Catalytic converter

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Catalytic_converter 

A three-way catalytic converter on a gasoline-powered 1996 Dodge Ram

Simulation of flow inside a catalytic converter

A catalytic converter is an exhaust emission control device which converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually used with internal combustion engines fueled by gasoline or 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 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. These "two-way" oxidation converters combine oxygen with carbon monoxide (CO) and unburned hydrocarbons (HC) to produce carbon dioxide (CO₂) and water (H₂O).

"Three-way" converters, which also reduce oxides of nitrogen (NO_x), were first commercialized by Volvo on the California-specification 1977 240 cars. When U.S. federal emission control regulations began requiring tight control of NOx for the 1981 model year, most all automakers met the tighter standards with three-way catalytic converters and associated engine control systems. Oxidation-only two-way converters 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 fuel-rich or stoichiometric combustion to successfully reduce NO_x.

Although catalytic converters are most commonly applied to exhaust systems in automobiles, they are also used on electrical generators, forklifts, mining equipment, trucks, buses, locomotives, motorcycles, and on ships. They are even used on some wood stoves to control emissions. 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. 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. 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 smokestacks, and later developed catalytic converters for warehouse forklifts running on unleaded gasoline. In the mid-1950s, he began research to develop catalytic converters for gasoline engines used on cars and was awarded United States Patent 2,742,437 for his work.

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, creating the first production catalytic converter in 1973.

The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the 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 hydrocarbons (HC, chemical compounds in fuel of the form C_mH_n) to produce carbon dioxide (CO₂) and water (H₂O). 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 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.

To lower harmful NO_x emissions, a twin-catalyst system was developed in the 1970s – this added a separate (rhodium/platinum) catalyst which reduced NO_x ahead of the air pump, after which a two-way catalytic converter (palladium/platinum) removed HC and CO. 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.

William C. Pfefferle developed a catalytic combustor for gas turbines in the early 1970s, allowing combustion without significant formation of nitrogen oxides and carbon monoxide. 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.

Construction

Cutaway of a metal-core converter

Ceramic-core converter

The catalytic converter's construction is as follows:

1. The catalyst support or substrate. For automotive catalytic converters, the core is usually a ceramic 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 (FeCrAl) are used in applications where particularly high heat resistance is required. 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 at Corning Glass, for which they were inducted into the National Inventors Hall of Fame in 2002.
2. 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 rough, irregular surface, which increases the surface area compared to the smooth surface of the bare substrate.
3. Ceria or ceria-zirconia. These oxides are mainly added as oxygen storage promoters.
4. 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 reduction catalyst, palladium is used as an oxidation catalyst, and platinum is used both for reduction and oxidation. Cerium, iron, manganese, and nickel are also used, although each has limitations. Copper can also be used in most countries. Nickel is not legal for use in the European Union because of its reaction with carbon monoxide into toxic nickel tetracarbonyl.

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 400 °C (750 °F) 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

Two-way

A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:

1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
2. Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: C_xH_2x+2 + [(3x+1)/2]O₂ → xCO₂ + (x+1)H₂O (a combustion reaction)

The two-way catalytic converter is widely used on diesel engines 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 oxides of nitrogen, manufacturers briefly installed twin catalyst systems, with an NO_x reducing, rhodium/platinum catalyst ahead of the air pump, which led to the development of the three-way catalytic converter. The two-way catalytic converter also continued to be used on certain, lower-cost cars in some markets such as Europe, where NO_x emissions were not universally regulated until the introduction of the Euro 3 emissions standard in 2000.

Three-way

The three-way catalytic converters have the additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO₂) (both together abbreviated with NO_x and not to be confused with nitrous oxide (N₂O)). NO_x are precursors to acid rain and smog.

