Study On Molecular Simulation,Preparation Of Rare Earth Doped TiO₂ And Its Application On NO Degradation

With the rapid development of transportation industry,traffic environment problem have become increasingly prominent,and the mass automobile exhaust has profoundly endangered human body and urban environment.Semiconductor photocatalysis technology has been a new effective way in environment pollution control for following advantages,such as mild reaction conditions,no secondary pollution,and low cost.However,nano TiO₂ large band gap and low quantum utilization ratio limit its application range.Doping is an effective way to improve the photocatalytic activity of TiO₂,in which the unique 4f and 5d electrons of rare earth elements can broaden the range of TiO₂ light response and improve the quantum conversion efficiency.But there are many different kinds of rare earth,so the development and improvement job of photocatalyst has much uncertainty.Molecular simulation technology,which can calculate the electronic structure of the material on the atomic scale and explain the material microscopic and macroscopic properties,to provide a theoretical basis for rare earth ions modified and predict the catalytic activity of the material.In this paper,the Materials Studio software was used to construct the rare earth doped TiO₂ super cell model and explain the modification mechanism from the point of view of electronic structure,and it was also predicted that the catalytic activity of the modified material.Based on the theoretical analysis,the rare earth modified photocatalyst with visible light activity was synthesized by ultrasonic assisted sol-gel method.It was sdudied by TG-DSC,XRD,SEM,XPS and UV-Vis that the effect of fabrication methods on physical and chemical properties and photocatalytic activity of nanocatalyst.The nitric oxide（NO）was used as degradation object for its representation within vehicle exhaust emission,and the NO degradation property was researched under different La-TiO₂ dosages or environment conditions.The theoretical simulation calculation on geometry structure,band structure and charge distribution was carried out based on first-principles theory.The calculation result indicated that doping rare earth ions into TiO₂ can cause lattice expansion,change charge distribution and form a new inner electric field which was conducive to the separation of the photogenerated charge carriers.The highest occupied molecular orbital of doped semiconductor moved down and photogenerated holes oxidation capacity was improved.Doped La,Ce increased the band gap of TiO₂ to 2.299ev and 2.27ev,respectively,and both introduced intermediate levels at the top of valence band.Doped Sm produced impurity levels at the bottom of conduction band,which connected with conduction band and narrowed band gap to 2.16ev.The induced impurity levels contributed to the separation of charge carriers and extension of the photoabsorption region.Whitin the same doping concentration,Sm introduced the maximum density of impurity levels in which the unoccupied molecular orbitals were effective shallow trap for electron-hole pairs.With tetrabutyltitanate as a precursor,La-doped nanoparticles TiO₂ were prepared using ultrasonic assisted sol-gel method.The influence of doping content,heat-treatment temperature and heat-treatment time on microstructure and photocatalytic activity of modified catalyst were investigated by physical and chemical testing.Doped La promoted grain growth when the content was 0.03%but degreased the grain size with content of La increasing.La inhibited the crystallization of anatase TiO₂ but did not change the phase transition temperature of rutile TiO₂ significantly.The binding energy of Ti decreased,and the number of hydroxyl radical adsorbed on the surface of TiO₂ increased.The absorption ability in the UV region had significant improvement while the absorption wavelength edges shifted to a long wave direction.The orthogonal test indicated that influence of process parameters on the photoactivity from most significant to least significant was heat-treatment temperature,dosage of La,heat-treatment time.The optimal synthesis conditions of La-TiO₂ were doping content for 0.5%,heat-treatment temperature for 550℃,heat-treatment time for 2h.La doped TiO₂ showed high degradation activity of NO under visible light.The ultimate degradation of 1.25ppm NO under 26w UV lamp and 33w halogen lamp were 65.4%and 76%respectively,and up to 96.9%of 0.5ppm NO under halogen lamp.The visible light response Sm doped TiO₂ photocatalyst was synthesized by ultrasonic assisted sol-gel method and the influence of fabrication methods on physical and chemical properties and photocatalytic activity of nanocatalyst were sdudied.Phase transition temperature range was broadened in Sm-doped TiO₂,and the size of TiO₂ grain decreased with the higher Sm content and the lower treating temperature.XPS analysis indicated that partial Ti4+ ion has converted to Ti3+,O/Ti atomic ratio reduced,and the number of oxygen vacancies and adsorbed hydroxyl on the surface of lattice increased.UV-Vis showed doping Sm iron into TiO₂ lead to a red-shift of absorption spectra and the increase of absorption in ultraviolet light region.The orthogonal test indicated that influence of process parameters on the photoactivity from most significant to least significant was:Sm ion content,calcination temperature,calcination time.The optimal modification process was as follows,0.2%Sm,calcination temperature for 500℃,calcination time for 3h.Sm doping improved the photodegradation activity.Under 26w UV light and 33w visible light irradiation,the degradation rate of 1.5ppm concentration of NO was 74.85%and 86.52%,respectively.With the increase of NO concentration,the photocatalytic activity decreased,but the total amount of degradation was improved.