| Cerium(Ce),as the element with the highest abundance among rare earth elements,is famous for its abundant reserves,low prices,high oxygen storage capacity and stable chemical properties of its oxide form cerium oxide(CeO2).In the last decade,CeO2-based materials have received increasing attention in the field of photocatalysis,and can even be used as a substitute for traditional TiO2 photocatalysts in applications of wastewater treatment,water splitting to produce hydrogen or oxygen and carbon dioxide reduction.However,as a photocatalyst material,CeO2 still presents some problems that need to be solved,such as weak absorption capacity of visible light,lack of active reaction sites and high recombination rate of electron-hole pairs,which greatly hinder its applications as a low-cost photocatalyst.The performance improvement is generally attributed to two aspects:one is the modification itself and the other is the addition of other materials to make up for its own shortcomings.The former can be carried out in terms of regulating morphology,increasing active oxygen vacancy concentration and controlling energy band structure.The later is mainly focused on the addition of other components to form heterogeneous structured photocatalysts.In this dissertation,a series of CeO2-based photocatalysts were synthesized via different modification methods,such as morphology was controlled to increase the active crystal surface and active sites;rare earth ions were doped to regulate the band gap and extend the spectrum response range;noble metal Ag was deposited to enhance the surface plasmon resonance effect and improve the visible light response;composite heterostructures of YCeO2/PCN and CeO2/Ce-MOF photocatalysts were constructed to construct internal electric fields and inhibit the recombination of photogenerated carriers.In addition,an indoor pollutant gas of acetaldehyde(CH3CHO),was used as degradation model to explore the related mechanism of synergistic enhancement of the photocatalytic performance of CeO2 by these strategies.Hence,the specific research contents mainly consist of the following five parts:1.Investigation of CeO2 photocatalysts and their photocatalytic performance investigation CeO2 photocatalysts with different morphologies and different types of ions doped-CeO2 products were prepared by solvothermic method.The comparison data showed that the flowerlike CeO2(CeO2-F)has the best degradation performance for acetaldehyde pollutants,and the rare earth(RE)ions doped-CeO2 showed better CH3CHO degradation performance under the same conditions.The formation energies of(111),(110)and(100)crystal planes of CeO2 cell before and after doping were evaluated by means of density function theory calculation.The calculated results show that Sm-CeO2 exhibit relatively negative energy for all crystal planes,and Sm ions are more prone to substitution and doping on active(100)crystal planes.Flowerlike morphology of CeO2 usually exposes more(110)and(100)active crystal faces,which also means that they own more active sites and are conducive to photocatalytic reactions.Therefore,CeO2-F photoctalyst was selected to investigate the influence of different RE ions doping in next chapter.2.Preparation of different rare earth metal ions(Sm,Y,La,Nd)-doped CeO2 photocatalysts and their photocatalytic activities investigationThis chapter aims to further explore the influence of different rare earth(RE)metal ions doping on the photocatalytic performance of CeO2.Herin,CeO2 photocatalysts doped with RE ions(RE=Sm,Y,La,Nd)were prepared by hydrothermal method.The comparison results showed that Sm doped CeO2(SC)had better photooxidation activity,and the CH3CHO degradation activity was 2.24-times higher than that of undoped CeO2.X-ray photoelectron spectroscopy and Raman spectrum test results indicated doping can increase the concentration of oxygen vacancy(Ov)in CeO2,and abundant Ov can be used as the capture center of photogenerated electrons,then forming doping transition states between conduction band and valence band.Therefore,the recombination rate of electrons and holes were effectively limited,and the photooxidation degradation performance was significantly improved.In addition,highly active hydroxyl radicals(·OH)and superoxide radicals(·O2-)are efficient intermediates with remarkable oxidation capacity,and their generation can further improve the photocatalytic activity.3.Synthesis of samarium-doped CeO2 photocatalyst modified by Ag quantum dots and its photocatalytic property investigationAlthough the samarium(Sm)ions