Catalytic converters

Also known as Catalytic converter, Autocatalyst, Three-way catalyst

A catalytic converter is a device in a petrol car's exhaust that uses a catalyst to turn carbon monoxide, unburnt fuel and nitrogen oxides into less harmful gases. It has been fitted to new petrol cars in the United States since the mid-1970s, and it depends on lead-free petrol.

CommercialDemonstratedReviewed 22 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

A catalytic converter is a device in the exhaust system of a petrol car. It contains a catalyst that helps harmful exhaust gases react into less harmful ones. The pollutants it targets are carbon monoxide, unburnt fuel (hydrocarbons) and nitrogen oxides.

The idea grew out of the smog problems of the early 1950s. Eugene Houdry applied for a patent in 1952 on a catalyst structure whose description includes treating engine exhaust fumes. The early converters had a serious problem: leaded petrol coated the catalyst and stopped it working. In the United States, the 1970 Clean Air Act required a 90 percent cut in emissions from new cars by 1975. First generation converters were built for that period, and unleaded petrol was introduced because lead in petrol can damage them.

Technical explanation

Modern petrol cars use a three-way catalyst. According to the Wikipedia article on Carl D. Keith, Keith and John Mooney developed a three-way catalyst at Engelhard that treats hydrocarbons, carbon monoxide and nitrogen oxides in a single catalyst bed. From 1981, the US EPA timeline records that sophisticated three-way catalysts with on-board computers and oxygen sensors appeared in most new cars.

The patent history is mixed. US Patent 2,742,437, filed on 29 September 1952 and granted on 17 April 1956 to Eugene J. Houdry (assignee Oxy-Catalyst, Inc.), is titled 'Catalytic structure and composition'. Its description discusses using the catalyst to treat internal combustion engine exhaust fumes, but none of its claims mention engines or exhaust. Secondary accounts date the first production converter to 1973 and credit Engelhard engineers, but the sources checked for this profile did not confirm either point, so they are not stated here.

The measured effects are reported in different ways. The National Medal of Technology and Innovation page quotes the claim that today's cars are 98 percent cleaner than those built 40 years ago. This is a fleet-level comparison, and the page does not give the method behind it. The Wikipedia article on Carl D. Keith reports that a three-way catalyst eliminates 97 percent of tailpipe hydrocarbons, 96 percent of carbon monoxide and 90 percent of nitrogen oxides, citing a Business Wire release of 15 November 2001 that was not checked. No peer-reviewed measured reduction figure was checked for this profile.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

US Patent 2,742,437, filed on 29 September 1952 and granted on 17 April 1956 to Eugene J. Houdry (assignee Oxy-Catalyst, Inc.), describes in its description the use of the catalyst to treat internal combustion engine exhaust fumes, although none of its claims mention engines or exhaust.

Sources: [1] · Description and claims

Congress passed the first major Clean Air Act in 1970, requiring a 90 percent reduction in emissions from new automobiles by 1975.

Sources: [2] · 1970 entry

The EPA's timeline records that first generation catalytic converters were built in 1975, and that unleaded gasoline was introduced because lead in gasoline may cause disintegration of catalytic converters.

Sources: [2] · 1975 entry

Sophisticated three-way catalysts with on-board computers and oxygen sensors appeared in most new cars from 1981, according to the EPA timeline.

Sources: [2] · 1981 entry

Reported, unverified Stated by a company or team but not independently confirmed.

According to the Science History Institute, Houdry's converter used a catalyst, usually platinum, inside a metal casing, and leaded petrol coated that catalyst so that exhaust gases could not reach it.

Sources: [3] · Lead problem section · medium confidence

The US Environmental Protection Agency required catalytic converters on all cars from the 1975 model year, according to a secondary historical account.

Sources: [3] · Lead problem and 1975 section · medium confidence

The National Medal of Technology and Innovation page states that today's cars are 98 percent cleaner than those built 40 years ago. The page does not give the method behind this fleet-level comparison.

Sources: [4] · Emissions comparison · medium confidence

The Wikipedia article on Carl D. Keith states that a three-way catalyst eliminates 97 percent of tailpipe hydrocarbons, 96 percent of carbon monoxide and 90 percent of nitrogen oxides from automobile and light truck engines. The article cites a Business Wire release of 15 November 2001, which was not checked.

Sources: [5] · Efficiency paragraph · low confidence

Keith and John Mooney developed a three-way catalyst at Engelhard that treats hydrocarbons, carbon monoxide and nitrogen oxides in a single catalyst bed, according to the Wikipedia article on Carl D. Keith.

Sources: [5] · Lead paragraph · medium confidence

Limitations and open problems

  • Leaded petrol coats the catalyst and stops it working (Science History Institute, 2015), so the device depends on lead-free fuel.
  • The sources checked for this profile name platinum as the usual catalyst metal. No supply or price figures for platinum-group metals were verified.
  • The sources checked here do not give verified figures for performance under other operating conditions, such as cold starts.
  • Most reduction figures found are fleet comparisons or unverified secondary claims. No peer-reviewed measured reduction figure was verified for this profile, so the numbers in this record should be read with that caveat.
  • The sources checked for this profile do not address carbon dioxide, so this profile makes no claim about CO2 effects.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • Converters that use much less platinum-group metal, or none, if cheaper catalyst chemistry can match current performance.
  • Exhaust treatment tuned for engines running on hydrogen or other new fuels, where the exhaust chemistry differs from petrol.

Timeline

  1. 17 Apr 1956US Patent 2,742,437, 'Catalytic structure and composition', granted to Eugene J. Houdry and assigned to Oxy-Catalyst, Inc. Sources: [1]
  2. 1970Congress passes the first major Clean Air Act, requiring a 90 percent reduction in emissions from new automobiles by 1975. Sources: [2]
  3. 1975First generation catalytic converters are built, and unleaded petrol is introduced because lead can damage them, according to the EPA. Sources: [2]
  4. 1981Sophisticated three-way catalysts with on-board computers and oxygen sensors appear in most new cars. Sources: [2]
  5. 2002Carl Keith and John Mooney receive the National Medal of Technology and Innovation for the three-way catalytic converter. Sources: [4]

Glossary

Catalyst
A material that speeds up a chemical reaction without being used up in it.
Three-way catalyst
A catalyst that oxidises carbon monoxide and hydrocarbons while reducing nitrogen oxides, all at once.
Platinum-group metals
A group of rare metals, including platinum, palladium and rhodium, that are used in catalytic converters.
Unleaded petrol
Petrol without added lead, which is needed because lead damages the catalyst.

Sources

  1. patent
    Catalytic structure and composition (US Patent 2,742,437)
    Eugene J. Houdry. United States Patent and Trademark Office, via Google Patents, 17 Apr 1956. Accessed 22 Oct 2026.
  2. government report
    Timeline of Major Accomplishments in Transportation, Air Pollution, and Climate Change
    US Environmental Protection Agency, 14 Oct 2026. Accessed 22 Oct 2026.
  3. other
    Clean Machine
    Jacob Roberts. Science History Institute, Distillations magazine, 13 Jan 2015. Accessed 22 Oct 2026.
  4. other
    Carl D. Keith, National Medal of Technology and Innovation laureate
    National Medal of Technology and Innovation, 2002. Accessed 22 Oct 2026.
  5. other
    Carl D. Keith
    Wikipedia, 22 Oct 2026. Accessed 22 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 22 Oct 2026
Published on owner instruction (car lineage tech tree, 2026-10-22)165371a
Sources and claims re-checked against originals409337d
Initial profile drafted from primary sourceseaac9eb