Investigation of Low-Temperature OHC and RHC in NH₃-SCR over Cu-CHA Catalysts: Effects of H₂O and SAR

A transient kinetic approach was applied to independently investigate the oxidation half-cycle (OHC) and reduction half-cycle (RHC) of NH3-selective catalytic reduction (SCR) at low temperature (150-200 degrees C). Three model Cu-exchanged chabazite (Cu-CHA) samples with fixed Cu loading (similar to 1.8% w/w) and different silica-to-alumina ratios (SARs = 10-17-25) were investigated under dry and wet conditions. We confirmed the following: (i) OHC proceeds via second- and first-order kinetics in Cu-I and O-2, respectively, with O-2 (+H2O) alone able to completely reoxidize Cu-I sites; (ii) RHC proceeds via second- and first-order kinetics in Cu-II and NO, respectively, according to a Cu/NO = 1:1 stoichiometry. Notably, coupling RHC and OHC rates resulted in an accurate prediction of the steady-state standard SCR conditions, providing the consistent closure of the SCR redox chemistry. Unexpectedly, we revealed the impact of H2O to vary depending on the catalyst formulation. At high SAR, water inhibits the RHC and promotes the OHC. As a result, a limited impact was observed on steady-state deNO(x) activity, while the Cu-oxidation state was significantly enhanced by H2O. With decreasing SAR, however, the H2O effect on RHC gradually shifts from inhibition to promotion, while the OHC is always promoted. At fixed water content, we revealed the RHC rate to decrease with decreasing Al density, with minor influence observed on the OHC; as a result, a lower deNO(x) activity was observed upon increasing SAR. Remarkably, the application of transient kinetic analysis to decouple RHC and the OHC greatly facilitated the identification of complex H2O and SAR effects on the global SCR redox chemistry.