The electro-oxidation of formaldehyde has been investigated at thermally prepared noble metal oxides, especially RuO2, in aqueous solution. Studies were carried out on thin layers of oxide supported on Ta or Ti, and on thin, Teflonbonded oxide layers attached to a glassy carbon rotating disk electrode. The catalytic activity of RuO2 for HCHO oxidation was considerably greater than that of IrO2 and Rh203. At RuO2, formaldehyde was observed to undergo oxidation to formate at potentials from ca. 0.75-1.25V vs. reference hydrogen electrode (RHE), and to carbonate at higher potentials. The results suggest that HCHO (and HCOOH) oxidation is mediated by higher valent states 'of the oxide metal ions electrogenerated at the electrode surface. Values of I0 -3 cm s -1 (geometric electrode area based) and 10 -6 cm s -1 (real area based) were estimated for the heterogeneous rate constant for HCHO oxidation at RuO,.. The activity for HCHO oxidation is correlated with the catalytic activity of the oxides for the oxygen gas evolution reaction.Formaldehyde is an important reducing agent used in electroless metal deposition, where an electrochemical type mechanism involving anodic oxidation of the HCHO along with cathodic reduction of the metal ion is considered to be involved (1, 2). Further, formaldehyde belongs to the group of C1 compounds, which also includes methanol and formic acid, which is the subject of considerable attention in electrochemical energy conversion research (3). The electro-oxidation of formaldehyde has been the subject of a number of investigations at metallic electrodes (4-6). In alkaline solution, formate has been generally found to be the product of HCHO oxidation, and the evolution of H2 gas derived from the HCHO may also be observed during the oxidation reaction depending on the nature of the catalyst and the experimental conditions (4, 7). In a number of instances it has been observed that metals which are catalytically active for HCHO oxidation exhibit significantly less activity when anodic oxides are formed on their surfaces, e.g., Au (8). On the other hand, the oxidation of certain organic molecules, including HCHO, at Ni is observed only in the presence of an anodic oxide layer in which the Ni species are oxidized beyond the Ni(II) state (9, 10). This suggests that certain metal oxides should exhibit catalytic activity for the electro-oxidation of HCHO.The class of noble metal oxides merits attention as potential electrocatalysts for organic oxidation reactions in view of the high catalytic activity possessed by certain members of this class, e.g., RuQ, for the C12 and 02 generation reactions (11,12). Benzaldehyde has been shown to undergo electro-oxidation at RuO2 (13). There appear to be no reports in the literature of studies of HCHO oxidation at oxide electrodes. In this paper, a study of HCHO oxidation at a number of thermally prepared noble metal oxides, principally RuQ, is described.
ExperimentalSome of the oxide electrodes used in this work were prepared essentially according to the stand...