Hydrogen tungsten surface covering

The heat of adsorption of nitrogen on tungsten at room temperature was found to be 95,000 calories per mole and constant over the whole fraction of the surface which it will cover—that is, about 60%. The adsorption is instantaneous. In Fig. 21 is [shown the heat of adsorption of hydrogen on a tungsten surface covered 60% with nitrogen. 

When hydrogen is adsorbed on a tungsten surface covered with one layer of oxygen, hydrogen and oxygen are not desorbed at 1150°K. even in 20 min, (11). Evidently H is held more firmly on 0-W than it is on clean W. However, at 1200°K. H2O is desorbed from the 100 region in a short time at 1300°K. H2O is desorbed on the 111 plane at 1500°K. clean tungsten is obtained in 2 min. 

In this section a method for the direct calorimetric determination of heats of adsorption on evaporated metal films is described and results for the heals of adsorption of hydrogen on nickel, iron, and tungsten are reported. In all cases the heats of adsorption decrease with the fraction of surface covered in a mode that can satisfactorily be explained by interaction of adsorbed atoms. A criterion for mobility of the adsorbed atoms is developed... 

Fig. 18. Heats of adsorption of hydrogen on evaporated tungsten films at 23°C. as a function of surface covered.

A paper by 0. Beeck (33d) has only recently come to the author s notice. Working with evaporated films of metal of area of the order of 10,000 cm. Beeck has confirmed all Roberts results on the speed, extent, and heat of adsorption as a function of surface covered, for the adsorption of hydrogen on tungsten. Nickel shows a similar behavior. This makes extremely likely the view that adsorption on tungsten powder is complicated in some fashion, if not by solution, as suggested above, then by some kind of physical or chemical heterogeneity of the surface, as in the Halsey and Taylor picture. 

Not only the hydrogen adsorption but also the nitrogen adsorption as observed by Davis (17a), as well as by Beeck and coworkers fits well into this picture. It was observed by Beeck et al. that 60 % of the surface of evaporated tungsten films could be covered by nitrogen at room temperature, 50% being adsorbed instantaneously with a constant heat... 

The heats of chemisorption of hydrogen on nickel and iron are nearly identical and decrease from about 30,000 calories for the sparsely covered surface to about 18,000 calories for the completely covered surface. The heat of chemisorption on tungsten decreases from 45,000 calories to about 13,000 calories as a function of surface coverage (Roberts, also Beeck and coworkers). The lower values of Frankenburg for higher... 

Oxygen and tungsten reacted in a totally different manner. A minute pressure of oxygen formed an adsorbed film over the whole surface of the filament. The presence of this film was shown in three different ways it was found to diminish the thermionic emission of electrons from the heated wire at 1,500° K. it was found to inhibit the dissociation of hydrogen at the surface of the wire and it was shown by quantitative studies that the rate of reaction of the filament covered by adsorbed oxygen, with further supplies of the gas, was so great as to require the presence of a monatomic film of adsorbed oxygen, already combined with the surface of the filament for, p. 1150). 

Furthermore, if a film of nickel or tungsten, which is very active in the exchange reaction (too fast to be measured) is covered with D2 at 78° K and the excess D2 removed, only about 1 % of the preadsorbed D2 will exchange with H2 circulated over the film at 78° K. It follows, therefore, that only this 1 % of the surface is active in the exchange reaction, while this is clearly not so for hydrogenation reactions. I believe similar results to the above have been obtained in Holland by Schuit and his coworkers. 

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