H2O-assisted oxidation resolves the Cu-CHA redox cycle during low temperature NH3-selective catalytic reduction

Operando UV-Vis spectroscopy combined with transient kinetic analysis is used to directly quantify the kinetics of the oxidation half-cycle (OHC) in low-temperature NH₃-SCR over Cu-CHA. Operando UV-Vis measurements further provide direct evidence that H₂O promotes the OHC via a NO-independent mechanism and induces the formation of an additional stable Cu(II) species under O₂/H₂O conditions, indicating that H₂O participates in the kinetically relevant step of Cu(I) reoxidation. Dispersion-corrected DFT calculations reveal the molecular origin of this promotion: free-energy analysis predicts a H₂O-assisted dissociation of the [Cu2(NH3)4O2]2+ peroxo intermediate, enabling redistribution of oxidative equivalents required to complete Cu(I) oxidation via a H₂O₂-mediated pathway. This mechanism provides a molecular-level explanation for the experimentally observed promoting effect of H₂O on the OHC. The calculated activation parameters are in excellent agreement with experiment, establishing quantitative consistency between the proposed molecular mechanism and macroscopic kinetics. Together, these results resolve the Cu redox chemistry of low-temperature NH₃-SCR by elucidating the transformation from the peroxo Cu(II) intermediate to the linear diamine Cu(I) species while satisfying the experimentally observed stoichiometry, which involves one NO converted per Cu equivalent in each redox cycle. Overall, this work provides a kinetically-consistent closure of the NH₃-SCR catalytic cycle by integrating the oxidation and reduction half-cycles into a unified redox mechanism that identifies H₂O as a key mediator of Cu redox dynamics.