The effect of Si/Al ratio of zeolite supported Pd for complete CH4 oxidation in the presence of water vapor and SO2

Abstract

The catalytic properties of palladium supported on H-beta and H-SSZ-13 zeolite with different silica to alumina ratio (SAR) have been evaluated for complete CH4 oxidation in the presence and absence of water vapor and together with SO2. Different SAR was successfully obtained by dealumination of the zeolites in an acidic solution at elevated temperature. Flow reactor experiments showed that the SAR of the zeolite support greatly impacts the catalytic activity, especially in the presence of water vapor. Pd supported on a highly siliceous beta zeolite expressed high and stable CH4 conversion in the presence of water vapor, whereas the activity for Pd supported on zeolites with low SAR or Al2O3 decreased over time due to accumulative water deactivation. Hence, an increased SAR of the zeolite support clearly correlates to a lower degree of water deactivation. We suggest that this is a result of the high hydrophobicity of the siliceous zeolites. The results also imply that the catalytic activity in the presence of water vapor is influenced more by the SAR than the type of the zeolite framework. The CH4 oxidation activity was also enhanced with increasing SAR under dry conditions. This was addressed to the formation of more Pd particles in relation to ion-exchanged Pd2+ species and changes of the oxidation-reduction behavior of the Pd. The high number of acidic sites in zeolites with low SAR provided higher dispersion of Pd particles and formation of more monoatomic Pd2+ species, whereas almost exclusively Pd particles of larger sizes were formed on the highly siliceous zeolites. The monoatomic Pd2+ species, mostly formed on zeolites with low SAR, were oxidized and reduced at significantly higher temperatures than Pd in the particle form. Hence, complete reduction or oxidation of the Pd supported on highly siliceous zeolites can be achieved at lower temperatures. Moreover, compared to Pd/Al2O3, the zeolite supported Pd expressed higher sensitivity to SO2. However, the major part of the catalytic activity could be regenerated more easily using siliceous zeolites as supports compared to Al2O3. We suggest that this is an effect of the lower SO2 adsorption on the zeolite supports than on the Al2O3 support, which results in the formation of more PdSOx species upon SO2 exposure. On the other hand, the low SO2 adsorption on the zeolites also results in less spillover of sulfur species from the support to the active PdO, which explains the facilitated regeneration.