Operando X-ray Absorption Spectroscopy Studies of Methane Oxidation Catalysts
Operando spectroscopy is an experimental method in catalysis that combines measurements of catalytic activity and selectivity with simultaneous in situ spectroscopic characterization of the catalyst. Since the structure and composition of the catalyst are probed under reaction conditions, the method can provide valuable information to elucidate the mechanism of the studied reaction. In this thesis, operando spectroscopy has been performed using X-ray absorption fine structure (XAFS) as an in situ characterization technique to study catalysts during the total oxidation of methane. Methane is a strong greenhouse gas and catalytic oxidation can be used, for example, to remove uncombusted methane from the exhausts of vehicles fueled with natural gas or biogas.
Palladium is the most active metal for the total oxidation of methane, but the oxidation state of palladium can change rapidly during reaction conditions, which has an effect on the catalytic activity. Time-resolved XAFS was used to investigate Pd/Al2O3 catalysts under transiently changing reaction conditions for methane oxidation, by performing oxygen pulse-response experiments, so the conditions periodically change from net-reducing to net-oxidizing. Simultaneously, the outlet concentrations of reactants and products were monitored with mass spectrometry. The XAFS data show that palladium in the catalyst is readily reduced and oxidized when the feed gas composition is changed from reducing to oxidizing. The highest activity for methane oxidation is found over bulk-oxidized palladium (PdO), while surface oxidized palladium in comparison is less active. Similar experiments were also performed for a Pd/CeO2 catalyst. Compared to the alumina supported catalyst, palladium supported on CeO2 is oxidized more rapidly in an oxidizing atmosphere, and is reduced at a slower rate in a reducing atmosphere. This shows that CeO2 is able to stabilize Pd in the oxidized state which, could be beneficial for the methane oxidation activity.
Moreover, methane oxidation under cycling conditions was investigated for bimetallic Pd-Pt/Al2O3 catalysts. Pd-Pt catalysts have attracted interest due their higher long-term stability compared to Pd-only catalysts. In our studies we found that the calcination temperature during catalyst preparation is of high importance for the morphology of the catalysts. For catalysts calcined at 500°C, Pd and Pt are not alloyed but are found as separate Pd and Pt nanoparticles. However, for catalysts calcined at 800°C alloyed Pd-Pt nanoparticles are found as well as monometallic Pd nanoparticles. The methane conversion was higher over the alloyed Pd-Pt nanoparticles calcined at 800°C. In similarity to the Pd-only catalyst, high activity for methane oxidation is connected to a high amount of oxidized palladium in the catalyst. Although XAFS is only available at synchrotron light sources, the ability to investigate the active phase of catalysts with high time resolution, under reaction conditions, makes it a highly useful technique in catalysis research.