by Chris Woodford. Last updated: September 7, 2019.
Blackened buildings and choking
streets—if that's your experience
when you open the front door in the morning, you probably live in a big
city like Los Angeles, London, Paris, or Beijing. Cars, buses, and
trucks have been a great gift to the world, because they help us move
ourselves (and the things we need) quickly and efficiently. But their
engine pollution spoils the places where we live and
harms our health. Fortunately, most vehicles are now fitted with
pollution-reducing units called catalytic
converters (sometimes known as "cats" or "cat-cons"), which turn
the harmful chemicals in vehicle exhausts into harmless gases such as
steam. Let's take a closer look at these brilliant gadgets and how they
Artwork: The basic concept of a catalytic converter: sitting between your car's engine and tailpipe, it takes in dirty air and removes a significant amount of pollution from it using chemical catalysts.
Why engines make pollution
Photo: The columns of the Parthenon in Athens, Greece have been blackened by vehicle pollution. Athens is one of the world's most traffic-polluted cities. Photo by Michael M. Reddy courtesy of
U.S. Geological Survey.
Car engines run on gasoline or diesel,
which are made from petroleum. Most of our petroleum is formed when the
remains of tiny sea creatures rot down, heat up, and get squeezed by
layers of sea-bed rocks. Petroleum is made up of hydrocarbons
(molecules built from carbon and hydrogen atoms)
because living organisms are mostly made from those atoms too.
In theory, if you burn any kind of hydrocarbon fuel with oxygen from the air, you release a lot
of energy and make nothing but carbon dioxide and water, which are clean and relatively harmless. In practice, though, gasoline is a mixture
of about 150 different chemicals, not just hydrocarbons but additives too, and it doesn't burn as cleanly as we'd like.
That means you generally get some
air pollution as
a byproduct. The pollutant gases made by car engines include a
poisonous gas called carbon monoxide, as well as VOCs (volatile organic
compounds) and nitrogen oxides that cause "smog" (the sort of choking,
cloudy vehicle pollution we all know and hate).
What is a catalytic converter?
Pollutant gases are made of harmful molecules, but those molecules
are made from relatively harmless atoms. So if we could find a way of
splitting up the molecules after they leave a car's engine and before
they get pumped out into the air, we could crack the problem of
pollution—or some of it, anyway. That's the job that a catalytic converter does.
Photo: An experimental new catalytic converter
is tested underneath a car. Picture courtesy of Southwest Research Institute and
US Department of Energy/National Renewable Energy Laboratory (DoE/NREL).
These gadgets are much simpler than they sound. A catalyst
is simply a chemical that makes a chemical reaction go faster without itself
changing in the process. It's a bit like an athletics coach who stands
by the side of the track and shouts at the runners to go faster. The
coach doesn't run anywhere; he just stands there, waves his arms about,
and makes the runners speed up. In a catalytic converter, the
catalyst's job is to speed up the removal of pollution.
The catalyst is made from platinum or a similar, platinum-like metal
such as palladium or rhodium.
A catalytic converter is a large metal box, bolted to the underside of your car, that has two pipes coming out of it. One of them (the converter's "input") is connected to the engine and brings in hot, polluted fumes from the engine's cylinders (where the fuel burns and produces power). The second pipe (the converter's "output") is connected to the tailpipe (exhaust). As the gases from the engine fumes blow over the catalyst, chemical reactions take place on its surface, breaking apart the pollutant gases and converting them into other gases that are safe enough to blow harmlessly out into the air.
One very important thing to note about catalytic converters is that they require you to
use unleaded fuel, because the lead in conventional fuel
"poisons" the catalyst and prevents it from taking up the pollutants in exhaust
What happens inside the converter?
Photo: Engineers are constantly trying to improve the performance of
catalytic converters, for example, by developing catalysts that work more effectively at
lower temperatures. This is an example of a low-temperature oxidation catalyst made from tin oxide and platinum. Photo by CPL Bryant V courtesy of NASA Langley Research Center (NASA-LaRC).
