Band engineering of metal oxide heterostructures for catalysis applications
Supported metal oxides are used as catalysts for a wide variety of industrially and environmentally important reactions such as the selective oxidation of hydrocarbons and alcohols and the selective catalytic reduction of nitrogen oxides. The support often plays an important role in the activity and selectivity of such catalysts, beyond just the provision of mechanical support and large surface area. V2O5 supported on TiO2 is an example of a catalyst that is widely used, where the synergy between the support and the overlayer is taken advantage of. The support effect has often been ascribed to the electronegativity of the support cation which affects the electron density on the metal–oxygen bond in the overlayer. However, this effect may not hold for thicker overlayers and for doped supports. In the current work, a model is proposed in which the supported catalyst is considered as a heterostructure. Most metal oxides are (wide band-gap) semiconductors and hence a semiconductor heterojunction is formed when one oxide is deposited on another. This conception of supported oxide catalysts allows for the use of heterojunction physics to predict the electron richness at the surface of the catalyst. Moreover, effect of overlayer thickness and support doping can be easily determined using such a model. Thus quantitative estimates of surface electron richness were obtained for the system of V2O5/TiO2. It is shown that modification of overlayer thickness and the carrier concentration in the support can lead to modification of the surface electron richness (represented by the surface Fermi level) of the catalyst. A semi-empirical model was also developed to relate the Fermi level of oxide catalysts to their activity. Using these models, the quantitative variation of catalytic activity with the heterostructure parameters (overlayer thickness, support carrier concentration) was determined, for the test reaction of partial oxidation of methanol to formaldehyde. Results from experiments using thin films of polycrystalline oxides (V2O5 supported on TiO2) and methanol oxidation as the test reaction, matched qualitatively with the model predictions. The quantitative enhancement (> 10x) in rate obtained by reducing the overlayer thickness was better than the model predictions. Surface potential measurements combined with kinetic data proved the validity of the model relating Fermi level and catalytic activity, and showed directions for further development of the heterojunction model to predict the surface electron richness.
supported catalysts, metal oxide, semiconductor heterojunction, band engineering
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign