View source for Catalytic converter

{{Short description|Exhaust emission control device}}
{{Use dmy dates|date=February 2021}}
[[File:DodgeCatCon.jpg|300px|thumb|A three-way catalytic converter on a gasoline-powered 1996 [[Ram Pickup|Dodge Ram]]]]
[[File:Catalytic-converter-simulation-velocity-streamlines.jpg|thumb|Simulation of flow inside a catalytic converter]]

A '''catalytic converter''' is an [[vehicle emissions control|exhaust emission control]] device which converts toxic gases and [[pollutant]]s in [[exhaust gas]] from an [[internal combustion engine]] into less-toxic pollutants by [[catalysis|catalyzing]] a [[redox]] [[chemical reaction|reaction]]. Catalytic converters are usually used with internal combustion engines fueled by [[gasoline]] or [[diesel fuel|diesel]], including [[lean-burn]] engines, and sometimes on [[kerosene]] heaters and stoves.

The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]]'s stricter regulation of exhaust emissions, most gasoline-powered vehicles starting with the 1975 model year are equipped with catalytic converters.<ref name='Petersen'>{{cite book |last1=Petersen Publishing |title=The Petersen Automotive Troubleshooting & Repair Manual |chapter=The Catalytic Converter |editor=Erwin M. Rosen |publisher=Grosset & Dunlap |year=1975 |location=New York, NY |page=493 |isbn=978-0-448-11946-5 |quote=For years, the exhaust system ... remained virtually unchanged until 1975 when a strange new component was added. It's called a catalytic converter...}}<!-- |access-date=14 December 2011 --></ref><ref name='GM_advert'>{{cite news |title=General Motors Believes it has an Answer to the Automotive Air Pollution Problem |date=12 September 1974 |work=The Blade: Toledo, Ohio |access-date=14 December 2011 |url=https://news.google.com/newspapers?id=9tBOAAAAIBAJ&dq=catalytic-converter&pg=6404%2C6576523}}</ref><ref name='Sentinel_1974'>{{cite news |title=Catalytic Converter Heads Auto Fuel Economy Efforts |date=11 November 1974 |work=The Milwaukee Sentinel |access-date=14 December 2011 |url=https://news.google.com/newspapers?id=FXVQAAAAIBAJ&dq=catalytic-converter&pg=6134%2C2245045 }}{{Dead link|date=February 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> These "two-way" converters combine [[oxygen]] with [[carbon monoxide]] (CO) and [[unburned hydrocarbon]]s (HC) to produce carbon dioxide (CO<sub>2</sub>) and water (H<sub>2</sub>O). 

Although two-way converters on gasoline engines were rendered obsolete in 1981 by "three-way" converters that also reduce [[NOx|oxides of nitrogen]] ({{NOx}}),<ref name="Palucka">{{cite journal |last=Palucka |first=Tim |date=Winter 2004 |title=Doing the Impossible |url=http://www.americanheritage.com/articles/magazine/it/2004/3/2004_3_22.shtml |journal=Invention & Technology |volume=19 |issue=3 |archive-url=https://web.archive.org/web/20081203124718/http://www.americanheritage.com/articles/magazine/it/2004/3/2004_3_22.shtml |archive-date=3 December 2008 |access-date=14 December 2011}}</ref> they are still used on lean-burn engines to oxidize particulate matter and hydrocarbon emissions (including diesel engines, which typically use lean combustion), as three-way-converters require [[Air-fuel ratio|fuel-rich]] or [[stoichiometric]] combustion to successfully reduce {{NOx}}.

Although catalytic converters are most commonly applied to [[exhaust system]]s in automobiles, they are also used on [[electrical generator]]s, [[forklift]]s, mining equipment, [[truck]]s, [[bus]]es, [[locomotive]]s, [[motorcycle]]s, and on ships. They are even used on some [[wood-burning stove|wood stoves]] to control emissions.<ref>{{cite web|title= Choosing the Right Wood Stove|url=https://www.epa.gov/burnwise/choosing-right-wood-stove|work=Burn Wise|publisher=US EPA|access-date=2 January 2012}}</ref> This is usually in response to government regulation, either through environmental regulation or through health and safety regulations.

==History==
Catalytic converter prototypes were first designed in [[France]] at the end of the 19th century, when only a few thousand "oil cars" were on the roads; these prototypes had inert clay-based materials coated with [[platinum]], [[rhodium]], and [[palladium]] and sealed into a double metallic cylinder.<ref>{{Cite book|title=Airvore ou la face obscure des transports; chronique d'une pollution annoncée|last=Castaignède|first=Laurent|publisher=écosociété|year=2018|isbn=9782897193591|location=Montréal (Québec)|pages=109–110 and illustration p. 7|oclc=1030881466}}</ref> A few decades later, a catalytic converter was patented by [[Eugene Houdry]], a French mechanical engineer. Houdry was an expert in catalytic oil refining, having invented the catalytic cracking process that all modern refining is based on today.<ref name="Csere1988p63">{{Cite journal|last= Csere|first= Csaba|author-link= Csaba Csere|date=January 1988|title= 10 Best Engineering Breakthroughs|journal= [[Car and Driver]]|volume= 33|issue= 7 |page= 63}}</ref> Houdry moved to the United States in 1930 to live near the refineries in [[Philadelphia]] and develop his catalytic refining process. 

When the results of early studies of [[smog]] in Los Angeles were published, Houdry became concerned about the role of smokestack exhaust and automobile exhaust in [[air pollution]] and founded a company called Oxy-Catalyst. Houdry first developed catalytic converters for [[Chimney|smokestacks]], called "cats" for short, and later developed catalytic converters for warehouse forklifts that used low grade, unleaded gasoline.<ref>"[https://books.google.com/books?id=49gDAAAAMBAJ&pg=PA134 Exhaust Gas Made Safe]" ''[[Popular Mechanics]]'', September 1951, p. 134, bottom of page</ref> In the mid-1950s, he began research to develop catalytic converters for [[Petrol engine|gasoline engines]] used on cars and was awarded United States Patent [[:File:US2742437 Houdry Auto Catalyst.pdf|2,742,437]] for his work.<ref>"[https://books.google.com/books?id=biYDAAAAMBAJ&dq=popular+science+1930&pg=PA83 His Smoke Eating Cats Now Attack Traffic Smog]". ''[[Popular Science]]'', June 1955, pp. 83-85/244.</ref>

