过剩电子对乙二醇在 R-TiO2(110)表面光化学断键选择性的影响

程序升温脱附谱（ Temperature-Programmed Desorption, TPD），是一种研究表面化学反应的强有力工具。论文作者使用自行搭建的基于超高灵敏质谱的 TPD 装置，深入探究了乙二醇（ EG）在金红石（ R） TiO2(110) 表面的光化学反应，并分析了过剩电子对光催化反应断键选择性（ C-O vs C-C）的影响。
过剩电子在基于过渡金属氧化物的表面催化反应中扮演了至关重要的作用，然而关于过剩电子在光催化反应断键选择性中所起到的作用的相关研究却鲜有报道。以乙二醇 / 金红石 TiO2(110) 体系为研究对象，论文作者比较了乙二醇在还原性和氧化性的金红石 TiO2(110) 表面上的光化学反应。 在还原性金红石TiO2(110) 表面上，当紫外光照时，乙二醇分子更倾向于通过C-O 键的断裂生成乙醛（ CH3CHOTi），而不是通过 C-C 键的断裂生成甲醛（ CH2OTi）；而在氧化性 R-TiO2(110) 表面上，随着表面预吸附 O2 量的增加，更多的乙二醇分子通过 C-C 键的断裂生成了甲醛；与此同时，通过 C-O 键断裂生成的CH3CHOTi 则基本不变。进一步分析表明，过剩电子很可能是造成乙二醇光催化选择性断键发生显著变化的关键原因。乙二醇光解离生成CH2OTi 属于半反应，会在表面留下过剩电子，因此，在还原性表面上，过剩电子的累积抑制 CH2OTi 的生成。预先吸附 O2 降低了表面过剩电子浓度，因而会促进 CH2OTi 的生成。这项工作有助于加深我们对过剩电子在 TiO2 光催化中所扮演作用的理解。未来，我们有望通过调控TiO2 及其他光催化剂上表面过剩电子的浓度，实现对表面光催化反应断键选择性的调节。
 

Temperature-Programmed Desorption (TPD) is a powerful tool for studying surface chemical reactions. Using a home-built TPD apparatus based on ultrahighly sensitive mass spectrometry detection system, the author has systematically explored the photochemical reaction of ethylene glycol (EG) on R-TiO2(110), and elucidated the influence of excess electrons on the selectivity of bond cleavage (C-O vs C-C) in the photocatalytic reaction.
Excess electrons play an important role in the surface catalytic reactions on various transition metal oxides. However, few studies have been performed on the role of excess electrons in the selectivity of bond cleavage in photocatalytic reactions. Taking EG / rutile TiO2(110) as a model system, the author studied the photochemical reactions of EG on reduced and oxidized rutile(R-) TiO2(110) surfaces. On the reduced R-TiO2(110) surface, when irradiated by UV light, EG molecules prefer to form acetaldehyde (CH3CHOTi) through the cleavage of the C-O bond at high EG coverage. With the increasing pre-adsorbed O2 on the surface, the C-C bond cleavage of EG into CH2OTi is largely enhanced and becomes dominant. Conversely, the yield of CH3CHOTi through the cleavage of the C-O bond remains nearly unchanged. Further analysis showed that the significant changes in the selectivity of bond cleavage in EG photocatalysis are most likely due to the excess electrons contributed by Ov and Tiint. The formation of CH2OTi from EG photolysis on R-TiO2(110) is a hole-mediated half-reaction, leaving excess electrons on the R-TiO2(110) surface. Therefore, on the reduced surface, the accumulation of excess electrons will inhibit the formation of CH2OTi. Pre-adsorption of O2 can decrease excess electron concentration, thus promoting the formation of CH2OTi. In the future, we may tune the photocatalytic selectivity by regulating the concentrations of excess electrons on R-TiO2 and other photocatalysts.
 

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