Study Of Complexes Containing Transition Metals And Their Heterogeneous Composites For Catalytic Water Oxidation

Hydrogen is an ideal substitute for traditional fossil fuels as a green and environmentally friendly renewable energy source,which is of great significance for solving the world energy crisis and sustainable development in the future.A method that uses the earth's abundant water resources as raw materials to convert solar energy into hydrogen energy through a water splitting reaction is an effective approach for hydrogen evolution.Water splitting involves two half reactions,namely water oxidation and proton reduction.The water oxidation reaction,which involves the participation of four electrons and four protons,is a much slower kinetic process and is considered to be the"bottleneck"of the water splitting reaction.At present,transition metal complexes show good water oxidation activity in water oxidation reactions.However,metal complexes still have many challenges,such as difficulty in recovery and poor stability in water oxidation reaction.These challenges limit their application value in industrial production.In this paper,the complexes of transition metals Fe,Ru and the heterogeneous materials Bi VO₄,C₃N₄,Melem,MIL-101(Cr)are combined by simple physical mixing or covalent bonding to form composite catalysts.Then,the water oxidation catalytic activity and stability of these composite catalysts were systematically studied in the photocatalytic,electrocatalytic,and chemical water oxidation reactions,respectively.The relationship between the structure and stability of the catalysts was also discussed.The main contents are as follows:(1)Four Fe-based complexes with polypyridine structure were synthesized and characterized.The chemically stable Bi VO₄ was used to replace[Ru(bpy)₃]²⁺that was applied in dye-sensitized system as a light-harvesting material in water oxidation reaction.Bi VO₄ has attracted widespread attention in the field of photocatalysis,because it has a suitable band gap(?2.4 e V)and is easy to prepare.However,the photo-generated electrons and holes of Bi VO₄ are easy to recombine,resulting in poor photocatalytic water oxidation activity.These Fe complexes used as cocatalysts can effectively improve the separation efficiency of photo-generated carriers in Bi VO₄.Thereby the water oxidation activity of Bi VO₄ is enhanced,which leads to an oxygen yield of 99.1%.By investigating the water oxidation stability of the Fe complex in this system,we found that the Fe complexes were decomposed during water oxidation reaction,and Fe OOH nanoparticles were generated in situ on the 110 face of Bi VO₄.Fe OOH was proved to be the true co-catalyst of Bi VO₄,and the remaining Fe complexes in solution had little effect on the photocatalytic activity of Bi VO₄.It was investigated by ¹H NMR that the polypyridine ligand TPA was decomposed during water oxidation reaction,which maybe the main reason that the Fe complexes containing the polypyridine ligand was unstable in the Bi VO₄ photocatalytic system.(2)Metal-organic framework materials(MOFs)have attracted wide attention in the field of catalysis due to their excellent specific surface area,porosity,and easy adjustment and modification.The organic ligands containing carboxyl groups were connected with MIL-101(Cr)-NH₂ containing rich amino groups via amide bonds.Three Ru complexes were successfully in situ synthesized on the modified frameworks of MOFs by a series of synthetic processes.The three anchored complexes were characterized by XAS and was proved that the structures of them anchored in the MOFs framework were:[Ru(terpy-Ac)(pic)₂Cl]⁺,[Ru(terpy)(isc)Cl₂],[Ru(terpy)(isc)(pic)₂]²⁺.Through the investigation and analysis among the structure,catalytic activity and stability of the Ru complexes acted as the catalytic center,we found that the modulation of the ligand in axial and equatorial plane will change the active site of the catalyst,thereby changing the water oxidation activity and stability of the anchoring catalyst.The XAS tests show that the molecules containing the[Ru-H₂O]structural unit are the true water oxidation catalysts for this type of complex.The composite catalyst synthesized through this construction strategy can not only change the stability and active site of the catalyst by modifying and adjusting the structure of the complex,but also realize the recovery of the precious metal complex.(3)Three Ru-based complexes were connected to the framework of graphitic carbon nitride(C₃N₄)through amide bonds to synthesize three catalysts,namely Ru-CN-1,Ru-CN-2,and Ru-CN-3.Ru-CN-1 not only showed the best activity in the chemical water oxidation reaction using ammonium cerium nitrate as an oxidant,but also showed the best activity in the visible light-driven water oxidation reaction.The anchoring of the Ru complex not only extends the position of the optical absorption band edge of the C₃N₄ material from?450 nm to?700 nm,but also improves the separation efficiency of photo-generated electrons and holes in the C₃N₄ material.C₃N₄ exhibits the properties of molecular catalysts through the anchoring of Ru complexes,which provides a reference for the design and synthesis of catalysts in the future.(4)Melem,which has strong conductivity and abundant terminal amino groups,is a monomer of graphite phase carbonitride materials.By covalent attachment of an organic ligand to the Melem backbone,a Ru-based complex was synthesized in situ on the Melem backbone.The obtained catalyst Ru-Melem not only exhibited chemical water oxidation activity,but also exhibited strong water oxidation activity in electrocatalytic water oxidation reaction.By comparison with the corresponding molecular catalysts,it was found that after the Ru complex was anchored to Melem,its electrocatalytic water oxidation activity and stability were greatly improved.The possible reason for this improvement is the immobilization of the molecular catalyst,which improves the charge transfer efficiency of the catalytic system and inhibits the decomposition of the metal complex caused by the intermolecular interaction.This work provides a new idea for the application research of metal complexes in electrochemical reactions,which is very meaningful.