Preparation Of N-doped TiO₂ Molecular Imprinted Photocatalyst And Study Of Its Selective Photocatalytic Activity Under Visible-light

To improve the degradation ability of TiO₂ for low concentration and high toxicity organic pollutants under visible light, the combination with doping and molecular imprinting thchnique has become a hot research topic at present. Two kinds of molecular imprinting technique were adopted for preparing nitrogen doped TiO₂ imprinted photocatalysts ion the paper. The preparation conditions were explored. The selected samples were characterized by XRD, TEM, XPS, FT-IR, UV-Vis diffuse reflectance spectrophotometer and N2 adsorption-desorption. The degradation kinetics of target degradation matter of SA and comparison of benzoic acid and methyl orange under visible light were studied. The visible light activity, selectivity and its mechanism of TiO₂ molecularly imprinted photocatalyst for salicylic acid were investigated.The sample of N-TiO₂-MIP was synthesized by the surface molecular imprinting technique using nitrogen doped TiO₂ as the stroma, salicylic acid as the template molecule, MAA as the functional monomer, EGDMA and AIBN as crosslinking agent and initiator, respectively. The best preparation conditions were that 4/1 of MAA/SA mole ratio, 3:1 of N-TiO₂/SA mole ratio and 12 h of reaction time. An organic modified layer was formed on the surface of nitrogen doped Ti O2 by the surface molecular imprinting process, which not changed the crystal form of nitrogen doped TiO₂ of anatase, but the particle size slightly increased from 12.8nm to 16.4nm. A large number of small holes at 2.2nm were formed in the surface organic modification layer. The shape of hysteresis loop and the distribution of pore were different from the nitrogen TiO₂. The shape of hysteresis loop changed from H2 to H4 of IV in the type of IUPAC. And the distribution of pore changed from normal distribution to even distribution at 3-8nm in addition to the most probable value at 2.2nm, which induced the larger specific surface area as 1.7 times compared to substrate. The red shift of the N-TiO₂-MIP absorption band edge was caused by nitrogen doping and the intensity of light absorption decreased by surface molecular imprinting. The visible light degradation rate was 72.6% for N-TiO₂-MIP. It showed that a good selectivity for the target pollutants of salicylic acid for N-TiO₂-MIP, which the selectivity coefficient at 1.1¹.7 for salicylic acid, benzoic acid and methyl orange.The sample of N-TiO₂-SA was synthesized by a improved molecularly imprinted sol-gel technique, which used urea as the nitrogen source and salicylic acid as the template molecule. The best preparation condition was 3:1 for butyl titanate and salicylic acid molar ratio. The sample was anatase, and the particle size increased by molecular imprinting. The shape of hysteresis loop and the distribution of pore were same to the substrate, but the specific surface area was increased 1.3 times. The N1 s of N-TiO₂-SA was detected by XPS, and the red shift of the N-Ti O2-SA absorption band edge was caused by nitrogen doping. The visible light degradation rate was 90.6% for N-TiO₂-SA. It showed that a good selectivity for the target pollutants of salicylic acid for N-TiO₂-SA, which the difference value of degradability rate constant, for salicylic acid, benzoic acid and methyl orange, were 2.0 and 2.5, respectively.The compared results for two methods showed that the visible-light activity of the surface molecular imprinting technique was lower than that of the improved molecularly imprinted sol-gel technique. The reason was that the layer of organic molecular imprinted polymer on the surface hindered contaction and weakened photic excitation for substrate. Due to demanding condition and more complicated preparation technique, the improved molecularly imprinted sol-gel technique was better than the surface molecular imprinting technique.