(768b) Design of Supported Catalysts by Principles of Semiconductor Heterojunctions

Supported metal oxides are used as catalysts in a variety of applications including the partial oxidation of alcohols, selective catalytic reduction (SCR) of NOx, oxidative dehyrogenation of alkanes and hydrodesulfurization. The role of the support in influencing the reactivity of the overlayer, beyond just the provision of mechanical stability and high surface area, has been recognized. It has also been shown that modification of the support of the overlayer, by doping, alters the catalytic performance. Considering that several transition metal oxides that are typically used as catalysts, including V₂O₅ and TiO₂, are semiconductors, we present a novel approach to understanding the performance of and designing supported catalysts - from the perspective of semiconductor heterojunctions. The supported catalyst is considered as a heterostructure with a thin overlayer. Heterojunction physics is used to estimate the Fermi level position at the surface of a supported catalyst for various doping levels of the substrate and for various thicknesses of the overlayer. The Fermi level position is then linked to the reactivity through empirical correlations involving surface acidity values. Experiments confirm this basic picture in the case of methanol oxidation over thin film V₂O₅/TiO₂ catalysts. Rates were measured at pressures of 1.5-2 torr. The carrier concentration in the polycrystalline anatase substrate is controlled over a range of 1.5 orders of magnitude via an unconventional means - film thickness, which indirectly influences the concentration of electrically active donor defects at grain boundaries. Over this range, the reaction rate constant varies by more than a factor of 6, and is well described by the heterojunction model.