Study On Photocatalytic Oxidation Of Toluene-sulfur Dioxide Over TiO₂/SiO₂ Binary Semiconductor

In recent years, heterogeneous photocatalytic oxidation (PCO) has been increasingly concerned in removing the organic pollutants present in waste gas streams, and is potentially more energy efficient, and regarded as a promising technique. Removing the trace harmful gases from the air by heterogeneous photocatalysis has been one of the most concerning fields by researchers.In our country, atmospheric pollution belongs to lampblack mode. The contents of SO₂ increased with the increasing motors, and concentration of benzene and toluene in the air presents an uptrend. The situation of the pollution that sulfur dioxide and organic compounds can action and bring out photochemical smoke is severely, which is relevant to the formation of photochemical smoke. In addition, the pollution of volatile organic compounds (VOCs) is the problem of indoor air, and benzene and similar compounds are primary pollutants of airtight environment. Toluene, moreover, is a widespread reagent used for the preparation of many compounds, such as benzaldehyde, benzyl alcohol, benzoic acid, chloro derivatives. Therefore, how to degrade toluene and sulfur dioxide and the products of their reactions is an imperative problem to resolve. So far, semiconductor photocatalysis have been rapidly developed and being a promising process for remedying environmental air pollution by volatile organic compounds.TiO₂ is the most widely used photocatalyst, due to its great capacity for oxidation, low cost, long-term chemical stability and non-toxicity. To improve the mechanical strength, thermal stability, and surface area of TiO₂, TiO₂/SiO₂ supported oxides have been considered as advanced support materials as substitutes for pure TiO₂. In this paper, the studies were carried out that photocatalytic oxidation of toluene and sulfur dioxide on TiO₂/SiO₂ binary semiconductor prepared by a sol-gel method, and studied the deactivation and regeneration of photocatalysts.The supported catalysts were prepared by a sol-gel method. TBOT and TEOS are original materials proceeding hydrolytic decomposition and condensation polymerization in the organic mediator (such as ammonia liquor). After gelation and drying processes, the nanoparticles were calcined at 600°C. These binary TiO₂/SiO₂ photocatalysts have a lot of merits, for instance, photoresponse red shift, delayed recombination, high catalysis activity, large specific surface area, and can adsorb organic compounds than TiO₂.The effects of initial concentration of toluene, oxygen content, and water vapor content and light intensity on the photocatalytic degradation were studied over semiconductor TiO₂/SiO₂. The photocatalytic degradation rate increased with increasing the initial concentration of toluene, and maintained almost constant when the concentration of toluene rose to 81.30μmol/L, however, when increased furthermore, the partially oxidized intermediates such as benzaldehyde, benzyl alcohol and benzoic acid accumulated on the surface of photocatalyst, and held the active position, which led to the less activity. The reaction rate increased with increasing oxygen content, and when it was higher than 25%, the reaction rate is highest.In the low oxygen contents, the capacity of capturing electrons and utilization increased with the increasing oxygen contents, which led to the improvement of photocatalytic reaction rate, but when exceeded 25%, the oxygen adsorbed on the surface of TiO₂/SiO₂ are saturated and restricted the adsorption of reactants on the surface of photocatalysts leading to the decreased reaction rate of toluene. Although the photocatalytic degradation rate was not as good as the rate when the oxygen content was 20%, the optimal content of oxygen came close to 25%. Consequently, there are definite practical values that oxygen content is controlled at 20% because atmosphere oxygen content is approximately 20%. For the influence of water vapor in the gas-phase photocatalytic degradation rate of toluene, there was an optimum concentration of water vapor (0.4%). The presence of H₂O can maintain a certain concentration of the radical·OH species on the surface. Adequate H₂O can provide more radicals, and improve the capacity of oxidization. H₂O and reactants can adsorb on the surface competitively, and the adsorption of toluene decreased leading to decreased activity and reaction rate. The photocatalytic degradation rate increased with light intensity but the rate approached to optimum when the light intensity rose to 3.0mW/cm². The more is electron holes, the more is irradiation intensity. When the light intensity rose to 3.0mW/cm², the photons adsorbed on the surface saturated, and parts of the electron holes recombined. At this moment, reaction rate increased slowly and reached a constant gradually.In the system of C₆H₅CH₃-O₂-TiO₂/SiO₂, the degradation of toluene was promoted in the presence of SO₂, but the degradation of intermediate products was inhibited when the reaction continued. In the meanwhile, the degradation of SO₂ was also inhibited by the presence of toluene. It is because of formation of the intermediate products, which accumulated on the surface of the photocatalyst and difficult to further oxidation to CO₂. In the system of SO₂-O₂-TiO₂/SiO₂, the added toluene decreased the degradation rate of SO₂. This is because that toluene took up the active position of photocatalyst, which led to less oxidization of SO₂.The deactivation and regeneration of binary semiconductor TiO₂/SiO₂ used to degrade toluene-sulfur dioxide were mainly studied. The activities of TiO₂/SiO₂ semiconductor decreased along with redundance using times in the course of reaction. It is learned that the activities decreased obviously when using a few times through the oxidation reaction. The cause is that the intermediate products produced during the photocatalytic oxidation covered the active center of TiO₂/SiO₂ surface, so that the photocatalytic reaction could not go on. The deactivated catalysts were calcined for an hour under the temperature of 500°C. It was found that the catalysts of TiO₂/SiO₂ binary semiconductors almost returned their reaction activities. It accounted for that the organic materials covering over the surface of the photocatalyst have been eliminated.