Reduction at low potential

It was found that underpotentially deposited germanium strongly enhances the reduction rate of nitrate. The reduction of nitrite is enhanced to a lesser extent, whereas germanium is inactive for NO and hydroxylamine reduction. It is of interest that the well-known inhibition of the nitrate reduction at low potentials was absent for germanium-modified electrodes. 

Cobalt porphyrins have been some of the most studied catalysts for oxygen reduction, due in part to their strong interaction with molecular oxygen and the ease with which they catalyze the reduction at low potentials. Anson and coworkers [97] performed a study of the simplest of cobalt porphyrins, cobalt porphine, in the hope of gaining a baseline for the observed reactions of other porphyrins. Instead, they observed a very different process from most other monomeric cobalt porphyrins. 

In the presence of Li+, H20 is reduced to LiOH, as evidenced by spectral studies [30-46], We have spectral evidence that LiOH is not the final stable product of H20 reduction at low potentials and that it is further reduced to Li20. It is quite possible that the LiOH surface films detected by some spectral studies on noble metal electrodes polarized in Li salt solutions result from hydration of the Li20 initially formed. Hence, in the case of H20 contamination, the water is reduced at an onset potential of 1.5 V (Li/Li+) to LiOH, which is further reduced to Li20 at lower potentials. The surface film thus formed may contain Li20 in an inner layer and LiOH at different stages of hydration in outer layers. These surface films passivate the electrodes, and thus water reduction becomes kinet-ically limited by the rate of diffusion of water through the surface films. 

It undergoes a quasireversible one-electron oxidation (E0 = + 0.26 V AEp(02Vs- ) = 160 mV), coupled to subsequent chemical complications, which impart to the monoanion a lifetime of about 3 s. Speculatively, the geometrical strain induced by the one-electron removal (see the large peak-to-peak separation) might be responsible for the lability of this monoanion. The dianion also undergoes a fast declustering, two-electron reduction at low potential values (Ep = - 1.70 V). 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

Scheme 29. Proposed Mechanism for BOB Reductive Decomposition on a Graphitic Surface at Low Potentials...

Flavodoxins are a group of flavoproteins which function as electron carriers at low potential in oxidation-reduction systems. The proteins of this group contain one molecule of FMN as their prosthetic group, but, in contrast to ferredoxins, do not contain metals such as iron. 

Detailed and fundamental studies of the mechanistic aspects of the electrochemical reduction and oxidation of nitric oxide were carried out by the Eindhoven group [82, 83] on a series of metals (Pt, Pd, Rh, Ru, Ir, and Au) both in the case of polycrystalline and well-defined single-crystal surfaces. It was found that the reduction process at all metals studied shows a high selectivity with respect to N2O production at high potentials while at low potentials the formation of NH3 is the dominant... 

If possible, the cell should be undivided to minimize the construction cost and also the energy consumption (see goal 1). The application of a controlled reaction at the auxiliary electrode taking place at low potential allows for the use of undivided cells in many cases. For oxidations, the cathodic process at the auxiliary electrode may be a proton reduction under formation of hydrogen. For reductions, the anodic process may be the oxidation of formate or oxalate under production of carbon dioxide [68] or the dissolution of sacrificial anodes [69] (see also Sec. V.B). 

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

We examined the representative esters, y-butyrolactone (BL), methyl formate (MF), and methyl acetate (MA). Figures 16 and 17 show FTIR spectra measured (ex situ) from noble metal electrodes polarized to low potentials in LiC104 solutions of BL and MF, respectively [30,39], As shown in these figures, at the onset reduction potential of around 1.3-1.2 V (Li/Li+), stable surface films precipitate on the electrode surfaces. Table 1 shows the spectral analysis for the surface films formed on noble metals at low potentials in BL. The conclusion drawn from the spectroscopic study is that the major surface compound formed is the dilithiated cyclic P-keto ester, which is similar to the electrolysis product of BL in TAA salt solutions (Scheme 2). 

Flence, aged surface films formed on nonactive electrodes at low potentials in alkyl carbonate solutions of these two salts contain LiF and other salt reduction products of the Li PF, Li BFy,... 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

Most of the commonly used salts in nonaqueous systems comprise anions that are reactive and may be reduced at noble metal electrodes at low potentials. In the presence of cations such as Li+, salt anion reduction may precipitate insoluble surface species on the electrodes and thus become the dominant surface film forming process. The criteria chosen here for the reactivity of the various salt anions used are the onset potential of their reduction on noble metal electrodes and to what extent their reduction on the electrodes dominates the surface film chemistry. In this respect, the commonly used salt anions can be divided into three... 

In the case of LiC104, there is some spectroscopic evidence of anion reduction below 1.5 V [39], The stable surface species which may precipitate due to the Cl()4 reduction is, among others, Li20. We have no spectroscopic evidence for precipitation of stable LiC10x (x = 1-3) or for LiCl onto noble metals at potentials above those of Li bulk deposition. In any event, the above salt anion reduction processes do not dominate the overall surface film formation on nonactive electrodes at low potentials in most aprotic solvents. Thus, both anions can be considered as only moderately reactive. The onset potential for the reduction of the anions from the third group is about 2 V (Li/Li+). This is clearly demonstrated in Figures 18 and 19, which show FTIR spectra measured in situ from... 

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