Fabrication And Environmental Applications Of Iron-Based Reducing Metals Confined In Mesoporous Carbon

Reducing nanomaterials have been widely used in environmental remediation and exihibit excellent treatment on many pollutants such as organic compounds,heavy metals and non-metallic inorganic species.Nanoscale zero-valent iron(nZVI),which is a typical reductive material,has large surface area and high reducing ability.Researches on the design and application of nZVI gradually become an attracting research area based on the degradation of pollutants.The fundamental scientific researches and limiting factors in environmental applications of nZVI are environmental stability and mobility in porous intermedia.In order to improve the migration efficiency of nZVI in porous media,the conventional methods are often used at the cost of reducing nZVI activity,which greatly limits the environmental application of nZVI.The development of novel environmental nanomaterials is of great significance to the development of environmental remediation technology and its basic theoretical research.Mesoporous carbon materials are porous carbon materials with pore size between 2-50 nm,high specific surface area and through pore channels,and play an important role in adsorption as well as catalytic applications.Nevertheless,the biggest challenge of mesoporous carbon is the simplex structure,so it is desperate for modification of functional materials to meet the requirement of different applications.In this study,a novel method was proposed for the dispersion of nZVI nanoparticles with the confinement of mesoporous carbon.The functionalization of mesopore carbon and stabilization of nZVI were realized at the same time.First of all,nZVI nanoparticles act as active sites in materials.Due to the excellent electron donor capacity of nZVI,nZVI play an outstanding role in organic degradation,heavy metal removal/packaging,eutrophic water restoration and other aspects.In addition,the confinement effects can be concluded in following three aspects: 1)Space confinement.Reducing the size of the particles,thus the increase of activity;2)Reactant enrichment and rapid mass transfer.Improving the enrichment of targeted pollutants and enhancing the removal efficiency of pollutants;3)Electronic modulation of support.Redistribution of electrons with noticeable effects on a couple of atomic layers at the interface and might be accompanied,in some cases,by a change in oxidation state of the metal atoms comprising the nanoparticle.Based on the research basis above,this study focuses on the material design and environmental application of iron-based nanoparticles confined in mesoporous carbon from the following three parts:Firstly,the composite,in which highly dispersed nZVI nanoparticles are confined in the channel of mesoporous carbon(nZVI@MC),was synthesized by chelation of acetyl acetone and iron precursor.The morphology and phase composition of nZVI@MC were investigated using solid-phase characterization methods.The results showed that the synthesized material had a high specific surface area(~500 m2/g)and ordered mesoporous channels(~4.2 nm).nZVI nanoparticles(10-20 nm),10 wt% of nZVI@MC material,were evenly dispersed in the stripe-like channel structure.The single particle is partially exposed to the channel and partially encapsulated in the carbon wall,so as to realize the fixation of nZVI particles.The settlement performance of nZVI@MC materials was mainly carried out by Turbiscan stability analyzer.The results showed that the suspension performance of the composites in aqueous solution was greatly improved by surface charge modification and the difference in carbon/iron density.Taking the migration experiment of nanomaterials in sand column as an example,4 g-Fe/L nZVI@MC can penetrate the sand column successfully under gravity,confirming its high mobility.This result brings a new idea for nZVI environmental applications,especially groundwater remediation.In addition,the mechanism of the removal of different heavy metals by nZVI@MC was investigated.The kinetic study showed that the removal of heavy metals by nZVI@MC conformed to the pseudosecond-order kinetic reaction process,and had a high removal capacity for different heavy metal ions.Further analysis of solution chemistry and characterization of the nZVI@MC material after using