Explainer: What is a catalyst? | Science News for Students

Explainer: What is a catalyst?

These substances help speed up chemical reactions so that they proceed efficiently
Feb 27, 2017 — 12:00 pm EST
platinum rocks

The element platinum can be used as a catalyst in some chemical reactions.


Catalysts are the unsung heroes of the chemical reactions that make human society tick. A catalyst is some material that speeds up chemical reactions. With a helping hand from a catalyst, molecules that might take years to interact can now do so in seconds. Factories rely on catalysts to make everything from plastic to drugs. Catalysts help process petroleum and coal into liquid fuels. They’re key players in clean-energy technologies. Natural catalysts in the body — known as enzymes — even play important roles in digestion and more.

During any chemical reaction, molecules break chemical bonds between their atoms. The atoms also make new bonds with different atoms. This is like swapping partners at a square dance. Sometimes, those partnerships are easy to break. A molecule may have certain properties that let it lure away atoms from another molecule. But in stable partnerships, the molecules are content as they are. Left together for a very long period of time, a few might eventually switch partners. But there’s no mass frenzy of bond breaking and rebuilding.

Catalysts make such a breaking and rebuilding happen more efficiently. They do this by lowering the activation energy for the chemical reaction. Activation energy is the amount of energy needed to allow the chemical reaction to occur.  The catalyst just changes the path to the new chemical partnership. It builds the equivalent of a paved highway to bypass a bumpy dirt road. A catalyst doesn’t get used up in the reaction, though. Like a wingman, it encourages other molecules to react. Once they do, it bows out.

Enzymes are biology’s natural catalysts. They play a role in everything from copying genetic material to breaking down food and nutrients. Manufacturers often create catalysts to speed processes in industry.

One technology that needs a catalyst to work is a hydrogen fuel cell. In these devices, hydrogen gas (H2) reacts with oxygen gas (O2) to make water (H2O) and electricity. These systems can be found in a hydrogen vehicle where they create the electricity to power the engine. The fuel cell needs to separate the atoms in molecules of hydrogen and oxygen so that those atoms can reshuffle to create new molecules (water). Without some assistance, though, that reshuffling would take place very slowly. So the fuel cell uses a catalyst — platinum — to propel those reactions along.

catalytic converter
Today’s cars rely on a catalytic converter, like the one shown in cross-section here. Such devices help break down exhaust gases into chemicals (such as water) that are less toxic to the environment.

Platinum works well in fuel cells because it interacts just the right amount with each starting gas. Platinum’s surface attracts the gas molecules. In effect, it pulls them close together so that it encourages — speeds along — their reaction. Then it lets its handiwork float free.

For years, other technologies have relied on platinum catalysts, too. To remove harmful pollutants from exhaust gases, for instance, cars now rely on catalytic converters.

But platinum has some downsides. It’s expensive, for one. (People like to use it in fancy jewelry.) And it isn’t easy to obtain.

Some other catalysts have risen to superstar status. These include metals with chemical properties similar to platinum’s. Among them are palladium and iridium. Like platinum, however, both are expensive and hard to get. That’s why the hunt is on for less costly catalysts to use in fuel cells.

Some scientists think that carbon molecules might work. They certainly would be less costly and readily abundant. Another option might be to use enzymes similar to those found inside living things.

Power Words

(for more about Power Words, click here)

activation energy     (in chemistry) The minimum energy needed for a particular chemical reaction to take place.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bond     (in chemistry) A semi-permanent attachment between atoms — or groups of atoms — in a molecule. It’s formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

catalyst    A substance that helps a chemical reaction to proceed faster. Examples include enzymes and elements such as platinum and iridium.

catalytic converter     A device made of honeycomb-shaped ceramic structures that is put onto the tailpipe of a vehicle. As exhaust gases flow through it, they encounter two different types of catalysts, each able to foster a different type of chemical reaction. One or more metals, usually platinum, rhodium, palladium — and sometimes even gold — coat the inside of the system. All of the walls of the device’s honeycomb structure increase greatly the area of catalyst-covered surfaces now available to react with the exhaust. As the gases from the engine hit these metal coated surfaces, they break apart the pollutants, turning them into less harmful materials. A sensor in the converter also measures how much oxygen is in the exhaust. If it finds too much, it tells a computer to adjust the air-to-fuel ratio in the engine so that it will burn more cleanly.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemical bonds     Attractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

electricity     A flow of charge, usually from the movement of negatively charged particles, called electrons.

engine     A machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor.

enzymes     Molecules made by living things to speed up chemical reactions.

exhaust     (in engineering) The gases and fine particles emitted — often at high speed and/or pressure — by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.

fuel cell     A device that converts chemical energy into electrical energy. The most common fuel is hydrogen, which emits only water vapor as a byproduct.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.

iridium     Discovered in 1803, its name comes from the Latin for rainbow. It’s a hard, brittle and corrosion-resistant metal in the platinum family. Slightly yellowish, the principle use for this element is as a hardener for platinum. Indeed, its melting point is more than 2,400° Celsius (4,350° Fahrenheit). The element’s atomic number is 77.

manufacturing     The making of things, usually on a large scale.

metal     Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

oxygen     A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

palladium     A soft, ductile, steel-white, tarnish-resistant, metallic element occurring naturally with platinum, especially in gold, nickel, and copper ores.

petroleum     A thick flammable liquid mixture of hydrocarbons. Petroleum is a fossil fuel mainly found beneath the Earth’s surface. It is the source of the chemicals used to make gasoline, lubricating oils, plastics and many other products.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

platinum     A naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.


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