(484d) Probing the Oxidation and Reduction Halves of the Cu Redox Cycle during Standard NH3-Selective Catalytic Reduction of NOx on Cu-SSZ-13

The selective catalytic reduction of NO_x (x=1,2) using NH₃ as a reductant (NH₃-SCR) is a leading strategy for NO_x abatement in oxygen-rich automotive exhaust to meet increasingly stringent emissions constraints. 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 two types of isolated Cu sites, both divalent Cu²⁺ cations compensating paired framework Al atoms and monovalent [CuOH]⁺ complexes at isolated framework Al atoms, which are solvated by NH₃ during reaction [2]. Two different kinetic regimes that are characterized by different apparent orders, activation energies, and steady-state Cu(I)/Cu(II) distributions in the Cu(I)/Cu(II) redox cycle have been identified under standard SCR conditions, in which O₂ is used as an oxidant. The rate-limiting steps during standard SCR depends on the Cu density (determined by the Si/Al and Cu/Al) of the Cu-SSZ-13 catalysts. One regime corresponds to SCR cycles limited by Cu(I)->Cu(II) oxidation with O₂ and the other regime corresponds to cycles limited by Cu(II)->Cu(I) reduction, which we probe in operando (varying Cu density, gas pressures and temperature) and in transient XAS experiments. O₂-oxidation during low temperature SCR requires two Cu(I) sites to be oxidized, in contrast to reduction occurring at single, isolated Cu(II) sites. Therefore, standard SCR rates depend on Cu cation density and distribution in Cu-SSZ-13 zeolites when Cu(I) oxidation steps are kinetically-relevant, but are independent of Cu density otherwise. We probe the nature of oxidation and reduction half-cycles independently by measuring their intrinsic kinetics. In-situ monitoring of Cu(I)/Cu(II) fractions under oxidation and reduction-limited conditions showed the existence of two different pools of Cu sites, those that are within diffusion distances of another Cu cation and can be oxidized with O₂ in standard SCR cycles, and others that are stranded and isolated from other Cu.

[1] C. Paolucci, A.A. Verma, S.A. Bates, V.F. Kispersky, J.T. Miller, R. Gounder, et al., Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13, (2014) 12022–12027. doi:10.1002/ange.201407030.

[2] C. Paolucci, A.A. Parekh, I. Khurana, J.R. Di Iorio, J. Albarracin, A. Shih, et al., Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations In SSZ-13 Zeolites, J. Am. Chem. Soc. (2016) 1–63. doi:10.1021/jacs.6b02651.