Catalytic converters were further developed by a series of engineers including [[Carl D. Keith]], [[John J. Mooney]], Antonio Eleazar, and Phillip Messina at [[Engelhard]] Corporation,<ref>{{Registration required|date=January 2011}} [https://www.nytimes.com/2008/11/15/us/15keith.html "Carl D. Keith, a Father of the Catalytic Converter, Dies at 88"]. ''[[The New York Times]]''. 15 November 2008.</ref><ref>{{cite news |last1=Roberts |first1=Sam |title=John J. Mooney, an Inventor of the Catalytic Converter, Dies at 90 |url=https://www.nytimes.com/2020/06/25/climate/john-j-mooney-dead.html |website=The New York Times |date=25 June 2020 }}</ref> creating the first production catalytic converter in 1973.<ref>[[Staff writer]] (undated). "[http://www.referenceforbusiness.com/history/En-Ge/Engelhard-Corporation.html Engelhard Corporation]". referenceforbusiness.com. Retrieved 7 January 2011.</ref>{{Unreliable source?|date=January 2011|reason=appears to be user-generated database. how vetted?}}

The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]]'s new exhaust emissions regulations, most gasoline-powered vehicles manufactured from 1975 onwards are equipped with catalytic converters. Early catalytic converters were "two-way", combining [[oxygen]] with [[carbon monoxide]] (CO) and [[unburned hydrocarbon]]s (HC, chemical compounds in fuel of the form C<sub>m</sub>H<sub>n</sub>) to produce [[carbon dioxide]] (CO<sub>2</sub>) and [[water]] (H<sub>2</sub>O).<ref name="Palucka"/><ref name="Petersen"/><ref name="GM_advert"/><ref name="Sentinel_1974"/> These stringent emission control regulations also resulted in the removal of the [[antiknock agent]] [[tetraethyl lead]] from automotive gasoline, to reduce lead in the air. Lead and its compounds are [[catalyst poisoning|catalyst poisons]] and foul catalytic converters by coating the catalyst's surface. Requiring the removal of lead allowed the use of catalytic converters to meet the other emission standards in the regulations.<ref name="HoudryCHF">{{cite web|title=Eugene Houdry|url=https://www.sciencehistory.org/historical-profile/eugene-houdry|website=Science History Institute|access-date=27 October 2016|date=June 2016}}</ref> 

To lower harmful {{NOx}} emissions, a twin-catalyst system was developed in the 1970s {{endash}} this added a separate (rhodium/platinum) catalyst which reduced {{NOx}} ahead of the air pump, after which a two-way catalytic converter (palladium/platinum) removed HC and CO.<ref name=Twigg>{{cite journal | last = Twigg | first = Martyn V. | date = 2005-04-15 | title = Controlling automotive exhaust emissions: successes and underlying science | journal = [[Philosophical Transactions of the Royal Society A]] | volume = 363 | issue = 1829 | pages = 1013–1033 | doi = 10.1098/rsta.2005.1547 | pmid = 15901550 | bibcode = 2005RSPTA.363.1013T | s2cid = 37283846 }}</ref> This cumbersome and expensive system was soon made redundant, after it was noted that under some conditions the initial catalyst also removed HC and CO. This led to the development of the three-way catalyst, made possible by electronics and engine management developments.<ref name=Twigg/>

[[William C. Pfefferle]] developed a catalytic combustor for [[gas turbine]]s in the early 1970s, allowing combustion without significant formation of [[nitrogen oxide]]s and carbon monoxide.<ref name=Carter>{{cite journal |last1=Carter |first1=Robert N. |last2=Smith |first2=Lance L. |last3=Karim |first3=Hasan |last4=Castaldi |first4=Marco |last5=Etemad |first5=Shah |last6=Muench |first6=George |last7=Boorse |first7=R. Samuel |last8=Menacherry |first8=Paul |last9=Pfefferle |first9=William C. |title=Catalytic Combustion Technology Development for Gas Turbine Engine Applications |journal=MRS Proceedings |date=1998 |volume=549 |doi=10.1557/PROC-549-93 }}</ref><ref name=BioMed>Worthy, Sharon. "[http://news.bio-medicine.org/biology-news-2/Connecticut-chemist-receives-award-for-cleaner-air-technology-4451-1 Connecticut chemist receives award for cleaner air technology] {{Webarchive|url=https://web.archive.org/web/20171201132238/http://news.bio-medicine.org/biology-news-2/Connecticut-chemist-receives-award-for-cleaner-air-technology-4451-1/ |date=1 December 2017 }}". ''Bio-Medicine''. 23 June 2003. Retrieved 11 December 2012.</ref> Four-way catalytic converters have also been developed which also remove particulates from engine exhaust; since most of these particulates are unburned hydrocarbons, they can be burned to convert them into carbon dioxide.<ref>https://www.greencarcongress.com/2014/12/20141210-basf.html {{Bare URL inline|date=August 2024}}</ref><ref>{{cite journal |last1=Millet |first1=Claire-Noelle |last2=Chédotal |first2=Romain |last3=Da Costa |first3=Patrick |title=Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration |journal=Applied Catalysis B: Environmental |date=August 2009 |volume=90 |issue=3–4 |pages=339–346 |doi=10.1016/j.apcatb.2009.03.026 |bibcode=2009AppCB..90..339M }}</ref>

==Construction==
[[File:Aufgeschnittener Metall Katalysator für ein Auto.jpg|thumb|Cutaway of a metal-core converter]]
[[File:Pot catalytique vue de la structure.jpg|thumb|Ceramic-core converter]]

The catalytic converter's construction is as follows:

# The [[catalyst support]] or [[substrate (materials science)|substrate]]. For automotive catalytic converters, the core is usually a [[ceramic]] [[monolith (catalyst support)|monolith]] that has a [[honeycomb structure]] (commonly square, not hexagonal). (Prior to the mid 1980s, the catalyst material was deposited on a [[packed bed]] of alumina pellets in early GM applications.) Metallic foil monoliths made of [[Kanthal (alloy)|Kanthal]] (FeCrAl)<ref name="Verbrennungsmotoren Band 2">{{cite book|last1=Pischinger|first1=Univ.-Prof. Dr.-Ing. Stephan|title=Verbrennungsmotoren Band 2|date=2011|publisher=Lehrstuhl Für Verbrennungskraftmachinen|location=Aachen, Germany|page=335|edition=24}}</ref> are used in applications where particularly high heat resistance is required.<ref name="Verbrennungsmotoren Band 2"/> The substrate is structured to produce a large [[surface area]]. The [[cordierite]] ceramic substrate used in most catalytic converters was invented by [[Rodney Bagley]], [[Irwin Lachman]], and [[Ronald Lewis (scientist)|Ronald Lewis]] at [[Corning Glass]], for which they were inducted into the [[National Inventors Hall of Fame]] in 2002.<ref name="Palucka"/>
# The washcoat. A washcoat is a carrier for the catalytic materials and is used to disperse the materials over a large surface area. [[Aluminum oxide]], [[titanium dioxide]], [[silicon dioxide]] e.g. [[colloidal silica]] or a mixture of [[silica]] and [[alumina]] can be used. The catalytic materials are suspended in the washcoat prior to applying to the core. Washcoat materials are selected to form a [[surface roughness|rough]], irregular surface, which increases the surface area compared to the smooth surface of the bare substrate.<ref>{{cite book |doi=10.1002/14356007.a03_189.pub2 |chapter=Automobile Exhaust Control |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2009 |last1=Votsmeier |first1=Martin |last2=Kreuzer |first2=Thomas |last3=Gieshoff |first3=Jürgen |last4=Lepperhoff |first4=Gerhard |isbn=978-3-527-30385-4 |s2cid=137166382 |url=https://publications.rwth-aachen.de/search?p=id:%22RWTH-2021-01987%22 }}</ref>
# [[Ceria]] or [[ceria-zirconia]]. These oxides are mainly added as oxygen storage promoters.<ref name="KašparFornasiero1999">{{cite journal|last1=Kašpar|first1=J.|last2=Fornasiero|first2=P.|last3=Graziani|first3=M.|title=Use of CeO2-based oxides in the three-way catalysis|journal=Catalysis Today|volume=50|issue=2|year=1999|pages=285–298|issn=0920-5861|doi=10.1016/S0920-5861(98)00510-0}}</ref>
# The catalyst itself is most often a mix of [[precious metals]], mostly from the [[platinum group]]. [[Platinum]] is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions and historically high cost. [[Palladium]] and [[rhodium]] are two other precious metals used, though as of February 2023, platinum has become the least expensive of the platinum group metals. Rhodium is used as a [[redox|reduction]] catalyst, palladium is used as an [[redox|oxidation]] catalyst, and platinum is used both for reduction and oxidation. [[Cerium]], [[iron]], [[manganese]], and [[nickel]] are also used, although each has limitations. Nickel is not legal for use in the European Union because of its reaction with carbon monoxide into toxic [[nickel tetracarbonyl]].{{citation needed|date=October 2013}} [[Copper]] can be used in most countries, with a notable exception in [[Japan]].{{Clarify|cummins uses copper|date=October 2015}}

