The nature of stored NOx and mechanistic aspects of the reduction of NOx stored over a model Pt-K/Al2O3 catalyst sample are investigated in this paper and a comparison with a model Pt-Ba/Al2O3 catalyst is also made It is found that at 350 degrees C on both the catalysts the storage proceeds with the initial formation of nitrites followed by the oxidation of nitrites to nitrates A parallel pathway involving the direct formation of nitrates species is also apparent at saturation only nitrates are present on the catalyst surface over both Pt-K/Al2O3 and Pt-Ba/Al2O3 However whereas bidentate nitrates are present in remarkable amounts on Pt-K/Al2O3 along with ionic nitrates only very small amounts of bidentate nitrates were observed on Pt-Ba/Al2O3 Under nearly isothermal conditions the reduction of the stored NOx with H-2 occurs via an in series two-steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate like for the Pt-Ba/Al2O3 catalyst sample However higher N-2 selectivity is observed in the case of the Pt-K/Al2O3 catalyst due to the similar reactivity of the H-2 + nitrate and NH3 + nitrate reactions Accordingly ammonia once formed readily reacts with surface nitrates to give N-2 and this drives the selectivity of the reduction process to N-2 Notably a strong inhibition of H-2 on the reactivity of NH3 towards nitrates is also pointed out due to a competition of H-2 and NH3 for the activation at the Pt sites Finally the effect of water and CO2 on the reduction process is also addressed Water shows a slight promotion effect on the reduction of the nitrates by H-2 and no significant effect on the reduction by ammonia whereas CO2 has a strong inhibition effect due to poisoning of Pt by CO formed upon CO2 hydrogenation CO2 (C) 2010 Elsevier Inc All rights reserved