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

Authors: 
Khurana, I. - Presenter, Purdue University
Shih, A. J., Purdue University
LI, S., University of Notre Dame
Jones, C., Purdue University
Yezerets, A., Cummins Inc.
Delgass, W. N., Purdue University
Miller, J. T., Purdue University
Schneider, W. F., University of Notre Dame
Gounder, R., Purdue University
Ribeiro, F. H., Purdue University
Selective catalytic reduction of NOx using NH3 as a reductant (NH3-SCR) is one of the most efficient emission control strategies for abatement of NOx 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)

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