64 PHENOMENA, ATOMS, AND MOLECULES 



accommodation coefficient is a purely surface property, these experiments 

 prove that these two forms of hydrogen film are both monatomic films 

 which probably represent two kinds of chemical binding by valence forces. 

 At temperatures below liquid air temperature a clean tungsten surface 

 would undoubtedly show also van der Waals' adsorption of hydrogen. 

 This would thus be a third kind of adsorbed film of hydrogen on tungsten. 



Catalytic Action of Surfaces. A monatomic film of oxygen on tungsten 

 at 1,500° acts as a catalytic poison for nearly all the reactions which would 

 otherwise occur in contact with the tungsten surface (15). Thus the dis- 

 sociation of hydrogen into atoms at i,500°K is stopped by a trace of 

 oxygen, as is also the decomposition of ammonia, methane or cyanogen. 

 The effect of the oxygen is to cover the surface so that the other gas cannot 

 make contact with the tungsten surface. 



Similarly hydrogen and carbon monoxide act as catalytic poisons on 

 platinum surfaces (36). The rate at which carbon monoxide and oxygen 

 combine in contact with platinum is proportional to the pressure of oxygen 

 and inversely proportional to the pressure of the monoxide. The reaction 

 velocity depends on that fraction of the platinum surface which is not 

 covered by adsorbed molecules of carbon monoxide. The oxygen molecules 

 that can become adsorbed in these vacant spots and so be adsorbed on the 

 platinum surface can thus react with adjacent adsorbed molecules of car- 

 bon monoxide. 



In numerous cases investigated, the action of a surface in catalyzing a 

 gas reaction involves just such interaction between molecules adsorbed in 

 adjacent elementary spaces on the surface. On this basis it is possible to 

 develop (7) (13) (36) a «law of mass action* for the velocity of surface 

 reactions by which the observed reaction velocities can be explained quan- 

 titatively. It has been pointed out (15) that reactions of this sort are 

 extremely sensitive to the actual distances between the atom« in the sur- 

 face of the catalyst. Thus with many surfaces there is only a relatively 

 small fraction of the surface on which the reaction can occur with extreme 

 rapidity, while over the larger part of the surface it takes place at a 

 negligible rate. A quantum theory explanation of the importance of the 

 spacing of the atoms in the catalyst has recently been given by Sherman 

 and Eyring (37). 



The presence of a second layer of adsorbed atoms or molecules, 

 although this second layer may cover only a minute fraction of the sur- 

 face, is often of great importance in the mechanism of gas reactions 

 brought about by a surface catalyst. 



Thus in the oxidation of heated tungsten filaments in oxygen at low 

 pressures by which WO3 is formed, the oxygen molecules incident on a 

 surface already nearly completely covered by a monatomic film of ox3^gen 



