Determination of active palladium species in ZSM-5 zeolite for selective reduction of nitric oxide with methane

摘要

The active site in ZSM-5 zeloite-supported palladium, which shows the catalytic activity for NO reduction with methane as a reducing agent, has been investigated qualitatively and quantitatively by means of NO chemisorption and NaCl titration, comparing with PdO supported on silica. Palladium species in 0.4 wt.% Pd loaded H-ZSM-5 can adsorb NO equimolarly after calcination at 773 K , and almost all the NO was desorbed at around 673 K, while the palladium species on Pdo/SiO_2 hardly adsorbed NO. The palladium species in Pd(0.4)/H-ZSM-5 are ion-exchangeable with Na~+ in NaCl solution, indicating that they exist in a cationic state of an isolated Pd~(2+). This method for quantitative analysis of the isolated Pd~(2+) cations is named as 'NaCl titration'. The amount of the isolated Pd~(2+) cationic species increased with increasing palladium content on Pd/H-ZSM-5, and PdO co-existed above 1 wt.%. The amount of the isolated Pd~(2+) cation was unchanged after the reaction of NO_2-CH_4, NO_2-CH_4-O_2, or CH_4-O_2at 673 K, while the adsorbed amount of NO per the Pd~(2+) as determined by NO-TPD decreased after the NO_2-CH_4-O_2 reaction. It was found by NaCl titration that the catalytic activity of Pd/H-ZSM-5 for NO_2-CH_4-O_2 reaction increased with increasing amount of the isolated Pd~(2+) cationic species up to 0.7 wt.%, while the increase in the amount of PdO led to decrease in selectivity towards NO_2 reduction. The palladium species that are active and selective for NO reduction with CH_4 will be proposed.