A large number of anodic oxygen-transfer reactions were studied at Au electrodes in both acidic and alkaline media. Results o f competitive adsorption studies are interpreted and support the conclusion that adsorption is a prerequisite to subsequent oxygen-and electron-transfer steps. Many of these oxidation reactions gave the voltammetric appearance of reversible waves, even though the observed half-wave potential values were shifted hundreds o f millivolts positive of the thermodynamic potentials. A catalytic mechanism is proposed in which adsorbed hydroxyl radicals (AuOH) participate in the oxygen-transfer step. The absence of pH effects on half-wave potentials for several inorganic compounds suggests that the electron transfer precedes the deprotonation step for these reactants.
Ratios of the change in surface mass corresponding to a change in charge
false(normalΔm/normalΔqfalse)
were determined for the electrodeposition of thin films of pure
PbO2
and Bi‐doped
PbO2
at Au‐film electrodes in
0.1M HClO4
, using an electrochemical quartz microbalance. The value
normalΔm/normalΔq=1.26±0.04 normalmg C−1
obtained for pure
PbO2
films is in good agreement with the theoretical value of 1.24 mg C−1. A larger value,
normalΔm/normalΔq=1.32±0.03 normalmg C−1
, obtained for the Bi‐doped
PbO2
films, is concluded to result from the co‐deposition of some
ClO4−
with Bi(V) in the mixed‐oxide film. Bismuth(III) was determined to be anodically adsorbed as Bi(V) at pure
PbO2
surfaces for
E>1.45V
vs.
normalAg/normalAgCl
with
normalΔm/normalΔq=1.72±0.02 normalmg C−1
in
0.1M HClO4
and
1.30±0.03 normalmg C−1
in
1M HNO3
. The
PbO2‐normalfilm
electrodes with adsorbed Bi(V) are active for various anodic oxygen‐transfer reactions, including the oxidations of Mn(II) to
MnO4−
and DMSO to
DMSO2
. Experimental results are interpreted to be consistent with a mechanism proposed previously, in which the Bi(V) sites have a lower overpotential for anodic discharge of
H2O
to produce
O2
. It has been proposed that adsorbed hydroxyl radicals (.OHads) generated in the
O2
evolution mechanism are consumed by oxygen‐transfer steps, required in many oxidation processes.
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