showed that the presence of nZVI in nZVI@MC material played a crucial role in the removal of heavy metals.For Zn2+,Cd2+ ions with REDOX potential E0 lower than E0(Fe2+/Fe),they were mainly removed by adsorption process.However,Cr6+,Cu2+,and Ni2+ ions with E0 higher than E0(Fe2+/Fe)were mainly removed by reduction/precipitation process.On the basis of the synthesis method of mesoporous carbon-confined mateiral,we explored the feasibility in electrochemical technology.For oxygen reduction reaction(ORR),we carried out the structural design of iron-based bimetal confined in nitrogendoped mesoporous carbon,as an electrocatalytic cathode material,in order to explore the relationship between structure and effect of electrochemical reducing materials.In order to reduce the amount of Pt group precious metals used in ORR,iron-Palladium bimetallic materials were used as catalysts in this study.Meanwhile,in order to further improve the electrical conductivity of carbon materials,the carbonization temperature was raised from 600℃ to 900℃.The structure of mesoporous carbon-confined Pd Fe bimetal material(Pd Fe@MC)is similar to nZVI@MC.Pd Fe bimetal exists in the form of alloy and the doping ratio of Pd and Fe is about 8.81 wt% and 9.21 wt%,respectively.After subsequent nitrogen doping treatment,nitrogen is uniformly doped in the whole mesoporous matrix,mainly in pyridine nitrogen configuration,which can optimize the charge distribution and improve the Fermi level of adjacent C atoms,so as to promote the adsorption and reduction reaction of O2.The electrochemical analysis methods were used to prove that Fe-Pd alloy has good ORR catalytic activity,and nitrogen doping can greatly improve the diffusion-limiting current density of polarization curve.In this study,the number of electron transfer n and the electrochemical active surface area(ECSA)were calculated.It was proved that Pd Fe bimetal can modulate the ORR process with four-electron transfer,and nitrogen doping can can greatly improve the ECSA of the catalyst,thus increasing the active site of ORR catalyst on cathode.The study of the structure-effect of the nitrogen-doped iron-based metal confined in mesoporous carbon provides guidance for the synthesis and environmental application of these materials.Finally,after clarifying the feasibility of the electrocatalytic applications of ironbased bimetal confined in nitrogen-doped mesoporous carbon material and the inherence of its high activity and stability,for the electrocatalytic reduction of nitrate in water,we further synthesized a non-noble metal catalyst,nitrogen-doped mesoporous carbon confined iron-copper bimetallic material(N-Cu Fe@MC).The morphology and structure of Cu Fe@MC material are similar to that of Pd Fe@MC,but the difference is that in Cu Fe@MC nanocomposite,Cu Fe bimetal is mainly in the form of copper and iron.Results of XPS spectrum showed that nitrogen mainly existed in pyrrolic nitrogen,which could enhance the stability of the catalyst.In addition,a large number of oxygen vacancies(OVs)were generated during carbonization process,which were conducive to the adsorption of nitrate.In this study,the removal efficiency of nitrate and the selectivity to nitrogen of different catalysts were tested in Na2SO4 and Na Cl electrolyte systems.In LSV,CV tests,reduction peaks of both processes,NO3-to NO2-and NO2-to NO,were detected in both electrolyte systems,while oxidation peak for NH4+ to N2 was detected in Na Cl electrolyte.Moreover,coupled with the results of solution chemistry,it was proved that NH4+ could be further transformed to N2 by Cl O-,which generated by the oxidation of Cl-on the surface of anode.The optimal catalyst NCu Fe@MC exhibits the removal efficiency of 95.6% and selectivity to nitrogen of 92.1%in 0.1 M Na Cl electrolyte for 12 h catalysis.The removal capacity was 3757 mg-N/gCu Fe.Then,the excellent catalytic performance of N-Cu Fe@MC was proved under different initial concentration of nitrate and different p H of solution.The same electrode prepared by N-Cu Fe@MC was cycled 16 times without the decrease of removal efficiency and N2 selectivity,which proved the superior stability of catalyst NCu Fe@MC.This study provides a basis for further application of mesoporous carbonconfined iron-based reducing metals to the remediation of eutrophication in water bodies at a later stage.