Studies in contact potentials,,: II. The competitive adsorption of hydrogen atoms and molecules on tungsten

Surface potential measurements show the adsorption of hydrogen on tungsten to be complex. Besides the negative effect due to the familiar atomic film, a positive effect is observed. Experimental evidence reveals two important points, viz, (i) the positive effect is due to a molecular adsorption, and (ii) at the approach of saturation, the latter is able to compete with the atomic adsorption. The properties of the mixed adsorption of atoms and molecules on a surface are examined theoretically. Three main conclusions are reached. The competition between the atomic and molecular adsorptions may give rise to a special mode of adsorption, consisting in the replacement of atoms by molecules and taking place near saturation on a surface with practically no vacant sites. Through substitution, a molecular adsorption may compete with an atomic adsorption with double the differential heat of adsorption. The saturated mixed film is normally non‐stoichiometric. The model provides a satisfactory explanation of some peculiarities of the positive effect. The high heat of the molecular adsorption (up to 8 kcal/mole) is to be attributed essentially to the charge transfer forces, the contribution of which is much greater than that of the dispersion forces. Using the above ideas, it is shown that the evidence regarding the catalytic hydrogen exchange at low temperature is in favour of the Rideal mechanism. Substitutional adsorption is an activated process. It may account for a part of certain effects frequently attributed to dissolution.