Effect of Surface Irregularities on Water Structures at Metal Surfaces

Thomas Ludwig, Stanford University

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The presence of a solvent at a solid catalyst surface affects the chemical kinetics of catalytic reactions in primarily two ways. The first is by direct solvent-adsorbate interactions, through which the solvent stabilizes key intermediates to varying degrees. The second is by interactions with the catalyst surface, which can result in a significant change in the surface's work function. To accurately model these effects, it is necessary to determine the solvent's interfacial structure. A common practice for determining this structure is to find the single structure that globally minimizes the potential energy as calculated by dispersion-corrected density functional theory (DFT) calculations. Although many surfaces have been characterized in this way, the effects of surface irregularities such as defects, step sites and adsorbates remains unclear. In this work, global optimization of water structures on copper surfaces is carried out to determine the effects of irregularities. Preliminary results indicate that the orientation of adsorbed water molecules is influenced by surface irregularities, e.g., undercoordinated sites seem to promote hydrogen-up adsorption, while a hydrogen-down orientation is favored at terrace sites. Current work aims to identify such trends more thoroughly, but the accuracy of using a single optimal structure may fundamentally depend on the properties of the water structure at experimental conditions. Whereas a solid-like water structure can be well approximated as a single optimal structure, a less ordered liquid-like water structure is generally more accurately treated as a thermal distribution. Ab initio molecular dynamics simulations are thus being carried out to investigate the equilibrium properties of interfacial water. Ensemble averages of properties such as intermediate stabilizations and work function changes can then also be compared with properties calculated from single minimal-energy water structures to determine the conditions at which using a single optimal solvent structure is sufficiently accurate.

Abstract Author(s): Thomas Ludwig, Jens K. Norskov