Nitrates and fast SCR reaction in NOx removal from Diesel engine exhausts

We review in the present paper the key mechanistic features of the NH(3)-NO/NO(2) Fast SCR reaction over vanadium and Fe-zeolite commercial catalysts: this is the main reaction occurring in the urea-SCR technology, that is currently considered the best technique for the abatement of NOx in the exhausts of Diesel engines. A detailed investigation by means of dynamic reactive experiments led to the proposal of an original scheme which accounts for stoichiometry, selectivity and intrinsic kinetics of the global SCR process: NO(2) forms surface nitrites and nitrates via dimerization and disproportionation/heterolytic chemisorption, NO reduces nitrates to nitrites, and NH(3) enables the rapid and selective decomposition of nitrites to nitrogen via formation of unstable ammonium nitrite.