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. Substances that slow a catalyst's effect in a chemical reaction are
called inhibitors. Substances that increase the activity of catalysts are
called promoters, and substances that deactivate catalysts are called catalytic
poisons.
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 dissociation 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 slowest 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. A
Nanomaterial-based catalyst is an example of a heterogeneous catalyst.
Analogous events associated with substrate binding and product dissociation
apply to homogeneous catalysts.
Although catalysts are not consumed by the reaction itself,
they may be inhibited, deactivated, or destroyed by secondary processes. In
heterogeneous catalysis, typical secondary processes include coking where the
catalyst becomes covered by polymeric side products. Additionally,
heterogeneous catalysts can dissolve into the solution in a solid–liquid system
or evaporate in a solid–gas system.
