二元合金Ag50Rh50从液态到非晶的分子动力学计算

本文对贵重金属银铑合金Ag50Rh50的液态结构和激冷过程进行了分子动力学模拟研究,原子间作用势采用紧束缚势,模拟在施加了周期性边界折常压状态下进行。采用了偶关联函数,键对分析技术和键取向序参, 分子动力学模拟计算方法揭示了Ag50Rh50的液态结构存在原子偏聚特征以及在快速凝固过程形成原子偏聚的不均匀非晶,并与同族过渡金属进行了非晶形成能力的比较。     

二十面体准晶对非晶形成影响的模拟

采用分子动力学模拟技术,以液态金属Ni为例,研究了在不同冷却条件下形成晶体及非晶的过程.模拟采用镶嵌原子法(EAM)作用势,得到了不同温度、不同冷却速度下Ni的径向分布函数以及原子组态变化的重要信息,利用键对分析技术探讨了二十面体准晶对非晶形成的影响.     

贵金属Au的液态结构分子动力学模拟

用分子动力学模拟方法在1573－200K的温度范围内对液态Au的微正则系综进行了模拟研究。模拟采用或嵌原子相互作用势对时间和空间的平均，得到了不同温度下Au的润分布函数及原子组态变化的重要信息，并利用键对分析技术对模拟结果作了深入讨论．     

耐蚀合金Au3 Cu高温冷却过程中能量及结构转变的分子动力学模拟

用分子动力学模拟方法对液态Au3Cu冷却过程进行了研究,考察了不同冷却速度下Au3Cu结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术.计算结果表明,冷却速度对液态Au3Cu能量及结构转变有重要影响,给出了不同冷却速度下液态Au3Cu结构转变的微观信息.     

耐蚀合金Al3Ni和Ni3Al液态冷凝过程的比较研究

采用F-S多体势对液态合金Al3Ni和Ni3Al在不同冷却速度下的微观结构及其转变机制进行了分子动力学模拟,得到了不同冷速下各温度的双体分布函数;采用HA键型指数法对其结构进行了分析,结果表明: Al3Ni在两种冷速下均以非晶的形式出现,只是慢冷时体系的有序度略有升高;而Ni3Al的结构及能量转变受冷速影响较大,快冷时形成非晶,而慢冷时出现明显结晶;同样冷速下Al含量较少的Ni3Al体系的有序度高,更易形成晶体,晶体的形成过程中有能量突变.     

碘原子在银(110)面吸附的能带理论研究

采用基于密度泛函理论的第一性模守恒赝势平面波分子动力学方法研究了碘原子在Ag(110)面的吸附性质。首先对银体相性质和Ag(110)面的驰豫进行了计算,验证了生成的赝势的可靠性;随后对碘原子在Ag(110)表面各吸附位的性质进行了研究,最稳定的吸附位是短桥位。另外,本文还考虑了碘原子吸附对Ag(110)表面结构性质的影响。     

铁磁性物质Co的液态结构分子动力学模拟

用分子动力学模拟方法在１８７３－３００Ｋ的温度范围内对液态Ｃｏ的微正则系综进行了模拟研究,模拟采用ＥＡＭ相互作用势,对时间和空间的平均,得到了不同温度下Ｃｏ的双体分布函数及原子组态变化的重要信息,当冷却速度较慢时,液态金属Ｃｏ最终形成晶态,当冷却速度较快时,液态Ｃｏ最后形成了非晶态,双体分布函数随的变化规律说明液态金属随温度的降低,有序度不断增强,利用键对分析技术对模拟结果作了深入分析,液态金属中     

金属Cu熔化结晶过程的分子动力学模拟

采用常温、常压分子动力学模拟技术,研究了在周期性边界条件下,由864个Cu原子构成的模型系统的熔化、结晶过程。原子间相互作用势采用EAM势。模拟结果表明：在连续升温过程中,金属Cu在1520 K熔化;以不同的冷速进行冷却,在较慢冷却条件下,液态Cu在1010 K结晶;当冷速较快时,液态Cu形成非晶态。分析了升降温过程中熔体偶分布函数、原子体积、能量、MSD随温度的变化特征。     

液态合金NiAl凝固过程中微观结构转变的分子动力学模拟

采用分子动力学模拟方法对液态NiAl凝固过程进行了研究,考察了不同冷却速度下液态NiAl结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术.计算结果表明,冷却速度对液态NiAl结构转变有重要影响,在不同的冷却速度下, NiAl凝固过程出现了明显不同,冷速为4×1013和4×1012 K/s时, NiAl快速凝固为无序的非晶体结构;而在较慢的8×1011 K/s冷速下, NiAl凝固为晶态结构.给出了不同冷却速度下液态NiAl结构转变的微观信息.     

