Catalysts And Catalytic Converters

Emma Schofield, from Johnson Matthey Technology Centre, joined us to explain what catalysts are and how they work...

Chris - First of all, everyone uses the word 'catalytic converter' but what is a catalyst and why is it important?

Emma - A catalyst is a substance that makes a chemical reaction happen more easily. You can get some really stroppy reactions in which you want to rearrange the atoms in them to make something useful, but it's just not playing. The starting material just isn't interested into becoming the product that you want. If you put the right catalyst into the reaction, you can make this reaction happen either more quickly or using a lot less energy. Often you have to go into high temperatures and pressures to get the reaction to work.

Chris - How do they do that?

Emma - Imagine that you've got yourself a beach cottage and the beach is about a mile away from your cottage. Between you and the beach there is this massive great mountain. You have several options for reaching the beach. One of them is that you put lots of energy into it, so you put lots of energy to walk up the mountain and down the other side. Not great. The second option is to walk all the way around the mountain but it takes a heck of a long time. But if someone's gone and dug a tunnel from one side to the other then you can get to the beach pretty quickly and with a lot less energy. This is exactly what a catalyst does. So the catalyst does for a chemical reaction what the tunnel does for you: it takes an alternative pathway that allows the reaction to happen a lot more easily.

Chris - And just like a tunnel, it's not used up in the reaction. It's available forever if you like?

Emma - Yeah, and that's why you only need a very small amount of catalyst when you have a chemical reaction. Because although the catalyst is changed during the reaction, it's regenerated at the end of it. So each little atom or each little molecule of catalyst that's in there can go on a react with hundreds and thousands of millions of reactant molecules.

Chris - Sounds fantastic, but how do we find these things? Why isn't there a catalyst for everything? Why isn't there a catalyst for my homework? How do we discover the chemicals that do these clever jobs?

Emma - There are quite a lot of metals that are used as catalysts because there are two different types of catalysts: homogeneous catalysts and heterogeneous catalysts. In homogeneous catalysts, the reactants and the products are in the same phase. So if there are gases reacting, the catalyst will also be a gas. In heterogeneous reaction, the reagents are in a different phase from the catalyst. An example of homogeneous would be making plastic bags - high density polyethylene. The ethene and the catalyst are all going on in the same phase, in solution. That would be a homogeneous catalyst. In a heterogeneous catalyst, that would be carbon monoxide turning into carbon dioxide. That would happen on platinum metal.

Chris - They've got expensive tastes these things!

Emma - Platinum is very expensive but you often find that some of the most expensive metals turn out to be the best catalysts.

Chris - Why is platinum so good? What's special about the metal? How does it do what it does?

Emma - When people ask questions like this, a scientist's usual answer is: oh well, it's quantum. Actually it's to do with how well these gas molecules can stick to the surface. Platinum is very good at sticking molecules onto the surface of it. That's very important because that's where the reaction actually happens. The other thing that platinum is good for is, well, when you have a molecule, the atoms in the molecule are stuck together with chemical bonds, which are electrons. Platinum is very good at rearranging these electrons and allowing the molecules to turn into something else. Again, forming this alternative pathway by which a chemical reaction can happen.

Chris - So if you could zoom in to the surface of the platinum, what would it look like to make it so sticky and that things like it?

Emma - We always imagine it as lots of little balls stuck next to each other. One of the aims of being a catalyst chemist, which is what I am, is to try and make as much surface as possible. So we have our tiny little pieces of platinum which are stuck onto a ceramic support. We want as much platinum on the surface and as little platinum in the middle of these balls as possible. The platinum is part of the periodic table which has lots of d-orbitals, and it's these magic d-orbitals that makes it so good at catalysis and making things stick to it. So it very easily forms bonds with lots of different types of molecule.

Chris - And it brings them together in just the right way that they want to get married or do whatever you want them to do...

Emma - And provides them with a route which requires so little energy that it can happen essentially spontaneously or with very little energy on the metal's surface.

Chris - Ok, so turning now to what comes out of your exhaust pipe, how does a catalytic converter on a car actually work? What are they doing?

Emma - The catalyst on a catalytic converter is essentially a can which is next to the engine. What it does is it purifies the exhaust gases. If it was ideal, we'd just get carbon dioxide and water out when fuel was burnt.

Chris - I'm sure people would argue that it would be ideal if we just had water and burning hydrogen, which is what Fraser is going to be talking to us about in a minute...

Emma - The problem being that you put fuel in at the beginning and you can destroy atoms as you go along. But what we also get out is carbon monoxide, which is a poison and binds so strongly to the blood that you can't bind oxygen anymore. There are also what's called NOx gases, which are oxides of nitrogen responsible for acid rain; and hydrocarbons which come together with NO-x to form smog. This is why in the 1970s Los Angeles got buried under this cloud of photochemical smog and what triggered all of the legislation about car pollutants. Also what comes out are particulates, which are essentially soot. This is linked with respiratory illnesses as well as cancer. So we obviously don't want these coming out of the backs of our cars. We need to put the catalytic converter between the engine and the exhaust pipe to catch these things as they go out. Inside the catalytic converter we have the monolith and the metal. The monolith is a ceramic and it's a honeycomb with a very large surface area and it's coated to give it an even greater surface area. If you spread it out it would cover about three football pitches. On the channels of this monolith you have little globules of the metal platinum, palladium, rhodium in various mixtures depending on whether you're a petrol or a diesel car, and these are so small that we call them nanoparticles. This is what we were talking about before. As these gases go past, which is a very quick reaction, it goes from the engine through the catalytic converter in less than a tenth of a second.

Chris - So it must be very fast.

Emma - Yes and it wouldn't happen normally unless there was a catalyst there.

Chris - So how much of the gases does the catalytic converter scavenge or convert? Does it do the lot?

Emma - It causes about a 90% decrease in the amount of pollution coming out, and what you mostly get out of the other end is nitrogen, water and carbon dioxide.

Chris - So it does a good job but there was a motivation for people to stop using leaded fuel because it makes your brain rot and causes dementia but also lead's quite toxic to catalysts.

Emma - Exactly. If you think about these little metal particles, the lead will stick onto the surface of them. The more you reduce the amount of surface there is, the less chance these pollutant gases have of sticking to the surface and making the catalyst catalyse.

Chris - So it's better to do without lead if we can for more reason than one.

Emma - It's better to do without lead and it's better to do without sulphur in petrol too, because sulphur is responsible, or used to be responsible when we had high sulphur petrol, for this eggy smell some people associate with catalytic converters. Now there's less sulphur in fuel, this is much less of a problem.

Chris - Now Emma, it's impossible to miss your t-shirt and on the subject of noisy engines, I was wondering if this was what we're talking about! It says NOISE. What is NOISE and why are you here today?

Emma - NOISE is the New Outlooks In Science and Engineering campaign. It's a group of young scientists who are there to give an alternative image for what scientists are like. Chris, when you think about the stereotype of a scientist, what is it that springs to mind?

Chris - Glasses more powerful than the Hubble space telescope, shocking teeth, 1960s get-up and muttering unintelligibly in a way that no-one can understand.

Emma - And the words fun and dynamic don't really feature in those descriptions.

Chris - But that's why people listen to the Naked Scientists!

Emma - And that's why NOISE is there. We need to change this. We're the new generation of young scientists and we have a website www.noisemakers.org.uk where there's this whole group of scientists that do lots of fun science that we want to tell people about. We have a snowboarding physicist and we have somebody who does robotics who is a scuba diver. The idea is to point out to people, especially kids who are thinking of going to university, that there's more to being a scientist than a white coat!