Cu-promoted small pore zeolites (Cu-CHA) are the state-of-the-art catalysts for NOx emissions control from Diesel engines via Selective Catalytic Reduction by ammonia (NH3-SCR). Their superior hydrothermal stability and notable low-temperature (LT) deNOx activity make Cu-CHA catalysts both practically and fundamentally eye-catching. In this context, great attention has been paid recently to the LT NH3-SCR reaction mechanism. Despite a general consensus about the involvement of Cu in a cyclic redox mechanism (Cu2+ ↔ Cu+), the detailed catalytic scheme is still debated. It is well accepted that dimeric Cu+(NH3)2 species are implicated in the activation of gaseous-O2 molecules in the Oxidation Half Cycle (OHC: Cu+ → Cu2+) of LT-SCR, but a clear mechanistic understanding of the kinetically relevant Reduction Half Cycle (RHC: Cu2+ → Cu+) is still missing. In this respect, a key question is whether isolated Cu2+-cations or rather dimeric Cu2+ complexes are involved in RHC. Herein, we report a systematic investigation of the LT-RHC redox chemistry over Cu-CHA. At this scope we combine transient-response methods (TRM), in situ/operando spectroscopy (FTIR, UV/Vis-NIR), chemical trapping and probe reaction experiments with transient kinetic modelling and computational approaches (DFT & AIMD). First, we show that the speciation of Cu2+-sites (ZCuOH and Z2Cu) involved in RHC can be easily characterized using TRM techniques. By exposing NH3-saturated Cu-CHA to reductive NO pulses, we then observe that NH3-ligands coordinated to Cu2+-cations are preferentially consumed in LT-RHC prior to the NH4+ stored on the zeolite Brønsted acid sites. Eventually, we reveal that the rate of Cu2+-reduction exhibits a quadratic dependence on Cu2+, highlighting the role of binuclear Cu2+-NH3 complexes as the active centers for LT-RHC. According to DFT calculations, Cu2+-pairs originate from the coexistence of two mobile Cu2+(OH)(NH3)x monomers in the same cage, which are in turn generated either directly from ZCuOH, once mobilized by NH3 ligands, or via hydrolysis of NH3-ligated Z2Cu sites, as probed by transient CO oxidation experiments. Such paired Cu2+ complexes account for the NO oxidative activation to intercage mobile HONO, which concurrently reduces Cu2+ to Cu+. These findings highlight for the first time the role of binuclear Cu2+-species in the LT-RHC chemistry over Cu-CHA, which is crucial in view of developing the next generation of SCR catalysts.

Dual site redox mechanism in the reduction half cycle of low-T NH3-SCR over Cu-CHA catalysts

ACS Spring 2022

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