Pathways for Carbon Dioxide Reduction

If carbon dioxide is reduced directly to give products of interest, the reduction potentials for the half-cell reactions in an aqueous solution of pH 7 are as follows  

In aprotic solvents, the reduction of carbon dioxide to form carbon monoxide and carbonate ion2 is also possible  

These reactions can be easily combined, if necessary, with an anodic reaction such as oxygen evolution to estimate the thermodynamic standard free energy, AG  

The values of E° for Eqs. (15)-(20) indicate that if multielectron reductions of C02 take place, for example, by using suitable catalysts, the potentials required are much less negative than that for single-electron transfer, C02/C0J, and are also less negative 

A calculation of the temperature dependence of the free energy for the reactions in Eqs. (15)-(18), and hence the electrochemical potential, showed that with an increase in temperature, formic acid formation became more unfavorable.4 In the case of formaldehyde, methanol, and methane formation, the calculation indicated a positive shift in the reduction potential, but of very small magnitude ca. 30 mV for a temperature change from 300 to 500 K, and ca. 20 mV from 500 to 1200 K.4 

---