ADSORPTION ISOTHERM AND THE STATES OF ADSORPTION

162
J. HORIUTI and T. Toy A
because of the contribution from the imperfections mentioned above and at
higher coverage on the other hand, since the preference of r-state adsorption
on the (llO)-plane is more or less counterbalanced by the repulsive interaction
of r-adatoms, which raises the energy of the adatoms.
The theoretical isotherms thus worked out accounts in the main quantitatively
for the observed ones 12 ) but the expected deviations reveal themselves
at lower and higher extremities of coverage as shown in § 7; the
deviation at lower coverages is explained qualitatively attributing it to the
imperfections, whereas that at higher coverge is satisfactorily reproduced by
ascribing it to the s·state adsorption on the (llO)-plane.
It might now be questioned how to reconcile the above conclusion with
the experimental results of the differential isotopic method 13 ) that the portion
adsorbed later is desorbed initially in preference to that adsorbed earlier as
observed by isotopic tracer, which is taken as the exclusive evidence for the
distribution model, on account of the inability of the crystal plane model
to explain it as based on the simple assumption of the repulsive potential
proportional to the coverage. It was shown by one of the present authors W
as summarised in § 8 that the experimental result of the differential isotopic
method is adequately derived from the crystal plane model, if allowed properly
for the repulsive interaction and for the appearance mentioned above of the
s-state on the same plane at higher coverage.
On these grounds it is concluded that the distribution model is inapplicable
but the crystal plane model with the above extension is the sound basis of
accounting for the observed isotherms.
Detailed accounts will be given of the above points in the following sections.
§ 1. Energy of Single Adatom
Consider a hydrogen adatom in the absence of interaction on a perfect
crystal plane of a metal, which will be called a single adatom in what follows.
The energy Wr of a single adatom at the r-state is determined by the equation
10 )
based on the quantum-mechanical theory of adsorption developed by one of
the present authors 10 ), where
Ro: repulsive potential of the adatom due to metal electrons and metal ions,
E+: energy of the H+ -state of the adatom averaged over the vacant energy
( 1)