Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu²⁺ and [Cu^II(OH)]⁺ ions. A redox reaction mechanism has also been established, where Cu ions cycle between Cu^I and Cu^II oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (Cu^II → Cu^I) is reasonably well-understood, that for the oxidation half-cycle (Cu^I → Cu^II) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH₃-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated Cu^I ions via formation of a transient [Cu^I(NH₃)₂]⁺-O₂-[Cu^I(NH₃)₂]⁺ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics.