EFFECTS OF THE ELECTROLYTE IDENTITY AND THE PRESENCE OF ANIONS ON THEREDOX BEHAVIOR OF IRREVERSIBLY ADSORBED BISMUTH ON PT(111)

The voltammetric profile in sulfuric acid electrolytes of high coverag e bismuth monolayers on Pt(lll) having a fractional coverage (theta) o f 0.39 exhibits two pairs of peaks located positive and negative, resp ectively, of the redox peak associated with the 0.33 coverage Bi/Pt(11 1) adlayer. The Delta E-fwhm (full width at half-maximum) values for t hese peaks are 10 and 20 mV, respectively, suggesting that the redox e vents involve significant near-neighbor interactions and may correspon d to phase transitions within the bismuth adlayer. The formal potentia ls of these redox peaks shift by 65 mV per pH unit, suggesting that tw o protons are involved in the two-electron redox reaction of the bismu th adatoms, This provides evidence that the two pairs of redox peaks c ould correspond to the abrupt formation of [Bi(OH)(2)](ad) and [BiO](a d) from Bi-ad. Upon continuous potential scanning in sulfuric acid ele ctrolyte, these peaks decay while two new peaks grow in, with an isopo tential point forming between the two anodic peaks. The new, latter pe ak corresponds to that found for the 0.33 coverage Bi/Pt(111) adlayer. The peaks that grow in upon potential scanning could arise from the i nterconversion of the hydroxide and oxide network to a stable, long-ra nge hydrogen-bonded (bi)sulfate network upon bismuth oxidation. The pe ak associated with the 0.33 coverage Bi/Pt(111) adlayer could then be ascribed to the abrupt formation of this (bi)sulfate network upon bism uth oxidation. In perchloric acid electrolytes the two pairs of high c overage bismuth redox peaks are also present, but one increases monoto nically while the other decreases upon potential scanning, without the appearance of any new peaks. The high coverage peaks are thus associa ted with the formation of a long-range hydroxide and oxide network upo n bismuth oxidation. The presence of halide anions (Cl-, Br-, I-) dram atically affects the voltammetric profile of Bi/Pt(lll), Initially, ch loride appears to form a complex with the bismuth adlayer. Upon potent ial scanning, the redox peak ascribed to the chloride complex decays w hile that associated with the stable bismuth adlayer reemerges, Bromid e and iodide anions both cause the complete desorption of the bismuth adlayer, suggesting that they have significantly stronger bonding inte ractions to Pt(lll) than does bismuth.