Use
A catalyst is a substance which, in small quantities,
facilitates a reaction or favorably alters the rate of a
chemical reaction, without itself undergoing any
permanent chemical change. This phenomenon is called
catalysis. Acid-based catalysis is an important type of
catalysis.
Catalysts that have the same phase as the reactants are
known as homogeneous catalysts while those having a
different phase are called heterogeneous catalysts.
Generally, a catalyst that increases the reaction rate is
known as a positive catalyst while one that slows down
the rate is known as a negative catalyst, retardant or
inhibitor.
Some examples of catalysts and their roles are as
follows: (i) Protein catalysts, called enzymes, speed up
many chemical reactions in living organisms. These play
a key role in all metabolic or biological processes and in
some manufacturing processes, such as the fermentation
industry. (ii) Catalytic converters, in automobiles,
employ a mixed oxide bed over which the exhaust gases
pass after they are mixed with additional air. These
catalysts promote oxidation of carbon monoxide and
hydrocarbons to carbon dioxide and water.
Catalysts may be inorganic, organic or complexes of
organic groups and metal halides. (Zeigler-Natta
cataIysts). They may be gases, liquids or solids. In some
cases, their action is undesirable and destructive, as in the
oxidation of iron to iron oxide, which is catalyzed by
water vapor (rusting). The life of an industrial catalyst
varies from 1000 to 10,000 hours, after which it must be
replaced.
Catalysts are highly specific in their action and
application and very essential for many industrial
chemical reactions, as in petroleum refining and organic
syntheses. The activity of a solid catalyst is centerd in a
small fraction of its surface; adding promoters increases
the surface area by increasing the porosity or the number
of active points. Substances that act as poisons clog and
weaken the catalyst surface, thus decreasing the catalytic
activity.