Spectroscopic and kinetic investigation of the reduction and oxidation half-cycles of NOx selective catalytic reduction with ammonia on Cu-SSZ-13
Ishant Khurana, Purdue University
Selective catalytic reduction of NOx using NH3 as a reductant (NH3-SCR) is used as an emission control strategy for abatement of NOx in lean-burn and diesel exhaust. The low temperature (473 K) NOx SCR reaction on Cu-SSZ-13 occurs via a Cu(II)/Cu(I) redox cycle  on both isolated Cu2+ sites and [CuOH]+ complexes, which are solvated by NH3 during reaction .
Cu(I) oxidation during SCR involves two mobile, NH3-solvated Cu(I) sites to activate O2, but subsequent mechanistic steps to close the SCR cycle remain unresolved. To resolve these details, intrinsic contributions from oxidation and reduction half-cycles were extracted from measured rate date to interpret the origin of kinetic differences with changes in Cu ion density. Here, we perform steady state operando XAS and transient experiments to unravel additional details of the SCR mechanism. This SCR cycle enables identification of zeolite structural parameters as well as gas conditions, on measured reaction kinetics. 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 energies and Cu site pairing statistics . These intrinsic rate constants can rationalize the origin of SCR kinetic differences on Cu-SSZ-13 zeolites with changes in Cu ion density. This approach also enables estimating the maximum fraction of Cu sites that are active during steady-state SCR, for a given Cu-SSZ-13 catalyst composition (Si/Al and Cu/Al).
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