Selectivity is a major challenge in oxidation of short-chain alkanes over metal oxides. The reactions have been studied extensively over V-containing monolayer and bulk mixed oxide catalysts. However, the often complex structural chemistry of the catalysts limits conceptual understanding and rational catalyst design. Activity and selectivity of bulk catalysts are frequently attributed to the bulk crystal structure and the exposure of specific surface facets. One example is the ab plane of the so-called “M1 phase” (ICSD 55097) of MoVTeNb oxide. High selectivity of MoVTeNb oxide in oxidation and ammoxidation of propane has been attributed to site isolation in ensembles cut out from the ideal crystal structure of the ab plane. Site isolation is a fundamental principle in oxidation catalysis, which is based on the hypothesis that reactive surface sites must be spatially isolated from each other in defined arrays on a catalyst surface to achieve selectivity. In the present contribution the origin of site isolation is discussed based on the surface properties of highly crystalline vanadium and manganese oxide-based catalysts that show different selectivity in oxidation of propane. Phase purity of the catalysts has been confirmed by X-ray diffraction, and electron microscopy. Surface sensitive synchrotron-based X-ray photoelectron spectroscopy, and soft X-ray absorption spectroscopy have been applied to investigate surface composition and oxidation state of the elements under conditions of propane oxidation. Generally, it is observed that the surface of working oxidation catalysts differs from the bulk crystal structure in terms of a self-supported monolayer irrespective of the catalyst performance. For example, MoVTeNb M1 crystallites are covered by a thin layer, which is enriched in V5+ and Te4+. The concentration of V5+ is enhanced at the surface of MoV oxide composed of the M1 structure. Nano-structured MnWO4 serves as support that stabilizes a defect-rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at reaction temperature as shown by temperature-programmed oxidation / reduction and confirmed by UV-vis DR, and Raman spectroscopy. High selectivity generally involves reversible changes of the catalyst surface under reaction conditions implying that site isolation is related to reversible surface restructuring. The impact of structural and electronic properties of the bulk on surface structure and dynamics will be discussed.

Structural and dynamic aspects of site isolation at the surface of selective oxidation catalysts

251st National Meeting (2016)

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