Cao Yang, Zhang Qiaozhi, Yu Iris K M, Tsang Daniel C W
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 999077, China.
Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117576, Singapore.
Proc Natl Acad Sci U S A. 2024 Oct 15;121(42):e2408770121. doi: 10.1073/pnas.2408770121. Epub 2024 Oct 10.
Catalytic oxidation through the transfer of lattice oxygen from metal oxides to reactants, namely the Mars-van Krevelen mechanism, has been widely reported. In this study, we evidence the overlooked oxidation route that features the in situ formation of surface OH species on Cu catalysts and its selective addition to the reactant carbonyl group. We observed that glucose oxidation to gluconic acid in air (21% O) was favored on low-valence Cu sites according to X-ray spectroscopic analyses. Molecular O was activated in situ on Cu/Cu forming localized, adsorbed hydroxyl radicals (*OH) which played the primary reactive oxygen species as confirmed by the kinetic isotope effect (KIE) study in DO and in situ Raman experiments. Combined with DFT calculations, we proposed a mechanism of O-to-*OH activation through the *OOH intermediate. The localized *OH exhibited higher selectivity toward glucose oxidation at C1HO to form gluconic acid (up to 91% selectivity), in comparison with free radicals in bulk environment that emerged from thermal, noncatalytic hydrogen peroxide decomposition (40% selectivity). The KIE measurements revealed a lower glucose oxidation rate in DO than in HO, highlighting the role of water (HO/DO) or its derivatives (e.g., *OH/*OD) in the rate-determining step. After proving the C1-H activation step kinetically irrelevant, we proposed the oxidation mechanism that was characterized by the rate-limiting addition of *OH to C1=O in glucose. Our findings advocate that by maneuvering the coverage and activity of surface *OH, high-performance oxidation of carbonyl compounds beyond biomass molecules can be achieved in water and air using nonprecious metal catalysts.
通过晶格氧从金属氧化物转移到反应物的催化氧化，即Mars-van Krevelen机理，已被广泛报道。在本研究中，我们证明了一条被忽视的氧化途径，其特征是在铜催化剂上原位形成表面OH物种，并将其选择性地加成到反应物羰基上。根据X射线光谱分析，我们观察到在低价铜位点上，空气中（21% O）葡萄糖氧化为葡萄糖酸更为有利。分子O在Cu/Cu上原位活化，形成局部吸附的羟基自由基（OH），动力学同位素效应（KIE）研究和原位拉曼实验证实，这些自由基是主要的活性氧物种。结合密度泛函理论（DFT）计算，我们提出了一种通过OOH中间体将O活化成OH的机理。与大量环境中由热的、非催化的过氧化氢分解产生的自由基（40%选择性）相比，局部的OH对葡萄糖在C1HO处氧化形成葡萄糖酸表现出更高的选择性（高达91%选择性）。KIE测量结果表明，DO中的葡萄糖氧化速率低于HO中的，突出了水（HO/DO）或其衍生物（如OH/OD）在速率决定步骤中的作用。在证明C1-H活化步骤在动力学上不相关后，我们提出了以OH向葡萄糖中C1=O的限速加成反应为特征的氧化机理。我们的研究结果表明，通过控制表面OH的覆盖度和活性，可以使用非贵金属催化剂在水和空气中实现超越生物质分子的羰基化合物的高效氧化。