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 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:

Reduction of nitrogen oxides to nitrogen (N₂)

• ${\displaystyle \text{C}+2{\text{NO}}_2\to {\text{CO}}_2+2\text{NO}}$
• ${\displaystyle \text{CO}+\text{NO}\to {\text{CO}}_2+\frac{1}{2}{\text{N}}_2}$
• ${\displaystyle 2\text{CO}+{\text{NO}}_2\to 2{\text{CO}}_2+\frac{1}{2}{\text{N}}_2}$
• ${\displaystyle {\text{H}}_2+\text{NO}\to {\text{H}}_2\text{O}+\frac{1}{2}{\text{N}}_2}$

Oxidation of carbon, hydrocarbons, and carbon monoxide to carbon dioxide

• ${\displaystyle \text{C}+{\text{O}}_2\to {\text{CO}}_2}$
• ${\displaystyle \text{CO}+\frac{1}{2}{\text{O}}_2\to {\text{CO}}_2}$
• ${\displaystyle a{\text{C}}_x{\text{H}}_y+b{\text{O}}_2\to c{\text{CO}}_2+d{\text{H}}_2\text{O}a,b,c,d,x,y\in \mathbb{Z}}$

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 oxygenated 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 computerized closed-loop feedback fuel injection system using one or more oxygen sensors (also known as Lambda Sonds or sensors). Other variants combined three-way converters with carburetors 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 NO_x 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–fuel ratios near the stoichiometric point. 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 NO_x by the engine increases, and the efficiency of the catalyst at reducing NO_x falls off rapidly. However, the conversion of HC and CO is very efficient due to the available oxygen, oxidizing to H₂O and CO₂. 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 NO_x is good, and the production of NO_x by the engine decreases. To maintain catalyst efficiency, the air–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 NO_x 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–fuel ratio to become slightly lean, improving HC and CO oxidation while storing additional oxygen in the catalyst material, at a small penalty in NO_x reduction efficiency. Then the air–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.

Closed-loop control under light to moderate load is accomplished by using one or more oxygen sensors in the exhaust system. When oxygen is detected by the sensor, the air–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–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–fuel ratio by the sensor, as well as the sigmoidal response of the oxygen sensors. Typical control systems are designed to rapidly sweep the air–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.

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–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.

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. Sulfur-free or low-sulfur fuels eliminate or minimize problems with hydrogen sulfide.

Diesel engines

For compression-ignition (i.e., diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst (DOC). DOCs contain palladium or platinum 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 NO_x, so NO_x 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 NO_x emissions under lean exhaust conditions, selective catalytic reduction (SCR) and the NO_x adsorber.

Instead of precious metal-containing NO_x absorbers, most manufacturers selected base-metal SCR systems that use a reagent such as ammonia to reduce the NO_x into nitrogen and water. 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. As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.

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 carburetors that provided a relatively rich air-fuel ratio. Oxygen (O₂) 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 (NO_x 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 sensors 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 (NO_x 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 fuel. Other common catalyst poisons include sulfur, manganese (originating primarily from the gasoline additive MMT), and silicon, which can enter the exhaust stream if the engine has a leak that allows 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 antiwear additives such as zinc dithiophosphate (ZDDP). Beginning in 2004, a limit of phosphorus concentration in engine oils was adopted in the API SM and 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]} The increased exhaust temperature can sometimes vaporize or sublimate the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is usually not possible because of lead's high boiling point.

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

Emissions regulations vary considerably from jurisdiction to jurisdiction. Most automobile spark-ignition engines in North America have been fitted with catalytic converters since 1975, 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. 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.

In the United States, it is a violation of Section 203(a)(3)(A) of the 1990 amended 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, 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 automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.

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 Euro 1 emission standards.

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. 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.

Dangers

Carburetors 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.

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.

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 750i. 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. BMW later introduced the same heated catalyst, developed jointly by Emitec, Alpina, and BMW, in its 750i in 1999.

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.

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 stoichiometric point, which means more fuel is consumed than in a lean-burn engine. This means approximately 10% more CO₂ emissions from the vehicle.
• Catalytic converter production requires palladium or platinum; part of the world supply of these precious metals 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.
• The extreme heat of the converters themselves can cause wildfires, especially in dry areas.

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 trucks and truck-based SUVs, 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.

Pipecutters are often used to quietly remove the converter but other tools such as a portable reciprocating saw can damage other components of the car, such as the Oxygen sensor, wiring or fuel lines, with potentially dangerous consequences.

