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

Abstract

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 5vol.% 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 10h of exposure in CH4O2H2O mixture at 450°C over the Pd/Al2O3 sample, while the decrease was only from 88% to 60% when catalyst was exposed to CH4O2H2ONO mixture. We propose that the reason is that the NO reacts with the hydroxyl species to form HNO2, which reduces the water deactivation effect.