How Do Catalytic Converters Work?

Published On February 15, 2017 |

Exhaust Tech: Everything You Need To Know About Catalytic Converters

Few parts on a car are surrounded with more mystery and misconception than the catalytic converter. How could it not be? It’s a sealed box with no user serviceable parts, it’s expensive, and has a somewhat generic sounding name? Why do cats cost so much? How are they made? Hopefully we’ll be able to shed some light on this metal mystery and clear up some confusion with a series of blog posts.

If we’re going to explain how catalytic converters work, first we’re going to have to talk a little about their basic construction. The most important part of a catalytic converter is the ceramic matrix. This honeycomb is made predominantly of a ceramic called cordierite. The honeycomb is created via an extrusion process in which lengths of honeycomb are squeezed through a die and supported by computer controlled jets of air that keep the honeycomb straight as it leaves the machine.

Once the honeycomb is fired and set, it receives a washcoat of various oxides combined with the precious metals that function as the actual catalyst. The washcoat is used because it most evenly disperses the metals throughout all the pores in the ceramic matrix. The metals are generally mixed so as to best utilize their individual properties. Most catalytic converters in the United States use some combination of platinum, palladium, and rhodium. Outside of the US, copper has been tried, but will form dioxin, a toxic substance with carcinogenic properties. In other places in the world, materials like nickel, cerium in washcoat and manganese in cordierite are used but again, each has its disadvantages.

Earlier catalytic converters were of a two-way type, but due to stricter environmental regulations, a more elaborate 3-way converter has become the norm. Cats work using what is called a redox reaction. This means that once the catalyst is up to operating temperature (anywhere from 500 to 1200 degrees Fahrenheit) both an oxidation reaction and reduction reaction are occurring simultaneously. That sounds a little complicated but what it means is that molecules are simultaneously losing and gaining electrons. Redox reactions are extremely common. Things like photosynthesis and rust are both good examples of redox reactions.

The ceramic matrix is fragile, and wouldn’t survive long hanging under a car if it was left to fend for itself. This is where the metal shell comes in. Most catalytic converters feature more than one ceramic “brick” in a shell. The shell helps the cat manage heat and keep the ceramic bricks in their ideal operating temperature range. Between the steel of the shell and the ceramic matrix is a heat resistant insulating barrier that locks the bricks in place inside the shell and limits the amount of vibration that the ceramic sees.

MagnaFlow uses vermiculite based mat for this, though other manufacturers may do things differently. Once assembled, the shells are welded shut and flow tested before being boxed and shipped to retailers and customers. MagnaFlow tests every cat that comes off of our assembly line for air tight performance so you, as a customer or installer, can be sure that it’s going to be good.

The Chemistry Behind The Converter

The catalytic converter works on what is known as a redox reaction. This means that as the spent gases exit the combustion chamber and start making their way through the matrix portions of the converter, the chemicals in the gases are gaining and losing electrons and as such their properties change. Exhaust gas leaving the combustion chamber is made up of a number of different chemicals, these include: carbon dioxide, carbon monoxide, nitrous oxide, nitric oxide, oxygen, water, and unburnt fuels. Some of these chemicals are much more harmful than others both to humans directly, and to the environment as a whole. A typical 3-way catalytic converter has two matrices, each coated with different precious metals. The first matrix is the reduction catalyst and is typically coated with platinum and rhodium. The second matrix is the oxidation catalyst and is coated with platinum and palladium.

In the first stage of the catalytic converter, the reduction stage, the goal is to remove the nitrous oxide and especially the nitric oxide, which when introduced to air quickly changes into nitrogen dioxide, which is very poisonous. The reduction stage works because the nitrogen molecule in the nitrogen oxides wants to bond much more strongly with the metals of the catalyst than it does with its oxygen molecules and the oxygen molecules would rather bond with each other, forming o2, which is the type of oxygen that we breathe. Once the oxygen molecules break off from their nitrogen molecules, the nitrogen molecules move along the surface of the catalyst looking to make friends with another nitrogen molecule. One it finds one, it bonds and becomes the stable, harmless nitrogen we find in our atmosphere. Once it becomes atmospheric nitrogen, its bond with the walls of the catalyst is weakened and the gas moves along to the second phase of the catalytic converter, oxidation.

Once the gases have finished in the reduction stage of the catalytic converter, and we’ve eliminated all the nitrogen oxides, we are left with atmospheric nitrogen, atmospheric oxygen, carbon dioxide, carbon monoxide, water, and unburned fuel. The oxidation stage of the catalytic converter uses platinum and palladium which want to bond with the various oxides. The oxygen molecules bond with the surface of the catalyst and break up and eventually find carbon monoxide molecules to bond with creating carbon dioxide. The carbon dioxide bond again stronger than the bond with the catalyst and moves through the matrix, allowing the process to begin again. At the same time that this is happening, some of the freed up oxygen molecules being to bond with the unburned fuel (hydrocarbons) and are changed into water and more carbon dioxide.

MagnaFlow has learned over the years, the perfect catalyst lets oxidation and reduction happen simultaneously, thanks to a brick design that allows enough airflow and the correct spacing to let the gasses travel at the most efficient speed for each application.

Modern catalytic converters are nearly 90% efficient at removing nitrogen oxides and carbon monoxide from your vehicle’s exhaust gases. They only get to this level of efficiency once they reach their operating temperature which can from 500 to 1200 degrees Fahrenheit which can take up to six miles of normal driving. One thing we haven’t touched on is the fact that one of the biggest byproducts of the catalytic converter process is carbon dioxide and that carbon dioxide is what is known as a greenhouse gas. This means that large amounts of carbon dioxide can become trapped in the atmosphere and provide an insulation effect, thereby warming the planet. We realize that politically speaking, this is a very contentious issue right now but it is important to understand what people are talking about when they talk about cars and pollution.

Many people who are interested in extracting every last ounce of performance out of their car think that their catalytic converter is robbing them of horsepower. When catalytic converters were first introduced in the 1970s, this was true. They were a new technology then and not terribly efficient. Many engines hadn’t been designed to work specifically with them and they were applied as a band-aid to meet increasingly strict anti-pollution legislation. This is no longer the case. The modern 3-way catalytic converter is extremely efficient both in terms of its function and in the way it allows exhaust gases to flow through it. Furthermore, every modern combustion engine has been designed specifically to work with a catalyst, so removing them can cause many other problems including poor performance and reduced economy. This doesn’t take into account the extremely stiff fines that can be imposed for being caught without one. Look at it this way, if Dodge can release a car with a 6.2 liter supercharged V8 engine that is conservatively rated at 707 horsepower and it has full emissions controls, how much of an impediment can your catalyst really be?

Thanks to Magnaflow for the taking the time to educate us on catalytic converters!

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About The Author

Tim King is the Editor and Co-Founder of Late Model Garage.

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