Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, China.
J Am Chem Soc. 2011 Jun 29;133(25):9938-47. doi: 10.1021/ja203468v. Epub 2011 Jun 7.
Whether gold is catalytically active on its own has been hotly debated since the discovery of gold-based catalysis in the 1980s. One of the central controversies is on the O(2) activation mechanism. This work, by investigating aerobic phenylethanol oxidation on gold nanoparticles in aqueous solution, demonstrates that gold nanoparticles are capable to activate O(2) at the solid-liquid interface. Extensive density functional theory (DFT) calculations combined with the periodic continuum solvation model have been utilized to provide a complete reaction network of aerobic alcohol oxidation. We show that the adsorption of O(2) is very sensitive to the environment: the presence of water can double the O(2) adsorption energy to 0.4 eV at commonly available edge sites of nanoparticles (4 nm) because of its strongly polarized nature in adsorption. In alcohol oxidation, the hydroxyl bond of alcohol can break only with the help of an external base at ambient conditions, while the consequent α-C-H bond breaking occurs on pure Au, both on edges and terraces, with a reaction barrier of 0.7 eV, which is the rate-determining step. The surface H from the α-C-H bond cleavage can be easily removed by O(2) and OOH via a H(2)O(2) pathway without involving atomic O. We find that Au particles become negatively charged at the steady state because of a facile proton-shift equilibrium on surface, OOH + OH ↔ O(2) + H(2)O. The theoretical results are utilized to rationalize experimental findings and provide a firm basis for utilizing nanoparticle gold as aerobic oxidation catalysts in aqueous surroundings.
自 20 世纪 80 年代发现金基催化以来，金自身是否具有催化活性一直备受争议。其中一个核心争议点是关于 O(2) 活化机制。这项工作通过研究水相中金纳米粒子上的有氧苯乙醇氧化，证明金纳米粒子能够在固液界面上活化 O(2)。我们利用广泛的密度泛函理论（DFT）计算结合周期性连续体溶剂化模型，提供了有氧醇氧化的完整反应网络。我们表明，O(2)的吸附对环境非常敏感：由于其在吸附中的强极化性质，水的存在可以将 O(2)的吸附能提高到通常存在于纳米粒子（~4nm）边缘的约 0.4eV，增加了一倍。在醇氧化中，只有在环境条件下有外部碱的帮助，醇的羟基键才能断裂，而随后的α-C-H 键断裂发生在纯 Au 上，无论是在边缘还是在平台上，反应势垒为 0.7eV，这是决速步骤。α-C-H 键断裂产生的表面 H 可以很容易地通过 O(2)和 OOH 通过 H(2)O(2)途径去除，而不涉及原子 O。我们发现由于表面上易于发生质子迁移平衡，Au 颗粒在稳态下带负电荷，OOH + OH ↔ O(2) + H(2)O。理论结果被用于合理化实验结果，并为在水相环境中利用纳米金颗粒作为有氧氧化催化剂提供了坚实的基础。
