“Good catalysts efficiently move protons and electrons around, taking them from some molecules and placing them onto others to produce the desired product,” he explained. “Nature has many ways of doing this. Under the right conditions, the hydroxyl groups on the diimine ligand of the catalyst help hydrogen react with carbon dioxide, which is difficult to do. We thought we could improve the reactivity by placing the pendent bases near the metal centers, rather than in peripheral positions.”
Once the Brookhaven team understood how Himeda’s catalysts worked, Hull realized that a novel ligand that had been synthesized by collaborators Brian Hashiguchi and Roy Periana of The Scripps Research Institute for an entirely different purpose would possibly be ideal for accomplishing this goal. The Brookhaven group designed a new iridium metal catalyst incorporating this new ligand.
Collaborator David Szalda of Baruch College (City University of New York) determined the atomic level crystal structure of the new catalyst to “see” how the arrangement of its atoms might explain its function.
Tests of the new catalyst revealed superior catalytic performance for storing and releasing H2 under very mild reaction conditions. For the reaction combining CO2 with H2, the scientists observed high turnovers at room temperature and ambient pressure; for the catalytic decomposition of formic acid to release hydrogen, the catalytic rate was faster than any previous report.
“We were able to convert a 1:1 mixture of H2 and CO2 to formate (the deprotonated form of formic acid) at room temperature, successfully regenerate H2, and then repeat the cycle. It’s a design principle we are very fortunate to have found,” said Hull.
The regenerated high-pressure gas mixture (hydrogen and carbon dioxide) is quite pure; importantly, no carbon monoxide (CO) — an impurity that can ‘poison’ fuel cells and thus reduce their lifetime — was detected. Therefore, this method of storing and regenerating hydrogen might have a use in hydrogen fuel cells.
Further efforts to optimize the hydrogen storage process are ongoing using several catalysts with the same design principle.
“This is a wonderful example of how fundamental research can lead to the understanding and control of factors that contribute to the solution of technologically important problems,” Muckerman concluded.
This research was funded by the DOE Office of Science, a Goldhaber Distinguished Fellowship, and by the Japanese Ministry of Economy, Trade, and Industry.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.