surface
gas molecule →physisorbed molecule E adsH(exothermic)surface
physisorbed molecule →dissociated atoms
Ebond energies (endothermic)
surface
dissociated atoms →chemisorbed atoms
Ebond energies (exothermic)In the second step, bond energies are broken, which is always endothermic.
Conversely, in the third step bonds are formed, which is always accompanied
by a release of energy (that is, exothermic). At this point, chemisorbed atoms
can react on the surface with little or no activation energy:
surface
chemisorbed atoms →products Ebond energies (exothermic)Desorption of products is the final step of the catalyzed reaction.
In order to get to the last, no-activation-energy step, it is important that in
the first three steps, the exothermicity be greater than the endothermicity.
Otherwise the process is overall endothermic and, if one ignores entropy ef-
fects, not spontaneous. (There are some cases where entropy factors are im-
portant, but we will not discuss them here.) However, in some cases the energy
balance is such that reactant molecules will spontaneously adsorb and dissoci-
ate, allowing them to react with little or no activation energy on the surface.
Two examples are the reactions between H 2 and O 2 to make H 2 O and H 2 and
N 2 to make NH 3. In both cases, the presence of the right catalyst surface pro-
vides the right energy balance between endothermic and exothermic processes,
and the reactions proceed relatively quickly. Without a catalyst, the rate of the
reaction is barely perceptible.
Because of the interplay of the energetics of the first three reaction steps
above, what acts as a good catalyst for one reaction may be a very poor cata-
lyst for another! The H 2 /O 2 reaction works well with a palladium or platinum
catalyst, whereas the ammonia reaction uses an iron-based catalyst.
Identification of the right catalyst for the right reaction is still an intense area
of research.
Finally, catalysis is not confined to well-defined, Miller-indexed metal sur-
faces. One area of recent interest is in the use of clay minerals to catalyze re-
actions. You may think of “clay” as a rather gooey and unstructured material,
but in reality it has a highly defined, three-dimensional structure. In some clays
called zeolites,there are pores in which molecules can enter, and then be ad-
sorbed. Figure 22.25 shows a diagram of what a pore within a zeolite clay looks
like. Thanks in part to the three-dimensional structure of the pore, only the
right reactant molecules can be adsorbed and a particular reaction promoted.
In fact, it is thought that clay minerals such as these are the future ofdesigned
catalysts that can be used to promote any given chemical reaction—if the pore
is just right.22.7 Summary
Surfaces are everywhere, and are more important in physical chemistry than
they seem at first glance. A surface has different thermodynamic properties
than a bulk material does. This is due to an imbalance in forces that are
found at a surface. This imbalance is the root cause of things like surface ten-788 CHAPTER 22 Surfaces
(a)
(b)
Figure 22.25 Certain clay minerals have cat-
alytic properties. Clays are composed of alu-
minum oxide units interspersed with silicon ox-
ide units, and can have pores for molecules to
enter and react. (a) A common building block of
aluminosilicate clays (the open spheres are O, the
dark spheres are Si or Al). (b) Part of a zeolite
structure, which is a common type of clay used in
heterogeneous catalysis.