Transient Kinetic Analysis of Low-Temperature NH₃-SCR over Cu-CHA Catalysts Reveals a Quadratic Dependence of Cu Reduction Rates on Cu^II

Abstract

We combine gas phase Transient Response Methods with Transient Kinetic Analysis to investigate the reduction half-cycle (RHC: CuII → CuI) of the Standard SCR redox mechanism over Cu-CHA (chabazite) catalysts. The results confirm that NO + NH3 can readily reduce CuII at low temperatures (150–220 °C) according to a Cu:NO:NH3:N2 = 1:1:1:1 stoichiometry. The observed CuII reduction dynamics are invariant with the CuII speciation. Unexpectedly, the CuII reduction rates show a quadratic dependence on CuII, which is hardly compatible with the so far proposed single-site RHC mechanisms. The second order kinetics are found to apply under both dry and wet conditions (0% and 2% H2O v/v in the feed gas, respectively) across different temperatures, space velocities, and NO feed concentrations over two powdered Cu-CHA catalysts with different Cu loadings as well as over a commercial Cu-CHA washcoated honeycomb monolith catalyst. Another unprecedented finding is that H2O significantly inhibits the CuII reduction rate and lowers the RHC apparent activation energy. These findings provide for the first time a complete kinetic description of the low-temperature RHC reaction cascade and, from a mechanistic perspective, strongly suggest a dinuclear-CuII mediated RHC pathway, which may renew interrogations on the current mechanistic understanding of the CuII reduction pathway in the low-temperature NH3-SCR redox chemistry over Cu-CHA.