Photochemical reduction, carbon dioxide

The essence of natural photosynthesis is the use of photochemical energy to split water and reduce CO2. Molecular oxygen is evolved in the reaction, although it appears at an earlier stage in the sequence of reactions than the reduction of carbon dioxide. Photochemical processes produce compounds of high chemical potential, which can drive a multistep synthetic sequence from CO2 to carbohydrate in a cyclic way. Reaction (16) is quite endoergic and thus thermodynamically very improbable in the dark (AG° = 522 kJ per mole of CO2 converted). Production of one molecule of oxygen and concomitant conversion of one molecule of carbon dioxide require the transfer of four electrons ... 

Kuwabata S, Nishida K, Tsuda R, Inoue H, Yoneyama H (1994) Photochemical reduction of carbon dioxide to methanol using ZnS microcrystaUite as a photocatalyst in the presence of methanol dehydrogenase. J Electrochem Soc 141 1498-1503... 

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). 

For a fuel, an electron source is needed. Water is the ultimate electron source from an economical point of view. Water photolysis is the simplest among the chemical conversion systems of solar energy. Photochemical reduction of nitrogen or carbon dioxide to produce ammonia or hydrocarbons with the electrons from water is also an attractive system of conversion. 

These products are the most important components of the living matter of plants, e.g. cellulose. In these simple terms photosynthesis is the photochemical oxidation of water, and reduction of carbon dioxide, by means of a photoactivated catalyst which in green plants is the molecule chlorophyll. 

The end result of the photochemical part of photosynthesis is the formation of 02, NADPH, and ATP. Much of the oxygen is released to the atmosphere, but the NADPH and ATP are utilized in a series of dark reactions that achieve the reduction of carbon dioxide to the level of a carbohydrate (fructose). A balanced equation is... 

Although water photolysis is the simplest photochemical conversion system, carbon dioxide reduction is still an attractive research subject as a synthetic model for C02 reduction in photosynthesis. There are numerous reports on chemical and photochemical C02 reduction,245 but it is not the aim of this chapter to review these works. 

Photochemical, Electrochemical, and Photoelectrochemical Reduction of Carbon Dioxide... 

Ishitani O, Inoue C, Suzuki Y, Ibusuki T. Photocatalytic reduction of carbon dioxide to methane and acetic acid by an aqueous suspension of metal-deposited Ti02. J Photochem Photobiol A Chem 1993 72 269-71. 

From the point of view of organic synthesis, the overall process consists of the formation of carbohydrates (CH20) by the reduction of carbon dioxide. The essence of the process is the use of photochemical energy to split water and concomitantly to reduce C02. Many proteins and small molecules are involved in photosynthetic machinery. Inorganic species are in the centre of photosynthesis as pigments in light harvesting, substrates, products, catalysts, and electron transfer mediators. 

Khan MMT, Rao NN, Chatterjee D. A novel photosynthetic mimic reaction catalysed by K[Ru(H-EDTA)C1]-2H20 reduction of carbon dioxide to formate and formaldehyde in the presence of an aqueous suspension of Pt-CdS-Ru02. / Photochem Photobiol A Chem 1991 60 311-18. 

The reduction of carbon dioxide to formate in the presence of FDH represents a photochemical C02-fixation process. In addition to MV+, the enzyme recognizes other electron carriers such as 2,2 -bipyridinium radical cations [184]. Reduction of nitrate (N03 ) to nitrite (NO2 ) and subsequently the reduction of nitrite to ammonia in the presence of NitraR and NitriR, respectively, allows the sequential 8e ... 

Natural photosynthesis is the most important of the many interesting photochemical processes known in biology. Not only was the evolution of the Earth s atmosphere dependent on it, but it is also the main route by which the free energy of the environment is made available to the living world. Green plants, algae and cyanobacteria make use of sunlight to drive a thermodynamically uphill reaction, the reduction of carbon dioxide to carbohydrates by water ... 

Evidence has been obtained that the photodecarboxylation of benzannelated acetic acids in aqueous solution proceeds via carbanion intermediates 5H-dibenzo[a,d]cyclohepten-5-carboxyl-ic acid, for example, undergoes photodecarboxylation with a quantum efficiency of close to unity. The reactions of the -nitrobenzyl anion, generated by photochemically induced loss of carbon dioxide from -nitrophenylacetate anion, have been examined and a mechanism proposed to account for the formation of the corresponding bibenzyl derivative. Reductive decarboxylation of carboxylic... 

Photochemical carbon dioxide reduction with metal complexes Differences between cobalt and nickel macrocycles... 

Our research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO2 reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis. X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here we summarize our research on photochemical carbon dioxide reduction with metal macrocycles. 

The challenging photochemical reduction of carbon dioxide to formate is catalyzed by Ru" [111] (cf. Section 3.3.4). For example, with the 2,2 -bipyridine-ruthenium(II) complex the active species is formed by photolabilization. Water renders the system more efficient with quantum yields up to 15%. Methanol is the photoproduct when CO2 is reduced with Ti02 in propene carbonate/2-propanol... 

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