石墨狭缝中甲烷吸附的分子动力学模拟

用分子动力学模拟研究石墨狭缝中甲烷的吸附,考察狭缝宽度和温度对甲烷吸附的影响.模拟发现甲烷在石墨狭缝中出现分层现象,吸附层中甲烷具有类液特征,第一吸附层内甲烷中总有两个氢原子的连线与另外两个氢原子的连线分别位于平行于狭缝壁的两个平面内,游离层中甲烷呈现气体的特征;碳原子间的平均作用势说明吸附层中甲烷分子间结合能力大于游离层,吸附态是甲烷在石墨狭缝中的主要赋存形式之一;伦敦力以及由吸附层净电荷产生的电场力是甲烷吸附和分层的主要原因;甲烷的吸附量随狭缝宽度增大或温度升高而减少,当狭缝宽度小于16.46Å时,甲烷仅以吸附形态存在.甲烷在第一吸附层中的扩散能力最弱、游离层中最强,甲烷扩散系数随狭缝宽度的增大或温度的升高而增大.     

氢在多壁碳纳米管中的吸附储存

利用巨正则系综蒙特卡罗(GCMC)的方法模拟了氢在多壁碳纳米管中的吸附,氢气分子之间、氢气分子和碳原子之间的相互作用势能采用Lennard-Jones势能模型。模拟了不同结构参数(管内径、管壁数、管壁间距)的多壁碳纳米管在77K和298K下的吸附等温线,分析了多壁碳纳米管的管内径、管壁数以及管壁间距对吸附性能的影响。模拟结果表明:多壁碳纳米管的管壁数和管壁间距对吸附性能的影响较明显;管壁数越少,管壁间距越大,其吸附性能越好;多壁碳纳米管的管内径对其吸附性能的影响甚微。     

Modification of chemisorption properties by electronegative adatoms: H2 and CO on chlorided,sulfided, and phosphided Ni(100)

The effect of preadsorbed electronegative atoms Cl, S and P on the adsorption-desorption behavior of CO and H₂ on Ni(100) has been studied using thermal desorption, LEED and AES. It is found that the presence of the electronegative atoms causes a reduction of the adsorption rate, the adsorption bond strength and the capacity of the Ni(100) surface for CO and H₂ adsorption. The poisoning effect becomes stronger with increasing electronegativity of the preadsorbed atoms. In the case of CO adsorption the most tightly bound β₂ -state is suppressed most significantly. The variation of the initial sticking coefficient of CO as a function of the adlayer precoverage shows that at low Cl and S coverages the effective number of the influenced Ni surface atoms is more than four. The observed reduction of CO and H₂ adsorption and the difference in the poisoning effect of Cl, S and P is generally interpreted in terms of the changes in the surface electron density in the presence of electronegative atoms.     

First-principles studies on the Au surfactant on polar ZnO surfaces

The surfactant effect of Au in ZnO nanostructures growth is studied using first-principles slab calculations based on density functional theory. The atomic structure and electronic properties of one monolayer of Au atoms on polar ZnO surfaces are examined. It is found that (1) one monolayer (ML) of Au capping layer on the ZnO polar surfaces may modify the growing properties of ZnO nanostructures by enhancing the binding energy by 0.41 eV/atom for Zn adsorption on the polar surfaces; (2) the Au adlayer on the polar ZnO surfaces seems more active for the adsorption of Zn atoms, which may be at the very heart of the effect that Au acts as catalyst for the growth of the ZnO nanostructures; and (3) total energy calculations show that the gold on-top geometry is energetically favorable than the gold diffused geometry, which may be useful to understand the phenomenon that Au particles are only found at the end of ZnO nanostructures during the growth process.     

