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

Abstract

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu 2+ 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 NH 3 -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 (NH 3 ) 2 ] + –O 2 –[Cu I (NH 3 ) 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.