Research On The Construction Of TiO₂-based Composite System And Photocatalytic Conversion Of Small Molecules

With the rapid development of industrial production and social economy,energy shortage and environmental problems are becoming to two main problems the world is currently facing.Therefore,from practical point of view,it is of great significance to develop a green and sustainable technology to alleviate the environmental and energy crisis.Photocatalytic technology,a chemical reaction over photocatalyst using solar energy under relatively mild reaction conditions,is expected to achieve the goal of converting solar energy into chemical energy directly.The photocatalytic technology has been gradually recognized as a promising chemical transformation to simultaneously alleviate energy shortage and environmental problem.As a typical semiconductor photocatalyst,TiO₂ has been widely investigated in photocatalytic conversion.However,it still suffers from low utilization rate of visible light and serious photogenerated electron-hole recombination loss,which severely restrict the photocatalytic performance of TiO₂photocatalyst.To overcome this limitation,this thesis is completed from two perspectives of improving the electron-hole separation efficiency and light utilization ability of TiO₂.We investigate the photocatalytic conversion of small molecules from three aspects:heat-assisted strategy,photothermal materials combining and decorating porous polymer on TiO₂.The main contents of this research are as follows:（1）To improve the photocatalytic conversion efficiency of TiO_2,we introduce a polypyrrole photothermal film into the gas-solid phase reaction system.Under the photothermal film assisted photocatalytic system,the TiO₂ shows high photocatalytic performance especially for CH₄ production,i.e.,18.3μmol g^-1 h^-1,which is 3.5 times higher than that the pure TiO₂ system.The photothermal film provides the hot water vapor for the reaction system,which promotes more and more effective protons to participate in the reaction.Meanwhile,the local photothermal effect will enhance the movement of gas molecules and charge carriers.In addition,the photothermal effect provides energy for the activation of CO₂ and the formation of C-H in the photocatalytic reaction.The superior photocatalytic performance towards CO₂ conversion can be attributed to the photothermal synergetic catalysis.（2）Inspired by the heat-assisted photocatalytic strategy,we introduce graphene into TiO₂photocatalyst.A glucose-assisted solvothermal strategy is developed to fabricate TiO₂-graphene（TiO₂-G）.The TiO₂ nanoparticles were uniformly adhered on the graphene surface with assistance of glucose to the formation of TiO₂-G composite with strong interfacial interactions.The introduction of graphene brought insignificant impact on the surface area and CO₂ uptake ability of TiO₂,however it greatly improved the charge carrier separation and surface temperature of photocatalyst,giving a high CH₄ production rate of 26.7?μmol?g^-1?h^-1,which is 5.1 times higher than that over pure TiO₂.A remarkable photothermal effect is discovered in the optimized TiO₂-G composite and demonstrated to have great impact on the photocatalytic efficiency based on the temperature dependent kinetic analysis.The synergetic effect of the increased electron mobility and surface temperature dramatically enhanced the photocatalytic efficiency.Moreover,the measurements of oxidation and reduction products from CO₂ and H₂O are of great significance in the complete understanding of photocatalytic mechanism.（3）In order to broaden the visible light absorption,a porous hypercrosslinked polymer-TiO₂-graphene composite structure（HCP-TiO₂-FG）with enrich porous structure and large specific surface area was successfully constructed by in situ knitting strategy.The HCP-TiO₂-FG demonstrate enhanced photocatalytic CO₂ reduction performance under visible light can be ascribed substantial CO₂ capture ability.We investigate the adsorption capacity of the composite photocatalyst by adsorption kinetics and adsorption isotherm models,this composite shows high adsorption capacity.Moreover,the HCP-TiO₂-FG composite shows high photocatalytic degradation performance of SDZ and 4-CP under visible light.The enhanced photocatalytic performance can be ascribed to large specific surface area,visible-light absorption,and photo-generated charge separation efficiency.Due to the composite unique porous structure,it provides channels for the transfer of adsorption molecules from the surface to the internal micropores,which is beneficial to the photocatalytic oxidation of organic pollutant molecules on the catalyst surface.Moreover,we also make research on the photocatalytic degradation mechanism,and present possible degradation pathway.