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. Credit: American Chemical Society
It 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. 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 to make products 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. It 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.
The commonly accepted mechanism for direct synthesis of H2O2 essentially states that H2, O2 atoms bind adjacent to one another on the catalyst surface and then react, Wilson said. To better understand what was going on, he spent over a year building a reactor, fine-tuning experimental procedures, then collecting and analyzing reaction rate data. “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 grad student in Flaherty’s lab.
Instead of reacting together on the surface of the catalyst (the palladium cluster), the H atoms dissociate into 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,” Wilson said.
“The reason this is critical,” Flaherty said, “is because it gives us guidance for how to make the next generation of these materials. This is all motivated by trying to make hydrogen peroxide more cheaply so it can be manufactured more easily, so we can use it in place of chlorine. But we didn’t know how to go about making a catalyst that was better than what we have now.”
The research group is now looking into another catalyst, gold-palladium, which has been shown in previous work to be very selective towards H2O2. Flaherty’s lab is exploring different ways of “coupling this chemistry directly with reactions that use hydrogen peroxide for green oxidations within very short length scales,” that is, micrometers away, Flaherty said. “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.” http://engineering.illinois.edu/news/article/14948
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