Synthesis Of TiO₂/Metal-organic Frameworks And Their Applications In Photocatalytic Oxidation Of Volatile Organic Compounds

Volatile organic compounds（VOCs）are the most important precursors of ozone,photochemical smog and PM_2.5,which are harmful to human health and natural ecological environment.Therefore,it is urgent to control VOCs pollution.Semiconductor photocatalytic oxidation technology has been identified as a promising method for indoor air purification because it can oxidate VOCs into CO₂ and H₂O under the mild operating conditions in the presence of photocatalysts.The key issue of photocatalytic technology is the development of stable and high-performance photocatalytic materials.TiO₂ is the most common photocatalyst in water treatment and air pollution control because of its relatively high photocatalytic activity,non-toxic,low cost and chemical stability.Metal-organic frameworks（MOFs）have drawn extensive attention due to its high specific surface areas,tunable pore size,wide spectral response and high adsorption capacity.Therefore,this paper selected photocatalytic oxidation of VOCs over TiO₂/MOF materials as the research system.The main research contents and results are as follows:1.TiO₂-UiO-66-NH₂ composites with different mass ratios are successfully prepared by a simple solvent evaporation method for photocatalytic oxidation of toluene and acetaldehyde under UV light condition.The composite exhibits superior removal efficiency for toluene and acetaldehyde than the pure TiO₂,UiO-66-NH₂and other TiO₂-based porous materials during a720 min of long-term evaluation under flowing conditions.The as-prepared 75TiO₂-25UN composites exhibit the highest photocatalytic activity and CO₂ selectivity.The conversion of75TiO₂-25UN for toluene and acetaldehyde are 72.7%and 70.74%respectively,which is about 9.7 and 10.5 times higher than that of UiO-66-NH₂,respectively.In addition,the CO₂production of 75TiO₂-25UN for toluene and acetaldehyde are 21.1 and 14.3 times higher than that of pure UiO-66-NH₂,respectively.N₂ adsorption-desorption and SEM results reveal that high surface area can enhance the dispersion of TiO₂ and the efficient contact of VOCs.Photoelectrochemical properties indicate that photoinduced carriers can be efficiently separated and transferred via the interface of two components.The outstanding deactivation resistance is achieved over 75TiO₂-25UN sample due to the efficient charge transfer and oxygen-rich condition.The possible mechanism of VOCs oxidation is proposed based on radical scavenger experiments,which proved that superoxide radical（·O₂^-）and hole（h⁺）are the main active species.2.TiO₂@UiO-66 composite with intimate contact interfaces is fabricated by in situ solvothermal method for efficient removal of organic pollutant（toluene,formaldehyde and Rhodamine B）under UV light condition.Compared with TiO₂,UiO-66 and TiO₂-UiO-66prepared by evaporation and mechanical methods,TiO₂@UiO-66 exhibits superior photocatalytic activity for oxidation of toluene into CO₂ during long-time reaction under flowing conditions.The optimal composite 4TiO₂@U exhibits the highest toluene conversion（66.59%）and CO₂ production（103.92 ppm）,which are 3.27 and 4.10 times higher than that of UiO-66 as well as 1.81 and 1.67 times higher than that of TiO₂,respectively.The superior performance of TiO₂@UiO-66 can be attributed to（1）the well-matched band structure and intimate contact interfaces between TiO₂ and UiO-66 in TiO₂@UiO-66 composite can effectively separate and transfer photogenerated electrons;（2）the synergistic effect between UiO-66 and TiO₂ in TiO₂@UiO-66 composite can not only promote the adsorption of toluene and desorption of CO₂,but also provide sufficient active sites for photocatalytic reaction.The conversion of formaldehyde over 4TiO₂@U reaches 83%after 78 h of continuous photocatalytic reaction without deactivation.In the case of Rhodamine B,the reaction rate constant of 4TiO₂@U is 8.63 and 1.82 times higher than that of UiO-66 and TiO₂,respectively.The·O₂^-and h⁺are the main active species for the degradation of toluene.Meanwhile,the in situ FTIR results point out that toluene can be oxidized by active species to benzaldehyde and benzoic acid,then further oxidized to oxalic acid,and finally mineralized into CO₂ and H₂O.3.The sandwich-like composite of Pt@MIL-101/TiO₂ is fabricated by the double solvent method（DSM）coupled with solvent evaporation method for photocatalytic oxidation of single VOC components（toluene and acetaldehyde）and their mixture during long-term evaluation under flowing condition.Compared with MIL-101,TiO₂,Pt@MIL-101 and MIL-101/TiO₂,the Pt@MIL-101/TiO₂ exhibits superior photacatalytic performance for toluene oxidation under UV conditions.Meanwhile,Pt@MIL-101/TiO₂ also shows excellent photocatalytic activity（75.30%）and CO₂ selectivity（76.34%）for acetaldehyde degradation.Compared with the single VOC components degradation,Pt@MIL-101/TiO₂ has a synergistic effect on the simultaneous degradation of toluene and acetaldehyde,and the conversions for toluene and acetaldehyde are 64.63%and 68.13%respectively.The conversion of Pt@MIL-101/TiO₂ for toluene in binary pollutant mixture is about 3.28,1.44 and 1.64 times higher than that of Pt@MIL-101,MIL-101/TiO₂ and MIL-101/Pt/TiO₂,respectively.In addition,the conversion of Pt@MIL-101/TiO₂ for acetaldehyde in binary pollutant mixture is2.03,1.32 and 1.37 times higher than that of Pt@MIL-101,MIL-101/TiO₂ and MIL-101/Pt/TiO₂,respectively.The superior activity of Pt@MIL-101/TiO₂ can be attributed to（1）the sandwich-like structure can spatially separate of photoinduced carriers;（2）The hydrophobicity and dipole moment of toluene and acetaldehyde are significantly different,which promote the simultaneous degradation of toluene and acetaldehyde by h⁺and·O₂^-,respectively.