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

近年来，由于大气中CO_2浓度的增加，引起了许多环境问题。最严重的问题之一是由于温室效应的累积而导致的气温上升。因此，对二氧化碳减排的研究引起了人们的关注。然而，CO_2的还原非常困难，而且是在高温高压下进行的。在本研究中，CO和CO_2在电极介体的均相催化下被还原为甲醇，并对其还原机理进行了探讨。结果表明：(1)CO和CO_2以较高的电流效率还原为甲醇.(2)红外光谱和核磁共振谱均表明存在H^&lt；13&gt；COOH，由此推测CO和CO_2的还原是通过甲酸甲酯进行的.(3)质量分析测得的同位素分布表明生成了Cd_3OH和^&lt；因此，CO和CO_2的电催化还原按如下方案进行：首先在中心金属和甲醇之间形成弱配位键，然后插入CO_2(或CO)形成中间金属-配体-甲酸盐络合物，该络合物最后被电极介体还原，消耗溶液中的质子生成甲醇和初始金属络合物。

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.