VOLTAMMETRIC AND RADIOACTIVE LABELING STUDIES OF SINGLE-CRYSTAL AND POLYCRYSTALLINE RHODIUM ELECTRODES IN SULFATE-CONTAINING ELECTROLYTES

Adsorption of sulfate/bisulfate anions on single crystal Rh(111) (ordered/disordered) and polycrystalline rhodium electrodes in perchloric acid solution was studied by the use of cyclic voltammetry and the radioactive labeling method. The ordered Rh(111) surface was characterized by LEED and Auger spectroscopy to verify its well-defined character. Details of the surface chemistry of the anions interacting with the three rhodium substrates are different. This is considered as evidence that a long-range order of the metal electrodes alters the structure of chemisorbed hydrogen and oxygen significantly and that this structure is affected by the reversibly interacting species. Likewise, the adsorbed layer of sulfate anions on Rh(111) is more stable than that on other rhodium surfaces due to a favorable spatial configuration of the anions and surface water molecule network, which are proposed to be cross linked by hydrogen bonding. Overall, the adsorption reversibility of sulfate was confirmed with respect to the bulk concentration of the adsorbate and the electrode potential (provided that the surface oxidation is avoided). Radiochemical data reveal that, in the studied bulk concentration range (up to about 10(-3 M), the uptake of anions by Rh(111) is limited to ca. 40% of the theoretical maximum coverage. The interaction of sulfate with Pt(111), and corresponding voltammetry in perchloric acid electrolyte are also brought into focus. It is concluded that only a part of the Pt(111) "butterfly" can be accounted for by adsorption of high energy hydrogen. No high energy hydrogen has been found to exist on Rh(111), as inferred earlier in our laboratory.