Investigating the Structure of Molybdenum Oxides and Sulfides for Catalytic DehydrogenationPublic
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Molybdenum oxides and sulfides are earth-abundant materials known to catalyze a wide array of reactions, including dehydrogenation, hydrotreating, and higher alcohols synthesis. In particular, alkane and alcohol dehydrogenation are of interest given recent shifts in the energy landscape away from traditional petroleum feedstocks and towards natural gas and renewable energy sources. Extensive research into the structure of these Mo-based catalysts highlight the anisotropic nature of these materials, which make probing their active sites particularly challenging. Molybdenum oxides and sulfides can exhibit a range of morphologies, which in turn leads to significant differences in catalytic performance. This dissertation seeks to elucidate the active site of molybdenum oxides and sulfides for C–H bond activation by developing a robust site counting method and establishing structure-function relationships, which can be used to rationally design molybdenum-based catalysts for catalytic dehydrogenation.Through a combination of spectroscopic characterization and catalytic tests, different morphologies of molybdenum oxides and sulfides are first demonstrated to be stable, selective catalysts for light alkane dehydrogenation. Kinetic studies to determine apparent activation energies highlight the importance of coordinatively unsaturated Mo atoms as catalytically active sites for C–H bond activation. Several structure-function relationships between Mo surface density, Mo reduction, and catalytic performance are also established. Namely, Mo reduction plays a key role in generating active sites for both alkane dehydrogenation and coke formation. Partially reduced Mo4+ is the active Mo oxidation state, but additional reduction to lower oxidation states will promote coking and subsequent catalyst deactivation. Application of selective methanol chemisorption and surface reaction as a robust site counting method provides further insights into the catalyst surface structure. Specifically, reduced Mo species exhibits an increase in the number of basic surface sites distinct from the alkali-doped support. The structure-function relationships determined for molybdenum oxides and sulfides along with the development of a robust active site counting method have important implications for advancing our understanding of these highly anisotropic materials and enabling rational catalyst design.
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