Active site requirements in oxidation and reduction halves of the Cu redox cycle during Standard NH3-Selective Catalytic Reduction of NOx on Cu-SSZ-13
Ishant Khurana, Purdue University
In order to meet increasingly stringent NOx (x=1,2) emissions constraints, Selective Catalytic Reduction of NOx using NH3 as a reductant (NH3-SCR) is used as one of the leading strategies for NOx abatement in oxygen-rich automotive exhaust. It was shown that low temperature (473 K) NOx SCR on Cu-SSZ-13 occurs via a Cu(II)/Cu(I) redox cycle 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 . By integrating theory and experimental approach under reaction conditions (in-situ and operando), we show that NH3 solvation of Cu(I) sites under low temperature standard SCR conditions imparts mobility to these single Cu(I) sites, enabling them to dynamically and reversibly pair and activate O2 in the oxidation half of Cu(II)/Cu(I) redox cycle. Involvement of two Cu(I)(NH3)2 sites during oxidation half of SCR justifies the commonly observed dependence of SCR on spatial density of Cu sites, also resulting in appearance of two different kinetic regimes, characterized by different apparent orders, activation energies, and steady-state Cu(I)/Cu(II) distributions. One regime corresponds to SCR cycles limited by Cu(I)->Cu(II) oxidation with O2 and the other regime corresponds to Cu(II)->Cu(I), which we probe in operando by varying reaction parameters (temperatures, gas pressures) while collecting steady-state kinetic data and XAS spectra. 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. 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 further 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.