CONTRASTING SURFACE BEHAVIOR OF RH (111) AND PT (111) ELECTRODES

Low energy electron diffraction and voltammetric measurements of the Rh (111) electrode were conducted and compared with the corresponding surface and electrochemical characteristics of Pt(111). Rhodium is unstable upon exposure to water vapor or liquid water, but retains its well-defined character after immersion to aqueous media. This is reflected in the voltammetric behavior of the clean surface, as well as the manner in which carbon monoxide and iodine are adsorbed from solution. That is, a monolayer of an oxygen-containing species, assembled into (2 x 2) surface structure, can either be reduced by the voltammetric treatment or replaced by adsorbing solution components without causing system disorder. The voltammetry of the Rh(111) electrode, while exhibiting the main features of several metallic single crystal surfaces, differs significantly from that of platinum electrodes normalized to the same 2D geometry. From the voltammetric behavior, it is concluded that adsorption of high energy hydrogen is not taking place on Rh (111). Equally important, the packing density and the surface structure of the Rh (111) - CO differs from its Pt (111) - CO analog. While iodine chemisorption from the gas phase leads to the development of several surface structures known from the corresponding platinum work, preferential formation of the Pt (111) (square-root 3 x square-root 3) R30(0)-I structure against the Pt (111) (square-root 7 x square-root 7) R19.1(0)-I was demonstrated. Both electronic and structural factors contribute to the contrasting surface behavior brought to focus in this work.