Heterogeneous catalysis

{{Short description|Type of catalysis involving reactants & catalysts in different phases of matter}} [[Image:Hydrogenation on catalyst.svg|thumb|[[Hydrogenation]] of [[ethene]] on a catalytic solid surface (1) Adsorption (2) Reaction (3) Desorption]]

'''Heterogeneous catalysis''' is [[catalysis]] where the [[Phase (matter)|phase]] of catalysts differs from that of the [[reagents]] or [[product (chemistry)|product]]s.<ref name=":1">{{Cite journal|last=Schlögl|first=Robert|date=9 March 2015|title=Heterogeneous Catalysis|journal=Angewandte Chemie International Edition|volume=54|issue=11|pages=3465–3520|doi=10.1002/anie.201410738|pmid=25693734|hdl-access=free|hdl=11858/00-001M-0000-0025-0A33-6}}</ref> The process contrasts with [[homogeneous catalysis]] where the reagents, products and catalyst exist in the same phase. Phase distinguishes between not only [[solid]], [[liquid]], and [[gas]] components, but also [[immiscible]] mixtures (e.g., [[oil]] and [[water]]), or anywhere an interface is present.

Heterogeneous catalysis typically involves solid phase catalysts and gas phase reactants.<ref name=":02">{{Cite book|last=Rothenberg|first=Gadi|title=Catalysis: concepts and green applications|date=17 March 2008|publisher=Wiley-VCH|isbn=978-3-527-31824-7|location=Weinheim [Germany]|oclc=213106542}}</ref> In this case, there is a cycle of molecular adsorption, reaction, and desorption occurring at the catalyst surface. Thermodynamics, mass transfer, and heat transfer influence the [[Reaction rate|rate (kinetics) of reaction]].

Heterogeneous catalysis is very important because it enables faster, large-scale production and the selective product formation.<ref>{{Cite journal|last=Information.|first=Lawrence Berkeley National Laboratory. United States. Department of Energy. Office of Scientific and Technical|title=The impact of nanoscience on heterogeneous catalysis|journal=Science|date=2003|volume=299|issue=5613|pages=1688–1691|publisher=Lawrence Berkeley National Laboratory|doi=10.1126/science.1083671|oclc=727328504|pmid=12637733|bibcode=2003Sci...299.1688B|osti=824849 |s2cid=35805920|url=https://digital.library.unt.edu/ark:/67531/metadc780930/}}</ref> Approximately 35% of the world's GDP is influenced by catalysis.<ref name=":6">{{Citation|last1=Ma|first1=Zhen|title=Heterogeneous Catalysis by Metals|date=2006-03-15|encyclopedia=Encyclopedia of Inorganic Chemistry|editor-last=King|editor-first=R. Bruce|publisher=John Wiley & Sons, Ltd|doi=10.1002/0470862106.ia084|isbn=978-0-470-86078-6|last2=Zaera|first2=Francisco|editor2-last=Crabtree|editor2-first=Robert H.|editor3-last=Lukehart|editor3-first=Charles M.|editor4-last=Atwood|editor4-first=David A.}}</ref> The production of 90% of chemicals (by volume) is assisted by solid catalysts.<ref name=":02" /> The chemical and energy industries rely heavily on heterogeneous catalysis. For example, the [[Haber–Bosch process]] uses metal-based catalysts in the synthesis of [[ammonia]], an important component in fertilizer; 144 million tons of ammonia were produced in 2016.<ref>{{Cite web|url=https://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/mcs-2018-nitro.pdf|title=United States Geological Survey, Mineral Commodity Summaries|date=January 2018|website=USGS}}</ref>

== Adsorption == [[Adsorption]] is an essential step in heterogeneous catalysis. Adsorption is the process by which a gas (or solution) phase molecule (the adsorbate) binds to solid (or liquid) surface atoms (the adsorbent). The reverse of adsorption is [[desorption]], the adsorbate splitting from adsorbent. In a reaction facilitated by heterogeneous catalysis, the catalyst is the adsorbent and the reactants are the adsorbate.