In 2023, bipartisan legislation to combat catalytic converter theft was introduced in the 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.

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. Apart from damaging other systems of the vehicle, theft can also cause death and injury to thieves.

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. 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. 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. 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.

From 2019 to 2020, thieves in the United Kingdom were targeting older-model hybrid cars (such as 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.

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. In 2023, however, a study of various samples of the drug concluded that its alleged origin from catalytic exhausts was found to be unsubstantiated.

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. 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 750 °C (1,380 °F). Modern catalytic-converter designs are not as susceptible to temperature damage and can withstand sustained temperatures of 900 °C (1,650 °F). 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.

The oxygen sensor is the basis of the 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 O₂ levels. The O₂ 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 O₂ 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. 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.

NO_x 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 NO_x 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.

Aluminium oxide

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Aluminium_oxide

 

Aluminium(III) oxide
(Aluminium oxide)

Names
IUPAC name Aluminium oxide
Systematic IUPAC name Aluminium(III) oxide
Other names Dialuminium trioxide
Identifiers
CAS Number	1344-28-1
3D model (JSmol)	Interactive image Interactive image
ChEMBL	ChEMBL3707210
ChemSpider	8164808
DrugBank	DB11342
ECHA InfoCard	100.014.265
EC Number	215-691-6
PubChem CID	9989226
RTECS number	BD120000
UNII	LMI26O6933
CompTox Dashboard (EPA)	DTXSID1052791

InChI

SMILES
Properties
Chemical formula	Al2O3
Molar mass	101.960 g·mol−1
Appearance	white solid
Odor	odorless
Density	3.987 g/cm3
Melting point	2,072 °C (3,762 °F; 2,345 K)
Boiling point	2,977 °C (5,391 °F; 3,250 K)
Solubility in water	insoluble
Solubility	insoluble in all solvents
log P	0.31860
Magnetic susceptibility (χ)	−37.0×10−6 cm3/mol
Thermal conductivity	30 W·m−1·K−1
Refractive index (nD)	nω = 1.768–1.772 nε = 1.760–1.763 Birefringence 0.008
Structure
Crystal structure	Trigonal, hR30
Space group	R3c (No. 167)
Lattice constant	a = 478.5 pm, c = 1299.1 pm
Coordination geometry	octahedral
Thermochemistry
Std molar entropy (S⦵298)	50.92 J·mol−1·K−1
Std enthalpy of formation (ΔfH⦵298)	−1675.7 kJ/mol
Pharmacology
ATC code	D10AX04 (WHO)
Hazards
GHS labelling:
Pictograms
NFPA 704 (fire diamond)	0 0 0
Flash point	Non-flammable
NIOSH (US health exposure limits):
PEL (Permissible)	OSHA 15 mg/m3 (total dust) OSHA 5 mg/m3 (respirable fraction) ACGIH/TLV 10 mg/m3
REL (Recommended)	none
IDLH (Immediate danger)	N.D.
Related compounds
Other anions	aluminium hydroxide aluminium sulfide aluminium selenide
Other cations	boron trioxide gallium(III) oxide indium oxide thallium(III) oxide
Supplementary data page
Aluminium oxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).  verify (what is  ?) Infobox references

Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al₂O₃. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It occurs naturally in its crystalline polymorphic phase α-Al₂O₃ as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al₂O₃ is used as feedstock to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.

Natural occurrence

Corundum is the most common naturally occurring crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and their laser qualities by traces of chromium. Sapphires come in different colours given by various other impurities, such as iron and titanium. An extremely rare δ form occurs as the mineral deltalumite.

History

The field of aluminium oxide ceramics has a long history. Aluminium salts were widely used in ancient and medieval alchemy. Several vintage textbooks cover the history of the field. A 2019 textbook by Andrew Ruys contains a detailed timeline on the history of aluminium oxide from ancient times to the 21st century.

Properties

Aluminium oxide in its powdered form

Al₂O₃ is an electrical insulator but has a relatively high thermal conductivity (30 Wm⁻¹K⁻¹) for a ceramic material. Aluminium oxide is insoluble in water. In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium oxide (4 nm thickness) forms on any exposed aluminium surface in a matter of hundreds of picoseconds. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The aluminium oxide generated by anodising is typically amorphous, but discharge-assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide in the coating, enhancing its hardness.

Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminium oxide is on the EPA's Toxics Release Inventory list if it is a fibrous form.

Amphoteric nature

Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.

Al₂O₃ + 6 HF → 2 AlF₃ + 3 H₂O

Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄ (sodium aluminate)

Structure

Corundum from Brazil, size about 2×3 cm.

The most common form of crystalline aluminium oxide is known as corundum, which is the thermodynamically stable form. The oxygen ions form a nearly hexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al³⁺ center is octahedral. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R3c (number 167 in the International Tables). The primitive cell contains two formula units of aluminium oxide.

Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase that can be tetragonal or orthorhombic. Each has a unique crystal structure and properties. Cubic γ-Al₂O₃ has important technical applications. The so-called β-Al₂O₃ proved to be NaAl₁₁O₁₇.

Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) octahedral Al-O present. Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing. The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm³. The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO₄ units, approaching the local structural arrangements found in amorphous alumina.

Production

See also: List of countries by aluminium oxide production

Aluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), along with impurities of iron oxides and hydroxides, quartz and clay minerals. Bauxites are found in laterites. Bauxite is typically purified using the Bayer process:

Al₂O₃ + H₂O + NaOH → NaAl(OH)₄

Al(OH)₃ + NaOH → NaAl(OH)₄

Except for SiO₂, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe₂O₃ is removed. When the Bayer liquor is cooled, Al(OH)₃ precipitates, leaving the silicates in solution.

NaAl(OH)₄ → NaOH + Al(OH)₃

The solid Al(OH)₃ Gibbsite is then calcined (heated to over 1100 °C) to give aluminium oxide:

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

The product aluminium oxide tends to be multi-phase, i.e., consisting of several phases of aluminium oxide rather than solely corundum. The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control.

Sintering Process

The Sintering Process is a high-temperature method primarily used when the Bayer Process is not suitable, especially for ores with high silica content or when a more controlled product morphology is required. Firstly, Bauxite is mixed with additives like limestone and soda ash, then heating the mixture at high temperatures (1200 °C to 1500 °C) to form sodium aluminate and calcium silicate.^[24] After sintering, the material is leached with water to dissolve the sodium aluminate, leaving behind impurities. Sodium aluminate is then precipitated from the solution and calcined at around 1000 °C to produce alumina. This method is useful for the production of complex shapes and can be used to create porous or dense materials.

Applications

Aluminium oxide output in 2005

Known as alpha alumina in materials science, and as alundum (in fused form) or aloxite in mining and ceramic communities, aluminium oxide finds wide use. Annual global production of aluminium oxide in 2015 was approximately 115 million tonnes, over 90% of which was used in the manufacture of aluminium metal. The major uses of speciality aluminium oxides are in refractories, ceramics, polishing and abrasive applications. Large tonnages of aluminium hydroxide, from which alumina is derived, are used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.

Over 90% of aluminium oxide, termed smelter grade alumina (SGA), is consumed for the production of aluminium, usually by the Hall–Héroult process. The remainder, termed specialty alumina, is used in a wide variety of applications which take advantage of its inertness, temperature resistance and electrical resistance.

Fillers

Being fairly chemically inert and white, aluminium oxide is commonly used as a filler for plastics. Aluminium oxide is a common ingredient in sunscreen and is often also present in cosmetics such as blush, lipstick, and nail polish.

Glass

Many formulations of glass have aluminium oxide as an ingredient. Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.

Catalysis

Aluminium oxide catalyses a variety of reactions that are useful industrially. In its largest scale application, aluminium oxide is the catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for dehydration of alcohols to alkenes.

Aluminium oxide serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler–Natta polymerizations.

Gas purification

Aluminium oxide is widely used to remove water from gas streams.

