Ghosh Richa, Hopping Geoffrey M, Lu Jordan W, Hollyfield Drew W, Flaherty David W
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
Department of Chemical and Biomolecular Engineering, University of Illinois Urbana-Champaign, Urbana, Illinois 61801 United States.
J Am Chem Soc. 2025 Jan 15;147(2):1482-1496. doi: 10.1021/jacs.4c08948. Epub 2024 Dec 11.
Rates and selectivities for the partial oxidation of organic molecules on reactive electrodes depend on the identity and prevalence of reactive and spectator species. Here, we investigate the mechanism for the epoxidation of 1-hexene (CH) with reactive oxygen species formed by electrochemical oxidation of water (HO) on gold (Au) in an aqueous acetonitrile (CHCN) electrolyte. Cyclic voltammetry measurements demonstrate that oxygen (O) evolution competes with CH epoxidation, and the Au surface must oxidize before either reaction occurs. Raman spectroscopy reveals reactive oxygen species and spectators (CHCN) on the active anode as well as species within the electrochemical double layer. The Faradaic efficiencies toward epoxidation and the ratios of epoxide formation to O evolution rates increase linearly with the concentration of CH and depend inversely on the concentration of HO, which agree with analytical expressions that describe rates for reaction between CH and chemisorbed oxygen atoms (O*) and exclude proposals for other forms of reactive oxygen (e.g., O*, OOH*, OH*). These findings show that the epoxidation and O evolution reactions share a set of common steps that form O* through electrochemical HO activation but then diverge. Subsequently, epoxides form when O* reacts with CH through a non-Faradaic process, whereas O evolves when O* reacts with HO through a Faradaic process to form OOH*, which then deprotonates. These differences lead to distinct changes in rates in response to electrode potential, and hence, disparate Tafel slopes. Collectively, these results provide a self-consistent mechanism for CH epoxidation that involves reactive O*.
反应性电极上有机分子部分氧化的速率和选择性取决于反应性物种和旁观物种的特性及丰度。在此，我们研究了在乙腈（CH₃CN）水溶液电解质中，1 - 己烯（CH）在金（Au）电极上通过水（H₂O）的电化学氧化形成的活性氧物种作用下进行环氧化反应的机理。循环伏安法测量表明，析氧（O₂）反应与CH环氧化反应相互竞争，且在任何一个反应发生之前，Au表面必须先被氧化。拉曼光谱揭示了活性阳极上的活性氧物种和旁观物种（CH₃CN）以及电化学双层内的物种。环氧化反应的法拉第效率以及环氧化物生成速率与O₂析出速率的比值随CH₃CN浓度呈线性增加，且与HO浓度呈反比，这与描述CH₃CN与化学吸附氧原子（O*）之间反应速率的分析表达式一致，并排除了其他形式活性氧（如O₂*、OOH*、OH*）的相关假设。这些发现表明，环氧化反应和析氧反应共享一组通过电化学活化HO形成O的共同步骤，但随后发生分歧。随后，当O通过非法拉第过程与CH₃CN反应时形成环氧化物，而当O通过法拉第过程与HO反应形成OOH，然后OOH去质子化时则析出O₂。这些差异导致速率随电极电位发生明显变化，进而导致不同的塔菲尔斜率。总的来说，这些结果为涉及活性O的CH₃CN环氧化反应提供了一个自洽的机理。