Upon failure, a catalytic converter can be recycled into [[scrap]]. The precious metals inside the converter, including platinum, palladium, and rhodium, are extracted.

==Placement of catalytic converters==
Catalytic converters require a temperature of {{convert|400|C|F|sigfig=2}} to operate effectively. Therefore, they are placed as close to the engine as possible, or one or more smaller catalytic converters (known as "pre-cats") are placed immediately after the exhaust manifold.

==Types==
==={{anchor|oxidation|two-way|2-way|oxi-cat}} Two-way===
A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:
# [[Oxidation]] of [[carbon monoxide]] to [[carbon dioxide]]: {{nowrap|2CO + O<sub>2</sub> → 2CO<sub>2</sub>}}
# Oxidation of [[unburned hydrocarbon|hydrocarbons]] (unburnt and partially burned fuel) to carbon dioxide and [[water]]: {{nowrap|C<sub>''x''</sub>H<sub>2''x''+2</sub> + [(3''x''+1)/2]O<sub>2</sub> → ''x''CO<sub>2</sub> + (''x''+1)H<sub>2</sub>O}} (a combustion reaction)

The two-way catalytic converter is widely used on [[diesel engine]]s to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in American and Canadian automobile markets until 1981. Because of their inability to control [[NOx|oxides of nitrogen]], manufacturers briefly installed twin catalyst systems, with an {{NOx}} reducing, rhodium/platinum catalyst ahead of the air pump, which led to the development of the three-way catalytic converter.<ref name=Twigg/> The two-way catalytic converter also continued to be used on certain, lower-cost cars in some markets such as Europe, where {{NOx}} emissions were not universally regulated until the introduction of the Euro 3 emissions standard in 2000.<ref name=Euro3>{{CELEX|31998L0069|text=Directive 98/69/EC of the European Parliament and of the Council of 13 October 1998 relating to measures to be taken against air pollution by emissions from motor vehicles and amending Council Directive 70/220/EEC}}</ref>

==={{anchor|TWC|3-way|three-way}} Three-way===
The three-way catalytic converters have the additional advantage of controlling the emission of [[nitric oxide|nitric oxide (NO)]] and [[nitrogen dioxide|nitrogen dioxide (NO<sub>2</sub>)]] (both together abbreviated with [[NOx|{{NOx}}]] and not to be confused with [[nitrous oxide|nitrous oxide (N<sub>2</sub>O)]]). [[NOx|{{NOx}}]] are precursors to [[acid rain]] and [[smog]].<ref name=TWC>{{cite journal|title=Automotive Catalytic Converters: Current Status and Some Perspectives|author1=Kaspar, Jan |author2=Fornasiero, Paolo |author3=Hickey, Neal |journal=Catalysis Today|year=2003|volume=77|issue=4 |pages=419–449|doi=10.1016/S0920-5861(02)00384-X}}</ref>

Since 1981, the three-way (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent [[vehicle emissions control|vehicle emission regulations]] that in effect require three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way catalytic converter does three simultaneous tasks:<ref name="TWC"/>

''[[Redox|Reduction]] of nitrogen oxides to [[nitrogen]] (N<sub>2</sub>)''
* <math>\text{C}+2\text{NO}_{2}\,\rightarrow\,\text{CO}_{2}+2\text{NO}</math>
* <math>\text{CO}+\text{NO}\,\rightarrow\,\text{CO}_{2}+\frac{1}{2}\text{N}_{2}</math>
* <math>2\text{CO}+\text{NO}_{2}\,\rightarrow\,2\text{CO}_{2}+\frac{1}{2}\text{N}_{2}</math>
* <math>\text{H}_{2}+\text{NO}\,\rightarrow\,\text{H}_{2}\text{O}+\frac{1}{2}\text{N}_{2}</math>
''[[Redox|Oxidation]] of carbon, hydrocarbons, and carbon monoxide to carbon dioxide''
* <math>\text{C}+\text{O}_{2}\,\rightarrow\,\text{CO}_{2}</math>
* <math>\text{CO}+\frac{1}{2}\text{O}_{2}\,\rightarrow\,\text{CO}_{2}</math>
* <math>a\,\text{C}_{x}\text{H}_{y}+b\,\text{O}_{2}\,\rightarrow\,c\,\text{CO}_{2}+d\,\text{H}_{2}\text{O}\qquad a,\,b,\,c,\,d,\,x,\,y\in\mathbb{Z}</math>
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the [[stoichiometric]] point. For gasoline combustion, this ratio is between 14.6 and 14.8 parts air to one part fuel, by weight. The ratio for [[autogas]] (or [[liquefied petroleum gas]] LPG), [[natural gas]], and [[ethanol]] fuels can vary significantly for each, notably so with [[oxygenate]]d or alcohol based fuels, with [[E85]] requiring approximately 34% more fuel, requiring modified fuel system tuning and components when using those fuels. Engines fitted with regulated 3-way catalytic converters are equipped with a [[engine control unit|computerized]] [[Electronic feedback loop|closed-loop]] [[feedback]] [[fuel injection]] system using one or more [[oxygen sensor#Automotive applications|oxygen sensors]] (also known as Lambda Sonds or sensors). Other variants combined three-way converters with [[carburetor]]s equipped with feedback mixture control were used. An unregulated three-way converter features the same chemical processes but without the oxygen sensor, which meant higher {{NOx}} emissions, particularly under partial loads. These were low-cost solutions, typically used for retrofitting to older cars or for smaller, cheaper cars.

