(307d) Free Energies of Adsorption Calculated through Potential of Mean Force Ab Initio Molecular Dynamics in Metal-Exchanged Zeolites
The reactions studied are the adsorption of small molecules (X) on the zeolite Cu-SSZ-13. We consider several small molecules, such as NH3 and H2O, chemically adsorbing to exchanged metal cation active sites (*) as represented by equation * + X - > *X. Ab initio molecular dynamics (AIMD) simulations are performed on zeolite-adsorbate complexes to explore their finite temperature behavior. Cu is mobilized within the zeolite after adsorption, resulting in a vibrational spectrum that significantly deviates from that obtained using HA.
The potential of mean force (PMF) method is thus implemented which includes anharmonic entropy in the AIMD simulations. In the PMF simulations one selected reaction coordinate (RC) is constrained. â??Gads is then calculated by integration of the average constraint force over the range of RCÂ from the adsorbed state to the state where no interaction exists between the adsorbate and Cu site. Metadynamics is also used to explore the free energy of the system where Gaussian potential hills are added to the free energy surface. After both the product and reactant wells are sampled the added Gaussian hills are used to reconstruct the real free energy landscape. These two methods give the same â??Gads, which are significantly more exergonic than those obtained by HA. Lastly we propose an empirical correlation between adsorbate gas phase entropies and adsorption entropies backed out from â??Gads=â??Hads-Tâ??Sads. This allows a quick estimation of â??Gads using only reaction energies from static DFT calculations and gas phase entropies of the adsorbates from statistical mechanical calculations.
Â  C. Paolucci, A. A. Parekh, I. Khurana, J. R. Di Iorio, H. Li, J. D. Albarracin Caballero, A. Shih, T. Anggara, W. N. Delgass, J. T. Miller, F. H. Ribeiro, R. Gounder and W. F. Schneider, Journal of the American Chemical Society, (2016) DOI: 10.1021/jacs.6b02651.
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