Selective electrocatalysis of anodic oxygen-transfer reactions at chemically modified, thin-film lead dioxide electrodes

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Chang, Hsiangpin
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Dennis C. Johnson
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The strategy to modify PbO[subscript]2 electrodes for electrocatalysis of oxygen-transfer (O-t) reactions is to incorporate spatially separated catalytic sites into the PbO[subscript]2 surface. The rate of ·OH radical formation is promoted at these sites, as is the rate of the O-t reaction, for the latter was demonstrated to be proportional to the former;The electrodeposition of PbO[subscript]2 was studied as a special case of a heterogeneous O-t reaction. Soluble Pb(IV) species were detected with a rotated ring-disc electrode as intermediate products of the PbO[subscript]2 deposition. These unstable Pb(IV) species are partially responsible for the catalytic activities of in-situ depositied PbO[subscript]2 electrode, as studied by voltammetry, spectrophotometry, and flow-injection analysis;Chemically modified PbO[subscript]2 is electrodeposited by addition of Bi[superscript]3+, As(III), or Cl[superscript]- to the Pb[superscript]2+ plating bath. The electrochemical stability and catalytic activity of the Bi-doped PbO[subscript]2 (Bi-PbO[subscript]2) increase with increased Bi/Pb atomic ratio in the mixed oxide which were determined by X-ray photoelectron spectroscopy;Microcracked, ultra-thin films of Bi-PbO[subscript]2 were found at Au, Pt, GC, and Ti electrodes after consecutive deposition and stripping of the thick oxide films. The ultra-thin film has higher catalytic activity and stability than the original film, due to an enhanced Bi surface concentration. The morphology and structure of the ultra-thin films were studied by scanning electron microscope and X-ray diffraction;The surface of pure PbO[subscript]2 can be modified by Bi[superscript]3+, As(V), and Cl[superscript]- by electroadsorption. The adsorption method to modify PbO[subscript]2 is fast and efficient for screening new catalysts. The catalytic activity of Bi[superscript]3+-adsorbed PbO[subscript]2 was determined to result in well-defined, mass transport-limited voltammetric plateaus for the anodic reactions Mn[superscript]2+ → MnO[subscript]4[superscript]-, Cr[superscript]3+ → CrO[subscript]4[superscript]-, (CH[subscript]3)[subscript]2SO → (CH[subscript]3)[subscript]2SO[subscript]2, and (CH[subscript]2)[subscript]4SO → (CH[subscript]2)[subscript]4SO[subscript]2. The E[subscript]1/2 values for these varied processes are virtually the same. The E[subscript]1/2 values are a function of the surface concentration of the catalyst and only slightly influenced by the identity of the catalyst;An O-t mediation mechanism was proposed for the electrocatalysis observed at the chemically modified PbO[subscript]2. The catalysts serve as O-t mediators, which are different from electron-transfer mediators by not undergoing any redox change during the catalysis. ftn[superscript]1This work was performed in Ames Laboratory under Contract No. W-7405-Eng-82 with the U.S. Department of Energy.

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Sun Jan 01 00:00:00 UTC 1989