Synthesis Of Ternary Hybrid TiO₂-SiO₂-POMs And Its Application In Degrading Rhodamine B Under Visible Light

Photocatalysis possesses broad application potentials in many aspects such as environmental protection, new energy, efficient bacterial-killing and self-cleaning materials, etc. The design and synthesis of high-efficient photocatalyst has aroused general concerns since the catalyst was applied into the degradation of organic or inorganic contaminants. Photocatalysis is a green technique. The photocatalyst can take good advantages of solar energy resources, and meanwhile it is a green technology. Titanium dioxide (TiO2), due to its relatively mature synthesis techniques as well as its relatively higher activity and stability, has become the photocatalyst universally studied. However, the band gap of the two main active structures of TiO2, known as Anatase and Rutile, is3.2eV and3.0eV respectively, and thus it is difficult to use the visible light with a wavelength longer than400nm to oxidize the organic pollutants in waste water. Moreover, TiO2can only take use of approximately4%in solar light, mainly in the ultraviolet (UV) light. The phenomena restrict TiO2for the effective utilization of solar energy. On the other hand, hole (h+)-electron (e-) pairs generated under illumination easily recombine, resulting in deactivation of the catalyst and the loss of photocatalytic activity.In recent years, polyoxometalates (POMs) have become a hot spot because of their similar catalytic mechanism to TiO2. POMs with acidic property, redox catalytic activity, excellent selectivity, mild reaction conditions and could be promising new catalysts in treatment of waste water. The gand gap of POMs ranges from3.1eV to4.6eV. Some latest studies indicate that the excitation light of POMs is mainly UV light or near-UV light, unfortunately POMs’utilization of solar energy is low as well.The main task of this thesis aims at broadening the spectra response of the photocatalytic material and improving the stability of the active sites. We designed and synthesized the ternary hybrid catalysts with high photocatalytic activity, In view of the problems in traditional TiO2photocatalyst, we targeted to take improvement measures. Mainly the thesis consists of the following aspects:(1) The design and synthesis of the ternary hybrid photocatalyst TiO2-SiO2-POMs;(2) Investigation of the activity mechanism of the ternary hybrid photocatalyst TiO2-SiO2-POMs;(3) Evaluation of the catalytic performance over TiO2-SiO2-POMs for degrading Rhodamine B under visible light.1. The design and synthesis of the ternary hybrid photocatalyst TiO2-SiO2-POMs.Taking TiO2as the central element, we introduced photoactive POMs into the catalyst system and then synthesized the new catalyst through introducing silica (SiO2) for the formation of ternary hybrid catalyst. At first, POMs were introduced by impregnation method, and SiW12O404-was found more effective than PW12O403-and newly-synthesized [Mn2(H2O)2Fe2(P2W15O56)2]14-Three synthesis methods, such as stepped sol-gel method, one-step method and cosolvent induced gelation method (CIG) were investigated. The result indicated that the catalyst synthesized by stepped sol-gel method stood out due to its simple operation and high activity.2. Investigation of the activity mechanism of the ternary hybrid photocatalyst TiO2-SiO2-POMs.The catalytic activity and stability were enhanced over ternary hybrid TiO2-SiO2-POMs compared to monomer, In this paper, a variety of characterization methods, such as diffuse reflectance spectroscopy (DRS), X-ray polycrystalline diffraction (XRD) and time-resolved microwave conductivity (TRMC), were applied for catalyst characterization. The introduction of SiO2significantly increases the specific surface area of the catalyst and promotes a high dispersion of TiO2. DRS study shows that the introduction of SiO2reduces the band gap of the catalyst and enhances the activity of the catalyst in the visible region. XRD characterization results indicate that in the ternary hybrid catalyst, a mount of Anatase-type TiO2increases after SiO2introduction, beneficial to enhancing the activity under visible light. In addition, hydrophobic SiO2pre-concentrates the pollutants in solution, improving the reaction efficiency. TRMC results reveal that the introduced POMs eminently improve the photoactivity, the combination of TiO2and POMs accelerates the lateral charge transfer as h+â†'h+and e-â†'e-, and it stabilizes the h+-e-pairs. The synergistic effect amongst TiO2, POMs and SiO2prompts the ternary hybrid TiO2-SiO2-POMs to obtain a higher activity and stability in the photocatalytic reaction under visible light.3. Evaluation of the catalytic performance over TiO2-SiO2-POMs for degradingRhodamine B under visible light.Rhodamine B (RB) is a typical sort of organic dyes. The photodegradation of RB was selected as the probe photocatalytic reaction because of its universal significance in the photocatalytic oxidation of organic contaminants in waste water. After optimization, the reaction conditions are selected as follows:ternary hybrid TiO2-SiO2-SiW12O404-catalyst in0.1mmol/L Rhodamine B solution is1.4g/L, and wherein the molar ratio of TiO2-SiO2-SiW12O404-is1:2:0.007. Air is selected as the oxidant to be bubbled into the solution, with100mL of100ppm Rhodamine B as the object material. Prior to the reaction, the reaction system were at a stirring rate of700rpm for150minutes to reach adsorption-desorption equilibrium in dark, and then the light source (wavelength greater than420nm) was turned on to carry out the reaction. When the reaction processed at a temperature of60°C for60minutes, the conversion of RB exceeded99%. The reaction follows the first-order kinetic equation, with kinetic constant k as0.163min-1.