Although humic acid (HA) can inhibit TiO2 photocatalysis, it can sensitize TiO2 and induce significant visible-light (VL) activity in phenol degradation. This favorable effect of HA was negligible on phos-phate-modified TiO2 (P-TiO2), but significantly stronger on Nafion-modified TiO2 (Nf-TiO2). The reaction rate constants for phenol degradation on Nf-TiO2 increased from (0.003±0.001) to (0.025±0.003) min-1 when the reaction was performed in the presence of 20 mg/L HA. The different effects of HA on P-TiO2 and Nf-TiO2 photocatalysis cannot be attributed to adsorption changes, be-cause the adsorption capacities of P-TiO2 and Nf-TiO2 were only slightly lower than that of TiO2 at an initial HA concentration of 20 mg/mL. Scavenger tests, electron paramagnetic resonance spectros-copy, and H2O2 detection were taken to understand the low VL activity of the P-TiO2/HA suspension. The main active species for phenol degradation in the TiO2 and Nf-TiO2 suspensions were superox-ide radicals. There were negligible amounts of superoxide radicals in the P-TiO2/HA suspension, possibly because a direct four-electron oxygen reduction reaction occurred. The better VL activity of Nf-TiO2 was rationalized on the basis of Mott–Schottky and electrochemical impedance plots. Nafion modification resulted in cathodic shifts of the energy band positions, increased electron density, and less resistance to electron transfer across the interface between TiO2 and electrolytes. All these factors facilitated electron transfer and improved the production of active species. Phosphate modi-fication therefore did not improve the VL response of HA sensitized TiO2, and low concentrations of HA can facilitate VL photocatalytic degradation of organic pollutants on Nafion surface-modified TiO2.%光催化水处理技术有望成为一种有效去除水中难降解有机污染物的方法.尽管人们已经研制了大量的新型光催化剂,但在环境应用中纳米TiO2仍是最受欢迎的催化剂.由于光催化反应在表面发生,反应动力学主要取决于TiO2的表面性质,因此,表面修饰是调控TiO2光催化反应的重要手段,其中Nafion和磷酸根表面修饰TiO2简单可行.Nafion修饰可以通过离子交换吸附富集阳离子底物提高光催化降解效率;而磷酸根修饰则可以增强对弱吸附底物的降解活性.另一方面,水中共存的天然有机物(NOM)例如腐植酸(HA)将抑制TiO2光催化降解目标污染物的活性,同时又将通过光敏化诱导可见光催化性能.因此,研究如何调控NOM对光催化反应的影响将有助于光催化水处理技术的工程应用.本文以HA作为代表性的NOM,研究了磷酸根和Nafion两种不同修饰方法对HA敏化TiO2性能的影响规律.可见光降解苯酚的实验结果表明,HA的敏化可以提高TiO2可见光降解苯酚的效率.磷酸根修饰TiO2抑制了HA的敏化作用,而Nafion修饰则增强了HA的敏化作用并提高可见光降解苯酚的活性.当HA浓度为20 mg/L时,Nafion修饰TiO2降解10 mg/L苯酚的反应速率常数由(0.003±0.001)min-1提高至(0.025±0.003)min-1.表面修饰引起的反应活性差异不能归因于HA的吸附容量的变化,因为吸附实验表明在光催化反应条件下,两种表面修饰的TiO2对HA的吸附容量并无显著差异.添加不同的自由基捕获剂发现,HA敏化TiO2及Nafion修饰TiO2降解苯酚的主要活性物质为超氧自由基,而磷酸根修饰TiO2的光催化反应中并没有超氧自由基.通过检测超氧自由基的电子自旋共振信号进一步证实了这一点.测试发现,光催化反应中Nafion修饰TiO2产生的H2O2增加,而磷酸根修饰的几乎不产生H2O2,说明可能发生了四电子的氧还原反应.莫特-肖特基曲线(Mott–Schottky)的测试结果表明,Nafion修饰TiO2的平带电位从-0.54 V负移到-0.85 V,多数载流子电子的密度提高了62%.电化学阻抗结果显示,Nafion修饰TiO2具有较小的界面电子迁移阻抗.这些变化有利于提高电子迁移速率,抑制复合并增加超氧自由基的生成,从而提高光催化性能.因此对TiO2进行恰当的表面修饰,将有助于强化HA的敏化作用,并提高可见光催化降解目标有机污染物的活性.
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