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 (petrol) 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 US 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 (), were first commercialized by Volvo on the California-specification 1977 240 cars. When U.S. federal emission control regulations began requiring tight control of for the 1981 model year, almost 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 .
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. Rarely, devices similar to catalytic converters are used in appliances such as wood stoves to control emissions.
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."Exhaust Gas Made Safe" Popular Mechanics, September 1951, p. 134, bottom of page 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."His Smoke Eating Cats Now Attack Traffic Smog". Popular Science, June 1955, pp. 83-85/244.
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 CmHn) 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 emissions, a twin-catalyst system was developed in the 1970s this added a separate (rhodium/platinum) catalyst which reduced 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.Worthy, Sharon. "Connecticut chemist receives award for cleaner air technology ". Bio-Medicine. 23 June 2003. Retrieved 11 December 2012. 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
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 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. 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. Ceria or ceria-zirconia. These oxides are mainly added as oxygen storage promoters. 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 rather than being scrapped. The precious metals inside the converter – the platinum, palladium, and rhodium – are extracted.
Placement of catalytic converters Catalytic converters require a temperature of 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: Oxidation of carbon monoxide to carbon dioxide: Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: (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 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 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 and not to be confused with nitrous oxide (N2O)). 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 which 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)
Oxidation of carbon, hydrocarbons, and carbon monoxide to carbon dioxide
These three reactions occur most efficiently when the engine runs 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 by the engine increases, and the efficiency of the catalyst at reducing 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 is good, and the production of by the engine decreases.
To maintain catalyst efficiency, the airfuel ratio must stay close to stoichiometric and not remain rich or lean for too long. Therefore, engines fitted with regulated 3-way catalytic converters have a closed-loop feedback carburetor or fuel injection engine control system to manage a continuous rich-lean balance required for effective 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 reduction efficiency. Then the airfuel 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.