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ski kreyòl ayisyen հայերեն bahasa indonesia ido íslenska italiano 日本語 patois ქართული қазақша ಕನ್ನಡ 한국어 kurdî кыргызча latina lietuvių latviešu македонски മലയാളം монгол bahasa melayu नेपाली nederlands norsk bokmål novial occitan ਪੰਜਾਬੀ polski پنجابی português runa simi română русский srpskohrvatski српскохрватски simple english slovenčina slovenščina chishona српски srpski sunda svenska kiswahili தமிழ் తెలుగు ไทย tagalog türkçe українська اردو oʻzbekcha ўзбекча tiếng việt winaray 吴语 閩南語 bân lâm gí 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikiquote wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from catalyst process of increasing the rate of a chemical reaction catalyst redirects here for other uses see catalyst disambiguation not to be confused with catalist or cathalistis for the stage of metabolism see catabolism a range of industrial catalysts in pellet form an air filter that uses a low temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature it can also remove formaldehyde from the air catalysis k ə ˈ t æ l ɪ s ɪ s kə tal iss iss is the increase in rate of a chemical reaction due to an added substance known as a catalyst 1 2 ˈ k æ t əl ɪ s t kat əl ist catalysts are not consumed by the reaction and remain unchanged after the reaction 3 if the reaction is rapid and the catalyst is recycled quickly a very small amount of catalyst often suffices 4 mixing surface area and temperature are important factors in reaction rate catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product in the process of regenerating the catalyst the rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the noncatalyzed mechanism however the noncatalyzed mechanism does remain possible so that the total rate catalyzed plus noncatalyzed can only increase in the presence of the catalyst and never decrease 5 catalysis may be classified as either homogeneous whose components are dispersed in the same phase usually gaseous or liquid as the reactant or heterogeneous whose components are not in the same phase enzymes and other biocatalysts are often considered as a third category furthermore a nanocatalyst is a nanosize catalyst used in the field of applied nanoscience catalysis is nearly ubiquitous in the chemical industry 6 estimates are that 90 of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture 7 the term catalyst is derived from greek καταλύειν kataluein meaning loosen or untie the concept of catalysis was invented by chemist elizabeth fulhame based on her novel work in oxidation reduction experiments 8 9 general principles edit example edit an illustrative example is the effect of catalysts to speed the decomposition of hydrogen peroxide into water and oxygen 2 h 2 o 2 2 h 2 o o 2 this reaction proceeds because the reaction products are more stable than the starting compound but this decomposition is so slow that hydrogen peroxide solutions are commercially available in the presence of a catalyst such as manganese dioxide this reaction proceeds much more rapidly this effect is readily seen by the effervescence of oxygen 10 the catalyst is not consumed in the reaction and may be recovered unchanged and re used indefinitely accordingly manganese dioxide is said to catalyze this reaction in living organisms this reaction is catalyzed by enzymes proteins that serve as catalysts such as catalase another example is the effect of catalysts on air pollution and reducing the amount of carbon monoxide development of active and selective catalysts for the conversion of carbon monoxide into desirable products is one of the most important roles of catalysts using catalysts for the hydrogenation of carbon monoxide helps remove this toxic gas and produce useful materials 11 units edit the si derived unit for measuring the catalytic activity of a catalyst is the katal which is quantified in moles per second the productivity of a catalyst can be described by the turnover number ton and the catalytic activity by the turn over frequency tof which is the ton per time unit the biochemical equivalent is the enzyme unit for more information on the efficiency of enzymatic catalysis see the article on enzymes catalytic reaction mechanisms edit main article catalytic cycle in general chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction mechanism reaction pathway having a lower activation energy than the noncatalyzed mechanism in catalyzed mechanisms the catalyst is regenerated 12 13 14 15 as a simple example occurring in the gas phase the reaction 2 so 2 o 2 2 so 3 can be catalyzed by adding nitric oxide the reaction occurs in two steps 2 no o 2 2 no 2 rate determining no 2 so 2 no so 3 fast the no catalyst is regenerated the overall rate is the rate of the slow step 15 v 2 k 1 no 2 o 2 an example of heterogeneous catalysis is the reaction of oxygen and hydrogen on the surface of titanium dioxide tio 2 or titania to produce water scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation the dissociated surface bound o and h atoms diffuse together the intermediate reaction states are ho 2 h 2 o 2 then h 3 o 2 and the reaction product water molecule dimers after which the water molecule desorbs from the catalyst surface 16 17 reaction energetics edit generic potential energy diagram showing the effect of a catalyst in a hypothetical exothermic chemical reaction x y to give z the presence of the catalyst opens a different reaction pathway shown in red with lower activation energy the final result and the overall thermodynamics are the same catalysts enable pathways that differ from those of uncatalyzed reactions these pathways have lower activation energy consequently more molecular collisions have the energy needed to reach the transition state hence catalysts can enable reactions that would otherwise be blocked or slowed by a kinetic barrier the catalyst may increase the reaction rate or selectivity or enable the reaction at lower temperatures this effect can be illustrated with an energy profile diagram in the catalyzed elementary reaction catalysts do not change the extent of a reaction they have no effect on the chemical equilibrium of a reaction the ratio of the forward and the reverse reaction rates is unaffected see also thermodynamics the second law of thermodynamics describes why a catalyst does not change the chemical equilibrium of a reaction suppose there was such a catalyst that shifted an equilibrium introducing the catalyst to the system would result in a reaction to move to the new equilibrium producing energy production of energy is a necessary result since reactions are spontaneous only if gibbs free energy is produced and if there is no energy barrier there is no need for a catalyst then removing the catalyst would also result in a reaction producing energy i e the addition and its reverse process removal would both produce energy thus a catalyst that could change the equilibrium would be a perpetual motion machine a contradiction to the laws of thermodynamics 18 thus catalysts do not alter the equilibrium constant a catalyst can however change the equilibrium concentrations by