Abrasion

Aluminium oxide is used for its hardness and strength. Its naturally occurring form, corundum, is a 9 on the Mohs scale of mineral hardness (just below diamond). It is widely used as an abrasive, including as a much less expensive substitute for industrial diamond. Many types of sandpaper use aluminium oxide crystals. In addition, its low heat retention and low specific heat make it widely used in grinding operations, particularly cutoff tools. As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards. Aluminium oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. It is also used in microdermabrasion, both in the machine process available through dermatologists and estheticians, and as a manual dermal abrasive used according to manufacturer directions.

Paint

Main article: Alumina effect pigment

Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.

Biomedical applications

Aluminium oxide is a representative of bioinert ceramics. Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints. In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance. It is also used for manufacturing dental implants, joint replacements, and other medical devices.

Composite fiber

Aluminium oxide has been used in a few experimental and commercial fiber materials for high-performance applications (e.g., Fiber FP, Nextel 610, Nextel 720). Alumina nanofibers in particular have become a research field of interest.

Armor

Some body armors utilize alumina ceramic plates, usually in combination with aramid or UHMWPE backing to achieve effectiveness against most rifle threats. Alumina ceramic armor is readily available to most civilians in jurisdictions where it is legal, but is not considered military grade.

Abrasion protection

An aluminium oxide layer can be grown as a protective coating on aluminium by anodizing or by plasma electrolytic oxidation (see the "Properties" above). Both the hardness and abrasion-resistant characteristics of the coating originate from the high strength of aluminium oxide, yet the porous coating layer produced with conventional direct current anodizing procedures is within a 60–70 Rockwell hardness C range which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with plasma electrolytic oxidation, the coating is porous only on the surface oxide layer while the lower oxide layers are much more compact than with standard DC anodizing procedures and present a higher crystallinity due to the oxide layers being remelted and densified to obtain α-Al2O3 clusters with much higher coating hardness values circa 2000 Vickers hardness.

Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas. They are not suitable for areas with high impact forces as these tiles are brittle and susceptible to breakage.

Electrical insulation

Aluminium oxide is an electrical insulator used as a substrate (silicon on sapphire) for integrated circuits, but also as a tunnel barrier for the fabrication of superconducting devices such as single-electron transistors, superconducting quantum interference devices (SQUIDs) and superconducting qubits.

For its application as an electrical insulator in integrated circuits, where the conformal growth of a thin film is a prerequisite and the preferred growth mode is atomic layer deposition, Al₂O₃ films can be prepared by the chemical exchange between trimethylaluminium (Al(CH₃)₃) and H₂O:

2 Al(CH₃)₃ + 3 H₂O → Al₂O₃ + 6 CH₄

H₂O in the above reaction can be replaced by ozone (O₃) as the active oxidant and the following reaction then takes place:

2 Al(CH₃)₃ + O₃ → Al₂O₃ + 3 C₂H₆

The Al₂O₃ films prepared using O₃ show 10–100 times lower leakage current density compared with those prepared by H₂O.

Aluminium oxide, being a dielectric with relatively large band gap, is used as an insulating barrier in capacitors.

Other

Aluminium residue from a '50s vintage ice cream scoop.

Before the advent of domestic plastics aluminium ice cream scoops would with wear and tear leave aluminium residue.

In lighting, translucent aluminium oxide is used in some sodium vapor lamps. Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamps.

In chemistry laboratories, aluminium oxide is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 when in water) and neutral formulations. Additionally, small pieces of aluminium oxide are often used as boiling chips.

Health and medical applications include it as a material in hip replacements and birth control pills.

It is used as a scintillator and dosimeter for radiation protection and therapy applications for its optically stimulated luminescence properties.

Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation contains a percentage of silica depending on the temperature rating of the material. The insulation can be made in blanket, board, brick and loose fiber forms for various application requirements.

It is also used to make spark plug insulators.

Using a plasma spray process and mixed with titania, it is coated onto the braking surface of some bicycle rims to provide abrasion and wear resistance.

Most ceramic eyes on fishing rods are circular rings made from aluminium oxide.

In its finest powdered (white) form, called Diamantine, aluminium oxide is used as a superior polishing abrasive in watchmaking and clockmaking.

Aluminium oxide is also used in the coating of stanchions in the motocross and mountain bike industries. This coating is combined with molybdenum disulfide to provide long term lubrication of the surface.