by Chris Woodford. Last updated: July 24, 2016.
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. Picture courtesy of Southwest Research Institute and US Department of Energy/National Renewable Energy Laboratory (DoE/NREL).
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 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 squeeze 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, 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 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. That's the job that a catalytic converter does.
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 DoE National Renewable Energy Laboratory.
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 gases.
What happens inside the converter?
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 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 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.
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.