Editing Catalysis (section)

==Heterogeneous catalysis==
{{Main|Heterogeneous catalysis}}

[[File:Zeolite-ZSM-5-vdW.png|thumb|right|The microporous molecular structure of the [[zeolite]] ZSM-5 is exploited in catalysts used in refineries]]
[[File:Ceolite nax.JPG|thumb|Zeolites are extruded as pellets for easy handling in catalytic reactors.]]
Heterogeneous catalysts act in a different [[phase (matter)|phase]] than the [[reactants]]. Most heterogeneous catalysts are [[solid]]s that act on substrates in a [[liquid]] or gaseous [[reaction mixture]]. Important heterogeneous catalysts include [[zeolite]]s, [[alumina]],<ref>{{cite journal |last1=Shafiq |first1=Iqrash |last2=Shafique |first2=Sumeer |last3=Akhter |first3=Parveen |last4=Yang |first4=Wenshu |last5=Hussain |first5=Murid |title=Recent developments in alumina supported hydrodesulfurization catalysts for the production of sulfur-free refinery products: A technical review |date=2020-06-23 |journal=Catalysis Reviews |volume=64 |issue=1 |pages=1–86 |issn=0161-4940 |doi=10.1080/01614940.2020.1780824 |s2cid=225777024}}</ref> higher-order oxides, graphitic carbon, [[transition metal]] [[oxide]]s, metals such as [[Raney nickel]] for hydrogenation, and [[vanadium(V) oxide]] for oxidation of [[sulfur dioxide]] into [[sulfur trioxide]] by the [[contact process]].<ref name=Housecroft>{{cite book |last1=Housecroft |first1=Catherine E. |last2=Sharpe |first2=Alan G. |title=Inorganic Chemistry |date=2005 |publisher=Pearson Prentice-Hall |isbn=0130-39913-2 |page=805 |edition=2nd}}</ref>

Diverse mechanisms for [[reactions on surfaces]] are known, depending on how the adsorption takes place ([[Langmuir-Hinshelwood-Hougen-Watson|Langmuir-Hinshelwood]], [[Eley–Rideal mechanism|Eley-Rideal]], and Mars-[[Dirk Willem van Krevelen|van Krevelen]]).<ref name=doi>Knözinger, Helmut and Kochloefl, Karl (2002) "Heterogeneous Catalysis and Solid Catalysts" in Ullmann's ''Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a05_313}}</ref> 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.<ref>{{cite book |date=2006 |last1=Appl |first1=Max |chapter=Ammonia |title=Ullmann's Encyclopedia of Industrial Chemistry |isbn=3527306730 |doi=10.1002/14356007.a02_143.pub2}}</ref> The reacting [[gas]]es [[adsorption|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.<ref>{{cite web |title=Chemistry of Vanadium |date=2013-10-03 |website=Chemistry LibreTexts |language=en |url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/3_d-Block_Elements/Group_05%3A_Transition_Metals/Chemistry_of_Vanadium |access-date=2022-07-08}}</ref> 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]].<ref name=Housecroft/> Many heterogeneous catalysts are in fact nanomaterials.

Heterogeneous catalysts are typically "[[catalyst support|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]].<ref>{{cite journal |last1=Chadha |first1=Utkarsh |last2=Selvaraj |first2=Senthil Kumaran |last3=Ashokan |first3=Hridya |last4=Hariharan |first4=Sai P. |last5=Mathew Paul |first5=V. |last6=Venkatarangan |first6=Vishal |last7=Paramasivam |first7=Velmurugan |title=Complex Nanomaterials in Catalysis for Chemically Significant Applications: From Synthesis and Hydrocarbon Processing to Renewable Energy Applications |date=2022-02-08 |journal=Advances in Materials Science and Engineering |language=en |volume=2022 |pages=e1552334 |issn=1687-8434 |doi=10.1155/2022/1552334 |doi-access=free}}</ref>

===Electrocatalysts===
{{Main|Electrocatalyst}}
In the context of [[electrochemistry]], specifically in [[fuel cell]] engineering, various metal-containing catalysts are used to enhance the rates of the [[half reaction]]s 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 [[electrode]]s in a fuel cell, this platinum increases the rate of [[oxygen]] reduction either to water or to [[hydroxide]] or [[hydrogen peroxide]].