APPLICATIONS OF IN SITU MAGNETIC RESONANCE SPECTROSCOPY FOR STRUCTURAL ANALYSIS OF OXIDE-SUPPORTED CATALYSTS
Jaegers, Nicholas Ryan
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Nuclear Magnetic Resonance (NMR) stands as an invaluable non-destructive technique to probe catalytic materials. The rise of in situ NMR has enabled detailed structural analysis of materials under tightly controlled conditions that are relevant for the chemistry of interest. Herein, the utilization of such technology has been described as it pertains to supported oxide catalysts—in particular, supported vanadium oxide materials. Metal oxides are notoriously challenging to characterize due to the distribution of species (monomer, dimer, polymer, and bulk oxides) they possess, often under the same conditions, as well as their sensitivity to the chemical environment surrounding the active center. 51V NMR is extensively used to understand the structure of vanadia-based catalysts under different environmental conditions. Dehydrated structures are analyzed for silica, titania, and titania/silica supports where the active structures for emissions controls applications are proposed. Vanadia materials under hydrated conditions are also considered where dramatic changes in the surface species towards both less support-coordinated structures and oligomers are present. Dry materials which have previously experienced harsh hydrothermal treatment or have undergone reaction cycles are shown to redisperse on the surface, including dispersion of the bulk V2O5 oxide phase after a single redox cycle of ethanol oxidative dehydrogenation. Finally, the interactions of water and other small molecules with the acid sites of MFI zeolite under strictly controlled environments are explored by monitoring both the interacting chemical constituent and the active site of the oxide. In each case, a firm control over the environment of the materials has enabled the observation of the catalysts under relevant conditions to better understand the nature of the active species. These studies represent a foundation for the wealth of information in situ NMR is capable of providing to the understanding of catalytic systems.