Electrocatalytic function of Bi(V) sites in heavily-doped PbO2-film electrodes applied for anodic detection of selected sulfur compounds

The role of Bi(V) sites at the surfaces of Bi(V)-doped beta-PbO2 film electrodes is examined as it relates to electrocatalysis of the anodic O-transfer reactions of organic sulfur-containing compounds, with emphasis on cysteine, in acidic media. Contrary to earlier speculation, these sites are concluded not to decrease the overpotential for O-transfer reactions by decreasing the net overpotential for anodic discharge of H2O. The possibility of electron-transfer mediation by a surface-bound Bi(III)/Bi(V) redox couple also is rejected on the basis of the voltammetric response for cysteine at PbO2 electrodes doped with Al(III), a species that cannot possibly function as an electron-transfer mediator. Data obtained by X-ray photoelectron spectroscopy support the conclusion that the Bi(V) sites function for preadsorption of the reactant species in the anodic O-transfer mechanism. This conclusion is believed to be pertinent to the anodic response mechanisms of all organic sulfur compounds at Bi(V)-doped PbO2-film electrodes. The benefit of preadsorption is concluded to result from: (i) desolvation of reactant species, which minimizes the requirement for transfer of O-atoms by an improbable tunneling mechanism; and (ii) the increased residence time of reactant species at the electrode surface. (C) 1998 Elsevier Science S.A. All rights reserved.