Pd-based catalysts are efficient for removal of methane emissions from natural gas vehicle exhaust. However, the reactivity of these catalytic systems in methane oxidation decreases with time-on-stream in the presence of high concentration of H2O and trace amount of SO2. We have therefore examined the effect of water inhibition on Pd/Pt/Ce/Al2O3 using both ramping experiments as well as transient experiments at constant temperature. We have found that the water inhibition consists of two paths; first water adsorption on the active sites, which gives an immediate deactivation and in addition, slow deactivation due to buildup of surface hydroxyle species. These data were also used to develop a kinetic model which could describe the experiments well. Another effect that was observed was that the addition of NO to the reaction mixture resulted in an increased activity in the presence of NO and we propose that the reason for this is that the water inhibition is weakened in the presence of NO. We examined the differences in activity and water deactivation using lean, stoichiometric and rich conditions and clear differences were observed. Higher methane oxidation activity was observed for increasing oxygen concentration in dry feed, which is likely due to the facilitation of palladium oxide formation. Moreover, the observed water inhibition was significantly larger in lean conditions compared to stoichiometric conditions and we propose that the hydroxyl species formation, which blocks the active sites, are facilitated by large oxygen excess. Sulfur poisoning is another issue that deactivates these catalysts and we have studied the poisoning and regeneration of Pd containing catalysts. Interestingly, the addition of Pt to Pd/Al2O3 catalyst resulted in a catalyst that was more resistant to sulfur poisoning. For example, after 1h the CH4 conversion was lowered to 5% level but in the case of PtPd sample, the conversion reached 10% after 3 h.

Methane oxidation over Pd containing catalysts for lean and stoichiometric conditions

254th National Meeting (2017)

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