Electrolytes in electroreduction

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

The most important factor in electrolytic reduction (electroreduction) is the nature of the metal used as a cathode. Metals of low overvoltage - platinum (0.005-0.09 V), palladium, nickel and iron - give generally similar results of reduction as does catalytic hydrogenation [727]. Cathodes made of metals of high overvoltage such as copper (0.23 V), cadmium (0.48 V), lead (0.64 V), zinc (0.70 V) or mercury (0.78 V) produce similar results to those of dissolving metal reductions. 

Another important factor in electroreduction is the electrolyte. Most electrolytic reductions are carried out in more or less dilute sulfuric acid but some are done in alkaline electrolytes such alkali hydroxides, alkoxides or solutions of salts like tetramethylammonium chloride in methanol [128] or lithium chloride in alkyl amines [729,130]. 

The influence of TU on Zn(II) electroreduction at the mercury electrode was also investigated in mixtures of water with DM F [63, 64], methanol [62, 63, 65], ethanol, and acetone [66, 67] with NaCl04 as a supporting electrolyte. In all these mixtures, the rate of Zn(II)/Zn(Hg) process was accelerated by adsorbed TU molecules. It was postulated that the composition... 

In mixed (0.8 - a ) M NaCl04 + x M NaF supporting electrolyte the electroreduction of Cd(II) was also studied by Saakes etal. [25]. The kinetic parameters were analyzed using CEE mechanism. The obtained chemical rate constants at both steps, kg 1 and kg 2, decreased with increasing NaF concentration. The data were corrected for nonspecific double-layer effect (Frumldn correction). The interpretation of CEE mechanism with parallel pathways connected with coexisting cadmium complexes was presented. 

Bode, H.E., Sowell, C.G., and Little, R.D., Electrolyte-assisted stereoselection and control of cyclization vs. saturation in electroreductive cyclizations. Tetrahedron Lett., 31, 2525, 1990. 

V.I. Shapoval, O.G. Tsiklauri and N.A. Gasviani, Significance of Acid-Base Properties of Molten Electrolyte in Kinetics of anion electroreduction, Soobsch. AN Gruz. SSR 88 (1977) 609-612. 

Recent work has involved the production of organosilanes and germanes, as small molecules or polymeric systems by electroreduction of the appropriate halo species at a reactive metal cathode (magnesium, aluminum, sometimes copper) in aprotic media. The same metal is used as a sacrificial anode, and the cell is undivided. Thus, with lithium perchlorate as electrolyte in THE solvent, a dichlorosilane such as PhMeSiCl2 gives a polysilane of Mn -- 3000 in 22% yield. This contrasts with earlier work at Hg cathodes in divided cells, where Si-0-containing polymers and cyclotetrasilanes were obtained. Simultaneous... 

The electrochemical conversion of CO2 into useful products, such as fuels, requires one to supply CO2 gas to the electrolysis cell by bubbling CO2 gas continuously in the electrolyte, at a constant flow rate. This can be done at atmospheric pressure (1 bar) or at elevated pressures (30-60 bars). One can perform the electrolysis at high pressure and/or low temperature to increase the otherwise low solubility of CO2 in the electrolyte solution, or in organic solvents, in which CO2 solubility is higher than in water [80]. An alternative approach proceeds to the saturation of the electrolyte in CO2 prior to electrolysis, and keeps the system under continuous stirring over the electroreduction step [81]. 

GDEs have been used successfully in the electroreduction of CO2 to formic acid on tin or lead cathodes, in aqueous solution [57,135,136]. Hence, Kwon and Lee [136] reported the efficient use of nanolayered lead electrodes, prepared by stepwise potential deposition. By this technique, a nanostructured Pb layer forms, which consists of particles and platelets in hexagonal and cubic crystalline form. In electroreductions conducted in aqueous 0.1 mol/ dm KHCO3 supporting electrolyte, at 10 mA/cm current density and a temperature of 5°C, cubic lead surface secured the highest Faradaic yield (94.1%), while polycrystalline smooth Pb films enabled only 52.3% yield. The authors suggested that indirect reduction of CO2 by adsorbed hydrogen atoms (shown in Equations 1.16a, 1.16b and 1.16c) is more likely than direct electroreduction of CO2 molecules. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

The electroreductive cyclization of the furanone 118 (R = -(CH2)4CH=CH— COOMe Scheme 36) using a mercury pool cathode, a platinum anode, a saturated calomel reference electrode, and a degassed solution of dry CH3CN containing -Bu4NBr as the electrolyte, gave the spirocyclic lactones 119 and 120 in a ratio 1.0 1.1 (Scheme 37)(91T383). 

Bouroushian M, Kosanovic T, Loizos Z, SpyreUis N (2000) On a thermodynamic description of Se(IV) electroreduction and CdSe electrolytic formation on Ni, Ti and Pt cathodes in acidic aqueous solution. Electrochem Commun 2 281-285... 

Jiang R, Chu D. 2000. Remarkably active catalysts for the electroreduction of O2 to H2O for use in acidic electrolyte containing concentrated methanol. J Electrochem Soc 147 4605-4609. 

Lalande G, Faubert G, Cote R, Guay D, Dodelet JP, Weng LT, Bertrand P. 1996. Catalytic activity and stability of heat-treated iron phthalocyanines for the electroreduction of oxygen in polymer electrolyte fuel cells. J Power Sources 61 227-237. 

Chu D, Gilman S. 1994. The influence of methanol on O2 electroreduction at a rotating Pt disk electrode in acid electrolyte. J Electrochem Soc 141 1770-1773. 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

The participation of hydride species in this process is corroborated by the ability of complexes [( 75-C5Me5)RhIII(bpy)Cl]+ (M = Rh or Ir)28 and [Rhni(bpy)(PPh2Et)2(Cl)2]+32 in films to perform ECH of organics. Indeed, such complexes are readily transformed to hydride derivatives upon electroreduction in protic electrolytes (see Section 9.10.3.1.1). However, these complexes give more stable hydride species and are much less active than [Rh(bpy)2(Cl)2]+ complexes. 

Carbon electrodes modified by polymeric films of [Ru(bpy)(CO)2] appear to be efficient molecular cathodes for selective reduction of C02 into CO (rj >95%) especially in pure aqueous electrolyte, at a moderate overpotential (—1.2V vs. Ag AgCl).93 Strongly adherent thin films of [Ru(bpy)(CO)2]ra can also be easily prepared from the electroreduction of monobipyridyl mono-or binuclear complexes of Ru containing two leaving groups per Ru, such as [Ru(bpy)(CO)2Cl2], [Ru(bpy)(CO)2(MeCN)2]2+, [Ru(bpy)(CO)2(MeCN)]22+, and [Ru(bpy)(CO)2Cl]2.94"97... 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

It has been known since 1980 that polyazamacrocyclic complexes of Ni11 or Co11 are good candidates to catalyze electroreduction of C02 into CO even in aqueous electrolyte.9,151 The most extensively studied compound of this class of complexes is [Ni(cyclam)]2+ (5), which is a particularly... 

Coin-cyclam322-324 and Nin-cyclam322 catalyze the electroreduction of nitrate in aqueous electrolytes with good current efficiencies and turnover numbers, giving mixtures of ammonia, nitrite, and hydroxylamine at a variety of electrode materials. Mechanistic investigations suggested the adsorption of electroreduced Co1- and Ni1 cyclam onto the electrode surface,322 and the formation of an oxo-metal bond via reduction of coordinated nitrate.323... 

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