Palladium species in Pd/H-ZSM-5 zeolite catalysts for CH4-SCR

The active state of palladium for NO reduction with methane (CH₄-SCR) was investigated by comparing the catalytic activity of Pd/H-ZSM-5 with that of PdO/SiO₂. High catalytic activity for CH₄-SCR was given by Pd/H-ZSM-5 in the temperature range of 300–500 °C. PdO/SiO₂ catalyzed the reaction between NO₂ and CH₄ in the absence of oxygen, which retarded the reaction by consuming CH₄ in combustion. CH₄ combustion occurred on either zeolite-supported or silica-supported catalyst, while NO preferentially retarded the combustion on Pd/H-ZSM-5. NO was found to be chemisorbed on the palladium sites in zeolite, while it was hardly chemisorbed on PdO/SiO₂. NaCl titration showed that the palladium species in zeolite are Pd²⁺ cations content, on which NO is strongly chemisorbed resulting in high selectivity for CH₄-SCR.