# Catalysis **Catalysis** is the increase in the rate of a [[Chemical_reaction|chemical reaction]] brought about by a substance, the catalyst, that takes part in the reaction, is regenerated by it, and is therefore not consumed. A catalyst opens a different [[Reaction_mechanism|mechanism]] whose highest barrier, the [[Activation_energy|activation energy]] of its rate-determining step, is lower than that of the uncatalysed path; the energies of reactants and products are where they were, so the reaction reaches the same [[Chemical_equilibrium|equilibrium]], only sooner.[^af-catalyst][^af-keq] In the microsim below the reader slides the barrier height Ea from 40 to 120 kJ/mol above a concentration–time plot for the reversible reaction A ⇌ P, and the relaxation law `[A](t) = [A]eq + ([A]0 − [A]eq)·exp(−(kf + kr)·t)` answers what changes: the approach speeds up or slows down, but the finish line `[A]eq = [A]0/(1 + K)` never moves, because `K = kf/kr` is independent of Ea.[^boyd-reversible] On the Chemistry flagship's spine this page is the main article for Part VI — Reaction, section *Catalysis*, the sibling that follows the [[Arrhenius_equation|Arrhenius equation]]; the [[Enzyme|enzyme]] door opens from here toward [[Michaelis–Menten_kinetics|Michaelis–Menten kinetics]]. Catalysis is the way most chemistry is done, from the [[Contact_process|contact process]] to every reaction inside a living cell. Wilhelm Ostwald received the 1909 Nobel Prize in Chemistry "for his work on catalysis and for his investigations into the fundamental principles governing chemical equilibria and rates of reaction".[^nobel-1909] ## General principles A catalyst enters the mechanism, is bound or transformed in an early step and released in a later one, and ends the cycle unchanged in amount and form, so a small quantity can turn over a large quantity of reactant. The two defining facts, that the catalyst is not consumed and that the equilibrium is untouched, follow from one picture: the catalyst changes the path between reactants and products, not the endpoints.[^af-catalyst] ### Example The Portal Book's worked example compares two paths for the same two-step reaction. On path (a) the first step has an activation energy of 80 kJ and the second 20 kJ; on path (b) the first step's barrier is 70 kJ and the second is again 20 kJ. The reactant and product energies are identical on both diagrams, so path (b) is the catalysed one: the same overall reaction, the same enthalpy change, a lower rate-determining barrier.[^af-paths] At 298 K a 10 kJ/mol drop in the controlling barrier multiplies the rate constant by `exp(10000/(8.314 × 298))`, about 57 times, if the pre-exponential factor is unchanged (a derived figure): the difference between an hour of waiting and a minute.[^af-arrhenius] A gas-phase example the book also treats is the destruction of stratospheric [[Ozone|ozone]] by chlorine atoms, which are regenerated by their second step and so catalyse the net reaction 2 O₃ → 3 O₂.[^af-ozone] ### Units Catalytic activity is a rate, and its SI unit is the katal, one mole of reaction per second.[^bipm-si] Two other measures compare catalysts: the turnover number, the cycles a catalytic site completes before it dies, and the turnover frequency, the cycles per site per unit time, so that a catalyst is judged by how fast it works and how long it lasts. ### Catalytic reaction mechanisms A catalytic mechanism is a closed loop of [[Elementary_reaction|elementary steps]]: the catalyst binds a reactant, the bound species reacts, and the product leaves, freeing the site. The catalyst appears on the left of an early step and on the right of a later one, so it cancels from the overall equation while the intermediates it forms, and usually its own concentration, appear in the rate law.[^af-catalyst] The [[Steady_state_(chemistry)|steady-state approximation]] reduces such a loop to one expression; the saturating rate law of an enzyme is its best-known result. ### Reaction energetics On a reaction energy diagram the activation energy is the height of the [[Transition_state|transition state]] above the reactants and the enthalpy change ΔH is the height of the products above the reactants; the two are independent, and a catalyst changes only the first.[^af-ea] Because the forward and reverse reactions share the same transition state, lowering it lowers both barriers by the same amount, so the forward and reverse rate constants rise by the same factor and their ratio, the [[Equilibrium_constant|equilibrium constant]] `K = kf/kr`, is unchanged.