SENSITIZATION OF WATER DECOMPOSITION BY SOLIDS 73 



Richardson (1939) found that the quantum yield of this reaction is 

 of the order of 0.1 in weak light, and less at the higher light intensity. 

 The rate of peroxide formation shows a "light saturation" similar to 

 that occurring in photosynthesis, proving that the photochemical process 

 is coupled with a thermal process of limited velocity. (For example, 

 the photochemical decomposition of water adsorbed at the surface of 

 ZnO may be followed by the desorption of the reaction products.) 

 The mechanism of this reaction is unknown, but we may assume that 

 the primary process is the decomposition of adsorbed water into OH 

 and H, made possible by the large heats of adsorption of OH and H on 

 zinc oxide. 



light 



(4.9) H2O (adsorbed) ^ H (adsorbed) + OH (adsorbed) 



dark 



In order to enable reaction (4.9) to occur in the near ultraviolet (that is, 

 with light quanta of about 78 kcal per einstein, one einstein being 6 X 10^^ 

 quanta), the combined heat of adsorption of the radicals must be at 

 least 35 kcal larger than that of water. 



The assumed primary reaction (4.9) is of the type postulated by 

 van Niel for photosynthesis (c/. Eq. 7.1) — photochemical decomposition 

 of water, with zinc oxide serving as acceptor for both hydrogen atoms 

 and hydroxyl radicals. To explain why hydrogen peroxide is formed 

 only in presence of oxygen, we may assume that oxygen molecules 

 snatch away the adsorbed hydrogen atoms, thus preventing the reversal 

 of reaction (4.9), and leaving to the hydroxyl radicals no other way but 

 to recombine to "biradicals" H2O2. In this way, the primary photo- 

 chemical decomposition of water is again reduced to a " photautoxida- 

 tion," according to equation (4.7), with its comparatively small energy 

 conversion. 



The question arises as to whether the back reaction in (4.9) is com- 

 pletely effective in absence of oxygen, or whether some hydrogen atoms 

 succeed in recombining to hydrogen molecules, causing an equal num- 

 ber of hydroxyl radicals to recombine to H2O2 and giving the net effect 

 of sensitized water decomposition (4.4), a result much more significant 

 from the point of view of artificial photosynthesis than the photautoxi- 

 dation (4.7). Successful achievement of reaction (4.4) would leave us 

 with the problem of carbon dioxide reduction by molecular hydrogen 

 as the final stage of artificial photosynthesis — a reaction which re- 

 quires no additional conversion of energy. True, we do not yet know 

 how to conduct it in a reversible way, without spending considerable 

 energy on activation; but we shall see in chapter 5, that the so-called 

 "Knallgas bacteria" reduce carbon dioxide to carbohydrates in the dark, 

 by means of molecular hydrogen, with up to 40% of the theoretical yield. 



