On the Redox Mechanism of Low-Temperature NH3-SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle.

Cu-CHA is the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx in vehicle applications. Although extensively studied, diverse mechanistic proposals still stand in terms of e.g., nature of active Cu-ions and reaction pathways in SCR working conditions. Here we address the redox mechanism underlying low-temperature (LT) SCR on Cu-CHA via an unprecedented integration of chemical-trapping techniques, transient-response methods, operando UV-Vis-NIR spectroscopy with modelling tools based on transient kinetic analysis and density functional theory calculations. We show that the rates of the reduction half-cycle (RHC) of LT-SCR display a quadratic dependence on Cu II , thus questioning single-site mechanisms based on isolated Cu II -ions. We propose, instead, a Cu II -pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates accounts for Cu II reduction. These results highlight for the first time the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC cascade.