Catalysts

A catalyst is a chemical that speeds up a reaction without being used up itself. It is like a matchmaker for chemistry: it brings reacting molecules together and helps them do their thing, then walks away unchanged, ready to help again. Without catalysts, modern life would be unrecognisable. Catalysts are in your car exhaust, in laundry detergent, in your stomach, and inside every cell of your body. They are the unsung speed boosters that make the chemistry of life and industry possible.

• What it doesSpeeds up reactionsWithout being used up
• HowLowers activation energyMakes reactions easier to start
• In your bodyEnzymesCatalysts made of protein
• Car exhaustCatalytic converterCleans exhaust gases
• Famous lab catalystPlatinumUsed in petrol refining
• Speed up100x to 10^17x fasterSome are millions of times faster

How catalysts work

To start a chemical reaction, the reacting molecules usually have to collide hard enough to overcome a kind of "energy hill" called the activation energy. If they do not have enough energy, they just bounce off each other and nothing happens.

A catalyst provides a different route for the reaction, one that has a much lower activation energy hill. Molecules can now reach the products much more easily, so the reaction goes faster. Importantly, the catalyst itself is not used up; it does its job, comes free, and goes on to help the next pair of molecules.

Catalysts in your body

Your body runs thousands of chemical reactions every second. Almost all of them rely on catalysts called enzymes: large protein molecules shaped to fit specific reacting molecules like keys in locks. Each enzyme catalyses one particular reaction (or a small family of similar reactions).

Examples of enzymes:

• Amylase: in saliva, starts breaking down starch into sugar as you chew bread.
• Pepsin: in stomach acid, chops protein molecules into smaller pieces.
• Lipase: in the gut, breaks fats into fatty acids.
• DNA polymerase: copies DNA when cells divide.
• ATP synthase: builds the molecule (ATP) that carries energy around your cells.

Without enzymes, the chemistry of life would happen far too slowly to be useful. Some reactions that take milliseconds with the help of an enzyme would take thousands of years without one.

Fact Enzymes are typically millions or billions of times faster than uncatalysed reactions. The enzyme catalase, found in your liver and in raw potatoes, can break down 40 million molecules of hydrogen peroxide every second. Pour hydrogen peroxide on a piece of raw liver and watch it foam furiously. That foam is oxygen gas being released as catalase rips through the chemical.

Catalysts in industry

Modern industry is built on catalysts. Without them, dozens of crucial chemicals would be too expensive or too slow to make. Some examples:

• Haber process: nitrogen and hydrogen are combined to make ammonia for fertilisers, using an iron catalyst. This single reaction feeds nearly half the people on Earth.
• Catalytic cracking: huge molecules in crude oil are broken into smaller useful ones (like petrol) using zeolite catalysts. This makes most of the worlds fuel.
• Margarine making: vegetable oil + hydrogen + nickel catalyst -> a solid fat that can be spread like butter.
• Sulfuric acid manufacture: a vanadium oxide catalyst speeds up the conversion of SO2 to SO3, the key step in the contact process.
• Plastic manufacture: many plastics are made by linking small molecules into long chains, using catalysts to control the chain length and structure.

Catalytic converters

Every modern petrol or diesel car has a catalytic converter in its exhaust system. Inside is a honeycomb ceramic structure coated with a thin layer of platinum, palladium and rhodium. These metals are excellent catalysts for cleaning up exhaust gases.

As the hot exhaust flows through the honeycomb, the catalysts speed up reactions that:

• Convert poisonous carbon monoxide (CO) to carbon dioxide (CO2)
• Burn unburnt fuel molecules into CO2 and water
• Break down nitrogen oxides (NOx, a major cause of smog and acid rain) back into nitrogen and oxygen

The catalysts themselves are not used up. A converter can last 10 years or more. The amounts of precious metal used are tiny: just a few grams of platinum, palladium and rhodium per car. Even so, the worldwide demand for these metals from car makers is so high that thieves sometimes saw catalytic converters off the bottom of parked cars to sell for scrap.

Did you know? Biological washing powders contain enzymes (catalysts) that break down protein and grease stains. The enzymes are picked to work at the relatively low temperatures used in modern washing machines (30 to 40 degrees Celsius), saving lots of electricity compared with older powders that needed boiling water. Enzymes get washed away in the rinse, so the powder also contains plenty more for the next wash.

How conditions affect catalysts

Catalysts often only work properly within a narrow range of conditions.

• Temperature: most enzymes work best around 37 degrees Celsius (body temperature). Above 50 degrees, enzymes start to break apart and stop working. That is why high fever is dangerous and why cooking food kills any enzymes in it.
• pH: enzymes are sensitive to pH. Pepsin in your stomach works best at very acidic pH 2; trypsin in your intestines works best at slightly basic pH 8.
• Poisoning: some chemicals stick to a catalysts active sites and block them. Lead in old leaded petrol used to ruin catalytic converters. That is one reason petrol is now lead-free.

Try this Pour about 100 ml of supermarket hydrogen peroxide solution (the kind used as a mild antiseptic, around 3 to 6 per cent) into a tall glass (with adult supervision). Add a teaspoon of washing-up liquid and a tiny drop of food colouring. Sprinkle in half a teaspoon of dried yeast (the kind used for baking). The yeast contains catalase, an enzyme that catalyses the breakdown of H2O2 into water and oxygen. Within seconds the glass overflows with thick coloured foam. You have just watched a catalyst at work, generating thousands of oxygen bubbles in seconds where the uncatalysed reaction would take hours.

Deeper dive: how a German chemist saved the world from starvation

In 1900, the world had a problem. The human population had reached about 1.6 billion people and was growing fast. Crops needed nitrogen-rich fertilisers to grow well, but nitrogen was tricky. Although nearly 80 per cent of the air is nitrogen (N2), the gas is very unreactive. Plants cannot use it directly. They need nitrogen in the form of nitrates or ammonia.

Farmers had relied for centuries on natural sources: bird droppings (guano) from Pacific islands, mined nitrate deposits in Chile and animal manure from livestock. These were running out, and the world risked mass famine within a generation if a new source of nitrogen could not be found.

In 1909, German chemist Fritz Haber and engineer Carl Bosch worked out how to combine nitrogen gas from the air directly with hydrogen to make ammonia. The key was an iron catalyst, plus very high temperatures (around 450 degrees Celsius) and pressures (200 atmospheres). On its own, the reaction is so slow as to be useless. With the iron catalyst, it produces enough ammonia to feed the world.

The Haber-Bosch process is one of the most important inventions in human history. About half of the nitrogen in your body atoms today was first turned from air into ammonia in a Haber-Bosch reactor. The fertiliser made by the process feeds nearly half of all humans alive today. Without it, the world population would probably never have grown beyond about 3 to 4 billion.

The process also has a dark side. The same chemistry that made fertiliser also made it possible to mass-produce explosives, which helped fuel both World Wars. Today, the worlds Haber-Bosch reactors use about 2 per cent of all the worlds energy and release a lot of CO2. Chemists are looking for greener ways to make ammonia, perhaps with new biological catalysts copied from nitrogen-fixing bacteria in plant roots. The next chapter in this story is still being written.

For more, see what is a chemical reaction and endothermic and exothermic.