=== Types of adsorption === Two types of adsorption are recognized: [[physisorption]], weakly bound adsorption, and [[chemisorption]], strongly bound adsorption. Many processes in heterogeneous catalysis lie between the two extremes. The [[Lennard-Jones potential|Lennard-Jones model]] provides a basic framework for predicting molecular interactions as a function of atomic separation.<ref name=":2">{{Cite book |title=Principles and practice of heterogeneous catalysis|first1=J. M. |last1=Thomas |last2=Thomas |first2=W. J.|isbn=978-3-527-68378-9|edition=Second, revised |location=Weinheim, Germany|oclc=898421752|date=2014-11-19 }}</ref>

==== Physisorption ==== In physisorption, a molecule becomes attracted to the surface atoms via [[van der Waals force]]s. These include dipole-dipole interactions, induced dipole interactions, and London dispersion forces. Note that no chemical bonds are formed between adsorbate and adsorbent, and their electronic states remain relatively unperturbed. Typical energies for physisorption are from 3 to 10 kcal/mol.<ref name=":02" /> In heterogeneous catalysis, when a reactant molecule physisorbs to a catalyst, it is commonly said to be in a precursor state, an intermediate energy state before chemisorption, a more strongly bound adsorption.<ref name=":2" /> From the precursor state, a molecule can either undergo chemisorption, desorption, or migration across the surface.<ref name=":3">{{Cite journal|last=Bowker|first=Michael|date=2016-03-28|title=The Role of Precursor States in Adsorption, Surface Reactions and Catalysis|journal=Topics in Catalysis|volume=59|issue=8–9|pages=663–670|doi=10.1007/s11244-016-0538-6|pmid=21386456|issn=1022-5528|doi-access=free|url=https://orca.cardiff.ac.uk/id/eprint/91556/1/art_10.1007_s11244-016-0538-6.pdf}}</ref> The nature of the precursor state can influence the reaction kinetics.<ref name=":3" />

==== Chemisorption ==== When a molecule approaches close enough to surface atoms such that their [[Atomic orbital|electron clouds]] overlap, chemisorption can occur. In chemisorption, the adsorbate and adsorbent share electrons signifying the formation of [[chemical bond]]s. Typical energies for chemisorption range from 20 to 100 kcal/mol.<ref name=":02" /> Two cases of chemisorption are:

* Molecular adsorption: the adsorbate remains intact. An example is alkene binding by platinum. * Dissociation adsorption: one or more bonds break concomitantly with adsorption. In this case, the barrier to [[Dissociation (chemistry)|dissociation]] affects the rate of adsorption. An example of this is the binding of H₂ to a metal catalyst, where the H-H bond is broken upon adsorption.

== Surface reactions == [[File:Catalytic reaction coordinate.jpg|thumb|upright=1.35|Reaction Coordinate. (A) Uncatalyzed (B) Catalyzed (C) Catalyzed with discrete intermediates (transition states)]] Most metal surface reactions occur by [[chain propagation]] in which catalytic intermediates are cyclically produced and consumed.<ref>{{Cite book|last=Masel|first=Richard I.|url=https://books.google.com/books?id=kEpRAAAAMAAJ|title=Principles of Adsorption and Reaction on Solid Surfaces|date=22 March 1996|publisher=Wiley|isbn=978-0-471-30392-3|language=en|oclc=32429536}}</ref> Two main mechanisms for surface reactions can be described for A + B → C.<ref name=":02" />

* [[Reactions on surfaces#Langmuir–Hinshelwood mechanism|Langmuir–Hinshelwood mechanism]]: The reactant molecules, A and B, both adsorb to the catalytic surface. While adsorbed to the surface, they combine to form product C, which then desorbs. * [[Reactions on surfaces#Eley–Rideal mechanism|Eley–Rideal mechanism]]: One reactant molecule, A, adsorbs to the catalytic surface. Without adsorbing, B reacts with absorbed A to form C, that then desorbs from the surface.

