Agrios Alexander G, Gray Kimberly A, Weitz Eric
Institute for Environmental Catalysis, Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
Langmuir. 2004 Jul 6;20(14):5911-7. doi: 10.1021/la036165d.
The goal of this research was to investigate the formation and reactivity of charge-transfer complexes (CTCs) among a homologous series of chlorophenols on TiO2. We previously showed that 2,4,5-trichlorophenol (245TCP) forms a CTC with Degussa P25, a commercial preparation of TiO2. Here, we probe how light energy influences reactivity and product formation. Slurries of P25 containing 245TCP were irradiated at 360, 400, 430, 480, and 550 nm. At each wavelength, the amount of transformation of 245TCP correlates to the diffuse-reflectance absorbance of a 245TCP/P25 system, supporting the CTC as the cause of reaction. In addition, polymeric products are formed only under wavelengths that excite the CTC, indicating a different reaction mechanism for the CTC than for bandgap excitation of TiO2. We also found a higher quantum efficiency for CTC reactivity than for bandgap activation of the catalyst, suggesting that the photocatalytic efficiency and selectivity can be improved for certain compounds by designing catalytic materials that form CTCs with them. Furthermore, to determine how chlorine substitution patterns affected adsorption and sub-bandgap reactivity, P25 slurries containing phenol, 4-chlorophenol, 2,4-dichlorophenol, or 2,4,6-trichlorophenol were probed following dark contact or irradiation at 360, 430, or 550 nm. With respect to the extent of adsorption, complexation, reaction, and polymerization on P25, the behavior of 245TCP far exceeded that of the other chlorophenols. Among these chlorophenols, only 2,4-dichlorophenol produced a polymeric product. 245TCP is unique among this family of chlorophenols, which we attribute to a chlorine arrangement that leads to a favorable orbital overlap with TiO2 and sterically permits coupling reactions. Our results demonstrate the critical role that charge-transfer complexation can play in determining the rates and products of photocatalytic reactions.
本研究的目标是探究一系列氯酚在TiO₂上电荷转移络合物（CTC）的形成及反应活性。我们之前表明2,4,5-三氯苯酚（245TCP）与德固赛P25（一种商业化的TiO₂制剂）形成了一种CTC。在此，我们探究光能如何影响反应活性和产物形成。含有245TCP的P25悬浮液在360、400、430、480和550nm波长下进行辐照。在每个波长下，245TCP的转化量与245TCP/P25体系的漫反射吸光度相关，这支持了CTC是反应的起因。此外，聚合物产物仅在激发CTC的波长下形成，这表明CTC的反应机制与TiO₂的带隙激发不同。我们还发现CTC反应的量子效率高于催化剂的带隙活化，这表明通过设计能与某些化合物形成CTC的催化材料，可以提高其光催化效率和选择性。此外，为了确定氯取代模式如何影响吸附和亚带隙反应活性，在黑暗接触或在360、430或550nm波长下辐照后，对含有苯酚、4-氯苯酚、2,4-二氯苯酚或2,4,6-三氯苯酚的P25悬浮液进行了探究。就P25上的吸附、络合、反应和聚合程度而言，245TCP的行为远远超过其他氯酚。在这些氯酚中，只有2,4-二氯苯酚产生了聚合物产物。245TCP在这一系列氯酚中是独特的，我们将其归因于一种氯排列方式，这种排列方式导致与TiO₂有良好的轨道重叠，并且在空间上允许偶联反应。我们的结果证明了电荷转移络合在决定光催化反应速率和产物方面可以发挥关键作用。
