| A practical catalyst for emission control in automobiles must have both high activities over a wide range of temperatures and durability at high temperatures of operation. Particularly because of the latter, alumina-supported metal/metal oxide catalysts have been studied as promising catalysts for NO reduction under oxidizing conditions despite their lower activities compared to zeolite-based catalysts at low temperatures. In this thesis research, lean NOx reduction by propene over Au/γ-Al2O3 and Pt/γ-Al 2O3 was studied to elucidate the effect of metal crystallite size and over SnO2/γ-Al2O3 to elucidate the reaction mechanism.; The lean NOx reduction activity over Au/γ-Al2O 3 was found to depend on the crystallite size of Au, being higher for smaller Au crystallites. But, when Au particles were very small (<5 nm) and highly dispersed, the catalyst was active for propene combustion. Al 2O3 also contributes significantly to the overall activity of Au/Al2O3 catalysts at high temperatures (>500°C).; Over Pt/γ-Al2O3 the turnover rate of NO oxidation to NO2 was found to depend strongly on the Pt dispersion, unlike the NO reduction and C3H6 oxidation. Therefore, the reduction of NO does not involve the oxidation of NO to NO2. In the reduction of NO, the ratio of N2O/N2 in the product depended more strongly on the conversion of hydrocarbon than Pt dispersion, reaction temperature or NO conversion. This suggests a reaction mechanism, in which the formation of N2O is favored on an oxygen-covered surface, whereas the formation of N2 is favored on a surface covered with hydrocarbon intermediates.; The bifunctional nature of SnO2/γ-Al2O 3 was confirmed: SnO2 catalyzes the oxidation of propene to oxygenated hydrocarbon species like acrolein and acetaldehyde, and Al 2O3 catalyzes the reduction of NO by these oxygenates. The major reaction pathway of NO reduction over this catalyst was found to be: C3H6 → CH2CHCHO → CH3CHO → HCN → N2. This pathway was supported by the same N2 production efficiencies of both oxygenates and by 13C labeling experiments. It was also found that the NO reduction activity of Al2 O3 depends on its surface property. |
