The Study Of The Preparation Of Mesoporous Molecular Sieve Composite From Coal Fly Ash And Photocatalytic Degradation Of Phenol

In this paper,coal fly ash（CFA）from thermal power plants was utilized as Si and Al source to prepare aluminum-containing mesoporous molecular sieve Al-MCM-41,which was further used as photocatalyst support to synthesize composite photocatalyst.The composite photocatalyst greatly solved the problems of insufficient degradation of phenol by the monomer photocatalyst,and realized the purpose of‘utilizing waste to treat waste’.The main research contents are as the following two parts:The first part is the preparation of AgCl/α-FeOOH@MCM composite photocatalyst and its photocatalytic phenol degradation performance.Firstly,after extracting Si and Al elements from CFA by alkali-dissolving method,Al-MCM-41 mesoporous molecular sieve containing aluminum was prepared by hydrothermal method,then AgCl was supported on it by chemical precipitation method.Finally,α-FeOOH was combined with AgCl by hydrothermal method.XRD,SEM,TEM,XPS,and other characterization methods were carried out to investigate the crystal phase,morphology,and chemical composition of the materials.The results show that the AgCl particles with a diameter of ca.1μm are uniformly“embedded”on the Al-MCM-41 mesoporous molecular sieve with a diameter of ca.5μm,while the needle-likeα-FeOOH with a length of ca.170 nm are closely distributed on the surface of AgCl.The HRTEM image displayed that a heterojunction structure was formed between AgCl andα-FeOOH,which suggested that the supported heterojunction photocatalyst had been synthesized.The optical properties and electrochemical properties of the materials were explored by UV-vis DRS spectra,PL spectra,photocurrent curves,and electrical impedance.UV-vis DRS results showed that the light absorption band edge of AgCl/α-FeOOH@MCM was red-shifted from 584 nm to 700 nm compared withα-FeOOH,indicating its absorption capacity for visible light was enhanced.Under the illumination of a 400 nm excitation light source,the peak value of AgCl/α-FeOOH@MCM was significantly lower than that of AgCl andα-FeOOH,indicating that its photogenerated electron-hole（e^--h⁺）recombination rate was greatly reduced.The electrochemical results showed that AgCl/α-FeOOH@MCM exhibited an extremely strong photocurrent intensity with the lowest impedance compared with pristine AgCl andα-FeOOH,indicating that the migration efficiency of photogenerated carriers in AgCl/α-FeOOH@MCM were significantly improved.By degrading phenol as the target organic pollution,the results showed 20 mg·L^-1 of phenol solution could be completely degraded by 1.5 g·L^-1 of AgCl/α-FeOOH@MCM photocatalyst after 30 min visible light illumination at pH=7.And cyclic degradation experiments showed that AgCl/α-FeOOH@MCM could still maintain a 100%phenol degradation rate after five cycles.By comparing the XRD patterns before and after use,it was found that some Ag⁺were reduced on the surface of AgCl after five times use,then formed an Ag-AgCl/α-FeOOH@MCM system,which was further enhanced the photocatalytic performance by the SPR effect generated by Ag⁰.After kinetic studies,the phenol degradation process followed a first-order kinetic model,and the kinetic constants of AgCl/α-FeOOH@MCM for phenol degradation were 4.18 times than that of AgCl,and193.25 times than that ofα-FeOOH,respectively.The results of radical trapping experiments showed that the contribution rates of O₂·^-,h⁺,e^-,and·OH to phenol degradation were 92.80%,85.57%,79.78%,and 18.26%,respectively,indicating that O₂·^-,h⁺and e^-were the most important active species in the photocatalytic degradation process.In the second part,Ag₂O were loaded on the as-prepared Al-MCM-41 mesoporous molecular sieve by chemical precipitation method and then irradiated by a 300 W xenon lamp for 30 min to form the Ag/Ag₂O@MCM composite photocatalyst.The products were characterized by XRD,XPS,SEM,TEM,BET,and UV-vis DRS,and the results showed that Ag/Ag₂O@MCM had a mesoporous structure and Ag₂O with a diameter of ca.100 nm were tightly immobilized on the surface of the Al-MCM-41.Additionally,compared with Ag/Ag₂O,the visible light response range and photogenerated carrier separation efficiency of Ag/Ag₂O@MCM were both enhanced significantly,indicating that the molecular sieve not only acted as the carrier material,but its abundant acidic sites also played a significant role as the traps for carriers,which promoted the separation efficiency of e^--h⁺pairs.According to the results of phenol photodegradation experiments,1.0-Ag/Ag₂O@MCM had the best photocatalytic activity.When the dosage was 1.5 g·L^-1,pH=7,and the initial concentration of phenol solution was 20 mg·L^-1,1.0-Ag/Ag₂O@MCM could achieve a 100%removal rate of phenol under visible light irradiation for 30 min,and the degradation process followed a first-order kinetic model,and the kinetic constants of 1.0-Ag/Ag₂O@MCM for phenol degradation were 26.37 times and 28.12 times than that of Ag/Ag₂O,and pure Ag₂O,respectively.The degradation rate of phenol can still reach 95.25%after repeated use five times.The free radical trapping experiments showed that the contribution rates of O₂·^-,e^-,h⁺,and·OH to phenol degradation were 93.41%,86.22%,83.24%,and 27.66%,respectively,indicating that O₂·^-,e^-,h⁺were the most important active species in the photodegradation process.Therefore,a proposed mechanism of phenol photodegradation by Ag/Ag₂O@MCM was revealed,which can be attributed to the strong visible light responsiveness of Ag₂O,the strong SPR effect of Ag⁰,and the effect of Al-MCM-41 as the carriers traps.