Most heterogeneously catalyzed reactions are described by the Langmuir–Hinshelwood model.<ref>{{Cite journal|last=Petukhov|first=A.V.|date=1997|title=Effect of molecular mobility on kinetics of an electrochemical Langmuir–Hinshelwood reaction|journal=Chemical Physics Letters|volume=277|issue=5–6|pages=539–544|doi=10.1016/s0009-2614(97)00916-0|bibcode=1997CPL...277..539P|issn=0009-2614}}</ref>

In heterogeneous catalysis, reactants [[Surface diffusion|diffuse]] from the bulk fluid phase to [[adsorb]] to the catalyst surface. The adsorption site is not always an active catalyst site, so reactant molecules must migrate across the surface to an active site. At the active site, reactant molecules will react to form product molecule(s) by following a more energetically facile path through catalytic intermediates (see figure to the right). The product molecules then desorb from the surface and diffuse away. The catalyst itself remains intact and free to mediate further reactions. Transport phenomena such as heat and mass transfer, also play a role in the observed reaction rate.

== Catalyst design == [[File:Zeolite-ZSM-5-3D-vdW.png|thumb|Zeolite structure. A common catalyst support material in hydrocracking. Also acts as a catalyst in hydrocarbon alkylation and isomerization.]] Catalysts are not active towards reactants across their entire surface; only specific locations possess catalytic activity, called [[Active site|'''active sites''']]. The surface area of a solid catalyst has a strong influence on the number of available active sites. In industrial practice, solid catalysts are often porous to maximize surface area, commonly achieving 50–400 m²/g.<ref name=":02" /> Some [[mesoporous silicates]], such as the MCM-41, have surface areas greater than 1000 m²/g.<ref>{{Cite journal|last1=Kresge|first1=C. T.|last2=Leonowicz|first2=M. E.|last3=Roth|first3=W. J.|last4=Vartuli|first4=J. C.|last5=Beck|first5=J. S.|date=1992|title=Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism|journal=Nature|volume=359|issue=6397|pages=710–712|doi=10.1038/359710a0|issn=0028-0836|bibcode=1992Natur.359..710K|s2cid=4249872}}</ref> Porous materials are cost effective due to their high surface area-to-mass ratio and enhanced catalytic activity.

In many cases, a solid catalyst is [[Dispersion (chemistry)|dispersed]] on a supporting material to increase surface area (spread the number of active sites) and provide stability.<ref name=":02" /> Usually [[catalyst support]]s are inert, high melting point materials, but they can also be catalytic themselves. Most catalyst supports are porous (frequently carbon, silica, zeolite, or alumina-based)<ref name=":6" /> and chosen for their high surface area-to-mass ratio. For a given reaction, porous supports must be selected such that reactants and products can enter and exit the material.

Often, substances are intentionally added to the reaction feed or on the catalyst to influence catalytic activity, selectivity, and/or stability. These compounds are called promoters. For example, alumina (Al₂O₃) is added during ammonia synthesis to providing greater stability by slowing sintering processes on the Fe-catalyst.<ref name=":02" />

