Catalytic properties of Palladium Exchanged ZSM-5 Catalysts in the Reduction of Nitrogen Monoxide by Methane in the Presence of Oxygen: Nature of the Active sites

摘要

The catalytic activity of Pd-H-ZSM-5 catalysts containing 0.18 to 1.56 wt.-percent Pd in the reduction of NO by CH_4 in the presence of excess O_2 (lean mixture) was measured and the adsorption of NO was studied by FTIR. For all samples, NO adsorption results in the formation of a Pd~+ mononitrosyl complex (v NO = 1881 cm~(-1)) and adsorbed NO_2 (v NO = 2136 cm~(-1)) interacting with both Pd ions and acidic hydroxyl groups of the zeolite. The formation of NO_2 arises from the reduction of isolated Pd~(2+) cations and/or Pd~(2+) hydroxyl complexes bonded to the oxygen atoms of the zeolite framework into Pd~+ complexes still anchored to the framework. The amount of Pd complexes increases linearly with Pd content up to 0.6 wt.-percent Pd. The catalytic activity measurements indicate that two competitive reactions occur: the reduction of NO by CH_4 and the combustion of CH_4 by O_2. Samples containing less than 0.5 wt.-percent Pd exhibit high selectivity for NO reduction and the conversion of NO increases with the amount of Pd nitrosyl complexes detected by IR. For samples with higher Pd contents, the combustion of CH_4 is favored. The coexistence of isolated Pd complexes active for the SCR reaction and PdO aggregates active for the CH_4 combustion is suggested to explain the catalytic properties.