Ce/TiO2/g-C3N4异质结的制备及光催化降解PFOA反应机制

采用溶胶凝胶-超声混合法制备了铈/二氧化钛/石墨相氮化碳（Ce/TiO₂/g-C₃N₄）异质结催化剂,通过透射电子显微镜（TEM）、X-射线粉末衍射仪（XRD）、傅里叶变换红外光谱仪（FTIR）和紫外-可见分光光度计（UV-Vis）等表征了复合材料的结构、晶相、活性基团和光吸收性能,并考察了可见光下该催化剂对模拟全氟辛酸（PFOA）废水的降解效果.结果表明,Ce/TiO₂/g-C₃N₄分散性好,粒径小于20 nm,金属Ce的掺入及TiO₂/g-C₃N₄异质结的生成,有效抑制了光生电子和空穴对的复合,使复合光催化剂禁带宽度减小,光吸收范围增大,光催化性能得到明显提升.在pH值为2,PFOA浓度为4 mg·L^-1,催化剂投加量为1 g·L^-1的条件下,可见光照射180 min后,可降解94.4%的PFOA,脱氟率为38.6%.降解过程符合准一级反应动力学,速率常数为0.522 h^-1.通过HPLC-MS分析降解产物,同时结合氟元素物料平衡的计算结果,认为降解机制主要是以阴离子形式存在的PFOA吸附至表面带有大量正电荷的催化剂表面后,在光生空穴和活性自由基的作用下,以自由基机理逐步脱去CF₂生成短链的PFCAs,同时生成氟离子.

Fabrication of a Ce/TiO2/g-C3N4 heterojunction and the photocatalytic decomposition mechanism of perfluorooctanoic acid with visible light

A novel Ce/TiO₂/g-C₃N₄ heterojunction has been successfully fabricated using a sol-gel method combined with ultrasonic mixing. The photocatalysts were characterized using TEM, XRD, FTIR and UV-Vis spectroscopy. The photocatalytic activity was evaluated using the photocatalytic degradation of perfluorooctanoic acid (PFOA) under the illumination of a fluorescent lamp. The results reveal that the Ce/TiO₂/g-C₃N₄ heterojunction had good dispersion with particle sizes <20 nm. The incorporation of Ce ions and the formation of heterojunction inhibited the recombination of photo-generated electron-hole pairs, reduced the band gap and expanded the absorption region to visible light. The composite photocatalysts show excellent photocatalytic activity. The degradation of PFOA followed pseudo-first order reaction kinetics and the rate constant was 0.522 h^-1. Using 1 g·L^-1 of this catalyst with an initial PFOA concentration of 4 mg·L^-1 at pH 2.0, the degradation rate of PFOA over 180 min could reached 94.4%, and the defluorination rate was 38.6%. The degradation products were separated and analyzed using HPLC-MS, and combined with the calculation results obtained for the fluorine element balance. The degradation mechanism involved the adsorption of PFOA anions onto the surface of the composite, which displayed a large amount of positive charge, and under the action of photogenerated holes and active free radicals, CF₂ was gradually removed to form short-chain PCFAs and fluoride ions via a free radical mechanism.