CATALYTIC ACTIVITY OF SUPPORTED MANGANATES : INFLUENCE OF PREPARATION AND SUPPORT

Abstract

The hypothesis is that by a surface-specific grafting reaction, it is possible to deposit a catalytically active phase on top of an oxidic support. Each grafting reaction will result in a "monolayer" of the catalytically active oxide. Under "monolayer" we understand a layer of one atom thickness with the composition of the active phase. Thus, it should be possible to prepare systems with 1,2,3, ... layers by repeated grafting reactions. Other preparation methods (e.g. impregnation or mechanically mixing) may also give complete coverage of the support surface with the active phase, or may result in the deposition of crystallites, leaving part of the support surface exposed. The aim of the present study can be stated as follows: ( 1) to investigate the change in catalytic activity of an active layer as a function of increasing distance from the support. (2) to compare the surface structures and catalytic activities of supported catalysts prepared by grafting, impregnation, and mechanical mixing. (3) to study the influence of the support material on the catalytic activity of the overlayer. For the present study, manganate was chosen as the catalytically active material. It was necessary to develop a methodology for the characterization of powder surfaces. A number of techniques from the literature were tried and evaluated. These were: microreactor studies of catalyst activity, using CO oxidation, N2O decomposition, and isopropanol decomposition as test reactions, surface area and pore structure determination by N2 adsorption, temperature programmed desorption (TPD) of NH3, N2O, and isopropanol, temperature programmed reduction, surface composition studies by XPS, and the identification of crystalline phases by powder X-ray diffraction. It was found that the grafting reaction, using Mn(OEt)2 as the reagent, leads indeed to deposition of layered manganate on the surface. However, contrary to our initial expectation, each grafting step will deposit only about 1/2 of a full monolayer. The reason is that the bulky alkoxide groups prevent the manganate centres from approaching each other at the surface to the distance found in the manganese oxides. Impregnating the surface with manganate from an aqueous solution of manganese acetate leads to the deposition of small crystallites at the surface. This is deduced from the XPS results. Differences in the catalytic activity are also clearly seen and are in line with the model. After one grafting step (? ? 0.5), the activity per manganese centre is very low. This indicates that the catalytic reaction proceeds at a site consisting of at least 2 manganese ions at close spacing. The activity in the first monolayer is strongly influenced by the support, and is far lower on ?-alumina than on zirconia. The activity of manganese centers in the second layer is increased over that in the first layer and approaches that of Mn in bulk MnO2.