The Effect of Reducible Metal Oxides on the Structure and Activity of Supported Vanadium Oxide Catalysts Prepared by Atomic Layer Deposition for Cyclohexane Oxidative DehydrogenationPublic
Supported vanadium oxide materials have been extensively studied for alkane oxidative dehydrogenation (ODH) reactions due to their high activity and selectivity. The catalytic activity of supported VOx materials is influenced by the surface coverage of VOx sites and hence the distribution of V=O, V-O-V, and V-O-S (S, support) bonds. The impact of the type of V-O-S bonds is often investigated by varying the bulk oxide support and it is known that the activity of these catalysts is highly dependent on the nature of the support material. This dissertation seeks to develop a fundamental understanding of the contribution of reducible supports to the activity of VOx species in cyclohexane ODH as a model reaction of ODH of alkanes. The first part of this thesis focuses on investigating the interactions of VOx species with amorphous TiO2 domains deposited by atomic layer deposition (ALD) on an inert Al2O3 support. A combination of ALD and calcination procedures is shown to influence the surface site distribution. Preferential binding of VOx and TiO2 domains to each other on an Al2O3 support is demonstrated by UV Raman spectroscopy and confirmed by DFT. Varying distributions of V-O-V, V-O-Ti and V-O-Al bonds have an effect on the ease of reducibility of VOx sites. The interactions of VOx and TiO2 species are elucidated further under a reducing H2 environment at elevated temperatures. Reversible migration and aggregation of V and Ti atoms is detected upon heating in H2. Changes in the oxidation state of V, but not Ti, are observed by XPS. The elucidation of VOx and TiO2 speciation on the Al2O3 support in their oxidized state and upon reduction facilitates the understanding of the function of individual sites in catalytic reactions relying on a redox mechanism. The second part of this thesis investigates the catalytic activities of alumina-supported VOx-TiO2 materials in comparison with VOx/Al2O3 and VOx/TiO2 to determine the role the TiO2 support plays in the improvement of ODH activity of surface VOx sites in comparison to VOx/Al2O3. The ease of reducibility of VOx species cannot exhaustively explain the observed variability in ODH activity. The increased activity of VOx supported on TiO2 films above a monolayer TiO2 coverage is attributed to the formation of oxygen vacancies within the TiO2 structure. The use of ALD in the synthesis of mixed metal oxide materials enables the distinction between the contribution of V-O-Ti bonds and that of the bulk TiO2 structure to the catalytic activity of supported VOx domains. However, while the catalytic activity is dependent on the composition of the support, the selectivity-conversion trends remain unchanged. Finally, the study of alumina-supported mixed metal oxides is extended to catalytically active CeO2. The individual contributions of VOx and CeO2 sites to the cyclohexane ODH reaction mechanism are evaluated by studying CeO2, VOx/CeO2 and Al2O3 modified with CeO2 and VOx domains by ALD. The bare ceria support shows the highest activity and favored total oxidation to CO2. The catalytic behavior is dependent on the distribution of V-O-S bonds, and small clusters of CeO2 at low surface densities favor total oxidation similarly to exposed CeO2 surface sites in VOx/CeO2. The extensive study of the influence of reducible metal oxide domains on the structure and catalytic activity of supported VOx sites and the resulting assignment of specific catalytic functions to individual surface structures can lead to the rational design of alkane ODH catalysts with improved alkene yields.
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