Reducing nitrogen to ammonia takes place at ambient temperature and pressure in water when synthetic industrial diamond serves as the catalyst, report researchers from the University of Wisconsin – Madison. Illuminating hydrogen-terminated diamond with deep ultraviolet light prompts the reduction, says Robert J. Hamers, a professor of chemistry at the school.
Their experiments demonstrate a fundamentally new photo-catalysis mechanism in which electrons are emitted directly into the reactant fluid, using H-terminated diamond, a solid-state source of electrons with high reducing power, the researchers declare in a recent article in Nature Methods. "The stability and negative electron affinity of diamond distinguish it from traditional photo-emitters. While demonstrated here for N2 reduction to NH3, solvated electrons should also enable the reduction of other difficult-to-reduce species such as CO2," the paper states.
"The unique properties of diamond are best suited to very difficult reduction reactions that can't be accommodated with other catalysts such as metal oxides. So, N2 reduction is really an ideal reaction. But surely there are others as well, such as nitrogen oxides," notes Hamers.
"We're already better than the previously best known catalyst for room-temperature nitrogen reduction, but there's a long way to go." The need to use deep ultraviolet light for activation is a limitation, he admits.
"The next step is to try to achieve visible light activation. We observe some reaction using visible light, but it's very small. We are also investigating how to manipulate the properties of diamond, both the bulk and the surface properties, to improve efficiency."
"This is truly a different way of thinking about inducing reactions that may have more efficiency and applicability. We're doing this with diamond grit. It is infinitely reusable," Hamers says. "We've done that [regenerated the catalyst] many times already just by heating it in hydrogen gas or exposing it to a hydrogen plasma, in both cases to get rid of the surface oxygen. Full efficiency is restored."
The researchers also are studying the applicability of diamond catalyst to carbon dioxide reduction. "CO2 reduction is a bit more tricky because there are a number of different reaction products that can be produced, and also because there are other competing catalysts. But diamond can, in principle, catalyze the direct one-electron reduction of CO2 to form CO2-, while other catalysts have to go through more complex routes know as proton-coupled electron transfer reaction… and so are, in principle, less efficient than a direct one-electron reduction."