Site Requirements, Kinetics, and Product Inhibition for Alkane Oxidation over Non-stoichiometric Bulk Metal Oxides

Non-stoichiometric metal oxides represent an important class of materials with industrial applications including gas sensing, energy storage, photovoltaics, and are widely used as catalysts for fossil fuel conversion. This dissertation aims to improve the mechanistic understanding between the non-stoichiometric structure of metal oxides and its catalytic function for alkane oxidation and to help guide future developments and kinetic inquiries of metal oxide catalysts. The dissertation focused on Ni1-xO, a potential ethane oxidative dehydrogenation (ODH) catalyst and the catalytic function of non-stoichiometric oxygen (NSO), a site that is associated with the nickel deficiency of the material. In Chapter 2, we studied the relationship between non-stoichiometry of Ni1-xO with the observed catalytic performance. Thermal treatment of cubic NiO under 400-1000 °C irreversibly reduced the amount of non-stoichiometric oxygen of the samples and their catalytic performance (activity and selectivity towards ethylene) in a monotonic trend, indicating that non-stoichiometry of Ni1-xO with high density of NSO is beneficial to ethylene yield, offering ways to enhance the catalyst performance through reducing the calcination temperature and increasing the oxygen pressure during catalyst synthesis. In Chapter 3, we studied the active site requirement for the selective and unselective ethane oxidation. Besides NSO, surface lattice oxygen is also proposed to be active for ethylene and CO2 formation. The existence and density of two sets of active sites were studied by CO2 adsorption due to their distinct binding characteristics at 50-300 ºC. In consistence with the selective binding between CO2 and NSO under reaction condition, cofeeding CO2 with the reactant led to incomplete titration in ethylene and CO2 formation, which confirms the activity of two active domains in both ODHE and total oxidation for the first time. Turnover frequency over non-stoichiometric oxygen was found to be ten times higher than lattice oxygen, and the former were also more selective to ODH, which confirmed the beneficial role of NSO. In Chapter 4, we provided a mechanistic understanding of product inhibitory effects of water and CO2 on ethane oxidation network. The dissociative adsorption of H2O and the molecular adsorption of CO2 on the active sites cause a strong sensitivity in rate with contact as well as with increasing CO2 and H2O cofeed pressure. Using such system as an example, a framework for the kinetic inquiry of a catalytic system with product inhibition is developed by including both integral kinetic data and differential co-feed data in the kinetic model. In Chapter 5, we quantitatively evaluated the error of the kinetic parameter estimation caused by not including product in the measurement. The results suggested that reducing the contact time used for kinetic measurement could reduce the errors but had limitation in systems exhibiting power law rate expression over extended conditions.