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

Abstract

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.

Mechanism of Standard NH3–SCR over Cu-CHA via NO+ and HONO Intermediates

“…It is well established that the standard SCR mechanism is based on the Cu II → Cu I step (RHC) and the Cu I → Cu II step (OHC). 17 In our previous DFT study, 36 we showed multistep pathway for the OHC with O 2 as the sole oxidant with DFTcalculated activation energies (E a ) of 99−110 kJ/mol (reaction 1 in Table 1). In this study, the E a values of possible reactions during the initial stage of RHC (reactions 2−11 in Table 1) were calculated and compared.…”

“…5,8−16 It is well established that the standard SCR mechanism is based on the Cu II /Cu I redox cycles; the Cu II → Cu I step so-called reduction half-cycle (RHC) and the Cu I → Cu II step so-called oxidation half-cycle (OHC). 8,17 Assuming an OH − -coordinated Cu 2+ cation at zeolite oxygen (O Z − ) as an active site of Cu-zeolite catalysts, the Cu II → Cu I step (RHC) is described as…”

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

“…56,57 More recently, mobile [Cu 2+ (OH)(NH 3 ) x ] + complexes formed by NH 3 solvation of ZCu 2+ OH ions (stabilized by one framework negative charge), have been proposed to be relevant in the reduction half-cycle, as discussed in mode detail in Section 3. 58,59 Finally, the formation and separation of Cu pairs assisted by NH 3 solvation is the core of one of the recently proposed mechanisms for NH 3 -SCR, which includes the role of Brønsted sites to facilitate the decomposition to N 2 and H 2 O of HONO and H 2 NNO intermediates formed on Cu sites (Fig. 1, right panel, part a).…”

“…As mentioned above, the role of NH 3 in solvating and mobilizing Cu + and Cu 2+ ions in the low temperature NH 3 -SCR reaction has been proposed to be pivotal, in both the oxidation and reduction half-cycles. 3,52,58,59 The ability of NH 3 to solvate the cations, replacing framework oxygen atoms (O fw ) from their rst-shell coordination sphere has been proven by X-ray absorption and emission spectroscopies (XAS and XES). 50,51 More in detail, Extended X-ray Absorption Fine structure (EXAFS) showed the disappearance of the ngerprint for framework coordinated Cu-sites (second-shell peak at $2.3 A in the phase-uncorrected modulus of FT-EXAFS spectrum) when a Cu-CHA catalysts was contacted with the NH 3 -SCR reactants below 250 C. This was accompanied by a raise in the pre-edge peak at $8982.5 eV, deriving from 1s / 4p transition in the linear Cu + (NH 3 ) 2 complex in the corresponding X-ray absorption near edge structure (XANES) spectrum, and by modications in the XES spectra compatible with the substitution of O fw ligand with N atoms from the NH 3 molecule (Fig.…”

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNOx and NH3-SCO

“…The detailed reaction mechanism is still under debate, indicating the complexity of both RHC and OHC. 157 …”