Inside the converter, the gases flow through a dense honeycomb
structure made from a ceramic and coated
with the catalysts. The honeycomb structure means the gases touch a
bigger area of catalyst at once, so they are converted more quickly and
Typically, there are two different catalysts in a
- One of them tackles nitrogen oxide pollution using
a chemical process called reduction
(removing oxygen). This breaks up nitrogen oxides into nitrogen and
oxygen gases (which are harmless, because they already exist in the air
- The other catalyst works by an opposite chemical process called oxidation (adding
oxygen) and turns carbon monoxide into carbon dioxide. Another oxidation reaction turns unburned hydrocarbons in the exhaust into carbon dioxide and water.
In effect, three different chemical reactions are going on at the same time. That's why we talk about three-way catalytic converters. (Some, less-effective converters carry out
only the second two (oxidation) reactions, so they're called two-way catalytic converters.)
After the catalyst has done its job, what emerges from the exhaust is
mostly nitrogen, oxygen, carbon dioxide, and water (in the form of
How effective are catalytic converters?
Chart: Effectiveness of catalytic converters. Cats make a big difference to emissions, with three-way converters giving a significant extra benefit over two-way converters. Figures show pollutants in grams per kilometer at 80,000 kilometers. Chart drawn by Explain that Stuff.com using data for light-duty gasoline fueled vehicles from US EPA (1990), quoted in table 3.2 (page 75) of Air Pollution from Motor Vehicles: Standards and Technologies for Controlling Emissions, Faiz et al, World Bank, 1996.
Catalytic converters are mainly designed to reduce immediate, local air pollution—dirty air where you're driving—and this chart certainly seems to suggest that they're effective. Even so, people sometimes question whether they're really as green as they seem. It's important to remember that they reduce emissions rather than eliminate them completely.
One problem is that they only really work at high temperatures (over 300°C/600°F or so), when the engine has had chance to warm up. Early types of catalytic converters typically took about 10–15 minutes to warm up, so they were completely ineffective for the first few kilometers/miles of a journey (or any part of a very short journey).
Modern converters warm up in only 2–3 minutes; even so, significant emissions can still occur during this time.
Another issue is whether they increase greenhouse gas emissions. We think of carbon dioxide as a safe gas, because it's not toxic in everyday concentrations. Nevertheless, it isn't entirely harmless, because we now know it's the major cause of global warming and climate change. Some people believe catalytic converters make climate change worse because they turn carbon monoxide into carbon dioxide. In fact, the carbon monoxide your car produces would eventually turn into carbon dioxide in the atmosphere all by itself, so a catalytic converter makes no difference on that score: it simply reduces the carbon monoxide a car pumps into the street as it drives along, improving the local air quality.
But when it comes to climate change, auto engineers and environmentalists have long pointed out another serious issue. Although cats turn most nitrogen oxides into nitrogen and oxygen, they also produce small amounts of nitrous oxide (N2O) in the process, a greenhouse gas that's over 300 times more potent than carbon dioxide. The trouble is that with so many vehicles on the road, even small amounts of nitrous oxide add up to a major problem. Back in 2000, the
Intergovernmental Panel on Climate Change noted: "The introduction of catalytic converters as a pollution control measure in the majority of industrialized countries is resulting in a substantial increase in
N2O emissions from gasoline vehicles."
Fortunately, newer catalytic converters produce dramatically less nitrous oxide than older ones.
Even so, while catalytic converters have certainly helped us to tackle short-term air pollution, there are
concerns that, when it comes to long-term climate change, they could be making matters worse.
Do catalytic converters work for diesel engines?
Chart: Dirty diesels? Only a tiny proportion of the emissions from a diesel engine (about one percent) is pollution. That one percent consists mostly of nitrogen oxides (about 50 percent) and particulates, with relatively small amounts of carbon monoxide, hydrocarbons, and sulfur dioxide. Drawn using figures from The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems by İbrahim Aslan Reşitoğlu et al, Clean Technologies and Environmental Policy, January 2015, Volume 17, Issue 1, itself quoting figures from Diesel Emissions and their Control by M. Khair and W. Majewski. Society of Automotive Engineers, Inc., Warrendale, PA: 2006.
Diesel engines can and do use catalytic converters, but there are several important differences
from how they work in gasoline engines.
