Research On Performance And Mechanism Of Photocatalytic Toluene Degradation By Ag And/or Bi Modified TiO₂

Indoor volatile organic compounds?VOCs?,including various alcohols,aromatic compounds?benzene,toluene,xylene?,aldehydes and halogenated hydrocarbons,were mainly derived from furniture,decorations and building materials,etc.These volatile organic compounds have proven hazard to human tissues,organs and so on.Photocatalytic technology is a kind of end treatment for low concentration pollutants.It has some advantages of mild reaction conditions,low energy consumption and easy operation,and TiO₂ was the most favored photocatalytic material,which has a low cost,high chemical stability and non-toxicity.However,the photocatalytic activity and stability of TiO₂ in the photocatalytic VOCs removal were not very high.Beyond that,it was unclear to the mechanism of determining the catalytic activity and stability.In this paper,the activity and stability of TiO₂ for photocatalytic toluene degradation were improved by metal Bi or Ag modification.The properties of the materials and the mechanism for toluene degradation were analyzed by using XRD,TEM,HETEM,XPS,BET,UV-Vis,EPR and FTIR.The main results were as follows:?1?Photocatalytic toluene degradation over Bi/TiO₂ catalystsThe Bi/TiO₂ photocatalysts with Bi loadings less than 2%were prepared by a two-step hydrothermal method.The addition of Bi did not change the TiO₂ structure,but the photocatalytic activity for toluene degradation was significantly higher than that of TiO₂.The activity of1.5%Bi/TiO₂ catalyst was the highest.Although the addition of Bi resulted in the insufficient separation of the photogenerated electron-hole pairs,the production of reactive oxygen species(·O^2-and H₂O₂)was comparable to that of TiO₂,which could be attributed to the promotion of oxygen supply to the catalyst surface by metal Bi.The in-situ infrared spectroscopy measurements showed that the fatty acid species was the main intermediate on the surface of TiO₂,while the adsorbed benzaldehyde was the main species on the surface of Bi-modified TiO₂.Theoretically,fatty acid species are more likely to be oxidized than benzaldehyde.The afore-mentioned results can be attributed to the strong interaction between polar fatty acid species and the TiO₂ surface,which results in insufficient oxygen supply to the TiO₂ surface.This further suggests the pivotal role of oxygen supply to the catalyst surface in toluene degradation.?2?Photocatalytic toluene degradation over Ag/TiO₂ catalystsAg-modified TiO₂ photocatalysts?Ag/TiO₂?were prepared by using hydrazine hydrate as a reducing agent.Ag element was uniformly dispersed onto the catalyst surface in the form of Ag⁰,and the addition of Ag did not change the inherent physical structure of TiO₂.Oxygen adsorption experiments showed that the ability of the catalyst to adsorb oxygen follows the order of 1.0%Ag/TiO₂>0.4%Ag/TiO₂>TiO₂,that is,Ag promotes the adsorption of oxygen molecules on the surface of catalysts.The 1.0%Ag/TiO₂ catalyst exhibits the largest activity for H₂O₂production.The activities of Ag/TiO₂ photocatalysts for photocatalytic toluene degradation was significantly higher than that of TiO₂.This result was attributed to the increased capacity of Ag/TiO₂ photocatalysts to O₂adsorption,and thus increasing the production of reactive oxygen species(such as H₂O₂,·O^2-).?3?Photocatalytic degradation of toluene by Ag/Bi/TiO₂ catalystThe above-mentioned results showed that both Bi and Ag modification could promote the activity for photocatalytic toluene degradation.In order to probe whether there was a synergism between Bi and Ag co-modification,thus resulting in a further improvement of activity for toluene degradation,we prepared Ag and Bi co-modified photocatalysts?Ag/Bi/TiO₂?by using hydrazine hydrate as the reducing agent.The total content of Ag and Bi and the ratio of Ag to Bi were optimized.It was found that when the total content of Ag and Bi was 1wt%and the ratio of Ag to Bi was equal to 1,the catalyst exhibited the highest photocatalytic activity.However,the activity of Bi/Ag/TiO₂ was significantly lower than that of the Bi/TiO₂ catalysts,indicating that Bi and Ag had no synergistic effect.We also found that there was no significant difference in catalytic behavior over bimetallic co-modified TiO₂ and single metal modification,which further indicates that there was no synergistic effect between Ag and Bi.