Construction Of Active Sites Of Two Electrondeficient Transition Metals And Their Electrocatalytic Reduction Of Nitrate

Nitrate pollution has become one of the most common water pollution,and scholars have conducted extensive research on how to control nitrate pollution.Electrocatalytic reduction of nitrate（ENRR）is a green nitrogen removal technology（NO₃^--N）,which uses renewable energy to reduce nitrate to nitrogen（N₂）or ammonia（NH₃）.Among them,ammonia is a carbon-free fuel that can be used as a hydrogen storage carrier,and the ammonium sulfate generated by the sulfuric acid collection product ammonia can be used in fertilizer,medicine,textile and other aspects,so electrocatalytic reduction of nitrate synthetic ammonia has important ecological and social significance.Due to the complex process of electrocatalytic reduction of nitrate to ammonia and the accompanying hydrogen evolution reaction（HER）,the ammonia selectivity is low.Copper-based catalysts have been widely studied in ENRR because of their low price and variable valence state,but Cu(Cu^δ+,0<δ≤2)in the electron-deficient state is difficult to stabilize in the reduction reaction as the main active species.Therefore,in this thesis,a catalyst with Cu^δ+active site is constructed,the method of Cu^δ+stability and its performance and mechanism in ENRR are studied,and corresponding reaction-recovery devices are proposed for different concentrations of nitrogen wastewater.In addition,Cu-based catalysts have the risk of dissolution,and there is a problem of heavy metal contamination when applied to water purification.To solve this problem,a novel and efficient catalyst for the efficient conversion of nitrate to ammonia at a non-copper-based,oxygen-rich vacancy(O_vs)exposed to more Zn^δ+active sites（rich in O-vacancies,H-Zn O_xNPs）is constructed,and its performance and mechanism in ENRR are studied.The conclusions are as follows:（1）In this thesis,Cu（OH）₂/1,4-NDC is modified in situ by the organic ligand 1,4-naphthalic acid（1,4-NDC）to form Cu（OH）₂/1,4-NDC,which is due to the carboxyl energy in the organic ligand and the surface Cu²⁺firmly bonded;Then,a catalyst with Cu^δ+-1,4-NDC active site（Cu（OH）₂/1,4-NDC-AT）is prepared by electrochemical reduction of Cu²⁺on the Cu（OH）₂surface.Due to the high electronegativity of oxygen in the carboxyl group of the organic ligand,the Cu^δ+-1,4-NDC active site is formed by taking away the electrons at Cu⁰.And the stable carboxyl group is conducive to the stable existence of Cu^δ+.At the same time,XPS and Cu AES LMM are used to estimate the relative abundance of Cu species in different valence states on the catalyst surface,and the results showed that Cu^δ+existed stably after a long-term reaction of Cu（OH）₂/1,4-NDC-AT for 56.0 h,which elucidated that the catalyst modified by organic ligand could stabilize Cu^δ+.Finally,for low-concentration NO₃^--N wastewater,this thesis proposes a coupling technology of"ENRR+NH₃-N adsorption removal"to completely eliminate nitrogen species in water.For high-concentration NO₃^--N wastewater,the coupling technology of"ENRR+NH₃-N recovery"is proposed to recover nitrogen in water in the form of（NH₄）₂SO₄（recovery rate of about 98.0%）.（2）In this thesis,a novel catalyst H-Zn O_xNPs rich in O_vsand exposed to more Zn^δ+active sites is constructed by chemical precipitation.The experimental results show that H-Zn O_xNPs react for 4.0 h at-0.65 V vs.RHE working voltage,the nitrate removal rate is 99.8%,and the selectivity of ammonia is 91.6%,which is about 10%higher than that of Zn O nanoparticles（Zn O_xNPs）electrode and 14.1%more selectivity.Then,density functional theory（DFT）calculation is used to clarify the reasons for the higher selectivity of H-Zn O_xNPs in electrocatalytic reduction of nitrate product NH₃:O_vsin H-Zn O_xNPs,the adsorption energy of nitrate by the exposed Zn^δ+active site is lower than that without oxygen defect and the N-O bond is longer,so the oxygen defect is more active for NO₃^-adsorption,thereby enhancing the selectivity of the product ammonia.The results of electrochemical differential mass spectrometry（DEMS）shows that the reaction path of nitrate reduction to ammonia is NO₃*→NO₂*→NO*→N*→NH*→NH₂*→NH₃*.Finally,the ammonia experiment of treating nitrate nitrogen in the actual water body and recovering the product shows that the selectivity of the product ammonia is 81.0%after 240 min of reaction.In the ammonia recovery experiment,the nitrogen in the water body is completely recovered in the form of（NH₄）₂SO₄at 300 min.This work provides a theoretical method for the preparation of stable electron-deficient catalysts,and provides a theoretical basis for the treatment of NO₃^--N wastewater at different concentrations.A novel non-copper-based transition metal catalyst H-Zn O_xNPs is proposed,and the role of H-Zn O_xNPs in ENRR was clarified,which provids a new idea for solving the problem of nitrate pollution in water.