U.S. and UK researchers have developed a method of producing hydrogen peroxide on demand through a simple, one-step process. The method enables diluted H2O2 to be made directly from hydrogen and oxygen in small quantities on-site, making it more accessible to underdeveloped regions of the world, where it could be used to purify water.

Hydrogen peroxide is typically made in a multi-step, energy-intensive process that requires it to be produced in large quantities and shipped and stored in a highly concentrated form.

Scientists led by Graham Hutchings, professor of physical chemistry at Cardiff University in Wales, have found that bimetallic compounds consisting of palladium and any one of six other elements can effectively catalyze the hydrogenation of oxygen to form hydrogen peroxide. The new catalysts can be made by combining palladium with tin, cobalt, nickel, gallium, indium or zinc and, moreover, can be made without the need to pre-treat the catalyst support with nitric acid.

“Scientists have known for more than a century that palladium metal can catalyze the direct reaction of hydrogen and oxygen to make hydrogen peroxide,” says fellow researcher Christopher Kiely, professor of materials science and chemical engineering at Lehigh University. “Unfortunately, the palladium also rapidly hydrogenates or decomposes the hydrogen peroxide that is produced to form water.

“In 2009, we figured out that gold-palladium nanoparticles, supported on acid-washed activated carbon, could switch off the second undesired reaction. But gold is expensive. For the catalyst to be industrially competitive, the gold needs to be replaced with a cheaper metal.”

The one-step process allows hydrogen peroxide to be made on-site and used to purify water. Image credit: Pixabay.The group found that a catalyst composed of palladium and tin could carry out the reaction to form hydrogen peroxide just as effectively as the gold-palladium catalyst. Subsequent tests showed that another five metals, in combination with palladium, also performed very well.

They then used a variety of electron microscopy techniques to determine why palladium alloys caused the hydrogen peroxide that was produced to decompose and how this second reaction could be prevented. The answer had to do with variations in the size and composition of the metal alloy catalyst particles.

“When you make a catalyst, you always tend to generate catalyst particles that span a range of different sizes," says Kiely. "When we measured the composition of the gold-palladium particles, it turned out that the larger particles contained a lot of gold, while the smaller particles had a lot of palladium. Only the medium-sized particles had the right composition."

The group deposited a palladium-tin mixture onto a titanium dioxide (TiO2) support and observed that some of the tin spread out to form a very thin tin oxide layer over the TiO2, while the remainder was consumed in making palladium-tin alloy particles. They developed a three-step heat treatment process that induced the secondary tin-oxide support layer to encapsulate the ultra-small palladium-rich particles. This served to muzzle the nuisance particles and prevent them from catalyzing the hydrogenation and decomposition of the hydrogen peroxide.

“The heat treatment induced the tin-oxide layer to crawl over and effectively bury the small palladium-rich particles and stop them from working,” says Kiely. “More importantly, the larger palladium-tin alloy particles, which efficiently generate the hydrogen peroxide, were unaffected. This phenomenon is normally not desired: you don’t usually want the metal particles to be covered with thin oxide layers because that deactivates the catalysts. However, in this case we have managed to use the phenomenon selectively to deactivate only the small detrimental particles, while leaving the larger beneficial alloy particles free to do their work.”

“It was a long, hard slog to develop the material,” he says, “but the resulting final catalyst has excellent performance characteristics.”