SURFACE CHEMISTRY 63 



adsorption was pointed out by the writer (Ref. 24 pp. 1 399-1 400) in 191 8 

 and illustrated by the adsorption of carbon monoxide and of oxygen on 

 platinum. At the temperature of liquid air and with a clean platinum surface 

 in contact with carbon monoxide at a pressure of 16 baryes, the surface 

 concentration o of the adsorbed carbon monoxide was 3.9 X 10^* molecules 

 cm""-, but, as the temperature of the platinum gradually rose to 20°C, 

 decreased to 1.4 X 10^'* and then increased again to 3.9 X lO^"* and re- 

 mained nearl}' constant until 200° C had been reached. When oxygen was 

 brought into contact with platinum at liquid air temperature, 1.5 X 10^^ 

 molecules cm~^ were adsorbed immediately. On raising the temperature 

 to 20°C, increased very slowly (in 18 hours) to 3.0 X 10^^. At 36o°C 

 rose to 4.8 X 10^^. It was concluded «that at liquid air temperature 

 platinum adsorbs carbon monoxide in much the same way that glass does, 

 that is, by secondary valence forces. With a moderate rise in temperature 

 this gas is released, but in the neighborhood of room temperature the 

 reaction velocity becomes sufficient for the platinum to react (primary 

 valence) with the carbon monoxide to form a much more stable adsorbed 

 film.» 



In a discussion of the mechanism of the dissociation of hydrdgen on 

 tungsten filaments in 19 16 the writer considered (7) the 'possibility that 

 the adsorbed hydrogen exists on the surface in two forms and that the 

 velocity of interaction between these forms may determine the rate of 

 production of atomic hydrogen. The mathematical formulation which was 

 given is applicable to many cases of activated adsorption. 



Within recent years H. S. Taylor (34) and his co-workers have 

 discovered many cases of activated adsorption and have demonstrated their 

 importance in an understanding of contact catalysis. The velocity of the 

 activation reaction, which is often low even at a temperature of several 

 hundred degrees, has been measured and the activation energy calculated. 

 Other investigators have sought to explain these slow surface reactions 

 by assuming solution of the adsorbed gas in the underlying substance 

 or slow penetration into cracks or capillary spaces. 



That this cannot be a general explanation is proved by some experi- 

 ments (35) that Dr. Blodgett and I have made on the thermal accom- 

 modation coefficient of hydrogen in*contact with tungsten. At temperatures 

 from 20o°K to 6oo°K a stable adsorbed film of hydrogen on tungsten is 

 formed which gives an accommodation coefficient a of about 0.22. As the 

 temperature is raised, this film goes over slowly into a much more stable 

 film which is characterized by a value of a = 0.14. The reaction velocity is 

 such that the formation of the stable film requires several minutes at 

 6oo°K and a small fraction of a second at 900°, indicating an activation 

 energy of the order of at least 20 Kg calories per molecule. Since the 



