Photocatalytic Hydrogen Production Based on a Serial Metal-Salen Complexes and the Reaction Mechanism

Three metal-salen (Ni, Cu, Zn) complexes were firstly introduced into homogeneous photocatalytic hydrogen-evolution systems. Based on these complexes, we developed noble-metal-free hydrogen production systems using disodium salts of Eosin Y (EY2-) as photosensitizers and trimethylamine (TEA) as sacrificial donors. In a Ni(II)-salen/EY2-/TEA system, a TON of 362 based on Ni(II)-salen was achieved in 5.5 h with one time complement for EY2-. The stability study revealed the quick photodecomposition of EY2- in the presence of ether Ni(II)-salen, TEA or both chemicals, and quick dehalogenation of EY2- was observed in the presence of TEA under irradiation. Besides, relatively slower photodecomposition of Ni(II)-salen was also found. These factors should be responsible for the deactivation of the photolysis system. Investigation of electrocatalytic proton reduction by Ni(II)-salen showed a CECE mechanism. An overall catalytic rate constant of 7.62x10(6) M-2 s(-1) was achieved. Combined with the stability analysis, photo-induced electron transfer and electrochemistry investigation, for the first time, the photocatalytic hydrogen production mechanism for metal-salen complexes (e. g. Ni(II)-salen) was proposed.