氟离子对二氧化钛选择性光催化氧化环己烷的影响

在氧气和波长大于320nm的紫外光作用下,TiO2能够催化氧化环己烷,生成环己酮和环己醇中间产物.在水悬浮液中加入少量氟化钠,能够显著提高锐钛矿型TiO2的光催化活性.随着氟离子浓度的增加,环己酮和环己醇的生成量,以及环己醇对环己酮的物质的量比均随之增加尔后达到饱和,且几乎与溶液的起始pH值(3.0和5.5)无关.随着TiO2煅烧温度的上升,环己酮和环己醇的生成量是先增加后下降;氟离子存在时也呈现类似的变化趋势,但环己酮和环己醇生成量的增加幅度与氟离子在TiO2表面的吸附量高低不尽一致.此外,金红石型二氧化钛的光催化活性较差,且加入氟离子反而抑制环己酮和环己醇的产生.对这些影响因素进行了讨论.再次假设,在二氧化钛双电层内形成的氟氢键能够促进表面羟基自由基脱附,从而加快环己烷的氧化,提高环己酮和环己醇的产率.

Effect of Fluoride Ions on the Selective Photocatalytic Oxidation of Cyclohexane over TiO_2

Under UV light irradiation at a wavelength longer than 320 nm, TiO₂ could catalyze selective oxidation of hexane by O₂ into hexanone and hexanol intermediates. It was found that addition of a small amount of NaF into the aqueous suspension could result in significant enhancement in the photocatalytic activity of anatase-type TiO₂. As fluoride concentration was increased, the amount of hexanone and hexanol, and the molar ratio of hexanol to hexanone, were all increased toward saturation, while the initial pH (3.0 and 5.5) had little effect. As the sintering temperature of TiO₂ was increased, the amounts of hexanone and hexanol formed were first increased and then decreased. In the presence of NaF, a similar sintering temperature effect was exhibited, but the trend in the increased amounts of hexanone and hexanol was not in agreement with the changes in the amount of fluoride adsorbed on TiO₂. Moreover, the photocatalytic activity of rutile-type TiO₂ was low, and in the presence of NaF, the production of hexanone and hexanol was partially inhibited. The influencing factors are discussed. The hypothesis is presented again that fluoride hydrogen bonding formed in the double layers of TiO₂ can promote desorption of surface bound hydroxyl radical, thus accelerating hexane oxidation and improving the yields of hexanone and hexanol production.