Kinetic model for NH3 solvation, reduction and re-oxidation of active Copper sites in Cu-SSZ-13

Rohil  Daya, Cummins Inc.

In this work, we report on a kinetic model that describes the NH₃ solvation, reduction and re-oxidation of active Copper sites in Cu-SSZ-13 under SCR conditions. Experimentally determined Z₂Cu and ZCuOH number densities are provided as initial inputs for the kinetic model, along with density functional theory (DFT) estimated NH₃ binding energies. These parameters are further modified to predict the rate of NH₃ adsorption and desorption during TPD experiments on oxidized and reduced catalysts. Relative adsorption strengths and mobilities of surface species based on equilibrium constants and entropic penalties are consistent with the electrostatic considerations, first-principles calculations and observations from impedance spectroscopy. The resulting kinetic model is incorporated into a formalism that combines all framework aluminum sites (i.e. Z₂Cu, ZCuOH, ZH and ZCu) and treats mild hydrothermal aging as a reaction involving the transformation of ZCuOH sites to Z₂Cu sites.

NO + NH₃ titration data on NH₃-free and NH₃-saturated catalysts is utilized to estimate the rate parameters associated with the reduction of Cu^II sites, via two distinct pathways on NH₃-solvated and NH₃-free Cu^II sites. These pathways proceed with identical rates on all Cu^II sites, leading to no changes in reduction rates upon hydrothermal aging. Reactor data on oxidation of Cu^I sites under exposure to O₂, NO + O₂ and NO + NH₃ + O₂ is utilized to develop a comprehensive kinetic model of the oxidation half cycle (OHC), including the formation of dimeric Cu^II complexes at low temperatures. Enthalpic and entropic changes associated with activation of O₂ on pairs of ZCu(NH₃)₂ complexes are consistent with experimental data and first-principles calculations. Both OHC and RHC (reduction half cycle) models are validated extensively as a function of hydrothermal aging, feed concentration and temperature. Furthermore, spatially resolved N₂ measurements on the degreened catalyst are utilized to consolidate the proposed kinetic model.

Accurate predictions of the distribution of active sites as a function of catalyst aging, along with predictions of the evolution of these sites under reduction and oxidation limited conditions is an important foundation to enable quantitative descriptions of the changes in the oxidation state and SCR redox reaction rates during real-world aging.

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