(380c) Kinetic Study of the Reduction and Oxidation Half-Cycles during Selective Catalytic Reduction of NOx with Ammonia on Cu-SSZ-13

Selective catalytic reduction of NO_x using NH₃ as a reductant (NH₃-SCR) is one of the most efficient emission control strategies for abatement of NO_x in lean-burn and diesel exhaust. Previously, it was shown that low temperature (473 K) NOx SCR on Cu-SSZ-13 occurs via a Cu(II)/Cu(I) redox cycle [1] on both isolated Cu²⁺ sites and [CuOH]⁺ complexes, which are solvated by NH₃ during reaction [2].

Figure 1. Low-temperature SCR Cu(II)-Cu(I) redox cycle. (adapted from [2])

We have identified two distinct kinetic regimes that are characterized by different apparent reaction orders, activation energies, and steady-state Cu(I)/Cu(II) distributions under standard SCR conditions. We provide kinetic and spectroscopic evidence that one regime corresponds to SCR redox cycles limited by Cu(I)->Cu(II) oxidation with O₂, and that the other regime corresponds to SCR redox cycles limited by Cu(II)->Cu(I) reduction. We probe these different kinetic regimes in operando by varying reactant pressures and temperatures, on Cu-SSZ-13 samples of varying Cu ion density, as well as during transient XAS experiments. The relative rates of the two half-cycles depends on the Cu ion density (which is a function of the Si/Al and Cu/Al) in the Cu-SSZ-13 catalysts, because the O₂-assisted oxidation of Cu(I) involves two Cu(I) sites, while the NO+NH₃-assisted reduction occurs at single Cu(II) sites [2]. Thus, steady-state standard SCR rates depend on Cu ion density and distribution in Cu-SSZ-13 zeolites when Cu(I) oxidation steps are kinetically-relevant, but appear independent of Cu ion density otherwise. The intrinsic kinetics of reduction and oxidation half-cycles, as a function of Cu ion density, are interpreted using theory predictions of Cu(I) diffusion energetics and Cu site pairing statistics [2]. This approach enables estimating the maximum fraction of Cu sites that are active during steady-state SCR for a fixed Cu-SSZ-13 catalyst composition (Si/Al and Cu/Al).

References:

[1] Paolucci et al., J. Am. Chem. Soc., 1-63 (2016)

[2] Paolucci et al., Science 357, 898–903 (2017)