Convert Non-rare Earth Luminescent Materials Modified Nano Tio <sub> 2 </ Sub> Visible Light Catalytic Reduction Of Co <sub> 2 </ Sub> Research

Photocatalytic reduction of CO2would be like killing two birds with one stone in terms of saving our environment, which is considered to be the best way to solve energy and environment problems and is one of the hotspots in current photocatalysis research. This dissertation firstly presents an overview of the research progress on photocatalytic reduction of CO2at home and aborad. Aiming at the shortage of TiO2photocatlyst, the modification mechanisms and its trends related to TiO2at present are analyzed in detail. On this basis, the research background, basic principle and research progress of TiO2photocatalysis based on upconversion luminescent materials (ULMs) are introduced. The results obtained can be summarized as follows:1) A rare-earth free upconversion luminescent material BaF2:NaxAlyFx+3y (0.3<x<0.5,0<y<0.1, NaxAlyFx+3y is employed to denote NaF and NasAlF6for short), is synthesized by a hydrothermal method for the first time. It is a composite consisting BaF2, NaF and NasAlF6, their corresponding molar ratio is about10:1:1. The study of fluorescent spectrum indicates that it can convert visible light (550nm-610nm) into ultraviolet light (290nm-350nm), and two emission peaks at304nm and324nm are observed under the excitation of583nm at the room temperature. Subsequently, TiO2/BaF2:NaxAlyFx+3y composite is prepared by ultrasonic dispersion and annealing treatments for visible light photocatalysis based on upconversion emission. The light energy transfer from BaF2:NaxAlyFx+3y to TiO2under visible light is confirmed by fluorescence spectroscopy testing. It pohotocatalytic activity is evaluated by the photocatalytic reduction of CO2under visible light. The results show that the composite is an effective visible light photocatalyst, the corresponding methanol yield can reach179μmol/g-cat under certian conditions. Additionally, the mechanism of photocatalytic reduction of CO2on TiO2/BaF2:NaxAlyFx+3y is analyzed.2) Rare-earth free upconversion luminescent material CaF2:NaxAlyFx+3y (0.3<x<0.5,0<y<0.1), is synthesized by a hydrothermal method based on the above research. It contains BaF2, NaF and NasAlFe with a molar ratio of10:5:3.5. It can convert visible light (570nm-640nm) into ultraviolet light (300nm-360nm), and two emission peaks at316nm and336nm are observed under the excitation of610nm at the room temperature. Subsequently, TiO2/CaF2:NaxAlyFx+3y composite is prepared and its photocatalytic activity is evaluated by the photocatalytic reduction of CO2under visible light. The results show that light energy transfer can occur from CaF2:NaxAlyFx+3y to TiO2, and the composite is also an effective visible light photocatalyst for CO2reduction, the corresponding methanol yield can reach 137μmol/g-cat under certian conditions. The results of above studies indicate that ULMs can make TiO2with visible light photocatalytic activity.3) ULMs and metal ion are employed to modify TiO2together, and the visible light photocatalyst4wt%Fe-TiO2/BaF2:NaxAlyFx+3y composite is prepared by an improved sol-gel method. Results show that Fe+can inhibit the recombination of photoexcited electron and hole, and light energy transfer can be achieved from CaF2:NaxAlyFx+3y to Fe-TiO2based on upconversion emission. The experimental results indicate that the synergetic effect of Fe3+and BaF2:NaxAlyFx+3y can improve the visible light photocatalytic activity of TiO2. The orthogonal experiment results show that catalyst dosage and CO2flow rate have great influence on the methanol yield comparing with reaction time and reaction temperature. The methanol yiled can reach243.9μmol/g-cat under optimal conditions, the corresponding quantum efficiency ΦO is about0.57%, and energy efficiency ΦE is about0.17%. Simultaneously, the experimental results indicate that the activation energy of the photocatalytic reaction with4wt%Fe-TiO2/BaF2:NaxAlyFx+3y composite is about15.9kJ/mol, suggesting that the desorption event is a rate limiting step.4) The luminescence mechanisms of above two materials BaF2:NaxAlyFx+3y and CaF2:NaxAlyFx+3y are investigated preliminarily. The results show that they are not a simple physical mixture of BaF2(CaF2), NaF and Na3AlF6. The upconversion emission belongs to the excited state absorption based on the study of excitation spectra, emission spectra and phosphorescence life time of the samples. The upconversion process may be related to additional energy levels in BaF2and CaF2after the hydrothermal reaction and annealing treatment. Under optimal preparation conditions, the internal quantum yield and external quantum yield of BaF2:NaxAlyFx+3y can reach0.068and0.012, the corresponding absorption rate is0.180, and the internal quantum yield and external quantum yield of CaF2:NaxAlyFx+3y can reach0.064and0.004, the corresponding absorption rate is0.059.5) The light energy conversion and utilization model is established based on the upconversion luminescent material composite with TiO2as an example. The calculation formula for light absorption efficiency ηA, light conversion and utilization efficiency ηc and total light utilization efficiency η are deduced. Simultaneously, an upconversion luminescent material composite CaF2+3BaF2:NaxAlyFx+3y is synthesized by a one-step hydrothermal process. The emission peaks at316nm and336nm under the excitation of610nm, and the emission peaks at304nm and324nm under the excitation of583nm can be observed respectively, the phosphorescence lift time is greater than3ms. The internal quantum yield and external quantum yield are0.042and0.004respectively, the corresponding absorption rate is0.092. The results show that the upconcversion properties of BaF2:NaxAlyFx+3y and CaF2:NaxAlyFx+3y are not changed in the composite. However, there is little difference between the emission spectra of composite and individual upconversion luminescent materials, because of the superposition effect of their excitation spectra.6) The research progress on photocatalytic reactor for CO2reduction is reviewed. Subsequently, the characteristics of bubble-type, thin film-type and internal loop-type reactors used by our group are introduced, and their operation process, advantages and disadvantages are summarized respectively. Finally, a multifunctional photoreactor is designed and established by optimizing of the closed type internal loop reactor, which consists of five parts, including gas circulation system, photoreaction system, separation system, detection system and light sources. This multifunctional photoreactor can be applied to liquid-solid, gas-solid and gas-liquid-solid photocatalytic reactions. In this paper, it is applied to the photocatalytic reduction of CO2In summary. BaF2:NaxAlyFx+3y and CaF2:NaxAlyFx+3y are new type of ULMs without rare-earth elements, this discovery can extend the range of research on ULMs. Meanwhile, the study on photocatalytic reduction of CO2with TiO2modified by above two ULMs provides a new academic thought for the conversion of solar energy into chemical energy. Addationally, a light energy conversion and utilization model based on the upconversion luminescent material composite is proposed and established, which greatly enriches the light energy conversion and utilization ways. This dissertation not only provides a new research method for development of visible-light-driven photocatalysis system, but also finds out a reasonable way to solve the contradiction between energy and environment.