Abstract
Cu(2+) ions (ZCu(2+) (OH)(-) , Z(2) Cu(2+) ) are regarded as the NH(3) -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: Cu(2+) →Cu(+) ) and oxidation (Oxidaton half-cycle, OHC: Cu(+) →Cu(2+) ) semi cycles, whose associated mechanisms are still debated. We recently proposed CO oxidation to CO(2) as an effective method to probe the formation of multinuclear Cu(2+) species as the initial low-T RHC step. NH(3) pre-adsorption determined a net positive effect on the CO(2) production: by solvating ZCu(2+) (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 NH(3) feed phases, clearly showed that CO(2) production enhancements are correlated with the extent of Cu(2+) ion solvation by NH(3) . Analogies with the SCR-RHC phase are evidenced: the NH(3) -Cu(2+) presence ensures the characteristic dynamics associated with a second order kinetic dependence on the oxidized Cu(2+) fraction. These findings provide novel information on the NH(3) role in the low-T SCR redox mechanism and on the nature of the related active catalyst sites.