Preparation Of Au@Ag/TiO₂ And Photocatalytic Decomposition Of Gaseous Ozone

TiO₂ is a quite important photocatalyst duing to its high photocatalytic activity, stability,moderately-priced and non-toxic.But the light with energy bigger than 3.2 eV (λ＜385 nm) can excitate TiO₂ to produce the electrons and holes,and the electrons and holes are extremely easy combination.The application of TiO₂ photocatalysis technology was restricted because of above two factors.The research indicated that the means of depositing noble metal on TiO₂ or other metal oxide and sulfide,doping inorganic ions,photoactivating or surface reductive processing can conduct TiO₂ modification,and may improve its light absorption,increase the stable state light degradation quantum and the photocatalytic efficiency.TiO₂ modification can also extend the light responsible range of TiO₂ from the ultraviolet to the visible light region. The dominant way to conduct modification was depositing noble metal on TiO₂.TiO₂ catalyst modification by depositing Au was studied by our group.Many researches had been carried on the preparation method,influencing factor on catalytic activity and practical application of Au/TiO₂ catalyst.Au/TiO₂ catalyst had high initial activeness and long live of catalyst for eliminating ozone.But its activeness reduced easily because of Cl^- anions and water vapor existence,which was the crucial question to restrict Au /TiO₂ catalyst towards practicality.Therefore,preparing new-style catalysts which can resist poisonous Cl^- anions and water vapor was main task aftertime.In this thesis,Au seed and Au core Ag shell composite structure nano-particles were prepared using a sol method.The Au core Ag shell composite nanoparticles were loaded onto TiO₂ nanoparticles using a powder-sol method,allowing the generation of Au @ Ag/TiO₂ photocatalysts for the photocatalytic decomposition and elimination of ozone. The sols and photocatalysts were characterized by means of ultraviolet-visible light (UV-Vis) absorption and reflection spectrometry,X-ray photoelectron spectroscopy (XPS),X-ray diffraction(XRD),and transmission electron microscopy(TEM).The activity of the Au @ Ag/TiO₂ photocatalysts for the photocatalytic decomposition and elimination of ozone was evaluated.The results showed that:under the black light lamp illumination,the Au@Ag/TiO₂ catalyst can decompose ozone,but the initial activities and the stability were still improved.The effect of Cl^- anions on the photocatalytic activity of Au@Ag/TiO₂ catalysts was studied.The as-prepared Au@Ag/TiO₂ catalysts was collected as three parts.One part was washed with 2000 mL distilled water,another part was washed with 100 mL distilled water,and the third part was unwashed.Results showed that Au @ Ag/TiO₂ washed with 100mL distilled water can resist poisonous Cl^- anions.If in the presence of a proper amount of NaCl solid for Au @ Ag prepared,Au @ Ag/TiO₂ had better activity,stability,and better ability to resist poisonous Cl^- anions.The reason was as follows,on one hand,NaCl was capable of reducing the concentration of free Ag⁺ by adsorption on the surface of Ag particles forming AgCl and enhancing the formation of Au core Ag shell particles,leading to a better resistance to Cl^- anions of the catalysts. On the other hand,AgCl took part in the photocatalytic decomposition of ozone together with Au @ Ag/TiO₂ catalysts and had synergistic effect with the latter, resulting in a higher photocatalytic activity of Au @ Ag/TiO₂ catalysts.Au@Ag/TiO₂ catalysts were calcinated at 100℃,200℃,300℃,400℃and 500℃, individually.It was discovered that catalytic activity decreased with the calcination temperature increasing.The possible reason was that particle size of supported metal growed up and formed Ag₂O/TiO₂ coupled semiconductor,which caused photo-induced electrons and holes move from conduction band and valence band of TiO₂ to that of Ag₂O,and were combined on Ag₂O quickly. The domestic and foreign numerous literatures indicated that the effect of water vapor was various to catalytic reaction of different system.In this paper,dried feed gas was bubbled through distilled water to change its humidity at room temperature.The study indicated that when water vapor with a partial pressureof 2.06 KPa was led into dried feed gas,the catalytic lifetime and activities enhangced remarkably.When water vapor in existence,catalytic activities of catalysts washed with 2000 mL and 100 mL distilled water and unwashed were all enhanced largely.The activities of catalysts were washed with 100 mL distilled water washed and unwashed can keep above 80%for 250h.The photo-induced carrier separation efficiency's enhancement,temperature effect and Cl^- anion decompositing ozone continually,were possible reasons for increasing activity of catalysts.