原子尺寸差异与非晶形成能力

采用分子动力学模拟技术，研究了纯Au及AuCu合金的熔化、非晶化和晶化过程.模拟结果表明,在冷却速率为5×1011 K•s－1至4×1012 K•s－1的范围内，液态Au总是形成晶体，且冷速越快，结晶温度越低；而AuCu合金则形成非晶，且冷速越快，非晶转变温度越高.验证了原子尺寸的不匹配有利于非晶形成这一规律.     

Cu-Ni合金结构的分子动力学模拟

采用EAM作用势对Cu-Ni合金的结构特性进行了MD模拟研究.通过FZ结构因子可发现,Cu含量的变化对结构因子的波动影响很小,键取向序参数和键对也表现出相似的变化规律,这表明液态Cu-Ni合金对成份变化不敏感,体系中的化学序较弱.将Cu70Ni30合金熔体的FZ结构因子与Waseda的实验结果进行对比,发现二者吻合得较好,表明EAM势可以很好地描绘Cu-Ni合金的结构特性.在快速冷却过程中,除了Cu20Ni80合金外,其他合金成份的双体分布函数的第二峰都发生了劈裂,标志着体系最终形成了非晶结构,而Cu20Ni80合金的双体分布函数却表现出晶体峰的特征.通过对键取向序参数、键型指数以及铜镍原子的有效扩散系数的分析表明,在快速冷却过程中,Cu20Ni80合金最终形成了hcp晶体结构.     

Effects of Ag Atomic Segregation from Different Planes on the Size-Dependent Melting of Ag–Pd Clusters

This paper studies the effects of Ag atomic segregation from the inner (100) or (111) planes on the melting of Ag–Pd clusters with different sizes by a molecular dynamics simulation. The results show that Ag segregation leads to the atomic energy decreases with increasing the temperature. Furthermore, the effect of the (100) segregation is larger than that of the (111) segregation. Meanwhile, the influence of segregation on the energy decreases with increasing the cluster size. The melting points of the clusters without segregation are the largest, followed by the clusters with a (111) and (100) segregation.     

Calorimetric study on self-assembling of two kinds of monometallic nanoparticles in solution

Recently, we have reported a noble method of preparing Ag/Rh bimetallic nanoparticles with a pseudo-core/shell structure. We simply mix the dispersions of poly(N-vinyl-2-pyrrolidone)(PVP)-protected Ag and Rh nanoparticles in solution at room temperature. We found that the mixture of dispersions forms bimetallic nanoparticles in a pseudo-core/shell structure on standing. We call this process the ‘self-assembling’ or ‘self-organizing’. In this study we seek for a thermodynamic driving force for this process by determining the enthalpy of the interaction among three pairs of nanoparticles by isothermal titration calorimetry (ITC). The results indicate that the interaction between each pair is strongly exothermic, and that among the pairs studied here the strength of the exothermic interaction is in the order of Ag/Pt     

Microscopic structure and dynamics of molten Se50Te50 alloys

In this work we investigate the microscopic structure and dynamics of the molten equimolar alloy, Se(50)Te(50) using a combination of neutron and x-ray diffraction experiments, reverse Monte Carlo analysis, and first principles molecular dynamics. The range of temperatures studied covers the semiconductor/metal transition. From our results it can be seen that the latter is associated with an increase in coordination numbers and a reduced tendency to heterocoordination. In agreement with previous inelastic neutron scattering experiments, our molecular dynamics calculation predict a certain widening of the stretching vibrational modes band in connection with the increase of coordination and the presence of longer bonds in the metallic phase.     

Evolution of catalytic activity of Au-Ag bimetallic nanoparticles on mesoporous support for CO oxidation

We report a novel Au-Ag alloy catalyst supported on mesoporous aluminosilicate Au-Ag@MCM prepared by a one-pot synthesis procedure, which is very active for low-temperature CO oxidation. The activity was highly dependent on the hydrogen pretreatment conditions. Reduction at 550-650 degrees C led to high activity at room temperature, whereas as-synthesized or calcined samples did not show any activity at the same temperature. Using various characterization techniques, such as XRD, UV-vis, XPS, and EXAFS, we elucidated the structure and surface composition change during calcination and the reduction process. The XRD patterns show that particle size increased only during the calcination process on those Ag-containing samples. XPS and EXAFS data demonstrate that calcination led to complete phase segregation of the Au-Ag alloy and the catalyst surface is greatly enriched with AgBr after the calcination process. However, subsequent reduction treatment removed Br- completely and the Au-Ag alloy was formed again. The surface composition of the reduced Au-Ag@MCM (nominal Au/Ag = 3/1) was more enriched with Ag, with the surface Au/Ag ratio being 0.75. ESR spectra show that superoxides are formed on the surface of the catalyst and its intensity change correlates well with the trend of catalytic activity. A DFT calculation shows that CO and O2 coadsorption on neighboring sites on the Au-Ag alloy was stronger than that on either Au or Ag. The strong synergism in the coadsorption of CO and O2 on the Au-Ag nanoparticle can thus explain the observed synergetic effect in catalysis.     

