Oxygen vacancy clusters on ceria: Decisive role of cerium f electrons
Oxygen vacancy clusters on ceria: Decisive role of cerium f electrons
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DOI：
10.1103/physrevb.79.075433
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发表时间：
2009-02
影响因子：
3.7
通讯作者：
C. Zhang;A. Michaelides;D. King;S. Jenkins
中科院分区：
文献类型：
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作者：
C. Zhang;A. Michaelides;D. King;S. Jenkins
氧空位等缺陷决定了氧化铈的电子和化学性质。然而，对这些缺陷，特别是空位簇的基本了解很少。在这项工作中，我们使用密度泛函理论，除了哈伯德U长期调查各种氧空位，包括空缺单体，二聚体，三聚体，四聚体，其中亚表面的空缺也可以参与。结果表明，表面和亚表面空位的稳定性非常相似，由两个表面空位组成的空位二聚体在理论上是稳定的，但实验上没有报道;在实验观察到的两个空位三聚体中，三角形的表面空位团比含有亚表面空位的双线性表面空位团更稳定，这与某些实验结果一致，但与另一些实验结果不一致;在含有次表面空位的可能的四聚体中，线性空位四聚体是最稳定的，尽管它不如不含次表面空位的那些稳定。这些发现是合理化的电子变化方面的去除氧，即，本地化产生的多余的电子Ce f轨道。我们确定了还原Ce离子占据f态的能级与其配位数之间的相关性，这对于解释各种空位的形成能和稳定性至关重要。与实验进行了比较，并讨论了明显的差异。金吸附的空位簇的结果，并简要讨论了这些在催化的影响。
Defects such as oxygen vacancies dominate the electronic and chemical properties of ceria. However, fundamental understanding of such defects, especially clusters of vacancies, is sparse. In this work, we use density-functional theory with the addition of the Hubbard U term to investigate various oxygen vacancies, including the vacancy monomer, dimer, trimer, and tetramer, in which subsurface vacancies can also be involved. We show that the individual surface and subsurface vacancies have very similar stabilities; the vacancy dimer consisting of two surface vacancies, which is not reported experimentally, is stable in theory; between the two vacancy trimers observed in experiments, the triangular surface vacancy cluster is more stable than the double linear surface vacancy cluster containing a subsurface vacancy, which agrees with some experiments but disagrees with some others; and the linear vacancy tetramer emerges as the most stable among the possible tetramers containing subsurface vacancies, although it is less stable than those containing no subsurface vacancies. These findings are rationalized in terms of the electronic change upon the removal of oxygen, namely, the localization of resulting excess electrons on Ce f orbitals. We identify a correlation between the energy levels of the occupied f states of reduced Ce ions and their coordination numbers, which proves pivotal in interpreting formation energy and stability of various vacancies. Comparisons are made with experiments and apparent discrepancies are discussed. Results for gold adsorption on the vacancy clusters are presented, and the implications these have in catalysis are briefly discussed.