Local Bonding Effects in the Oxidation of CO on Oxygen-Covered Au(111) from Ab Initio Molecular Dynamics Simulations.

Abstract

A fully dynamical approach using ab initio molecular dynamics (AIMD) simulations is applied to the investigation of CO oxidation on O-covered Au(111). We investigate how the activity of gold depends upon temperature, oxygen coverage, and surface structure. On clean Au(111) at 500 K, CO binds transiently on top of Au atoms, spending a small fraction (∼7%) of the total simulation time adsorbed on the surface. The presence of O on the surface increases the residence time for CO by more than 4 times on a surface containing 0.22 ML of O. On the other hand, the probability for CO adsorption decreases with oxygen coverage from 31% at 0.22 ML of oxygen to 15% at 0.55 ML of oxygen. Our simulations show that the activity for CO reaction with O to yield CO2 decreases with increasing oxygen coverage. We attribute this decrease of activity to (1) the decrease in the CO adsorption probability as the oxygen coverage increases and (2) the decreasing amount of reactive chemisorbed oxygen (oxygen bound in a 3-fold site) with increasing total oxygen coverage. We show that oxygen bound in sites of local 3-fold coordination (chemisorbed oxygen) is nearly 2 times more reactive than other oxygen species observed on the surface, namely, surface and subsurface oxide. Our work demonstrates the value and feasibility of using AIMD to study surface reactions.