The Synthesis of Visible Light Driven Ceria-based Photocatalysts for Organic Contaminants Removal in Water

Author:Wen Xiao Ju

Supervisor:niu cheng gang


Degree Year:2018





Water is fundamental to the survival of humans and the natural world.With the rapid progress of social industrialization,an environmental problem of water pollution is becoming more and more prominent.Photocatalytic technology has great application prospects in wastewater treatment.The development of photocatalysts with visible light photocatalytic properties is the key to application.Cerium dioxide(CeO2)is an extremely versatile rare earth oxide with the properties of an n-type semiconductor.However,due to its poor absorption of visible light and its easy recombination of photogenerated carriers,its application in the field of visible light photocatalysis is restricted.In order to further improve its application in the field of visible light photocatalysis,CeO2 samples with two different morphologies were prepared by sol-gel and urea hydrolyzation methods,and two kinds of CeO 2 were used as carriers.A series of CeO2-based composites with good visible light response were prepared through in situ precipitation,water bath method,thermal decomposition,photoreduction and other methods.The obtained photocatalysts were characterized by various technologies,and applied to the removal of different organic pollutants in water.The degradation activities of the obtained photocatalysts were investigated and the degradation pathway of pollutants is explored.In view of the enhanced photocatalytic mechanism of the obtained photocatalysts,detailed studies and in-depth discussions were conducted.The main contents and results of this dissertation are as follows:(1)AgI/CeO2 composites were prepared by in-situ co-precipitation method using CeO2 prepared by sol-gel method as carrier.The obtained samples were analyzed via different characterizations.Under visible light irradiation,AgI/CeO2 composites exhibited good visible light photocatalytic degradation of RhB.Among them,the composite with the AgI content of 19.03 wt%exhibits the highest degradation ability.It can degrade 97.91%of RhB within 20 min.The photocatalytic degradation rate is0.1815 min-1.The enhancement of photocatalytic activities can be attributed to the suitable band gap position between AgI and CeO2,which could efficiently facilitate the separation of electron hole pairs.In addition,the dye adsorbed on the surface of the photocatalyst can also indirectly generate electrons due to photosensitization,further improving the photocatalytic efficiency.(2)In order to broaden CeO2 absorption in the visible light absorption region and improve the separation of photogenerated carriers,BiOI/CeO2 heterojunction photocatalyst was successfully prepared by a simple water bath method.The BiOI/CeO2 heterojunction photocatalyst not only has a good degradation performance towards the MO,but also can effectively degrade refractory pollutant BPA,and the degradation process is coincided well with the pseudo-first-order kinetics model.Among them,the BC-2(Bi/Ce molar ratio of 1:1)composite showed the highest photoactivity,and the degradation rates towards MO and BPA reached 93.75%and92.02%under visible light irradiation time of 40 min and 120 min.The removal rates gradually decreased with the increase of BPA concentrations.The pH of the solution is an important factor towards the BPA degradation.The enhanced photocatalytic degradation of composites is mainly due to the introduction of BiOI which broadens the visible light absorption region of CeO2,and forms an effective p-n heterojunction through intimate contact,which greatly improves the electron-hole pair transfer rate.(3)The shuttle-like CeO2 was prepared via urea hydrolysis method and a novel Ag2O/CeO2 p-n heterojunction photocatalyst was prepared by a two-step synthesis process.Under visible light irradiation,Ag2O/CeO2 heterojunction photocatalyst showed higher photocatalytic activity than pure Ag2O and CeO2 sample.The three-dimensional excitation-emission matrix fluorescence spectroscopy showed that the Ag2O/CeO2 composites can effectively destroy the conjugated heterocyclic structure of EFA molecule.The generated h+and?O2-are the main active groups involved in the degradation of organic pollutants under visible light irradiation.The possible mechanism of photocatalytic enhancement is the formation of p-n heterojunction between Ag2O and CeO2,which greatly enhances the separation of photogenerated carriers.The valence band offset(ΔECBO)and conduction band offset(ΔEVBO)of the Ag2O/CeO2 heterostructure were calculated to be-0.27 eV and-1.69eV,respectively.(4)A novel Z-scheme CeO2-Ag/AgBr heterojunction photocatalyst was prepared by in-situ loading and photoreduction method for exploiting novel CeO 2-based heterojunction photocatalysts.The CeO2-Ag/AgBr composite exhibited a highly photocatalytic activity toward the degradation CIP under visible light irradiation.Among them,the CAB-21.26 exhibited the best photocatalytic degradation of CIP,and its photocatalytic degradation kinetic constant was 0.02011 min-1.The study found that as the initial concentration of CIP increased,the degradation rate of CIP gradually decreased.In addition,the presence of bicarbonate and chloride ions has a more pronounced inhibitory effect on CIP degradation than the presence of sulfate and nitrate ions in the solution.Based on LC-MS analysis,three major photocatalytic degradation pathways for CIP were proposed.The generated h+,?O2-and?OH are the main active groups involved in photodegradation of organic pollutants.The enhanced photocatalytic performance on CeO2-Ag/AgBr composites is mainly through the formation of a Z-scheme heterojunction with silver nanoparticles as the recombination center.(5)A novel ternary Ag2CO3/CeO2/AgBr photocatalyst was prepared by in situ precipitation and acid etching route.Compared with the Ag2CO3/CeO2 and CeO2/AgBr binary composites,the ternary Ag2CO3/CeO2/AgBr composites exhibited improved visible light photocatalytic activity.The degradation rate of LVF was 87.63%within 40 min in presence of ACCAB-3 with the highest catalytic performance.Bas ed on LC-MS analysis,possible photocatalytic degradation pathways of LVF were proposed.The ACCAB-3 composite exhibited the highest mineralization capacity,and the TOC in the LVF aqueous solution could be reduced by 60.98%within 80 min.With the extension of the illumination time,the toxicity of the solution reduced and the biodegradability of the solution increased.After photocatalytic degradation for 40min,the disfection rate of the degradation solution towards E.coli decreased from the initial 52.61%to 15.18%.The enhanced photocatalytic degradation activity of the obtained photocatalyst is based on the formation of a dual Z-scheme heterojunction within the system,which greatly prolongs the lifetime of photogenerated carriers within the system.In addition,the photocatalyst has good recyclability and stability,and has great practical significance for the removal of antibiotics in waste water.