Photocatalytic Hydrogen Evolution From Specially Designed Noble-metal-free Photocatalysts

Hydrogenase, a kind of microbial enzyme existing in some microorganisms in nature, can catalyze the reversible oxidation of dihydrogen to protons and electrons efficiently: 2H++2e-?H2. The study of its structural and functional mimics has become a hot-spot research field in recent years since the crystal structure of [Fe Fe]-H2 ase was determined. Researchers have got some features of the hydrogenases by studying specially designed model complexes. However, for some reasons, so far there has been no breakthrough in mimicking [Fe Fe]-H2 ase active site. At the same time, some earth-abundant elements, such as cobalt or nickel based complexes show better photo/electro-chemical catalytic hydrogen evolution properties and can be more stable.A specially designed bridge, namely(R)-1, 1’-Binaphthalene-2, 2’-dithiol was introduced into the all carbonyl [Fe Fe]-Hase model complex(A). And then, the low-cost xanthene dyes were chosen as photosensitizers and triethylamine as sacrificial electron donor to construct a novel noble-metal-free homogeneous catalytic system under visible light for hydrogen production. The TON was up to 404 under the optimal conditions, which was a little better than the other similar photocatalytic systems.(R)-1, 1’-Binaphthalene-2, 2’-dithiol and cobalt salt or nickel salt were used to prepare two novel metal dithiolene complexes(B and C). An efficient homogeneous photocatalytic system was constructed by combination of the noble-metal-free target complexes as water reduction catalyst, EY2- as photosensitizer and triethylamine as the sacrificial electron donor under the irradiation of visible light. The maximum H2 evolution of 495/676 turnovers was recorded under the optimal conditions.The other two nickel quinolinethiolate complexes(D and E) that were prepared with 8-quinolinethiol were proved to be active catalysts for both the photocatalytic and electrocatalytic hydrogen production. When coupled with fluorescein as the photosensitizer and triethylamine as the sacrificial electron donor, these complexes exhibit activity that correlates with ligand electron donor ability for light-driven hydrogen generation. Catalyst D and E achieves over 5923 and 7634 TON of H2 after 8 hours under optimized conditions.The series of experimental results suggested that the deactivation of these systems was mainly due to the decomposition of the organic photosensitizers. Furthermore, the mechanism of H2 evolution was also briefly discussed by fluorescence spectrum and cyclic voltammetry studies.