| With the rapid development of industry,the situation of water pollution in the environment continues to intensify.The rapid development of the aerospace industry,the use of chemical fertilizers,and the domestic sewage lead to the accumulation of nitrate pollutant concentrations in groundwater or surface water.At present,various methods such as ion exchange,membrane separation,electrodialysis,biodegradation and catalytic degradation are used for the degradation of nitrate,but these methods are low efficiency and low selectivity.Therefore,developing new methods for high nitrate conversion rates and high value-added products is urgent.Electrocatalysis has high efficiency and environmental protection,which regarded as an effective method to remove nitrate pollutants,and can achieve the conversion to high value-added products(such as ammonia).This strategy can not only solve the problem of nitrate pollutants in water,but convert it to high value-added products.Therefore,improve the efficiency of energy utilization and alleviate the energy crisis and the greenhouse effect.The efficient design of nitrate reduction catalyst requires materials to meet the factors of large specific surface area,excellent conductivity and fast charge transfer.Therefore,a new two-dimensional material MXene developed by Yury gogotsi et al.has attracted.Compare with Graphene,MXene has excellent metal conductivity and rich surface functional groups.On the one hand,the linking and anchoring of groups and metal ions can be realized by means of the surface chemical reaction of MXene.On the other hand,theoretically,stacking any two dissimilar materials(at least one is semiconductor),charge will cross their interface to produce a built-in electric field until their Fermi levels align.The existence of built-in electric field plays an important role in ion adsorption and mass transfer.Using the surface chemical reaction of MXene,a metal doped MXene electrocatalyst with built-in electric field is constructed and applied to the electrocatalytic reduction of nitrate.In this paper,a series of metal-doped Ti3C2Tx MXene composites were designed and synthesized,and nano electrocatalysts with built-in electric fields were constructed in situ.The amount and size of active sites can be selectively controlled by adjusting the doping ratio between metal/group and Ti3C2Tx MXene.The built-in electric field in electrocatalyst was studied by theoretical calculation,and a home-made instrument was used for nitrate reduction reaction.The mechanism of nitrate reduction was explained theoretically and experimentally.The main research works are as following:(1)Investigation on surface chemical reaction of MXene:This chapter discusses the surface chemical reaction of MXene and achieves the successful surface modification of monolayer MXene by phosphonic acid groups.The phosphonic acid group can be attached to the surface of MXene through hydrogen bonding,and the degree of modification of MXene can be controlled by the content of phosphonic acid.Therefore,the metallic MXene exhibits semiconductor characteristic after phosphonic acid modification.In order to study the effect of surface modification,Ti3C2Tx-OP MXene-based memory devices were demonstrated.The understanding of the surface chemical reaction of MXene provides guidance for the next work,laying a foundation for the application of MXene in other fields.(2)Investigation on construction of CuCl_BEF electrocatalyst with built-in electric field by MXene surface chemical reaction for electrocatalytic reduction of nitrate to ammonia:Since we have a sufficient understanding of the structure and surface properties of Ti3C2Tx MXene,we have constructed MXene-based composites and explore its application potential in other fields.In the last chapter,phosphonic acid groups are controllably attached to the surface of MXene through hydrogen bonds.Next,due to the surface chemical reaction of MXene,CuCl_BEF electrocatalyst is constructed on the surface of MXene through electrostatic attraction,therefore,achieve efficient nitrate pollutant removal and electrocatalytic nitrate reduction to high valueadded ammonia.A model electrocatalyst is created by stacking CuCl(111)and rutile TiO2(110)layers together,in which a built-in electric field induced from TiO2 to CuCl(CuCl_BEF)is successfully formed.The built-in electric field formed on CuCl and Ti02 not only effectively triggers the accumulation of NO3-at the electrocatalyst interface,but lowers the energy barrier of the rate-determining step ON to ONH.Finally,92%nitrate conversion and 98.6%ammonia selectivity were achieved.(3)Investigation on construction of FeNPs@MXene nano electrocatalyst by MXene surface chemical reaction for electrocatalytic reduction of nitrate to ammonia:Although the selective conversion of nitrate to ammonia is achieved,the conversion rate of nitrate is not satisfactory.Due to the high solubility and mobility of nitrate,efficient nitrate conversion at low concentrations has not been reasonably resolved for a long time.Inspired by nitrate reductase with iron as the active center in plants,a novel FeNPs@MXene nano catalyst was constructed by the surface chemical reaction of MXene,and proved that the confinement effect of-Cl on the surface of MXene facilitates the formation of FeOOH nanoshuttles.The FeNPs@MXene electrocatalyst can realize the selective catalytic reduction of nitrate to ammonia with a nitrate conversion rate as high as 97.8%.The reliability of the electroreduction of nitrate to ammonia was proved by the precise analysis of in situ electrochemical online differential electrochemical mass spectro1etry and 15N isotope labeling experiment.Finally,we use DFT calculations and conducting atomic force microscopy revealed that the built-in electric field formed between FeOOH and MXene.(4)Investigation on construction of Cu-Fe bimetallic electrocatalyst by MXene surface chemical reaction for electrocatalytic reduction of nitrate to ammonia:In order to overcome the problem that single atom doped MXene can either achieve high nitrate conversion or high ammonia selectivity,copper-iron bimetallic doped MXene catalyst was constructed by using the surface chemical reaction of MXene.The copper ion center plays the role of adsorbing nitrate ions,regulating the proton-electron pair,and inhibiting the hydrogen evolution reaction.The iron ion center further promotes the conversion of nitrite to ammonia.The synergistic effect of bimetals to achieve the conversion of reactants and high selectivity of target products.We prepared CuFe@MXene electrocatalysts using MXene as the substrate material.The charge accumulation effect and proton-electron pair transfer in CuFe@MXene were demonstrated by SCS and EIS.The built-in electric field formed in CuFe@MXene is investigated and demonstrated using in situ differential electrochemical mass spectrometry and DTF.The synergistic effect of copper-iron bimetallic electrocatalysts achieves 100%nitrate conversion and up to 99.6%ammonia selectivity,as well as excellent cyclic reaction stability(14 cycles).The construction of this copper-iron bimetallic catalyst provides a new insight for other multi-step electroreduction reactions in the future. |
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