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 CuII sites, via two distinct pathways on NH3-solvated and NH3-free CuII sites. These pathways proceed with identical rates on all CuII sites, leading to no changes in reduction rates upon hydrothermal aging. Reactor data on oxidation of CuI 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 CuII 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.