State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, 18 Shuangqing Road, Beijing 100085, China.
Phys Chem Chem Phys. 2018 Jul 18;20(28):18978-18985. doi: 10.1039/c8cp03081b.
Heterogeneous catalytic reactions usually proceed at the surfaces of materials, where many intermediates, such as free radicals, usually were believed to be short-lived. Herein, surface long-lived superoxide radicals (O2˙-) were identified in UV-irradiated aqueous suspensions of TiO2 and other metal oxide nanoparticles using an online chemiluminescence system. From the decay dynamics process of O2˙-, a long-lived O2˙- radical was observed on anatase TiO2 at pH = 12. After separation of the photo-excited suspension via filtration, CL was detected from the particles but not the filtrate, thus confirming O2˙- surface adsorption. The unusual stability of O2˙- was also verified using density functional theory (DFT) calculations. The lifetimes of the radicals were estimated on the different kinds of semiconductor surface according to the decay dynamics curves, and followed the order: TiO2 > ZnO > SnO2 > CeO2 > Fe2O3. Furthermore, the function of surface long-lived O2˙- in TiO2 suspensions with regards to photochemical conversion was investigated using NBT as a chemical model; it was found that half of the molecules were reduced by the surface-adsorbed O2˙-. The finding of surface-stabilized, long-lived superoxide radicals may have important implications in relation to the chemistry, biology and toxicology of these radicals.
多相催化反应通常在材料表面进行，在材料表面会存在许多中间体，例如自由基，通常认为自由基的寿命都很短。在此，通过在线化学发光系统，在 TiO2 和其他金属氧化物纳米粒子的紫外辐照水悬浮液中鉴定到了表面长寿命超氧自由基（O2˙-）。从 O2˙-的衰减动力学过程中，在 pH = 12 时，锐钛矿 TiO2 上观察到了长寿命的 O2˙-自由基。将光激发悬浮液通过过滤分离后，从颗粒中检测到 CL，但在滤出液中未检测到，从而证实了 O2˙-的表面吸附。使用密度泛函理论（DFT）计算还验证了 O2˙-的不寻常稳定性。根据衰减动力学曲线，估算了自由基在不同半导体表面上的寿命，其顺序为：TiO2 > ZnO > SnO2 > CeO2 > Fe2O3。此外，通过将 NBT 用作化学模型，研究了 TiO2 悬浮液中表面长寿命 O2˙-在光化学转化中的作用；发现一半的分子被表面吸附的 O2˙-还原。表面稳定的长寿命超氧自由基的发现可能对这些自由基的化学、生物学和毒理学具有重要意义。