[^af-keq] The microsim on this page draws that argument. Above a concentration plot for the reversible first-order reaction A ⇌ P sits a schematic barrier diagram, a smooth curve through a reactant well, a summit and a product well. The reader's one control is Ea, from 40 to 120 kJ/mol in 1 kJ steps, starting at 80 kJ/mol. The model behind the curves is ILLUSTRATIVE and not taken from the book: both rate constants share one pre-exponential factor A, with `kf = A·exp(−Ea/(R·T))` and `kr = A·exp(−(Ea + ΔE)/(R·T))` for a fixed reaction energy ΔE, so that `K = kf/kr = exp(ΔE/(R·T))` is a constant the slider cannot touch. The concentration of A follows the exact solution `[A](t) = [A]eq + ([A]0 − [A]eq)·exp(−(kf + kr)·t)`, which the Portal Book gives as `ln(([A] − [A]eq)/([A]0 − [A]eq)) = −(kf + kr)·t`;[^boyd-reversible] starting from pure A, the material balance fixes `[A]eq = [A]0/(1 + K)`. As Ea is lowered the summit drops, kf and kr grow together, and the curve relaxes faster toward the same horizontal line; as Ea is raised the curve flattens and takes longer to arrive. A ghost curve stays at the 80 kJ default for comparison, and the HUD reads `K = kf/kr ; [A] = [A]eq + ([A]0 − [A]eq)·exp(−(kf + kr)·t)`. Over the slider's full range at 298 K the relaxation rate `kf + kr` changes by `exp(80000/(8.314 × 298))`, about 10¹⁴, while [A]eq does not change at all (derived). The sim's one rule, enforced in the code, is that Ea must never move K.[^af-keq] ### Related concepts A reagent is consumed; so is an initiator, such as a peroxide that starts a radical [[Polymer|polymerisation]]. A promoter improves a catalyst without being one, a support is the inert solid on which a catalyst is dispersed, and in autocatalysis a product catalyses the reaction that makes it. A catalyst does not make an impossible reaction possible: it accelerates only reactions already spontaneous in the [[Gibbs_free_energy|free-energy]] sense, toward the equilibrium they would reach on their own.[^af-catalyst] ### Classification Catalysts are classed by phase. A homogeneous catalyst is in the same phase as the reactants, typically dissolved in the same solution; a heterogeneous catalyst is in a different phase, usually a solid acting on a liquid or gas, so the reaction happens on its surface.[^af-classes] Enzymes are formally homogeneous but form a class of their own. ## Heterogeneous catalysis [[Heterogeneous_catalysis|Heterogeneous catalysis]] happens on a surface, in a cycle the Portal Book describes as adsorption of the reactants onto the solid, reaction among the adsorbed species, and desorption of the products.[^af-hetero] The rate is proportional to the number of surface sites in use rather than to the amount of solid, which is why catalysts are prepared as fine powders, porous pellets or [[Nanoparticle|nanoparticles]] on a support. When every site is occupied the rate no longer responds to the reactant concentration: the book's zero-order example is the decomposition of [[Ammonia|ammonia]] on hot [[Tungsten|tungsten]], whose concentration falls linearly with a half-life of 18 minutes in the worked problem, a "pseudo" order that holds only while the surface stays saturated.[^af-zero] The transition metals [[Platinum|platinum]], [[Palladium|palladium]], [[Nickel|nickel]] and [[Iron|iron]] are the classic surface catalysts, and Gerhard Ertl received the 2007 Nobel Prize in Chemistry "for his studies of chemical processes on solid surfaces", which turned [[Surface_science|surface science]] from a description of catalysts into an explanation of them.[^nobel-2007] ### Electrocatalysts An electrocatalyst is a surface catalyst for a reaction in which electrons cross an electrode. In the Portal Book's electrochemical model the rate is a current density that grows exponentially with the overpotential through the [[Butler–Volmer_equation|Butler–Volmer equation]], and the catalyst's quality is measured by the exchange current density, the rate at equilibrium.[^hav-bv] A better catalyst raises the exchange current and lowers the overpotential needed for a given current, which in [[Electrolysis_of_water|water electrolysis]] and in a [[Fuel_cell|fuel cell]] is energy saved; platinum for hydrogen and iridium or nickel oxides for oxygen are the working examples in [[Electrochemistry|electrochemistry]]. ## Homogeneous catalysis A homogeneous catalyst shares the phase of its reactants. In solution the classic cases are acid and base catalysis, in which a proton or hydroxide ion is transferred in and out of the reacting molecule, and transition-metal complexes whose metal centre binds, activates and releases the substrate in a defined cycle. Every catalyst molecule is available and identical, so homogeneous catalysts are often more selective than solid ones, at the price of separation from the product afterwards. Palladium-catalysed cross-coupling, honoured by the 2010 Nobel Prize in Chemistry, and olefin metathesis, honoured in 2005, are homogeneous cycles that reorganised organic synthesis.