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What is a catalytic converter? PH Explains

How a handful of precious metals can significantly reduce the environmental impact of a vehicle

By Lewis Kingston / Tuesday, August 7, 2018

A catalytic converter is an emissions control device that chemically converts toxic exhaust gases into less harmful emissions. The reactions that occur in a catalytic converter significantly reduce the pollutants emitted by a vehicle; this helps minimise air quality issues, particularly in congested urban areas, as well as the associated environmental and health problems.

How do catalytic converters work?
A catalytic converter, which is fitted into a car's exhaust system so that spent gases flow through it, is designed to tackle harmful engine emissions. Inside the converter are catalysing elements that react with by-products in the exhaust gas, in order to reduce the pollutants emitted. Modern catalytic converters are designed primarily to tackle harmful carbon monoxide, hydrocarbon and nitrogen oxide emissions.

At the heart of each converter is a honeycomb structure, often formed out of ceramics or steel foils; the ceramic core is most common in modern catalytic converters and is often called a 'monolith'. Within this ceramic monolith are thousands of tiny passageways, which increase the surface area available for the necessary reactions to take place.

During production, a 'washcoat' is applied to this core. The washcoat gives the flat surfaces of the core a rough finish, again boosting the surface area available, and also contains the catalysts themselves. Noble metals such as platinum are used as the catalysing elements, as they do not corrode and can easily withstand the conditions within the converter. Other metals that may be used include rhodium and palladium, each of which reacts with the exhaust gas in different ways to reduce toxic emissions.

These catalysing elements are then heated by the exhaust gas and react with the gases flowing through them. Two key processes, called oxidation and reduction, then take place within the converter. During oxidation, oxygen is added to the likes of carbon monoxide - turning it into carbon dioxide. Reduction, on the other hand, removes oxygen from nitrogen oxide. The by-product of this process is nitrogen and oxygen.

Common types of catalytic converter
Two-way converter: Often found in older petrol applications but also commonly used in diesels. These convert carbon monoxide to carbon dioxide and oxidise unburnt hydrocarbons into carbon dioxide and water.

Three-way converter: These more advanced converters are commonly used in petrol-engined cars. They expand on the capabilities of the two-way converter by further reducing nitrogen oxides into nitrogen and oxygen.

Diesel oxidation catalyst: These converters use oxygen in the exhaust gas to oxidise carbon monoxide into carbon dioxide. They also convert unburnt hydrocarbons into water and more carbon dioxide. Beneficially, diesel oxidation catalysts also reduce the smell and soot emissions of diesel engines.

Selective catalytic reduction: In this set-up, a fluid - such as urea - is injected into the catalyst. This causes a reaction that converts nitrogen oxides into nitrogen and water, greatly reducing nitrogen oxide emissions.

A brief history of automotive catalytic converters
The concept of a catalytic converter for automotive applications was first proposed by French chemist Michel Frenkel in 1909. The aim of his system, which is described as using nickel, copper or platinum, was to 'deodorize automobile engine exhaust gases'. Due to the low number of cars on the road and the cost of the device at the time, it seemingly went no further - but the development of catalytic converters did continue in the following decades.

It was French engineer Eugene Houdry that designed and patented what we would regard as the modern catalytic converter in 1952. He was based in America at the time and, during the late 1940s, the number of vehicles on the road had risen dramatically. This led to pollution becoming a serious problem in urban areas.

By 1965, the National Emissions Standards Act had been introduced, which set out a series of hefty emissions reduction targets for 1968 and on vehicles. Catalytic converters could help but the leaded petrol that was in use at the time would poison the converter, making it useless. As the regulations continued to tighten with the introduction of the US Clean Air Act in 1970, and smog worsened, the need for catalytic converters grew. The government then stepped in and began a wide rollout of unleaded filling stations, in order to support cars with catalytic converters.

Extensive development was being carried out by several companies, including General Motors and Engelhard, and catalytic converters began being adopted in the mid-1970s. Other countries soon followed suit, and, today, catalytic converters are a key component of any car's emissions control system.

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