Palladium monomers, dimers, and trimers on the MgO(001) surface viewed individually.
Palladium monomers, dimers, and trimers on the MgO(001) surface viewed individually.
复制标题
DOI：
10.1002/anie.200702444
复制
发表时间：
2007-11
影响因子：
--
通讯作者：
M. Sterrer;T. Risse;L. Giordano;M. Heyde;N. Nilius;H. Rust;G. Pacchioni;H. Freund
中科院分区：
文献类型：
--
作者：
M. Sterrer;T. Risse;L. Giordano;M. Heyde;N. Nilius;H. Rust;G. Pacchioni;H. Freund
人们早就知道，金属团簇的大小会极大地影响氧化物负载金属催化剂的催化活性为了理解产生这种效应的物理和化学原理，以及建立催化剂颗粒的结构和尺寸与它们的反应性之间的关系，对尺寸相关的几何性质和相应的电子特征的详细了解是必要的。[2-4]这一信息对于仅由几个原子组成的非常小的粒子特别重要，它们的性质不能用标度定律来预测。近年来，通过在氧化物表面沉积气相选择团簇的方法，研究了只有几个原子大小的负载金属颗粒的电子和几何性质。[5-8]然而，表面小簇的大小和形状，以及它们的吸附位置的性质——规则的表面位置或缺陷——往往仍然不清楚，特别是如果采用非成像技术。因此，我们对氧化物支撑的金属原子和小簇的大部分知识来自理论研究。本文中，我们使用低温扫描隧道显微镜（STM）实验研究了最小的氧化物负载金属颗粒，包括精确测定它们的化学计量和吸附位点，以Pd/MgO为模型系统。Pd/MgO体系特别有趣，因为MgO支持的钯纳米颗粒是在簇生长和结构以及催化活性方面研究得最好的模型催化剂之一，[9-12]，甚至在单原子状态下也发现了催化活性。[6,13,14]虽然实验技术已经获得了关于氧化镁负载钯纳米颗粒结构的宝贵信息，但单原子和小簇的性质，如成核位置、键合机制、几何结构和扩散，只能从理论上得到。[16-23]本文报道了小Pd颗粒（Pd1, Pd2, Pd3）吸附在MgO薄膜规则表面上的几何形状、吸附位置和电子态。STM的操作和研究能力与密度泛函理论（DFT）获得的信息相结合，为粒子的结构和电子特性提供了详细的实验和理论描述。图1显示了在衬底温度为5-10 K下沉积Pd后获得的三单层MgO/Ag（001）薄膜表面的STM图像（参见实验细节的支持信息），以及代表MgO表面一个离子亚晶格的原子分辨率图像。大部分沉积的Pd被吸附为单个原子，只有一小部分形成聚集体，如图1中较亮的斑点所示在图2a中，显示了一小块MgO表面吸附了Pd原子（1-4）和小聚集体(5)。为了获得更好的可视化效果，将该图像倒置，并调整高度比例以减小Pd原子的表观尺寸（图2b）。从MgO表面的原子分辨图像中提取的离子亚晶格的叠加（见图1）表明，所有的Pd原子都位于叠加晶格上的同一类型的位置（见图2b）。电压脉冲（通常为1.5-2 V）在靠近Pd原子1位置的STM尖端处施加，诱导跃迁到新的位置（1*），这再次对应于等效的晶格位置（图2 c和d）。这一结果表明，与Au原子在MgO表面的吸附相反，钯原子在该表面只存在一个有利的吸附位点。
It has long been known that the size of metal clusters can tremendously influence the catalytic activity of oxide-supported metal catalysts.[1] To understand the physical and chemical principles that give rise to this effect, as well as to establish a relation between the structure and size of the catalyst particles and their reactivity, a detailed knowledge about the size-dependent geometrical properties and the corresponding electronic features is necessary.[2–4] This information is of particular importance for very small particles, constituting only a few atoms, whose properties cannot be predicted by scaling laws. Recently, progress has been made in studying the electronic and geometric properties of supported metal particles only a few atoms in size by depositing gasphase selected clusters on oxide surfaces.[5–8] However, the size and shape of the small clusters on the surface, as well as the nature of their adsorption site—regular surface sites or defects—often remains unclear, especially if non-imaging techniques are applied. Therefore, most of our knowledge of oxide-supported metal atoms and small clusters comes from theoretical studies. Herein, we use low-temperature scanning tunneling microscopy (STM) to investigate experimentally the smallest oxide-supported metal particles, including the exact determination of their stoichiometry and adsorption sites, using Pd/MgO as a model system. The Pd/MgO system is particularly interesting since MgO-supported palladium nanoparticles are among the best studied model catalysts in terms of cluster growth and structure,[9–12] as well as catalytic activity, which has been found even in the single-atom regime.[6, 13, 14] Although experimental techniques have yielded valuable information on the structure of MgO-supported palladium nanoparticles,[15] the properties of single atoms and small clusters, such as nucleation sites, bonding mechanism, geometric structure, and diffusion, are only available from theory.[16–23] Herein we report the geometry, adsorption sites, and electronic states of small Pd particles (Pd1, Pd2, Pd3) adsorbed on regular surface sites of MgO thin films. The manipulation and investigation capabilities of STM are combined with information obtained from density functional theory (DFT), to provide a detailed experimental and theoretical description of the structural and electronic properties of the particles. An STM image of the surface of a three-monolayer thin MgO/Ag (001) film acquired after deposition of Pd at a substrate temperature of 5–10 K is shown in Figure 1 (see theSupporting Information for experimental details), together with an atomically resolved image representing one ionic sublattice of the MgO surface. The majority of the deposited Pd is adsorbed as single atoms and only a small fraction forms aggregates, seen as the brighter spots in Figure 1.[24] In Figure2a a small area of the MgO surface with adsorbed Pd adatoms (1–4) and a small aggregate (5), is shown. For better visualization, this image has been inverted and the height scale has been adjusted to reduce the apparent size of the Pd adatoms (Figure 2 b). Superposition of the ionic sublattice extracted from atomically resolved images of the MgO surface (inset in Figure1) reveals that all the Pd adatoms are located on the same type of site on the superimposed lattice (Figure 2b). A voltage pulse (typically 1.5–2 V) applied with the STM tip near the location of the Pd adatom 1 induced a hopping to the new position (1*), which corresponds again to an equivalent lattice site (Figure 2 c and d). This result indicates the existence of only one favorable adsorption site for Pd on this surface, in contrast to Au atoms adsorbed on MgO …