[boswell] catalysis OF HYDROGENATION BY NICKEL 11 



to see, from this standpoint alone, why the reaction with ethylene 

 of hydrogen already absorbed by nickel, should cause the adsorption 

 of a much larger volume of hydrogen than has reacted with the 

 ethylene. Moreover, were this explanation correct, one would cer- 

 tainly expect to find a nickel complex, carrying a considerable amount 

 of adsorbed hydrogen, able to effect, to a measurable extent, the 

 hydrogénation of ethylene at 150°C., in the absence of free hydrogen. 

 Experiment (5) has shown that such is not the case. 



It seems a more satisfactory explanation to say that free hydro- 

 gen has reacted, not only with the ethylene but with something on 

 the surface of the catalyser complex other than unchanged nickel, 

 oxide. The conclusion is almost unavoidable, that oxygen in some 

 active form (possibly as negative hydroxyl groups, accompanied 

 by positive hydrogens) is present on the nickel surfaces, formed by 

 the reduction of nickel oxide at temperatures below 300°C., and that 

 it is with this special form of oxygen that the relatively large amount 

 of hydrogen, which disappeared in experiment (5), reacted. 



Confidence in this view is further increased by the fact, demon- 

 strated in experiment 10, that a nickel, free from oxygen, is a poor 

 catalyst for hydrogénations, and also by the fact that as a normal 

 nickel catalyser loses its oxygen, which it slowly does during hydro- 

 génations, it also gradually loses its catalytic activity, while a nickel 

 free from oxygen and carrying hydrogen alone, is almost useless as 

 a catalyst for hydrogénations. 



All these facts taken in conjunction point very strongly to the 

 conclusion that the reaction of hydrogen with ethylene is not only 

 accompanied by reaction of hydrogen with this special form of oxygen, 

 but that these two reactions are mutually dependent and simul- 

 taneous reactions. 



The writer suggests as a mechanism for hydrogénation, by a 

 normal nickel catalyser, the reactions given at the conclusion of 

 experiment 7. These four reactions seem to furnish an adequate 

 picture of the facts referred to above, as well as of the others dealt with 

 later in this paper. According to this mechanism the complex, 

 formed by the partial reduction of nickel oxide, which is active in 

 catalysing hydrogénation, consists of nickel oxide particles covered 

 by nickel surfaces carrying negative hydroxyl groups and positive 

 hydrogens, as represented in the following constitution, where only 

 one layer of hydroxyls and hydrogens is shown. 



