A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction (an oxidation and a reduction reaction). Catalytic converters are usually used with internal combustion engines fueled by either gasoline or diesel—including lean-burn engines as well as 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 must be equipped with catalytic converters. These "two-way" converters combine oxygen with carbon monoxide (CO) and unburned hydrocarbons (HC) to produce carbon dioxide (CO2) and water (H2O). In 1981, two-way catalytic converters were rendered obsolete by "three-way" converters that also reduce oxides of nitrogen (NOx); however, two-way converters are still used for lean-burn engines. This is because three-way-converters require either rich or stoichiometric combustion to successfully reduce NOx.
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, and motorcycles. They are also used on some wood stoves to control emissions. This is usually in response to government regulation, either through direct environmental regulation or through health and safety regulations.
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; it was constituted of an inert material coated with platinum, iridium and palladium, sealed into a double metallic cylinder.
A few decades later, a catalytic converter was patented by Eugene Houdry, a French mechanical engineer and expert in catalytic oil refining, who moved to the United States in 1930. When the results of early studies of smog in Los Angeles were published, Houdry became concerned about the role of smoke stack exhaust and automobile exhaust in air pollution and founded a company called Oxy-Catalyst. Houdry first developed catalytic converters for smoke stacks called "cats" for short, and later developed catalytic converters for warehouse forklifts that used low grade, unleaded gasoline. In the mid-1950s, he began research to develop catalytic converters for gasoline engines used on cars. He was awarded United States Patent 2,742,437 for his work.
Widespread adoption of catalytic converters did not occur until more stringent emission control regulations forced the removal of the antiknock agent tetraethyl lead from most types of gasoline. Lead is a "catalyst poison" and would effectively disable a catalytic converter by forming a coating on the catalyst's surface.
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 catalytic converter's construction is as follows:
- 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 pellets, especially 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.
- 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, 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 greatly increases the surface area compared to the smooth surface of the bare substrate. This in turn maximizes the catalytically active surface available to react with the engine exhaust. The coat must retain its surface area and prevent sintering of the catalytic metal particles even at high temperatures (1000 °C).
- Ceria or ceria-zirconia. These oxides are mainly added as oxygen storage promoters.
- The catalyst itself is most often a mix of precious metal. Platinum is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions and high cost. Palladium and rhodium are two other precious metals used. 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. Nickel is not legal for use in the European Union because of its reaction with carbon monoxide into toxic nickel tetracarbonyl. Copper can be used everywhere except Japan.
Placement of catalytic converters
Catalytic converters require temperature of 800 degrees Fahrenheit (426 °C) to efficiently convert harmful exhaust gases into inert gases, such as carbon dioxide and water vapor. Therefore, the first catalytic converters were placed close to the engine to ensure fast heating. However, such placing caused several problems, such as vapor lock. It occurred several minutes after the engine is shut off, as the heat from the catalytic converter influences fuel in the fuel lines, which makes it first boil and then aerate. This condition of fuel results in non-start condition which lasts until the engine and fuel in the lines cool down.
As an alternative, catalytic converters were moved to a third of the way back from the engine, and were then placed underneath the vehicle.
A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:
- Oxidation of carbon monoxide to carbon dioxide: 2 CO + O2 → 2 CO2
- Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O (a combustion reaction)
This type of 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-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.
Three-way catalytic converters (TWC) have the additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO2) (both together abbreviated with NOx and not to be confused with nitrous oxide (N2O)), which are precursors to acid rain and smog.
Since 1981, "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 has three simultaneous tasks:
- 2 CO + 2 NO → 2 CO2 + N2
- hydrocarbon + NO → CO2 + H2O + N2
- 2 H2 + 2 NO → 2 H2O + N2
Oxidation of carbon monoxide to carbon dioxide
- 2 CO + O2 → 2 CO2
Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water, in addition to the above NO reaction
- hydrocarbon + O2 → H2O + CO2
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 is slightly different for each, requiring modified fuel system settings when using those fuels. In general, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system using one or more oxygen sensors, though early in the deployment of three-way converters, carburetors equipped with feedback mixture control were used.
Three-way converters are effective when the engine is operated within a narrow band of air-fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen). Conversion efficiency falls very rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, leaving only oxygen stored in the catalyst available for the oxidation function.
Closed-loop engine control systems are necessary for effective operation of three-way catalytic converters because of the continuous balancing required for effective NOx reduction and HC oxidation. The control system must prevent the NOx reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material so that its function as an oxidation catalyst is maintained.
Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air–fuel ratio goes lean. When sufficient oxygen is not available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NOx reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.
Unwanted reactions can occur in the three-way catalyst, such as the formation of odoriferous hydrogen sulfide and ammonia. Formation of each can be limited by modifications to the washcoat and precious metals used. It is difficult to eliminate these byproducts entirely. Sulfur-free or low-sulfur fuels eliminate or reduce hydrogen sulfide.
For example, when control of hydrogen-sulfide emissions is desired, nickel or manganese is added to the washcoat. Both substances act to block the absorption of sulfur by the washcoat. Hydrogen sulfide forms when the washcoat has absorbed sulfur during a low-temperature part of the operating cycle, which is then released during the high-temperature part of the cycle and the sulfur combines with HC.
For compression-ignition (i.e., diesel engines), the most commonly used catalytic converter is the diesel oxidation catalyst (DOC). DOCs contain palladium, platinum, and aluminium oxide, all of which catalytically oxidize the hydrocarbons and carbon monoxide with oxygen to form carbon dioxide and water.
