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Catalysis - Wikipedia, the free encyclopedia
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Catalysis
From Wikipedia, the free encyclopedia
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"Catalyst" redirects here. For other uses, see
Catalyst (disambiguation).
Solid heterogeneous catalysts such as in automobile
catalytic converters are plated on structures designed to maximize their surface area.
A low-temperature oxidation catalyst used to convert
carbon monoxide to non-toxic
carbon dioxide at room temperature. It can also remove
formaldehyde from the
air.
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 interact with catalysts to slow the reaction are called inhibitors (or negative catalysts). 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
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 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. 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 c...
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