The mechanism of temporal potential oscillations that occur during galvanostatic formic acid oxidation on a Pt electrode has been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS). Carbon monoxide (CO) and formate were found to adsorb on the surface and change their coverages synchronously with the temporal potential oscillations. Isotopic solution exchange (from H13COOH to H12COOH) and potential step experiments revealed that the oxidation of formic acid proceeds dominantly through adsorbed formate and the decomposition of formate to CO2 is the rate-determining step of the reaction. Adsorbed CO blocks the adsorption of formate and also suppresses the decomposition of formate to CO2, which raises the potential to maintain the applied current. The oxidative removal of CO at a high limiting potential increases the coverage of formate and accelerates the decomposition of formate, resulting in a potential drop and leading to the formation of CO. This cycle repeats itself to give the sustained temporal potential oscillations. The oscillatory dynamics can be explained by using a nonlinear rate equation originally proposed to explain the decomposition of formate and acetate on transition metal surfaces in UHV.
The influence of the atomic-level structure of electrode surfaces on electrochemical oscillations has been
studied in a system of H2O2 reduction on Pt electrodes in acidic solutions. A current oscillation of another
type, named oscillation E, has been found to appear for an atomically flat single-crystal Pt(111) electrode, in
addition to previously reported oscillations, named oscillations A and B. Oscillation E does not appear for
atomically flat Pt(100), Pt(110), polycrystalline Pt, and Pt(111) with atomically nonflat surfaces. Mathematical
simulation by use of a model including an autocatalytic effect of adsorbed OH for dissociative adsorption of
H2O2, as a possible explanation, has reproduced the appearance of oscillation E, as well as observed correlations
between the appearance of oscillation E and the magnitudes of H2O2-reduction current and “negative” resistance.
It is discussed that an efficient autocatalytic mechanism works at the atomically flat Pt(111) surface, which
is responsible for the appearance of oscillation E at this surface.
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