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

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 + NH3 can readily reduce CuII at low temperatures (150-220 degrees C) according to a Cu:NO:NH3: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% 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 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 NH3-SCR redox chemistry over Cu-CHA.

Automated summary

NO and NH3 can readily reduce CuII at low temperatures according to a 1:1:1:1 stoichiometry over Cu-CHA catalysts. The reduction rates of Cu-II show a quadratic dependence on Cu-II, suggesting a dinuclear-Cu-II mediated RHC pathway. Water significantly inhibits the Cu-II reduction rate in the reaction cascade.