[[Sabatier principle]] can be considered one of the cornerstones of modern theory of catalysis.<ref>{{Cite journal|doi=10.1016/j.jcat.2014.12.033|title=From the Sabatier principle to a predictive theory of transition-metal heterogeneous catalysis|journal=Journal of Catalysis|volume=328|pages=36–42|year=2015|last1=Medford|first1=Andrew J.|last2=Vojvodic|first2=Aleksandra|last3=Hummelshøj|first3=Jens S.|last4=Voss|first4=Johannes|last5=Abild-Pedersen|first5=Frank|last6=Studt|first6=Felix|last7=Bligaard|first7=Thomas|last8=Nilsson|first8=Anders|last9=Nørskov|first9=Jens K.|doi-access=free}}</ref> Sabatier principle states that the surface-adsorbates interaction has to be an optimal amount: not too weak to be inert toward the reactants and not too strong to poison the surface and avoid desorption of the products.<ref>{{Cite journal|title=The Sabatier Principle Illustrated by Catalytic H2O2 Decomposition on Metal Surfaces|journal = Journal of Chemical Education|volume = 88|issue = 12|pages = 1711–1715|last1=Laursen|first1=Anders B.|last2=Man|first2=Isabela Costinela|date=2011-10-04|doi=10.1021/ed101010x|last3=Trinhammer|first3=Ole L.|last4=Rossmeisl|first4=Jan|last5=Dahl|first5=Søren|bibcode = 2011JChEd..88.1711L}}</ref> The statement that the surface-adsorbate interaction has to be an optimum, is a qualitative one. Usually the number of adsorbates and [[transition state]]s associated with a chemical reaction is a large number, thus the [[optimum]] has to be found in a many-dimensional space. Catalyst design in such a many-dimensional space is not a computationally viable task. Additionally, such optimization process would be far from intuitive. Scaling relations are used to decrease the dimensionality of the space of catalyst design.<ref name=":0">{{Cite journal|last1=Abild-Pedersen|first1=F.|last2=Greeley|first2=J.|last3=Studt|first3=F.|last4=Rossmeisl|first4=J.|last5=Munter|first5=T. R.|last6=Moses|first6=P. G.|last7=Skúlason|first7=E.|last8=Bligaard|first8=T.|last9=Nørskov|first9=J. K.|date=2007-07-06|title=Scaling Properties of Adsorption Energies for Hydrogen-Containing Molecules on Transition-Metal Surfaces|journal=Physical Review Letters|volume=99|issue=1|article-number=016105|doi=10.1103/PhysRevLett.99.016105|pmid=17678168|bibcode=2007PhRvL..99a6105A|s2cid=11603704 |url=http://orbit.dtu.dk/ws/files/4787818/Scaling_PRL.pdf}}</ref> Such relations are correlations among adsorbates binding energies (or among adsorbate binding energies and transition states also known as [[Bell–Evans–Polanyi principle|BEP relations]])<ref>{{Cite journal|last1=Nørskov|first1=Jens K.|last2=Christensen|first2=Claus H.|last3=Bligaard|first3=Thomas|last4=Munter|first4=Ture R.|date=2008-08-18|title=BEP relations for N2 dissociation over stepped transition metal and alloy surfaces|journal=Physical Chemistry Chemical Physics|volume=10|issue=34|pages=5202–5206|doi=10.1039/B720021H|pmid=18728861|bibcode=2008PCCP...10.5202M|issn=1463-9084}}</ref> that are "similar enough" e.g., OH versus OOH scaling.<ref>{{Cite journal|last1=Viswanathan|first1=Venkatasubramanian|last2=Hansen|first2=Heine Anton|last3=Rossmeisl|first3=Jan|last4=Nørskov|first4=Jens K.|date=2012-07-11|title=Universality in Oxygen Reduction Electrocatalysis on Metal Surfaces|journal=ACS Catalysis|volume=2|issue=8|pages=1654–1660|doi=10.1021/cs300227s|issn=2155-5435|doi-access=free}}</ref> Applying scaling relations to the catalyst design problems greatly reduces the space dimensionality (sometimes to as small as 1 or 2).<ref name=":7"/> One can also use micro-kinetic modeling based on such scaling relations to take into account the kinetics associated with adsorption, reaction and desorption of molecules under specific pressure or temperature conditions.<ref>{{Cite journal|last1=Medford|first1=Andrew J.|last2=Shi|first2=Chuan|last3=Hoffmann|first3=Max J.|last4=Lausche|first4=Adam C.|last5=Fitzgibbon|first5=Sean R.|last6=Bligaard|first6=Thomas|last7=Nørskov|first7=Jens K.