We combine gas phase Transient Response Methods with Transient Kinetic Analysis to investigate the reduction half-cycle (RHC: CuII → CuI) of the Standard SCR redox mechanism over Cu-CHA (chabazite) catalysts. The results confirm that NO + NH3 can readily reduce CuII at low temperatures (150–220 °C) according to a Cu:NO:NH3:N2 = 1:1:1:1 stoichiometry. The observed CuII reduction dynamics are invariant with the CuII speciation. Unexpectedly, the CuII reduction rates show a quadratic dependence on CuII, which is hardly compatible with the so far proposed single-site RHC mechanisms. The second order kinetics are found to apply under both dry and wet conditions (0% and 2% H2O v/v in the feed gas, respectively) across different temperatures, space velocities, and NO feed concentrations over two powdered Cu-CHA catalysts with different Cu loadings as well as over a commercial Cu-CHA washcoated honeycomb monolith catalyst. Another unprecedented finding is that H2O significantly inhibits the CuII reduction rate and lowers the RHC apparent activation energy. These findings provide for the first time a complete kinetic description of the low-temperature RHC reaction cascade and, from a mechanistic perspective, strongly suggest a dinuclear-CuII mediated RHC pathway, which may renew interrogations on the current mechanistic understanding of the CuII reduction pathway in the low-temperature NH3-SCR redox chemistry over Cu-CHA.

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As the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx, Cu-CHA has been extensively investigated in both its practical and fundamental aspects. Among the latter, how Z2Cu2+, an active site for SCR, participates in low-temperature (LT) SCR reactions remains debated. Here, we propose a scheme involving the hydrolysis of Z2Cu2+ to ZCu2+(OH)−, a thermodynamically and kinetically favorable process under LT-SCR conditions, based on multiple pieces of evidence from a probe reaction (transient CO oxidation), transient Cu2+ reduction kinetic runs, in situ FTIR spectroscopy, and first-principles calculations. Such an integrated investigation reveals unambiguously that the hydrolysis of Z2Cu2+ to ZCu2+(OH)− occurs facilely in the presence of NH3, which may thus reconcile the identical quadratic kinetics of Z2Cu2+/ZCu2+(OH)− reduction with the inactivity of Z2Cu2+ in the formation of Cu2+ pairs. Accordingly, we highlight that NH3-assisted hydrolysis plays a critical role in LT-SCR and should be taken into account especially when discussing SCR reaction details over Z2Cu2+.

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 CuII and CuI 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 CuI-diamines in activating O2 and reoxidizing CuI to CuII (oxidation half-cycle, OHC). The kinetically relevant reduction half-cycle (RHC) that reduces CuII to CuI 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.