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.