Mechanism of the Catalytic Reduction of CO and CO_2 at Ordinary Temperature and in Atmospheric Pressure

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Recently, many environmental problems has been pointed out which are caused by the increase of CO_2 in the atmosphere. One of the most serious problems is a rise in temperature due to the buildup of the greenhouse effect. Hence, the study on the reduction of carbon dioxide attracts attention. However, the reduction of CO_2 is very difficult, and it is carried out at elevated temperature and in high pressure.In the present study, CO and CO_2 was catalytically reduced to methanol in the presence of homogeneous catalysis with an electrode mediator, and the reduction mechanism was investigated. The results obtained are as follows:(1) CO and CO_2 were reduced to methanol with high current efficiency.(2) Both IR and ^<13>C-NMR spectra showed the existence of H^<13>COOH, and hence it was suggested that the reduction of CO and CO_2 proceed via meythylformate as an intermediate.(3) The isotope distribution measured by mass analysis showed the formation of CD_3OH and ^<13>CH_3OH.(4) Consequently, the electrocatalytic reduction of CO and CO_2 proceeds by the following scheme: a weak coordination bond is first formed between the central metal and methanol, followed by the insertion of CO_2 (or CO) to form an intermediate metal-ligand-formate complex, and this complex is finally reduced by the electrode mediator with the consumption of protons in solution to methanol and initial metal complex.