[^nobel-2010][^nobel-2005] ### Organocatalysis An organocatalyst is a small metal-free organic molecule, typically an amine or a thiourea, that binds the substrate through a transient covalent intermediate or a hydrogen-bonded pair and releases it transformed. The 2021 Nobel Prize in Chemistry went to Benjamin List and David MacMillan "for the development of asymmetric organocatalysis", the making of one mirror-image form of a product in preference to the other.[^nobel-2021] ### Photocatalysts A photocatalyst absorbs light and uses the excited state, or the electron–hole pair it produces in a [[Semiconductor|semiconductor]], to drive a reaction that the ground state cannot. Titanium dioxide under ultraviolet light, which oxidises organic films on self-cleaning glass, is the everyday example, and the light-driven splitting of water into [[Hydrogen|hydrogen]] and [[Oxygen|oxygen]] the long-sought industrial one. ### Enzymes and biocatalysts An [[Enzyme|enzyme]] is a protein, or occasionally an RNA molecule, whose folded shape holds a substrate in an active site where a few residues do the chemistry. Enzymes are fast and specific, and their kinetics follow the saturating law `v = Vmax·[S]/(KM + [S])` of [[Michaelis–Menten_kinetics|Michaelis–Menten kinetics]], the steady-state result for a catalyst that must bind its substrate before converting it.[^sauter-enzyme] Sidney Altman and Thomas Cech received the 1989 Nobel Prize in Chemistry "for their discovery of catalytic properties of RNA", the ribozymes,[^nobel-1989] and engineered enzymes now serve as industrial biocatalysts in water at mild temperatures. ## Significance Most large-scale chemistry is catalytic, for economic as much as scientific reasons: a catalyst lowers the temperature and pressure a process needs, raises its selectivity so that less raw material is wasted, and is not consumed. The subsections follow the pair's grouping, from energy and bulk chemicals to food and the environment. ### Energy processing A refinery is a sequence of catalytic units: cracking heavy fractions over acidic solids, reforming over platinum to raise octane, and hydrotreating over metal sulfides to remove sulfur before it reaches an engine or a downstream catalyst. In electrochemical energy conversion the catalyst is the electrode, and the Portal Book's fuel-cell and electrolyser chapters treat the catalyst layer as the place where the device's activation loss is set.[^hav-fuelcell] ### Bulk chemicals The [[Haber_process|Haber process]] fixes nitrogen as ammonia over an iron catalyst at high pressure, and [[Fritz_Haber|Fritz Haber]] received the 1918 Nobel Prize in Chemistry "for the synthesis of ammonia from its elements"; sulfuric acid is made by oxidising sulfur dioxide over vanadium pentoxide in the contact process; and the polyolefins of packaging are made with the Ziegler–Natta catalysts whose discoverers were honoured in 1963.[^nobel-1918][^nobel-1963] ### Fine chemicals Pharmaceuticals and agrochemicals are made in small tonnage but at high value, and their synthesis depends on selective homogeneous, organo- and biocatalysts that build one bond and one mirror image at a time. The chiral hydrogenation and oxidation catalysts honoured by the 2001 Nobel Prize in Chemistry are the archetype.[^nobel-2001] ### Food processing Hydrogenation of vegetable oils over nickel converts liquid oils into solid fats, and enzymes run through food technology: amylases break starch into sugars in brewing and baking, rennet curdles milk for cheese, and lactase removes lactose from milk. These are catalytic processes in the strict sense, with the catalyst recovered or left in a harmless trace. ### Environment A three-way catalytic converter passes exhaust over platinum, palladium and rhodium on a ceramic honeycomb, oxidising carbon monoxide and unburnt hydrocarbons while reducing nitrogen oxides, and the Portal Book treats it as the everyday example of heterogeneous catalysis.[^af-converter] The same chapter presents the other side of the ledger, the chlorine atoms from chlorofluorocarbons that catalyse the destruction of stratospheric ozone.