Three-way converters are effective when the engine is operated within a narrow band of air{{endash}}fuel ratios near the stoichiometric point.<ref name="The three way catalytic converter">{{Cite web|url=https://www.open.edu/openlearn/science-maths-technology/science/chemistry/the-three-way-catalytic-converter/content-section-1.3|title=The three way catalytic converter|access-date=31 January 2022|archive-date=31 January 2022|archive-url=https://web.archive.org/web/20220131002220/https://www.open.edu/openlearn/science-maths-technology/science/chemistry/the-three-way-catalytic-converter/content-section-1.3|url-status=dead}}</ref> Total conversion efficiency falls very rapidly when the engine is operated outside of this band. Slightly lean of stoichiometric, the exhaust gases from the engine contain excess oxygen, the production of {{NOx}} by the engine increases, and the efficiency of the catalyst at reducing {{NOx}} falls off rapidly. However, the conversion of HC and CO is very efficient due to the available oxygen, oxidizing to H<sub>2</sub>O and CO<sub>2</sub>. Slightly rich of stoichiometric, the production of CO and unburnt HC by the engine starts to increase dramatically, available oxygen decreases, and the efficiency of the catalyst for oxidizing CO and HC decreases significantly, especially as stored oxygen becomes depleted. However, the efficiency of the catalyst at reducing {{NOx}} is good, and the production of {{NOx}} by the engine decreases. To maintain catalyst efficiency, the air{{endash}}fuel ratio must stay close to stoichiometric and not remain rich or lean for too long.

Closed-loop engine control systems are used for effective operation of three-way catalytic converters because of this continuous rich-lean balance required for effective {{NOx}} reduction and HC+CO oxidation. The control system allows the catalyst to release oxygen during slightly rich operating conditions, which oxidizes CO and HC under conditions that also favor the reduction of NOx. Before the stored oxygen is depleted, the control system shifts the air{{endash}}fuel ratio to become slightly lean, improving HC and CO oxidation while storing additional oxygen in the catalyst material, at a small penalty in {{NOx}} reduction efficiency. Then the air{{endash}}fuel mixture is brought back to slightly rich, at a small penalty in CO and HC oxidation efficiency, and the cycle repeats. Efficiency is improved when this oscillation around the stoichiometric point is small and carefully controlled.<ref>{{Cite journal|last1= Brandt|first1= Erich|last2= Wang|first2= Yanying|last3= Grizzle|first3= Jessy|title= Dynamic Modeling of a Three Way Catalyst for SI Engine Exhaust Emission Control|journal= IEEE Transactions on Control Systems Technology|volume= 8|issue= 5|pages= 767–776|year= 2000|url= http://web.eecs.umich.edu/~grizzle/papers/TWC98.pdf|doi= 10.1109/87.865850}}</ref>

Closed-loop control under light to moderate load is accomplished by using one or more [[oxygen sensor#Automotive applications|oxygen sensors]] in the exhaust system. When oxygen is detected by the sensor, the air{{endash}}fuel ratio is lean of stoichiometric, and when oxygen is not detected, it is rich. The control system adjusts the rate of fuel being injected into the engine based on this signal to keep the air{{endash}}fuel ratio near the stoichiometric point in order to maximize the catalyst conversion efficiency. The control algorithm is also affected by the time delay between the adjustment of the fuel flow rate and the sensing of the changed air{{endash}}fuel ratio by the sensor, as well as the [[Sigmoid function|sigmoidal]] response of the oxygen sensors. Typical control systems are designed to rapidly sweep the air{{endash}}fuel ratio such that it oscillates slightly around the stoichiometric point, staying near the optimal efficiency point while managing the levels of stored oxygen and unburnt HC.<ref name="The three way catalytic converter"/>

Closed loop control is often not used during high load/maximum power operation, when an increase in emissions is permitted and a rich mixture is commanded to increase power and prevent exhaust gas temperature from exceeding design limits. This presents a challenge for control system and catalyst design. During such operations, large amounts of unburnt HC are produced by the engine, well beyond the capacity of the catalyst to release oxygen. The surface of the catalyst quickly becomes saturated with HC. When returning to lower power output and leaner air{{endash}}fuel ratios, the control system must prevent excessive oxygen from reaching the catalyst too quickly, as this will rapidly burn the HC in the already hot catalyst, potentially exceeding the design temperature limit of the catalyst. Excessive catalyst temperature can prematurely age the catalyst, reducing its efficiency before reaching its design lifetime. Excessive catalyst temperature can also be caused by cylinder misfire, which continuously flows unburnt HC combined with oxygen to the hot catalyst, burning in the catalyst and increasing its temperature.<ref>{{cite journal |last1=Cooper |first1=B. J. |title=Durability of Platinum-Containing Automotive Exhaust Control Catalysts: Improvements in Resistance to Thermal Degradation |journal=Platinum Metals Review |date=October 1983 |volume=27 |issue=4 |pages=146–155 |doi=10.1595/003214083X274146155 |s2cid=36970773 }}</ref>

====Unwanted reactions====
Unwanted reactions result in the formation of [[hydrogen sulfide]] and [[ammonia]], which poison catalysts. [[Nickel]] or [[manganese]] is sometimes added to the washcoat to limit hydrogen-sulfide emissions.{{Citation needed|date=May 2021}} Sulfur-free or low-sulfur fuels eliminate or minimize problems with hydrogen sulfide.

===Diesel engines{{Anchor|Diesel Oxidation Catalyst|DOC}}===
For compression-ignition (i.e., [[diesel engine|diesel]]) engines, the most commonly used catalytic converter is the ''diesel oxidation catalyst'' (DOC). DOCs contain [[palladium]] or [[platinum]] [[catalyst support|supported on]] [[alumina]]. This catalyst converts [[particulate matter]] (PM), hydrocarbons, and carbon monoxide to carbon dioxide and water. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates. These catalysts are ineffective for {{NOx}}, so {{NOx}} emissions from diesel engines are controlled by [[exhaust gas recirculation]] (EGR).

In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet federal emissions requirements. Two techniques have been developed for the catalytic reduction of {{NOx}} emissions under lean exhaust conditions, [[selective catalytic reduction]] (SCR) and the [[NOx adsorber|{{NOx}} adsorber]].

Instead of precious metal-containing {{NOx}} absorbers, most manufacturers selected base-metal SCR systems that use a [[reagent]] such as [[ammonia]] to reduce the {{NOx}} into [[nitrogen]] and water.<ref>{{Cite web|url=https://www3.epa.gov/ttncatc1/dir1/fscr.pdf|title=Air Pollution Control Technology Fact Sheet (PDF, US Environmental Protection Agency)}}</ref> Ammonia is supplied to the catalyst system by the injection of [[urea]] into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. The urea solution is also referred to as [[diesel exhaust fluid]] (DEF).

