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

Rohil  Daya, Cummins Inc.

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 turnover rates (TORs) are isolated on individual active sites, utilizing NH₃-temperature programmed desorption (TPD) experiments on Cu-SSZ-13 and the corresponding H-form SSZ-13 catalyst. NH₃ adsorption on Brønsted 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, NH₃ storage at low temperatures (<200°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. 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 , and the site-specific storage dynamics follow reported spectroscopic characterizations. 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 TORs (mean field approximation). An Arrhenius correlation is developed for the loss of Brønsted acid sites upon hydrothermal aging, yielding a similar activation energy for the aging process as reported by us previously. 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 NO_x emissions.

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