Platinum is widely used as a catalyst for the reduction of water to hydrogen and for the reverse reaction, the oxidation of hydrogen to water, because the corresponding multi-electronic processes take place on its surface without electrochemical overvoltage. This remarkable property is shared by a family of enzymes known as hydrogenases, which enable certain living microorganisms (bacteria, photosynthetic organisms) to use the reducing power of hydrogen for their bioenergetic metabolism or to get rid of excess electrons. Hydrogenases are metalloproteins that catalyze water-hydrogen interconversion not only without overpotential, but with impressive catalytic efficiencies. This explains the growing interest in these enzyme systems as potential catalysts in battery devices, electrolyzers and photolyzers, despite their high sensitivity to air. There are two classes of hydrogenases: those with an active site containing one Ni and one Fe atom, and those with an active site containing two Fe atoms. These active sites have the unique feature of containing cyanide and carbon monoxide ligands.
This lecture provides information on: (i) the physiological role of hydrogenases; (ii) the three-dimensional structure of proteins, revealing gas transfer channels, electron transfer chains and proton transfer chains; (iii) enzymatic mechanisms (metal-hydrogen, metal-hydride complexes); (iv) the first hydrogenase biopiles.
