Conversion of nitric oxide and methane over Pd/ZSM-5 catalysts in the absence of oxygen

摘要

We have studied the conversion of nitric oxide and methane on several H- and Na-ZSM-5 zeolite catalysts in the absence of oxygen. Our results suggest that the NO---CH4 reaction can be explained in terms of a mechanism that starts with a nitric oxide decomposition step followed by the surface reaction of methane with the product oxygen regenerating the active site. We have found that reduced Pd/ZSM-5 catalysts are active for the nitric oxide decomposition reaction but deactivate rapidly due to self-poisoning by product oxygen. By contrast, in the presence of methane these catalysts can exhibit high activity and stability under certain conditions. For instance, when the nitric oxide decomposition and the reaction of methane with the surface oxygen proceed at comparable rates the catalyst is stable but when the methane conversion is lower than that required to remove all the oxygen produced (stoichiometric methane conversion) the catalyst rapidly deactivates. Under some conditions the methane conversion may be higher than the stoichiometric requirement leading to the deposition of carbonaceous species. These carbonaceous deposits can promote the reaction by helping to remove the product oxygen.