Catalytic converters use noble metals to speed up chemical reactions. Vehicle emissions are therefore less harmful.

Do you have any friends studying for their driving test? Or it is you who are trying to master the rules of the road. What do you really know about cars? For example, did you know that noble metals cleanse engine exhaust gases?


Car exhaust fumes are also called vehicle emissions. They contain many substances, some of which are more harmful than others.

Your car’s engine probably runs on gasoline. Gasoline is a hydrocarbon. This fuel is mixed with the air in your car before it is burned. This process, called combustion, creates a lot of chemical byproducts.

Ø  Some of these by-products are absolutely safe. Air, for example, consists of 78% nitrogen gas (N 2). Some of this nitrogen reacts with oxygen during combustion. However, most of it is released as N 2 in the exhaust gases of an engine. Engine exhaust also contains water (H 2 O). Water is often seen flowing from exhaust pipes in winter.

Ø  Automobile engines also emit a lot of harmful substances. Some of them can be responsible for acid precipitation. This is the case with carbon dioxide (CO 2), nitrogen oxides ( NO x) and sulfur oxides .


Ø  Other vehicle emissions can cause health problems, such as cardiovascular disease and cancer. This is the case with unburned hydrocarbons, particles (carbon particles) and volatile organic compounds (VOCs).

Ø  Automotive engines also release carbon monoxide ( CO ). This poisonous gas can take the place of oxygen in the blood. It can even lead to suffocation if inhaled too much!

It all sounds very scary, doesn’t it? Fortunately, catalytic converters help make engine emissions less harmful. Here’s how.

What is a catalytic converter?

The catalytic converter was invented around 1950 by Eugène Houdry, a French mechanical engineer. He invented the catalytic converter to clean car exhaust gases.

The widespread use of catalytic converters spread around 1975. Governments began to want to reduce air pollution from cars. At that time, a large number of cars used leaded gasoline. Lead (Pb) can interfere with the proper functioning of a catalytic converter. Indeed, lead can cover the surface which reacts normally with the exhaust gases.

How does a catalytic converter work?

The catalytic converter of a car is connected to the exhaust pipe. A metal case contains a ceramic honeycomb structure. The honeycomb structure is covered with a mixture of platinum (Pt), palladium (Pd) and rhodium (Rh). These noble metals offer good resistance to oxidation, corrosion and acid. They are therefore resistant to bad weather and all the chemicals that an automobile engine lets out.

The noble metals in catalytic converters act as a catalyst. Catalysts are compounds which can start a chemical reaction without being themselves modified. The honeycomb structure inside a catalytic converter maximizes the contact area that gives rise to the reactions.

What chemical reactions are involved in a catalytic converter?

Catalytic converters use the reduction and oxidation reaction (redox) to reduce harmful emissions.

They use a reduction catalyst which is made of platinum and rhodium. Such a catalyst makes it possible to reduce nitrogen oxides (NO x) thanks to the elimination of nitrogen atoms in the molecules of nitrogen oxides (NO and NO 2 ). Thus, free oxygen forms an oxygen gas (O 2). Then the nitrogen atoms attached to the catalyst react with each other. This reaction creates gaseous nitrogen (N 2).

Catalytic converters also use an oxidation catalyst which is made of platinum or palladium. This converter makes it possible to reduce the quantity of hydrocarbons (HC) and carbon monoxide (CO). First, carbon monoxide and oxygen combine to form carbon dioxide (CO 2 ). Then unburned hydrocarbons and oxygen unite to form carbon dioxide and water.

Modern catalytic converters also use oxygen sensors. They are also sometimes called lambda probes. They control the additional amount of oxygen that is injected into the exhaust stream. Maintaining the correct amount of oxygen improves the efficiency of reduction and oxidation reactions.

Researchers are trying to determine if it is possible to use gold in catalytic converters. It may sound expensive, but gold is, in fact, cheaper than many other noble metals. And there are more! In fact, we could run out of metals like platinum within a few decades. In some places, people even steal catalytic converters to get their hands on the noble metals they contain!

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