|date=2015-03-01|title=CatMAP: A Software Package for Descriptor-Based Microkinetic Mapping of Catalytic Trends|journal=Catalysis Letters|volume=145|issue=3|pages=794–807|doi=10.1007/s10562-015-1495-6|s2cid=98391105|issn=1572-879X}}</ref> Such modeling then leads to well-known volcano-plots at which the optimum qualitatively described by the Sabatier principle is referred to as the "top of the volcano". Scaling relations can be used not only to connect the energetics of [[Radical (chemistry)|radical]] surface-adsorbed groups (e.g., O*,OH*),<ref name=":0" /> but also to connect the energetics of [[Closed shell|closed-shell]] molecules among each other or to the counterpart radical adsorbates.<ref>{{Cite journal |last1=Kakekhani |first1=Arvin |last2=Roling |first2=Luke T. |last3=Kulkarni |first3=Ambarish |last4=Latimer |first4=Allegra A. |last5=Abroshan |first5=Hadi |last6=Schumann |first6=Julia |last7=AlJama |first7=Hassan |last8=Siahrostami |first8=Samira |author-link8=Samira Siahrostami |last9=Ismail-Beigi |first9=Sohrab |date=2018-06-18 |title=Nature of Lone-Pair–Surface Bonds and Their Scaling Relations |journal=Inorganic Chemistry |volume=57 |issue=12 |pages=7222–7238 |doi=10.1021/acs.inorgchem.8b00902 |issn=0020-1669 |osti=1459598 |pmid=29863849 |s2cid=46932095}}</ref> A recent challenge for researchers in catalytic sciences is to "break" the scaling relations.<ref>{{Cite journal|last1=Chen|first1=Ping|last2=He|first2=Teng|last3=Wu|first3=Guotao|last4=Guo|first4=Jianping|last5=Gao|first5=Wenbo|last6=Chang|first6=Fei|last7=Wang|first7=Peikun|date=January 2017|title=Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation|journal=Nature Chemistry|volume=9|issue=1|pages=64–70|doi=10.1038/nchem.2595|pmid=27995914|bibcode=2017NatCh...9...64W|issn=1755-4349}}</ref> The correlations which are manifested in the scaling relations confine the catalyst design space, preventing one from reaching the "top of the volcano". Breaking scaling relations can refer to either designing surfaces or motifs that do not follow a scaling relation, or ones that follow a different scaling relation (than the usual relation for the associated adsorbates) in the right direction: one that can get us closer to the top of the reactivity volcano.<ref name=":7">{{Cite journal|last1=Nørskov|first1=Jens K.|last2=Vojvodic|first2=Aleksandra|date=2015-06-01|title=New design paradigm for heterogeneous catalysts|journal=National Science Review|volume=2|issue=2|pages=140–143|doi=10.1093/nsr/nwv023|issn=2095-5138|doi-access=free}}</ref> In addition to studying catalytic reactivity, scaling relations can be used to study and screen materials for selectivity toward a special product.<ref>{{Cite journal|last1=Schumann|first1=Julia|last2=Medford|first2=Andrew J.|last3=Yoo|first3=Jong Suk|last4=Zhao|first4=Zhi-Jian|last5=Bothra|first5=Pallavi|last6=Cao|first6=Ang|last7=Studt|first7=Felix|last8=Abild-Pedersen|first8=Frank|last9=Nørskov|first9=Jens K.|date=2018-03-13|title=Selectivity of Synthesis Gas Conversion to C2+ Oxygenates on fcc(111) Transition-Metal Surfaces|journal=ACS Catalysis|volume=8|issue=4|pages=3447–3453|doi=10.1021/acscatal.8b00201|osti=1457170|url=https://www.osti.gov/biblio/1457170}}</ref> There are special combination of binding energies that favor specific products over the others. Sometimes a set of binding energies that can change the selectivity toward a specific product "scale" with each other, thus to improve the selectivity one has to break some scaling relations; an example of this is the scaling between methane and methanol oxidative activation energies that leads to the lack of selectivity in direct conversion of methane to methanol.<ref>{{Cite journal|last1=Nørskov|first1=Jens K.|last2=Studt|first2=Felix|last3=Abild-Pedersen|first3=Frank|last4=Tsai|first4=Charlie|last5=Yoo|first5=Jong Suk|last6=Montoya|first6=Joseph H.|last7=Aljama|first7=Hassan|last8=Kulkarni|first8=Ambarish R.|last9=Latimer|first9=Allegra A.|date=February 2017|title=Understanding trends in C–H bond activation in heterogeneous catalysis|journal=Nature Materials|volume=16|issue=2|pages=225–229|doi=10.1038/nmat4760|pmid=27723737|bibcode=2017NatMa..16..225L|osti=1349287 |s2cid=11360569 |issn=1476-4660|url=https://escholarship.org/uc/item/2ww8j3wc}}</ref>

