Above: Platinum crystals. Catalytic converters use platinum to convert harmful nitrogen oxides to nitrogen and oxygen gases (Jurii, Wikimedia Commons)
Chances are, you or somebody you know is studying for their driver’s test. You may be mastering the rules of the road, but how much do you really know about your car? For example, did you know how much of the engine’s exhaust is cleaned by noble metals?
Vehicle emissions contain many substances. Some are harmless, some are not. But imagine if your car spewed out even more harmful substances than it already does. Luckily, cars contain catalytic converters to help keep the air clean.
Did you know? If you gave a lawnmower without a catalytic converter and an SUV with one the same amount of fuel, the lawnmower would blast around 100 times more pollutants into the air!
Your car’s engine is probably powered by gasoline, which is a hydrocarbon. Using this fuel produces various other chemicals. Nitrogen (N2) is a harmless emission and makes up most of the air sucked into the engine (78 per cent). Most of it comes out the other end unchanged. Engine exhaust also includes water (H2O), which you often see dripping onto the street in the winter.
Car engines also release a lot of harmful emissions. Some of these, like nitrogen oxides (NOx) and sulfur oxides (SOx), can cause acid rain. Others, like unburnt hydrocarbons, particulates, and volatile organic compounds from diesel engines can cause health problems like cardiovascular disease and cancer.
Car engines also emit carbon monoxide (CO). This poisonous gas can replace oxygen in your bloodstream. If you breathe enough of it, it can even suffocate you!
That all sounds pretty dangerous, doesn’t it? Fortunately, catalytic converters help make engine emissions less harmful. Here’s the story on how.
The catalytic converter was invented around 1950 by a French mechanical engineer named Eugène Houdry. The first catalytic converters cleaned the exhaust from smokestacks and forklifts. It took several decades before they were used in cars as well. One problem was the widespread use of leaded gasoline. Lead ruins catalytic converters by reacting badly with the precious metals inside.
The catalytic converters installed in cars today use these precious metals as catalysts. They speed up chemical reactions that convert the harmful emissions into more benign (less harmful) ones. This happens through the processes of reduction and oxidation, as well as the use of oxygen sensors.
Usually, chemical equations are balanced. A balanced equation has the same number of chemical elements on the left side as the right.
You’ll notice that the last equation on the left (Unburnt hydrocarbons + O2 + N2 => CO2 + H2O + N2) is not balanced.
This is because unburnt hydrocarbons (fuel) are made up of a bunch of different compounds - so balancing it is complicated! You can learn more about balancing equations here.
First of all, modern catalytic converters have a chamber where the oxygen atoms of potentially harmful oxides of nitrogen are removed. This is a type of chemical reaction called reduction. It converts those harmful oxides to nitrogen and oxygen gases, which are standard components of air. Typically, the metals rhodium and platinum facilitate this reaction.
2NO => N2 + O2
2NO2 => N2 + 2O2
2N2O => 2 N2 + O2
The second stage of the catalytic converter adds oxygen to unburnt hydrocarbons and carbon monoxide. This is a type of reaction called oxidation. The metals ruthenium or platinum and palladium are catalysts that turn these harmful emissions into carbon dioxide.
2CO + O2 => 2 CO2
Unburnt hydrocarbons + O2 + N2 => CO2 + H2O + N2
Finally, modern catalytic converters also use an oxygen sensor, which is sometimes called a lambda sensor. The oxygen sensor controls how much extra oxygen (air) needs to be pumped into the exhaust stream to produce a reaction. Using the correct amount of oxygen can make the chemical reactions you’ve just read about more efficient.
Et voilà! With the three components of catalyst converters working together, vehicles keep harmful emissions down...and keep the air clean.
Did you know? Catalytic converters can take a few minutes to kick in, so car exhausts pollute much more during this time. This is a great reason to walk short distances!
Researchers are studying whether gold could be used as a catalyst in catalytic converters. That might sound expensive, but gold is actually cheaper than many of the previously mentioned above—and there’s more of it! In fact, metals like platinum may run out in the next 20 or so years. Some thieves are even stealing catalytic converters just to get a hold of the metal inside!
So when you take your driving test, remember: you might be sitting on a box of platinum!