Dissociative Adsorption of O2 on Clean and CO-Precovered Pt Surfaces
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1084-S05-01
Dissociative Adsorption of O2 on Clean and CO-Precovered Pt Surfaces Bin Shan, Ligen Wang, Jangsuk Hyun, Yang Sang, Yujun Zhao, and John B Nicholas Nanostellar Inc, 3696 Haven Ave, Redwood City, CA, 94063 ABSTRACT It is generally accepted that CO oxidation on transition metals follows a LangmuirHinshelwood mechanism. The oxidation reaction takes place in two sequential steps where the oxygen molecule first dissociates into atomic oxygen and then reacts with an adsorbed CO to form CO2. One critical question concerning the reaction kinetics under high pressure is the probability of oxygen dissociation on a highly CO covered surface. On bare transition metal surfaces, molecularly adsorbed oxygen readily dissociates with little or no apparent activation barrier. In industrial diesel engine catalysis, the metal surface is initially packed with CO. Subsequent reactions such as oxygen dissociation must take place on a CO covered surface. In this paper, we performed density functional theory (DFT) calculations for O2 dissociation on Pt(111) in the presence of different CO adsorption environments. While several stable O2 molecular precursor states (top-bridge-top, top-fcc-bridge, and top-hcp-bridge) exist on a clean Pt(111) surface, these precursors become endothermic beyond a critical CO coverage of ~0.44 ML. Furthermore, the reaction path for CO oxidation via dissociated atomic oxygen becomes less favorable at higher CO coverage, primarily due to competitive adsorption and lateral repulsion. It was found that the oxygen dissociation barrier and the binding energies of atomic oxygen are linearly correlated.
INTRODUCTION The adsorption and dissociation of oxygen molecules plays an important role in the CO oxidation reaction on transition metal surfaces [1]. Even though oxygen dissociation on bare platinum metal surface proceeds with no apparent activation barrier, the situation is less clear on a CO covered surface [2]. There are many experimental and theoretical studies of oxygen dissociation reactions on platinum, where both molecular and atomic oxygen adsorption states have been observed [refs]. In order to gain insight into the effects of surrounding CO molecules on the oxygen dissociation path on Pt(111), we have performed density functional theory (DFT) calculations [3] with the nudged elastic band (NEB) method to locate the transition states [4]. Based on our simulations, we propose that there is a general repulsive effect of adsorbed CO molecules on both the oxygen precursor state and the dissociated atomic oxygen state. The effective result of the CO presence thus makes the overall oxygen dissociation route energetically less favorable. Our calculations offer a new perspective on the CO oxidation reaction path; for a Pt(111) surface packed with CO molecules, oxygen adsorption and dissociation only takes place in regions where the local CO concentration is relatively low. Some alternative mechanism might be responsible for CO oxidation under very high CO coverage. These findings give useful insight into
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