Chemists at the University of Kansas and the US Department of Energy’s (DOE) Brookhaven National Laboratory have unraveled the entire reaction mechanism for a key class of water-splitting catalysts. Their work was published in Proceedings of the National Academy of Sciences (PNAS).
It’s very rare that you can get a complete understanding of a full catalytic cycle. These reactions go through many steps, some of which are very fast and cannot be easily observed.—Brookhaven chemist Dmitry Polyansky, a co-author of the paper
Rapid intermediate steps make it difficult for scientists to decipher exactly where, when, and how the most important parts of a catalytic reaction occur—and therefore, if the catalyst is suitable for large-scale applications.
At the University of Kansas, associate professor James Blakemore was researching possible candidates when he noticed something unusual about one catalyst in particular. This catalyst, called a pentamethylcyclopentadienyl rhodium complex, or Cp*Rh complex, was demonstrating reactivity in an area where molecules are usually stable.
Metal complexes—molecules that contain a metal center surrounded by an organic scaffold—are important for their ability to catalyze otherwise difficult reactions. Typically, reactivity happens directly at the metal center, but in our system of interest, the ligand scaffold appeared to directly take part in the chemistry.—James Blakemore, corresponding author
So, what exactly was reacting with the ligand? Was the team really observing an active step in the reaction mechanism or just an undesirable side reaction? How stable were the intermediate products that were produced? To answer questions like these, Blakemore collaborated with chemists at Brookhaven Lab to use a specialized research technique called pulse radiolysis.
Pulse radiolysis harnesses the power of particle accelerators to isolate rapid, hard-to-observe steps within a catalytic cycle. Brookhaven’s Accelerator Center for Energy Research (ACER) is one of only two locations in the United States where this technique can be conducted, due to the Lab’s advanced particle accelerator complex.
We accelerate electrons, which carry significant energy, to very high velocities. When these electrons pass through the chemical solution we’re studying, they ionize the solvent molecules, generating charged species that are intercepted by the catalyst molecules, which rapidly alter in structure. We then use time-resolved spectroscopy tools to monitor the chemical reactivity after this rapid change occurs.—Brookhaven chemist David Grills, co-author
Spectroscopic studies provide spectral data, which can be thought of as the fingerprints of a molecule’s structure. By comparing these signatures to known structures, scientists can decipher physical and electronic changes within the short-lived intermediate products of catalytic reactions.
Pulse radiolysis allows us to single out one step and look at it on a very short timescale. The instrumentation we used can resolve events at one millionth to one billionth of a second.—Dmitry Polyansky
By combining pulse radiolysis and time-resolved spectroscopy with more common electrochemistry and stopped-flow techniques, the team was able to decipher every step of the complex catalytic cycle, including the details of the unusual reactivity occurring at the ligand scaffold.
One of the most remarkable features of this catalytic cycle was direct involvement of the ligands. Often, this area of the molecule is just a spectator, but we observed reactivity within the ligands that had not yet been proven for this class of compounds. We were able to show that a hydride group, an intermediate product of the reaction, jumped onto the Cp* ligand. This proved that the Cp* ligand was an active part of the reaction mechanism.—David Grills
Capturing these precise chemical details will make it significantly easier for scientists to design more efficient, stable, and cost-effective catalysts for producing pure hydrogen.
The researchers also hope their findings will provide clues for deciphering reaction mechanisms for other classes of catalysts.
This work was supported by the National Science Foundation and the DOE Office of Science.
Henker, Wade et al. (2023) “Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes” PNAS doi: 10.1073/pnas.2217189120