摘要/Abstract

已知氟离子和磷酸根能加快锐钛矿和P25 TiO₂光催化降解苯酚等有机物，但作用机理仍存在争议.合成了不含无机阴离子的锐钛矿，并在其表面沉积0.52 wt% Pt（Pt/TiO₂）.在初始pH 5.2的水溶液中，99%的氟离子和磷酸根分别为F^-和H₂PO₄^-.加入0.1～30 mmol/L阴离子，苯酚的光催化降解速率常数（k_obs）都增大，证实了氟离子和磷酸根均具有正效应.有趣的是，k_obs增加倍数均与阴离子吸附量呈线性关系，其中氟离子的斜率大于磷酸根，而Pt/TiO₂的斜率大于TiO₂.这表明阴离子的正效应源于吸附于固体表面的阴离子，并且氟离子的活性大于磷酸根.（光）电化学测试表明，氟离子和磷酸根分别抑制和促进O₂还原，但它们都促进苯酚氧化.此外，氟离子和磷酸根分别使TiO₂平带电位移动-159 mV和89 mV.前者有利于TiO₂价带与苯酚发生轨道重叠，后者有利于TiO₂导带分别与O₂发生轨道重叠，进而加快界面电荷转移.由于阴离子广泛存在，该结果将有助于半导体光催化的机理研究及其环境应用.
关键词: 锐钛矿TiO₂, 氟离子, 磷酸根, 吸附, 光电催化, 电荷转移
It is known that fluoride and phosphate in aqueous solution can accelerate the photocatalytic degradation of phenol over anatase or P25 TiO₂. But the mechanism still remains under debate. In this work, an anion-free anatase TiO₂ is prepared, followed by deposition with 0.52 wt% Pt (Pt/TiO₂). Reaction was performed in aqueous solution at initial pH 5.2, where 99% of anions were in the form of F^- or H₂PO₄^-. On the addition of 0.1~30 mmol/L anions, the rate constants of phenol degradation (k_obs) were all increased, confirming the positive effect of fluoride and phosphate, respectively. Interestingly, there was a linear relationship between the increase of k_obs and the amounts of anion adsorption, the slope of which became larger in the order of fluoride>phosphate, and Pt/TiO₂>TiO₂. These observations indicate that the positive effect of anions originates from the adsorbed anions on solid, and that fluoride was more active than phosphate. A (photo)electrochemical measurement showed that fluoride and phosphate were negative and positive, respectively, to O₂ reduction, but they were all beneficial to phenol oxidation. Furthermore, in the presence of fluoride and phosphate, the flat band potentials of TiO₂ were shifted by -159 and 89 mV, respectively. The former favors orbital overlapping of phenol with TiO₂ valence band, and the latter favors orbital overlapping of O₂ with TiO₂ conduction band, all of which promotes the interfacial charge transfers. Since inorganic anions are widely present, this result would benefit the mechanism study of a semiconductor photocatalyis and its application. As a reference, pure anatase was prepared from the hydrolysis of tetrabutyl titanate, followed by calcination in air at 400℃ for 2 h. The solid was then deposited with Pt, produced in situ from the photocatalytic reduction of H₂PtCl₆ in the presence of methanol. Solid was characterized with X-ray diffraction, N₂ adsorption, Raman, and X-ray photoelectron spectroscopy. After Pt deposition, anatase phase remained unchanged, but the solid pores were blocked by a mixture of Pt and PtO₂. Photoreactions were performed at room temperature under UV light at wavelengths equal to and longer than 320 nm. Organic compounds and inorganic anions were quantitatively analyzed with a high performance liquid and ionic chromatography, respectively. (Photo)electrochemical measurement was performed in a three-electrode compartment, where a Pt gauze was used as counter electrode, and a AgCl/Ag as reference electrode.
Key words: anatase TiO₂, fluoride, phosphate, adsorption, photoelectrocatalysis, charge transfer


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