Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Abstract

The number and type of Cu²⁺ species present on O₂-treated Cu-ZSM5 catalysts (Si/Al = 13.1–14.6) with varying Cu/Al ratios (0.12–0.60) were measured using temperature-programmed reduction in H₂ or CO and desorption of O₂ with He as the carrier. The effluent stream was monitored using mass spectrometry and the structure and oxidation state was determined in parallel by X-ray absorption spectroscopy. Isolated Cu²⁺ monomers and oxygen-bridged Cu²⁺ dimers interacting with Al–Al next nearest neighbor pairs were the predominant Cu species on these catalysts. The fraction of Cu present as dimers increased from 0.46 to 0.78 as Cu/Al ratios increased from 0.12 to 0.60, as expected from the decreasing average Cu–Cu distance with increasing Cu content. In contrast, monomers reached a plateau of ∼0.15 Cu²⁺/Al, suggesting that only some Al–Al pairs can interact with small Cu²⁺ monomer structures, while a much larger fraction can bind with larger oxygen-bridged Cu²⁺ dimers. The measured distribution of Cu dimers and monomers is consistent with the number and bond distances of Al–Al pairs for the Si/Al ratio in these ZSM5 samples. The distributions of Cu species obtained from the amount of CO₂ formed (from CO), the amount of H₂O formed (from H₂), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O₂ was significantly smaller than that removed with H₂ or CO, suggesting that only proximate Cu dimers autoreduce via recombinative desorption steps. NO decomposition turnover rates (normalized per Cu dimer) were nearly independent of Cu content, except at the lowest Cu/Al ratio, consistent with the involvement of Cu dimers as the active Cu species in NO decomposition redox cycles on Cu-ZSM5. Multiple O₂ and CO₂ peaks during desorption and reduction in CO suggest the presence of Cu dimers with varying oxygen binding energy and reactivity. The Cu dimers initially formed at low Cu/Al contents during exchange are less reducible, consistent with their lower NO decomposition turnover rates.