Lakhasly

Catalysts significantly alter reaction rates without being reactants or products, operating as biochemical enzymes essential for life or man-made solids crucial for over 50% of industrial chemical production. While speeding reactions, their key attribute is selectivity, changing only specific reaction rates. Catalyst action is complex; activity relates to physical surface structure, not just chemical constitution. Theories suggest catalysts form intermediates on their surface, influence molecules near it, or generate free radicals. Fundamentally, catalysts reduce the potential energy barrier (activation energy) for reactions, increasing rates without affecting equilibrium. Desirable catalysts possess large, accessible surface areas.

Solid Catalyzed Reactions
With many reactions, the rates are affected by materials which are neither re-
actants nor products. Such materials called catalysts can speed a reaction by a
factor of a million or much more, or they may slow a reaction (negative catalyst).
There are two broad classes of catalysts: those that operate at close to ambient
temperature with biochemical systems, and the man-made catalysts that operate
at high temperature.
The biochemical catalysts, called enzymes, are found everywhere in the bio-
chemical world and in living creatures, and without their action I doubt that life
could exist at all. In addition, in our bodies hundreds of different enzymes and
other catalysts are busily at work all the time, keeping us alive. We treat these
catalysts in Chapter 27.
The man-made catalysts, mostly solids, usually aim to cause the high-tempera-
ture rupture or synthesis of materials. These reactions play an important role in
many industrial processes, such as the production of methanol, sulfuric acid,
ammonia, and various petrochemicals, polymers, paints, and plastics. It is esti-
mated that well over 50% of all the chemical products produced today are made
with the use of catalysts. These materials, their reaction rates, and the reactors
that use them are the concern of this chapter and Chapters 19-22.
Consider petroleum. Since this consists of a mixture of many compounds,
primarily hydrocarbons, its treatment under extreme conditions will cause a
variety of changes to occur simultaneously, producing a spectrum of compounds,
some desirable, others undesirable. Although a catalyst can easily speed the rate
of reactions a thousandfold or a millionfold, still, when a variety of reactions
are encountered, the most important characteristic of a catalyst is its selectivity.
By this we mean that it only changes the rates of certain reactions, often a single
reaction, leaving the rest unaffected. Thus, in the presence of an appropriate
catalyst, products containing predominantly the materials desired can be obtained
from a given feed.
The following are some general observations.

1. The selection of a catalyst to promote a reaction is not well understood;
   therefore, in practice extensive trial and error may be needed to produce
   a satisfactory catalyst.
   18.1 The Rate Equation for Surface Kinetics 377


2. Duplication of the chemical constitution of a good catalyst is no guarantee
   that the solid produced will have any catalytic activity. This observation
   suggests that it is the physical or crystalline structure which somehow imparts
   catalytic activity to a material. This view is strengthened by the fact that
   heating a catalyst above a certain critical temperature may cause it to lose
   its activity, often permanently. Thus present research on catalysts is strongly
   centered on the surface structure of solids.


3. To explain the action of catalysts, it is thought that reactant molecules are
   somehow changed, energized, or affected to form intermediates in the
   regions close to the catalyst surface. Various theories have been proposed
   to explain the details of this action. In one theory, the intermediate is viewed
   as an association of a reactant molecule with a region of the surface; in
   other words, the molecules are somehow attached to the surface. In another
   theory, molecules are thought to move down into the atmosphere close to
   the surface and be under the influence of surface forces. In this view the
   molecules are still mobile but are nevertheless modified. In still a third
   theory, it is thought that an active complex, a free radical, is formed at the
   surface of the catalyst. This free radical then moves back into the main gas
   stream, triggering a chain of reactions with fresh molecules before being
   finally destroyed. In contrast with the first two theories, which consider the
   reaction to occur in the vicinity of the surface, this theory views the catalyst
   surface simply as a generator of free radicals, with the reaction occurring
   in the main body of the gas.


4. In terms of the transition-state theory, the catalyst reduces the potential
   energy barrier over which the reactants must pass to form products. This
   lowering in energy barrier is shown in Fig. 18.1.


5. Though a catalyst may speed up a reaction, it never determines the equilib-
   rium or endpoint of a reaction. This is governed by thermodynamics alone.
   Thus with or without a catalyst the equilibrium constant for the reaction
   is always the same.
   Without catalyst the complex
   has high potential energy
   resulting in low rate of reaction
   With catalyst the lower
   energy barrier allows
   higher rate of reaction
   Final

• I state
  Reactants Complex Products *
  Reaction path
  Figure 18.1 Representation of the action of a catalyst.
  378 Chapter 18 Solid Catalyzed Reactions

6. Since the solid surface is responsible for catalytic activity, a large readily
   accessible surface in easily handled materials is desirable. By a variety of
   methods, active surface areas the size of football fields can be obtained per
   cubic centimeter of catalyst.