Making Use of the δ Electrons in K₄Mo₂(SO₄)₄ for Visible-Light-Induced Photocatalytic Hydrogen Production

Abstract

Quadruply bonded dimolybdenum complexes with a σ 2 π 4 δ 2 electronic configuration for the ground state have rich metal-centered photochemistry. An earlier study showed that stoichiometric or less amount of molecular hydrogen was produced upon irradiation by ultraviolet light (λ = 254 nm) of K 4 Mo 2 (SO 4 ) 4 in sulfuric acid solution, which was attributed to the reductive capability of the ππ* excited state. To make use of the δ electrons for visible-light-induced photocatalytic hydrogen evolution, a multicomponent heterogeneous photocatalytic system containing K 4 Mo 2 (SO 4 ) 4 photosensitizer, TiO 2 electron relay, and MoS 2 cocatalyst is designed and tested. With ascorbic acid added as a sacrificial reagent, irradiation by artificial sunlight (AM 1.5) on the reaction in 5 M H 2 SO 4 has produced 13 400 μmol g −1 of molecular hydrogen (based on the Mo 2 complex), which is 30 times higher than the hydrogen yield obtained from the reaction of bare K 4 Mo 2 (SO 4 ) 4 with H 2 SO 4 under ultraviolet light irradiation. Further improvement of hydrogen evolution is achieved by addition of oxalic acid, along with an electron donor, which gives an additional 50% increase in H 2 yield. Spectroscopic analyses indicate that, in this case, a junction between the Mo 2 complex and TiO 2 is built by the oxalate bridging ligand, which facilitates charge injection and separation from the Mo 2 core. This Mo 2 −TiO 2 −MoS 2 system has achieved a high hydrogen evolution rate up to 4570 μmol g −1 h −1 . The efficiency of K 4 Mo 2 (SO 4 ) 4 as a metal-centered photosensitizer is also proved by parallel experiments with a dye chromophore, fluorescein, which presents comparable H 2 yields and hydrogen evolution rates. Most importantly, in this study, detailed analyses illustrate that the photocatalytic cycle with hydrogen gas as an outcome of the reaction is established by involvement of the δδ* excited state generated by visible light irradiation. Therefore, this work shows the potential of quadruply bonded Mo 2 complexes as photosensitizers for photocatalytic hydrogen evolution.

Enhancing the Air Stability of Dimolybdenum Paddlewheel Complexes: Redox Tuning through Fluorine Substituents

“…Additionally, the strong interaction between the Mo 2 -δ and π systems of equatorial chelating ligands [e.g., carboxylates ( − O 2 CR) and formamidinates ( − NN; Figure 1)] is ideal for facilitating and studying electron transfer in mixed-valence architectures 4−8 and in photoexcited states. 9,10 Consequently, Mo 2 PWCs are starting to find application in solar energy conversion, 11,12 molecular elec-tronics, 13 and catalysis. 14,15 However, there is one important caveat.…”

“…Their 3D structure allows the construction of large molecular arrays through combination with a broad scope of bridging ligands. − They are redox-active, undergoing one-electron oxidation processes, often at easily accessible potentials. Additionally, the strong interaction between the Mo 2 -δ and π systems of equatorial chelating ligands [e.g., carboxylates ( − O 2 CR) and formamidinates ( − NN; Figure )] is ideal for facilitating and studying electron transfer in mixed-valence architectures − and in photoexcited states. , Consequently, Mo 2 PWCs are starting to find application in solar energy conversion, , molecular electronics, and catalysis. , However, there is one important caveat. Mo 2 PWCs are air-sensitive, reacting readily with atmospheric oxygen (O 2 ), which could be seen as problematic for their translation into the aforementioned technologies.…”