Linking the selectivity of varying surface phases of a real palladium catalyst toward specific methane oxidation pathways with Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract: The study of heterogeneous catalysis, especially within methane oxidation, is of high importance in order to more efficiently remove unburned methane from bio-fuel exhausts. This is relevant as bio-fuel engines will replace those driven by fossil fuels in the coming decades. This thesis contributes to the understanding of that reaction in a 5 mbar environmental pressure regime over a polycrystalline, rough palladium catalyst. The work thus bridges both, the materials gap by using a real catalyst and the pressure gap by approaching industrial pressures. Various surface phase transitions between oxide, carbide, and metallic surfaces were induced by ramping the catalyst temperature between 350°C and 585°C. Meanwhile, changes in the products of the methane oxidation were observed with time-resolved Ambient Pressure X-ray Photoelectron Spectroscopy (tr-APXPS), it hence became possible to specifically relate certain surface phases to complete or partial oxidation of methane as well as carbon deposition on the surface. These results, especially the fact that carbon deposition already takes place when there is still oxide at the surface, might prove highly relevant for industrial applications. Furthermore, applying temperature ramps to enforce transitions in the catalyst's surface phase is a novelty first applied in the realm of this thesis. The successes of this technique open up possibilities for future research in catalysis. These contributions will, in the long run, help to fabricate better catalysts for bio-fuelled engines whose development will help transition to a more sustainable lifestyle.