doping strategy can effectively improve the performance of CeO2 photocatalyst,it still has the shortcomings of weak absorption of visible light and low utilization efficiency.Therefore,the third chapter is based on the Sm ions doping strategy and introducing Ag quantum dots for anchoring,and finally prepared the waxberrylike CeO2 photocatalyst(short named ASC).The results showed that the activity of ASC photocatalyst reached about 6.04-times and 5.83-times higher than that of CeO2 in CH3CHO removal and in H2 production from H2O reduction under visible light,respectively.In addition,through the analysis of transient photovoltage,surface photovoltage and density functional theory calculation results,the migration mechanism of photogenerated carriers in ASC photocatalyst is elaborated and the mechanism of photocatalytic performance improvement is revealed in depth.Sm doping reduces band gap of CeO2 and doping-related transition states can capture photoexcited electrons.Then these captured electrons are further transferred to the co-catalytic site of the anchored Ag quantum dots to participate in the reaction.Meanwhile,the enhancement effect of surface plasmon resonance(SPR)can improve the absorption and utilization efficiency of visible light,that means the photoredox performance of ASC photocatalyst can be significantly improved under visible light.4.Construction of yttrium-doped CeO2/PCN heterojunction photocatalyst and its photocatalytic performance investigationIn the fourth chapter,Y-doped CeO2 and PCN were in-situ coupled to prepare YCeO2/PCN(YCC)heterojunction photocatalysts,and the photocatalytic CH3CHO degradation activity under visible light reaches 8.50-times higher than that of CeO2 component.At the same time,the ability of H2 production of H2O reduction is 3.74-times higher than that of CeO2,this value is lower than that of ASC photocatalyst in the previous chapter,which can be attributed to the better reduction performance of Ag catalytic site.Further,this chapter also puts forward the possibility mechanism of Y-CeO2 and PCN complement each other in simultaneous improvement of photoredox performance:due to highly matched band structure position between the two components can effectively construct electron-transfer-typeheterojunction and establish an effective internal electric field,this built-in electric field can effectively separate photogenerated electrons and hole pairs and inhibit their recombination,which is definitely conducive to the synergistic promotion of photoredox performance.5.Design of isogenous S-scheme CeO2/Ce-MOF heterojunction and its photocatalytic efficiency investigationIn order to enhance the photoexcited carrier separation and migration ability and further improve the reaction rate on CeO2 photocatalysts surface,the construction of a redox center separation system that can improve exciton separation also has a significant effect.In the fifth chapter,a homologous step-scheme(S-scheme)photocatalyst of CeO2/Ce-MOF was prepared by growing CeO2 nanoparticles onto a cubic cerium-based metal-organic framework(CeMOF)via a one-pot hydrothermal treatment.Under visible light irradiation,the degradation of gaseous CH3CHO by CeO2/Ce-MOF photocatalyst was complete within 12 h(the reaction time of above-mentioned four chapters were 24 h).When compared with CeO2,the degradation efficiency was increased to 9.52-times,and the apparent quantum efficiency(AQE)at 420 nm reaches 7.15%.In addition,the S-scheme charge separation on CeO2/CeMOF photocatalyst is verified by in-situ XPS spectroscopy and KPFM characterization,and the rationality of the reaction pathway and related reaction mechanism of actual photocatalytic CH3CHO degradation is further verified by calculation based on DFT calculation combined with in-situ DRIFTS analysis.On the one hand,the improvement of photocatalytic efficiency is due to the effective photogenerative carriers’ separation ability and the extension of photoexciton reaction lifetime.On the other hand,the internal electric field created by the two components can recombine these useless electrons and holes,so as to retain abundant photocarriers with stronger catalytic reaction ability.This chapter shows that,in comparation with other modification strategies,the construction of isogenous S-scheme heterojunction photocatalysts can more effectively cooperatively improve the photocatalytic performance of seriously harmful gaseous VOCs pollutants removal efficiency on CeO2-based photocatalysts. |
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