Transient kinetic analysis of CO oxidation over a Cu-SSZ-13 catalyst proves complete conversion of ZCu2+(OH)- cations to binuclear Cu2+ species

The nuclearity of Cu2+ species active in the low-temperature Reduction Half Cycle (RHC) of NOx Selective Catalytic Reduction with NH3 over Cu-CHA catalysts is controversial. In the past, transient CO to CO2 oxidation protocols have been used to titrate binuclear Cu2+ species, and identified NH3-solvated ZCu2+(OH)- ions as their precursors. However, the prior results relied on asymptotic extrapolation due to the very slow CO oxidation kinetics. We herein present the results from a prolonged (24 h) CO titration experiment under dry conditions over an industrial Cu-SSZ-13 catalyst: the results demonstrate the conversion of all ZCu2+(OH)- ions to binuclear Cu2+ complexes and agree well with the extrapolation of a shorter (90 min) experiment, based on the assumption of the CO oxidation rate being second order in ZCu2+(OH)-. These outcomes confirm the adequacy of short CO oxidation tests to titrate ZCu2+(OH)- ions and support a Cu2+ pair mediated RHC pathway.