Catalytic converters
Last updated: October 23, 2007.
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
work.
Photo: An experimental new catalytic converter
is tested underneath a car. By courtesy of US Department of
Energy/National Renewable Energy Laboratory.
Why engines make pollution
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 squeeze by
layers of sea-bed rocks. Petroleum is made up of hydrocarbons
(molecules built from carbon and hydrogen atoms)
because living organisms are made from those things too.
In theory, if
you burn any kind of 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, there may be too
little oxygen (or too much) or there may be impurities in the engine or
the fuel you're burning. That means you generally get some 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).
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 courtesy of U.S. Geological Survey.
What is a catalytic converter?
Pollutant gases are made from 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. That's the job that a catalytic converter does.
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.
Photo: This scientist is working to develop
a new type of catalytic converter that can reduce automobile pollution by over 50 percent.
Photo by Warren Gretz courtesy of
National Renewable Energy Laboratory.
A catalytic converter is a metal box with chemical catalysts inside
it that sits underneath your car. There's a pipe going into one end of
the converter from the car engine and another pipe going out of the
other end of the converter to the car's exhaust. The catalysts make
chemical reactions happen that convert the molecules of
pollution into simple, harmless gases. These are much safes to pass
into the
outside air.
What happens inside the converter?
Photo: Catalytic converters don't cut the
number of
cars on our highways—but they certainly cut the pollution they make!
Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy
Laboratory.
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
efficiently. Typically, there are two different catalysts in a
catalytic converter:
- One of them tackles nitrogen oxide pollution using
a chemical reaction called reduction
(removing oxygen). This breaks up nitrogen oxides into nitrogen and
oxygen gases (which are harmless, because they already exist in the air
around us).
- The other catalyst works by an opposite chemical process
called oxidation (adding
oxygen) and turns carbon monoxide into carbon dioxide. Another
oxidation reaction turns any 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.
After the catalyst has done it's job, what emerges from the exhaust is
mostly nitrogen, oxygen, carbon dioxide, and water (in the form of
steam).
One final note: We generally think of carbon dioxide as a safe gas,
because it's
not toxic. Nevertheless, it isn't entirely harmless, because we now
know it's the major cause of global
warming and climate change.
