Electrochemical Oscillations during Reduction of Nitrate and Nitrite Ions at High Overpotential
The electrochemical reduction of nitrate ions, NO3-, has been extensively studied concerning the mechanism of the nitrate reduction, the influence of electrolysis conditions, and the applications of the nitrate reduction, etc. [1, 2]. The studies have revealed that Cu is a catalyst for the electrochemical reduction . Moreover, the nitrate reduction on Cu in acidic solutions was found to exhibit two types of electrochemical oscillations when it was accompanied by the hydrogen evolution reaction (HER) . The oscillations appeared both under potential controlled conditions and under current controlled conditions. One of the oscillations, named oscillation I, appeared at a higher potential or a lower current (in absolute value) region where hydrogen bubbles due to the HER evolved slightly. Another oscillation, named oscillation II, appeared at a lower potential or a higher current region where hydrogen bubbles evolved vigorously (see Figs. 1a and 1b).
An N-shaped negative differential resistance (N-NDR) plays an important role in most of the electrochemical oscillations, because an N-NDR characteristic causes a positive feedback mechanism. It was reported that the NO3- reduction was suppressed by adsorbed hydrogen atoms on a Cu surface, i.e., by reaction intermediates for the HER. Therefore, the N-NDR that caused oscillation I was ascribed to the suppression of the NO3-reduction, and the mechanism of oscillation I could be explained by the N-NDR .
On the other hand, the mechanism of oscillation II was unclear because the factor causing an N-NDR characteristic for oscillation II was not detected. Then, we have been investigating oscillation II, which yielded the following novel findings.
- Oscillation II also appeared during the nitrate reduction in non-acidic solutions (see Figs. 1c and 1d) .
- Recently, oscillation II was found to appear during the nitrite ions, NO2-(see Figs. 1e and 1f).
- The hydrogen bubbles evolved synchronously with oscillation II .
In this presentation, the mechanism of oscillation II will be discussed on the basis of the findings.
RESULTS and DISCUSSION
Figure 1 shows the current (I) – potential (E) curves for a Cu-wire electrode. When the solution was 0.1 M H2SO2 + 0.1 M KNO3 (Figs. 1a and 1b), the NO3- reduction occurred in the potential region below ca. 0.2 V, and the reduction of water occurred in the potential region below ca. -1.5 V. Oscillations I appeared as a current oscillation at around -0.6 V and as a potential oscillation in the potential range between -0.4 and -0.7 V, that is, it was accompanied by the HER due to the H+reduction. On the other hand, oscillation II appeared as a current oscillation in the potential region below ca. -1.7 V and as a potential oscillation which ranged lower than -1.8 V, that is, it was accompanied by the HER due to the water reduction. These results lead to the idea that oscillation I appeared when the local pH at the electrode surface was acidic whereas oscillation II did when it was basic.
When the solution was non-acidic, e.g., 0.2 M KNO3 (Figs 1c and 1d) and 0.2 M NaOH + 0.2 M KNO3 (data not shown), oscillation I did not appear whereas oscillation II appeared accompanied by the HER due to the water reduction , which supported the above idea. Furthermore, an oscillation, which was thought to be oscillation II because it synchronized with the behavior of the hydrogen bubble evolution, appeared when the solution was 0.2 M NaNO2(Figs. 1e and 1f). This indicates that the appearance of oscillation II was independent of the kind of reactive species. Thus, it can be concluded that the bubble evolution probably played an essential role in the appearance of oscillation II.
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Figure 1. The I – E curves for (a, b) 0.1 M H2SO4 + 0.1 M KNO3, (c, d) 0.2 M KNO3, and (e, f) 0.2 M NaNO2, measured (left) under potential controlled conditions and (right) under current controlled conditions.