Selective catalytic reduction of NO by H2 in O2 on Pt/BaO/Al2O3 monolith NOx storage catalysts

Abstract

Comprehensive steady-state experiments of the selective catalytic reduction of NO on a series of Pt, Pt/BaO, and BaO monolithic catalysts have been carried out to evaluate the light-off, NOx conversion and product distribution features as a function of the feed composition, temperature and catalyst composition. The reaction between NO and H2 produces a mixture containing N2O, NH3, and N2, the composition of which is a function of the catalyst temperature and NO/H2 ratio in the feed. NO inhibits the reaction at low temperatures as revealed by light-off temperature and supporting kinetic data. NOx conversions were found to be complete at space velocities below 90,000h−1 and above 100°C for Pt loadings exceeding 1.27wt.%. At low temperature and O2 concentration the NO–H2 reaction mainly produces N2O and is positive (negative) order in H2 (NO). The light-off temperature of the NO–H2 system is dictated by these kinetics as well as the Pt loading. Complementary theoretical analyses elucidate selected kinetic trends and the effect of Pt loading on the conversion versus temperature trends. The NO–H2–O2 data are interpreted with a phenomenological reaction network model. Particular attention focused on the production and consumption of ammonia, a problematic byproduct during conventional NOx storage and reduction (NSR). NH3 is a major product under O2 deficient conditions typical of the rich pulse in NSR, while N2 and N2O are the main products at higher O2 concentrations (lean conditions). NH3 oxidation ignites on Pt catalysts at 170–180°C; in the ignited state a mixture of N2, NO, NO2 and N2O is produced, the composition of which is sensitive to the NH3/O2 feed ratio and temperature. Experiments involving a feed containing H2, NH3, and NO show complete H2 conversion and negligible net NH3 conversion. For temperatures exceeding 150°C an equimolar mixture of the three components results in complete NO reduction by H2 with negligible conversion of NH3. The decomposition of NH3 is observed above 330°C but is kinetically inhibited by H2. A comparison of the Pt and Pt/BaO catalysts reveals similar steady-state behavior. The BaO catalyst exhibited a non-negligible but lower activity and a different product distribution than the Pt and Pt/BaO catalysts.