Contribution to the mechanism of NO reduction by CO over Pt/NaX zeolite

Abstract

The reaction of NO + CO was studied over Pt/NaX prepared by the decomposition of [Pt(NH 3) 4] 2+. The decomposition was carried out via calcination followed by reduction, by vacuum decomposition, and by decomposition in hydrogen, by ways which are known to lead to the formation of Pt clusters of different sizes and location. The NO reduction by CO was studied under static conditions for longer (20–30 min) and shorter (100 s) time intervals, and the reaction was followed by temperature programmed decomposition (TPD) of species adsorbed during the preceding isothermal reactions. The effect of various NO/CO ratios and of added oxygen was examined. The reactions of N 2O + CO were compared with those of NO + CO. The increasing size of Pt clusters enhances the reduction of NO by CO, but it is complicated at lower reaction temperatures (below 230°C) by the poisoning of active Pt centres, especially by adsorbed CO. Smaller Pt clusters exhibit higher preference towards NO adsorption from NO + CO mixtures than the larger Pt clusters. The incomplete reduction of NO to N 2O proceeds under our experimental conditions below 230°C, and is accompanied by the formation of adsorbed species. N 2O formation is enhanced by the increased NO/CO ratio and by the addition of oxygen. The reduction of nitrous oxide occurs much slower than that of nitric oxide, and therefore N 2O could play a role only as a surface intermediate in the CO + NO reaction.