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PNNL studies rhodiumbased catalyst for hydrogen storage S cientists from Pacific Northwest National Laboratory in the US have revealed new details of a catalyst that enhances the extraction of hydrogen from a molecule in which it has been stored. The results are a step towards designing catalysts for use in hydrogen applications such as fuel cells.
The PNNL scientists combined experimental and theoretical studies to identify the characteristics of the catalyst, which comprises a cluster of rhodium, boron, and other atoms. The catalyst chemically reacts with ammonia borane, a molecule that stores hydrogen densely, to release the hydrogen as a gas. Their results, which reveal molecular details of the dehydrogenation of dimethylaminoborane by rhodium clusters, were reported recently in the Journal of the American Chemical Society [DOI: 10.1021/ ja901480u].
One way to achieve a fuel system in which hydrogen is stored safely and discharged easily is by storing hydrogen as part of a larger molecule. The molecule that contains hydrogen atoms, in this case ammonia borane, serves as a structural support. The catalyst plucks the hydrogen from the ammonia borane as needed to run the device.
PNNL chemists in the Institute for Interfacial Catalysis studied a rhodium-based catalyst that performs this job fairly well, but which could be improved. Initial work showed that the catalyst works as a molecule with a core of four rhodium atoms in a tetrahedron, with each corner decorated with boron and other elements. But the rhodium and other atoms could line up in dozens of configurations in the molecule.
The team wanted to know which of the numerous structures was the real catalyst, as well as how the atoms worked together to remove the hydrogen from ammonia borane. First, they followed the catalyst-ammonia borane reaction with several technologies. They then used computer models to construct a theoretical molecular configuration that accounted for all the data. To test whether this structure worked properly, the team performed a computer simulation of an analysis of that catalytic structure reacting with ammonia borane. Then they compared the simulated data with real data, which matched very well, suggesting their structure was close to reality.
The chemical nature of the structure allowed the team to outline the chemical reaction between the catalyst and the ammonia borane. The results suggest that the active catalyst picks off hydrogen from a particular spot on the ammonia borane molecule: a nitrogen atom in the molecule holding onto two hydrogen atoms. First, the catalyst plucks one hydrogen atom off, which makes the bond between the remaining hydrogen and boron unstable. So the molecule spits off the second hydrogen, and the two hydrogen atoms form molecular hydrogen (H 2 ), which is released as a gas.
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