Splitting hydrogen: Neutrons reveal clues

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  • Published: May 1, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Splitting hydrogen: Neutrons reveal clues

Neutron perspective

New clues from neutron diffraction studies regarding the behaviour of a hydrogen-splitting catalyst based on the active site of a hydrogenase enzyme could accelerate us towards a hydrogen economy by providing the means to develop more efficient hydrogen fuel cells. Credit: Wiley/Angew et al

New clues from neutron diffraction studies regarding the behaviour of a hydrogen-splitting catalyst based on the active site of a hydrogenase enzyme could accelerate us towards a hydrogen economy by providing the means to develop more efficient hydrogen fuel cells.

A fuel catalyst fed on hydrogen gas must split the molecule as one step in the conversion of chemical to electrical energy. Now, Morris Bullock of the US Department of Energy's Pacific Northwest National Laboratory in Richland, Washington state, and colleagues, researchers have used neutron diffraction studies to capture a snapshot of one such catalyst grasping each half of the hydrogen molecule it has torn in two. This neutron snapshot lends credence to an earlier hypothesis of the mechanism and could provide new insights into how to develop and even more efficient fuel cell catalyst.

They explain where the hydrogen halves reside in their catalytic structure designed to mimic the active site of one of nature's hydrogen-splitting catalysts, a hydrogenase enzyme.

Lowering the price point

"The catalyst shows us what likely happens in the natural hydrogenase system," explains Morris. "The catalyst is where the action is, but the natural enzyme has a huge protein surrounding the catalytic site. It would be hard to see what we have seen with our catalyst because of the complexity of the protein."

Although invented during the rapidly evolving science and technology of the nineteenth century, fuel cells, and in particular, hydrogen-powered fuel cells, are being investigated today as a possible alternative to electricity generation that involves burning fossils fuels or vehicle propulsion that avoids the same. If a sustainable and renewable source of molecular hydrogen can be found, perhaps through solar-powered splitting of water, then we would have a power source for fuel cells the side product from which in generating electricity would simply be water, at least at the local level and excluding energy and materials costs elsewhere in the chain. Electric vehicles powered by "clean" hydrogen or domestic electricity generators might then be feasible. If renewable power is used to store energy in molecular hydrogen, these fuel cells can be carbon-neutral. But at the moment, the noble metal catalysts used in current fuel cells mean they are not yet of sufficiently low cost to be commercial viable for everyday use.

To make fuel cells less expensive, the researchers have turned to natural hydrogenase enzymes for inspiration. These enzymes break hydrogen for energy in the same way a fuel cell would. But nature does not need rare, expensive platinum, for instance, and can run its enzymes with much more abundant and readily available iron, for example. Biomimetic catalysts too might use iron or even nickel at their core. In this research one of the most critical steps is simply the breaking of the covalent bond that holds the two hydrogen atoms together in a hydrogen molecule. However, for the bond's energy to be released ready to generate electricity the split must be asymmetrical with a positively charged proton and a negatively charged hydride ion released in the process.

Nucleus

X-ray crystallography is almost always the mainstay of enzyme structure determination and analyses of heavy metal catalysts. However, to observe the active site in action as it were with any kind of useful precision when hydrogen is involved, requires neutrons rather than X-rays for the diffraction studies. Chemical techniques and X-ray methods have provided good estimates of the positions of the hydrogen ions, but neutron diffraction has now pinned down the precise positions without resorting to imagination.

Bullock, Tianbiao "Leo" Liu and their colleagues at the Center for Molecular Electrocatalysis at PNNL, collaborated with colleagues at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee to locate the proton and hydride in their iron-based catalyst. Key to success was the ability to build much larger crystals than can be used with X-ray crystallography; some forty times larger.

"We were designing a molecule that represented an intermediate in the chemical reaction, and it required special experimental techniques," Liu said. "It took more than six months to find the right conditions to grow large single crystals suitable for neutron diffraction. And another six months to pinpoint the position of the split dihydrogen molecule."

The neutron studies showed that the "barbell-shaped" hydrogen molecule snuggles into the core of their catalyst. But, once split, the hydride bonds to the more "positive" iron at the centre while the proton binds to a nitrogen atom with its overdose of electrons on the other side of the catalytic core. This was what the researchers had anticipated would happen on the basis of theoretical understanding and earlier studies. But, science requires solid evidence for hypotheses not assumptions. In this form, the hydride and the proton form a rarely seen di-hydrogen bond, which is much stronger than the well-known hydrogen bonds seen in biological systems and between water molecules. The distance between the nucleus of the hydride and the proton as trapped by the catalyst is much shorter than a conventional hydrogen bond, the team reports, although it is longer than a typical covalent bond. Indeed, this di-hydrogen bond is the shortest of its kind identified in any system so far, the team adds.

They suggest that the rare strength of their di-hydrogen bond plays an important role in how well the catalyst balances tearing the hydrogen molecule apart and putting it back together. This balance allows the catalyst to work efficiently. "We're not too far from acceptable with its efficiency," says Bullock. "Now we just want to make it a little more efficient and faster." As such, "We are continuing to modify these catalysts in an attempt to increase the rate of the catalytic reaction," Bullock told SpectroscopyNOW.com

Related Links

Angew Chem, 2014, online: "Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model Investigated by Neutron Diffraction"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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