EPR studies of electron and hole trapping in titania photocatalysts
EPR studies of electron and hole trapping in titania photocatalysts
复制标题
DOI：
10.1016/j.cattod.2011.08.039
复制
发表时间：
2012-03-17
期刊：
影响因子：
5.3
通讯作者：
Howe, Russell F.
中科院分区：
文献类型：
--
作者：
Macdonald, I. Ross;Rhydderch, Shona;Howe, Russell F.
低温下的原位EPR光谱用于观察当二氧化钛光催化剂在反应物分子存在下用紫外-可见光照射时形成的顺磁产物。在没有反应物的情况下，真空辐照会产生捕获空穴（O-）和捕获电子（Ti3+）的弱EPR信号。当使用高光子通量时，当辐照停止时，捕获的电子信号的强度显着增强。这一过程在恢复辐照后是完全可逆的，并归因于一旦辐照停止，EPR不可见的导带电子被捕获。当辐照恢复时，被捕获的电子被激发回导带。在被吸附的有机化合物存在的情况下，有机分子捕获价带空穴的产物被检测到。甲基自由基是由吸附在乙酸上的价带空穴攻击而形成的。价带空穴也能劈开碳硅键，从四甲基硅烷形成甲基自由基。苄基三甲基硅烷衍生物通过所有四个碳硅键的裂解形成甲基自由基和苄基自由基。这些观察结果与光催化有机反应的相关性，其中碳-碳键形成发生通过自由基中间体进行了讨论。(C) 2011 Elsevier b.v.版权所有
In situ EPR spectroscopy at cryogenic temperatures is used to observe paramagnetic products formed when titania photocatalysts are irradiated with UV-visible light in the presence of reactant molecules. Irradiation in vacuo, in the absence of reactants, produces weak EPR signals of trapped holes (O-) and trapped electrons (Ti3+). When high photon fluxes are used, the intensities of the trapped electron signals are enhanced dramatically when irradiation is stopped. This process is completely reversible on restoring the irradiation, and is attributed to a trapping of EPR invisible conduction band electrons once irradiation is stopped. The trapped electrons are excited back into the conduction band when irradiation is resumed. In the presence of adsorbed organic compounds, products of valence band hole trapping by the organic molecules are detected. Methyl radicals are formed by attack of valence band holes on adsorbed acetic acid. The valence band holes are also able to cleave carbon-silicon bonds, forming methyl radicals from tetramethylsilane. Benzyltrimethylsilane derivatives form both methyl radicals and benzyl radicals through cleavage of all four carbon-silicon bonds. The relevance of these observations to photocatalysed organic reactions in which carbon-carbon bond formation occurs via radical intermediates is discussed. (C) 2011 Elsevier B. V. All rights reserved.