No abstract

It is well accepted that the electrooxidation of ethanol on Pt in aqueous solution proceeds via two parallel pathways: C1-pathway, which is the complete oxidation of ethanol to CO 2 via CO ad intermediate, and C2-pathway, which produces acetaldehyde and also acetic acid with further oxidation. Water plays important roles for the oxidation, i.e., for the oxidative removal of CO ad in the C1-pathway and for the oxidation of acetaldehyde to acetic acid in the C2-pathway. In the present work, however, we show that the ethanol oxidation proceeds in the absence of water. Detailed study using surface-enhanced infrared absorption spectroscopy and high-performance liquid chromatography reveals that CO ad reacts with ethanol to form ethyl formate, which is designated as C3-pathway, and also that the ethanol oxidation produces ethyl acetate, which is designated as C4-pathway.

We have previously reported that the hydrogen evolution reaction (HER) in acidic water electrolysis shows a potential oscillation with amplitude of about 1 V. The oscillation, named HER oscillation, is accompanied with a periodic change in the evolution rate of hydrogen bubbles, i.e., hydrogen bubbles evolve more vigorously at low potentials than at high potentials, which has led us to propose a mechanism for HER oscillation (J. Electroanal. Chem., 713, 39 (2014)). In order to obtain a deeper insight into the mechanism of HER oscillation, this present work studies the effect of high pressure (e.g. 0.7 MPa) on the oscillation and current-potential curves. It reveals that any N-shaped negative differential resistance characteristics are not involved in HER oscillation unlike the majority of electrochemical oscillations. It also shows that a solution-stirring effect due to the hydrogen bubble evolution, which causes an enhancement of convection near the electrode surface, plays an essential role in HER oscillation. We thus conclude that the appearance of HER oscillation can be explained by considering that the enhancement occurs only at low potentials at which hydrogen bubbles evolve vigorously.

A nitrobenzene droplet moves in a self-propelled manner on Au electrode surface during the Sn electrodeposition in H 2 SO 4 solution because the interfacial tension of solid-water interface acting on the front side of the droplet is larger than that on the rear side. The electrodeposition, which increases the interfacial tension, is suppressed by a side reaction such as hydrogen evolution reaction and by adsorbed nitrobenzene molecules. If the electrode surface is non-uniform, the electrodeposition as well as the side reaction takes place non-uniformly on the electrode surface, leading to an initiation of the droplet motion. If the electrode surface is uniform, the motion is initiated when nitrobenzene molecules are adsorbed non-uniformly on the surface. Once the motion is initiated, the imbalance is created spontaneously: the electrodeposition occurs less efficiently at the rear side because it is suppressed by nitrobenzene molecules remaining on the surface. When the electrodeposition is performed in HNO 3 solution, the formation of aniline by the reduction of the non-uniformly adsorbed nitrobenzene induces the Marangoni effect and consequently the droplet continually changes in shape and moves like an amoeba. This amoeba-like motion is also self-propelled because nitrobenzene molecules remaining on the rear side are reduced to form aniline molecules.

A new type of oscillation, which we name regeneration oscillation, is investigated in detail. The regeneration oscillation is the ordinary potential oscillation with the higher turning potential lower than 0.85 V appearing repeatedly after the potential stays at a value higher than 1.0 V for a long time, of the order of 10 min or an hour. The oscillation has been observed during the oxidation of methanol, formaldehyde, and formic acid at 315 K when their concentrations are high, 1 or 10 mol/L (M), in the presence of high chloride ion concentrations, such as 10 −2 M, at a current far lower than the maximum current for the appearance of ordinary oscillation. The reason for the appearance of regeneration oscillation has been found by voltammetry and surface-enhanced infrared absorption spectroscopy to be due to the presence of two potentials for oxidizing adsorbed CO, one shifting to a value higher than 1.0 V and the other remaining unchanged, with increasing chloride ion concentration.