Car Chemistry - Catalytic Converters

I hope you all had a great bank holiday weekend! I went back to Kent, where I grew up, to see my parents and some friends. I was chatting to my Dad, who's a leader for the local scout group (30th Strood in case you were wondering), and he was telling me that the exhaust from the group's minibus was stolen. Why steal just the exhaust? Apart from the fact that a whole minibus is harder to get away with. Well the answer is what's inside the exhaust system of modern vehicles, the catalytic converter (CC). You've probably heard of these, and might be familiar with what they do, but how do they work and what's so valuable about them?

Normally you wouldn't see the CC on your car, its usually quite close to the engine in order to heat it up (temperatures of about 800 degrees Celsius are required for it to work). Pictured above is an example of one.

The purpose of the CC is to make a vehicle more environmentally friendly. We know that cars emit CO₂, a greenhouse gas. This is a product of the combustion reaction used to drive the wheels. Complete combustion of hydrocarbons (petrol) is described by the following chemical reaction:

C_nH_2n+2 + n/2 O₂ = n CO₂ + 2n+2/2 H₂O

This means that all the fuel is burnt in the presence of enough oxygen to form only water and CO₂, if this was what happened in the engine, then there'd be no need for the CC, as these products can't really be processed  into anything friendlier to the environment. The key word above is "enough". If there is insufficient oxygen for the above reaction to take place, incomplete combustion occurs. This is a little trickier to describe but it goes like this:

Hydrocarbon + not enough O₂ = CO + soot + H₂O

Carbon monoxide or CO is bad news. It's extremely toxic and is an unwanted byproduct of combustion in an engine. Here is where the CC comes in, using a catalyst (something that makes chemical reactions go faster without being used up itself) it oxidises CO to CO₂. Additionally it converts (hence the name) unburnt fuel to CO₂ and H₂O. So this is all very useful, it means that toxic gas isn't being spat out of exhaust pipes, especially in areas where there are a lot of people and cars like cities.

But why steal it? I briefly mentioned above that there is some substance called the catalyst which promotes the useful chemical reactions. In most CCs, this catalyst is platinum. Platinum is a precious metal even more expensive than gold. It's rare and not super easy to get hold of, both contributing to its cost. Because of this, existing platinum products are highly recycled. Scrapyards will strip out most of the valuable metals before disposing of an old car. They'll also pay good money for old exhausts because they can sell on the precious metals.

This does beg the question, why use it if it's such a target for thieves? Quite simply because it works the best!

If you want to find your local scout group and get involved here is where to go. I went to 21st Medway beavers, cubs, scouts and explorers and couldn't recommend it more!

If you’re interested in hearing about my work, you can follow me on Twitter @StubbingScience and the work of GlamSci can be found @GlamSci.