Photocatalytic Oxidation And Aqueous Absorption Combined Process For Nitrogen Oxides Treatment

For the wet absorption of nitrogen oxides (NOx) from flue gases, the main obstacle is its rather low efficiency, since the solubility of NO is quite low. Thus, the oxidation of NO to increase the solubility of NOx is the key technology in this process. This dissertation presented the theoretic and experimental study on the photocatalytic oxidation (PCO) of NO on TiO₂-based photocatalyst, which was prepared by different methods including sol-gel method and hydrothermal method. The effects of transition metal ion doping and noble metal ion doping on the photocatalytic activity were also investigated in this dissertation. Furthermore, a novel combined process combining the photocatalytic oxidation with the aqueous absorption was developed and experimentally studied.Firstly, the PCO behavior of NO at high concentrations was discussed. And the relationship between the reaction rate and several key factors (relative humidity, space time, inlet concentration, oxygen percentage) was also investigated. The experimental results illustrated that the reaction kinetic of NO photocatalytic oxidation fitted well with the Langmuir-Hinshelhood model. The results from Fourier transform infrared spectrometry (FT-IR) showed that the possible explanation for the catalyst deactivation was the deposition of nitric acid and nitrous acid on the TiO₂ surface. Fortunately, the activity of the photocatalyst could be regenerated with a simple efficient heat treatment (500℃, 30 min).Secondly, with the attention to photoactivity for NO oxidation, the optimization of the preparation conditions by sol-gel method and hydrothermal method was carried out in the dissertation. The characterizations for the physicochemical properties of the catalysts prepared under different conditions were investigated by XRD, TEM, TG-DSC, SEM and UV-Vis methods. It was found that the preparing conditions have great effects on the PCO activities of the catalysts. And the optimal values of the two preparing methods were discussed, respectively. Furthermore, the composite TiO₂ films (using Degussa P25 as filler) coating on the woven glass fabric were prepared by a modified sol-gel process, and the experimental results show that the photoactivity of composite TiO₂ film could have an obvious improvement compared to the single films.Thirdly, a series of transition metal ion-doped (such as Zn²⁺,Fe³⁺,Cr³⁺,Mn²⁺ and Mo⁶⁺) TiO₂ catalysts with different ion content had been prepared by sol-gel method or wet impregnation method. These catalysts were examined with respect to their photocatalytic activity under different preparing methods, metal ion types and the doping contents. An enhancement of the photocatalytic activity was accomplished for the Zn²⁺ doped TiO₂ catalysts which synthesized by sol-gel method or wet impregnation method. Also, a series of nano-sized noble metal ion-doped (Ag⁺,Au³⁺ and Pt⁴⁺) TiO₂ catalysts with different ion contents was prepared by a wet impregnation method and examined with the respect to their behaviors for the photocatalytic oxidation of NO. It was found that the Pt⁴⁺ doped TiO₂ catalyst had the highest photoactivity for NO, which was attributed to the lengthened lifetime of electron-hole pairs.Finally, a novel process combining photocatalytic oxidation with aqueous absorption was developed in this study for the treatment of nitrogen oxides from the simulated flue gases. In this process, sodium sulfite (Na₂SO₃) was employed as the absorbent. The effects of the operation parameters of this combined process (relative humidity, space time, inlet concentration, and so on) were investigated and the reaction mechanism was proposed. The results indicated that the relative humidity and oxygen percentage were the key factors of the combined process. And Na₂S₂O₃ was an excellent oxidation inhibitor to prevent the excessive oxidation of sulfite.