CHEMICAL REDUCTION OF CARBON DIOXIDE 79 



1. Chemical Reduction of Carbon Dioxide 



So far, carbon dioxide has been reduced in vitro only by means of the 

 strongest available reductants, or at high temperatures. 



Fenton (1907) described the reduction of carbon dioxide to formaldehyde by 

 magnesium, while Bredig and Carter (1914) have achieved the reduction of carbon 

 dioxide to formic acid by means of hydrogen and palladium. Reactions of this type 

 are of no use in artificial photosynthesis. Imagine, for example, that we would begin 

 by reducing carbon dioxide with magnesium, and — to provide a similar start at the 

 other end of the reaction chain — oxidize water with fluorine. We would thus obtain 

 magnesium oxide and hydrogen fluoride as the first reaction products. Bringing these 

 two compounds together wall lead to the formation of magnesium fluoride, and the 

 completion of the reaction cycle would now require the photochemical dissociation of this 

 salt into metal and halogen, a task considerably more difficult than photosjm thesis itself. 



One comparatively mild reductant which has been credited with the capacity to 

 reduce carbon dioxide, was hydrogen peroxide. Kleinstuck (1918) working in Wislicenus' 

 laboratory, found that phosgene, diphenyl carbonate and carbonate ions, can be reduced 

 to formaldehyde by heating with hydrogen peroxide under pressure. Wislicenus (1918) 

 corrected the results, stating that the reduction product obtained from alkah carbonates 

 (or bicarbonates) and hydrogen peroxide is formic acid, and suggested that the process 

 involves the formation of percarbonic acid as an intermediate: 



OOH 

 (4.18a) H2O2 + H2CO2 > OC + H2O 



OH 

 (4.18b) H2CO4 > H2CO2 + O2 



(4.18) H2O2 + H2CO3 > H2O + O2 + H2CO2 - 44 kcal 



Thunberg (1923), while faihng to confirm most of Kleinstiick's results, claimed that 

 formaldehyde can be obtained by boiUng lead carbonate with hydrogen peroxide. 



Thunberg and Weigert (cf. page 70) have used these results as basis for a theory 

 according to which photosynthesis consists of a photochemical decomposition of water 

 into hydrogen and hydrogen peroxide, and a nonphotochemical reduction of carbon 

 dioxide by the latter two compounds: 



light 



(4.19a) 2 H2O > (H202)aq. + H2 - 79 kcal 



(4.19b) CO2 + H2 + (H202)aq. > {CH2O} + O2 + H2O - 33 kcal 



(4.19) CO2 + 2 H2O > {CH2O! + H2O + O2 - 112 kcal 



However, not only reaction (4.18), in which hydrogen peroxide alone reduces carbon 

 dioxide, but even reaction (4.19b), in which hydrogen peroxide is assisted by hydrogen, 

 is endothermal to such an extent that it cannot occur spontaneously at low temperatures. 

 Some doubts may be entertained as to the reliability of Thunberg's experiments; 

 but, even if they are correct, they do not point a way by which carbon dioxide can be 

 reduced at low temperatures. The energy accumulated in the oxidation of water to 

 peroxide by oxygen (23 kcal per mole) is much too small to enable the product to reduce 

 carbon dioxide without an external supply of energy. The oxidation-reduction potential 

 of the system O2-H2O2 (- 0.27 volt at pH 7) is much too negative to bring about the 

 reduction of the system H2CO3-H2CO2, or H2CO3-H2CO, whose potentials (at the same 

 pH) are above + 0.4 volt (cf. Table 9,1 V). 