==Catalyst deactivation== Catalyst deactivation is defined as a loss in catalytic activity and/or selectivity over time.

Substances that decrease the reaction rate are called '''poisons'''. Poisons chemisorb to the catalyst surface and reduce the number of available active sites for reactant molecules to bind to.<ref name=":4">{{Cite journal|last=Bartholomew|first=Calvin H|date=2001|title=Mechanisms of catalyst deactivation|journal=Applied Catalysis A: General|volume=212|issue=1–2|pages=17–60|doi=10.1016/S0926-860X(00)00843-7|doi-access=free|bibcode=2001AppCA.212...17B }}</ref> Common poisons include Group V, VI, and VII elements (e.g. S, O, P, Cl), some toxic metals (e.g. As, Pb), and adsorbing species with multiple bonds (e.g. CO, unsaturated hydrocarbons).<ref name=":2" /><ref name=":4" /> For example, sulfur disrupts the production of methanol by poisoning the Cu/ZnO catalyst.<ref name=":5">{{Cite book|title=Fundamental concepts in heterogeneous catalysis|author=Nørskov, Jens K.|others=Studt, Felix., Abild-Pedersen, Frank., Bligaard, Thomas.|isbn=978-1-118-89202-2|location=Hoboken, New Jersey|oclc=884500509|date=2014-08-25}}</ref> Substances that increase reaction rate are called '''promoters'''. For example, the presence of alkali metals in ammonia synthesis increases the rate of N₂ dissociation.<ref name=":5" />

The presence of poisons and promoters can alter the [[activation energy]] of the rate-limiting step and affect a catalyst's selectivity for the formation of certain products. Depending on the amount, a substance can be favorable or unfavorable for a chemical process. For example, in the production of ethylene, a small amount of chemisorbed chlorine will act as a promoter by improving Ag-catalyst selectivity towards ethylene over CO₂, while too much chlorine will act as a poison.<ref name=":2"/>

Other mechanisms for catalyst deactivation include:

* [[Sintering]]: when heated, dispersed catalytic metal particles can migrate across the support surface and form crystals. This results in a reduction of catalyst surface area. * [[Fouling]]: the deposition of materials from the fluid phase onto the solid phase catalyst and/or support surfaces. This results in active site and/or pore blockage. * [[Coking]]: the deposition of heavy, carbon-rich solids onto surfaces due to the decomposition of hydrocarbons<ref name=":4" /> * Vapor-solid reactions: formation of an inactive surface layer and/or formation of a volatile compound that exits the reactor.<ref name=":4" /> This results in a loss of surface area and/or catalyst material. * [[Phase transition|Solid-state transformation]]: solid-state diffusion of catalyst support atoms to the surface followed by a reaction that forms an inactive phase. This results in a loss of catalyst surface area. * Erosion: continual attrition of catalyst material common in fluidized-bed reactors.<ref>{{Cite journal|last=Forzatti|first=P|date=1999-09-14|title=Catalyst deactivation|journal=Catalysis Today|volume=52|issue=2–3|pages=165–181|doi=10.1016/s0920-5861(99)00074-7|s2cid=19737702|issn=0920-5861}}</ref> This results in a loss of catalyst material.

In industry, catalyst deactivation costs billions every year due to process shutdown and catalyst replacement.<ref name=":4"/>

==Industrial examples== In industry, many design variables must be considered including reactor and catalyst design across multiple scales ranging from the subnanometer to tens of meters. The conventional heterogeneous catalysis reactors include [[Batch reactor|batch]], [[Continuous reactor|continuous]], and [[Fluidized bed reactor|fluidized-bed reactors]], while more recent setups include fixed-bed, microchannel, and multi-functional [[Heterogeneous catalytic reactor|reactors]].<ref name=":2" /> Other variables to consider are reactor dimensions, surface area, catalyst type, catalyst support, as well as reactor operating conditions such as temperature, pressure, and reactant concentrations.

