From Wikipedia,
Catalysis is the change in
rate of a
chemical reaction due to the participation of a substance called a
catalyst. Unlike other
reagents that participate in the chemical reaction, a catalyst is
not consumed by the reaction itself. A catalyst may participate in
multiple chemical transformations. Catalysts that speed the reaction
are called positive catalysts. Catalysts that slow the reaction are
called negative catalysts, or inhibitors. Substances that increase the
activity of catalysts are called promoters.
Catalytic reactions have a lower rate-limiting
free energy of activation than the corresponding uncatalyzed
reaction, resulting in higher reaction rate at the same temperature.
However, the mechanistic explanation of catalysis is complex.
Catalysts may affect the reaction environment favorably, or bind to
the reagents to polarize bonds, e.g.
acid catalysts for reactions of
carbonyl compounds, or form specific intermediates that are not
produced naturally, such as osmate
esters
in
osmium tetroxide-catalyzed
dihydroxylation of
alkenes,
or cause
lysis of reagents to reactive forms, such as
atomic hydrogen in
catalytic hydrogenation.
Kinetically, catalytic reactions are typical
chemical reactions, i.e. the reaction rate depends on the
frequency of contact of the reactants in the rate-determining step.
Usually, the catalyst participates in this slow step, and rates are
limited by amount of catalyst and its "activity". In
heterogeneous catalysis, the diffusion of reagents to the surface
and diffusion of products from the surface can be rate determining.
Analogous events associated with
substrate binding and product dissociation apply to homogeneous
catalysts.
