Catalysis by manganese oxide monolayers .2. Zirconia support

The catalytic and structural properties of two different preparations of manganese oxide supported on high surface area zirconia are compared. The catalyst were prepared by grafting from an alkoxide precursor and by conventional aqueous wet impregnation technique. Reaction of manganese ethoxide with the hydroxyl groups at the surface of the zirconia support is highly specific and allows manganese oxide to be deposited layer by layer. The zirconia used in this study has a mesoporous structure; pores with diameters less than or equal to 5.8 nm were found to be inaccessible to the manganese ethoxide precursor. Hence, only about half of the total surface area is available for the reaction with ethoxide. A quantitative evaluation of the XPS intensities for manganese and zirconia shows that about 2.5 layers of manganate are deposited after five grafting steps. Activity tests made on these catalysts confirm the monolayer buildup. The specific activity of the isolated manganese ions which result from the first grafting step is very low; the specific activity increases by an order of magnitude once the first monolayer is completed, and shows a further increase in the second and third layers. In contrast, impregnation with manganese acetate results in small manganate crystallites, which most likely form inside the pores. The specific activity of these catalysts and their selectivity towards acetone in isopropanol decomposition are very similar to that of bulk manganese dioxide whereas the grafted samples show a high selectivity towards propene.