[^af-ozone] ## History The word was coined by Jöns Jacob Berzelius, who grouped under it a series of reactions, from the fermentation of sugar to the ignition of hydrogen by platinum, in which a substance appeared to act by its mere presence.[citation needed] The physical meaning came with the founders of [[Chemical_kinetics|chemical kinetics]]: Ostwald's definition of a catalyst as a substance that changes the rate without appearing in the final product, and without altering the equilibrium, is the one still used, and it earned the 1909 Nobel Prize.[^nobel-1909] Industrial catalysis arrived with the fixation of nitrogen, recognised by Haber's 1918 prize,[^nobel-1918] and the mechanistic understanding of surface catalysis with the ultra-high-vacuum surface-science methods recognised by Ertl's in 2007.[^nobel-2007] The prizes of 1963, 1989, 2001, 2005, 2010 and 2021 cited above mark the widening of the subject into polymers, RNA, chiral synthesis, metathesis, cross-coupling and organocatalysis; the barrier picture drawn in the microsim has survived every extension unchanged. ## Inhibitors, poisons, and promoters An inhibitor slows a reaction, and a catalyst poison is an inhibitor that acts on the catalyst, binding to its active sites more strongly than the reactants do. Lead poisons the platinum of a catalytic converter, which is why leaded fuel disappeared with the converter's arrival; sulfur compounds poison nickel and platinum catalysts, which is why refinery feeds are hydrotreated first; and carbon monoxide poisons the platinum anode of a hydrogen fuel cell. In enzyme kinetics a competitive inhibitor competes with the substrate for the active site and raises the apparent KM without changing Vmax, so the inhibition can be overcome by more substrate, whereas a non-competitive inhibitor lowers Vmax itself.[^sauter-enzyme] A promoter is the opposite of a poison: a substance with little activity of its own that improves a catalyst's activity, selectivity or lifetime, such as the potassium and aluminium oxides added to the iron of an ammonia catalyst. The [[Steady_state_(chemistry)|steady-state]] sim on this flagship shows product inhibition, in which the reaction's own product reverses the first step. ## Prebiotic catalysis in the origin of life Before enzymes existed something had to catalyse the reactions that made them, and three candidates are studied. Mineral surfaces such as clays and metal sulfides adsorb and orient small organic molecules and can catalyse their joining, and the metal-sulfide theory places the first metabolism on such surfaces at hydrothermal vents. Small metal ions, the ancestors of enzyme cofactors, catalyse many of the reactions the cofactors do today. And RNA, which can both store information and catalyse reactions, is the basis of the RNA-world hypothesis, in which ribozymes of the kind discovered by Altman and Cech preceded protein enzymes;[^nobel-1989] all three remain hypotheses under test. ## See also - [[Enzyme]] - [[Heterogeneous_catalysis]] - [[Surface_science]] - [[Arrhenius_equation]] - [[Chemical_kinetics]] - [[Michaelis–Menten_kinetics]] - [[Haber_process]] - [[Reaction_coordinate]] ## References [^af-catalyst]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7 Catalysis, p. 828 (a catalyst lowers the activation energy of the rate-determining step through a different mechanism; reactant and product energies are unchanged; the catalyst is not consumed). https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first [^af-keq]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7, pp. 834–835 (K = kf/kr; a catalyst changes kf and kr equally, so K is unchanged). [^af-paths]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7, Example 17.15, pp. 828–829 (two reaction diagrams: first-step barrier 80 kJ on path (a) and 70 kJ on path (b); second step 20 kJ on both). [^af-ea]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.5 and §17.6, pp. 818, 828 (activation energy as the transition-state height above the reactants; ΔH as the product–reactant difference). [^af-arrhenius]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.5, pp. 819–820 (the Arrhenius equation; R = 8.314 J mol⁻¹ K⁻¹). The 57-fold and 10¹⁴-fold factors in the text are derived from it with a fixed pre-exponential factor. [^af-ozone]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7 Catalysis, pp. 828–835 (chlorine-atom catalysis of ozone decomposition; page to pin). [^af-classes]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7 Catalysis, pp. 828–835 (homogeneous and