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

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 ( TONcatlim ${TON_{cat}^{\lim } }$ ). 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 TONcatlim ${TON_{cat}^{\lim } }$ . It indicates that exceptionally high TONcatlim ${TON_{cat}^{\lim } }$ 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 TONcatlim ${TON_{cat}^{\lim } }$ values and to provide keys for performance improvement. Turnover numbers cannot be directly used as a benchmarking figure of merit for homogeneous molecular catalysts in photoinduced hydrogen production. However, kinetic analysis allows rationalization of the increase of turnover numbers as the concentration of catalyst is decreased in the context of limitation of the process by degradation of the photosensitizer. This provides a way for photocatalytic systems benchmarking.