On kinetic modeling of change in active sites upon hydrothermal aging of Cu-SSZ-13

Dynamic changes in the state of a commercial Cu-SSZ-13 catalyst as a function of hydrothermal aging are explained through a unified and quantitative theoretical model. NH3 adsorption and desorption rate constants are identified on individual active sites, utilizing NH3-temperature programmed desorption (TPD) experiments on a commercial Cu-SSZ-13 and a model H-SSZ-13 catalyst. NH3 adsorption on 81-misted acid sites is described by a Type-II BET isotherm model to account for NH3 hydrogen bonded to a tetrahedral NH4+ ion. NH3 adsorption on different types of Copper sites is modeled with identical energetics, utilizing a Temkin isotherm model to account for minor site heterogeneity and lateral interactions between adsorbates. In the model, NH 3 storage at low temperatures (< 200 degrees C) is captured by a Physisorbed site to account for NH3 bound to extra-framework Al species and additional low temperature adsorption on Copper sites and Bronsted acid sites. The adsorption enthalpies and entropic penalties on individual sites in the kinetic model are consistent with the binding energies and entropies reported from first principles density functional theory (DFT) calculations by Paolucci et al., 2016, and the site-specific storage dynamics follow reported spectroscopic characterizations (Giordanino et al., 2014). Changes in NH3 -temperature programmed desorption (TPD) peaks are then used as a probe to identify the transformation of individual active sites as a function of hydrothermal aging time and temperature, assuming fixed site-specific turnover rates (mean field approximation). An Arrhenius correlation is developed for the loss of Bronsted acid sites upon hydrothermal aging, yielding a similar activation energy for the aging process as reported by Luo et al., 2018. The quantification of different types of Copper sites is hypothesized, and the limitations of the mean field approximation at extreme aging temperatures are discussed. The systematic quantification of active sites as a function of hydrothermal age provides a foundation for improved understanding and modeling of the SCR reaction mechanism, and serves as a guide to better catalyst design for stricter durability requirements and lower NOx emissions.