Complete methane oxidation over alumina and zeolite supported palladium for emission control
The use of methane-based fuels, such as natural gas and biogas, gives lower emissions of for instance carbon dioxide (CO2) and particulate matters compared to traditionally used liquid fossil fuels. However, the exhaust gases contain significant levels of unburnt methane (CH4) residuals, which are desirable to minimize since CH4 has a high global warming potential. This can be accomplished by completely oxidizing the CH4 remains to CO2 and water using a catalytic converter. Palladium-based catalysts provide high CH4 oxidation activity, however, low temperatures and exposure to deactivating compounds, such as water vapor and sulfur dioxide (SO2), are challenging. It is however possible to optimize the catalytic properties by a careful selection of the support material which the palladium (Pd) is dispersed upon. Therefore, Pd supported on different materials, more specifically zeolites, alumina (Al2O3) and barium (Ba) promoted Al2O3, have been investigated in this thesis under various conditions using catalytic activity measurements combined with detailed characterization.
The results show that zeolite supported Pd is a promising candidate for the future CH4 oxidation catalyst. The use of zeolites with high silicon content significantly improved the CH4 oxidation activity in the presence of water vapor, which is ascribed to limited hydroxyl formation on the hydrophobic zeolite surface. In addition, the formation of ion-exchanged Pd2+ species is minimized in zeolites with low aluminum content. The formation of ion-exchanged Pd2+ species and Pd sintering appear to be important deactivation routes of Pd/zeolites, especially upon treatment at high temperatures and in the presence of water vapor. Zeolite supported Pd is also generally sensitive to SO2, however, the regeneration after SO2 poisoning is easier compared to for Pd/Al2O3. Hence, two major challenges for Pd/zeolite materials are stabilization of dispersed Pd particles and sulfur poisoning.
The catalytic properties can be altered by the addition of promoters, which was tested by adding Ba to Pd/Al2O3. It was found that a content of up to 2 wt.% Ba in Pd/Al2O3 does not provide electronic promotion of the Pd, however, the Ba addition improves the catalytic activity in the presence of water vapor and facilitates regeneration after water deactivation.
Whilst methane oxidation typically is tested under lean conditions, it was here also evaluated under stoichiometric and rich conditions for Pd/Al2O3. The presence of water vapor and SO2 caused substantial deactivation under stoichiometric conditions. Treatment of Pd/Al2O3 under rich conditions resulted in severe deactivation, due to reduction of active PdO into less active metallic Pd. Regeneration under stoichiometric conditions was difficult due to poor Pd re-oxidation.