Appraising Multinuclear Cu2+ Structure Formation in Cu-CHA SCR Catalysts via Low-T Dry CO Oxidation with Modulated NH3 Solvation.

Cu2+ ions (ZCu2+ (OH)- , Z2 Cu2+ ) are regarded as the NH3 -SCR (SCR=selective catalytic reduction) active site precursors of Cu-exchanged chabazite (CHA) which is among the best available catalysts for the abatement of NOx from Diesel engines. During SCR operation, copper sites undergo reduction (Reduction half-cycle, RHC: Cu2+ →Cu+ ) and oxidation (Oxidaton half-cycle, OHC: Cu+ →Cu2+ ) semi cycles, whose associated mechanisms are still debated. We recently proposed CO oxidation to CO2 as an effective method to probe the formation of multinuclear Cu2+ species as the initial low-T RHC step. NH3 pre-adsorption determined a net positive effect on the CO2 production: by solvating ZCu2+ (OH)- ions, ammonia enhances their mobility, favoring their coupling to form binuclear complexes which can catalyze the reaction. In this work, dry CO oxidation experiments, preceded by modulated NH3 feed phases, clearly showed that CO2 production enhancements are correlated with the extent of Cu2+ ion solvation by NH3 . Analogies with the SCR-RHC phase are evidenced: the NH3 -Cu2+ presence ensures the characteristic dynamics associated with a second order kinetic dependence on the oxidized Cu2+ fraction. These findings provide novel information on the NH3 role in the low-T SCR redox mechanism and on the nature of the related active catalyst sites.