SENSITIZED REDUCTION OF CARBON DIOXIDE 85 



thus playing the part of true photocatalysts, or did they also serve as re- 

 ductants? Of course, to reduce carbon dioxide by uranyl salts or ferrous 

 salts would be an important success, since the oxidation-reduction po- 

 tentials of these substances are far below the potential of the system 

 CO2-H2CO. Still, this reduction would represent only one-half of photo- 

 synthesis, the remaining half being the reduction of the oxidized cata- 

 lyst (e. g., ferric iron) by water, leading to the liberation of oxygen 

 (as in Hill's experiments with isolated chloroplasts). It is, however, 

 improbable that Moore and Webster have achieved even that much, 

 since Baur and Rebmann (1922), in attempts to repeat their experiments, 

 have failed to observe any formation of formaldehyde, oxalic, glyoxalic 

 or formic acid, not to speak of evolution of oxygen. 



(a) The Experiments of Baly and Dhar 



The subsequent development of the subject, in two long series of 

 publications, one by Baly and coworkers in Liverpool (1927-1940) and 

 the other by Dhar and coworkers in Allahabad (India) (1925-1933), 

 brought many spectacular claims, but no convincing results. One is 

 therefore tempted to dispense with their presentation altogether; but 

 wishing the reader to be able to form his own opinion as to the validity 

 of claims which have been repeated so persistently (lastly, in Baly's 

 monograph. Photosynthesis, published in 1940), we will review in some 

 detail the experiments on which these claims were based. 



Baly's experiments have attracted most attention, because of the 

 reputation of the author, and of the comparatively large yields of organic 

 matter which he and his coworkers claimed to have obtained in their 

 first investigations. 



Baly, Davies, Johnson and Shanassy (1927), employed white powders (barium sul- 

 fate or alumina) as sensitizers, and used ultraviolet light. Baly, Stephan and Hood 

 (1927) went over to the use of colored powders (basic carbonates of nickel and cobalt), 

 illuminated by incandescent lamps. Carbon dioxide was bubbled for several hours 

 through illuminated vessels containing these powders suspended in water; the solution 

 was then separated from the powder and evaporated. A gummy residue was obtained 

 which gave some aldehyde and sugar reactions (reduction of Benedict's solution, MoUsch 

 test; Rubner test; osazone formation). The carbonates soon lost their "catalytic" 

 capacity; Baly attributed this to their oxidation by the oxygen produced by photo- 

 synthesis (no direct test for oxygen production was ever attempted). The yield of 

 "artificial carbohydrates," obtained by Baly and Davies (1927) was up to 75 mg. in 

 two hours, in a vessel with a surface of 300 sq. cm., i. e. "about equal to the yield of 

 natural photosynthesis on an equal area covered by vegetation." Baly and Hood 

 (1929) found that the rate of "artificial photosynthesis" increased between 5° C. and 

 31°, and declined between 31° and 41°, Uke that of natural photosynthesis. 



Difficulties in reproducing these first promising results soon arose, and 

 the next ten years were spent on attempts to prepare reliable catalysts. 



