Structure and performance of zeolite supported Pd for complete methane oxidation

The influence of zeolite support materials and their impact on CH oxidation activity was studied utilizing Pd supported on H-beta and H-SSZ-13. A correlation between CH oxidation activity, Si/Al ratio (SAR), the type of zeolite framework, reduction-oxidation behaviour, and Pd species present was found by combining catalytic activity measurements with a variety of characterization methods (operando XAS, NH -TPD, SAXS, STEM and NaCl titration). Operando XAS analysis indicated that catalysts with high CH oxidation activity experienced rapid transitions between metallic- and oxidized-Pd states when switching between rich and lean conditions. This behaviour was exhibited by catalysts with dispersed Pd particles. By contrast, the formation of ion-exchanged Pd and large Pd particles appeared to have a detrimental effect on the oxidation-reduction behaviour and the conversion of CH . The formation of ion-exchanged Pd and large Pd particles was limited by using a highly siliceous beta zeolite support with a low capacity for cation exchange. The same effect was also found using a small-pore SSZ-13 zeolite due to the lower mobility of Pd species. It was found that the zeolite support material should be carefully selected so that the well-dispersed Pd particles remain, and the formation of ion-exchanged Pd is minimized. 4 4 3 4 4 2+ 2+ 2

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

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 CH 4 oxidation in the presence and absence of water vapor and together with SO 2 . 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 CH 4 conversion in the presence of water vapor, whereas the activity for Pd supported on zeolites with low SAR or Al 2 O 3 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 CH 4 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 Pd 2+ 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 Pd 2+ species, whereas almost exclusively Pd particles of larger sizes were formed on the highly siliceous zeolites. The monoatomic Pd 2+ 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/Al 2 O 3 , the zeolite supported Pd expressed higher sensitivity to SO 2 . However, the major part of the catalytic activity could be regenerated more easily using siliceous zeolites as supports compared to Al 2 O 3 . We suggest that this is an effect of the lower SO 2 adsorption on the zeolite supports than on the Al 2 O 3 support, which results in the formation of more PdSO x species upon SO 2 exposure. On the other hand, the low SO 2 adsorption on the zeolites also results in less spillover of sulfur species from the support to the active PdO, which explains the facilitated regeneration

Complete methane oxidation over Ba modified Pd/Al2O3: The effect of water vapor

The effect on complete methane (CH 4 ) oxidation activity by an addition of up to 2 wt.% barium (Ba) promoter to alumina supported palladium (Pd/Al 2 O 3 , 2 wt.% Pd) was investigated. The catalyst samples were characterized with various techniques; temperature programmed oxidation (TPO), temperature programmed reduction with CH 4 (CH 4 -TPR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Flow reactor was used to investigate the CH 4 oxidation activity in the presence and in the absence of water vapor and also to evaluate the possibility to regenerate the catalytic activity after water vapor exposure. The results from the TPO and the CH 4 -TPR experiments together with the XPS analysis gave no evidences for electronic promotion of the catalytic activity by addition of 0.5–2 wt.% Ba. This goes in line with the CH 4 conversion in dry gas condition, which was not affected by the Ba addition. However, we observed that an addition of Ba to Pd/Al 2 O 3 enhances the CH 4 oxidation activity in the presence of water vapor, hence mitigates the effect of water deactivation. Interestingly, it was also seen that after water vapor exposure, the CH 4 oxidation activity could be regenerated to greater extent for the Ba promoted samples, particularly for the regeneration temperatures of 500–600 \ub0C. Our results clearly show that the support influences the water deactivation of the active palladium sites and that the addition of barium is beneficial for the catalyst regeneration

The influence of gas composition on Pd-based catalyst activity in methane oxidation - inhibition and promotion by NO

The individual influence, as well as the combined effect of H2O and NO on the activity of Pd/Al2O3, PtPd/Al2O3 and PtPd/CeAl2O3 catalysts in complete methane oxidation under lean conditions were investigated. Under temperature-programmed ramping experiments the activity was severely inhibited in the presence of 5 vol.% H2O in the reaction mixture. We propose that this is due to blocking by both water and hydroxyl species. Under the influence of NO without water in the gas flow, it was found that the methane oxidation activity was partly suppressed, due to blocking of active sites. Indeed TPD performed after ramping experiments showed NOx storage on the catalyst. Contrary to the negative effect of NO in the dry case, the promotional NO effect on the activity was observed when water was co-fed, comparing the case with only water presence. The promotional NO effect was confirmed with isothermal experiments, where e.g. the methane conversion decreased from initial 96% to 25% after 10 h of exposure in CH4-O-2-H2O mixture at 450 degrees C over the Pd/Al2O3 sample, while the decrease was only from 88% to 60% when catalyst was exposed to CH4-O-2-H2O-NO mixture. We propose that the reason is that the NO reacts with the hydroxyl species to form HNO2, which reduces the water deactivation effect

Deceleration of SO2 poisoning on PtPd/Al2O3 catalyst during complete methane oxidation

The inhibiting effect of SO2 on the catalytic activity of the monometallic Pt/Al2O3 and Pd/Al2O3, as well as on bimetallic PtPd/Al2O3 catalyst for the complete oxidation of methane under lean conditions has been studied. Flow reactor experiments, in-situ DRIFT spectroscopy and characterization with XPS, STEM-EDX were performed. It was found that the addition of Pt to the Pd/Al2O3 resulted in a catalyst that was more robust towards sulfur poisoning. XPS results revealed residual sulfates on catalyst surface after regeneration. This was confirmed with EDX analysis, which demonstrated that sulfur was accumulated in noble metal particles and especially in the region of the particle rich in Pt. Although the catalyst has been deactivated in the presence of both SO2 and H2O, an additional presence of NO in the gas mixture of reactants resulted in an increased lifetime of the sample under reaction conditions. This NO effect strongly depends on the temperature of experiments and is most intense at a temperature close to 550 degrees C A postponed inhibition caused by the addition of NO may be explained by the DRIFTS results, which demonstrated that the presence of NO lowers the sulfate formation and mostly surface sulfites are observed that increase the lifetime of the catalyst during SO2 exposure

An Experimental and Kinetic Modelling Study for Methane Oxidation over Pd-based Catalyst: Inhibition by Water

The water inhibition of methane oxidation over a bimetallic Pt-Pd on CeO2-Al2O3 catalyst was investigated and the experimental data were used to develop a kinetic model, consisting of only three reaction steps. In the model, the water effect was assigned to the adsorption of H2O on surface sites, as well as to the formation and accumulation of surface hydroxyl groups. These two effects were accounted by the model, which could well describe the experimental data obtained under various conditions