The NO/H-2/O-2 reaction has been studied under lean-burn conditions in the 100-400 degreesC range over 0.1 wt% Pt supported on La0.5Ce0.5MnO3 (mixed oxide containing LaMnO3, CeO2, and MnO2 phases). For a critical comparison, 0.1 wt% Pt was supported on gamma -Al2O3 and tested under the same reaction conditions. The maximum in the NO conversion has been observed at 140 degreesC (74% conversion) for the Pt/La0.5Ce0.5MnO3 and at 125 degreesC (66% conversion) for the Pt/gamma -Al2O3 catalyst using a GHSV of 80,000 h(-1). Addition of 5% H2O in the feed stream influenced the performance of the catalyst in a positive way. In particular, it widened the operating temperature window of the catalyst above 200 degreesC with appreciable NO conversion and had no negative effect on the stability of the catalyst for a 20-h run on reaction stream. Remarkable N-2 selectivity values in the 80-90% range have been observed on the Pt/La0.5Ce0.5MnO3 catalyst in the 100-200 degreesC range either in the absence or in the presence of water in the feed stream. This result is reported for the first time for the NO/H-2/O-2 lean-deNO(x) reaction at least on Pt-based catalysts. A maximum specific integral reaction rate of 397 mu mol of N-2/s.g of Pt metal was measured at 140 degreesC during reaction with 0.25% NO/1% H-2/5% O-2/5% H2O/He gas mixture on the 0.1 wt% Pt/La0.5Ce0.5MnO3 catalyst. This value was found to be higher by 40% than that observed on the 0.1 wt% Pt/gamma -Al2O3 catalyst at 125 degreesC, and it is the highest value ever reported in the 100-200 degreesC range. A TOF value of 0.49 s(-1) was calculated at the Pt/La0.5Ce0.5MnO3 catalyst. This value was found to be higher by 40% than that observed on the 0.1 wt% Pt/gamma -Al2O3 catalyst at 125 degreesC, and it is the highest value ever reported in the 100-200 degreesC range. A TOF value of 0.49 s(-1) was calculated at 140 degreesC for the Pt/La0.5Ce0.5MnO3 catalyst. Temperature-programmed desorption (TPD) of NO and transient titration experiments of the catalyst surface following reaction have revealed important information concerning several mechanistic steps of the present catalytic system. A hydrogen-assisted NO dissociation step and a nitrogen-assisted mechanism for N-2 and N2O formation are proposed to explain all the transient experiments performed in a satisfactory manner. (C) 2001 Academic Press.