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The Dissociation Equilibrium of Hydrogen and its Adsorption on Tungsten

Published online by Cambridge University Press:  24 October 2008

J. K. Roberts
J. K. Roberts
Affiliation:
Trinity College
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Extract

The various processes occurring at the surface when hydrogen is adsorbed on tungsten are considered together with the dissociation equilibrium of hydrogen in the gas phase. The form of the adsorption isotherm is deduced from the principle of detailed balancing and is in agreement with that obtained by Fowler using a statistical method. A detailed interpretation of the experimental results now available shows that either (a) measurements of the rate of removal of the adsorbed film of oxygen on tungsten do not measure the rate of evaporation of oxygen atoms or (b) it is not possible to obtain a general first approximation formula giving the rate of evaporation of adsorbed atoms in terms of the heat of desorption. The desorption of hydrogen from tungsten is discussed and it is shown that the agreement between the temperature at which the film evaporates at an appreciable rate and that deduced from a desorption formula of the type mentioned in (b) assuming that the hydrogen evaporates as atoms must at present be regarded as a coincidence.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 32 , Issue 1 , January 1936 , pp. 152 - 157
Copyright
Copyright © Cambridge Philosophical Society 1936

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References

* Langmuir, , Journal Amer. Chem. Soc. 54 (1932), 2798.CrossRefGoogle Scholar

† Simple is meant to exclude such processes as those which involve the recombination of two separate adsorbed atoms on the surface and subsequent evaporation as a molecule. This proviso is added here.

‡ Roberts, , Trans. Faraday Soc. 31 (1935), 1710.CrossRefGoogle Scholar

* Langmuir, and Villars, , Journal Amer. Chem. Soc. 53 (1931), 495.CrossRefGoogle Scholar

† It should be mentioned that the arguments in this paper would be unaffected if instead of this particular desorption formula that proposed by Langmuir, (Journal Amer. Chem. Soc. 54 (1932), 2798CrossRefGoogle Scholar) on the basis of his empirical vapour pressure equation was used.

‡ Roberts, , Proc. Roy. Soc. A, 152 (1935), 445.CrossRefGoogle Scholar

§ It is difficult to state this without appearing to imply a criticism of this excellent and fundamental work by Langmuir and Villars, but the whole question appears to be of such essential importance that the possibility of their interpretation being incorrect must be considered.

* Giauque, , Journal Amer. Chem. Soc. 52 (1930), 4816CrossRefGoogle Scholar. He uses 102,800 for the heat of dissociation instead of the more recent value 101,000, but as we are only concerned with orders of magnitude his results are used.

* This agrees with the form deduced by Fowler, , Proc. Camb. Phil. Soc. 31 (1935), 262CrossRefGoogle Scholar, from statistical considerations.

* Assuming of course that at room temperature the only important process by which the film is formed is the adsorption of the two atoms of a molecule striking the surface.

† Roberts, , Proc. Roy. Soc. A, 152 (1935), 460.Google Scholar

‡ Langmuir, , Journal Amer. Chem. Soc. 54 (1932), 2798CrossRefGoogle Scholar; Roberts, , Trans. Faraday Soc. 31 (1935), 1710.CrossRefGoogle Scholar