Coadsorption of water and sodium on the Ru(001) surface

Abstract

The coadsorption of water and sodium on a Ru(001) surface has been studied as a model system of the interaction of adsorbed water with electropositive ions. A variety of surface sensitive methods were employed: electron stimulated desorption ion angular distributions (ESDIAD), thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The coadsorption results for temperatures above 80 K have been compared and contrasted to results obtained when water and Na are adsorbed separately on Ru(001). For Na coverages less than a critical value (0.25 monolayer), water adsorbs and desorbs in molecular form. However, even for low Na coverages, the presence of adsorbed Na strongly influences the desorption kinetics and causes an increase in the binding energy of water to Ru; in addition, the local bonding geometry of water is altered. The H + ESD yield from adsorbed water monomers increases substantially in the presence of Na. For Na coverages greater than 0.25 monolayer, the adsorbed two-dimensional Na structures observed by LEED at 80 K become disordered when water is adsorbed, and water dissociates even at 80 K to form a hydroxide-like species with the OH molecular axis perpendicular to the surface. The hydroxide decomposes upon heating, and leads to hydrogen desorption between 350 and 600 K. The striking dependence of the water surface chemistry on the coverage of preadsorbed Na has been associated with the electropositive properties of the Na-Ru layer. The influence of Na on the adsorption of water on Ru(001) is important for understanding processes occurring at the metal-electrolyte interface in an electrochemical cell as well as the catalytic promotion of transition metal catalysts by adsorbed alkalis.