Electrode reduction mechanism

The electrode reduction mechanism of benzenodicarbonitrile isomers was examined by polarography, cyclic voltammetry and controlled potential electrolysis in DMF solutions at a Hg cathode. 1,2- and 1,4-dicyanobenzenes were reduced in two successive steps under polarographic conditions, where the first step corresponds to a quasi-reversible one-electron transfer. Cyclic voltammetric experiments provided more information on the electrode reduction mechanism and allowed one to suggest the mechanistic scheme for 1,2-and 1,4-dicyanobenzenes shown in Scheme 16. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

The third class of redox species are couples located near the conduction band of WSe2- The only outer-sphere example found, which is suitable for use in aqueous electrolytes, is Ru(NH3)e3+. Its reduction is characterized by an immediate onset upon accumulation in the semiconductor and a tafel slope of 130 mV/decade. The reduction mechanism appears to be direct reduction of the Ru(NH3)e3+ by electrons from the accumulation layer. The only member of the forth class of redox species is triiodide ion. It is characterized by adsorption onto the semiconductor surface as was demonstrated by the first application of chronocoulometry to a semiconductor electrode (another demonstration of the reproducibility and low background currents on... 

The spectroscopic identification of the y3-keto ester and the reduction mechanism leading to it have been previously described by Aurbach and co-workers when studying the surface chemistry of yBL-based electrolytes on various electrodes polarized to low potentials. 123,208,209... 

Figure 28. Svensson s macrohomogeneous model for the i— 1/characteristics of a porous mixed-conducting electrode, (a) The reduction mechanism assuming that both surface and bulk diffusion are active and that direct exchange of oxygen vacancies between the mixed conductor and the electrolyte may occur, (b) Tafel plot of the predicted steady-state i— V characteristics as a function of the bulk oxygen vacancy diffusion coefficient. (Reprinted with permission from ref 186. Copyright 1998 Electrochemical Society, Inc.)...

The convolution-deconvolution voltammetry, combined with digital simulation techniques, was applied [36] to determine the electrochemical and chemical parameters for the Cd(II)/Cd(Hg) system in aqueous NaNOs solution. The agreement between experimental and theoretical data indicated that the reduction mechanism at the mercury electrode proceeds via consisting in chemical step (C) followed by charge transfer step (E)-so-called CE mechanism [37]. 

The reduction of the Cd(2,2,2) + complex on mercury electrodes was studied in aqueous solutions free and saturated with -pentanol and n-octanol [86] and also in acetonitrile [87]. The corresponding reduction mechanism was established and the kinetic parameters were calculated. 

The reduction mechanism of Cd(II)-fer rone complexes accumulated on static mercury electrode was studied using square wave voltammetry (SWV) [92]. The electrochemical behavior of Cd(II) complexes with cysteine and folic acid was investigated. Folic acid forms adducts... 

The ORR mechanism is very complex and involves a number of different adsorbate intermediates. In addition to its complex reduction mechanism, the relatively high thermodynamic electrode potential of the ORR causes electrocatalytic surfaces... 

In some cases, the reduction mechanism of a metal complex varies with solvent. For example, the complex [Fe(bpy)3]2+ (bpy = 2,2 -bipyridine) in aqueous solutions is reduced at a dropping mercury electrode directly to metal iron by a two-electron process [Fe(II) —> Fe(0)]. In aprotic solvents, however, it is reduced in three steps, each corresponding to a reversible one-electron process, and the final product is [Fe(bpy)3], which is relatively stable [5] ... 

For many inorganic and organic substances, it is rare that the electrode reaction is simply an electron transfer at the electrode surface. In most cases, the electron transfer process is accompanied by preceding and/or following reactions, which are either chemical or electrochemical. For example, for the electrode reduction of substance A, mechanisms as described below can be considered ... 

Interestingly, the reduction mechanism of 2-benzopyrylium salts 10 at the dropping mercury electrode involves the disproportionation of a radical intermediate 304 in acid to 10 and 107 (72MI3). 

Beley et al. and others studied tetra-azamacrocyclic Ni complexes, similar to those presented in Section 11.2, in aqueous and organic solvents for the mediated reduction of C02 [98-100], In aqueous 0.1 M KN03 solution at a potential of-1.2 V (versus SCE), Ni(II)-cyclam dichloride (cyclam = 1,4,8,11-tetra-azatetradecane) reduced C02 to CO with 96% faradaic efficiency at Hg electrodes. The mechanism involved a first electron reduction of the species which coordinated C02, followed by C02 protonation, and a second electron transfer to yield CO and OH (as dis-... 

Tetraalkyl ammonium (TAA) salts are characterized by very low reduction potentials, along with good solubility in many organic solvents. Thus, nonaqueous solutions composed of such salts (e.g., tetrabutyl ammonium perchlorate and organic solvents such as ethers, esters, and alkyl carbonates) can be electrolyzed using noble metal electrodes. In contrast to lithium salt solutions, in TAA-based solutions there is no precipitation of insoluble products on the electrode, which leads to its passivation. Therefore, it is possible to isolate and identify the electrolysis products and thus outline precise reduction mechanisms for the various systems. 

We have studied the electrolysis of y-butyrolactone (BL) and methyl formate (MF) in TBAP solutions. A typical voltammogram of y-BL/TBAP with a gold electrode is also shown in Figure 1. Butyrate (CH3CH2CH2COO ) and a cyclic (3-keto ester were identified as the major electrolysis products. The latter is a product of a nucleophilic attack of y-BL anion (in the a position) on the carbonyl center of another molecule [3], The FTIR spectra of this product, as well as its lithiated derivative, are shown in Figure 2. The basic reduction mechanisms of y-BL, based on the above product analysis, as well as on other arguments [3], are presented in Scheme 2. 

Hcncc, another reduction mechanism of EC (or PC) oil active electrodes can be ... 

In the case of C02 contamination, we have strong evidence that its reduction on noble metal electrodes in nonaqueous systems in the presence of Li ions (and the absence of water) forms Li2C03 and CO [17], Figures 20 and 21 show typical FTIR spectra obtained from noble metal electrodes polarized to low potentials in C02-saturated nonaqueous Li salt solutions and provide clear evidence for Li2C03 formation as the major surface species that is precipitated [15,39], The C02 reduction mechanism for the reaction appears in the literature [43] and is described in the following equations ... 

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