Spontaneous formation of core/shell bimetallic nanoparticles: a calorimetric study

We showed recently that low entropy core/shell structured nanoparticles form spontaneously from the physical mixture of a dispersion of Ag nanoparticles and that of another noble metal (Rh, Pd, or Pt) at room temperature. Here we use isothermal titration calorimetry (ITC) and show that the initial step of such a spontaneous process is strongly exothermic. When the alcohol dispersion of poly(N-vinyl-2-pyrrolidone) (PVP)-protected Rh nanoparticles (average diameter 2.3 nm) was titrated into the alcoholic dispersion of PVP-protected Ag nanoparticles, a strong exothermic enthalpy change, DeltaH, was observed: DeltaH = -908 kJ/mol for Ag(S) nanoparticle (average diameter 10.8 nm) and -963 kJ/mol for Ag(L) nanoparticles (average diameter 22.5 nm). The strength of interaction increases in the order of Rh/Ag > Pd/Ag > Pt/Ag. This strong exothermic interaction is considered as a driving force to from low entropy bimetallic nanoparticles by simple mixing of two kinds of monometallic nanoparticles. We show also that exothermic interactions occur between a pair of noble metal nanoparticles themselves by using ITC.     

Hydrogen-storage properties of solid-solution alloys of immiscible neighboring elements with Pd

Rh and Ag are the elements neighboring Pd, which is well known as a hydrogen-storage metal. Although Rh and Ag do not possess hydrogen-storage properties, can Ag-Rh alloys actually store hydrogen? Ag-Rh solid-solution alloys have not been explored in the past because they do not mix with each other at the atomic level, even in the liquid phase. We have used the chemical reduction method to obtain such Ag-Rh alloys, and XRD and STEM-EDX give clear evidence that the alloys mixed at the atomic level. From the measurements of hydrogen pressure-composition isotherms and solid-state (2)H NMR, we have revealed that Ag-Rh solid-solution alloys absorb hydrogen, and the total amount of hydrogen absorbed reached a maximum at the ratio of Ag:Rh = 50:50, where the electronic structure is expected to be similar to that of Pd.     

XPS revelation of tungsten edges as a potential donor-type catalyst

We report an efficient yet simple technology of photoelectron spectroscopic purification for identifying the capability of, and direction of charge flow in, a catalyst in a reaction, which has enabled the finding, for the first time, of the similarity of the valence band of tungsten edges to that of Rh adatoms and Ag/Pd alloy and hence suggested that W undercoordinated atoms could be a suitable candidate for replacing the costly Rh adatoms and Ag/Pd alloy as a cheaper, richer, and efficient donor-type catalyst for CO and NO oxidation applications. The new technology and new findings will be stimulating to the community for new catalyst design and identification and provide a better understanding of the electronic process of a catalytic reaction associated with undercoordinated atoms.     

A reflection absorption infrared spectroscopy and density-functional theory investigation of methanol dehydrogenation on Rh(111)V alloy surfaces

The dehydrogenation reaction of methanol on a Rh(111) surface, a Rh(111)V subsurface alloy, and on a Rh(111)V islands surface has been studied by thermal-desorption spectroscopy, reflection absorption infrared spectroscopy, and density-functional theory calculations. The full monolayer of methanol forms a structure with a special geometry with methanol rows, where two neighboring molecules have different oxygen-rhodium distances. They are close enough to form a H-bonded bilayer structure, with such a configuration, where every second methanol C-O bond is perpendicular to the surface on both Rh(111) and on the Rh(111)V subsurface alloy. The Rh(111)V subsurface alloy is slightly more reactive than the Rh(111) surface which is due to the changes in the electronic structure of the surface leading to slightly different methanol species on the surface. The Rh(111)V islands surface is the most reactive surface which is due to a new reaction mechanism that involves a methanol species stabilized up to about 245 K, partial opening of the methanol C-O bond, and dissociation of the product carbon monoxide. The latter two reactions also lead to a deactivation of the Rh(111)V islands surface.