The influence of a small amount of active sites on the adsorption kinetics of nitrogen on ruthenium

Recently, Dahl et al. [S. Dahl, A. Logadottir, R.C. Egeberg, J.H. Larsen, I. Chorkendorff, E. Törnqvist, J.K. Nørskov, Phys. Rev. Lett. 83 (1999) 1814; S. Dahl, E. Törnqvist, I. Chorkendorff, J. Catal. 192 (2000) 381] have proposed very interesting hypothesis that the rate of dissociation and adsorption of nitrogen on Ru(0 0 0 1) facet is totally dominated by the presence of a small amount of step sites on Ru(0 0 0 1) terraces. Following this idea, a kinetic model, based on applying the Statistical Rate Theory approach, was developed in order to explain if such mechanism is able to explain the observed features of the system N₂/Ru(0 0 0 1). As a result, it was stated that the activation barrier for adsorption on the active (step) sites is equal to 36 kJ/mol; in turn, the adsorption energy of nitrogen atoms on the active sites is 43 kJ/mol. It implies that the rate of adsorption via the active sites is much faster than direct adsorption on the three-fold hollow sites; moreover, the occupation of the active sites is always close to zero at the investigated temperatures, so they are not blocked and may act as an indirect channel for adsorption. Thus, the rate of nitrogen adsorption on Ru(0 0 0 1) surface is governed by the rate of diffusion of nitrogen atoms from the active sites into the three-fold hollow sites. The analysis of thermodesorption spectra revealed an important role of repulsive interactions between the N atoms adsorbed on the hollow sites, the associated interaction parameter between nearest neighbors was estimated to be 5 kJ/mol. The presence of small amount of gold on Ru(0 0 0 1), apart of blocking the active sites, seems to remove the repulsion between nitrogen atoms.     

异质原子对液体气泡成核影响的分子动力学模拟

采用分子动力学方法模拟了池沸腾中液体层加入异质原子对气泡成核的影响.分析了异质原子能量参数对液体起始气泡成核时间和温度的影响及其机理.结果表明,当异质原子能量参数小于液氩能量参数时,液体起始气泡成核时间缩短,起始温度降低.当异质原子剂能量参数大于液氩能量参数时,液体起始气泡成核时间增加,起始温度升高.异质原子在壁面上的吸附及在液体中的扩散行为影响固液界面性质,较大能量参数的异质原子扩散系数较小,更多能量参数较大的原子吸附在固体表面上使得壁面势能壁垒增加,导致沸腾时间延迟,液体需要吸收更多的热量克服势能壁垒,进而提高沸腾起始温度.能量参数较小的异质原子扩散系数较大,异质原子更容易分散到液体中,使得壁面附近液体层势能减小,液体层更容易气泡成核行为.     

Effect of the degree of alloying of PtRu/C (1:1) catalysts on ethanol oxidation

The effect of alloying degree on the ethanol oxidation activity of a PtRu/C catalyst with a Pt/Ru atomic ratio of 1:1 was investigated by measurements in a half-cell and in a single direct ethanol fuel cell. The increase of the amount of Ru alloyed from one third to two thirds of the total Ru content in the catalyst clearly resulted in a decrease of the ethanol oxidation activity. As the amount of the highly active hydrous ruthenium oxide was near the same, the lower activity of the PtRu/C catalyst with higher alloying degree was mainly ascribed to the presence of an excessive number of Ru atoms around Pt active sites, hindering ethanol adsorption on Pt sites. The reduced ethanol adsorption could be also related to the decreased Pt–Pt bond distance and to the electronic effects by alloying.     

A study of laser action on a free-molecular flow of atoms in a capillary

We found experimentally that resonance laser irradiation of a rarefied flow of atoms in a capillary causes their density at the exit to decrease. We established a dependence of the change in atomic density on the excitation intensity. The observed effect is shown to be related to the flow heating caused by hyperelastic collisions of excited atoms with the capillary wall. We describe the gas heating in a capillary taking into account the energy relaxation of hot atoms as they collide with the wall. We show that our experimental results can be explained by assuming that the energy accommodation coefficient for hot atoms during their collisions with the wall is close to unity.     

Diffraction and selective adsorption in the corrugated hard wall model

Numerical calculations are carried out for the elastic scattering of thermal energy atoms from a perfect crystalline surface with a square unit cell. The surface is treated as an infinitely-repulsive corrugated hard wall with an attractive square well in front of the hard wall. Closed, as well as open and bound, channels are included in the calculation, and multiple scattering effects are treated explicitly. It is concluded from these numerical studies that the single scattering approximation is inadequate, that closed channels must be included in any numerical calculations, and that selective adsorption can readily be understood within the framework of this simple model. Moreover, selective adsorption minima as well as maxima are found in this calculation without the necessity of invoking inelastic processes. The general quantitative agreement of the calculated results with the HeLiF diffraction experiments, the relative ease of handling more complicated corrugated surfaces, and the small amount of computer time required for these calculations suggest that the hard wall model is ideally suited in a parameterization scheme.     

