Electrocatalysis of anodic oxygen-transfer reactions at modified lead dioxide electrodes

摘要

This dissertation describes results of the studies of anodic oxygen-transfer reactions at the electrochemically deposited lead dioxide electrodes modified by incorporation of spatially separated catalytic sites at the electrode surface. These active surface sites were created either by co-depositing the PbO[subscript]2 films with dopants (i.e., Bi[superscript]3+, As[superscript]5+, Cl[superscript]- and OAc[superscript]-) or by oxidative electrosorption of Bi[superscript]3+ at the pre-deposited oxide films to produce the Bi[superscript]5+-adsorbed PbO[subscript]2. The incorporated cations and anions were speculated to substitute for the surface Pb[superscript]4+ and O[superscript]2- ions, respectively;Studies showed that the dopants influence (i) the deposition kinetics, (ii) the electrocatalytic properties, (iii) the surface morphologies, and (iv) the preferential orientations of the exposed crystallite planes of the doped oxides, as studied by cyclic voltammetry, scanning electron microscopy (SEM), and X-ray diffraction spectrometry (XRD). Results obtained from X-ray fluorescence spectrometry (XRF) revealed that the electrode activity was related to the densities of dopants in the modified electrodes, which were controlled by the concentration ratio of (dopant) / (Pb[superscript]2+) in the deposition solutions;The anion-doped PbO[subscript]2 electrodes exhibited significant catalytic activities for anodic O-transfer reactions for numerous compounds in 1 M H[subscript]2SO[subscript]4 when compared with that in 1 M HClO[subscript]4. The HSO[subscript]4[superscript]- ions were concluded to be adsorbed at the electrode surface by an ion-exchange mechanism with the exchangeable anions at the electrode surface (i.e., Cl[superscript]- for Cl-PbO[subscript]2 and OAc[superscript]- for OAc-PbO[subscript]2). Investigations of mass changes at electrode surface resulting from the anion exchange were performed using an Electrochemical Quartz Crystal Microbalance (EQCM);Catalytic production of adsorbed hydroxyl radicals ([superscript].OH[subscript] ad) was concluded to be a prerequisite for an O-transfer reaction as well as the O[subscript]2 evolution process. Enhanced rates of O[subscript]2 evolution were observed at modified PbO[subscript]2 electrodes, which suggested a catalyzed mechanism for the anodic discharge of H[subscript]2O to form the adsorbed OH radicals. This reaction was concluded to be the rate-limiting step for both O-transfer and O[subscript]2-evolution processes.