How a Catalytic Converter works

Internal combustion is nasty business. Fuel and air get burned to create pressure for driving the crankshaft to power the drivetrain, but that combustion rips the molecules apart, releasing toxic gasses that quickly find their way into the atmosphere. The nastiest pollutants are carbon monoxide, unburned hydrocarbons and nitrogen oxide. The EPA classifies these as greenhouse gasses and they must be reduced significantly in all gasoline-burning engines. The muffler on your car filters the sound, but it does very little to reduce the toxic components of the exhaust. That task falls on the much-maligned catalytic converter.

A scientific mystery, the catalytic converters or “cat” for short, are not serviceable, you don’t know when they fail, and cost a small fortune to replace when they do. So why not take a few minutes and learn a little more about how they function, how they are made, and why it is important to make sure they are in good working order.

Most vehicles these days have multiple cats, typically four, to convert as much of the nasty gases to water and CO2.

Most vehicles these days have multiple cats, typically four, to convert as much of the nasty gases to water and CO2.

Most of the mystery lies in the construction of a catalytic converter. The most important part is the ceramic matrix. Shaped like a honeycomb, the matrix is made predominantly of a ceramic material called cordierite. The honeycomb is created through an extrusion process in which lengths of the honeycomb shape 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 ceramic 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 evenly disperses the metals throughout all the pores of the matrix. The metals are generally mixed so as to best utilize their individual properties, most cats 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 each has its disadvantages.

The honeycomb matrix is ceramic, which has to be fired in a kiln to set. It is not that much different from your high school pottery class mixed with a really fancy play-doh press.

The honeycomb matrix is ceramic, which has to be fired in a kiln to set. It is not that much different from your high school pottery class mixed with a really fancy play-doh press.

Originally developed throughout the 1930s and 40s for industrial smoke stacks, catalytic converter inventor Eugene Houdry began to develop a cat for automobiles in the 1950s. The first cats were mandated in 1975. These catalytic converters were of a two-way type, that combined oxygen with the carbon monoxide and unburned hydrocarbons to form CO2 and water. As the science progressed, more strict environmental regulations brought about a change to 3-way converters in 1981. These more advanced cats also reduce nitrogen oxide.

Cats work using what is called an oxidation-reduction (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. These types of reactions are extremely common; photosynthesis and rust are both good examples of redox reactions.

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.

The honeycomb is wrapped in a special mat to absorb vibrations.

The honeycomb is wrapped in a special mat to absorb vibrations.

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.

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 1,200 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, which is a known 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. 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.

To protect the matrix and direct the gases through it, a steel or stainless steel tube encases it.

To protect the matrix and direct the gases through it, a steel or stainless steel tube encases it.

Damage to a cat can occur through physical damage, such as smashing the case on a speed bump, or chemical, such as allowing a contaminant to cause “catalyst poisoning”, which coats the honeycomb, keeping it from doing it’s job. The most common cat killing chemicals are lead, sulfur, manganese, zinc, and phosphorous. Now if you are a hot rodder, you might be able to think of a couple of those that are pretty important to you. Yes, that is right, the reason both zinc and phosphorous have been severely limited from engine oil is due to the fact that the fumes from oil getting into the exhaust kills the cats.  

Physical damage comes from the fact that the ceramic matrix is fragile; it is just a thin ceramic brick full of holes. 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. Exhaust manufacturer MagnaFlow uses vermiculite based mat for this, though other manufacturers may do things differently.

The outside casing is just as important, this keeps road debris from damaging the inner structure where the magic happens.

The outside casing is just as important, this keeps road debris from damaging the inner structure where the magic happens.

Hot rodders who are interested in extracting every last ounce of performance out of their car may 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 extreme 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 with full emissions controls, how much of an impediment can your cats really be?

If you have a high-performance engine that is in a vehicle subject to emissions testing and requires a cat system, there are some options. The factory cat(s) are designed to perform up to the level of performance for the stock engine, but when you increase the performance of the engine, the exhaust needs to perform better. This is where the aftermarket shines. Instead of doing a cat-delete or risk damaging your valuable cats (they fetch a pretty penny at the scrap yard), look at upgrading to an aftermarket cat, such as the direct-fit and universal-replacement cats from Magnaflow. You have several options, including California CARB legal, New York legal, or the Federal EPA legal models, all of which are available at your local NAPA auto parts store.

This diagram from Magnaflow shows the overall construction and reactions for a catalytic converter.

This diagram from Magnaflow shows the overall construction and reactions for a catalytic converter.

A plugged up or damaged cat will severely hamper your engine’s performance and fuel economy. You usually don’t know it is damaged until the check engine light comes on and you get a malfunction code. Most engines these days have four or more cats, so replacing them can be a costly affair. Regular fuel treatments such as Sea Foam will help remove contaminants from the honeycombs. For most vehicles, installing a new catalytic convertor is as simple as unbolting the old one and bolting a new one on, but be forewarned, the constant heat cycles have a bad habit of freezing the bolts in place. It is a good idea to soak them in penetrating oil several times over the course of a couple days to help loosen them up.

To learn more about NAPA AutoCare, visit www.NAPAAutoCare.com.

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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1 Comment

  • Jc Stamps

    September 29, 2016 at 2:02 PM

    Reply

    Thank you for the valuable information.

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