Study On The Modification Of G-C₃N₄ And The Performance Of Photoreduction Of Cr???

Hexavalent chromium(Cr(VI))pollution,a kind of heavy metal pollution,has become an urgent environmental issue.Compared with conventional methods of removing Cr(?),semiconductor photocatalytic technology,which has been considered as a promising method,is more economical,more adaptive and no secondary pollution.Graphite-like carbon nitride(g-C3N4),a metal-free polymer semiconductor,has great potential due to its significant advantages such as appropriate band gap(about 2.7 eV),high chemical stability,low cost and nontoxicity.Nevertheless,the photocatalytic activity of bulk g-C3N4 still needs to be improved,because of its small specific surface area,high recombination rate of photogenerated electrons and holes,and low photocatalytic activity.However,as far as the current research is concerned,it is difficult to meet the needs of practical applications for the performance of g-C3N4 by single modification.Therefore,a variety of modification methods are used together to explore the possibility of mutual promotion to achieve the purpose of synergistic modification,which is of great significance for the development of g-C3N4 and other photocatalysts.This paper adopts the methods of hydrothermal treatment and acid treatment to modify g-C3N4 and apply it to the photoreduction of Cr(VI).The actual effects of the separate treatment and the combined treatment were compared,and the reason for the influence of the treatment conditions on the photocatalytic activity was studied through material characterization.The main contents of this thesis are summarized as follows:1.First,g-C3N4 was prepared by thermal condensation of melamine.Then,g-C3N4 was placed in acid solution with heat treatment.A series of modified samples(CN-HCl-1?7?CN-HNO3-1?7?CN-H2SO4-1?7)were obtained by treating in different types of acid(HCl,HNO3 and H2SO4)with different concentrations(1.0,3.0,6.0 mol/L)at 20-80? for 3-8 h.The obtained samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The photocatalytic activity of the modified g-C3N4 samples were studied by photoreduction of Cr(VI)under xenon lamp.The results supported that:(1)after different kinds of acid treatment,the photocatalytic activity of the samples all increased to some extent;(2)acid treatment can tune the electronic and surface property,improve charge separation,and add acid sites on the surface of g-C3N4;(3)there were different optimum conditions for different types of acid.(4)g-C3N4 sample(CN-H2SO4-7),which was obtained by treating in 3.0 mol/L H2SO4 solution at 80? for 8h,exhibited the highest photocatalytic activity.2.First,g-C3N4 was prepared by thermal condensation of melamine.Then,to obtain the modified samples(PCN-120?PCN-150?PCN-180),g-C3N4 was treated by hydrothermal treatment in deionized water at 120-180? 12 h.The obtained samples were characterized by XRD,SEM and TEM,and the photocatalytic activity of the modified g-C3N4 samples were studied by photoreduction of Cr(?)under xenon lamp.The results showed that:(1)samples treated by hydrothermal treatment have larger specific surface and its surface property was tuned;(2)the photocatalytic activity of the products improved unobviously at a low treatment temperature,while the photocatalytic activity of the product was greatly improved when the treatment temperature was increased to 180?.3.First,g-C3N4 was prepared by thermal condensation of melamine.According to the results of previous experiments,g-C3N4 was treated by hydrothermal treatment in deionized water at 180? 12 h,then the resultant products were treated in acid under optimum condition,and a series of modified samples(PCN-HCl?PCN-HNO3?PCN-H2SO4)were obtained.The obtained samples were characterized by XRD,SEM and TEM,and the photocatalytic activity of the modified g-C3N4 samples were studied by photoreduction of Cr(VI)under xenon lamp.The results suggested that,the photocatalytic activity of samples modified by hydrothermal synergistic acid treatment was higher than those modified by hydrothermal treatment or acid treatment only.And among all the modified samples,the g-C3N4 treated by hydrothermal synergistic HNO3 treament exhibited the best photocatalytic activity,of which photoreduction ratio could reach 99%for 10mg/L of Cr(VI)within 30 min.