Structure and Catalytic Property of Coherent Spinel Surface Layers on Hexaaluminate Microcrystals

Abstract

Formation of coherent surface layers of Mn3O4 on hexaaluminate microcrystals has been studied as a structural modification to enhance the catalytic activity for methane combustion. The spinel oxide surface layer was successfully produced on planar microcrystals of hexaaluminate by employing the air oxidation of an aqueous Mn(II) solution. As evident from TEM, XPS, and CO2 chemisorption measurements, as-prepared surface layers completely covered the basal plane of planar microcrystals. This characteristic structure appears to be a result of liquid phase epitaxy at the structurally coherent interface between hexaaluminate and spinel. A completely different structure was observed for the samples from the conventional impregnation method, which contain insular Mn3O4 particles dispersed on hexaaluminate. The catalytic activity for methane combustion was evaluated as a function of the Mn3O4 loading and the metal composition of the spinel surface layer. It was revealed that the higher specific activity was observed over air-oxidation-derived samples than over the impregnated samples. Partial substitution of Fe for Mn on the surface layer was effective in enhancing the combustion activity as a result of promoting the reduction/oxidation property.