Carbon dioxide reduction nonaqueous solutions

The mechanism of carbon dioxide reduction in aqueous and nonaqueous solutions was investigated by several authors. It is now generally accepted that the reduction of carbon dioxide to formate ions is a multistep reaction with the intermediate formation of free radicals CO2 and HCO2 either in the solution or adsorbed on the electrode ... 

Mechanisms of carbon dioxide reduction in both aqueous and nonaqueous solutions have been studied mainly at metal electrodes. 

Electrochemical reduction of carbon dioxide is usually conducted in aqueous or nonaqueous electrolyte solutions at cathodes made of various materials. As a result, various organic substances can form. The most common reactions are as follows ... 

Reduction of carbon dioxide takes place at various metal electrodes. The main products are formic acid in aqueous solutions and oxalate, CO, and formic acid in nonaqueous solutions. An indium electrode is the most potential saving for C02 reduction. Due to the difference in optimum conditions between those for C02 reduction to formic acid and those for formic acid reduction to further reduced products, direct reduction of C02 in aqueous solutions without a catalyst to highly reduced products seems to be difficult at metal electrodes. However, catalytic effects of metal electrodes themselves have recently become more clear for example, on Cu, methane was detected, while on Ag and Au, CO was produced effectively in aqueous solutions. Furthermore, at a Mo electrode, methanol was obtained. The power efficiency is, however, still low at any electrode. 

Electrochemical reduction of carbon monoxide in dry nonaqueous media at moderate to low pressures leads to the formation of the 1,3-cyclobutanedione dianion (squarate) at current efficiencies, up to about 45% depending on the cathode material [1,2]. In aqueous solution, electroreduction can lead to the formation of methane and other hydrocarbon products. The role of the metal/adatom in determining the extent of CO and hence hydrocarbon formation during the reduction of carbon dioxide is related to the ability of the electrode material to favor CO formation (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and stabilize HCCO [3, 4]. 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

In undivided cells the counterlectrode reaction must be chosen with care, as the products from it obviously must not interfere with the proper electrode reaction. This becomes a problem especially in nonaqueous solvents in such media, hydrogen evolution is generally an acceptable counterlectrode reaction for oxidations for reductions, the oxidation of oxalates and formates to carbon dioxide or azide to nitrogen [452] may be a solution to the problem. 

In nonaqueous solution azomethine compounds are usually stable, and several types have been reductively coupled with alkyl halides or carbon dioxide. Benzophenone anil (III) gives thus on reduction in DMF-TBAI in the presence of methyl chloride a mixture of N- and C-methylated and N,C-dimethylated products [28] ... 

The solubility of carbon dioxide in aqueous and non-aqueous solutions depends on its partial pressure (via Henry s law), on temperature (according to its enthalpy of solution) and on acid-base reactions within the solution. In aqueous solutions, the equilibria forming HCO3 and CO3 depend on pH and ionic strength the presence of metal ions which form insoluble carbonates may also be a factor. Some speculation is made about reactions in nonaqueous solutions, and how thermodynamic data may be applied to reduction of CO2 to formic acid, formaldehyde, or methanol by heterogenous catalysis, photoreduction, or electrochemical reduction. 

Direct excitation of the transition metal complex active in COj reduction was demonstrated in a photosystem composed of tricarbonyl(2,2 -bipyridin-ium)rhenium(I), /ac-Re(bpy)(CO)3X (X = C1, Br) as a light-active compound and a homogeneous catalyst [140-142]. Photosensitized reduction of carbon dioxide to CO proceeds in nonaqueous solutions (i.e., dimethylform-amide) including the rhenium(I) complex and TEOA as the sacrificial electron donor. The quantum efficiency for CO formation in the system corresponds to

Mechanistic studies show that the primary step in... 

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