10.4 Photochemical Reactions for CO 2 Conversion
For convenience of the reader, we report here again the table of the reduction
potentials of CO 2 in water (Table 10.1) as they are relevant to the use of solar light
for driving chemical reactions. The solar light-driven reduction of CO 2 has been
attempted using solid and soluble catalysts and is a complex process (Fig. 10.2). In
solution, it was described by Nobel Laureate J.M. Lehn in 1983 using [Re(dipy)
(CO) 3 Cl] (Fig. 10.3) and Ru-metal systems [5].
Fig. 10.1 Solar Spectrum. From the left: UV (200–400 nm), Vis (400–700 nm) IR (700–
2500 nm) [3b]
Table 10.1 Potential of reduction of CO 2 in water
Reaction
E° [V] versus SHE at pH 7 in water
1. CO 2 + e
− ! CO 2
−
−1.9 (−2.1 in organic solvents)
2. CO 2 + 2H
+ + 2e
− ! HCOOH
−0.61
3. CO 2 + 2H
+ + 2e
− ! CO + H 2 O
−0.52
4. 2CO 2 + 12H
+ + 12e
− ! C 2 H 4 + 4H 2 O
−0.34
5. CO 2 + 4H
+ + 4e
− ! HCHO + H2O
−0.51
6. CO 2 + 6H
+ + 6e
− ! CH 3 OH + H2O
−0.38
7. CO 2 + 8H
+ + 8e
− ! CH 4 + 2H 2 O
−0.24
8. 2H
+ + 2e
– ! H 2
−0.42
180
10 Solar Chemistry and CO 2 Conversion