Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

The reduction (RHC: CuII-* CuI) and oxidation (OHC: CuI-* CuII) half-cycles of low-temperature (150-200 degrees C) NH3-selective catalytic reduction (SCR) were investigated by transient response methods over a model Cu-CHA catalyst (Cu loading = 1.8% w/w; silica to alumina ratio = 25). In line with previous findings: i) RHC proceeded via second-order kinetics in CuII and with an equimolar stoichiometry between CuII reduced and NO consumed; ii) complete reoxidation of reduced NH3- solvated Cu sites was achieved by exposing the catalyst to O2 + H2O only, no other species (e.g., NO) were found to be relevant to the OHC pathway. Dedicated kinetic tests highlighted a second order dependence of the OHC rate on CuI and a first-order dependence on O2. Coupling the RHC and OHC kinetic models, independently developed from transient tests, resulted in an accurate description of Standard SCR turnovers, predicting steady-state NO conversion, N2 formation, and bed-average Cu-oxidation states, as well as light-off and extinction transients closely consistent with experimental measurements. These results demonstrate the rational dissection of the complex steady-state SCR reaction network into two redox half-cycles and provide a stoichiometrically and kinetically consistent closure of the Standard SCR redox cycle over Cu-CHA catalysts.