First principles simulation of electrochemical reactions at the metal/liquid interface: Adsorption of sulfuric acid anions on platinum electrodes and the interactions between water and carbon monoxide adsorbed on platinum and platinum-ruthenium alloy

In the present research work, a computational methodology to study electrochemical reactions and adsorption processes at the metal/liquid interface has been developed and tested. It was found that finite clusters can be employed to artificially charge and discharge the metal electrode and therefore simulate an electrode potential within the half-cell approximation. With this approach, small changes in the electrode potential can be simulated allowing the study of potential dependant electrochemical processes. This method is capable of predicting accurate adsorption and activation energies for different electrocatalytic reactions.;The method was employed to study the electrochemical adsorption of sulfuric acid anions on low-index single crystal surfaces of platinum electrodes, Pt(111), Pt(110) and Pt(100). The results from the adsorption process of sulfuric acid anions on Pt(111) show that the anion adsorption begins with the coordination of bisulfate on a two-fold site at potentials near 0.30 V versus the reversible hydrogen electrode. The results also indicate that the sharp current spike on the hydrogen voltammogram of Pt(111) in sulfuric acid solution arises from a rapid conformational change---deprotonation of the adsorbed bisulfate (HSO4-) to form a stable ion pair (H 3O+-SO42-)---leading to a strong binding of the sulfate ion to a three-fold site on the Pt(111) surface in the potential range of 0.45--0.60 V versus the reversible hydrogen electrode. At potentials above ca. 0.70 V, the stable species on the platinum surface is the sulfate ion.;The adsorption process of sulfuric acid anions on Pt(110) shows a different behavior compared to that on Pt(111). On the Pt(110) surface, bisulfate and sulfate are coordinated only in a two-fold site. Similar to Pt(111), a stable ion pair (H3O+-SO42-) is formed on the Pt(110) surface. Preliminary results from the adsorption of sulfuric acid anions on Pt(100) shows that the anions are only coordinated on two-fold site. The ion pair (H3O+-SO4 2-) on the Pt(100) is not as stable as on the other Pt(111) and Pt(110) surfaces.;The computational method was also applied to study the interaction of interfacial water with adsorbed CO on the platinum and platinum-ruthenium alloy electrodes. The theoretical study reveals a new interpretation of interactions between water and CO adsorbed on platinum and platinum-ruthenium alloys under electrochemical conditions, which rationalizes the observed quantitative relationship between infrared intensities for adsorbed CO and water, exhibiting a high-frequency O-H stretch (ca. 3650--3600 cm-1). The theoretical model indicates that this observed feature is due to a water molecule firmly hydrogen-bonded to CO(bridge).