AVS 63rd International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS1+AS+HC+NS-TuM

Paper SS1+AS+HC+NS-TuM11
Hindered Translator and Hindered Rotor Models for Calculating the Entropy of Adsorbed Species

Tuesday, November 8, 2016, 11:20 am, Room 104D

Session: Surface Dynamics, Non-Adiabaticity, and Theory and Modeling of Surface and Interfacial Phenomena
Presenter: Lynza H. Sprowl, Oregon State University
Authors: L.H. Sprowl, Oregon State University
C.T. Campbell, University of Washington
L. Arnadottir, Oregon State University
Correspondent: Click to Email
Adsorbed species on surfaces are important for a range of applications including heterogeneous catalysis, corrosion processes, and film growth. The need for a fast and accurate way to predict equilibrium constants and rate constants for surface reactions is important for understanding reaction kinetics and for building microkinetic models of catalytic reactions. Here a method to calculate partition functions and entropy of adsorbed species is presented. Instead of using the vibrational frequencies estimated from density functional theory and the harmonic oscillator approximation to calculate the partition function for all modes of motion, we use hindered translator and hindered rotor models for the three modes of motion parallel to the surface, two translations and one rotation. The energy barriers for translation and rotation were determined using density functional theory and the nudged elastic band method for four different adsorbates on a platinum surface: methanol, propane, ethane, and methane. The hindered translator model was used to calculate the entropy contributions from the two translations parallel to the surface and the hindered rotor model was used to calculate the entropy contribution from the rotation about the axis perpendicular to the surface. When combined with the vibrational entropy contributions and the concentration related entropy contributions, this gives the total entropy of the adsorbate on the surface. The total adsorbate entropies were found to agree well with experimental results, with an average absolute value of the error of only 1.1R or 8% for the four adsorbates. This new model should be useful to future researchers in surface chemistry, since it provides more accurate predictions of standard-state entropies and partition functions, and thus more accurate equilibrium constants and rate constants for surface reactions than provided by the standard harmonic oscillator approximation.