sites, and then reach a plateau, showing the intervention of acidity in the SCR. This is true only for Rh but not for Pt. The investigation of the elemental steps on Rh/sulfated TiO2 by in situ diffuse reflectance spectroscopy permits to clarify a few points: the oxidation of propene, presumably to acetaldehyde, occurs by the reaction with nitrates adsorbed on the support. The further oxidation of this intermediate by NO2 yields an isocyanate, which can be hydrolysed to ammonia.