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 ZCu^IIOH 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 ZCu^IIOH dimerization is essential for NH₃ oxidation at high temperature.