Catalysis
is the process of increasing the rate of a chemical reaction by adding a
substance known as a catalyst, which is not consumed in the catalyzed reaction
and can continue to act repeatedly. Because of this, only very small amounts of
catalyst are required to alter the reaction rate in principle.
In
general, chemical reactions occur faster in the presence of a catalyst because
the catalyst provides an alternative reaction pathway with lower activation
energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst
usually reacts to form a temporary intermediate, which then regenerates the
original catalyst in a cyclic process. A substance which provides a mechanism
with higher activation energy does not decrease the rate because the reaction
can still occur by the non-catalyzed route. An added substance which does
reduce the reaction rate is not considered a catalyst but a reaction inhibitor.
Catalysts
may be classified as either homogeneous or heterogeneous. A homogeneous
catalyst is one whose molecules are dispersed in the same phase (usually
gaseous or liquid) as the reactant's molecules. A heterogeneous catalyst is one
whose molecules are not in the same phase as the reactant's, which are
typically gases or liquids that are adsorbed onto the surface of the solid
catalyst. Enzymes and other biocatalysts are often considered as a third
category