Chemistry Of Producing Pure Hydrogen From Non-Fossil Fuels Revealed

Chemists have laid bare the complete reaction mechanism for an important group of 'water-splitting' catalysts, taking scientists closer to making pure hydrogen from renewable energy sources.

The chemists at the University of Kansas and the U.S. Department of Energy's Brookhaven National Laboratory achieved this through pulse radiolysis experiments, the results of which are published in the journal Proceedings of the National Academy of Sciences.

'Understanding how the chemical reactions that make clean fuels like hydrogen work is very challenging," said co-author James Blakemore, associate professor of chemistry, whose research in Lawrence, Kansas, forms the basis of the discovery.

"Our paper presents data hard-won from specialised techniques to understand how a certain catalyst for hydrogen generation does the job. Implementing these (techniques) allowed us to get a full picture of how to make hydrogen from its constituent parts, protons and electrons," said Blakemore.

Blakemore took his research from KU to Brookhaven for research using pulse radiolysis as well as other techniques.

Pulse radiolysis is a method of initiating fast reactions to study reactions occurring on a timescale faster than approximately one hundred microseconds when simple mixing of reagents is too slow and other methods of initiating reactions have to be used.

"It's very rare that you can get a complete understanding of a full catalytic cycle," said Brookhaven chemist Dmitry Polyansky, a co-author of the paper. "These reactions go through many steps, some of which are very fast and cannot be easily observed."

Blakemore and his collaborators made their discovery by studying a catalyst that is based on a pentamethylcyclopentadienyl rhodium complex, which is [Cp*Rh] for short. They focused on the Cp* (pronounced C-P-'star') ligand paired with the rare metal rhodium because of hints from prior work showing the suitability of this combination for the work.

"Our rhodium system turned out to be a good target for the pulse radiolysis.

"The Cp* ligands, as they're called, are familiar to most organometallic chemists, and really chemists of all stripes.

"They're used to support many catalysts and can stabilise a variety of species involved in catalytic cycles. One key finding of this paper gives fresh insight into how the Cp* ligand can be intimately involved in the chemistry of hydrogen evolution," said Blakemore.

Besides producing clean hydrogen, Blakemore stressed that the findings could lead to other improved chemical processes as well.

"In our work, we hope that chemists will see a study about how a common ligand, Cp*, can enable unusual reactivity.

"This unusual reactivity is relevant to the hydrogen story, but it's actually bigger than this because Cp* is found in so many different catalysts.

"Chemists normally think of catalysts as being based on metals. In this way of thinking, if you're making a new molecule, the metal is the key actor that brings the constituent parts together.

"Our paper shows that this isn't always the case. Cp* can be involved in stitching the pieces together to form products," said Blakemore.