heterogeneous catalysts defined; page to pin). [^af-hetero]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7 Catalysis, pp. 828–835 (steps of heterogeneous catalysis: adsorption, reaction, desorption; page to pin). [^af-converter]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.7 Catalysis, pp. 828–835 (the automobile catalytic converter; page to pin). [^af-zero]: Flowers, Paul; Neth, Edward; Robinson, William et al. (2019). *Chemistry: Atoms First 2e*. OpenStax. Chapter 17 Kinetics, §17.4 Integrated Rate Laws, pp. 812–813, 816 (zero-order decomposition of ammonia on tungsten; t½ = 18 min; zero order as conditional, "pseudo" behaviour). [^boyd-reversible]: Boyd, W. Christopher (2025). *Exploring Inorganic and Organometallic Chemistry*. Chapter 10 Kinetics, pp. 275–276 (reversible first-order reaction A ⇌ P: ln(([A] − [A]eq)/([A]0 − [A]eq)) = −(k1 + k−1)t). https://open.umn.edu/opentextbooks/textbooks/exploring-inorganic-and-organometallic-chemistry [^hav-bv]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Chapter 1 Electrochemistry, pp. 30–35 (Butler–Volmer and Tafel kinetics; the exchange current density). https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling [^hav-fuelcell]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Chapter Fuel cells, pp. 132–153, and Chapter Electrolysers, pp. 158–173 (catalyst layers and activation losses; page to pin). [^sauter-enzyme]: Sauter, Thomas; Albrecht, Marco (2023). *Introduction to Systems Biology: Workbook for Flipped-Classroom Teaching*. Enzyme kinetics and the Michaelis–Menten lecture, pp. 134–151 (page to pin). https://open.umn.edu/opentextbooks/textbooks/introduction-to-systems-biology-workbook-for-flipped-classroom-teaching [^bipm-si]: Bureau International des Poids et Mesures (2019). *The International System of Units (SI)*, 9th edition, Table of coherent derived units with special names (the katal, mol/s, for catalytic activity). https://www.bipm.org/en/publications/si-brochure [^nobel-1909]: The Nobel Prize. "The Nobel Prize in Chemistry 1909 — Wilhelm Ostwald." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/1909/summary/ [^nobel-1918]: The Nobel Prize. "The Nobel Prize in Chemistry 1918 — Fritz Haber." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/1918/summary/ [^nobel-1963]: The Nobel Prize. "The Nobel Prize in Chemistry 1963 — Karl Ziegler, Giulio Natta." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/1963/summary/ [^nobel-1989]: The Nobel Prize. "The Nobel Prize in Chemistry 1989 — Sidney Altman, Thomas R. Cech." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/1989/summary/ [^nobel-2001]: The Nobel Prize. "The Nobel Prize in Chemistry 2001 — William S. Knowles, Ryoji Noyori, K. Barry Sharpless." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/2001/summary/ [^nobel-2005]: The Nobel Prize. "The Nobel Prize in Chemistry 2005 — Yves Chauvin, Robert H. Grubbs, Richard R. Schrock." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/2005/summary/ [^nobel-2007]: The Nobel Prize. "The Nobel Prize in Chemistry 2007 — Gerhard Ertl." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/2007/summary/ [^nobel-2010]: The Nobel Prize. "The Nobel Prize in Chemistry 2010 — Richard F. Heck, Ei-ichi Negishi, Akira Suzuki." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/2010/summary/ [^nobel-2021]: The Nobel Prize. "The Nobel Prize in Chemistry 2021 — Benjamin List, David W. C. MacMillan." NobelPrize.org, Nobel Prize Outreach. https://www.nobelprize.org/prizes/chemistry/2021/summary/ ## External links - *Chemistry: Atoms First 2e* (OpenStax, 2019), Portal Book 051: https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first - *Electrolysers, Fuel Cells and Batteries: Analytical Modelling* (2024), Portal Book 053: https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling - The Wikipedia pair's External links section lists the pair's own links. <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Catalysis.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Catalysis* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Catalysis.html" data-title="Catalysis"></div> *Built from `MICROSIM_GUIDE/specs/sims/Catalysis.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Catalysis) : [Wikitube](https://en.wikitube.io/wiki/Catalysis) · pinned revision [1373692352](https://en.wikipedia.org/w/index.php?oldid=1373692352) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Chemistry]]. --- *Matter & Energy Cluster child articles, wave 1 · 2026-09-11 · drafted · Chemistry row K34 · sim pending (matter/Catalysis).*