Pd nanoparticles fixed in zeolites promote low-temperature NO reduction via hydrogen spillover

H₂-selective catalytic reduction is a promising NO_x control technology owing to its high intrinsic activity at low temperatures. We investigated catalysts composed of Pd NPs fixed in ZSM-5, which demonstrated enhanced activity for NO reduction by H₂ at T ≤ 150 ℃, achieving over 95 % NO_x conversion and N₂ selectivity at 100 ℃. This system was particularly suited for studying the influence of hydrogen spillover on NO reduction activity because the fixed Pd NPs cannot interact with NO, while ZSM-5 can mediate hydrogen spillover and activate NO away from the Pd NPs on the Brønsted acid sites. Quantifying the kinetic isotope effect, and using catalytic measurements and in-situ infrared spectroscopy, we elucidate the reaction mechanism governing the catalytic activity. Under reaction conditions (NO + O₂ flow at 100 ℃), it was found that NO is activated as nitrosyl complexes on in-situ formed Pd cations as well as charge-compensating NO⁺ species at the cation exchange sites of ZSM-5, with the latter species serving as active intermediates. At this stage, the activated NO⁺ species on Brønsted acid sites undergo selective reduction to N₂ by reacting with spillover hydrogen from Pd NPs. Furthermore, the fixed Pd structure reduces the susceptibility to O₂ chemisorption, enabling the Pd NPs to preserve their metallic state against oxidative poisoning for efficient H₂ dissociation. Our study provides insights into the characterization of zeolite-fixed metal NPs, which is distinct from conventional analyses that focus on exposed metal sites. Moreover, this study highlights the fixation of metal NPs on zeolites as an effective strategy for NO reduction by H₂.