[[Diesel exhaust]] contains relatively high levels of particulate matter. Catalytic converters remove only 20–40% of PM so particulates are cleaned up by a soot trap or [[diesel particulate filter]] (DPF). In the U.S., all on-road light, medium, and heavy-duty diesel-powered vehicles built after 1 January 2007, are subject to diesel particulate emission limits, and so are equipped with a 2-way catalytic converter and a diesel particulate filter.{{Citation needed|date=May 2021}} As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system.{{Citation needed|date=May 2021}} This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.<ref>{{cite web|url= http://www.epa.gov/otaq/highway-diesel/regs/f00057.pdf |title=Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements|date=2015-08-19}}&nbsp;{{small|(123&nbsp;KB)}}</ref>

===Lean-burn spark-ignition engines===
For [[lean-burn]] [[spark-ignition]] engines, an oxidation catalyst is used in the same manner as in a diesel engine. Emissions from lean burn spark ignition engines are very similar to emissions from a diesel compression ignition engine.

==Installation==
Many vehicles have a close-coupled catalytic converter located near the engine's [[exhaust manifold]]. The converter heats up quickly, due to its exposure to the very hot exhaust gases, allowing it to reduce undesirable emissions during the engine warm-up period. This is achieved by burning off the excess hydrocarbons which result from the extra-rich mixture required for a cold start.

When catalytic converters were first introduced, most vehicles used [[carburetor]]s that provided a relatively rich [[air–fuel ratio|air-fuel ratio]]. Oxygen (O<sub>2</sub>) levels in the exhaust stream were therefore generally insufficient for the catalytic reaction to occur efficiently. Most designs of the time therefore included [[secondary air injection]], which injected air into the exhaust stream. This increased the available oxygen, allowing the catalyst to function as intended.

Some three-way catalytic converter systems have air injection systems with the air injected between the first ({{NOx}} reduction) and second (HC and CO oxidation) stages of the converter. As in two-way converters, this injected air provides oxygen for the oxidation reactions. An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period. This causes unburned fuel to ignite in the exhaust tract, thereby preventing it reaching the catalytic converter at all. This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or [[operating temperature]].

Most newer vehicles have [[electronic fuel injection]] systems, and do not require air injection systems in their exhausts. Instead, they provide a precisely controlled air-fuel mixture that quickly and continually cycles between lean and rich combustion. [[Oxygen sensor]]s monitor the exhaust oxygen content before and after the catalytic converter, and the [[engine control unit]] uses this information to adjust the fuel injection so as to prevent the first ({{NOx}} reduction) catalyst from becoming oxygen-loaded, while simultaneously ensuring the second (HC and CO oxidation) catalyst is sufficiently oxygen-saturated.

==Damage==
[[Catalyst poisoning]] occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, so that they cannot contact and react with the exhaust. The most notable contaminant is [[lead]], so vehicles equipped with catalytic converters can run only on [[unleaded gasoline|unleaded]] fuel. Other common catalyst poisons include [[sulfur]], [[manganese]] (originating primarily from the gasoline additive [[methylcyclopentadienyl manganese tricarbonyl|MMT]]), and [[silicon]], which can enter the exhaust stream if the engine has a leak that allows [[antifreeze|coolant]] into the combustion chamber. [[Phosphorus]] is another catalyst contaminant. Although phosphorus is no longer used in gasoline, it (and [[zinc]], another low-level catalyst contaminant) was widely used in engine oil [[AW additive|antiwear additives]] such as [[zinc dithiophosphate]] (ZDDP). Beginning in 2004, a limit of phosphorus concentration in engine oils was adopted in the [[American Petroleum Institute|API]] SM and [[motor oil#ILSAC|ILSAC]] GF-4 specifications.

Depending on the contaminant, catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time.{{Citation needed|date=May 2021}} The increased exhaust temperature can sometimes vaporize or sublimate the contaminant, removing it from the catalytic surface.{{Citation needed|date=May 2021}} However, removal of lead deposits in this manner is usually not possible because of lead's high boiling point.<ref>{{Cite web|url=https://autorideofreading.com/blog/which-cars-are-least-likely-to-have-catalytic-converter-stolen/|title= Which cars are least likely to have catalytic converter stolen?|publisher=Auto Ride Of Reading|date= July 15, 2022}}</ref>

Any condition that causes abnormally high levels of unburned hydrocarbons (raw or partially burnt fuel or oils) to reach the converter will tend to significantly elevate its temperature bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction. These conditions include failure of the upstream components of the exhaust system (manifold or header assembly and associated clamps susceptible to rust, corrosion or fatigue such as the exhaust manifold splintering after repeated heat cycling), ignition system (e.g., coil packs, primary ignition components, distributor cap, wires, ignition coil and spark plugs) or damaged fuel system components (e.g., fuel injectors, fuel pressure regulator, and associated sensors). Oil and coolant leaks, perhaps caused by a head gasket leak, can also cause high unburned hydrocarbons.

==Regulations==
{{More citations needed section|date=January 2017}}
Emissions regulations vary considerably from jurisdiction to jurisdiction. Most automobile spark-ignition engines in North America have been fitted with catalytic converters since 1975,<ref name="Palucka"/><ref name="Petersen"/><ref name="GM_advert"/><ref name="Sentinel_1974"/> and the technology used in non-automotive applications is generally based on automotive technology. In many jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its direct and immediate replacement. Nevertheless, some vehicle owners remove or "gut" the catalytic converter on their vehicle.<ref name="gaisun"/><ref>{{Cite news|title=Some of Us Can Only Afford a Clunker|work=[[The Palm Beach Post]]|date=23 February 1996}}</ref> In such cases, the converter may be replaced by a welded-in section of ordinary pipe or a flanged "test pipe", ostensibly meant to check if the converter is clogged by comparing how the engine runs with and without the converter. This facilitates temporary reinstallation of the converter in order to pass an emission test.<ref name="imptune"/>

In the United States, it is a violation of Section 203(a)(3)(A) of the 1990 amended [[Clean Air Act (United States)|Clean Air Act]] for a vehicle repair shop to remove a converter from a vehicle, or cause a converter to be removed from a vehicle, except in order to replace it with another converter,<ref name=51FR150>[http://www.epa.gov/compliance/resources/policies/civil/caa/mobile/amccpolicy.pdf Sale and Use of Aftermarket Catalytic Converters], US Environmental Protection Agency, US Federal Register Volume 51</ref> and Section 203(a)(3)(B) makes it illegal for any person to sell or to install any part that would bypass, defeat, or render inoperative any emission control system, device, or design element. Vehicles without functioning catalytic converters generally fail emission inspections. The [[aftermarket (automotive)|automotive aftermarket]] supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.<ref>{{Cite book|title=Mazda MX-5 Miata|last=Tanner|first=Keith|page=120}}</ref>

