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

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 NO_x 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.