The Preparation Of CQDs/MOFs And Their Photocatalytic Removal Performance Of NO_x

Semiconductor photocatalytic oxidation technology to remove Nitrogen oxides?NO_x?is a promising air purification technology.As a kind of semiconductor photocatalytic material,the porous metal organic frameworks?MOFs?material has the advantages of large specific surface area,excellent adsorption performance and easy functionalization,which has attracted extensive attention in the treatment of atmospheric pollutants.But its?wide band gap,limited spectral response range and low carrier separation efficiency result in poor photocatalytic performance,it is still challenging to effectively improve the visible light responsiveness of MOFs.Studies have shown that the unique up-conversion fluorescence properties and electron mediators of carbon quantum dots?CQDs?make it plays an important role in the construction of wide spectral response photocatalysts.In view of the above problems,first,we synthesized ZIF-8 in three different reaction systems and tested the photocatalytic removal performance of NO.Then,CQDs/ZIF-8 and N/CQDs/MIL-125?Ti?photocatalysts?N/CM?Ti??were synthesized,and the mechanism of photocatalytic NO_x removal was discussed.The research works of this text are as follows:?1?ZIF-8 was synthesized with Zn?NO₃?₂·6H₂O or Zn?OH?₂ as zinc source and 2-methyl imidazole?2-mIm?as ligand.Three different synthesis methods of ZIF-8?in three different synthesis systems?were investigated:synthesis of ZIF-8 in H₂O solvent,CH₃OH solvent and CH₃OH/NH₃·H₂O mixed solvent,respectively.The physicochemical properties of the samples were preliminarily analyzed by XRD,SEM,FTIR and BET characterizations.The photocatalytic performance was evaluated by NO_x analyzer,and it was found that ZIF-8 has photocatalytic removal activity for NO only under ultraviolet light irradiation.SEM results showed that the morphology of the samples synthesized in three different systems is significantly different:in the aqueous phase system,with the increase of precursor concentration,the morphology of ZIF-8 change from two-dimensional leaf shape?ZIF-L?to three-dimensional cube?ZIF-8?,the specific surface area was significantly enhanced,and the photocatalytic activity also increased;the morphology of ZIF-8 synthesized in CH₃OH solvent and CH₃OH/NH₃·H₂O solvent is quite different.?2?CQDs/ZIF-8 photocatalysts with different CQDs loading amount?0.1mL,0.5 mL and 2.0 mL?were synthesized by a surface soaking process.The physical and chemical properties of the composite materials such as structure,morphology and light absorption properties were analyzed in detail.The activity test results showed that 0.5-CQDs/ZIF-8 had the best photocatalytic removal performance for NO_x?⁴3%?under visible light irradiation.Photocurrent test and electrochemical impedance?EIS?test results showed that the carrier separation efficiency of the composite is nearly 1.6 times higher than that of ZIF-8.The up-conversion fluorescence spectroscopy?UCPL?,radical capture experiments at single wavelength light irradiation and ESR characterization confirmed the up-conversion properties of CQDs played a role in drving ZIF-8as a visible light responsive photocatalyst,however,the sample was obviously deactivated after three cycles of use.?3?MIL-125?Ti? was successfully synthesized by a hydrothermal method,and N/CM?Ti?composites with different N/CQDs loading amount?0.5 Vol%,2.5 Vol%and 5.0 Vol%?were synthesized by a surface soaking process.The material with 2.5 Vol%loading amount of N/CQDs had the best visible light photocatalytic activity and good stability for NO_x.Furthermore,characterization techniques such as XRD,XPS,DRS,SEM,BET and TEM were developed for the analysis of physicochemical properties.The results of XPS test indicating that N/CQDs are combined with MIL-125?Ti?in the form of N-Ti-O bond,which is favorable to facilitate carrier migration.Photocurrent test and electrochemical impedance spectroscopy test results show that the carrier separation efficiency of 2.5 Vol%N/CM?Ti?composite is higher than that of MIL-125?Ti?.There are two possible electron migration paths under visible light excitation:On the one hand,visible light excites N/CQDs to produce photoexcited electrons that migrate to the Ti^?-Ti^? metal center;On the other hand,the H₂BDC ligand of MIL-125?Ti?absorbs the ultraviolet light from N/CQDs to generate photoexcited electrons,and then migrates to the Ti^?-Ti^? metal center metal center.