Mechanistic Insight into the Oxygen Reduction Reaction on the Mn-N4/C Single-Atom Catalyst: The Role of the Solvent Environment
Mechanistic Insight into the Oxygen Reduction Reaction on the Mn-N4/C Single-Atom Catalyst: The Role of the Solvent Environment
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DOI：
10.1021/acs.jpcc.0c00352
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发表时间：
2020-04-02
影响因子：
3.7
通讯作者：
Wang, Yang-Gang
中科院分区：
文献类型：
--
作者：
Cao, Hao;Xia, Guang-Jie;Wang, Yang-Gang
无铂族金属(PGM)催化剂具有优良的催化性能，其设计对能量转换的应用至关重要。最近，M-N-x(M=Mn，Fe，Co等)嵌入石墨烯的单原子催化剂在催化氧还原反应(ORR)方面得到了广泛的研究。尽管ORR是在液相中运行，但很少有机理研究考虑溶剂化效应。在本工作中，我们用显式水分子从头算分子动力学(AIMD)模拟和密度泛函理论(DFT)计算研究了溶剂化效应对MNN4-石墨烯的ORR机理的影响。研究发现，溶剂环境能有效地促进底物向O-2的电荷转移，导致超氧物种向过氧化氢物种的转化。这也使得ORR优选地通过解离途径进行，在该途径中，O-2可以容易地以“侧向”类型的形式吸附在单一的Mn位上，导致O-O键在质子化之前可能断裂。此外，溶剂水分子还通过延长Mn-O键和周围的氢键来提高*O和*OH中间体的质子化步骤的反应性。最后，在计算的自由能路径的基础上，液相模型比气相模型对过电势给出了更准确的估计，这与实验观察到的结果是一致的。本工作为理解M-NX/C单原子催化剂表面-液界面上的ORR反应机理提供了详细的信息。
The design of platinum group metal (PGM)-free catalysts is crucial to the application of energy conversion due to their excellent catalytic capability. Recently, M-N-x (M = Mn, Fe, Co, etc.) single-atom catalysts embedded in graphene have been extensively studied with respect to catalyzing the oxygen reduction reaction (ORR). Although the ORR is operated in the liquid phase, few mechanistic studies have taken the solvation effect into consideration. In the present work, we have performed ab initio molecular dynamics (AIMD) simulations as well as density functional theory (DFT) calculations to investigate the influence of the solvation effect on the mechanisms of ORR on MnN4-graphene by using explicit water molecules. It is found that the solvent environment can effectively promote the charge transfer from the substrate to O-2, leading to the transformation from superoxide species to peroxide species. This also makes the ORR preferably proceed via a dissociative pathway, where O-2 can be easily adsorbed on the single Mn site in the form of a "side-on" type, leading to the probable rupture of the O-O bond before being protonated. Furthermore, the solvent water molecules also raise the reactivity of protonation steps for *O and *OH intermediates by the elongation of the Mn-O bond with the assistance of the surrounding hydrogen bonds. Finally, on the basis of the calculated free-energy pathway, the liquid-phase model gives a more correct estimation for the overpotential than the gas-phase model, which is consistent with the experimental observation. The present work provides detailed information for understanding the reaction mechanisms of ORR at the surface-liquid interface on the M-Nx/C single-atom catalyst.