Atomic-scale Interfacial Studies of Supported Catalysts Using Synchrotron X-rays

Das, Anusheela

doi:https://doi.org/10.21985/n2-ke5h-rn56

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Atomic-scale Interfacial Studies of Supported Catalysts Using Synchrotron X-rays

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Interfacial science brings together diverse areas of interest such as electronic materials, quantum materials, bio-membranes and catalysts. In-situ X-ray characterization techniques can be used to understand the assembly of atoms, molecules and supported nanoparticles at interfaces in complex environments. This thesis work focuses on the use of various X-ray characterization techniques to study redox-induced changes at catalyst-support interface. During redox reactions catalytic systems undergo complex atomic-scale changes. Improving subsequent catalytic properties requires an understanding of these structural and chemical state changes. Vanadium and molybdenum oxides, which represent commercially important materials, form a fascinating class of compounds concerning their crystal structure as well as their electronic properties. The rich and diverse chemistry of these oxides results from the fact that the metal ions can adopt different formal oxidation states in the oxides, ranging from +2 to +5 in the case of vanadium and from +4 to +6 in the case of molybdenum. The versatility of the properties of both oxides leads to their use as active and selective catalysts in many reactions belonging to redox or acid-base processes. The primary focus of this thesis is on the use of X-ray standing wave excited X-ray photoelectron spectroscopy to follow atomic positional and chemical state changes of vanadium oxide and molybdenum oxide thin film catalyst supported on rutile TiO2 (110). This gives us 3D composite atomic maps that include the bulk substrate atoms, and surface V or Mo atoms as well as surface O atoms. These results have shown good agreement with density functional theory predictions. The thesis work also extends to X-ray absorption spectroscopy studies of powder molybdenum oxide catalysts which investigates the local coordination and bond lengths around the scattering Mo atom. All the X-ray measurements were made at the two different synchrotron sources: the Advanced Photon Source (APS) in USA and the Diamond Light Source (DLS) in UK. The XSW-XPS studies were carried out with VOx and MoOx thin films grown by atomic layer deposition on -TiO2 (110) single crystals. The in-situ X-ray studies were also supported by ex-situ atomic force microscopy and low-energy electron diffraction. Thus, the goal of this thesis work is to explore novel X-ray characterization techniques to study various catalytic systems and get a detailed understanding of the dynamic changes in the atomic scale structures of the catalyst/support interface.

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