Kinetics and thermodynamics of ammonia solvation on Z2Cu, ZCuOH and ZCu sites in Cu-SSZ-13 – Implications for hydrothermal aging

Abstract

Temperature and ammonia (NH3) pressure dependent surface coverage of Cu-amine complexes formed on Z2Cu, ZCuOH and ZCu sites in Cu-SSZ-13 are described using a kinetic model. Consistent with results from in-situ electron paramagnetic resonance (EPR) spectra upon NH3 solvation [47] and quantification from NH3 temperature programmed desorption (TPD) experiments [33], the model considers a maximum NH3/Cu ratio of 2 for Z2Cu sites, 1 for ZCuOH sites and 2 for ZCu sites. 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 [3]. These parameters are further modified to predict the rate of NH3 adsorption and desorption during TPD experiments on oxidized and reduced catalysts. Relative adsorption free energies of surface species based on equilibrium constants are consistent with the expected nature of NH3 solvated Cu sites. Relative mobility estimates based on the entropy of Cu-amine complexes are consistent with electrostatic considerations, first-principles calculations and observations from impedance spectroscopy [3,68,69]. Desorption pre-exponentials on CuII sites are validated via in-situ diffuse reflectance infrared fourier transform spectroscopic (DRIFTS) experiments during NH3 adsorption. The resulting kinetic model is incorporated into a formalism that combines all framework aluminum sites and treats mild hydrothermal aging as a reaction involving the transformation of ZCuOH sites to Z2Cu sites. NH3 storage at 200 °C on Cu sites increases upon reduction. Modifications to the NH3 coordination on Brønsted acid (ZH) sites in the reduced catalyst upon hydrothermal aging are predicted by the model and confirmed experimentally. Accurate predictions of the distribution of Z2Cu, ZCuOH and ZH sites as a function of catalyst aging, along with predictions of the extent of NH3 solvation of these sites is an important foundation to enable quantitative descriptions of the changes in the oxidation state and SCR redox reaction rates during real-world aging.