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

The number and type of Cu2+ species present on O-2- 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 H-2 or CO and desorption of O-2 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 similar to0.15 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 H-2), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O-2 was significantly smaller than that removed with H-2 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 O-2 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.