Structure Sensitive Reaction Channels of Molecular Hydrogen on Silicon Surfaces

The ability of the Si(001) surface to adsorb H2 molecules dissociatively increases by orders of magnitude when appropriate surface dangling bonds are terminated by H atoms. Through molecular beam techniques the energy dependent sticking probability at different adsorption sites on H-precovered and stepped surfaces is measured to obtain information about the barriers to adsorption, which decrease systematically with an increase in coadsorbed H atoms. With the help of density functional calculations for interdimer adsorption pathways, this effect is traced back to the electronic structure of the different adsorption sites and its interplay with local lattice distortions.     

硫原子在镍基合金825(001)面吸附的第一性原理研究

元素硫在镍基合金表面吸附产生严重的电化学腐蚀,为从原子尺度研究硫腐蚀机理,采用第一性原理方法,构建并优化了镍基合金825的晶胞结构模型,计算分析了S原子在镍基合金825耐蚀性较差面(001)晶面的吸附及电子转移情况.结果表明:Ni原子占据顶角, Cr原子和Fe原子对称占据面心是镍基合金825稳定的晶胞结构;原子S在镍基合金825(001)面上最稳定的吸附位为四重穴位,吸附能为-6.51 eV; S吸附前后的态密度(DOS)和二维电荷差分密度图(DCD)对比发现,镍基合金825中Fe与S之间电荷偏移明显,形成离子键,易生成腐蚀产物Fe_xS_y. S的吸附对镍基合金825中Cr原子的电子分布影响不大,且合金中Cr和Ni抑制了合金中Fe与S之间的相互作用,从而提高了合金耐蚀性.     

Mechanical properties of two-dimensional graphyne sheet under hydrogen adsorption

In the present work, the mechanical properties of graphyne, a class of graphene allotropes with carbon triple bonds, subjected to the hydrogen chemisorption are studied using a first-principles density functional approach. Two configurations for the maximum of hydrogen adsorption are considered: (I) adsorption of hydrogen atoms on carbon atoms at the two opposite sides of graphyne sheet and (II) adsorption of hydrogen atoms on carbon atoms at the same side of graphyne sheet. Formation energy for hydrogenated graphyne (H-graphyne) corresponding to these states of adsorption is calculated and it is indicated that state (I) is more stable than state (II). Density functional calculations within the generalized gradient approximation (GGA) in the harmonic elastic deformation range are performed to obtain the elastic constants of graphyne and H-graphyne in state (I). This study shows that H-graphyne has an in-plane stiffness of 125 N/m and a Poisson's ratio of 0.23. It is observed that the in-plane stiffness of H-graphyne is lower than that of graphyne. This clearly reveals the destructive effect of hydrogen adsorption on the mechanical properties of graphyne. The results of this paper are helpful for the design of future nanodevices in which H-graphyne acts as their basic element.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

H,Cl和F原子钝化Cu_2ZnSnS_4(112)表面态的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法系统研究了Cu_2ZnSnS_4体相的晶格结构、能带、态密度及表面重构与H,Cl和F原子在Cu_2ZnSnS_4(112)表面上的吸附和钝化机理.计算结果表明:表面重构出现在以金属原子Cu-Zn-Sn终止的Cu_2ZnSnS_4(112)表面上,并且表面重构使表面发生自钝化;当单个H,Cl或F原子吸附在S原子终止的Cu_2ZnSnS_4(112)表面上时,相比于桥位(bridge)、六方密排(hcp)位和面心立方(fcc)位点,三种原子均在特定的顶位(top)吸附位点表现出最佳稳定性.当覆盖度为0.5 ML时,无论H,Cl还是F原子占据Cu_2ZnSnS_4(112)表面的2个顶位均具有最低的吸附能.以S原子终止的Cu_2ZnSnS_4(112)表面在费米能级附近的电子态主要由价带顶部Cu-3d轨道和S-3p轨道电子贡献,此即表面态.当H,Cl或F原子在表面的覆盖度达0.5 ML时,费米能级附近的表面态降低,其中H原子钝化表面态的效果最佳,Cl原子的效果次之,F原子的效果最差.表面态降低的主要原因在于吸附原子从S原子获得电子致使表面Cu原子和S原子在费米能级处的态密度峰几乎完全消失.     

