MERCURY-SENSITIZED DECOMPOSITION OF WATER 71 



must be considered whenever the oxidation of water takes place in the 

 presence of air. 



1. Decomposition of Water in Ultraviolet Light 



The direct photochemical decomposition of water into hydrogen and 

 oxygen according to equation (4.3) was described by Coehn (1910) and 

 Coehn and Grote (1912). A " photostationary state" is established in 

 ultraviolet-illuminated water vapor, with light accelerating both its 

 decomposition and the recombination of hydrogen and oxygen (Berthelot 

 and Gaudechon 1910). 



The decomposition according to (4.4), i. e., with the formation of 

 hydrogen peroxide, was discovered by Thiele (1908) and Kernbaum 

 (1909). Tian (1916) suggested the existence, in ultraviolet-illuminated 

 liquid water, of a stationary state involving photochemical formation 

 and decomposition of hydrogen peroxide. If oxygen is present, all 

 hydrogen formed by (4.4) is taken away, making it possible for hydrogen 

 peroxide to accumulate, and the net effect is a peroxide formation 

 according to equation (4.6). 



In these investigations, a mercury arc was used, and the decomposition was caused 

 mainly by the first resonance line of mercury (189 mp), which is strongly absorbed by 

 quartz walls and only weakly absorbed by water. A more rapid decomposition can be 

 achieved by means of a hydrogen discharge tube with a fluorite window, as used by 

 Terenin and Neujmin (1934, 1935, 1936). The active wave lengths are 130-140 m/i, 

 which fall into the second absorption band of water. In this region, water decomposes 

 into OH* + H (the asterisk indicating electronic excitation), as proved by the emission 

 of OH bands in fluorescence. The primary process in the first absorption band of 

 water, situated below 178-179 m^, may be either: 



H2O* -^ OH + H, or H2O* ^ H2 + O (Goodeve and Stein, 1931) 



or (in the liquid state) : 



H2O* + H2O -* H2O+ + H2O- 



The direct photochemical decomposition of water solves a large part 

 of the diflficulties involved in artificial photosynthesis (it accumulates 

 energy and liberates oxygen), but it does not solve all of them, because 

 neither hydrogen molecules nor hydrogen atoms prove capable of reducing 

 carbon dioxide. The reaction between molecular hydrogen and carbon 

 dioxide will be discussed in more detail on page 83; as to atomic hydrogen, 

 Harteck (1933) found that the admission of hydrogen atoms to carbon 

 dioxide gas does not produce more than traces of formaldehyde. 



2. Mercury-Sensitized Decomposition of Water 



We now go over to sensitized photodecompositions of water. The 

 first extension of the photochemically active range towards the visible 

 can be achieved by using mercury vapor as a sensitizer. 



