低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu13团簇以不同接触条件与Cu(001)表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大.     

低温下Cu13团簇负载于Cu(001)表面上结构变化的分子动力学研究

应用分子动力学方法研究温度为10和50 K时具有二十面体结构的Cu₁₃团簇以不同接触条件与Cu（001）表面结合后的结构变化,原子间的相互作用势采用Johnson的嵌入原子方法模型.通过基于原子密度分布函数的分析表明,负载团簇与表面的结合能主要受团簇与载体相接触的最低层原子数及这些原子所具有的不同几何构型影响,同时更高层的原子呈现出不同的几何结构.温度为10 K时,负载团簇的初始位置对团簇几何结构和结合能影响较大. 关键词： 分子动力学 团簇 表面 计算机模拟     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综(NVT)分子动力学,模拟了在10 K条件下,Au959团簇负载于MgO(100)表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大.     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综（NVT）分子动力学,模拟了在10 K条件下,Au₉₅₉团簇负载于MgO（100）表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大. 关键词： 团簇 分子动力学 计算机模拟 表面     

PdSi_n(n=1-15)团簇结构、稳定性与电子性质的密度泛函研究

使用卡里普索(CALYPSO)预测团簇可能结构,运用密度泛函理论(DFT)的杂化密度泛函B3LYP,对PdSi_n(n=1-15)团簇的几何结构与电子性质进行了计算,并讨论了团簇的平均结合能、能隙、二阶能量差分以及电子自然布局和极化率.研究结果表明:PdSi_n(n=1-15)团簇的基态构型由平面结构向立体结构演化,最终形成笼形结构;在Sin中掺杂Pd原子增强了团簇的稳定性;PdSi_4与PdSi_(12)团簇是幻数结构,PdSi_4的稳定性和密堆性最好;NCP和NEC分析表明,在PdSi_n基态团簇中,电荷从Si原子向Pd原子转移,在Pd原子内部发生了spd杂化;Si-Si键之间较强的相互作用力是PdSi_4和PdSi_(12)团簇基态结构更加稳定的原因;PdSi_n团簇中原子间的相互作用伴随n值的增大而不断增强.     

分子动力学模拟Cu(010)基体对负载Co-Cu双金属团簇熔化过程的影响

纳米团簇负载到基体上的结构演化和热稳定性是其走向技术应用的关键. 本文用分子动力学结合嵌入原子方法模拟了具有二十面体初始结构的Co₂₈₁Cu₂₈₀ 混合双金属团簇在Cu(010)基体上的熔化过程, 考察了基体的Cu原子可以自由移动(自由基体)和固定(固定基体)两种条件对负载团簇熔化的影响. 发现基体条件对团簇的熔化有明显的影响. 在自由基体上团簇原子的温度-能量曲线存在明显的团簇熔化时的能量突变点, 熔点为1320 K, 低于固定基体上团簇的熔点1630 K. 在升温过程中团簇的二十面体结构会在基体表面发生外延生长. 外延团簇随着温度增加发生表面预熔, 预熔原子会逐渐向基体表面扩散形成薄层, 直至完全熔化. 自由基体上团簇原子的嵌入行为会使原子的分布状态产生不同于固定基体上的演变.     

Cu_(87)团簇在升温与急冷过程中结构变化的原子尺度模拟

本文采用基于嵌入原子法的正则系综分子动力学方法在原子尺度上计算了包含87个原子的Cu87金属团簇在连续升温和急冷降温时的结构演化过程.根据原子平均势能、对分布函数、原子堆积结构和主要原子键对数目随温度的变化表明,温度的不同极大地影响团簇内的原子堆积结构.在升温过程中,随着温度的升高,团簇内原子堆积结构出现由密排六方、二十面体直到无序堆积的变化.在急冷降温过程中,随着急冷温度的降低,团簇内出现一定数量的二十面体和面心立方局域结构、数量不一的HCP,FCC和二十面体局域结构,直到急冷温度较低时的一定数量的二十面体局域结构.     

Pd_nAl~(±)(n=1—8)二元团簇的密度泛函理论研究

采用密度泛函理论(DFT)方法,在BPW91/LANL2DZ水平下详细研究了PdnAl±(n=1—8)团簇的几何结构、稳定性和电极化率,并与Pdn+1和PdnAl(n=1—8)团簇进行了比较.结果表明:除n=6,8外,PdnAl团簇和Pdn+1团簇的基态结构保持了相同的构型,除Pd2Al±和Pd6Al+外,PdnAl±团簇的基态结构与PdnAl团簇的保持了相同的结构.且在PdnAl和PdnAl±团簇基态结构中,Al原子位于较高的配位点上.分裂能和二阶能量差分的分析结果表明Pd4,Pd3Al和Pd3Al±团簇相对其他团簇具有较高的稳定性.极化率的研究表明富Pd团簇的非线性光学效应强,容易被外场极化.Pd6,Pd5Al和Pd3Al±团簇的极化率各向异性不变量为最小值,说明相应团簇对外场的各向异性响应最弱,各方向的极化率大小变化不大.     

