Research reveals mechanism direct synthesis hydrogen peroxide

Research reveals mechanism for direct synthesis of hydrogen peroxide

9:20 AM, 25th January 2016
Research reveals mechanism for direct synthesis of hydrogen peroxide
Instead of reacting together on the surface of the catalyst, the hydrogen atoms dissociate into their components, protons and electrons. The protons enter the surrounding solution of water and methanol, while the electrons flow through the palladium itself into oxygen molecules. © ACS

CHAMPAIGN, US: From the polyurethane that makes our car seats to the paper made from bleached wood pulp, chlorine can be found in a variety of large-scale manufacturing processes. But while chlorine is good at activating the strong bonds of molecules, which allows manufacturers to synthesize the products we use on a daily basis, it can be an insidious chemical, sometimes escaping into the environment as hazardous byproducts such as chloroform and dioxin.

As a result, scientists and companies have been exploring a more environmentally benign alternative to chlorine—hydrogen peroxide, or H2O2. But it is an expensive reactant. Hydrogen peroxide is typically made in big, centralized facilities and requires significant energy for separation, concentration, and transportation.

A handful of large-scale facilities around the globe have begun to produce H2O2 using the current process, but at the same facilities as the polyurethane precursors, which results in significant cost and energy savings and reduces environmental impact. Ideally smaller-scale factories would also be able to make hydrogen peroxide on site, but this would require a completely different set of chemistry, direct synthesis of H2O2 from hydrogen and oxygen gas, which has long been poorly understood according to University of Illinois researchers.

New research from David Flaherty, assistant professor of chemical and biomolecular engineering, and graduate student Neil Wilson reveals the mechanism for the direct synthesis of H2O2 on palladium cluster catalysts, and paves the way to design improved catalysts to produce H2O2 to use in place of harmful chlorine, regardless of the scale of the production facility.

The research is published in the Journal of the American Chemical Society.

The commonly accepted mechanism for direct synthesis of H2O2 essentially states that hydrogen and oxygen atoms bind adjacent to one another on the catalyst surface and then react, Wilson said.

“What people thought was happening is after the hydrogen atoms broke apart and they’re adsorbed onto the palladium surface, that they just reacted with the oxygen on the surface. But that’s not really consistent with what we saw,” said Wilson, a fourth-year graduate student in Flaherty’s lab and first author of the article, "Mechanism for the direct synthesis of H2O2 on Pd Clusters: Heterolytic reaction pathways at the liquid–solid interface."

Instead of reacting together on the surface of the catalyst (the palladium cluster), the hydrogen atoms dissociate into their components-protons and electrons. The protons enter the surrounding solution of water and methanol, while the electrons flow through the palladium itself into oxygen molecules.

“When oxygen comes down onto the surface, it can react with pairs of protons and electrons to form hydrogen peroxide,” said Wilson.

Researchers will now have a better sense of what is happening at the catalyst surface and an appreciation for the role of proton and electron transfer processes in this chemistry. It was not recognized that the oxygen on the surface reacted with liquid phase species, and that the formation of H2O2 by direct synthesis is, therefore, strongly influenced by the solution itself. However, the formation of water (the undesired side reaction) is mostly influenced by properties of the catalyst surface.

“Now that we understand what’s happening on the surface, we can start pushing towards rational catalyst design,” said Wilson. The research group is now looking into another catalyst, gold-palladium, which has been shown in previous work to be very selective towards H2O2. “People still don’t entirely know why gold-palladium is so selective,” Wilson added, but it seems that this new mechanistic insight will help to explain the selectivity of these materials.

“If we can put these H2O2 formation catalysts very close to something which performs the oxidation reaction, we can avoid the entire problem or concentrating and transporting hydrogen peroxide,” said Flaherty.

© University of Illinois News

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