Research On The Designing Preparation And Properties Of Zinc Titanate(Gallate)-based Solid Solution Photocatalysts

Photocatalytic CO2 reduction has attractive potential applications for solving environmental and energy problems.The key to building an efficient photoreduction CO2 reaction system is the design of semiconductor photocatalytic materials.Improving the spectral effect range,quantum efficiency,activity and cycling stability of photocatalytic materials is undoubtedly the core of the design of CO2 reduction photocatalyst.This paper is based on designing semiconductors band-gap engineering theory,using Zn2TiO4 as a matrix for the solid solution photocatalyst,Zn2Ti1-xGexO4 and Zn2Ti1-xSnxO4 solid solutions photocatalysts were synthesized with Na2SO4 and K2SO4 as molten salt media through high-temperature molten salt technology.Simultaneously,Zn(1+3x)/2Ga1-2xFexGexO2+2x(0?x?0.15)solid solution was synthesized by expanding the system.Solid-solution photocatalysts catalytic conversion CO2 and H2O to carbon-based fuels as research target to investigate the photocatalytic reaction performance and possible enhancement mechanism of CO2 reduction performance on these three types of solid solution materials.The main research contents are given as follows:(1)The nano-Zn2Ti1-xGexO4(0?x?0.15)solid solution was prepared by molten salt method at 1000°C for 12 h.It was found that the pseudocubic Zn2GeO4 can be introduced into the cubic spinel Zn2TiO4 to form a Zn2Ti1-xGexO4(0?x?0.15)replacement solid solution.Under simulated sunlight irradiation,it was found that CO2 and H2O to generate CH4 and CO could reach 0.4 ?mol h-1 g-1 and 11.8 ?mol h-1 g-1,with Zn2Ti0.9Ge0.1O4 solid solution reacts,respectively.In addition,this photocatlytic activity is stable in the 200-hour cycle test.However,the rate of foemation of CH4 and CO was only 0.2 ?mol h-1 g-1 and 8.88 ?mol h-1 g-1,using pure Zn2TiO4 under same light conditions.The analysis of catalyst composition,structural characterization and theoretical calculations illustrates that Ge concentration could reduce the band gap of the solid solution and the hole effective mass of the carrier decrease,and the delocalization of the conduction/valence optimization,which in turn leads to more light absorption and higher carrier diffusion.Therefore,the Zn2Ti1-xGexO4(0?x?0.15)solid solution photocatalyst exhibits higher photocatalytic reduction CO2 on the preparation of carbon-based fuels performance.(2)Nanosized Zn2Ti1-xSnxO4(0?x?1.0)continuous solid solution photocatalysts were prepared by molten salt method at 1000? for 12 h.The activity test results show that under the simulated sunlight irradiation,when x = 0.6,the activities of the Zn2Tio.4Sn0.6O4 solid solution catalytic conversion CO2 and H2O to generate CH4 and CO to reach 0.16 ?mol h-1 g-1 and 17 ?mol h-1 g-1,respectively.And the activity was stable in the 200-hour cycle test.The rates of CH4 and CO formation were 0.8 and 2 times the photocatalytic activity of pure Zn2TiO4 under the same conditions,respectively.Combining with catalyst characterization and DFT theoretical calculation results,the introduction of cubic spinel Zn2SnO4 in cubic spinel Zn2TiO4 not only enhanced the oxidized capacity of Zn2TiO4 semiconductor valence band,but also improved the valence band delocalization performance.The enhanced ability of valence band oxidation and the increase of hole-migration rate result in Zn2Ti1-xSnxO4(0?x?1.0)solid solution exhibiting higher photocatalytic reduction of CO2 performance.(3)Na2SO4 and K2SO4 were used as the molten salt medium.The high-temperature molten salt method was used to simultaneously introduce the lanthanum cubic spinel ZnFe2O4 and the pseudocubic Zn2GeO4 to the cubic spinel ZnGa2O4 crystal structure.Ternary Zn(1+3x)/2Ga1-2xFexGexO2+2x(0?x?0.15)solid solution was successfully prepared.The structural characterization and performance evaluation of the bonded materials revealed that the prepared ternary solid solution photocatalyst is not only composed of uniform and short nanorods,but also can convert CO2 and H2O into CO and O2 under simulated sunlight irradiation.Specifically,it has a certain photocatalytic activity under visible light conditions.The DFT theoretical calculation results show that the introduction of cubic spinel ZnFe2O4 and the pseudocubic Zn2GeO4 to the cubic spinel ZnGa2O4 crystal structure includes the addition of Ge and Fe elements in the ZnGa2O4 crystal structure.The interaction between the valence band and the conduction band Ge-O,the interaction between Ge 4s 4p and O 2p leads to the existence of antibonding and bonding orbitals in the conduction band and valence band,which leads to the p-d orbital exclusion.The result of p-d repulsion causes the valence band to move upwards.At the same time,the conduction band position is reduced because the Fe 3d orbital forms a new continuous energy level in the conduction band of the bulk photocatalyst.The upward movement of the valence band and the downward movement of the conduction band lead to the reduction of the forbidden band width and the expansion of the light absorption range of the solid solution photocatalyst.On the other hand,the Zn(1+3x)/2Ga1-2xFexGexO2+2x(0?x?0.15)ternary solid solution photocatalyst has improved band structure delocalization performance,which means higher carrier migration.Ability to improve the redox reaction rate of H2O and CO2.Therefore,the ternary solid solution photocatalyst formed by the pseudocubic 2n2GeO4,cubic spinel ZnFe2O4 and cubic spinel ZnGa2O4 show a higher photocatalytic reduction CO2 performance.