CH4 partial oxidation mechanism of LaFeO3 oxygen carrier in chemical looping reforming
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摘要: 本文基于密度泛函理论（DFT）计算揭示了化学链重整过程中LaFeO3载氧体的CH4部分氧化反应机理，通过系统研究CH4吸附活化、H2和CO形成以及氧扩散等基元反应步骤，构建了CH4部分氧化反应网络。研究发现，CH4发生逐步脱氢反应形成H原子，其中CH3脱氢反应所需要克服的能垒（1.50 eV）最高，是CH4逐步脱氢反应的限速步骤。载氧体表面H2形成有两种可能的路径，其中H原子从O顶位迁移到Fe顶位，然后与另外O顶位的H原子成键形成H2分子是主要途径。由于其相对较低的能垒（1.27 eV），CO的形成过程较易发生。氧扩散需要克服1.35 eV的能垒，表明氧扩散过程需要在高温下进行且扩散速率较低。通过比较各基元反应能垒，发现H2形成是LaFeO3载氧体CH4部分氧化反应动力学的限速步骤，而H迁移是限制H2形成的关键，加快H迁移是增强LaFeO3载氧体性能的主要途径。基于DFT计算研究系列A/B位点掺杂LaFeO3载氧体的H迁移过程，有望实现潜在A/B位点有效掺杂剂的快速筛选，指导高性能LaFeO3载氧体的设计开发。Abstract: Density functional theory (DFT) calculations were employed to reveal the CH4 partial oxidation mechanism of LaFeO3 oxygen carrier during chemical looping reforming. The CH4 partial oxidation reaction network was constructed by systematically studying the elementary reaction steps, including CH4 adsorption activation, H2 and CO formation, and oxygen diffusion. It was found that CH4 undergoes a gradual dehydrogenation reaction to form H atoms, and the energy barrier (1.50 eV) of CH3 dehydrogenation is the highest, which is the rate-limiting step. There are two possible paths for H2 formation on the surface of oxygen carrier. It is the main route that the H atom from O-top site to Fe-top site bonds with another H atom on O-top site to form H2 molecule. Due to its relatively low energy barrier (1.27 eV), the CO formation process is easier to occur. Oxygen diffusion needs to overcome an energy barrier of 1.35 eV, indicating that it occurs at high temperatures and the diffusion rate is low. By comparing the energy barrier of each elementary reaction, it was found that the H2 formation is the rate-limiting step of CH4 partial oxidation kinetics for LaFeO3 oxygen carrier. The H migration is the key to limiting H2 formation, and accelerating the H migration is the main approach to improve the performance of LaFeO3 oxygen carrier. Based on DFT calculations, the H migration of A/B site doped LaFeO3 oxygen carriers could be studied, which is expected to achieve the rapid screening of potential A/B site effective dopants and guide the design and development of high-performance LaFeO3 oxygen carriers.
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图 2 (a) LaFeO3正交晶胞；(b) LaFeO3准立方超胞；(c) FeO2-终端LaFeO3(010)表面
Figure 2 (a) LaFeO3 orthorhombic unit cell; (b) LaFeO3 pseudocubic cell; (c) FeO2-terminated LaFeO3(010) surface
图 3 CHx (x = 0-4)和H物种在LaFeO3(010)表面上最稳定的吸附构型、结构参数和吸附能（键长单位Å）
Figure 3 The most stable adsorption configurations, structural parameters and adsorption energies of CHx (x = 0–4) and H on Ca2Fe2O5(010) surface (the bond lengths are in Å)
图 4 LaFeO3(010)表面CH4脱氢反应的初态、过渡态和终态
Figure 4 Initial state (IS), transition state (TS), and final state (FS) of CH4 dehydrogenation on LaFeO3(010) surface
图 5 LaFeO3(010)表面CH3脱氢反应的初态、过渡态和终态
Figure 5 Initial state (IS), transition state (TS), and final state (FS) of CH3 dehydrogenation on LaFeO3(010) surface
图 6 LaFeO3(010)表面CH2脱氢反应的初态、过渡态和终态
Figure 6 Initial state (IS), transition state (TS), and final state (FS) of CH2 dehydrogenation on LaFeO3(010) surface
图 7 LaFeO3(010)表面CH脱氢反应的初态、过渡态和终态
Figure 7 Initial state (IS), transition state (TS), and final state (FS) of CH dehydrogenation on LaFeO3(010)
图 8 LaFeO3(010)表面CH4逐步脱氢反应的势能图
Figure 8 Potential energy of CH4 gradual dehydrogenation on LaFeO3(010) surface
图 9 LaFeO3(010)表面H2形成的反应路径和势能图
Figure 9 Reaction pathway and potential energy of H2 formation on LaFeO3(010) surface
图 10 LaFeO3(010)表面H2形成的初态、过渡态和终态
Figure 10 Initial state (IS), transition state (TS), and final state (FS) of H2 formation on LaFeO3(010) surface
图 11 LaFeO3(010)表面CO形成的反应路径和势能图
Figure 11 Reaction pathway and potential energy of CO formation on LaFeO3(010) surface
图 12 LaFeO3(010)表面氧扩散路径和势能图
Figure 12 Pathway and energy profiles of oxygen diffusion for LaFeO3(010) surface
图 13 LaFeO3(010)表面CH4部分氧化的反应网络（数字表示各基元反应的能垒，VO表示表面氧空位）
Figure 13 Reaction network of CH4 partial oxidation of on LaFeO3(010) surface (the energy barrier of each elementary steps is given, VO represents the surface oxygen vacancy)
表 1 LaFeO3载氧体CH4部分氧化过程涉及的基元反应
Table 1 The elementary steps for CH4 partial oxidation over LaFeO3 oxygen carrier
No. Elementary steps CH4 adsorption activation 1 CH4 + * → CH4* 2 CH4* + * → CH3* + H* 3 CH3* + * → CH2* + H* 4 CH2* + * → CH* + H* 5 CH* + * → C* + H* Surface reaction 6 H + * → H* 7 2H* → H2(g) 8 C + * → C* 9 C* + OS (Ca2Fe2O5) → CO(g) Oxygen diffusion 10 OB (Ca2Fe2O5) → OS (Ca2Fe2O5) * and X* represent an unoccupied active site and the adsorbed species, respectively. OS and OB denote the surface oxygen and bulk oxygen of LaFeO3(010) surface, respectively. 
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