Bifunctionality of palladium-based catalysts used in the reduction of nitric oxide by methane in the presence of oxygen

We have studied the selective catalytic reduction (SCR) of nitric oxide by methane over palladium on several acidic and non-acidic supports in the presence of excess oxygen, We have found that the acidity of the support promotes the nitric oxide conversion. Both palladium and acid sites are necessary to achieve high catalytic activity. On Pd/H-ZSh4-5 catalysts, the level of palladium loading is important. The activity and selectivity for nitric oxide reduction pass through a maximum with increasing Pd wt.-%. On low palladium loading catalysts, the activity and selectivity also reach a maximum with increasing oxygen concentration. The H-ZSM-5 support was not unique in promoting the activity of palladium, but it was the most effective among the acidic supports investigated when palladium was directly supported on it. Enhancements in activity and selectivity were also observed when acidic materials were mechanically mixed with a Pd/SiO2 catalyst. In this case, the most effective material to promote the activity of palladium was sulfated zirconia. Therefore, it seems that the acidity is more important than the zeolitic structure in determining the activity of these catalysts. The observed catalytic behavior is consistent with a two-step bifunctional mechanism previously proposed for the nitric oxide reduction on Ga and In/H-ZSM-5 catalysts.