Electrochemical Reduction of Metal Salts

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Metal nanostructures Chemical and electrochemical reduction of metal salts in solution and at liquidlliquid interface [67- 74, 84] Electrochemical synthesis from sacrificial anode [59, 95, 96]... 

Microelectrodes with high real surface areas and well-defined periodic nanostmctures have recently attracted much interest because of their potential applications in electrocatalysis and electroanalysis [94-96]. These electrode systems can be prepared, using templating techniques, from lyotropic liquid crystalline phases of nonionic surfactants [94,95]. In particular, the normal topology hexagonal (Hj) liquid crystalline phase has been used as a template for the synthesis of mesoporous metal thin films via the electrochemical reduction of metal salts dissolved in the aqueous domain of the liquid crystalline phases [119, 120]. 

Traditionally, mesoporous metals have been elaborated by using mesoporous silica as a hard template. In 1997, Attard s research group first reported that mesoporous platinum can be produced by the chemical or electrochemical reduction of metal salts dissolved in the aqueous domains of a hexagonal lyotropic liquid crystal (LLC) phase architecture. " They have shown that the reduction of platinum salts in this system led to platinum whose nanostructure was a cast of the LLC... 

Using electrons for the electrolytic reduction of metal salts, Reetz and coworkers have introduced a further variation to the tetraalkylammoniumhalide-stabilization mode [192-198]. The overall electrochemical process can be divided into the following steps (i) oxidative dissolution of the sacrificial Metbuik anode, (ii) migration of Met ions to the cathode, (iii) reductive formation of... 

Preparation of amalgams electrochemical reduction on an Hg cathode According to Guminski (2002), the electrochemical reduction of metallic ions on an Hg cathode from aqueous or non-aqueous solvents (as well as from molten salts) allows the introduction of both soluble and insoluble metals into the Hg phase. Some amalgams may be prepared by simultaneous reduction of Hg2+ and Men+ from their solutions. On the other side, noble metal (Pd, Pt, Ag, Au) amalgams may by obtained by reduction of Hg2+ on noble metal electrodes. 

Indolines and indoles were prepared by a direct electrochemical reduction of arenediazonium salts. As a result, radical intermediates were generated from which 3,4-disubstituted tetrahydrofuran skeleta were constructed <02OL2735>. A short and stereoselective total synthesis of furano lignans was realized by radical cyclization of epoxides using a transition-metal radical source <02JOC3242>. Other preparations of tetrahydrofurans using radical cyclization include the synthesis of novel amino acids L-bis-... 

Bernard, M.-C., A. Chausse, E. Cabet-Deliry, M. M. Chehimi, J. Pinson, F. Podvorica, and C. Vautrin-Ul. Organic layers bonded to industrial, coinage, and noble metals through electrochemical reduction of aryldiazonium salts. Chem. Mater. 15, 2003 3450-3462. 

ZnO nanoflower morphologies usually include nanowires, nanorods, nanorings, nanoneedles, etc. The synthetic techniques of nanomaterials include oxidation of elemental metals, reduction of metal salts, thermal decomposition of relatively unstable compounds, or electrochemical route. Various other metal nanoflowers have also been synthesized by various workers from time to time. 

Redox reactions. The chemical reduction of metal salts by sodium boron hydride is a common procedure in the preparation of metals and metal alloys. Nanoparticles of iron, FeZrB, FeCoB and FeCoB have been obtained from the corresponding sulphate salts, and NdFeB compounds from neodymium and iron chloride salts. Cobalt nanoparticles have been prepared from cobalt acetate using 1,2 dodecanediol as a mild reducing agent. FePt nanoparticles are produced by the decomposition of iron pentacarbonyl and the reduction of platinum tetrachloride complexes in an organic solvent. Redox reactions can also be produced by electrochemical methods, or in the solid phase. ... 

As alternatives to amphiphilic betaines, a wide range of cationic, anionic, and non-ionic surfactants including environmentally benign sugar soaps have been successfully used as colloidal stabilizers [201]. Electrochemical reduction of the metal salts provides a very clean access to water soluble nanometal colloids [192]. 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

Electrochemical deposition of metals and alloys involves the reduction of metal ions from aqueous, organic, and fused-salt electrolytes. In this book we treat deposition from aqueous solutions only. The reduction of metal ions in aqueous solution is... 

Many investigators have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents because these solvents dissolve much more C02 than water. With the exception of methanol, however, no hydrocarbons were obtained. The solubility of C02 in methanol is approximately 5 times that in water at ambient temperature, and 8-15 times that in water at temperatures below 0°C. Thus, studies of electrochemical reduction of C02 in methanol at —30°C have been conducted.148-150 In methanol-based electrolytes using Cs+ salts the main products were methane, ethane, ethylene, formic acid, and CO.151 This system is effective for the formation of C2 compounds, mainly ethylene. In the LiOH-methanol system, the efficiency of hydrogen formation, a competing reaction of C02 reduction, was depressed to below 2% at relatively negative potentials.152 The maximum current efficiency for hydrocarbon (methane and ethylene) formation was of 78%. 

One-electron reduction of pyrylium salts, with dissolving metals or electrochemically, gives dimers (e.g. 382) via pyranyl radicals (80AHC(27)46). 

It is pertinent to note that the chemistry of the 1,2-diseleno ligands is less extensive than that of the analogous sulfur chelates. However, it is interesting to note that chemical or electrochemical reductive coupling of CSe2 has provided 1,2-diseleno anions which can react with a variety of metal salts to give the corresponding square planar bis chelates (Scheme 5).47 48... 

Several studies have been made of the effect of added metal ions on the pinacol/alcohol ratio. Addition of antimony(m) chloride in catalytic amounts changes the product of the electrochemical reduction of acetophenone in acidic alcohol at a lead electrode from the pinacol in the absence of added metal salt to the secondary alcohol in its presence53. Antimony metal was suspected to be an intermediate in the reduction. Conversely, addition of Sm(in) chloride to DMF solutions of aromatic aldehydes and ketones54 and manganese(II) chloride to DMF solutions of hindered aromatic ketones55 results in selective formation of pinacols in excellent yields. When considering these results one should keep in mind the fact that aromatic ketones tend to form pinacols in DMF even in the absence of added metal ions1,29,45. 

Fig. 10.8 Schematic experimental set-up for the deposition of metal nanoparticles by plasma electrochemical reduction of a metal salt dissolved in an ionic liquid at room temperature.

---