Catalytic converters have been mandatory on all new gasoline cars sold in the [[European Union]] and the [[United Kingdom]] since January 1, 1993 in order to comply with the [[European emission standards|Euro 1 emission standards]].<ref>{{cite web |title=Euro emissions standards {{!}} AA |url=https://www.theaa.com/driving-advice/fuels-environment/euro-emissions-standards |website=www.theaa.com |access-date=31 December 2022}}</ref>

==Effect on exhaust flow==
Faulty catalytic converters as well as undamaged early types of converters can restrict the flow of exhaust, which negatively affects vehicle performance and fuel economy.<ref name="gaisun">{{Cite news|title=Kits to Foil Auto Pollution Control Are Selling Well|author=Crutsinger, Martin|work=[[The Gainesville Sun]]|date=29 September 1982}}</ref> Modern catalytic converters do not significantly restrict exhaust flow. A 2006 test on a 1999 Honda Civic, for example, showed that removing the stock catalytic converter netted only a 3% increase in maximum horsepower; a new metallic core converter only cost the car 1% horsepower, compared to no converter.<ref name="imptune">{{cite web |url=http://www.importtuner.com/features/0610impp_catalytic_converter_removal/viewall.html |title=Beat the Law |date=1 October 2006 |website=Import Tuner |access-date=9 January 2011 |archive-url=https://web.archive.org/web/20140228203151/http://www.importtuner.com/features/0610impp_catalytic_converter_removal/viewall.html |archive-date=28 February 2014}}</ref>

==Dangers==
[[Carburetor]]s on pre-1981 vehicles without feedback fuel-air mixture control could easily provide too much fuel to the engine, which could cause the catalytic converter to overheat and potentially ignite flammable materials under the car.<ref>{{Cite news|publisher=The Bulletin{{Clarify|date=December 2010|reason=which one? there are several such named publications}}|date=14 June 1976|title=Catalytic Converter Still Controversial after Two Years of Use|author=Ullman, Owen}}</ref>

===Warm-up period===
Vehicles fitted with catalytic converters emit most of their total pollution during the first five minutes of engine operation; that is, before the catalytic converter has warmed up sufficiently to be fully effective.<ref name="National Synchotron Light Source">[https://web.archive.org/web/20120417183354/http://www.nsls.bnl.gov/about/everyday/catalytic.asp Catalytic converters], nsls.bnl.gov</ref>

In the early 2000s it became common to place the catalyst converter right next to the exhaust manifold, close to the engine, for much quicker warm-up.
In 1995, [[Alpina]] introduced an electrically heated catalyst. Called "E-KAT", it was used in Alpina's B12 5,7 E-KAT based on the BMW [[BMW E38|750i]].<ref name=":0">{{cite web|url= http://www.alpina-automobiles.com/en/alpina-world/milestones/|title= Milestones|access-date= 5 June 2015|website= alpina-automobiles.com|url-status= dead|archive-url= https://web.archive.org/web/20150630081940/http://www.alpina-automobiles.com/en/alpina-world/milestones/|archive-date= 30 June 2015}}</ref> [[Resistance heating|Heating coils]] inside the catalytic converter assemblies are electrified just after the engine is started, bringing the catalyst up to operating temperature very quickly to qualify the vehicle for [[low emission vehicle]] (LEV) designation.<ref name=":1">{{cite web|last=Edgar|first=Julian|title=Goodbye 12 volts... hello 42 volts!|url=http://autospeed.com/cms/title_Goodbye-12-volts-hello-42-volts/A_0319/article.html|publisher=Autospeed|access-date=2 January 2012|date=5 October 1999|quote=The current model BMW 750iL has a maximum electrical load of 428 amps (5.9&nbsp;kW)! In this car, over half of the maximum load is from the short-term electrical heating of the catalytic converters|archive-url=https://web.archive.org/web/20120528225424/http://autospeed.com/cms/title_Goodbye-12-volts-hello-42-volts/A_0319/article.html|archive-date=28 May 2012|url-status=dead}}</ref> BMW later introduced the same heated catalyst, developed jointly by Emitec, Alpina, and BMW,<ref name=":0"/> in its 750i in 1999.<ref name=":1"/>

Some vehicles contain a pre-cat, a small catalytic converter upstream of the main catalytic converter which heats up faster on vehicle start up, reducing the emissions associated with cold starts. A pre-cat is most commonly used by an auto manufacturer when trying to attain the Ultra Low Emissions Vehicle (ULEV) rating, such as on the [[Toyota MR2]] Roadster.<ref>{{cite web|url=http://www.toyotaownersclub.com/forums/topic/70844-pre-cats-what-you-should-know/|title=Pre-cats - What You Should Know|website=Toyota Owners Club - Toyota Forum|date=11 September 2007 |access-date=15 April 2018}}</ref>

===Environmental effect===
Catalytic converters have proven to be reliable and effective in reducing noxious tailpipe emissions. However, they also have some shortcomings in use, and also adverse environmental effects in production:
* An engine equipped with a three-way catalyst must run at the [[Fuel mixture|stoichiometric point]], which means more fuel is consumed than in a [[Lean burn|lean-burn]] engine. This means approximately 10% more CO<sub>2</sub> emissions from the vehicle.{{Citation needed|date=August 2022}}
* Catalytic converter production requires [[palladium]] or [[platinum]]; part of the world supply of these [[precious metal]]s is produced near [[Norilsk]], Russia, where the industry (among others) has caused Norilsk to be added to ''Time'' magazine's list of most-polluted places.<ref>{{cite news |first=Bryan |last=Walsh |title=Norilsk, Russia |work= The World's Most Polluted Places |url=http://www.time.com/time/specials/2007/article/0,28804,1661031_1661028_1661022,00.html |archive-url=https://web.archive.org/web/20071031012612/http://www.time.com/time/specials/2007/article/0,28804,1661031_1661028_1661022,00.html |url-status=dead |archive-date=31 October 2007 |publisher=[[Time (magazine)|Time]] |access-date=7 January 2011|date=12 September 2007}}</ref>
* The extreme heat of the converters themselves<ref>{{cite web|url=https://www.pe.com/2016/09/06/heres-how-to-avoid-accidentally-starting-the-next-firestorm/|title=Here's how to avoid accidentally starting the next firestorm|date=6 September 2016|website=pe.com}}</ref> can cause [[wildfires]], especially in dry areas.<ref>{{cite web|url=http://www.ocregister.com/2008/11/18/catalytic-converter-started-fires-are-common/|title=Catalytic converter-started fires are common|date=18 November 2008|website=ocregister.com|access-date=15 April 2018}}</ref><ref>{{cite web|url=http://fox5sandiego.com/2017/06/29/brush-fire-prompts-traffic-alert-on-eastbound-sr-52/|title=Catalytic converter blamed for causing SR-52 brush fire|date=29 June 2017|website=fox5sandiego.com|access-date=15 April 2018}}</ref><ref>{{cite web |url=http://www.kusi.com/story/35861061/rapid-moving-fire-burning-near-lake-jennings-in-lakeside |title=Crews have 400-acre Jennings Fire in Lakeside 100 percent contai - KUSI News - San Diego, CA |access-date=2017-07-14 |url-status=dead |archive-url=https://web.archive.org/web/20170714034830/http://www.kusi.com/story/35861061/rapid-moving-fire-burning-near-lake-jennings-in-lakeside |archive-date=14 July 2017}}</ref>

