Tereshchuk Polina, Freire Rafael L H, Ungureanu Crina G, Seminovski Yohanna, Kiejna Adam, Da Silva Juarez L F
São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970, São Carlos, SP, Brazil.
Phys Chem Chem Phys. 2015 May 28;17(20):13520-30. doi: 10.1039/c4cp06016d.
Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO2), fundamental issues such as the role of the TM atoms in the change in the oxidation state of Ce atoms are still not well understood. In this work, we report a theoretical investigation based on static and ab initio molecular dynamics density functional theory calculations of the interaction of 13-atom TM clusters (TM = Pd, Ag, Pt, Au) with the unreduced CeO2(111) surface represented by a large surface unit cell and employing Hubbard corrections for the strong on-site Coulomb correlation in the Ce f-electrons. We found that the TM13 clusters form pyramidal-like structures on CeO2(111) in the lowest energy configurations with the following stacking sequence, TM/TM4/TM8/CeO2(111), while TM13 adopts two-dimensional structures at high energy structures. TM13 induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from Ce(IV) (itinerant Ce f-states) to Ce(III) (localized Ce f-states). There is a charge flow from the TM atoms to the CeO2(111) surface, which can be explained by the electronegativity difference between the TM (Pd, Ag, Pt, Au) and O atoms, however, the charge is not uniformly distributed on the topmost O layer due to the pressure induced by the TM13 clusters on the underlying O ions, which yields a decrease in the ionic charge of the O ions located below the cluster and an increase in the remaining O ions. Due to the charge flow mainly from the TM8-layer to the topmost O-layer, the charge cannot flow from the Ce(IV) atoms to the O atoms with the same magnitude as in the clean CeO2(111) surface. Consequently, the effective cationic charge decreases mainly for the Ce atoms that have a bond with the O atoms not located below the cluster, and hence, those Ce atoms change their oxidation state from IV to III. This increases the size of the Ce(III) compared with the Ce(IV) cations, which builds-in a strain within the topmost Ce layer, and hence, also affecting the location of the Ce(III) cations and the structure of the TM13 clusters.
尽管对负载在二氧化铈（CeO₂）上的过渡金属（TM）团簇进行了广泛研究，但诸如TM原子在Ce原子氧化态变化中所起的作用等基本问题仍未得到很好的理解。在这项工作中，我们报告了一项基于静态和从头算分子动力学密度泛函理论计算的理论研究，该研究针对由大表面晶胞表示的未还原CeO₂(111)表面与13原子TM团簇（TM = Pd、Ag、Pt、Au）之间的相互作用，并对Ce f电子中的强在位库仑相关性采用了哈伯德修正。我们发现，TM₁₃团簇在CeO₂(111)上以最低能量构型形成类似金字塔的结构，具有以下堆叠顺序：TM/TM₄/TM₈/CeO₂(111)，而TM₁₃在高能量结构中采用二维结构。TM₁₃会使位于最顶层Ce层中的少数Ce原子（16个中的3个）的氧化态从Ce(IV)（巡游的Ce f态）变为Ce(III)（局域的Ce f态）。存在从TM原子到CeO₂(111)表面的电荷流动，这可以用TM（Pd、Ag、Pt、Au）与O原子之间的电负性差异来解释，然而，由于TM₁₃团簇对下层O离子施加的压力，电荷并非均匀分布在最顶层O层上，这导致位于团簇下方的O离子的离子电荷减少，而其余O离子的离子电荷增加。由于电荷主要从TM₈层流向最顶层O层，电荷无法以与清洁的CeO₂(111)表面相同的量级从Ce(IV)原子流向O原子。因此，有效阳离子电荷主要对于与不在团簇下方的O原子形成键的Ce原子减少，因此，那些Ce原子将其氧化态从IV变为III。这使得Ce(III)的尺寸相对于Ce(IV)阳离子增大，这在最顶层Ce层内产生了应变，因此，也影响了Ce(III)阳离子的位置和TM₁₃团簇的结构。
