Palladium Catalyzed Methane Oxidation for Emission Control
The use of methane based fuels, such as natural gas and biogas, benefits from lower emissions of CO2, CO, NOx, non-methane HC, SOx and particulate matters compared to traditionally used liquid fossil fuels. However, the exhaust gases contain significant levels of unburnt CH4 residuals, which are desirable to minimize since CH4 is a strong greenhouse gas with high global warming potential. Methane removal can be accomplished by completely oxidize the CH4 remains to CO2 and H2O using a catalytic converter. Palladium based catalyst provides the highest activity for complete CH4 oxidation. However, the major challenges for such catalytic system are to maintain high activity at low temperatures and in the presence of water vapor and sulfur containing compounds, which severely deactivates Pd based catalysts. Since the nature of the support material, which the Pd is dispersed upon, highly influences the catalytic properties, one strategy to obtain such properties is to modify the support material with promoters and/or to use different types of support materials. Therefore, the effect of adding Ba promoter to Pd/Al2O3 and the use of zeolite supports (beta and SSZ-13) with various silica-to-alumina ratios (SAR) have been fundamentally investigated with various characterization techniques in combination with catalytic activity tests. It was found that an addition of up to 2 wt.% Ba to Pd/Al2O3 does not provide electronic promotion of the Pd; however, the addition of Ba improves the catalytic activity in the presence of water vapor due to suppressed water deactivation. Furthermore, it was also seen that the addition of Ba facilitates the regeneration of the catalytic activity after exposure to water vapor. Although no differences in the rate of water adsorption/desorption or in the type of formed hydroxyl species could be observed, it cannot be excluded that Ba influences the surface chemistry, which may be the reason for the enhanced tolerance to water vapor. Moreover, it was shown that zeolite supported Pd is a promising catalytic system for CH4 oxidation. By increasing the SAR of the zeolite supports by dealumination with oxalic acid, the CH4 oxidation activity was markedly improved under wet conditions, but also in the absence of water vapor. Indeed, Pd supported on highly siliceous zeolites showed almost no accumulative water deactivation at all, which most likely is related to the high hydrophobicity of the siliceous zeolites. The improved activity in the absence of water vapor for Pd supported on zeolites with high SAR was ascribed to the formation of more PdO as well as higher ratio of PdO particles in relation to ion-exchanged Pd2+ species. However, the zeolite supported Pd was more rapidly deactivated than Pd/Al2O3 by SO2. We suggest that this is a result of the lower sulfur adsorption on the zeolites than on the Al2O3, which results in formation of more PdSO4 species. Nevertheless, the zeolite based samples, especially those with high SAR, could be regenerated much easier than Pd/Al2O3 after SO2 poisoning. We suggest that this is related to the low sulfur storage capacity of the zeolite support, which thus results in less spill-over of sulfur species from the support to the Pd in the absence of SO2.