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Diamonds Mediate Photochemistry

Materials: Ejected electrons serve as powerful reducing agents under mild conditions

by Mitch Jacoby
July 1, 2013 | A version of this story appeared in Volume 91, Issue 26

Credit: Robert Hamers
Shining light (purple arrow) on H-capped diamond (red and black) ejects electrons (yellow), which are quickly surrounded by water molecules (green and red) and can react with protons (orange) and N2 (blue) to produce ammonia (blue and red).
A schematic showing how a diamond immersed in water reacts with light to catalyze an ammonia-producing reaction.
Credit: Robert Hamers
Shining light (purple arrow) on H-capped diamond (red and black) ejects electrons (yellow), which are quickly surrounded by water molecules (green and red) and can react with protons (orange) and N2 (blue) to produce ammonia (blue and red).

The sparkle and beauty of diamonds may have made them Marilyn Monroe’s best friend, but a curious electronic property is sure to endear them to chemists as well. Researchers show that a diamond property known as negative electron affinity (NEA) can be exploited under gentle conditions to drive the same industrially important reactions that are normally carried out at high temperatures and pressures (Nat. Mat. 2013, DOI: 10.1038/nmat3696). The finding could lead to energy savings and facilitate reactions that are otherwise difficult to carry out.

Hydrogen-capped diamond, a common form of the material, readily emits electrons when it’s irradiated with ultraviolet light. University of Wisconsin, Madison, chemists have now tapped into the easily stimulated photoemission, a consequence of diamond’s NEA, by using diamond as an electron source to drive chemical reactions.

The team demonstrates that shining UV light on diamond that is immersed in nitrogen-charged water causes electrons to be emitted and quickly interact with water molecules. These solvated electrons function as potent reducing agents that can react with protons and nitrogen to form ammonia.

Typically the nitrogen-to-ammonia reaction is practiced on an industrial scale at temperatures near 500 °C and pressures in the 150- to 250-atm range. The harsh conditions are required because N2 binds weakly to solid-state catalysts and the reaction proceeds via high-energy intermediates. In contrast, the diamond-mediated reaction occurs in water at room temperature.

The team, which includes Di Zhu, Linghong Zhang, Rose E. Ruther, and Robert J. Hamers, detected solvated electrons directly by using a technique known as transient absorption spectroscopy. And they used a method based on UV-visible absorption spectroscopy to measure yields of ammonia produced in specially designed photochemical cells. The team confirmed the origin of the ammonia by purging the cells with isotopically labeled N2. They also compared various forms of diamond and report that even inexpensive diamond grit, a common polishing compound, stimulates reduction of nitrogen to ammonia. Now the group is focusing on understanding the underlying reaction mechanism.

This study neatly demonstrates that diamond’s NEA can be applied to reducing nitrogen to ammonia in a liquid, says Northwestern University’s Robert P. H. Chang, a diamond specialist. He adds that this is a unique way to mediate reduction processes when solid-state catalysts pose difficulties.

Describing the study as “very impressive,” Princeton University chemist Andrew B. Bocarsly, an expert in CO2 electroreduction (see also page 21 of this issue), notes that “these findings may help provide new and important mechanistic insight into the redox chemistry of CO2.” Better understanding in that area may lead to ways to use CO2 as a feedstock for making fuels and chemicals, he notes.



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