Cooperative Roles of Cu Ions and Brønsted Acid Sites in NOx Selective Catalytic Reduction by Cu-Chabazite Catalysts

The standard NH3 selective catalytic reduction (SCR) of NOx over Cu-exchanged chabazite (Cu-CHA) zeolites is widely deployed in heavy-duty diesel aftertreatment yet remains mechanistically unresolved, particularly regarding closure of the Cu redox cycle and the role of framework Brønsted acid sites. Here, we combine density functional theory, ab initio molecular dynamics, and metadynamics simulations to develop a molecularly detailed mechanism for the low-temperature (<523 K) standard SCR reaction. Cu2+-nitrite species, ZCu2+(NO2)-(NH3)3, formed via NO activation over O2-bridged binuclear Cu dimers, are proposed as key intermediates in the reduction half-cycle whose fate depends on the local framework Al environment: in the absence of proximal NH3-covered Brønsted acid sites, H2O promotes conversion to mononuclear ZCu2+OH(NH3)3 sites, while proximal NH3-covered Brønsted acid sites catalyze the transformation to Z2Cu2+(NH3)4. HONO is predicted to be a key intermediate in the reduction half-cycle whose decomposition to N2 and H2O is catalyzed by NH3-covered Brønsted acid sites via proton-shuttle mechanisms. Competing reduction pathways of mononuclear ZCu2+OH(NH3)3 sites, including direct NH3/NO coupling and HONO-mediated routes, have comparable apparent activation energies and are therefore kinetically complementary rather than mutually exclusive. The proposed catalytic cycle offers a resolution to the debate regarding single- versus dual-site Cu2+ reduction half-cycle mechanisms. The standard SCR redox cycle is predicted to be a cooperative, composition-dependent process governed by an interplay between Cu speciation and framework Brønsted acid sites. This composition-dependent nature calls for a new generation of kinetic models that explicitly account for the distribution of local cage environments across practical catalyst samples. Consequently, no single universal catalytic cycle applies across all catalyst compositions, implying control of the microscopic environment as a lever for optimization of Cu-CHA catalysts.