Mechanisms and site requirements for NO and NH3 oxidation on Cu/SSZ-13

Abstract

Two series of Cu/SSZ-13 catalysts were synthesized via aqueous solution and solid-state ion exchange using SSZ-13 supports of varying Si/Al ratios. The isolated and multinuclear Cu content of these catalysts were determined by H2 temperature programmed reduction (H2-TPR). Multinuclear Cu in these catalysts, including in situ Cu-dimers formed from ZCuIIOH coupling and permanent CuO clusters, are active species for dry NO oxidation. NH3 oxidation on these catalysts follows an internal SCR (i-SCR) mechanism, i.e., a portion of NH3 is first oxidized to NO, then NO is selectively reduced by the remaining NH3 to N2. NH3 oxidation displays distinct kinetic behavior below ∼300 °C and above ∼400 °C. At low temperature the results indicate that NH3-solvated mobile Cu-ions are the active centers. CuO clusters, when present, also contribute to the low temperature activity by catalyzing NH3 oxidation to NO. At high temperature, in situ Cu-dimers and CuO clusters catalyze NH3 oxidation to NO, and isolated Cu-ions catalyze SCR to realize the cascade turnovers. For both NO and NH3 oxidation, Cu-dimers balanced by framework charges of close proximity appear to be more active than Cu-dimers balanced by distant framework charges. However, the former Cu-dimers are less stable than the latter and tend to split into monomers in the presence of vicinal Brønsted acid sites. Via density functional theory (DFT) calculations, the i-SCR mechanism for low temperature NH3 oxidation, i.e., the energetic favorability for the involvement of the NO intermediate, is justified. The DFT results also agree with experimental data that the formation of Cu-dimers from ZCuIIOH dimerization is essential for NH3 oxidation at high temperature.