Transient Kinetic Analysis of Low-Temperature NH 3 -SCR over Cu-CHA Catalysts Reveals a Quadratic Dependence of Cu Reduction Rates on Cu II

We combine gas phase Transient Response Methods with Transient Kinetic Analysis to investigate the reduction half-cycle (RHC: Cu II → Cu I ) of the Standard SCR redox mechanism over Cu-CHA (chabazite) catalysts. The results confirm that NO + NH 3 can readily reduce Cu II at low temperatures (150–220 °C) according to a Cu:NO:NH 3:N 2 = 1:1:1:1 stoichiometry. The observed Cu II reduction dynamics are invariant with the Cu II speciation. Unexpectedly, the Cu II reduction rates show a quadratic dependence on Cu II, 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% H 2 O 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 H 2 O significantly inhibits the Cu II 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-Cu II mediated RHC pathway, which may renew interrogations on the current mechanistic understanding of the Cu II reduction pathway in the low-temperature NH 3 -SCR redox chemistry over Cu-CHA.