[[File:Heterrogenous catalysis across scales.png|thumb|Schematic representation of a heterogeneous catalytic system from the subnanometer to industrial scale.]]

Some large-scale industrial processes incorporating heterogeneous catalysts are listed below.<ref name=":6" /> {| class="wikitable" style = "text-align:center" |- !Process !Reactants, Product/s (not balanced) !Catalyst !Comment |- |Sulfuric acid synthesis ([[Contact process]]) |SO₂ + O₂, SO₃ |vanadium oxides |Hydration of SO₃ gives H₂SO₄ |- |Ammonia synthesis ([[Haber–Bosch process]]) |N₂ + H₂, NH₃ |iron oxides on [[alumina]](Al₂O₃) |Consumes 1% of world's industrial energy budget<ref name=":02" /> |- |Nitric acid synthesis ([[Ostwald process]]) |NH₃ + O₂, HNO₃ |unsupported Pt-Rh gauze |Direct routes from N₂ are uneconomical |- |Hydrogen production by [[Steam reforming]] |CH₄ + H₂O, H₂ + CO₂ |Nickel or K₂O |Greener routes to H₂ by [[water splitting]] actively sought |- |[[Ethylene oxide]] synthesis |C₂H₄ + O₂, C₂H₄O |[[silver]] on [[alumina]], with many promoters |Poorly applicable to other alkenes |- |Hydrogen cyanide synthesis ([[Andrussov oxidation]]) |NH₃ + O₂ + CH₄, HCN |Pt-Rh |Related [[ammoxidation]] process converts hydrocarbons to [[nitrile]]s |- |Olefin polymerization [[Ziegler–Natta polymerization]] |[[propylene]], [[polypropylene]] |[[Titanium trichloride|TiCl₃]] on [[magnesium chloride|MgCl₂]] |Many variations exist, including some [[homogeneous catalysis|homogeneous examples]] |- |Desulfurization of petroleum ([[hydrodesulfurization]]) |H₂ + R₂S (idealized organosulfur impurity), RH + H₂S |[[Molybdenum|Mo]]-[[Cobalt|Co]] on alumina |Produces low-sulfur hydrocarbons, sulfur recovered via the [[Claus process]] |}

[[File:Haber-Bosch-En.svg|center|thumb|upright=2.75|Process flow diagram illustrating the use of catalysis in the synthesis of [[ammonia]] (NH₃)]]