First-principles study of hydrogen adsorption on titanium-decorated single-layer and bilayer graphenes

The adsorption of hydrogen molecules on titanium-decorated （Ti-decorated） single-layer and bilayer graphenes is studied using density functional theory （DFT） with the relativistic effect. Both the local density approximation （LDA） and the generalized gradient approximation （GGA） are used for obtaining the region of the adsorption energy of H2 molecules on Ti-decorated graphene. We find that a graphene layer with titanium （Ti） atoms adsorbed on both sides can store hydrogen up to 9.51 wt% with average adsorption energy in a range from -0.170 eV to 0.518 eV. Based on the adsorption energy criterion, we find that chemisorption is predominant for H2 molecules when the concentration of H2 molecules absorbed is low while physisorption is predominant when the concentration is high. The computation results for the bilayer graphene decorated with Ti atoms show that the lower carbon layer makes no contribution to hydrogen adsorption.     

The adsorption of fluor-carbon complexes on GaAs(110) studied by electron energy loss spectroscopy

The room temperature adsorption of CF₃COOH, CH₃COOH and CO on cleaved GaAs(110) surfaces has been studied by vibrational electron energy loss spectroscopy (HRELS), second derivative electron energy spectroscopy (ELS) and electron diffraction (LEED). CO does not adsorb on the GaAs surfaces in measurable quantities. Acetic acid CH₃COOH is dissociatively adsorbed as an acetate bonded to Ga surface atoms with the split-off hydrogen on As surface atoms. The fluorated acid CF₃COOH decomposes via an acetate intermediate CF₃COO into active CF₃ groups which adsorb on Ga surface atoms. The split-off hydrogen sticks to surface As atoms while the generated CO₂ desorbs. The adsorption models are consistent with the LEED c(2×2) superstructure observed after saturated adsorption of both acids.     

H2S在Fe(100)面吸附的第一性原理研究

基于密度泛函理论第一性原理, 在广义梯度近似下, 研究了表面覆盖度为0.25 ML (monolayer)时硫化氢分子在Fe(100)面吸附的结构和电子性质, 并与单个硫原子吸附结果进行了对比. 结果表明: 硫化氢分子吸附在B2位吸附能最小为-1.23 eV, 最稳定, B1位吸附能最大为-0.01 eV, 最不稳定; 并对硫化氢分子在B1位和B2位吸附后的电子态密度进行了分析, 也表明了吸附在B2位稳定, 且吸附在B2位后硫化氢分子几何结构变化不大; 将硫化氢中硫原子吸附与单个硫原子吸附的电子性质进行了比较, 发现前者吸附作用非常微弱; 同时对吸附后的Fe(100)面进行了对比, 单个硫原子吸附的Fe(100)面电子态密度出现了一系列峰值且离散分布, 生成了硫化亚铁, 表明在硫化氢环境下, 主要是硫化氢析出的硫原子发生了吸附. 关键词： 第一性原理 Fe(100)表面 吸附能 硫化氢     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

本征及掺杂磷烯对2,3,7,8-TCDD吸附机理的计算模拟研究

2,3,7,8-四氯二苯并对二噁英(2,3,7,8-TCDD)是二噁英家族中危害人类和环境最显著的一种.设计一种高效,灵敏的吸附剂来检测和去除2,3,7,8-TCDD对人类和环境的影响是亟需解决的问题.本研究利用基于密度泛函理论(DFT)的计算模拟方法探索了本征磷烯对2,3,7,8-TCDD的吸附机理,并详细考察了掺杂Ti, Fe, Ca, Al金属原子后磷烯对2,3,7,8-TCDD吸附的影响.研究结果表明2,3,7,8-TCDD初始构型会影响磷烯对其吸附,当平躺于磷烯表面时有较大的吸附.而且掺杂金属原子的磷烯对2,3,7,8-TCDD的吸附也存在较大的影响,掺杂金属原子均增大了磷烯对2,3,7,8-TCDD的吸附,其中Ca掺杂磷烯>Fe掺杂磷烯>Ti掺杂磷烯>Al掺杂磷烯.研究结论对于2,3,7,8-TCDD的处理带来了新的思考方向,有望为二噁英的检测和去除提供有用的理论指导.