NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al2O3 powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H-2. NO pulsing and NO-H-2 pump-probe experiments demonstrate the effect of catalyst temperature, NO-H2 pulse delay time and H-2/NO ratio on N-2, N2O and NH3 selectivity. At lower temperature (1150 degrees C) decomposition of NO is negligible in the absence of H-2, indicating that N-O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H-2 at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N2O or recombine to form N-2. In excess H-2, higher temperatures and longer delay times favor the production of N-2. The longer delay enables NO decomposition on reduced Pt with the role of H-2 being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N-2 formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination. (C) 2009 Elsevier B.V. All rights reserved.