==Theft==
{{see also|2020–2022 catalytic converter theft ring}}
Because of the external location and the use of valuable precious metals including [[platinum]], [[palladium]] and [[rhodium]], catalytic converters are a target for thieves. The problem is especially common among late-model [[pickup truck]]s and truck-based [[SUV]]s, because of their high ground clearance and easily removed bolt-on catalytic converters. Welded-on converters are also at risk of theft, as they can be easily cut off. The [[Toyota Prius]] catalytic converters are also targets for thieves. The catalytic converters of hybrids need more of the precious metals to work properly compared to conventional internal combustion vehicles because they do not get as hot as those installed on conventional vehicles, since the combustion engines of hybrids only run part of the time.<ref>{{cite news|title= Catalytic converters make 2004-09 Toyota Prius unlikely theft target |date= 18 November 2021 |url= https://www.iihs.org/news/detail/catalytic-converters-make-2004-09-toyota-prius-unlikely-theft-target |work= IIHS |access-date= 15 April 2023}}</ref><ref>{{cite news|url=http://www.chroniclet.com/2007/07/31/underhanded-thieves/|title=Underhanded thieves|last=Medici|first=Joe|date=31 July 2007|work=Chroniclet.com|archive-url=https://web.archive.org/web/20070928035937/http://www.chroniclet.com/2007/07/31/underhanded-thieves/|archive-date=2007-09-28|url-status=dead}}</ref><ref>{{cite news |url= http://www.newsweek.com/id/88793 |author=Murr, Andrew |date=9 January 2008|title=An Exhausting New Crime&nbsp;— What Thieves Are Stealing from Today's Cars|work= Newsweek|access-date= 7 January 2011}}</ref>

[[Pipecutter]]s are often used to quietly remove the converter<ref>{{Cite news|url=https://www.rte.ie/news/claire-byrne-live/2019/1211/1098450-catalytic-converters-are-being-stolen-for-valuable-metals/|title=Catalytic converters are being stolen for valuable metals|date=11 December 2019|work=RTÉ.ie | publisher = Raidió Teilifís Éireann}}</ref><ref>{{Cite web|url=https://sfbayca.com/2019/11/29/cotati-police-bust-catalytic-converter-theft-suspects/|title=Cotati police bust catalytic converter theft suspects|date=29 November 2019}}</ref> but other tools such as a portable [[reciprocating saw]] can damage other components of the car, such as the [[alternator]], wiring or fuel lines, with potentially dangerous consequences.

In 2023, bipartisan legislation to combat catalytic converter theft was introduced in the [[United States Senate|U.S. Senate]]. The Preventing Auto Recycling Thefts Act (PART Act) would mandate catalytic converters in new vehicles to come with traceable identification numbers. Additionally, the legislation would make catalytic converter theft a federal criminal offense.<ref>{{Cite web |last=Pietrasz |first=Jake |date=2023-02-02 |title=Vance introduces legislation to address catalytic converter thefts |url=https://www.13abc.com/2023/02/02/vance-introduces-legislation-address-catalytic-converter-theft/ |access-date=2023-03-15 |website=13ABC |language=en}}</ref>

=== Statistics ===
Rising metal prices in the U.S. during the 2000s commodities boom led to a significant increase in converter theft. A catalytic converter can cost more than $1,000 to replace, more if the vehicle is damaged during the theft.<ref>{{Cite news|title=Thieves Nationwide Are Slithering Under Cars, Swiping Catalytic Converters|date=9 February 2021|work=The New York Times|url=https://www.nytimes.com/2021/02/09/climate/catalytic-converter-theft.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/02/09/climate/catalytic-converter-theft.html |archive-date=2021-12-28 |url-access=limited}}{{cbignore}}</ref><ref>{{cite news|url=https://www.nbcnews.com/id/wbna23117250|title=Stolen in 60 Seconds: The Treasure in Your Car&nbsp;— As Precious Metals Prices Soar, Catalytic Converters Are Targets for Thieves|author=Johnson, Alex|date=12 February 2008|work=NBC News|access-date=7 January 2011}}</ref><ref>{{Cite news|title=Converters Taken by Car Lot Thieves|date=2 July 2009|work=PoconoNews}}</ref>  Apart from damaging other systems of the vehicle, theft can also cause death and injury to thieves.<ref>{{cite web |last1=Razek |first1=Raja |title=A man was crushed to death while allegedly attempting to steal a catalytic converter, police say |url=https://www.cnn.com/2023/03/12/us/georgia-man-death-catalytic-converter-theft/index.html |access-date=13 March 2023 |website=cnn.com |date=12 March 2023 |publisher=[[CNN]]}}</ref>

Thefts of catalytic converters rose over tenfold in the United States from the late 2010s to early 2020s, driven presumably by the rise in the price of precious metals contained within the converters.<ref>{{Cite web |date=2022-08-12 |title=WATCH: Catalytic converter thefts are skyrocketing. Here's why |url=https://www.pbs.org/newshour/nation/watch-catalytic-converter-thefts-are-skyrocketing-heres-why |access-date=2022-12-26 |website=PBS NewsHour |language=en-us}}</ref> Study findings reveal an average price elasticity of 1.98, which means that a 10 percent increase in the price of metal leads to an approximate 20 percent increase in thefts.<ref name="link.springer.com">{{cite journal|last1=Stickle |first1=Ben |last2=Rennhoff |first2=Adam |last3=Morris |first3=Charles |last4=Fritts |first4=Samuel|title=Catalytic converter theft:An examination of the elasticity of crime |url=https://link.springer.com/article/10.1057/s41300-024-00207-6 |journal=Crime Prevention and Community Safety|date=2024 |volume=26 |issue=2 |pages=139–156 |doi=10.1057/s41300-024-00207-6 |access-date=29 June 2024}}</ref> According to the [[National Insurance Crime Bureau]], there were 1,298 reported cases of catalytic converter theft in 2018, which increased to 14,433 in 2020.<ref>{{Cite web |last=Pender |first=Caelyn |date=2022-08-09 |title=Why do people steal catalytic converters? |url=https://www.kron4.com/news/why-do-people-steal-catalytic-converters/ |access-date=2023-03-15 |website=KRON4 |language=en-US}}</ref> In 2022, it was reported that the number of catalytic converter thefts in the United States sharply rose to 153,000 total thefts for the year.<ref name="link.springer.com"/>

