Synthesis and characterization of manganese and cerium oxide catalysts supported on gamma-alumina and catalytic application in deNOx reactions and oxidative dehydrogenation of propane.

摘要

This dissertation contains studies carried out on synthesis and characterization of manganese and cerium oxides on gamma-alumina supports to achieve higher loadings of well-dispersed sub-monolayers. These were further investigated for their application in oxidative dehydrogenation of propane to propene and as potential catalysts for nitrogen oxide reduction in lean burn diesel engine exhausts.; Manganese ethoxide synthesized in situ in ethanol was successfully used to graft up to 4.6% Mn on alumina at 60°C, well above the best-reported value of 2.02%. Based on TGA, N2 adsorption and TPR analysis it was found that the oxidation states of manganese present on the surface to be mainly 2+ and 3+. Sequential grafting results in much higher loadings but in multilayers.; Impregnation of manganese oxides was carried out using Mn(NO3) 2 in THF eliminating the difficulties faced by using aqueous solutions of manganese nitrate and acetate. At low loadings manganese was found to be dispersed and mostly single layered with oxidation states of mainly 4+ and 3+. Grafting cerium oxide was not successful but impregnation led to a dispersed layer at low loadings.; Supported manganese oxide catalysts were better than the reported pure manganese oxide catalysts for ODP with propane conversion of ∼20% propene yield ∼7%. Diluting the Mn catalysts with Zr and additives such as Ce, Cu, Fe and K did not improve the catalysts. However the addition of Ni with Mn resulted in higher selectivities toward propene (50%) resulting in yield as high as 9.53%. However, these catalysts were not as good as the commercial ODP catalysts.; 1%Pt/Mn/Al was found to be a working catalyst for the reduction of NOx in lean burn diesel engine exhausts. Both the grafted and impregnated manganese catalysts resulted in converting more than 35% of NOx at 200°C with 15% oxygen in the feed. Mechanistic studies had led to identifying Mn as the active redox site and Pt plays a supporting role by reducing the CO concentration on the surface and enhancing the redox properties of manganese. Dispersion of Pt and Mn oxide on the surface is crucial for the activity of the catalyst. Mn/Ce/Al also shows continuous de-NOx activity.