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The adsorption of hydrogen 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

A method for studying the adsorption of hydrogen on a clean tungsten surface has been developed which depends on the fact that the accommodation coefficient of neon is different for a bare surface and for a surface with an adsorbed film on it.

It has been shown that on a bare tungsten surface, saturation occurs at a partial pressure of hydrogen of 4.10−4 mm. of mercury and probably much lower, so that the adsorption is of the nature of chemi-sorption.

Important deductions can be drawn concerning the process of activation if any for this chemi-sorption of hydrogen on a bare tungsten surface.

The accommodation coefficient of neon with a tungsten surface covered with a hydrogen film of the type investigated is 0·17 at 295°K, and 0·32 at 79°K.

The experiments described were carried out during my tenure of a Moseley Studentship at the Cavendish Laboratory, and I should like to thank Lord Rutherford for his interest in the work and the Council of the Royal Society for their continued support. I should also like to thank Professor Rideal for his interest in the results.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 30 , Issue 1 , January 1934 , pp. 74 - 79
Copyright
Copyright © Cambridge Philosophical Society 1934

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References

* Roberts, , Proc. Roy. Soc. A, 129, p. 146 (1930)CrossRefGoogle Scholar; 135, p. 192 (1932); 142, p. 518 (1933).

† Rupp, , Ann. d. Physik, 5, p. 453 (1930).CrossRefGoogle Scholar

‡ See, for example, Thomson, G. P., Proc. Roy. Soc. A, 133, p. 1 (1931)CrossRefGoogle Scholar; Rupp, , Ann. d. Physik, 13, p. 101 (1932)CrossRefGoogle Scholar; Farnsworth, , Phys. Rev. 43, p. 900 (1933)CrossRefGoogle Scholar, and earlier papers.

* An upper limit to the energy of activation for the type of adsorption observed can be obtained by assuming that only molecules which strike the surface with translational energy greater than the energy of activation can be adsorbed. We assume further (1) that the hydrogen is adsorbed as atoms, (2) that the spacing in the adsorbed film is double that of the metal atoms in the surface (cf. the paper by Germer cited below), and (3) that after ten minutes at 79° K. the film is four-fifths occupied—i.e. we assume that if this was not the case we should certainly detect a difference in the rates of attaining the final equilibrium temperature at 79° and 295° K. The choice of the fraction four-fifths is arbitrary. In this way we obtain the result that the energy of activation is about 2000 calories per mol. H2 or a little less. It is important to note that a comparison of the curves at 79° and at 295° K. shows that the instrument used, and not necessarily the adsorption process, is fixing the time of ten minutes. The meaning of this result is, therefore, that the energy of activation lies between 0 and 2000 calories per mol. H2; and, with partial pressures of hydrogen which up to the present it has been possible to measure satisfactorily, the instrument cannot resolve these limits.

It is important to mention that the energy of activation calculated in this way may not be compared as to order of magnitude with values usually quoted in connection with the conception of “activated” adsorption, since the latter values are calculated on an entirely different basis.

* It should be mentioned that at these higher partial pressures it is necessary to make a correction for the heat carried away from the wire by the hydrogen, since hydrogen owing to its smaller mass and to the fact that it has rotational as well as translational energy is a more efficient agent than neon at the same pressure. In making the correction it was assumed that the accommodation coefficient of hydrogen on a tungsten surface was 0·22. This is the “initial” value given by Blodgett, and Langmuir, (Phys. Rev. 40, p. 78 (1932))CrossRefGoogle Scholar and refers to tungsten flashed in vacuo to which hydrogen is then introduced.

† Germer, (Zeit. f. Physik, 54, p. 408 (1929))CrossRefGoogle Scholar has discussed the observation by the electron diffraction method of the gradual formation of adsorbed films on a bare nickel crystal due to residual traces of gas. The nature of the gas could not be determined, but it may of course have been hydrogen. The spacing of the adsorbed layer was twice that of the nickel atoms in the surface of the crystal.

* As the result of a certain line of reasoning Blodgett, and Langmuir, (Phys. Rev. 40, p. 104 (1932)CrossRefGoogle Scholar—a portion of the last sentence on the page) say “…we believe that we never have a bare W surface in contact with hydrogen at low temperatures, but instead of this an HW surface…”. The present work provides strong experimental evidence that this view is correct even at very much lower pressures than were considered by them.

† Such a picture has been proposed by Taylor, and Rideal, (Faraday Soc. Discussion on Adsorption of Gases, pp. 137, 147 (1932))Google Scholar. They imagine the hydrogen molecules to be first adsorbed by van der Waals' forces and then to receive the necessary energy of activation from the surface and pass to the chemi-sorbed state.