BIFUNCTIONALITY OF PALLADIUM-BASED CATALYSTS USED IN THE REDUCTION OFNITRIC-OXIDE BY METHANE IN THE PRESENCE OF OXYGEN

We have studied the selective catalytic reduction (SCR) of nitric oxid e by methane over palladium on several acidic and non-acidic supports in the presence of excess oxygen, We have found that the acidity of th e support promotes the nitric oxide conversion. Both palladium and aci d sites are necessary to achieve high catalytic activity. On Pd/H-ZSh4 -5 catalysts, the level of palladium loading is important. The activit y and selectivity for nitric oxide reduction pass through a maximum wi th increasing Pd wt.-%. On low palladium loading catalysts, the activi ty and selectivity also reach a maximum with increasing oxygen concent ration. The H-ZSM-5 support was not unique in promoting the activity o f palladium, but it was the most effective among the acidic supports i nvestigated when palladium was directly supported on it. Enhancements in activity and selectivity were also observed when acidic materials w ere 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 bifunct ional mechanism previously proposed for the nitric oxide reduction on Ga and In/H-ZSM-5 catalysts.