Study of the electrochemical reduction of dioxygen in acetonitrile in the presence of weak acids
Study of the electrochemical reduction of dioxygen in acetonitrile in the presence of weak acids
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DOI：
10.1021/jp055296f
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发表时间：
2006-01-12
影响因子：
3.3
通讯作者：
Evans, DH
中科院分区：
文献类型：
--
作者：
Singh, PS;Evans, DH
已经在乙腈中在玻碳电极上研究了分子氧的电化学还原。第一步是可逆的单电子还原形成超氧化物。在氢键供体（水、甲醇、2-丙醇）存在的情况下，超氧化物与供体形成复合物，导致电势发生正向变化，可以通过分析该电势以获得这些复合物的形成常数。较强的酸会导致超氧化物质子化，然后还原生成 HO2-。在没有氢键供体的情况下，超氧化物的还原发生在非常负的电势下，并且第二个还原峰沿着电势轴非常拉长，表明转移系数 α 的值很小。氢键供体 HA 的添加使该峰发生正向移动，但形状没有明显变化。在第二个峰发生的反应是协同质子和电子转移 (CPET)，其中电子转移到超氧化物，质子从 HA 转移到超氧化物，在协同过程中形成 HO2- 和 A(-)。对该反应的标准电势的估计表明，第二次还原总是在高驱动力下发生，这解释了观察到的较小的 α 值。与 CPET 一致，检测到三个氢键供体的动力学同位素效应（HA 与 DA）。随着水浓度的增加，第二个峰的正向移动被解释为形式势变化的结果，因为水既是该过程中的反应物，又是阴离子氢键稳定作用的参与者。
The electrochemical reduction of dioxygen has been studied in acetonitrile at glassy-carbon electrodes. The initial step is the reversible one-electron reduction to form superoxide. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the potential that can be analyzed to obtain formation constants for these complexes. Stronger acids result in protonation of the superoxide followed by reduction to produce HO2-. In the absence of hydrogen-bond donors, the reduction of superoxide occurs at very negative potentials, and this second reduction peak is very much drawn-out along the potential axis, indicating a small value of the transfer coefficient, alpha. The addition of hydrogen-bond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton and electron transfer (CPET) in which the electron is transferred to superoxide and a proton is transferred from HA to the superoxide, forming HO2- and A(-) in a concerted process. An estimation of the standard potential for this reaction shows that the second reduction always occurs at a high driving force, which explains the small value of alpha that is observed. Consistent with a CPET, a kinetic isotope effect, HA versus DA, was detected for the three hydrogen-bond donors. The increasing positive shift of the second peak with increasing water concentration has been interpreted as being a consequence of the change in the formal potential, as water is both a reactant in the process and a participant through the hydrogen-bond stabilization of the anions.