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Researchers use neutron crystallography to show outcome of hydrogen cleavage by catalyst; helping to build better fuel cell catalyst
24 April 2014
|Neutron crystallography shows this iron catalyst gripping two hydrogen atoms (red spheres). This arrangement allows an unusual dihydrogen bond to form between the hydrogen atoms (red dots). Source: Liu et al. Click to enlarge.|
Using neutron crystallography, researchers at Pacific Northwest National Laboratory (PNNL) and their colleagues at Oak Ridge National Laboratory (ORNL) have shown for the first time precisely where the hydrogen halves end up in the structure of a molecular catalyst—an iron hydrogenase inspired by a natural hydrogenase enzyme—that breaks down hydrogen. A paper on their study is published in Angewandte Chemie International Edition.
The view confirms previous hypotheses and provides insight into how to make the catalyst work better for energy uses—i.e., for fuel cells—as an alternative to platinum.
Use of hydrogen as a fuel by hydrogenase enzymes in nature requires heterolytic cleavage of the H—H bond into a proton (H+) and a hydride (H-), a reaction that is also a critical step in homogeneous catalysts for the hydrogenation of C=O and C=N bonds. Iron, which is earth-abundant and inexpensive, is essential in the active site of both the [FeFe] hydrogenase and the [NiFe] hydrogenase, spurring intense efforts to model their reactivity with synthetic iron complexes. An understanding of the catalytic oxidation of H2 by hydrogenases provides insights into the design of synthetic catalysts that are sought as cost-effective alternatives to the use of the precious metal platinum in fuel cells.
Crystallographic studies on the [FeFe] hydrogenase enzyme were critical to understand its reactivity, but the key H—H bond cleavage step is not readily observed experimentally in natural hydrogenases. Limitations on the precise location of hydrogen atoms by X-ray diffraction can be overcome by use of neutron diffraction, though its use is often limited by the difficulty of obtaining suitable crystals and by the scarcity of neutron sources.—Liu et al.
One of the barriers to fuel cell commercialization is the high cost of the precious metal catalysts commonly used. To make fuel cells less expensive, researchers have turned to natural hydrogenase enzymes for inspiration. These natural enzymes break hydrogen for energy in the same way a fuel cell would, but use inexpensive iron or nickel at their core rather than an expensive previous metal. Researchers have thus been designing catalysts inspired by hydrogenase cores and testing them.
An important step in breaking a hydrogen molecule so the bond’s energy can be captured as electricity is to break the bond unevenly. Instead of producing two equal hydrogen atoms, the catalyst must produce a positively charged proton and a negatively charged hydride.
The physical shape of a catalyst, along with electrochemical information, can reveal how it does that. So far, scientists have determined the overall structure of catalysts with inexpensive metals using X-ray crystallography, but hydrogen atoms can’t be located accurately using X-rays. Based on chemistry and X-ray methods, researchers have had to make do with a best guess for the position of hydrogen atoms.
Morris Bullock, Tianbiao Liu and their colleagues at the Center for Molecular Electrocatalysis at PNNL, one of DOE’s Energy Frontier Research Centers, collaborated with scientists at the Spallation Neutron Source at Oak Ridge National Laboratory (earlier post) in Tennessee to find the proton and hydride. Using a beam of neutrons allows researchers to pinpoint the nucleus of atoms that form the backbone architecture of their iron-based catalyst.
The catalyst shows us what likely happens in the natural hydrogenase system. 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—Morris Bullock
To use their iron-based catalyst in neutron crystallography, the team had to modify it chemically so it would react with the hydrogen molecule in just the right way. Neutron crystallography also requires larger crystals as starting material compared to X-ray crystallography.
We were designing a molecule that represented an intermediate in the chemical reaction, and it required special experimental techniques. 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 H2 molecule.—Tianbiao Liu
The structure, they found, confirmed theories based on chemical analyses. For example, the barbell-shaped hydrogen molecule snuggles into the catalyst core. On being split, the negatively charged hydride attaches to the iron at the center of the catalyst; meanwhile, the positively charged proton attaches to a nitrogen atom across the catalytic core. The researchers expected this set-up, but no one had accurately characterized it in an actual structure before.
In this form, the hydride and proton form a type of bond not commonly seen by scientists: a dihydrogen bond. The energy-rich chemical bond between two hydrogen atoms in a molecule is a covalent bond and is very strong. Another bond called a hydrogen bond is a weak one formed between a slightly positive hydrogen and another, slightly negative atom. The dihydrogen bond seen in the structure is much stronger than a single hydrogen bond. Measuring the distance between atoms reveals how tight the bond is. The team found that the dihydrogen bond was much shorter than typical hydrogen bonds but longer than typical covalent bonds. In fact, the dihydrogen bond is the shortest of its type so far identified, the researchers report.
These results now provide a full picture for the first time, illustrating structures and reactivity of the dihydrogen complex and the product of the heterolytic cleavage of H2 in a functional model of the active site of the [FeFe] hydrogenase enzyme.—Liu et al.
This unusually strong dihydrogen bond likely plays into 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. Now we just want to make it a little more efficient and faster.—Morris Bullock
This work was supported by the Department of Energy Office of Science.
Tianbiao Liu, XiaopingWang, Christina Hoffmann, Daniel L. DuBois, and R. Morris Bullock (2014) “Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model Investigated by Neutron Diffraction,” Angewandte Chemie International Edition, doi: 10.1002/anie.201402090
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