JACS: 通过精确合成以原子精度揭示钙钛矿氧化物中的氧空穴-性能关系

DOI:https://doi.org/10.1021/jacs.4c08643

原文链接：https://pubs.acs.org/doi/full/10.1021/jacs.4c08643

钙钛矿氧空穴浓度及分布等对其各方面性质有着重要影响，在催化、SOFC电极等应用都会注重氧空穴的含量表征，构筑其与性能之间的重要关联。本公众号最近推送了几篇关于氧空穴的文章，推荐大家阅读。

本论文则注重揭示在催化中的氧空穴和性能之间的关系，该思路可以在SOFC电极设计中予以借鉴。

由于传统粉末催化剂的晶面、表面位置、缺陷等各异，且结构本身十分复杂，因此，了解钙钛矿氧化物中氧空位原子结构对催化性能的根本影响仍然具有挑战性。通过定量合成四面体（LaCoO_2.5-T）、金字塔（LaCoO_2.5-P）和八面体（LaCoO₃）外延薄膜作为模型催化剂，展示了缺氧钙钛矿中活性位点几何构型在 CO 氧化模型反应过程中的反应顺序：CoO₄四面体 > CoO₆ 八面体 > CoO₅ 金字塔。常压 Co L-edge和 O K-edge XAS 光谱阐明了实际催化过程中活性位点电子结构的动态演变，并强调了缺陷几何结构的重要作用。此外，原位XAS和共振非弹性X射线散射光谱以及密度泛函理论计算直接揭示了CoO₄位点高反应性的性质，以及衍生的能带结构中的浅受体缺陷能级有利于反应气体的吸附和活化，导致催化反应速率比CoO₅位点提高了23倍以上。

Figure 1. Atomic structure characterizations for epitaxial thin films. HAADF-STEM images of (a) LaCoO_2.5-T, (b) LaCoO₃, and (c) LaCoO_2.5-P. The scale bar is 2 nm. (d–f) Corresponding periodic change of out-of-plane La–La distances in (a–c). (g–i) Simulated atomic images and crystal structures of LaCoO_2.5-T, LaCoO₃, and LaCoO_2.5-P, where La (green), Co (blue), and O (red) atoms are presented. The scale bar is 0.5 nm.

Figure 2. Surface chemical property of thin films. (a) synchrotron hard X-ray-based Co K-edge XANES spectra, (b) normalized reaction rate of CO oxidation, (c) O₂-TPD profiles, (d) surface adsorbed species (in situ synchrotron soft X-ray O K-edge XAS spectra under different reaction conditions, Figures S3–S11), and (e) O 1s XPS spectra of LaCoO_2.5-T, LaCoO₃, and LaCoO_2.5-P.

Figure 3. Dynamical evolutions of active sites. Synchrotron soft X-ray-based Co L-edge XAS spectra of LaCoO_2.5-T, LaCoO₃, and LaCoO_2.5-P under vacuum conditions using (a) TEY mode and (b) FY mode. Electronic structure evolution of the Co 3d center position in LaCoO_2.5-T, LaCoO₃, and LaCoO_2.5-P: (c) TEY mode and (d) FY mode. Average valence state changes of Co ions in LaCoO_2.5-T, LaCoO₃, and LaCoO_2.5-P under different conditions: (e) TEY mode and (f) FY mode, where a large intensity ratio of peak C/peak B in Co L-edge XAS implies a higher oxidation state. Dynamic structure variations of catalytic sites are obtained by in situ Co L-edge XAS spectra

Figure 4. DFT calculations of active sites in the LaCoO_2.5-T thin film. (a) Top view of the CoTet and CoOct sites of the LaCoO_2.5-T model and the CoOct site of the LaCoO₃ model. (b) Free energies of adsorption for CO and O₂ at different active sites of LaCoO_2.5-T and LaCoO₃ thin films. (c) Gibbs free-energy diagrams for the CO oxidation model reaction over CoTet and CoOct sites of LaCoO_2.5-T and CoOct sites of LaCoO₃ models. (d) Structures of intermediates at CoTet sites of LaCoO_2.5-T during CO oxidation. Green, light blue, blue, dark blue, pink, orange, and dark spheres represent La, CoTet in LaCoO_2.5-T, CoOct in LaCoO_2.5-T, CoOct in LaCoO₃, lattice OL, adsorbed OAds, and C atoms, respectively.

Figure 5. Relationship between the oxygen vacancy and electronic structure. Synchrotron soft X-ray-based O K-edge mapping of resonance inelastic scattering (mRIXS) of (a) LaCoO_2.5-T, (b) LaCoO₃, and (c) LaCoO_2.5-P. Schematic band structures of (d) LaCoO_2.5-T, (e) LaCoO_3, and (f) LaCoO_2.5-P, which are obtained by analyzing normalized XAS spectra and XES spectra of Co and O elements in corresponding mRIXS maps in (a–c) and Figures S33–S36. (g) Schematic illustrations of intrinsic activity and geometry configurations of different active-site structures in thin films.

总而言之，作者以定量的氧空穴有序外延薄膜为模型催化剂，阐明了氧缺陷型钙钛矿钴氧化物中真正的活性位点结构。还证明了钙钛矿氧化物中活性位点几何结构的固有反应性顺序：CoO₄四面体>CoO₆八面体>CoO₅金字塔。原位XAS和RIXS光谱相结合，阐明了氧缺陷型钙钛矿中不同活性位点结构的电子结构特征、动态演变及其在催化过程中的重要作用。CoO₄四面体的高催化活性归功于价带附近衍生的浅受体缺陷能级，这有利于反应气体和CoTet位点之间的电子转移，从而降低氧分子的活化能垒。在 LaCoO_2.5-T 薄膜体系中，CoO₆八面体主要起到稳定 CoO₄ 四面体活性位点结构的作用。结合电子和几何描述符，我们对原子尺度的研究揭示了钙钛矿氧化物中的氧空位-性能关系，为高性能钙钛矿氧化物的设计和对反应或传感机制的理解提供了重要指导。