Chemical reactions on surfaces play a critical role in many technologies and industrial processes such a catalysis, batteries, fuel cells and pollution cleanup. Detailed mechanistic and kinetic understanding of the surface chemical reactions is critical to further optimize and design new more efficient technologies. Microkinetic models that can accurately predict catalytic reaction rates require estimation of the equilibrium and rate constants for different elementary reactions steps involving adsorbed species.Rate constant for surface reactions are often estimated using DFT combined with saddle-point finding methods (e.g., NEB) and harmonic transition state theory (HTST), which assumes that all adsorbates are localized harmonic oscillators with only vibrational modes.  We show here that this harmonic oscillator approximation significantly underestimates the entropies of many adsorbed species, which were recently found experimentally  to be almost 2/3 of the entropy of their gas-phase analogues, due to large contributions from nearly free translations and rotations of the adsorbates parallel to the surface.1  Knowing these entropies is necessary for accurately estimating the pre-exponential factor in their rate constantsHere we combine DFT calculations and the Hindered Rotor Density of States Interpolation Function of McClurg, Flagan and Goddard2 to predict the entropy associated with rotations and translation of adsorbates parallel to the surface for a range of temperatures. This allows us to more accurately estimate the partition functions, entropies and kinetic prefactors for adsorbates at catalytically relevant temperatures, in better agreement with experimental data.1 Campbell, C. T.; Sellers, J. R. V., The Entropies of Adsorbed Molecules. Journal of the American Chemical Society 2012,  134, 43, 18109-18115(JACS 2013 , 135, 37, 13998).2 McClurg, R. B.; Flagan, R. C.; Goddard, W. A., The hindered rotor density-of-states interpolation function. Journal of Chemical Physics 1997,  106, 6675-6680.

Calculations of rate constant for surface reactions using density functional theory

248th National Meeting (2014)

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