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 i nvestigated 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 th e temperature range of 300-500 degrees C. PdO/SiO2 catalyzed the reaction b etween NO2 and CH4 in the absence of oxygen, which retarded the reaction by consuming CH4 in combustion. CH4 combustion occurred on either zeolite-sup ported or silica-supported catalyst, while NO preferentially retarded the c ombustion on Pd/H-ZSM-5. NO was found to be chemisorbed on the palladium si tes 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.