- Instead of three-way catalysts, diesels use two-way oxidation catalysts
(which only tackle carbon monoxide and hydrocarbons), and ones specifically designed
to work with diesel exhausts, which are significantly cooler than gasoline exhausts.
- Since they don't have reduction catalysts,
diesel engines produce much higher tailpipe emissions of nitrogen oxides than gasoline engines. (There are various other mechanisms that diesels can use to tackle NOx emissions, but we won't go into details here.)
- Catalytic converters on diesel engines do help to reduce particulate emissions (mostly soot), though only slightly; specifically, they tackle one type of particulate known as the soluble organic fraction, SOF, made from hydrocarbons bound to soot. Diesel particulate filters (DPFs) have to be used to make a significant impact on an engine's soot emissions.
- Cars aside, diesel engines tend to power much bigger vehicles than gasoline engines (huge construction machines, for example),
with considerably greater exhaust output. Instead of a single catalytic converter fitted between the
engine and the tailpipe, they may have a number of individual units fitted in parallel to cope with the bigger exhaust
gas volume (as in the diagram below).
Artwork: Big diesel engines can produce much higher exhaust volumes, so they may need to use
multiple catalytic converters "in parallel." In this 1990s design by Caterpillar, the huge converter unit (gray) is around 1m (3.3ft) in diameter. Exhaust gas enters at the left (1), is evenly separated into streams by a flow distributor unit (2, blue), passes through one of seven separate catalytic converter units (3, red), is quietened by a noise muffler system (4, green), and exits, somewhat cleaned up, through the tailpipe (5).
Artwork from US Patent 5,578,277: Modular catalytic converter and muffler for internal combustion engine by Scott T. White et al, Caterpillar, courtesy of US Patent and Trademark Office.
Who invented the catalytic converter?
Whom do we thank for making streets and cities safer and cleaner? French chemical engineer
Eugene Houdry (1892–1962) patented what seems to have been the very first
catalytic converter in the United States, filing the invention on May 5, 1950 and receiving his
(US Patent 2,674,521: Catalytic converter for exhaust gases)
four years later on April 6, 1954. Houdry had previously invented catalytic cracking, the industrial process by
which the many large complex organic chemicals in petroleum are separated into dozens of useful products, including gasoline.
After that, he experimented with making different kinds of vehicle fuels and making them cleaner.
Although he recognized the growing problem of air pollution, his ideas were far ahead of their time:
catalytic converters were "poisoned" by the lead additives used in gasoline to improve performance.
Fortunately, in the 1970s, people started to recognize the dangers of lead, a toxic heavy metal. In 1973, the US Environmental Protection Agency (EPA) released a report demonstrating how lead harmed people's health, which began the slow process for removing lead from gasoline. The first practical catalytic converters appeared shortly afterward, in the mid-1970s, and have been used in cars ever since.
Artwork: Eugene Houdry's original catalytic converter from his 1950 patent. It's essentially a set of concentric metal tubes (blue) through which the exhaust gases flow. Clean air is sucked in through ventilation holes (yellow) with the help of a venturi (orange). As in a modern cat, Houdry explains that "the deposited finely divided metal catalyst is preferably platinum," although other similar metals can be used; unlike a modern cat, the catalyst (green) isn't arranged in a honeycomb but mounted in sixteen separate rings (red) at intervals along the tube, with each one working in parallel. Artwork from US Patent 2,674,521: Catalytic converter for exhaust gases, courtesy of US Patent and Trademark Office.
Houdry invented the basic oxidation catalyst for tackling carbon monoxide. Improved, three-way catalytic converters, which could also tackle nitrogen oxides, were designed in the early 1970s by
Carl Keith (1920–1988), John Mooney (1929–), and chemical engineers at Engelhard Corporation. Apart from removing more pollutants, they start to purify the tailpipe gases much faster than earlier converters, so they're more effective on shorter journeys.
Artwork: In Carl Keith and John Mooney's improved design, there are two separate catalytic converters. Polluted gases flow from the engine (red, 10), and the exhaust manifold (orange, 11), through the first catalyst (green, 13) and then the second (25), some distance away, before exiting through the tailpipe (gray, 26). Artwork from US Patent 3,896,616: Process and Apparatus, courtesy of US Patent and Trademark Office.