Catalysts for Cleaner Environments and Future Energy

To do the experiment, you will need:

Slice of cheap white bread

How to do the experiment:

1 - Take half a slice of bread and chew it and chew it and chew it! Even if the bread becomes disgusting, you should still keep chewing amd remember - don't swallow or you'll spoil the experiment!

2 - Pay attention to how the flavour of the bread changes.

3 - Once you think you have the answer, you can swallow the bready mush or (this is probably best!) spit it out.

What's going on?

As you chew the bread, you may have noticed that it slowly tastes sweeter. Why?

Bread is made up of starch and starch is made by plants. But how do plants make starch in the first place?

When plants take in light, they convert it into sugars through a process called photosynthesis. Once the sugar is made, they need to store it in their cells. The problem is that when there is a lot of sugar in a cell, the proportion of water in that cell is less relative to a cell with hardly any sugar in it. If there are some cells with a lower water concentration (or more sugar) than others, then a concentration gradient is established. This can be thought of as a slope: at the top of the slope are cells with lots of water (and little sugar) and at the bottom of the slope are cells with relatively little water (and lots of sugar).

If you put lots of water at the top of a slope, then it will trickle down to the bottom. This is exactly what happens in cells: the cells with a high concentration of water have some of their water sucked out and it is taken up by the cells with a low concentration of water. This continues until all the cells have an equal concentration of water and the gradient (or slope) disappears. This process of water moving between cells depending on the concentration of other small molecules (such as sugar) is called osmosis.

So what's the problem? Well plant cells are surrounded by a tough cell wall made of cellulose, and this gives the cell strength. However, if the cell is full of sugar from photosynthesis and water from other cells rushes in by osmosis, then the cell wall starts to strain. If too much water enters the cell, the cell wall will eventually give way and explode - just like a balloon that's been blown one puff too far.

Exploded (and thus dead) cells are bad news for plants, so they've had to come up with a cunning storage solution. What they do is take all the sugar molecules and glue them together into a long chain. It is this chain of sugar molecules that we call starch. Because starch is a long molecule, it doesn't alter the overall water concentration (or osmotic pressure) of the cell like small sugar molecules do. This means that having starch in the cell doesn't establish a large concentration gradient, doesn't cause water to rush in, and the cells don't explode.

When we use plants to make food such as bread, it is still in its starchy (or long chain carbohydrate) form. This is great, but we can't absorb it into our bodies in this long chain form. The only way round it is to chop it up. Thanks to evolution, there's an enzyme in our spit called amylase which specifically cuts up starch and turns it back into small sugar molecules. This is why the bread starts to taste sweet after lots of chewing - the amylase enzyme is breaking down the starch and turning it into glucose.

Amylase isn't the only enzyme to break down food into molecules we can absorb. Another example is the enzyme lactase. This breaks down lactose (which we can't absorb) into galactose and glucose (which we can). People who stop producing lactase will stop breaking down lactose, meaning that lactose is left behind in the gut. Bacteria that live in the gut see this otherwise ignored lactose and begin to break it down themselves. This produces large quantities of gas, and explains why people with lactose intolerance find that they become rather windy after a glass of milk. The lactose is acting as the fuel for a large bacterial fermentation plant!

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!

There are a couple of reasons but it's unlikely that it's anything to do with the x-ray machine itself, but more to do with the phone being turned off. One explanation might involve it being cold when you went into the airport. Batteries work by a chemical reaction going on inside. When they get cold, the chemical reaction slows down, which means that they can't produce as much current. This makes the battery appear to be a lot emptier than it is. This is why you sometimes get a flat battery on a cold day. If you have a battery and you've been using it a lot, it will drain quickly and the voltage will go down. If you turn it off, that voltage will slowly build up as long as you're not drawing a current. This means that when Adamski turned his mobile phone back on, it was full.

When you do an emergency stop, the car is accelerating everything in it backwards. This means that everything relative to the car rushes forwards. The balloon is obviously floating in the air. But if it's a helium balloon, the air is heavier than the balloon itself. This means that the air wants to rush forwards more than the helium balloon is going to want to rush forward. The air will then push the helium balloon out of the way, making it move backwards.

There are two sides to this and that's the amount of carbon monoxide, which is what we call CO, and also the amount of soot which is much closer to pure carbon. The amount that's formed depends on the carbon to oxygen ratio in the mixture. Coal, for example, and many long-chain hydrocarbons have a much higher carbon content and it's necessary to have a much higher oxygen content in the mixture in order to avoid the formation of what we call lower oxides or soot. So if there's lots of carbon there it's much easier to get something wrong and not have enough oxygen to take it all the way to CO2.

I understand the point you're making and it's a good one. You're half right and half not right and the reason is as follows: the north pole is ice that's already floating. If that melts, you're quite right, it's never going to overflow because when it melts it will displace an equal volume of water as itself. As it's made of water it will just turn into water and never overflow. But here's the spanner in the works: Greenland for example and Antarctica are continents and there's land under there. The ice is not under water but sitting on land, so if that melts it's going to raise sea levels drastically. In fact there's enough ice locked up in Antarctica and Greenland to give us about 7 - 70 metres of sea level rise. There's a paper that was published in this week's edition of the journal Nature in which scientists have used two satellites in space. It's called GRACE but should be called BRACE because they work in concert with each other, and they work out how much mass there is on Earth underneath them by working out how fast one is accelerating because of gravity compared with the other one. What they've done is to watch Greenland for the last two years and they've found that Greenland has lost 248 cubic kilometres, plus or minus about 60, of water every single year in the past two to four years. It's gone up 250%, so it's a scary amount. That is enough to raise sea levels every single year by about 0.5 millimetres. That's Greenland on its own in one year. So if things really do take off with global warming, we're in trouble.

The calorie content, which is actually kilocalories, is the energy that would be obtained in an experiment were that food to be burned in an excess of oxygen.