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, NO x conversion and product distribution features as a function of the feed composition, temperature and catalyst composition. The reaction between NO and H 2 produces a mixture containing N 2 O, NH 3 , and N 2 , the composition of which is a function of the catalyst temperature and NO/H 2 ratio in the feed. NO inhibits the reaction at low temperatures as revealed by light-off temperature and supporting kinetic data. NO x conversions were found to be complete at space velocities below 90,000 h −1 and above 100 °C for Pt loadings exceeding 1.27 wt.%. At low temperature and O 2 concentration the NO–H 2 reaction mainly produces N 2 O and is positive (negative) order in H 2 (NO). The light-off temperature of the NO–H 2 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–H 2 –O 2 data are interpreted with a phenomenological reaction network model. Particular attention focused on the production and consumption of ammonia, a problematic byproduct during conventional NO x storage and reduction (NSR). NH 3 is a major product under O 2 deficient conditions typical of the rich pulse in NSR, while N 2 and N 2 O are the main products at higher O 2 concentrations (lean conditions). NH 3 oxidation ignites on Pt catalysts at 170–180 °C; in the ignited state a mixture of N 2 , NO, NO 2 and N 2 O is produced, the composition of which is sensitive to the NH 3 /O 2 feed ratio and temperature. Experiments involving a feed containing H 2 , NH 3 , and NO show complete H 2 conversion and negligible net NH 3 conversion. For temperatures exceeding 150 °C an equimolar mixture of the three components results in complete NO reduction by H 2 with negligible conversion of NH 3 . The decomposition of NH 3 is observed above 330 °C but is kinetically inhibited by H 2 . 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.

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