Theoretical study of the electronic structure and reactions involving oxygen-bridged CU-pairs in zeolite catalysts for lean nitric oxide abatement (Copper pairs)

Metal-ion-exchanged zeolite catalysts (e.g., Cu-ZSM-5) are promising candidates for lean NO_x automotive exhaust treatment. Despite extensive analysis, the relative importance of isolated metal ions and pairs of metal ions in catalytic chemistry is still unclear. In this investigation, two complementary approaches were used to address this problem: quantum-mechanics-based calculations to determine the structure and stability of paired Cu ions, and statistical modeling to determine the probability of favorable pairing sites in ZSM-5. First-principles electronic structure calculations show that both O- and O2-bridged Cu-ion pairs ([CuO_xCu]2+) are likely to be highly stable for conditions found in high-silica zeolites. Further, both types are stable over a wide range of Cu separations, and the latter exhibits a rich variety of structural isomers. These results provide insight into the role of Cu pairs in the catalytic chemistry of Cu-ZSM-5, including O₂ formation in the NO decomposition mechanism. For statistical modeling of metal ion pairs in ZSM-5, the channel structure of the zeolite was incorporated into the pairing requirements. Using a Monte Carlo procedure, the distribution of pairs as a function of distance and zeolite composition was then sampled. A significant number of the sites in the zeolite lattice were found to satisfy criteria for forming pairs. Implications of the combined results for NO decomposition and selective catalytic reduction are discussed.