Catalysis is the acceleration of a chemical reaction by a catalyst: a substance that participates in the reaction mechanism but is regenerated rather than consumed in the overall transformation. A catalyst changes how rapidly reactants become products without changing the reaction’s overall standard Gibbs energy change. Catalysis is fundamental to chemistry, biological processes, and industrial production, where it can improve reaction rates and favor desired products over competing byproducts. (goldbook.iupac.org)
Principles and reaction mechanisms
A reaction may be thermodynamically favorable yet proceed slowly because its reactants must overcome an energy barrier. A catalyst provides an alternative mechanism with a lower effective barrier, often described in introductory treatments as a reduction in activation energy. This mechanism may contain several intermediate steps rather than a single direct conversion. Its acceleration depends on the barriers along the catalytic pathway, not simply on the number of steps. (openstax.org)
Catalysts therefore affect kinetics rather than the underlying thermodynamics of the overall reaction. At a fixed temperature, they do not change the equilibrium constant for that reaction. They accelerate the approach to chemical equilibrium by facilitating both forward and reverse transformations; they cannot, by themselves, make an unfavorable equilibrium favorable. (goldbook.iupac.org)
A simple illustrative catalytic cycle is:
[ A+C\rightleftharpoons AC,\qquad AC+B\rightarrow P+C ]
Here, (C) is the catalyst, (AC) an intermediate, and (P) the product. Adding the steps gives the overall reaction (A+B\rightarrow P), with the catalyst canceled from the net equation. Regeneration does not mean that a catalyst remains chemically unchanged throughout the mechanism: temporary bonding and intermediate formation are central to many catalytic processes. (nobelprize.org)
Principal forms
In homogeneous catalysis, the reacting system involves a single phase. Familiar examples include reactions catalyzed by acids in solution. In heterogeneous catalysis, reaction occurs at or near an interface between phases, commonly between a solid catalyst and gaseous or liquid reactants. These classifications concern phase relationships, not whether the catalyst is biological, metallic, or organic. (goldbook.iupac.org)
Solid-catalyst mechanisms frequently involve adsorption of reactants, transformation on the surface, and desorption of products. Surface composition and atomic arrangement can influence individual reaction steps. Studies of ammonia synthesis on iron, for example, have connected macroscopic reaction rates with elementary events on the catalyst surface and shown how potassium promotion affects nitrogen adsorption. (nobelprize.org)
Organocatalysis uses small organic molecules as catalysts. It is particularly important in asymmetric catalysis, where a catalyst favors the formation of one enantiomer over its mirror-image counterpart. Such selectivity allows catalytic synthesis to control not only which compounds form but also their three-dimensional configuration. (nobelprize.org)
Autocatalysis occurs when a reaction product catalyzes the reaction that produces it. This differs from ordinary catalysis by an independently supplied substance. Under IUPAC terminology, a substance that decreases reaction rate is an inhibitor, not a “negative catalyst.” (goldbook.iupac.org)
Biological catalysis
Biocatalysis is catalysis carried out by biological catalysts, especially enzymes. Most enzymes are proteins, although catalytic RNA also exists. Enzyme catalysis underlies the chemical transformations of metabolism and many processes involved in genetic information transfer. (ncbi.nlm.nih.gov)
An enzyme binds its substrate at an active site, forming an enzyme–substrate complex. Catalysis can involve orienting reactants, stabilizing the transition state, transferring protons, or forming temporary covalent intermediates. Binding may also alter the conformations of both enzyme and substrate. These mechanisms explain why an enzyme is more than a passive surface on which reactants meet. (ncbi.nlm.nih.gov)
Enzyme activity can be regulated through interactions with other molecules. Such interactions may change the protein’s conformation and consequently its catalytic behavior. Despite their specificity and regulation, enzymes obey the same thermodynamic constraints as other catalysts: they accelerate reactions without changing their overall equilibrium. (ncbi.nlm.nih.gov)
Activity, selectivity, and durability
Catalyst performance includes both activity and selectivity. Activity concerns the speed of transformation under specified conditions; selectivity concerns the preference for desired products rather than alternative products. Turnover frequency expresses the number of reaction events per active site per unit time, allowing activity to be related to catalytic sites rather than simply to the mass of material present. (energy.gov)
Regeneration within a reaction cycle does not guarantee unlimited service life. Catalysts can lose performance through poisoning by contaminants, carbon deposition, particle sintering, structural changes, or physical attrition. The relevant mechanisms depend on the catalyst and operating environment. Research on biomass-conversion catalysts, for instance, distinguishes inorganic poisoning from carbon deposition and redistribution of metallic components. (energy.gov)
Applications and historical development
The Haber process illustrates industrial heterogeneous catalysis: promoted iron catalysts facilitate conversion of nitrogen and hydrogen into ammonia, a feedstock for fertilizer production. In electrochemistry, electrocatalysts facilitate reactions at electrode surfaces, including reactions used in fuel cells and hydrogen production. Catalytic efficiency can reduce the temperature, pressure, or energy requirements of chemical processing. (nobelprize.org)
Jöns Jacob Berzelius introduced the term catalysis in 1835. Wilhelm Ostwald subsequently helped establish its interpretation in terms of reaction rates rather than an unexplained catalytic force. He received the 1909 Nobel Prize in Chemistry for work on catalysis, chemical equilibria, and reaction rates. Later recognition included the 2021 prize for the development of asymmetric organocatalysis. (nobelprize.org)