Phenomenology of oscillatory electro-oxidation of formic acid at Pd: role of surface oxide films studied by voltammetry, impedance spectroscopy and nanogravimetry

Concepts of electrocatalysis are applied to examination of the oscillatory kinetics that are observed in the oxidation of formic acid in aqueous HClO4 and H2SO4 at Pd. The phenomenology and interpretation of such behavior at Pd are compared and contrasted with related phenomena observed at Pt electrodes reported in earlier work by the present authors and also in more recent literature. Special attention is given to the role of formation and reactivity of the surface oxide anodically formed at Pd, up to various potentials, studied by voltammetry and nanogravimetry, in generating oscillatory kinetics, particularly in oscillation transients that extend to ca. 1.6 V(RHE), unlike the behavior at Pt where they extend only to ca. 0.83 V. Remarkably reproducible oscillations are recorded up to 11.1 h of constant polarization, with periods of up to 1000 s, i.e. frequencies of 10−3 s−1, dependent on polarization current-density. Major effects of solution-stirring are shown to arise in the oscillatory behavior of formic acid oxidation at Pd. Also the effects of increase of temperature from 273.1 K, upwards, on the form of the oscillations are examined. The oscillatory behavior is attributable to a combination of the following factors, also recognized in our earlier works and other literature: (a) negative resistance over the parts of the anodic polarization curve associated with (b) S-shapes of such curves exhibiting hysteresis between positive-, and negative-directions of polarization and (c), importantly, the participation of autocatalysis in chemical/or electrochemical steps, e.g. reactions of an intermediate or H.COOH with oxide-film species at the Pd electrode. The appearance of both potential and current oscillations at Pd in the presence of H.COOH under conditions of stirring is a new and significant observation, not observed in previous studies.