Influence of total pressure on complete oxidation of methane over Pd/Al2O3 catalysts

Sammanfattning: Natural gas and biogas, which have methane as their main component, are interesting choices of fuel to reduce the anthropogenic emissions of greenhouse gases. However, as methane is a greenhouse gas, uncombusted methane should be removed from the combustion gases. Emission control catalysts can preferably be used to completely oxidise methane. This thesis aims to examine whether an increased total pressure can be utilized to enhance the methane oxidation reaction over Pd/Al2O3 catalysts. The effects of total pressure are studied by flow-reactor experiments and simulations. The prepared catalyst samples are characterised by N2-physisorption, CO-chemisorption and diffusive reflectance infrared Fourier transform spectroscopy. A multiscale model is developed to simulate the activity of methane oxidation over Pd/Al2O3 where the reaction kinetics are based on first-principles calculations. The results show that the oxidation of methane can be enhanced when the total pressure is increased above atmospheric pressure. However, the effect depends on the gas composition and reaction temperature. In a dry and oxygen rich feed gas composition, the activity benefits from an increased total pressure over the entire examined temperature range. The positive effect is attributed to a high fraction of available under-coordinated palladium and oxygen sites, which can dissociate the increased concentration of methane. When water or carbon dioxide is present in the feed gas these molecules adsorb on the under-coordinated palladium sites and through surface reactions block the palladium atom as adsorbed water, hydroxyl species and bicarbonate. The coverage of hindering species requires a higher temperature to regain available palladium and oxygen sites and the positive total pressure dependence on the oxidation of methane. If the temperature is too low, the simulations predict a negative effect of increased total pressure on the reaction. The multiscale simulations capture the experimental trends and indicate that support effects should be incorporated to the model for a more complete reaction mechanism.