大尺寸核/层Co-Pd合金团簇稳定结构研究

纳米尺度合金团簇具有特殊的光学、电、磁和催化等性质,在基础研究和应用领域吸引了广泛的兴趣.使用多体Gupta势函数描述Co-Pd团簇原子间相互作用,应用基于内核构建的自适应免疫优化算法确定最稳定结构.研究结果显示:98原子Co-Pd团簇结构可分为面心立方结构、Mackay二十面体、双二十面体、由双二十面体面面相连构成的结构和十面体结构.序列参数显示Co原子位于内层,而Pd原子位于外层.原子半径和表面能进一步解释了Co和Pd原子的分布规律.原子数目为147的Co-Pd团簇均为完整二十面体结构.     

Pd负载共价有机骨架COF-108上氢溢流机理的密度泛函理论研究

使用密度泛函理论研究了Pd负载共价有机骨架COF-108上的氢溢流机理,主要包括Pd_4团簇在COF-108上的各种沉积构型,H_2分子在Pd_4@COF-108中Pd_4团簇上的吸附和解离,H原子从Pd_4团簇向COF-108上的迁移以及H原子在COF-108表面的扩散.研究结果表明:1)Pd原子与HHTP或TBPM接触的数目越多,束缚能越大,沉积取向对束缚能的影响不大;2)H_2分子在Pd_4团簇上的解离过程是自动进行的,满足氢溢流发生的条件;3)只有位于Pd_4团簇桥位上的H原子能够迁移到基底表面上,迁移过程为吸热反应,稳定性较差,这表明H原子将在基底表面进一步扩散;4)过渡金属Pd的引入可大大降低H原子在基底表面的扩散势垒,使H原子在基底表面的扩散更容易进行.这些研究结果有助于理解氢溢流对COFs储氢性能影响的微观机理,进而为实验上定向制备性能优异的COFs基底材料提供有益指导.     

Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth

Fabrication of nanoclusters on a substrate is of great interest in studies of model catalysts. The key factors that govern the growth and distribution of metal on graphene have been studied by scanning tunneling microscopy (STM) based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moiré pattern formed on Ru(0001). Our experimental findings show that Pt and Rh form finely dispersed small clusters located at fcc sites on graphene while Pd and Co form large clusters at similar coverages. These results, coupled with previous findings that Ir forms the best finely dispersed clusters, suggest that both metal–carbon (M–C) bond strength and metal cohesive energies play significant roles in the cluster formation process and that the M–C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Furthermore, experimental results show Au behaves differently and forms a single-layer film on graphene, indicating other factors such as the effect of substrate metals and lattice match should also be considered. In addition, the effect of annealing Rh on graphene has been studied and its high thermal stability is rationalized in terms of a strong interaction between Rh and graphene as well as sintering via Ostwald ripening.     

XPS and XAES study of the interaction of palladium and copper overlayers with fullerene films

X-ray photoelectron and Auger spectroscopies were used to examine surface bonding and overlayer growth during palladium and copper deposition onto films of fullerene, C₆₀ The results were consistent with metal cluster formation on C₆₀. The observed positive core electron binding energy shifts in small metallic clusters supported on C₆₀ were shown to originate in metal-fullerene interaction accompanied by charge transfer. Palladium-fullerene intermixing at temperatures as low as ss 50°C was observed for small Pd coverages.     

Grain-boundary diffusion and solubility of helium in submicrocrystalline palladium

The diffusion and solubility of helium in palladium with a submicrocrystalline structure are investigated by thermal desorption of helium from He-saturated specimens at temperatures T = 293–508 K and saturation pressures P = 0.1–35 MPa. As the saturation pressure rises, the effective diffusion coefficient increases, exhibits a plateau, and then decreases to its initial value. Along with the four plateaus discovered earlier, the solubility versus saturation pressure dependence in the range 25.5–35.0 MPa demonstrates a fifth plateau, where the solubility is as high as (3.0 ± 0.4) × 10¹⁷ cm⁻³. It is shown that the helium diffuses along grain boundaries, at which clusters (traps) consisting of eight to ten vacancies are localized, and dissolves in these clusters. The high value of C _eff in the fifth plateau is explained by pairwise merging of adjacent vacancy clusters. From the D _eff(P) dependences, the vacancy clusters concentration is estimated as C* = 2.32 × 10¹⁶ cm⁻³. Within the experimental error, this value coincides with that obtained from the solubility data. Calculations of the energy of helium-defect interaction in submicrocrystalline Pd that are made using the molecular dynamics method support the experimental results.     

