Reaction of Carbon Dioxide Reduction

Unsaturated organic compounds that are present in the solution while these reactions proceed will also become adsorbed on the electrode and may act as acceptors for the radicals, yielding addition products  

In solutions containing more than one compound of the RCOOH type, asymmetric products of the crossed addition of different radicals may form together with the standard additive dimers. 

Electrochemical reduction of carbon dioxide has found no extensive application so far, yet it is of great interest for scientists in the fields of theoretical and applied electrochemistry. To a certain extent, it is analogous to the photochemical carbon dioxide reduction, but it involves no chlorophyll and yields simpler products. In recent years some books and reviews on this topic have been published (e.g., Taniguchi, 1989 Sullivan et al., 1993 Bagotsky and Osetrova, 1995). 

From the applied point of view, this reaction can be used to solve some important issues (1) production of organic subproducts (e.g., methanol, carbon monoxide, oxalic acid), which can be used for synthesizing many valuable organic substances (2) manufacture of synthetic fuels or energy-storage media and (3) removal and utilization of carbon dioxide in life-support systems for closed environments of spacecraft or submarines. 

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  

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The overall reaction of carbon dioxide reduction in the Calvin-Benson cycle (Fig. 17-14) becomes... 

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 ... 

Both CO and C02 are reduced by eh. The immediate product of the first reaction is CO-, which reacts with water, giving OH and the formyl radical the latter has been identified by pulse radiolysis. The product of carbon dioxide reduction, C02-, is stable in the condensed phase with an absorption at 260 nm. It reacts with various organic radicals in addition reactions, giving carboxylates with rates that are competitive with ion-ion or radical-radical combination rates. 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Darensbourg, D. J. In Enzymatic and Model Carboxylation and Reduction Reactions of Carbon Dioxide Utilization Aresta, M., Schloss, J. V., Eds NATO ASI Series C, Vol. 314 Kluwer Academic Dordrecht, 1990 pp. 43-64. 

Major limitation associated with carbon dioxide reduction is the accuracy of the analytical measurements employed. The photocatalytic process is a multielectron transfer process, hence the reaction leads to the formation of a variety of products like carbon monoxide, methane, higher hydrocarbons, alcohol, aldehydes, carboxylic acid etc., with some intermediates. The identification and quantification of the products are needed for the best selection of photocatalyst, comparison and elucidation of reaction mechanisms. Currently there is no standard analysis method that has been developed for product analysis of carbon dioxide reduction. Hence the results of these measurements also include the products derived from the carbon contamination invariably present in the reaction sys-... 

Carbon Monoxide Reduction, The Water Gas Shift Reaction and Reactions of Carbon Dioxide... 

The reaction of carbon dioxide with Li, Na, K, and Cs in rare gas and nitrogen matrices leads to the reduction of the heterocumulene with formation of M C02 species. Two geometric isomers of Li C02 were isolated in solid argon or krypton... 

Ru or Os, the resulting products are methane, ethylene, and hydrogen. These reduction reactions of carbon dioxide proceed via an electron transfer pathway involving hydrogen ions and metal-activated carbon dioxide rather than by a hydrogenation route from the hydrogen that may be also formed in the photoreduction (Refs. 128 and 129) ... 

Carbon dioxide is a linear molecule in which the oxygen atoms are weak Lewis (and Br0nsted) bases and the carbon is electrophilic. Reactions of carbon dioxide are dominated by nucleophilic attacks at the carbon, which result in bending of the O—C—O angle to about 120°. Figure 5.3 illustrates four very different nucleophilic reactions hydroxide attack on CO2 to form bicarbonate the initial addition of ammonia to CO2, which ultimately produces urea the binding of CO2 to a macrocyclic cobalt(I) complex, which catalyzes CO2 reduction and the addition of an electron to CO2 to yield the carbon dioxide radical ion. The first three also exemplify the reactivity of CO2 with respect to nucleophilic attack on the carbon. 

In this chapter, we discuss theoretical studies of some selected transition metal-catalyzed reactions of carbon dioxide to illustrate how important concepts and insights can be derived as a result of these studies. These selected reactions include hydrogenation of CO2 with Hj, coupling reactions of COj and epoxides, reduction of CO2 with organoborons, carboxylation of olefins with COj, and hydrocarboxy-lation of olefins with CO2 and Hj. They are fundamentally important reactions of carbon dioxide and have been intensively investigated experimentally and theoretically. This chapter is not intended to be a comprehensive review. Instead, we discuss the above-mentioned selected examples that we believe to be representative and important in the area of homogeneous catalysis of COj by transition metals from our own perspective. 

Of these reactions the oxidation (combustion) of carbon and the oxidation (roasting) of pyrite are both highly exothermic, while the reduction of iron oxide with hydrogen is slightly endothermic. Both the reaction of carbon dioxide with carbon and the reaction of ferrous oxide with carbon are strongly endothermic. It is perhaps of interest to make some comments on the characteristics of the five reactions listed. 

Products and Standard Reduction Potentials of Carbon Dioxide Reduction Reactions, at pH 6.8, Involving 2-18 Electrons Standard Reduction Potential... 

E.R. Keene and B.P. Sullivan, Mechanism of the electrochemical reduction of carbon dioxide catalyzed by transition metal complexes. In B.P. Sullivan, K. Krist and H.E. Guard (eds.). Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, Elsevier, New York, Amsterdam, 1993,118-140. 

Preparation. Thiophosgene forms from the reaction of carbon tetrachloride with hydrogen sulfide, sulfur, or various sulfides at elevated temperatures. Of more preparative value is the reduction of trichi oromethanesulfenyl chloride [594-42-3] by various reducing agents, eg, tin and hydrochloric acid, staimous chloride, iron and acetic acid, phosphoms, copper, sulfur dioxide with iodine catalyst, or hydrogen sulfide over charcoal or sihca gel catalyst (42,43). 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

The reduction of carbon dioxide is another of the basic electrochemical reactions that has been studied at modified electrodes. The reduction at Co or Ni phthalocyanine in acidic solution yields formic acid or carbon monoxide A very high selectiv-... 

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