Sythesis And Modification Of TiO₂ Nanotube Array Films And Study On Their Photocatalytic Reduction Performance Of Gas-phase CO₂

With the progress of human society,fossil fuel has been consumed in large quantities,which triggers the global energy crisis.Meanwhile,the consumption of fossil fuel releases excessive CO₂ into the atmosphere,which leads to the global warming.Therefore,how to solve the energy crisis and effectively reduce CO₂ to curb the global warming have become the major problems that need to be solved by all countries in the world.Photocatalytic CO₂ conversion technology has great potential in alleviating energy crisis and greenhouse effect because it can directly use solar energy and convert CO₂ into methanol,ethanol and other combustible compounds at room temperature and pressure.However,the bottleneck problem hindering the development of photocatalytic CO₂conversion technology is the extremely low CO₂ conversion efficiency.As a key factor affecting the reaction rate,the properties of photocatalytic materials are a hot topic in the research and attention of scholars all over the world.Nanotube array film has excellent photocatalytic properties due to its highly ordered tubular arrays,porous structures and large specific surface areas,so it has attracted much attention among researchers.In this paper,the TiO₂ nanotube array films with controllable morphology were prepared on the pure titanium sheet substrate by anodic oxidation method,and the photocatalytic reduction gas-phase CO₂ reaction system and micro-reactor were designed and constructed.Meanwhile,we studied the effects of different preparation process parameters of catalyst,volume flow rate of CO₂,light source and illumination intensity on products yield.In order to further enhance the CO₂ conversion efficiency and expand the response spectrum of the catalyst,we modified the TiO₂ nanotube array film.Forming element,chemical state and absorption spectrum of the photocatalyst were characterized by SEM-EDS,TEM,XRD,XPS,FTIR and UV-vis.Furthermore,the effects of the modifaction on the products yield of photocatalytic conversion CO₂ was deeply studied.Research contents and results will be briefly presentated as follows:?1?The effects of different preparation parameters of TiO₂ nanotube array?TNTA?on the photocatalytic reduction of CO₂ were studied.TiO₂ nanotube arrays synthesized by anodic oxidation on pure titanium sheet in electrolyte comprising ammonium fluoride?NH₄F?,water containing ethylene glycol.TNTA was prepared under different process parameters includeing the concentration of NH₄F in the electrolyte,the applied voltage,the calcination temperature,and the properties of the sample were tested under 365nm-wavelength ultraviolet light.The effects of different content of NH₄F in the electrolyte,applied voltage,and the calcination temperature on performance of TiO₂ nanotube arrays were researched.The experimental results show that the TiO₂ nanotube array has excellent photocatalytic performance due to their good crystal shape,porous morphology,and great specific surface area.The main product of CO₂ conversion is methanol.When NH₄F concentration is 0.25 M,the applied voltage is 60 V and the calcination temperature is 500 degree centigrade,the highest methanol yield is 138.23 nmol/?cm²-cat·h??about 258.57?mol/?g-cat·h??.According to the characterization analysis,the main factors affecting the performance of the catalyst include weight,specific surface area,crystallinity and the ability to absorb light which are mainly determined by the preparation process parameters of TNTA.Therefore,adjusting the technological parameters of anodic oxidation is a significant method for enhancing the efficiency of photocatalytic reduction of CO₂.?2?The effect of TNTA modified with graphene on the photocatalytic reduction of CO₂ was investigated.RGO-TNTA composite catalyst were synthesized by vapor-thermal treatment which made graphene oxide modifying on surface of uncrystallized TiO₂ nanotube arrays.Effects of different GO concentrations,light sources,illumination intensities and volume flow velocities of CO₂ on photocatalytic reduction performance of CO₂ were respectively studied.The experimental results show that the main product is methanol both under visible light and ultraviolet light.Photocatalytic performance of the RGO-TNTA composite catalyst under ultraviolet light and visible light respectively are1.26 times and 2.22 times as that of bare TNTA.The highest yields are up to 302.40nmol/?cm²-cat·h??about 565.70?mol/?g-cat·h??and 198.51 nmol/?cm²-cat·h??about371.36?mol/?g-cat·h??under ultraviolet light and visible light when the GO concentrations are 0.2 mg/mL and 0.3 mg/mL,the CO₂ volume flow velocities are 20.5mL/min and 25 mL/min,the light intensity are 36 mW/cm² and 80 mW/cm²,respectively.Modifying with graphene improves the mobility of surface free electron motivated by light.And C element doping widens the absorption light wavelength to visible range.TNTA crystallizes and forms relatively small size of crystal grain by vapor-thermal treatment,which increases the specific surface area and reduces the recombination rate of electron hole pair.So the performance of the photocatalytic reduction of CO₂ is greatly enhanced.?3?The effect of CdS/ZnS-sensitized TNTA on the photocatalytic reduction of CO₂was studied.Crystallized TiO₂ nanotube arrays cosensitized with CdS/ZnS quantum dots?CdS/ZnS QDs?was synthesized via SILAR cycle method.CdS/ZnS QDs deposited into the surface of TiO₂ nanotube arrays and its load depends on the number of SILAR cycles.The effects of SILAR cycle number,CO₂ volume flow rate and illumination intensity on the performance of photocatalytic reduction of CO₂ were respectively studied.The results show that the main product is methanol,while its performance under visible light is 2.73times as that of bare TNTA.The sample with optimal performance is treated by 10 SILAR cycles,and whose broad absorption band red shifts to 524 nm.The yield with the increase of light intensity is up to 255.49 nmol/?cm²-cat·h??477.95?mol/?g-cat·h??when the CO₂volume flow rate is 20.5 mL/min.Cds/ZnS QDs mainly widens the response wavelength range of the catalyst to visible light and partly inhibits the recombination of electron hole pair.So the performance of CdS/ZnS-TNTA is greatly enhanced under visible light.