How Does a Catalytic Converter Work?

​Most people are at least vaguely aware of what a catalytic converter does: "It cleans a car’s exhaust..."

But, of course, it's a bit more complex than that.

​So, for the curious ones out there, I am going to dive a bit deeper into the chemistry that goes on inside of a catalytic converter, and how it turns harmful exhaust fumes into less-harmful emissions.

​The secret is in the name: it uses a catalyst to convert the harmful exhaust fumes into less-harmful gases. First, let’s define what the word “catalyst” means: a catalyst is a substance which facilitates a chemical reaction without itself being consumed in the reaction. The three main catalysts in a catalytic converter are Platinum, Palladium, and Rhodium.

​When a combustion engine burns gasoline, it produces three toxic byproducts: nitrogen oxides, carbon monoxide, and un-burnt fuel. These products are harmful to the environment and to human health, and the output must be controlled. Luckily, the catalytic properties of the Platinum Group Metals (PGMs) have been understood since the late 1800s. So, in 1973, the technology was adapted for vehicles, and by 1975 it was mandated that all cars in the United States be produced with a catalytic converter. 

​Catalytic converters are also commonly known as “three-way converters”, due to the three reactions that occur within them:

1 - Reduction of nitrogen oxides (NOx)

NOx is a collective term for the pollutants Nitric Oxide (NO) and Nitrogen Dioxide (NO2). When the nitrogen oxides undergo reduction, they get converted into nitrogen gas (N2) which is environmentally harmless.

​Rhodium is the metal that is primarily responsible for catalyzing the reduction reaction.

2 - Oxidation of carbon monoxide (CO)

Carbon monoxide is notoriously bad for human health, and is a natural byproduct of burning any type of fuel. Oxidation is, simply put, the addition of oxygen to a compound; so, when CO is oxidized, it forms carbon dioxide (CO2), which is significantly less toxic than CO.

3 - Oxidation of un-burnt hydrocarbons (HCs)

Gasoline, oil, propane, coal, wood… all these things have something in common: they are made up of hydrocarbons. A hydrocarbon is any compound that consists entirely of carbon and hydrogen. These compounds make good fuel because they burn relatively easily, which releases energy that can then be used to heat a home, cook food, or power vehicles. Any fuel that manages to leave the engine without being burned will undergo oxidation once it reaches the catalytic converter, which breaks down the hydrocarbons and combines them with oxygen to form carbon dioxide (CO2) and water vapor (H2O).

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​Platinum and Palladium are both responsible for catalyzing the oxidation reactions.

Anatomy of a catalytic converter:

1. Stainless steel or cast-iron casing to protect the internal components.
2. Insulation to keep the inside of the converter hot, and the outside of the converter less hot.
3. Ceramic monolith (also known as the "honeycomb") that is made up of many small channels which the exhaust gases pass through.
4. Wash-coat layer that contains the catalyst metals, and is coated on the inside of the honeycomb.

So, to put it all together...

The chemical reactions that happen inside of a catalytic converter.

The exhaust gases (NOx, CO, and HCs) pass through the honeycomb channels, interacts with the precious metals in the wash-coat, and undergoes either reduction or oxidation. The heat from the exhaust warms up the catalytic converter, which makes the reactions faster and more efficient. By the time that the exhaust comes out of the tail-pipe, most of the toxic gases have been converted (catalytically) into carbon dioxide, nitrogen gas, and water. 

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​And it wouldn’t be possible without the catalytic properties of platinum, palladium, and rhodium. These three precious metals do a lot of work in keeping our air clean and healthy. Even after a car has reached its end-of-life, the precious metals can be recycled and re-manufactured into the next generation of catalytic converters.