- 2 CO + O2 → 2 CO2
- CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O
These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates (soot). These catalysts are not active for NOx reduction because any reductant present would react first with the high concentration of O2 in diesel exhaust gas.
Reduction in NOx emissions from compression-ignition engines has previously been addressed by the addition of exhaust gas to incoming air charge, known as exhaust gas recirculation (EGR). In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet new federal emissions requirements. There are two techniques that have been developed for the catalytic reduction of NOx emissions under lean exhaust conditions: selective catalytic reduction (SCR) and the lean NOx trap or 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. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also referred to as Diesel Exhaust Fluid (DEF), is AdBlue.
Diesel exhaust contains relatively high levels of particulate matter (soot), consisting largely of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction, so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). Historically, a DPF consists of a cordierite or silicon carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. Contemporary DPFs can be manufactured from a variety of rare metals that provide superior performance (at a greater expense). As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after January 1, 2007, must meet diesel particulate emission limits, meaning that they effectively have to be equipped with a 2-way catalytic converter and a diesel particulate filter. Note that this applies only to the diesel engine used in the vehicle. As long as the engine was manufactured before January 1, 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. During the re-generation cycle, most systems require the engine to consume more fuel in a relatively short amount of time in order to generate the high temperatures necessary for the cycle to complete. This adversely affects the overall fuel economy of vehicles equipped with DPF systems, especially in vehicles that are driven mostly in city conditions where frequent acceleration requires a larger amount of fuel to be burned and therefore more soot to collect in the exhaust system.
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.
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, enabling 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 (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 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 (NOx reduction) catalyst from becoming oxygen-loaded, while simultaneously ensuring the second (HC and CO oxidation) catalyst is sufficiently oxygen-saturated.
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 until recently 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. 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—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. Usually the upstream components of the exhaust system (manifold/header assembly and associated clamps susceptible to rust/corrosion and/or fatigue e.g. the exhaust manifold splintering after repeated heat cycling), ignition system e.g. coil packs and/or primary ignition components (e.g. distributor cap, wires, ignition coil and spark plugs) and/or damaged fuel system components (fuel injectors, fuel pressure regulator, and associated sensors) - since 2006 ethanol has been used frequently with fuel blends where fuel system components which are not ethanol compatible can damage a catalytic converter - this also includes using a thicker oil viscosity not recommended by the manufacturer (especially with ZDDP content - this includes "high mileage" blends regardless if its conventional or synthetic oil), oil and/or coolant leaks (e.g. blown head gasket inclusive of engine overheating). 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.
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.
Regulations for diesel engines are similarly varied, with some jurisdictions focusing on NOx (nitric oxide and nitrogen dioxide) emissions and others focusing on particulate (soot) emissions. This regulatory diversity is challenging for manufacturers of engines, as it may not be economical to design an engine to meet two sets of regulations.
Regulations of fuel quality vary across jurisdictions. In North America, Europe, Japan, and Hong Kong, gasoline and diesel fuel are highly regulated, and compressed natural gas and LPG (autogas) are being reviewed for regulation. In most of Asia and Africa, the regulations are often lax: in some places sulfur content of the fuel can reach 20,000 parts per million (2%). Any sulfur in the fuel can be oxidized to SO2 (sulfur dioxide) or even SO3 (sulfur trioxide) in the combustion chamber. If sulfur passes over a catalyst, it may be further oxidized in the catalyst, i.e., SO2 may be further oxidized to SO3. Sulfur oxides are precursors to sulfuric acid, a major component of acid rain. While it is possible to add substances such as vanadium to the catalyst washcoat to combat sulfur-oxide formation, such addition will reduce the effectiveness of the catalyst. The most effective solution is to further refine fuel at the refinery to produce ultra-low-sulfur diesel. Regulations in Japan, Europe, and North America tightly restrict the amount of sulfur permitted in motor fuels. However, the direct financial expense of producing such clean fuel may make it impractical for use in developing countries. As a result, cities in these countries with high levels of vehicular traffic suffer from acid rain, which damages stone and woodwork of buildings, poisons humans and other animals, and damages local ecosystems, at a very high financial cost.
Catalytic converters restrict the free flow of exhaust, which negatively affects vehicle performance and fuel economy, especially in older cars. Because early cars' carburetors were incapable of precise fuel-air mixture control, the cars' catalytic converters could overheat and ignite flammable materials under the car. A 2006 test on a 1999 Honda Civic showed that removing the stock catalytic converter netted a 3% increase in horsepower; a new metallic core converter only cost the car 1% horsepower, compared to no converter. To some performance enthusiasts, this modest increase in power for very little cost encourages the removal or "gutting" of the catalytic converter. 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 many jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its direct and immediate replacement. In the United States, for example, 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.
Vehicles fitted with catalytic converters emit most of their total pollution during the first five minutes of engine operation; for example, before the catalytic converter has warmed up sufficiently to be fully effective.
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.
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 impacts 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.
- Pieces of catalytic converters can cause wild fires, especially in dry areas
Because of the external location and the use of valuable precious metals including platinum, palladium, rhodium, and gold, catalytic converters are a target for thieves. The problem is especially common among late-model trucks and 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 tools with which thieves quickly remove a catalytic converter, such as a portable reciprocating saw, can often damage other components of the car, such as wiring or fuel lines, and thereby can have dangerous consequences. Rises in metal costs in the U.S. during recent years have led to a large increase in converter theft. A catalytic converter can cost more than $1,000 to replace.
Various jurisdictions now require on-board diagnostics to monitor the function and condition of the emissions-control system, including 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 are still sometimes 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). More-recent 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.
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 catalyst 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.
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