Development and Application of DFT Based Methods for Studying Transition Metal Oxide Catalysis
Creators: Jiang, Shang
Publisher: name: University of Alberta Library
Year: 2022

<p>Transition metal oxides (TMOs) are commonly used as catalysts and catalyst supports in a variety of chemical transformations. Computational tools like density functional theory (DFT) are often used to study TMO catalyzed reactions, as TMOs are strongly correlated systems which require the additional Hubbard U-correction to reduce the systematic error caused by excessive electron delocalization in DFT. However, commonly reported U values are optimized to reproduce bulk properties and fail to predict surface properties like surface-adsorbate interactions and catalytic reaction energetics. Meanwhile, experimental surface characterization techniques like X-ray Photoelectron Spectroscopy (XPS) have difficulties identifying key surface adsorbates or reaction intermediates corresponding to the XPS shifts observed. A synergistic application of XPS and DFT+U can be used to determine the surface specific U values, as well as unidentified adsorbed surface moieties on the TMO surface. NiO and Co3O4 were chosen as sample TMOs in this thesis. We use previously published experimental XPS shifts data to gauge the DFT+U calculated core level binding energy shifts with U values ranging from 0-6eV, for clean catalyst surface or probable adsorbates with potential surface vacancies. For NiO, the U value ~2eV is able to reproduce the experimental XPS O1s core level binding energy correctly and this surface specific U value of 2eV also helps in assigning the experimental observed shifts to adsorbed oxygen (+1.8 and +2.2eV), surface lattice oxygen on which hydrogen is dissociative adsorbed (+2.2eV) and the oxygen connected with hydrogen and carbon in adsorbed HCO2 closed to a Ni Vacancy site on the NiO surface. As for Co3O4, despite of the disagreement on the bulk property optimized U value, ~3eV of U value could successfully predict the experimentally observed XPS shifts, and these shifts are also identified to be surface lattice oxygen on which hydrogen is dissociative adsorbed, the oxygen connected with carbon and surface cobalt in adsorbed HCO3 for +1.5eV and the oxygen connected with carbon and surface cobalt in adsorbed HCO2 for +2.6eV. Then, we demonstrated the application of surface property optimized U value to elucidate demethylenation of benzyl alcohol to phenol on CuO surface in water and at room temperature. The surface U value of CuO, 4eV is benchmarked by the same method and is able to accurately capture the adsorption free energy of hydrogen on CuO(111) surface. Using a combination of catalyst characterization, chemical and acoustic analysis, isotope labelling and density functional theory computations, we reveal the molecular reaction mechanism, involving benzaldehyde as an intermediate. Water is not just a benign solvation medium but it directly participates in the chemistry by getting dissociated due to ultrasound and the OH from water gets adsorbed on the catalyst surface, inhibiting its recombination. The surface adsorbed OH from water plays a key role in activating the C-H bond in benzyl alcohol to form benzaldehyde and later incorporates itself into the phenyl ring to form phenol.</p>