By Rachel Berkowitz
During photosynthesis, plants use the Sun's energy to split water molecules into oxygen and hydrogen atoms. Hydrogenase enzymes then catalyze the oxidation, or loss of electrons, of molecular hydrogen (H2). When manmade fuel cells produce electricity by breaking the H–H bond and releasing electrons, they also need a catalyst to facilitate the reaction. Designing such a catalyst from abundant and inexpensive material is one focus of the Center for Molecular Electrocatalysis at Pacific Northwest National Laboratory (PNNL) in Washington State.
The catalyst of choice in fuel cells is currently platinum, which is a rare metal. Right now, a die-sized piece costs around $1000. To bring down the cost of fuel cells, a more common metal is needed. Because platinum’s role as a catalyst is not completely understood, developing an efficient and rugged substitute requires a more complete understanding of how platinum operates as a catalyst.
Protein crystallographers have demonstrated that natural hydrogenases, in contrast to manmade catalysts, contain cheap, abundant metals such as iron and nickel. Natural hydrogenases also contain small, dangling molecular chains, called pendant amines, that strategically position nitrogen atoms. The N atoms, which act as bases and help move protons during H2 oxidation, inspired the PNNL team led by Tianbiao Liu to synthesize an iron-based molecule that replicates the function of the natural enzyme.
In the new catalyst, the pendant amine is incorporated into a larger six-sided phosphorus-containing ring to keep it in the optimal position to interact with the hydrogen molecules. The active iron center of the pendant amine breaks the hydrogen molecule into a proton (H+) and a hydride (H−) ion. The proton lands on the catalyst's nitrogen, and the hydride transfers to the iron, which removes the hydride's electrons. Protons are then moved into solution so that the process can repeat.[1]
“The main focus of what we're looking at is attaching a [nitrogen] base to the ligand to lower the barrier for moving protons,” explains Morris Bullock, director of the PNNL Center. Splitting both hydrogen and deuterium (D2, a slightly heavier form of hydrogen) and using nuclear magnetic resonance spectroscopy, the researchers proved that the cleavage had occurred because atomic H and D recombined to form in solution.
“We've shown that we can heterolytically cleave H2 into H+ and H−," says Bullock. "But this is not a catalyst for the complete oxidation of H2."
Oxidation also releases protons into solution. An organic base external to the catalyst was therefore added to take up the dissolved protons. However, the base chemically attached itself to the iron in the catalyst, blocking the H2 from getting into the active site. Preventing that attachment, possibly by increasing the size of the organic base, is necessary for the catalyst to fully oxidize hydrogen.
The PNNL research aims to re-create the functional features of nature's version of a catalyst to get the right reactions to occur. Other research groups take a different approach. They create synthetic molecules whose structures mimic those of natural enzymes, and then they figure out how the molecule works.
The two approaches might seem equivalent, but they’re not. “The reason that the two approaches are not so related is that the structural models are possibly influenced by the protein into which they are embedded,” explains chemist Tom Rauchfuss of the University of Illinois, whose research aims to elucidate structure–function relationships to develop ways of manipulating enzymes. When it comes to technology, he favors the PNNL approach, but he points out that no one has yet developed a catalyst that is as fast and needs as little input energy as nature's enzymes.[2]
Whereas the goal is to produce catalysts that will lead to useful technological improvements, the current focus is on fundamental understanding of how the reactions work. And whether hydrogenase-inspired research will lead to new methods of chemical synthesis or more directly to energy conversion devices remains to be seen. But for now, Rauchfuss sees “a rich vein of basic chemical insights to be mined” in this most fruitful mechanistic approach.
References
- 1. T. L. Liu et al., J. Am. Chem. Soc. 134, 6257 (2012).
- 2. F. Gloaguen, T. B. Rauchfuss, Chem. Soc. Rev. 38, 100 (2009).
