An investigation of the NO/H-2/O-2 (lean-deNO(x)) reaction on a highly active and selective Pt/La0.5Ce0.5MnO3 catalyst

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 o xide 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 degr eesC (74% conversion) for the Pt/La0.5Ce0.5MnO3 and at 125 degreesC (66% co nversion) for the Pt/gamma -Al2O3 catalyst using a GHSV of 80,000 h(-1). Ad dition of 5% H2O in the feed stream influenced the performance of the catal yst in a positive way. In particular, it widened the operating temperature window of the catalyst above 200 degreesC with appreciable NO conversion an d 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 hav e been observed on the Pt/La0.5Ce0.5MnO3 catalyst in the 100-200 degreesC r ange 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 reacti on rate of 397 mu mol of N-2/s.g of Pt metal was measured at 140 degreesC d uring 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% t han 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. T his value was found to be higher by 40% than that observed on the 0.1 wt% P t/gamma -Al2O3 catalyst at 125 degreesC, and it is the highest value ever r eported in the 100-200 degreesC range. A TOF value of 0.49 s(-1) was calcul ated at 140 degreesC for the Pt/La0.5Ce0.5MnO3 catalyst. Temperature-progra mmed desorption (TPD) of NO and transient titration experiments of the cata lyst surface following reaction have revealed important information concern ing several mechanistic steps of the present catalytic system. A hydrogen-a ssisted NO dissociation step and a nitrogen-assisted mechanism for N-2 and N2O formation are proposed to explain all the transient experiments perform ed in a satisfactory manner. (C) 2001 Academic Press.