Methane oxidation over palladium oxide. From electronic structure to catalytic conversion
Abstract: Understanding how catalysts work down to the atomic level can provide ways to improve chemical processes on which our contemporary economy is heavily reliant. The oxidation of methane is one such example, which is important from an environmental point of view. Methane is a potent greenhouse gas and natural and biogas vehicles need efficient catalysts to prevent slip of uncombusted fuel into the exhaust. Commercial catalysts for methane oxidation are often based on palladium or platinum. Metallic palladium, however, is easily converted to palladium oxide when the engine is operated at oxygen rich conditions. In this thesis, various aspects of complete methane oxidation over PdO(101) are investigated with computational methods based on density functional theory (DFT). PdO(101) is the active surface for methane oxidation, and firstly, the reaction intermediates CO and H are studied in detail. Possible pathways for H2 adsorption, dissociation and eventual water formation are investigated, in connection to core-level spectroscopy experiments. Similarly, the adsorption configurations for carbon monoxide on clean and oxidized palladium are examined with a combination of DFT calculations, core-level and infrared spectroscopy. Secondly, a detailed kinetic model is constructed that describes the catalytic conversion of CH4 to CO2 and H2O over PdO(101). This is done in a first-principles microkinetics framework, where the kinetic parameters are obtained by applying density functional and transition state theory. The kinetic model provides a fundamental understanding of findings from reactor experiments, such as the rate limiting steps and poisoning behaviour, and shows qualitatively different behaviour of adsorbates on oxide as compared to metal surfaces. Lastly, limitations of the commonly used class of generalized gradient functionals are illustrated in the computation of several properties of adsorbates on metal oxide surfaces. These include core-level shifts and thermodynamic and reactive properties of adsorbates on the PdO(101) surface. Similarly, the description of several molecular and cooperative adsorption processes are also found to be sensitive to the applied exchange-correlation functional on the BaO(100), TiO2(110) and CeO2(111) surfaces.
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