92168.pdf

! 18 The solid-gas interface

Dissociation

energy of X2

Activation energy

for chemisorption

X+X

p Distance

Heat of

physical adsorption

Heat of

chemisorption

Figure 5.2 Potential energy curves for physical adsorption and chemisorption

surface than the relatively shallow minimum in the physical adsorp-
tion curve.
It can be seen from these curves that initial physical adsorption is a
most important feature of chemisorption. If physical adsorption were
non-existent, the energy of activation for chemisorption would be
equal to the high dissociation energy of the adsorbate gas molecules.
As it is, an adsorbate gas molecule is first physically adsorbed, which
involves approaching the solid surface along a low-energy path.
Transition from physical adsorption to chemisorption takes place at
the point where curves P and C intersect, and the energy at this point
is equal to the activation energy for chemisorption. The magnitude of
this activation energy depends, therefore, on the shapes of the