From 2019 to 2020, thieves in the United Kingdom were targeting older-model [[hybrid car]]s (such as [[Toyota Hybrid System|Toyota's hybrids]]) which have more precious metals than newer vehicles—sometimes worth more than the value of the car—leading to scarcity and long delays in replacing them.<ref>{{Cite web|date=2020-02-01|title=Catalytic converter theft: hybrid car owners face insurance nightmare|url=http://www.theguardian.com/money/2020/feb/01/catalytic-converter-thefts-hybrid-car-insurance-toyota-prius|website=the Guardian|language=en}}</ref>

In 2021 a trend emerged in the [[Democratic Republic of the Congo]] where catalytic converters were alleged to be stolen for use in illicit street drug production. The drug, a powder known as "bombé," was said to be a mixture of powdered pills/vitamins and pulverized honeycomb structures of catalytic converters.<ref>{{cite news |last1=Nyemba |first1=Benoit |title=Car-exhaust drug craze alarms Congo's capital |url=https://www.reuters.com/world/africa/car-exhaust-drug-craze-alarms-congos-capital-2021-09-27/ |access-date=8 October 2021 |work=Reuters |date=27 September 2021 |language=en}}</ref> In 2023, however, a study of various samples of the drug concluded that its alleged origin from catalytic exhausts was found to be unsubstantiated.<ref>{{cite journal |last1=Obadia |first1=Paul |last2=Kapia |first2=Patrice |last3=Bonneure |first3=Arne |last4=Duca |first4=Radu-Corneliu |last5=Creta |first5=Matteo |last6=Kayembe-Kitenge |first6=Tony |last7=Tytgat |first7=Jan |last8=Okitundu |first8=Daniel |last9=Nkulu |first9=Célestin |last10=Nemery |first10=Benoit |title=Bombé, an undetermined substance that caused an outbreak of illicit drug use in Kinshasa, Democratic Republic of the Congo |url=https://www.tandfonline.com/doi/full/10.1080/15563650.2023.2276040 |journal=Clinical Toxicology |date=2023 |volume=61 |issue=12 |pages=1055–1058 |doi=10.1080/15563650.2023.2276040 |pmid=38047882 |s2cid=265607501 |access-date=17 February 2024}}</ref>

==Diagnostics==
Various jurisdictions now require [[on-board diagnostics]] to monitor the function and condition of the emissions-control system, including the catalytic converter. Vehicles equipped with [[OBD-II]] diagnostic systems are designed to alert the driver to a misfire condition by means of illuminating the "check engine" light on the dashboard, or flashing it if the current misfire conditions are severe enough to potentially damage the catalytic converter.<ref>{{cite web |title=Can A Catalytic Converter Cause A Misfire? - Carsneedcare |url=https://carsneedcare.com/can-a-catalytic-converter-cause-a-misfire/ |website=carsneedcare |access-date=17 March 2023 |date=22 September 2022}}</ref> On-board diagnostic systems take several forms.

Temperature sensors are used for two purposes. The first is as a warning system, typically on two-way catalytic converters such as those used on LPG forklifts. The function of the sensor is to warn of catalytic converter temperature above the safe limit of {{convert|750|C|F}}. Modern catalytic-converter designs are not as susceptible to temperature damage and can withstand sustained temperatures of {{convert|900|C|F}}.{{Citation needed|date=October 2007}} Temperature sensors are also used to monitor catalyst functioning: usually two sensors will be fitted, with one before the catalyst and one after to monitor the temperature rise over the catalytic-converter core.{{Citation needed|date=May 2021}}

The [[oxygen sensor#Automotive applications|oxygen sensor]] is the basis of the [[feedback|closed-loop]] control system on a spark-ignited rich-burn engine; however, it is also used for diagnostics. In vehicles with OBD II, a second oxygen sensor is fitted after the catalytic converter to monitor the O<sub>2</sub> levels. The O<sub>2</sub> levels are monitored to see the efficiency of the burn process. The on-board computer makes comparisons between the readings of the two sensors. The readings are taken by voltage measurements. If both sensors show the same output or the rear O<sub>2</sub> is "switching", the computer recognizes that the catalytic converter either is not functioning or has been removed, and will operate a malfunction indicator lamp and affect engine performance. Simple "oxygen sensor simulators" have been developed to circumvent this problem by simulating the change across the catalytic converter with plans and pre-assembled devices available on the Internet. Although these are not legal for on-road use, they have been used with mixed results.<ref>{{Cite news|url=http://eponline.com/articles/2007/07/01/settlement-involves-illegal-emission-control-defeat-devices-sold-for-autos.aspx|title=Settlement Involves Illegal Emission Control 'Defeat Devices' Sold for Autos|date=1 June 2007}}</ref> Similar devices apply an offset to the sensor signals, allowing the engine to run a more fuel-economical lean burn that may, however, damage the engine or the catalytic converter.<ref>{{Cite news|work=[[Concord Monitor]]|title=Check Engine Lights Come on for a Reason|date=12 January 2003}}</ref>

[[Nitrogen oxide sensor|{{NOx}} sensors]] are extremely expensive and are in general used only when a compression-ignition engine is fitted with a selective catalytic-reduction (SCR) converter, or a {{NOx}} absorber in a feedback system. When fitted to an SCR system, there may be one or two sensors. When one sensor is fitted it will be pre-catalyst; when two are fitted, the second one will be post-catalyst. They are used for the same reasons and in the same manner as an oxygen sensor; the only difference is the substance being monitored.{{Citation needed|date=July 2015}}

==See also==
* [[Catalytic heater]]
* [[Cerium(III) oxide]]
* [[List of auto parts]]
* [[NOx adsorber|{{NOx}} adsorber]]
* [[Roadway air dispersion modeling]]

==References==
{{Reflist}}

==Further reading==
* Keith, C. D., et al. {{US patent|3441381}}: "Apparatus for purifying exhaust gases of an internal combustion engine". 29 April 1969
* Lachman, I. M. et al. {{US patent|3885977}}: "Anisotropic Cordierite Monolith" (Ceramic substrate). 5 November 1973
* Charles H. Bailey. {{US patent|4094645}}: "Combination muffler and catalytic converter having low backpressure". 13 June 1978
* Charles H. Bailey. {{US patent|4250146}}: '"Caseless monolithic catalytic converter". 10 February 1981
* Srinivasan Gopalakrishnan. {{Cite patent|country=GB|number=2397782}}: "Process And Synthesizer For Molecular Engineering of Materials". 13 March 2002.

{{Automotive engine|collapsed}}
{{Authority control}}

[[Category:Air pollution control systems]]
[[Category:French inventions]]
[[Category:Automotive technologies]]
[[Category:Carbon monoxide]]
[[Category:Catalysis]]
[[Category:Exhaust systems]]
[[Category:NOx control]]
[[Category:Products introduced in 1975]]