Enhanced oxygen binding through surface mediated ionic bonds

Palladium clusters deposited on TiO₂ supports constitute important oxidation catalysts. Addition of O₂ in presence of deposited palladium clusters results in dissociated O atoms whose binding strength controls the activity of a catalytic oxidation reaction. Here we demonstrate how the presence of even remote Pd sites can more than double the energy required to remove surface O atoms from a surface. This unusual increase in binding is shown to originate in a Coulomb interaction between Pd^δ+ and O^δ? where the otherwise semiconducting support remains almost neutral acting to mediate a charge transfer from Pd to O sites. The O atoms supported on lattice Ti sites are further shown to form composite TiO motifs that can exchange sites with Pd atoms with a minimal energy, opening the pathway to Ti migration. This behavior is proposed to be linked to their identical valence pool containing d-states.     

Structure and thermal properties of supported catalyst clusters for single-walled carbon nanotube growth

Structure and thermal properties of supported iron clusters were studied using molecular dynamics simulations. When supported clusters are in the liquid state, their surfaces have spherical curvature, whereas solid clusters form a layered crystalline structure. The cluster freezing (melting) point increases dramatically with increasing cluster-substrate interaction strength, and rapid diffusion of cluster surface atoms is observed below the freezing point.     

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd(3d_5/2) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the Pd---CO bond) on these surfaces. A similar correlation is seen for the Ni(2p_3/2) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the Ni---Ni and Pd---Pd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd(3d_5/2) XP binding energy of Pd/W(110) as a function of film thickness indicate that the Pd---W interaction affects the electronic properties of several layers of Pd atoms.     

Adsorbate-oxide interactions during the NO + CO reaction on MgO(100) supported Pd monolayer films

The potential energy diagram for the NO+CO reaction on 1, 2, and 3 monolayer (ML) Pd films supported by MgO(100) is calculated using density functional theory. Thin Pd films are generally found to be more reactive than thick films, with a notable exception for nitrogen adsorption on 2 ML Pd/MgO(100). For this system an attractive through-the-metal adsorbate-oxide interaction of 0.5 eV is identified. Nitrogen adsorption is consequently estimated to provide a thermodynamic driving force for the reconstruction of MgO(100) supported 3 ML (or thicker) Pd clusters into thinner Pd clusters.     

Effect of epitaxial strain on the atomic structure of Pd clusters on MgO(100) substrate

Morphology and atomic structure of supported Pd clusters on MgO(100) substrate are investigated theoretically using a mixed approach: a semi-empirical potential for the metal bonding within the cluster and a potential fitted to ab initio calculations for the metal-oxide interaction. We find that the clusters adopt a truncated pyramidal morphology in agreement with experimental results. The detailed study of the epitaxial relation as a function of cluster size shows the existence of a critical size around 3 nm where elastic strain due to the misfit between the substrate and the deposit is released by the introduction of interfacial dislocations.     

Interaction of ethylene with palladium clusters supported on oxidised tungsten foil

The reactivity with ethylene of palladium clusters supported on oxidised tungsten foil has been investigated by synchrotron radiation-induced photoelectron spectroscopy and temperature programmed desorption. The effect of the heat pre-treatment of the sample on the interaction strength with ethylene is demonstrated. Already at room temperature, adsorption of ethylene causes breaking of both the C-H and C-C bonds and the appearance of a highly reactive C₁ phase with unsaturated bonds. A part of this phase is oxidised to carbon monoxide by oxygen supplied by the support immediately after ethylene adsorption. Another part of ethylene is probably adsorbed in the form of ethylidyne. Heating at temperatures between 400 K and 500 K brings about the dissolution of the C₁ phase in the shallow subsurface region of the Pd clusters. Further oxidation of the C₁ phase by oxygen from the support proceeds at ∼600 K. Substantial reduction of the concentration of C₁ phase at room temperature is observed after heat pre-treatment of the sample at 500 K, while complete suppression of the room temperature ethylene chemisorption proceeds upon heat pre-treatment at 800 K. This effect is related to thermally induced encapsulation of palladium clusters in surface tungsten oxide.     

Pd diffusion on MgO(1 0 0): The role of defects and small cluster mobility

Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D-3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 × 10¹⁴ s⁻¹ for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at ?600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].