An Investigation Of VO_x Doped MgF₂ Photodegradation Catalyst

Photo-catalytic oxidation of pollutants over semiconductor catalysts has attracted a lot of attention of chemists since its' discovery at the beginning of the last decade.Comparing with the conventional catalytic degradation or burning process,photo-catalytic oxidation is efficient in terms of energy saving and completely removing of low concentration pollutants.Hence,photocatalysis is a potential technology that could find applications in environmental protection.Almost all of organic pollutants could be degrade in photocatalysis reaction.It is valid in the degradation of compounds those are not degradable by biotechnologies.The priority of photocatalysis degradation is that if visible light catalyst were discovered,solar lights could be used as the light source,which could save huge amount of energy for us..Because of the high activity,chemical stability,and low-toxicity of TiO₂,TiO₂ is widely used as photo-catalyst.However,TiO₂ has wide band-gap(3.2 eV),it is only active under ultraviolet lights.It is not possible to use solar lights as light source.Consequently, to design a visible light active catalyst is crucial in this area.In the current investigation,a type of VO_x/MgF₂ photocatalysts has been designed for visible light applications.Our investigation proved that VO_x/MgF₂ photocatalysts is active and stable in visible lights. The XRD,SEM,UV-vis,and BET characterizations of the catalysts indicate that the average particle size of VO_x/MgF₂ catalysts was about 10-20 nm.The catalysts contain both V₂O₅ and MgF₂ phases.The active phase is V₂O₅ and MgF₂ phase acted a role of retarding the recombination of photoelectron-hole pairs,and therefore,make the catalysts have high photo quantum efficience.The UV-Vis spectra of the catalysts show that the VO_x/MgF₂ catalysts have strong absorption in lights with wavelength below 600 nm, which indicates that these types of catalysts are active under visible light.This is consistent with the photodegradation investigations in acetone,methanol,ethanol,ether,and benzene. The doping of V₂O₅ in MgF₂ phase does not have apparent influence on the specific surface area,and therefore should not have significant influence on catalytic activity.The catalytic properties of the catalysts are found to be sensitive to the structure of the catalysts. In our investigation,the catalysts were calcined in the temperature range of 250 to 900℃. it was found that the catalysts were active when calcined between 250 to 450℃,the catalytic activity decreased when calcined between 450 to 500℃,and the catalysts were not active when calcined at temperature higher than 500℃.The XRD and Raman investigation show that the catalysts contain V₂O₅ and MgF₂ phases and no new phase formed when catalysts were calcined between 250 and 450℃,the V₂O₅ and MgF₂ phases transfer to new unknown phase(s)when catalysts were calcined between 450 and 500℃, the V₂O₅ and MgF₂ phases disappeared when catalysts were calcined above 500℃.With the increase of calcination temperature,the specific surface area of catalysts decreased,the UV-Vis spectra edge shift to the short wavelength direction,and the crystal size of catalyst increased.All of the investigations show that the structure of catalysts is the crucial factor to determine the catalytic properties of VO_x/MgF₂ photocatalysts.An active VO_x/MgF₂ photocatalyst must contain both V₂O₅ and MgF₂ phases and V₂O₅ phase must be uniformly distributed in MgF₂ phase.The catalyst must be calcined in a temperature region of 250 to 450℃.The investigation shows that the optimum concentration of V in VO_x/MgF₂ photocatalyst is 9.09%in tool.Therefore,9.09%VO_x/MgF₂ has the highest activity.The doping of Pd,Rh,and Pt could improve the activity of VO_x/MgF₂ catalyst.The best catalyst was found to be Pt/VO_x/MgF₂ catalyst.The optimum concentration of Pt in VO_x/MgF₂ was found to be 0.40wt%.The XRD investigation indicates that the doping of Pt did not change the structure of catalyst and also,both catalysts VO_x/MgF₂ and Pt/VO_x/MgF₂ are stable in catalytic reaction.There is no structure change in VO_x/MgF₂ and Pt/VO_x/MgF₂ catalysts after more than 20 h of online reaction.