Modification Of Silica Supported TiO₂ And Photocatalytic Degradation Of Acrylonitrile Under Simulant Solar Light Irradiation

Photocatalytic oxidation using semiconductor photocatalyst TiO₂ as an effect advance osidation processes（AOPs） attracts increasing interest in recent years due to its ability to mineralize most of organic pollutants, low cost, reaction at room temperature, nontoxicity and chemical stability of TiO₂. However, photocatalytic oxidation technique has many disadvantages like lower photocatalytic perfoemance, longer reaction time and limited light absorption of the semiconductor. In this paper, for the above problems, photocatalytic degradation of acrylonitrile as a target pollutant under simulant solar light irradiation as stable light source, with silica suppoted TiO₂ modified with HNO₃ impregnation, H2SO4 impregnation, F doping and N-F codoping is investigated. The main research contents and conclusions are as follows:Given that the Lewis basicity of acrylonitrile molecular, solid acids used widely in industrial organic synthesis was applied in this study, in order to obtain an environmentallly benigh photocatalyst with higher photocatalytic performance. The SO₄^2- /TiO₂/SiO₂ solid acids were prepared by impregnating amorphous TiO₂/SiO₂ with different concentrations of H2SO4 solution. The element S exists in the form of SO₄^2- highly dispersed in TiO₂ crystals. The grains growth is inhibited by sulfated modification, which results in small grain size. Sulfated modification is an effective way to increase the number of surface acid site. It improves the adsorption capacity for acrylonitrile, which is responsible for improving the photocatalytic activity. The surface acid sites of the modified sample are stable, which assures the better recycle performance.To shorten photoreaction time for acrylonitrile degradation, F doped TiO₂/SiO₂ composite oxides were prepared by sol-gel method using HF solution as fluorine source. The effects of TiO₂ loading, HF/Ti molar ratio in starting material and calcinations temperature on the physical and chemical properties, especially the nature of the surface active site, and the photocatalytic activities of the prepared composite oxides for acrylonitrile degradation were investigated. The results show that the highest photocatalytic activity for acrylonitrile degradation is obtained for a sample calcined at 450 oC with molar ratio（HF/Ti） of 1.1 and 36 wt.% TiO₂ loading. The presence of SiO₂ as support for TiO₂ loading assures the formation of nanoparticles with a large surface. The higher photocatalytic activity for F doped sample is attributed to the increase of number and strength of surface acid sites, not the improving of light absorption.In order to further improve the photocatalytic activity of the F doped sample for acrylonitrile degradation, N-F codoped TiO₂/SiO₂ composite oxides were synthesized by sol-gel method using HH4 F solution as nitrogen and fluorine source. The research shows that NH4 F doping can inhibit the agglomeration of TiO₂ powder effectively. N elements are doped into TiO₂, which cause a red shift of the absorption edge of TiO₂/SiO₂ composite oxide and play a part in improving the photocatalytic activity. Bronsted acid sites appear on the surface of the sample and Lewis acid sites are increased after N-F codoping, which played a major role in improving photocatalytic activity. The degradation ratio for acrylonitrile can reach to 71.5% after reaction for 6 min under simulant solar light with 1.0 g N-F codoped TiO₂/SiO₂ sample/180 ml solution, which provides a fast and effective method for removing acrylonitrile under solar light. Moreover, a tentative degradation pathway is proposed for acrylonitrile degradation under simulant solar light irradiation with the modified catalyst, based on the intermediates generated.Considering that a part of surface hydroxyl groups exist in the form of acid sites and surface hydroxyl groups play an important role in photocatalytic oxidation organic compounds, a model of surface hydroxyl groups of single layer was presented. The concentrations of surface hydroxyl groups of pure TiO₂/SiO₂, HNO₃ modification sample, H2SO4 modification sample, F doped sample and N-F codoped sample were caculated. The results show that the concentration of surface hydroxyl groups is positively correlated with the photocatalytic activity of the corresponding samples approximately, expect N-F codoped TiO₂/SiO₂. Based on the in situ IR analysis and the relavent literatures, one of the hydroxyl groups is Bronsted acid site, and another hydroxyl group can form hydrogen bond with pyridine. Moreover, both of them are beneficial to improve the photocatalytic activity.A comparative analysis of surface aids sites and photocatalytic activity between several modified catalysts was given and the results show that the photocatalytic activity of acrylonitrile degradation with the catalysts under similant solar light follows the same change trend as the surface acid sites. The samples after the modifications show superior photocatalytic activities, compared with the original TiO₂/SiO₂ and reported literatures. Moreover, acid modification for increasing of number and strength of surface acid sites is an effective modification method for photocatalysts, in order to remove organic pollutants with Lewis base groups by using photocatalytic oxidation technique.