(307c) Theoretical Challenges in Characterizing Cu Centers in the Zeolite SSZ-13

Understanding the true nature of active centers under realistic conditions and relate them to experimental observables is one of the major challenges in electronic structure theory today. This is especially true for characterization of heterogeneous catalysis, where a distribution of active centers is present. In this contribution we show our efforts to characterize Cu centers in the zeolite SSZ-13, a material highly efficient in cleaning car exhaust gases. It requires a combination of Density-Functional Theory (DFT), molecular dynamics simulations, thermodynamic modeling and post Hartree-Fock calculations to elucidate the true nature of the Cu-centers under different conditions.

As a first step we study vibrational spectroscopy of NO adsobed to Cu^II centers in zeolites using ab-initio molecular dynamics simulation. Already at room temperature the Cu cation changes its coordination regularly, which leads to a complex, multi-peak spectrum for all the studied sites, which are in excellent agreement with experimental measurements. Inspired by these results we investigated the mobilities of Cu in different oxidation states using ab-initio molecular dynamics simulations and found that Cu^I changes its coordination environment on a ps-timescale, while Cu^II remains fairly stable in its most preferred state. We furthermore tested the impact of this movement on UV-vis spectroscopy of Cu^I in a high Si/Al SSZ-13. Comparing theory, a combination of TDDFT and molecular dynamics simulations, and experiment for a defect-free and a normal zeolite sample reveals that upon photoexcitation the Cu-cation switches position with defect Na-atoms at finite temperature, which leads to highly different photoluminescence spectra. We combine these two ideas in studying vibrational spectra of CO adsorbed to Cu^I centers and find that again Cu and Na exchange positions. Additionally we develop a model to study the impact of water on the coordination environment and find that already at low temperatures Cu cations are fully solvated.

All these different aspects show the importance of including dynamical behavior of Cu cations in the computational models. Only when this mobility is accounted for correctly, it is possible to arrive at qualitatively and quantitatively correct predictions for different types of spectroscopies in these systems.