Synthesis, characterization, and catalytic applications of grafted metal oxide submonolayers on gamma-aluminum oxide

This dissertation reports studies directed at understanding the reactions in the synthesis of gamma-Al2O3 supported metal oxides through grafting techniques using metal alkoxides as precursors, and expansion of the applications of grafted metal oxide submonolayers on gamma-Al 2O3 in catalysis.; Various metal oxides were supported on gamma-Al2O3 through grafting techniques. It was found that the reaction between metal alkoxides and gamma-Al2O3 surface OH groups proceeds slowly. The grafted metal oxide is present on supports as single sites with submonolayer coverage. The maximum surface density of grafted sites is controlled intrinsically by the precursor molecular size. Based on analysis of OH surface density changes of gamma-Al2O3 upon Zr(OR)4 grafting, it was found that Zr(OR)4 reacts with gamma-Al 2O3 surface OH groups through protonolysis in a monopodal manner, resulting in a grafted fragment, ≡Al-O-Zr(OR)3, that converts to ≡Al-O-Zr(OH)3 by hydrolysis. Dehydration leads to the final grafted species, (≡Al-O)3ZrOH. These novel Zr-OH groups are stable against dehydroxylation up to 520°C. Results suggest that dimeric precursors give a triply-bridging oxygen between two Zr atoms and an Al atom in the final grafted product. Reactivity studies suggest that sequential grafting of Mn(OR)2 occurs with the participation of Mn-OH sites from the first grafting.; Grafting a ZrO2 submonolayer on Pd/Al2O3 can hinder the palladium sintering up to 850°C. Based on experimental results and literature sintering mechanisms, an explanation was proposed for this sintering impedance. The grafted ZrO2 submonolayer traps Pd atoms or PdO particles, impeding their migration. Grafted ZrO2 submonolayers also increase the thermal decomposition temperature of PdO. The differences among grafted ZrO2, TiO2, Al2O 3, and V2O5 submonolayers suggest that their mobilities play a crucial role in the sintering retardation influence.; Supported vanadium oxide catalysts, from grafting techniques, show higher activities and selectivities in oxidative dehydrogenation of propane to propylene than catalysts prepared from traditional impregnation, sol-gel, and thermal spreading. This is attributed to the fact that the grafting technique produces isolated single active sites, avoiding the formation of dimeric and polymeric vanadium species that reduce the catalytic activity and selectivity to propylene. Grafting techniques can also been used in synthesis of catalysts for various deNOx applications.