Turnover Number in Photoinduced Molecular Catalysis of Hydrogen Evolution: a Benchmarking for Catalysts?

Abstract

Development of devices for production of H₂ using light and a sustainable source of electrons may require the design of molecular systems combining a molecular catalyst and a photosensitizer. Evaluation of the efficiency of hydrogen production is commonly performed in homogeneous solution with a sacrificial electron donor and the report of the maximal turnover number vs catalyst ( $TO{N}_{cat}^{lim}$ ). This figure of merit is strongly dependent on deactivation pathways and does not by itself provide a benchmarking for catalysts. In particular, when the photosensitizer degradation is the primary source of limitation, a kinetic model, rationalizing literature data, shows that a decrease of the catalyst concentration leads to an increase of $TO{N}_{cat}^{lim}$ . It indicates that exceptionally high $TO{N}_{cat}^{lim}$ obtained at very low catalyst concentration shall not be considered as an indication of an exceptional catalytic system. We advocate for a systematic kinetic analysis in order to get a quantitative measure of the competitive pathways leading to $TO{N}_{cat}^{lim}$ values and to provide keys for performance improvement.

Keywords: Catalysis; Hydrogen Evolution; Photocatalysis; Photosensitizer; Turnover Number.