Preparation Of Photocatalysts TiO₂/ASC Modified By Mental And Non-mental And Investigation Of Photo-electron Behavior For NO Oxidation

In recent years, nitrogen oxides, NOx, has become one of the major air pollutants instead of sulfur dioxide, SO2, in China. The total NOx emission of China in 2013 was 22.273 million tons while 10.995 million tons for the first half of 2014. Those pollutions have led to more and more serious environmental problems, which promote district regulations proposed to control NOx emissions. Now, NOx emission control technology has become an issue of national problem need to be solved.Selective catalytic reduction of NOx (SCR) is the most used technology in industrial applications where NOx is reduced to N2 by NH3 at high temperature. 12.642 million tons of NH3 will be consumed to reduce all of the NOx emissions in 2013 and produce 20.794 million tons of N2 by use of SCR technology. Otherwise, catalytic oxidation of NOx gives nitrate as final product, which means 59.414 million tons of NH4NO3 obtained. As a result, catalytic oxidation of NOx is an environmentally friendly and energy-saving NOx control technology. Previous work has confirmed that semi-coke is an efficient catalyst support on catalytic oxidation of NO at low temperature with remarkable activities, but water resistance is the bottleneck for further industrial application. Photocatalysis is considered as a new efficient with low environmental requirements technology on degradation of pollutants and water is one of the sources of hydroxyl radical, which plays an important role in photocatalytic reaction process. Semi-cokes, as raw materials, were prepared by different activated methods, such as acid activation, alkalinity activation and hydrothermal activation. Photcatalytic oxidation of NO of TiO2 loaded on activated semi-coke (ASC) prepared by sol-gel was investigated.Acid-hydrothermal activation shows more positive effect on photocatalytic activities of TiO2/ASC than alkalinity-hydrothermal activation. Due to the "cavitation effect", ultrasonic-assisted sol-gel method is better than traditional sol-gel method. The various factors were investigated and the results showed that the TiO2/ASC photocatalysts were optimized with TiO2 dosage of 5.2% calcinated at 500 ℃ for 1h.The mechanism of photocatalytic degradation of pollutants is different because of the difference of objects and reaction environment and can be divided into two kinds, free radical oxidation and photogenerated-hole oxidation. In order to determine the mechanism of photocatalytic oxidation of NO in this paper, radical trapping agents were added one by one into the humidifier to find the active species that play major role in this process. Photocatalytic de-NOx activities decrease sharply with the addition of OH and ·O2- trapping agents. On the basis of these results, photocatalytic oxidation of NO process can be described that electron of valance band transits to conduct band with UV light irradiation producing photogenerated hole on conduct band. Oxidizing electron and hole attack O2 and H2O to OH and ·O2-, respectively, which have strong oxidation and almost no selectivity. NO is oxidized into NO2 and transforms into eventually NO3-.Doped with metal or nonmetal oxide has been proved to be an efficient way to enhanced photocatalytic activities of TiO2. M-TiO2/ASC (M=Cu、Fe、Ce、Bi、B) photocatalysts were prepared by ultrasonic-assisted sol-gel method with different amount of dopant. Investigation of photocatalytic de-NOx activities of those as-prepared photocatalysts shows that doped with mental and non-mental oxide enhance de-NOx activities of TiO2/ASC under both UV and vis irradiation. In detail, the best de-NOx activities are obtained when Ti:Cu=10:1, Ti:Fe=15:l, Ti:Ce=2:1, Ti:Bi=20:1 and Ti:B=20:1 and NO conversion becomes as high as 80% after 3.5h. Reaction conditions maintain consistent in all tests with NO concentration 300-500mg/m3,5% O2,8% H2O at 100℃.All photocatalysts are characterized by X-ray power diffraction (XRD), FT-IR, UV-Vis adsorption spectroscopy. XRD results show that, FeTiO3 appears in Fe-TiO2/ASC when Ti:Fe=1:1, and the diffraction peaks of CuO and Cu arise in Cu-TiO2/ASC when Ti:Cu=1:1, while bismuth exists in all Bi-TiO2/ASC samples. UV-Vis results show that doping amount of mental and non-mental vary the absorption edge position of doped-TiO2/ASC catalysts. The absorption edge position of all Cu-TiO2/ASC and B-TiO2/ASC catalysts moves toward longer wavelength, which is called red shift.Investigation of TiO2/ASC regeneration indicates that 100% NO oxidation ability can be recover after high temperature (400℃) regeneration with water, while only 80% by ammonia regeneration. After regeneration three times by high temperature regeneration with water, regenerated catalysts are still available with 80% NO conversion.Cu/Al2O3 catalysts show an interesting phenomenon on photocatalytic oxidation of CO.100% CO conversion is obtained under UV irradiation, sooner with addition of 2.0% TiO2. CO concentration increases rapidly with no irradiation but high CO conversion can be back with light on again. In spite of its remarkable ability for CO removal, the de-NOx activities are badly low.