The Three Core Functions of a Catalytic Converter

A modern three-way catalytic converter performs three primary functions: it oxidizes carbon monoxide (CO) into carbon dioxide (CO2), oxidizes unburned hydrocarbons (HC) into carbon dioxide and water (H2O), and reduces nitrogen oxides (NOx) into nitrogen (N2). These reactions dramatically cut harmful tailpipe emissions when the engine operates near the stoichiometric air–fuel ratio and the catalyst reaches operating temperature.

What Those Three Functions Mean on the Road

The catalytic converter sits in the exhaust stream and uses precious-metal catalysts to convert toxic combustion byproducts into less harmful gases. Below is a concise breakdown of the three functions it performs in gasoline engines equipped with a three-way catalyst (TWC).

1. Oxidation of carbon monoxide (CO) to carbon dioxide (CO2): Converts CO—a poisonous, colorless gas—into CO2 by adding oxygen over platinum/palladium catalyst sites. This reaction curbs a major health hazard from incomplete combustion.
2. Oxidation of unburned hydrocarbons (HC) to CO2 and H2O: Breaks down fuel fragments and volatile organic compounds into carbon dioxide and water vapor, reducing smog-forming pollutants and unburned fuel odors.
3. Reduction of nitrogen oxides (NOx) to nitrogen (N2): Uses rhodium (and sometimes platinum) to strip oxygen from NO and NO2, releasing harmless nitrogen gas and oxygen. This is critical for controlling smog and meeting NOx emission limits.

Together, these three reactions define the “three-way” functionality. When properly controlled, they can cut tailpipe CO, HC, and NOx by well over 90% compared with an uncatalyzed exhaust stream.

How the Converter Achieves These Reactions

The catalyst materials and structure

Automakers washcoat a ceramic or metallic honeycomb substrate with high-surface-area alumina, then apply precious metals—typically platinum and palladium for oxidation, and rhodium for NOx reduction. Cerium oxide (ceria) helps store and release oxygen during transient conditions to keep reactions balanced.

Operating conditions that matter

The catalyst needs heat (“light-off” typically around 250–300°C, with peak efficiency often 400–800°C). Short trips, cold starts, and degraded catalysts reduce conversion efficiency. Excessively rich or lean mixtures also impair performance, because oxidation reactions need oxygen while NOx reduction requires a near-stoichiometric mix.

Closed-loop control and sensors

Upstream oxygen (lambda) sensors guide the engine control unit to maintain an air–fuel ratio near 14.7:1 for gasoline. Many systems add a downstream sensor to monitor catalyst efficiency and diagnose faults. This precise control enables simultaneous oxidation of CO/HC and reduction of NOx—something older two-way catalysts could not do.

Variations Across Engine Types

Not every vehicle uses a three-way system in the same way. Different engine and fuel types necessitate different aftertreatment strategies.

• Gasoline engines (TWC): Use the full three-function catalyst under stoichiometric control for CO, HC, and NOx conversion.
• Diesel engines: Typically use an oxidation catalyst (for CO and HC), a diesel particulate filter (for soot), and selective catalytic reduction (SCR with urea/DEF) to cut NOx—separating the functions rather than combining all three in one brick.
• Hybrids and small engines: May face colder average exhaust temperatures, so designs focus on faster light-off and thermal management to keep the catalyst effective.

While configurations differ, the end goals are consistent: reduce CO, HC, and NOx to meet stringent emission standards without sacrificing engine performance.

Key Takeaways

The catalytic converter’s three functions—CO oxidation, HC oxidation, and NOx reduction—work in concert to minimize the most harmful components of exhaust. Their effectiveness depends on catalyst chemistry, temperature, and precise fuel–air control.

Summary

A catalytic converter in a three-way configuration performs three core tasks: it oxidizes carbon monoxide to carbon dioxide, oxidizes unburned hydrocarbons to carbon dioxide and water, and reduces nitrogen oxides to nitrogen. These reactions, enabled by platinum-group catalysts under tightly controlled engine conditions, are central to meeting modern emissions standards and protecting air quality.

What three things does a catalytic converter do?

The catalytic converter is designed to convert harmful emissions produced by an internal combustion engine to less-harmful elements: H2O (water), CO2 (carbon dioxide) and N2 (nitrogen).

What 3 emissions do catalytic converters help reduce?

Catalytic converters are used in exhaust systems to provide a site for the oxidation and reduction of toxic by-products (like nitrogen oxides, carbon monoxide, and hydrocarbons) of fuel into less hazardous substances such as carbon dioxide, water vapor, and nitrogen gas.

What is the function of the three-way catalytic converter?

The three-way catalytic converter currently permits the most efficient, safest and most reliable form of emission control in petrol engines. As the name implies, the three-way catalytic converter converts three pollutants: hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) into less harmful gases.

Where are 3-way catalytic converters used?

Area of application. By far the largest use of three-way catalytic converters is in gasoline and gas engines in passenger cars. Engines with three-way catalytic converters do not require an SCR catalytic converter. Retrofitting is possible, in which case an additional control unit and a lambda probe are required.