===Other examples=== *Reduction of [[nitrile]]s in the synthesis of [[phenethylamine]] with [[Raney nickel]] catalyst and hydrogen in [[ammonia]]:<ref>Organic Syntheses, Coll. Vol. 3, p.720 (1955); Vol. 23, p.71 (1943). https://web.archive.org/web/20120315000000*/http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf</ref> [[Image:NitrileHydrogenation.svg|thumb|upright=1.5|Nitrile hydrogenation]] * The cracking, [[isomerisation]], and reformation of [[hydrocarbons]] to form appropriate and useful blends of petrol. *In automobiles, [[catalytic converter]]s are used to catalyze three main reactions: **The [[oxidation]] of [[carbon monoxide]] to [[carbon dioxide]]: **:2CO(g) + O₂(g) → 2CO₂(g) **The [[redox|reduction]] of [[nitrogen monoxide]] back to [[nitrogen]]: **:2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g) **The [[oxidation]] of [[hydrocarbons]] to water and [[carbon dioxide]]: **:2 C₆H₆ + 15 O₂ → 12 CO₂ + 6 H₂O *This process can occur with any of [[hydrocarbons|hydrocarbon]], but most commonly is performed with [[petrol]] or [[Diesel fuel|diesel]]. *Asymmetric heterogeneous catalysis facilitates the production of pure enantiomer compounds using chiral heterogeneous catalysts.<ref>{{cite journal | last1 = Heitbaum | last2 = Glorius | last3 = Escher | year = 2006 | title = Asymmetric heterogeneous catalysis | journal = Angew. Chem. Int. Ed. | volume = 45 | issue = 29| pages = 4732–62| doi = 10.1002/anie.200504212 | pmid = 16802397 }}</ref> *The majority of heterogeneous catalysts are based on [[metal]]s<ref>{{cite journal |title=Heterogeneous single-atom catalysis |journal=Nature Reviews Chemistry |date=June 2018 |volume=2 |issue=6 |pages=65–81 |doi=10.1038/s41570-018-0010-1 |language=en |issn=2397-3358|last1=Wang |first1=Aiqin |last2=Li |first2=Jun |last3=Zhang |first3=Tao |s2cid=139163163 }}</ref> or [[oxide|metal oxides]];<ref>{{cite journal |title=Metal oxide redox chemistry for chemical looping processes |journal=Nature Reviews Chemistry |date=November 2018 |volume=2 |issue=11 |pages=349–364 |doi=10.1038/s41570-018-0046-2 |language=en |issn=2397-3358|last1=Zeng |first1=Liang |last2=Cheng |first2=Zhuo |last3=Fan |first3=Jonathan A. |last4=Fan |first4=Liang-Shih |last5=Gong |first5=Jinlong |s2cid=85504970 }}</ref> however, some chemical reactions can be [[carbocatalysis|catalyzed by carbon]]-based materials, e.g., oxidative [[dehydrogenation]]s<ref name=zhang>{{cite journal | last1 = Zhang | first1 = J. | last2 = Liu | first2 = X. | last3 = Blume | first3 = R. | last4 = Zhang | first4 = A. | last5 = Schlögl | first5 = R. | last6 = Su | first6 = D. S. | year = 2008 | title = Surface-Modified Carbon Nanotubes Catalyze Oxidative Dehydrogenation of n-Butane | journal = Science | volume = 322 | issue = 5898 | pages = 73–77 | doi = 10.1126/science.1161916 | pmid=18832641|bibcode = 2008Sci...322...73Z | hdl = 11858/00-001M-0000-0010-FE91-E | s2cid = 35141240 | hdl-access = free }}</ref> or selective [[oxidation]]s.<ref name=frank>{{cite journal | last1 = Frank | first1 = B. | last2 = Blume | first2 = R. | last3 = Rinaldi | first3 = A. | last4 = Trunschke | first4 = A. | last5 = Schlögl | first5 = R. | year = 2011 | title = Oxygen Insertion Catalysis by sp² Carbon | journal = Angew. Chem. Int. Ed. | volume = 50 | issue = 43 | pages = 10226–10230 | doi = 10.1002/anie.201103340 | pmid = 22021211 | doi-access = free | hdl = 11858/00-001M-0000-0012-0B9A-8 | hdl-access = free }}</ref> **[[Ethylbenzene]] + 1/2 O₂ → [[Styrene]] + H₂O **[[Acrolein]] + 1/2 O₂ → [[Acrylic acid]]

==Solid-Liquid and Liquid-Liquid Catalyzed Reactions== Although the majority of heterogeneous catalysts are solids, there are a few variations which are of practical value. For two immiscible solutions (liquids), one carries the catalyst while the other carries the reactant. This set up is the basis of biphasic catalysis as implemented in the industrial production of butyraldehyde by the hydroformylation of propylene.<ref>{{cite book|title=Aqueous-Phase Organometallic Catalysis: Concepts and Applications|publisher=Wiley-VCH|year=2004|editor=Boy Cornils |editor2=[[Wolfgang A. Herrmann]]}}</ref> {| class="wikitable" style="text-align:center" |- !Reacting phases !Examples given !Comment |- |solid + solution |hydrogenation of fatty acids with nickel |used for the production of [[margarine]] |- |immiscible liquid phases |[[hydroformylation]] of [[propene]] |aqueous phase catalyst; reactants and products mainly in non-aqueous phase |}

==See also== *[[Heterogeneous gold catalysis]] *[[Nanomaterial-based catalyst]]s *[[Platinum nanoparticles]] *[[Temperature-programmed reduction]] *[[Thermal desorption spectroscopy]]

==References== <references/>

== External links == * {{Commons category-inline|Heterogeneous catalysis}}

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[[Category:Catalysis]]