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Sour catalysis for Hydrogen production



Researchers at our laboratory [collaboration] have deciphered the structure of amorphous molybde​num sulfide, a catalyst capable to replace noble metals like platinum to produce hydrogen. This coordination polymer, which interests industrials due to its technological strengths and its abundance, is not structured as scientists thought.

Published on 21 March 2016
Water electrolysis, a promising option for hydrogen production, is still much too costly. This process requires platinum catalysts sold at a steep price. Scientists from CEA have been exploring production alternatives using various materials, such as noble-metal-free catalysts. One promising option in this group is molybdenum sulfides, in the amorphous form, in particular. These sulphides and their graphene-like crystal structure are also of great interest when it comes to applications in batteries, for electrochemical storage. They also represent a family of hydrogen production catalysts that has been extensively investigated due to their high activity under acidic conditions, and the advantage that molybdenum is an abundant, naturally occurring compound in the Earth's crust.

By combining several spectroscopy techniques, electrochemistry and surface characterization methods, scientists from our laboratory and the Biosciences team of iSm (the Molecular Sciences Institute) in Marseille, in collaboration with researchers from the University of Science and Technology of Hanoi (USTH), have revealed that the structure of these promising catalysts is not as expected, and that their catalytic efficiency relies on a new mechanism. Hitherto, the amorphous form was thought to be composed of chains of molybdenum ion dimers decked by sulphide ligands. The catalytic activity was interpreted to result from the addition of hydrogen atoms on the sulphide ligands of this structure. Now the scientists have discovered that the amorphous molybdenum sulfide is in fact a well-defined coordination polymer based on trimer patterns (as opposed to dimers) of molybdenum ions connected by disulphide ligands. Due to this discovery, the catalytic mechanism for hydrogen production was reinterpreted and is now associated with the formation of catalytic intermediates of the molybdenum hydride type. The latter is the active species in the catalytic reaction of hydrogen production. 

Molybdenum sulphide is a first example of amorphous material with a molecular structure for the production of hydrogen by proton reduction. It constitutes part of the reaction necessary for water electrolysis. These results pave the way for the rational optimization of this class of materials using the tools of coordination chemistry.

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