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Materials Science at Oregon State University

Modeling of Molecular Interactions and Chemical Reaction on Surfaces

Date: Thursday, Jan 19th
Presenter: Prof. Liney Arnadottir, OSU Materials Science and CBEE

Abstract


Adsorbed species on solid surfaces are involved in many reactions of great technological importance, especially in catalysis, electrocatalysis, separations, and fabrication of devices that require film growth (e.g., microelectronics and photovoltaics). Theoretical calculations of adsorbate-surface interactions are well established through density functional theory calculations and has been used to predict activity of new catalyst and alloys through Brønsted-Evans-Polanyi Relations based on adsorbate-surface interactions of key intermediates. These studies give great insights into the general activity of a catalyst but lacks accurate kinetic information critical for understanding of reaction kinetics and for building microkinetic models of catalytic reactions.
As such, there has been tremendous effort worldwide to learn how to predict reaction rates and equilibrium constants for elementary reactions involving adsorbates. Accurate prediction of reaction rates and equilibrium constants requires knowledge of the activation barrier for each elemental step but also the entropy of the adsorbates which is ill-defined. 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. This hindered translator and hindered rotor model accurately connects the two limiting cases for adsorbates on a surface, the 2D ideal lattice gas (harmonic oscillator) model and the 2D ideal gas (free translator) model, making it valid for all temperature ranges. To verify this model, density functional theory was used to calculate the translations and rotations of four adsorbates on a platinum surface: methanol, propane (adsorbed via middle C2 or end C1 carbon), ethane, and methane. These entropies were combined with the vibrational entropy contributions and the concentration related entropy contributions to give the total entropies of the adsorbates in good agreement with experimental results, with an average absolute value of the error of only 1.1R or 8% for the four adsorbates.

Bio:
Dr. Arnadottir received her M.Sc. and Ph.D. in chemical engineering at the University of Washington under the guidance of Dr. E.M. Stuve and Dr. H. Jonsson and a B.Sc. in Chemistry from the University of Iceland. At the University of Washington, Dr. Arnadottir combined experimental electrochemistry and theoretical chemistry to study the reaction mechanism of methanol oxidation on platinum for direct methanol fuel cell applications.
Starting in 2008, she was a post-doctoral researcher at NESAC/BIO at the University of Washington. NESAC/BIO is a state-of-the art surface analysis facility concentrating on bio-related surfaces. Under the guidance of Dr. Lara Gamble and Dr. David Castner, she used Time of Flight Secondary Ion Mass Spectrometry and X-ray spectroscopy to study protein orientation on self-assembly monolayers.
Dr. Arnadottir joined the faculty at Oregon State University as a tenure-track Assistant Professor in 2013. Dr. Arnadottir research interests include catalysis and atomic understanding of surface interactions and reaction mechanisms. The Arnadottir groups aims to combine of experimental and computational studies to gain fundamental insights into surface interactions at atomic level. Among her active projects, is a computational study of the Fischer Tropcsh reaction mechanism with the aim of finding optimal catalysts and operational conditions for improved CO utilization and narrower product distributions. Fundamental study on the use of statistical mechanics to improve computational predictions of prefactors for microkinetic models. Combined experimental-computational study of the structure of the Al2O3 – Pt interface and the initial stages of corrosion of nickel alloys in supercritical CO2 environment an density functional theory and reactive molecular dynamics studies of the role of salts in the initial states of corrosion. Dr. Arnadottir is actively collaborating on research in catalysis and reactor design, development of reaction theory, electro chemical ammonia synthesis and dimethylether fuel cell development with researchers from around the world.