reacting in a subsequent step it is then consumed as the reaction proceeds and thus it is also a reactant illustrative is the base catalyzed hydrolysis of esters where the produced carboxylic acid immediately reacts with the base catalyst and thus the reaction equilibrium is shifted towards hydrolysis the catalyst stabilizes the transition state more than it stabilizes the starting material it decreases the kinetic barrier by decreasing the difference in energy between starting material and the transition state it does not change the energy difference between starting materials and products thermodynamic barrier or the available energy this is provided by the environment as heat or light related concepts edit some so called catalysts are really precatalysts which convert to catalysts in the reaction for example wilkinson s catalyst rhcl pph 3 3 loses one triphenylphosphine ligand before entering the true catalytic cycle precatalysts are easier to store but are easily activated in situ because of this preactivation step many catalytic reactions involve an induction period in cooperative catalysis chemical species that improve catalytic activity are called cocatalysts or promoters in tandem catalysis two or more different catalysts are coupled in a one pot reaction in autocatalysis the catalyst is a product of the overall reaction in contrast to all other types of catalysis considered in this article the simplest example of autocatalysis is a reaction of type a b 2 b in one or in several steps the overall reaction is just a b so that b is a product but since b is also a reactant it may be present in the rate equation and affect the reaction rate as the reaction proceeds the concentration of b increases and can accelerate the reaction as a catalyst in effect the reaction accelerates itself or is autocatalyzed an example is the hydrolysis of an ester such as aspirin to a carboxylic acid and an alcohol in the absence of added acid catalysts the carboxylic acid product catalyzes the hydrolysis switchable catalysis refers to a type of catalysis where the catalyst can be toggled between different ground states possessing distinct reactivity typically by applying an external stimulus 19 this ability to reversibly switch the catalyst allows for spatiotemporal control over catalytic activity and selectivity the external stimuli used to switch the catalyst can include changes in temperature ph light 20 electric fields or the addition of chemical agents a true catalyst can work in tandem with a sacrificial catalyst the true catalyst is consumed in the elementary reaction and turned into a deactivated form the sacrificial catalyst regenerates the true catalyst for another cycle the sacrificial catalyst is consumed in the reaction and as such it is not really a catalyst but a reagent for example osmium tetroxide oso 4 is a good reagent for dihydroxylation but it is highly toxic and expensive in upjohn dihydroxylation the sacrificial catalyst n methylmorpholine n oxide nmmo regenerates oso 4 and only catalytic quantities of oso 4 are needed classification edit catalysis may be classified as either homogeneous or heterogeneous a homogeneous catalysis is one whose components are dispersed in the same phase usually gaseous or liquid as the reactant s molecules a heterogeneous catalysis is one where the reaction components are not in the same phase enzymes and other biocatalysts are often considered as a third category similar mechanistic principles apply to heterogeneous homogeneous and biocatalysis heterogeneous catalysis edit main article heterogeneous catalysis the microporous molecular structure of the zeolite zsm 5 is exploited in catalysts used in refineries zeolites are extruded as pellets for easy handling in catalytic reactors heterogeneous catalysts act in a different phase than the reactants most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture important heterogeneous catalysts include zeolites alumina 21 higher order oxides graphitic carbon transition metal oxides metals such as raney nickel for hydrogenation and vanadium v oxide for oxidation of sulfur dioxide into sulfur trioxide by the contact process 22 diverse mechanisms for reactions on surfaces are known depending on how the adsorption takes place langmuir hinshelwood eley rideal and mars van krevelen 23 the total surface area of a solid has an important effect on the reaction rate the smaller the catalyst particle size the larger the surface area for a given mass of particles a heterogeneous catalyst has active sites which are the atoms or crystal faces where the substrate actually binds active sites are atoms but are often described as a facet edge surface step etc of a solid most of the volume but also most of the surface of a heterogeneous catalyst may be catalytically inactive finding out the nature of the active site is technically challenging for example the catalyst for the haber process for the synthesis of ammonia from nitrogen and hydrogen is often described as iron but detailed studies and many optimizations have led to catalysts that are mixtures of iron potassium calcium aluminum oxide 24 the reacting gases adsorb onto active sites on the iron particles once physically adsorbed the reagents partially or wholly dissociate and form new bonds in this way the particularly strong triple bond in nitrogen is broken which would be extremely uncommon in the gas phase due to its high activation energy thus the activation energy of the overall reaction is lowered and the rate of reaction increases 25 another place where a heterogeneous catalyst is applied is in the oxidation of sulfur dioxide on vanadium v oxide for the production of sulfuric acid 22 many heterogeneous catalysts are in fact nanomaterials heterogeneous catalysts are typically supported which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes its cost supports prevent or minimize agglomeration and sintering of small catalyst particles exposing more surface area thus catalysts have a higher specific activity per gram on support sometimes the support is merely a surface on which the catalyst is spread to increase the surface area more often the support and the catalyst interact affecting the catalytic reaction supports can also be used in nanoparticle synthesis by providing sites for individual molecules of catalyst to chemically bind supports are porous materials with a high surface area most commonly alumina zeolites or various kinds of activated carbon specialized supports include silicon dioxide titanium dioxide calcium carbonate and barium sulfate 26 electrocatalysts edit main article electrocatalyst in the context of electrochemistry specifically in fuel cell engineering various metal containing catalysts are used to enhance the rates of the half reactions that comprise the fuel cell one common type of fuel cell electrocatalyst is based upon nanoparticles of platinum that are supported on slightly larger carbon particles when in contact with one of the electrodes in a fuel cell this platinum increases the rate of oxygen reduction either to water or to hydroxi...
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