Dynamic Binuclear Cu II Sites in the Reduction Half-Cycle of Low-Temperature NH3-SCR over Cu-CHA Catalysts

As the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx from lean-burn engines, Cu-exchanged chabazite zeolite (Cu-CHA) has been a spotlight in environmental catalysis because of its preeminence in DeNOx performance and hydrothermal stability. The microscopic cycling of active Cu cations between Cu-II and Cu-I in response to dynamic, macroscopic reaction conditions dominates SCR catalysis over Cu-CHA zeolites. In such cycling, Cu cations are solvated by gas-phase reactants, e.g., NH3, under low-temperature (LT) conditions, conferring peculiar mobility to Cu-NH3 complexes and making them act as mobilized entities during LT-SCR turnovers. Such motions provide LT-SCR-a typical heterogeneous catalytic process-with homogeneous features over Cu-CHA, but, differently from conventional homogeneous catalysis, the motions are tethered by electrostatic interactions between Cu cations and conjugate Al centers. These features affect distinctly the LT-SCR redox chemistry on Cu-CHA, resulting in, for example, the involvement of two Cu-I-diamines in activating O-2 and reoxidizing Cu-I to Cu-II (oxidation half-cycle, OHC). The kinetically relevant reduction half-cycle (RHC) that reduces Cu ll to Cu l is far less understood particularly within the context of such linked homo- and heterogeneous catalysis. Here, we focus on the LT-RHC chemistry over Cu-CHA and summarize observations from a series of recent, dedicated works from our group, benchmarking these findings against those closely relevant in the literature. We thus attempt to reconcile and rationalize results informed from independent, multitechnique evidence and to further progress mechanistic insights into LT-SCR catalysis, especially in the context of dynamic interconversion between mono- and binuclear Cu sites.