Science seeks alternatives to expensive, rare catalysts

By strict definition, a catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Being such a utilitarian concept, the word takes on other applications outside of the world of chemistry from which it evolved.

It has come to mean stimulus or a spark, such as an event that initiated a social movement or debate. The assassination of Archduke Ferdinand was a catalyst for World War I, and Hitler’s invasion of Poland catalyzed World War II.

From a chemical perspective, a catalyst lowers the activation energy required for a chemical reaction to proceed but is not consumed in the ensuing reaction.

A chemical reaction is nothing more than a reorganization of atoms. One of the simplest cases rearranges two oxygen atoms that share electrons to form a molecule of oxygen gas and two hydrogen atoms that share electrons to form a molecule of hydrogen gas.

In the new arrangement, the two hydrogen atoms bond with one oxygen atom to form a molecule of water. The other oxygen atom requires another molecule of broken hydrogen gas to make a water molecule, so it takes twice as much hydrogen as oxygen to make water from “scratch.”

Adding energy with fire or electric spark will cause an explosive reaction, but this is not catalysis since the activation energy came from an outside source.

An example of catalysis in the formation of water is using certain precious metals such as platinum, palladium, ruthenium or iridium. These expensive and difficult-to-get metals, especially platinum, interact with both oxygen and hydrogen just the right amount.

Platinum will not form compounds with either oxygen or hydrogen, but it acts as a matchmaker, pulling the molecules close together to speed the reaction. This is the principle of operation of fuel cells that combine hydrogen and oxygen to produce electricity that spacecraft have used since the days of the Apollo moon program.

Biological catalysts are so important that complex life processes could not proceed without them. Without enzymes, reactions between substrates, as reactant molecules are called before the reaction occurs, would occur too slowly for life processes to take place.

Biological enzymes break food into nutrients and help cells make energy or perform other cellular functions. They are usually proteins that have specific sites where substrate molecules will bind. Those sites hold the substrates in place like pieces of a jigsaw puzzle so that they can exchange electrons to react chemically.

The enzyme protein molecules might change shape by bending or stretching in order to place the substrate molecules strategically in position to overcome the activation energy and cause the reaction to proceed at a sufficiently fast rate to serve the purpose.

Because the precious metals are rare and mining has ecological repercussions, scientists are searching for new substances that can act as catalysts for the many reactions used in manufacturing.

Carbon is promising because it is cheap, abundant and can be assembled into many structures. Graphene nanotubes are flat sheets of orderly hexagonal rings like chicken wire rolled into tubes. When nitrogen or phosphorus atoms replace some of the carbon, they change the electric charge distribution and make the carbon act more like a metal.

Clumps of carbon nanotubes doped with nitrogen and aligned like fistfuls of spaghetti show promise as stand-ins for platinum inside fuel cells. Another doping method involves microwaving a mixture of carbon and a type of acid containing phosphorus into sticky soot that breaks down cellulose into smaller glucose units.

Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.