DETAILED ANALYSIS OF THE VOLTAMMETRY OF RH(111) IN PERCHLORIC-ACID SOLUTION

The preparation and surface treatment of a rhodium single crystal with (111) orientation are described. Surface characterization by scanning tunnelling microscopy (STM) in air, including topography and atomic-s cale pictures, is reported. Hexagonal surface domains one atomic diame ter thick are observed with edges parallel to the dense rows of the ba sal orientation; three or four atomic levels are included in this dist ribution. The atomic resolution reveals a surface lattice with the 2 x 2 periodicity, probably formed by oxygen adatoms. The voltammetric el ectrochemical study was carried out using perchloric acid solution whi ch is known to give a reduction of the perchlorate anion at an Rh elec trode. The reduction of perchlorate is revealed at low sweep rates. Th e effect of the formation of chloride anions or other anionic reductio n products on the voltammetric response of this electrode was investig ated in detail. A dramatic effect was found regarding the unusual reve rsible adsorption state around 0.6 V, which is very often compared wit h the unusual adsorption state of Pt(111) in the same solution. A quan titative analysis of the various adsorption states is given. The unusu al state is shown to be sensitive to the electrochemical oxidation of the surface above a potential threshold of 1.080 V (RHE), similar to t he behaviour of Pt(111). The presence of this state is thus shown to b e characteristic of two-dimensional long-range order.