A team from the University of Oxford (UK) and CNRS (France) has developed an optimized system for the photocatalytic production of hydrogen using an hydrogenase and photosensitizer co-attached to a TiO2 nanoparticle. A paper on their work was published online 23 November in the Journal of the American Chemical Society.
The titaniaphilic enzyme selected by Oxford’s Dr. Fraser Armstrong and his colleagues—Db [NiFeSe]-H (from the bacterium Desulfomicrobium baculatum)—is very stable even after prolonged exposure to air with a turnover frequency of 50 (mol H2) s-1 (mol enzyme)-1 upon visible light irradiation, when attached to a RuP-sensitized TiO2 at pH 7 and 25 °C (77 °F).
It thus provides a benchmark and reference on a per active site basis for future systems. Although this rate is lower than the maximum rate that hydrogenases can acquire, it is remarkably high considering the small driving force acting on the enzyme to produce H2 during irradiation. This direct-electron transfer controlled rate is 6 times higher than the one obtained by bimolecular (and probably) diffusion-controlled photo H2 production when replacing TiO2 by MV2+.
Our results show that, despite its large size and the complexity of its internal electron-transport system, Db [NiFeSe]-H is an excellent catalyst for artificial photosynthetic systems and proves the capability for achieving very efficient sunlight conversion without precious metals.—Reisner et al.
Hydrogenases—natural enzymes used by some microbes in their energy metabolism—can reduce protons into hydrogen at active sites composed or iron- or nickel/iron complexes. Importantly, the authors note, the electrochemical reaction is reversible, and hydrogenases can produce or oxidize H2 with just a minimum overpotential—as does the expensive metal catalyst platinum. (Earlier post).
Armstrong and colleagues note that the ability to attach hydrogenases with high electrocatalytic activity to a graphite electrode surface has been exploited recently in several “inspiring” demonstrations. They also recently reported on the stable electrochemistry of a [NiFeSe]-hydrogenase from Desulfomicrobium baculatum (Db [NiFeSe]-H) on a TiO2 electrode, leading to the development of a prototype solar H2 production system consisting of that hydrogenase and a synthetic ruthenium photosensitizer, co-attached to colloidal TiO2 nanoparticles.
The present study was focused on determining the importance of choosing a particular enzyme and photosensitizer and identifying operational conditions leading to optimal performance.
Erwin Reisner, Daniel J. Powell, Christine Cavazza, Juan C. Fontecilla-Camps and Fraser A. Armstrong (2009) Visible Light-Driven H2 Production by Hydrogenases Attached to Dye-Sensitized TiO2 Nanoparticles. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja907923r