(751b) Stabilization of Metastable Oxides Via Surface Modification

Metal oxides are utilized in diverse applications due to their ability to form single component crystals, multi-component mixed oxide systems, and supported oxides. During the preparation of a supported oxide catalyst, low surface energies enable increased atomic surface mobility and permit oxides to spread more readily than metals. This facilitates widespread coverage of the support, the ability to form monolayers, and offers synergetic catalytic effects between the surface oxide and its support. However, high surface mobility also makes an oxide susceptible to sintering, pore collapse, phase transformation, and catalytic deactivation at elevated temperatures. Therefore, it remains imperative to develop methods of oxide stabilization when designing catalysts for operation in extreme environments. This work provides a conceptual framework for the preservation of metastable oxides via the incorporation of a second, supported oxide, along the surface of a metastable support. The selection of an appropriate surface oxide is shown to ballast the underlying support while preserving its surface area, crystal structure, and catalytic functionality. This phenomenon is demonstrated by assessing the stability of common metastable oxides (e.g. tetragonal zirconia and anatase titania) when supporting a range of surface oxides (e.g. molybdenum oxide, silica, tungsten oxide, and ceria) during high temperature calcination. HRTEM, XRD, LEIS, N₂ adsorption, and TPR are utilized to evaluate reconstruction and catalytic activity at elevated temperatures. The insight provided by this work allows a rational approach for selecting surface oxides in order to limit catalytic reconstruction and assists in the design of robust catalysts for high temperature applications.