Synthesis And Water Oxidation Catalysis Of Nickel And Copper Complexes Of Flexible Polypyridine Ligands

Energy shortage and environmental degradation are two major issues that seriously threaten the sustainable development of human society.Renewable clean energy sources are thus urgently needed.Obtaining dihydrogen by splitting water and utilizing it as fuel is one ideal method to solve the problems on the energy and the environment.However,the water oxidation reaction（WOR）is a complicated and sluggish process involving four electrons and four protons,which is a bottleneck of water splitting.Therefore,it is vital to develop water oxidation catalysts with high efficacy and low overpotential.In recent years,transition metal complexes with excellent catalytic properties for WO R have attracted much attention.Among them,nickel and copper complexes are attractive due to the abundance and the low cost of both metals.In this work,series of nickel and copper complexes of flexible polypyridine were synthesized,and their activity for WOR studied.The kinetics and mechanism of the catalytic reactions were explored.The factors affecting the catalytic activity were investigated such as the ligand framework,the solution p H condition and the type of metal ions.The main work of this study includes the following parts:1.Complex[N i（tpbn）]（Cl O₄）₂（1）（tpbn=N,N,N’,N’-tetrakis（2-pyridylmethyl）-1,4-butane diamine）was synthesized and characterized.The structure of 1 was determined by X-ray single crystallography.The redox properties and electrocatalytic water oxidation activity of 1 were studied.1 is a precatalyst for WOR.In 0.1 M PBS at p H 11.0,a film was obtained after the electrolysis of complex 1 for 4 h.As the film was used as the working electrode,the onset potential of WOR was 0.97 V vs.Ag/Ag Cl.The current density for WOR was 88.0μA·cm^-2,the Faradaic efficiency was 82﹪,and the Tafel slope was 109.0m V/decade.The film on the ITO electrode surface was characterized by scanning electron microscopy（SEM）,Energy-dispersive X-ray analysis（EDX）,and X-ray photoelectron spectroscopy（XPS）.The results showed that the main component of the catalytic film was Ni O and Ni OOH.2.Complex[N i（tpen）]（Cl O₄）₂·0.5CH₃COCH₃（2）（tpen=N,N,N’,N’-tetrakis（2-pyridylmethyl）-1,2-ethylenediamine）was synthesized and characterized.The structure of 2was determined by X-ray single crystallography.The redox properties and electrocatalytic activity for WOR were studied under different p H conditions.In 0.1 M PBS at p H 8.0,2 is a homogeneous molecular catalyst for WOR.The onset potential for WOR was 1.00 V with an overpotential of 440.0 m V.Using an ITO working electrode for controlled-potential electrolysis（CPE）at 1.50 V,the catalytic current density for WOR reached 1.22 m A·cm^-2with a Faradaic efficiency of 89﹪.The rate constant k_cat for WOR was 0.04 s^-1.The kinetics study suggested a nucleophilic attack mechanism（WNA mechanism）for WOR catalyzed by 2,which involved a proton-coupled electron transfer（PCET）process（1H⁺/2e^-）forming a high-valence Ni^V for the formation of the O-O bond.In 0.1 M PBS at p H 11.0,2 is a precatalyst for WOR.A film on the ITO surface was obtained after the electrolysis of 2.Using the ITO with a film as working electrode,the onset potential for WOR appeared at 0.88 V.The CPE at 1.40 V gave a catalytic current density of 0.66 m A·cm^-2,a Faradaic efficiency of97﹪and the Tafel slope of 90.0 m V/decade.The film on the ITO surface was characterized by SEM,EDX and XPS.The results indicated that the main component of the film was N i O and Ni OOH,which was similar to the film from 1.3.Complexes[N i（tpmen）（H₂O）]（Cl O₄）₂（3）and[Cu（tpmen）]（Cl O₄）₂（4）（tpmen=N,N,N’-tris（2-pyridylmethyl）-N’-methylethylenediamine）were synthesized and characterized.Both3 and 4 were characterized by ultraviolet-visible spectroscopy,infrared spectroscopy and mass spectrometric analyses.The redox properties and catalytic activities of 3 and 4 for WOR were studied.3 was a precatalyst for WOR in 0.1 M boric acid buffer solution（BABS）.In 0.1M BABS at p H 11.0,a film was deposited on the ITO electrode surface after the electrolysis of 3.Using the ITO with a film as working electrode,the onset potential for WO R was 0.94 V.In CPE at 1.55 V,the current density for WOR reached 4.94 m A·cm^-2 with a Faradaic efficiency of 100﹪,and the Tafel slope of 115.0 m V/decade.In a p H 11.0 0.1 M PBS,4 is a homogeneous molecular catalyst for WOR.The onset potential was 0.90 V with an overpotential of 517.0 m V.Using an ITO electrode for working electrode,the current density for WOR reached 1.39 m A·cm^-2 with a Faradaic efficiency of 94%.The rate constant k_catwas0.24 s^-1.The kinetics study showed that the catalytic mechanism was a WN A pathway involving the 1H⁺/1e^-PCET process forming a high-valence Cu^IV for the formation of the O-O bond.