Salophen-based Coordination Organic Polymers And Their Derived Carbon Materials For Electrocatalytic Denitrification

The nitrogen cycle is an important cycle in nature.The burning of traditional fuels and the popularization of nitrogen fertilizers have caused the nitrogen cycle system seriously unbalanced.NO_Xis easily absorbed by water and alkali,and mainly forms NO₃^-solution.Drinking a large amount of groundwater contaminated with NO₃^-can cause diseases such as high blood pressure and blue baby syndrome.However,NO₃^-is stable in aqueous solution,so the degradation of NO₃^--N is an urgent problem to be solved.Electrochemical reduction technology has the advantages of convenience,rapidity,environmental protection,no secondary pollution,and low investment cost,and has been widely studied in chemical engineering.Converting NO₃^--N in the water into N₂-N through the electrolysis process is an environmental-friendly method.Therefore,searching for electrode materials that can efficiently reduce NO₃^--N and have higher nitrogen selectivity is the primary task of promoting the electrochemical degradation of NO₃^--N technology.In this paper,three types of new Metal-Salophen covalent polymers and their composite materials have been designed and synthesized.The metal ions coordinated by the covalent polymer form surface Lewis acid sites,while the pyrolyzed coordinated covalent polymer causes a small amount of metal particles to be wrapped by nitrogen-doped carbon to form a Schottky barrier coupling interface.The special Lewis acid site and the metal particle interface effectively promote the adsorption of NO₃^-in the water body and accept electrons to reduce it,and finally realize the high-efficiency electrocatalytic conversion of NO₃^--N into harmless nitrogen.The specific research content is as follows:（1）Using trialdehyde phloroglucinol（TP）and 3,3’-diaminobenzidine as monomers,a new type of TPDAB-Co covalent polymer compounded on the surface of carbon nanotubes（CNTs）was synthesized by Schiff base reaction.This non-pyrolyzed carbon nanotube-supported covalent polymer（TPDAB-Co@CNTs）exhibits good electrocatalytic performance during oxygen reduction（ORR）and NO₃^--N reduction（NO₃^--RR）processe.In 0.1 M KOH alkaline solution,the half-wave potential(E_1/2)of the ORR process can reach 0.89 V,and the power density in the practical application of zinc-air batteries can reach 156 m W cm^-2,which is better than commercial Pt/C catalyst.In 100 ppm Na NO₃and 0.1 M Na₂SO₄electrolyte,TPDAB-Co@CNTs catalytic NO₃^--N conversion rate can reach 87%,and N₂-N selectivity is as high as 97%,which is a very rare high-efficiency non-pyrolysis electricity catalytic denitrification material.The study found that the Co²⁺ion in the Co-Salophen structural unit has a high oxygen affinity,and acts as an effective catalytic active site in the electrocatalytic oxygen reduction（ORR）and NO₃^--N reduction（NO₃^--RR）processes.CNTs are used as wires to improve the electron transport capacity in the Co-Salophen unit.The rich mesoporous structure（40-60 nm）and the cotton-like fluffy morphology all promote the mass transfer process of the composite material and improve the electrocatalytic performance of the material.（2）Using the TP and 2,3,6,7,10,11-hexaaminotribenzene hexahydrochloride as monomers,select copper salt,and construct a complex on carbon by Schiff base reaction.The new Cu-Salophen covalent organic polymer material on the surface of the nanotubes is treated by high-temperature pyrolysis to obtain a small amount of NC porous carbon-coated Cu metal nanoparticle material（Cu-COP@CNTs-800）.The results of electrocatalytic NO₃^--RR show that although the material contains only a small amount of copper particles of 1.6 wt%,its NO₃^--RR catalytic performance is very effective.In 100 ppm Na NO₃and 0.1 M Na₂SO₄electrolytes,after 12 h of electrolysis,the conversion rate of NO₃-N is 66%,the selectivity of N₂-N reaches93%,and the Faraday efficiency is 13%.The effective electrocatalytic performance of Cu-COP@CNTs-800 can be attributed to the special porous nitrogen-doped carbon-coated Cu nanoparticles.This semi-closed core-shell structure facilitates the penetration of the electrolyte and the copper metal particles and promotes the metal.The catalytic activity of the copper particles also effectively prevents the severe corrosion of metallic copper during long-term electrolysis.（3）Using TP and o-phenylenediamine as monomers and silver nitrate as silver salt,a new type of Ag-Salophen-based covalent organic framework compounded on the surface of carbon nanotubes was synthesized through Schiff base reaction.A low-silver（1.99 wt%）CNTs@CNx@Ag-800 material was prepared by high-temperature pyrolysis at 800°C under the protection of nitrogen.The Ag-N-C material achieves high-efficiency catalysis of the NO₃^--RR process under ultra-low overpotential（-0.29 V）.The conversion rate of NO₃^--N electrolysis within 24 h is33%,and the selectivity of N₂-N is as high as 100%.Its high-efficiency catalytic effect at lower over-points is related to the unique"veins-leaves-apples"morphology formed by Ag-Salophen-derived carbon materials.The thin-layer carbon shell-wrapped Ag metal particles strengthened the corrosion resistance of the silver particles.The morphological composition and catalytic performance of the material remained unchanged during continuous electrolysis for 192 h.At the same time,the electrocatalytic NO₃^--RR performance of the silver metal particles was activated.