过渡族金属掺杂Al(111)表面对氢分子催化分解的影响

为了探求过渡金属催化剂对催化合成储氢材料NaAlH₄效果的影响, 本文采用第一性原理方法研究了多种金属原子取代Al (111)表面铝原子形成的合金表面对氢的催化分解的影响. 计算结果表明, Sc, V, Fe, Ti原子掺杂的表面对氢分子分解具有催化作用. H₂在对应的掺杂表面催化分解所需要的活化能分别为0.54 eV, 0.29 eV, 0.51 eV, 0.12 eV. H原子在Sc, V, Ti掺杂表面扩散需要的活化能分别为0.51 eV, 0.66 eV, 0.57 eV. 同时, 过渡金属掺杂在Al表面时倾向于分散分布, 增加掺杂表面的掺杂原子个数, 掺杂表面的催化效果体现为单个掺杂过渡金属原子的催化效果. 本研究将为金属掺杂Al (111)表面催化加氢合成NaAlH₄提供理论参考.     

H2 分子在Li3N(110)表面吸附的第一性原理研究

采用第一性原理方法研究了H₂分子在Li₃N(110)晶面的表面吸附. 通过研究H₂/Li₃N(110)体系的吸附位置、吸附能和电子结构发现: H₂分子吸附在N桥位要比吸附在其他位置稳定,此时在Li₃N(110)面形成两个-NH基,其吸附能为1.909 eV,属于强化学吸附;H₂与Li₃N(110)面的相互作用主要是H 1s轨道与N 关键词： 第一性原理 3N(110)')" href="#">Li₃N(110) 2')" href="#">H₂ 吸附和解离     

Li掺杂对MgH2(001)表面H2分子扩散释放影响的第一性原理研究

本文利用基于密度泛函理论的第一性原理分别研究了MgH₂(001)表面H原子扩散形成H₂分子释放出去的可能路径及金属Li原子掺杂对其影响. 研究结果表明: 干净MgH₂(001)表面第一层释放H原子形成H₂分子有两种可能路径, 其释放能垒分别为2.29和2.50 eV; 当将Li原子替代Mg原子时, 两种H原子扩散释放路径的能垒分别降到了0.31和0.22 eV, 由此表明Li原子掺杂使MgH₂(001)表面H原子扩散形成H₂释放更加容易.     

H原子在Al(111)和Ag(111)面上的吸附

本文用电荷自洽的EHT方法,研究了H原子在Al(111)和Ag(111)面上的吸附,结果指出:在Al(111)面上,H以原子状态吸附在某些对称位置上,它也能渗透到表面层中去,成为填隙原子;H₂分子在表面处发生解离吸附。在Ag(111)表面上,H原子有可能以分子状态吸附,H—H键平行于表面,这与高分辨率电子能量损失谱所得到的实验结果一致;但H₂分子在Ag(111)表面也可能发生解离吸附。 关键词：     

碳硼富勒烯衍生物C18B2M(M=Li,Ti, Fe)的储氢性能计算研究

本文使用密度泛函理论(density functional theory, DFT)中的广义梯度近似(generalized gradient approximation, GGA)研究了经碱金属原子Li、过渡金属原子Ti和Fe原子修饰的富勒烯C₁₈B₂M(M=Li, Ti, Fe)的储氢性能. 研究发现, C₁₈B₂由于B的替代掺杂, 比C₂₀对金属原子具有更高的结合能. 由平均吸附能分析可知: C₁₈B₂Li对H₂的吸附能力较弱, C₁₈B₂Fe对H₂的吸附能力过强, 而C₁₈B₂Ti对H₂的平均吸附能介于0.45-0.59 eV 之间, 介于物理吸附和化学吸附之间 (0.2-0.6 eV), 因此可以实现常温下的可逆储氢. C₁₈B₂M(M=Li, Ti, Fe)能够吸附的H₂数目最多分别为4, 6和4. 由储氢机理分析可知: C₁₈B₂Li主要通过碱金属离子激发的静电场来吸附H₂, 而C₁₈B₂Ti和C₁₈B₂Fe主要通过金属原子与H₂之间的Kubas作用来吸附H₂. 由于C₁₈B₂Ti既有较大的储氢数目, 又可以实现可逆储氢, 因此有望开发成新型纳米储氢材料.     

单原子Al修饰空位缺陷V2C（MXene）对H2气体表面吸附的第一性原理研究

基于第一性原理计算方法,对含空位缺陷的V₂C(MXene)在不同位点修饰单原子Al的相关性能进行系统研究.研究表明,几何优化后得到含空位缺陷的V₂C稳定结构表面能为-3075.53 J/m²,单原子Al修饰本征V₂C单原子的吸附能为1.5511eV、单原子Al修饰空位缺陷V₂C的吸附能为-2.0763 eV,这表明含空位缺陷的V₂C,由于单原子Al的修饰可以明显改善晶体结构稳定性.进一步从态密度、分波态密度、吸氢能力研究发现,各体系态密度和分波态密度均出现分波越过费米能级的现象,表现出较强的金属性;V₂C吸附H₂气体分子吸附能为-7.5867 eV,而空位缺陷V₂C和单原子Al修饰空位缺陷V₂C两个体系对H₂气体分子的吸附能仅为-0.9851 eV、-2.7130 eV,均未能进一步改善V₂C对H₂气体分...     

NH3分子在LiH（100）表面吸附的第一性原理研究

采用第一性原理方法研究了NH₃分子在LiH（100）晶面的表面吸附情况.通过研究LiH（100）/NH₃体系的吸附位置、吸附能和电子结构,发现NH₃分子在LiH（100）晶面主要是化学吸附,初始位置为NH₃分子中N-H键在Li顶住时失去一个H原子,并在LiH（100）面形成NH₂基,其吸附能为0.511 eV,属于强化学吸附,吸附作用最强.此时NH₂基与附近H原子和Li原子之间为离子键作用,NH₂基中N—H键为共价键;NH₃分子中另一个H原子与LiH表面的一个H原子形成一个H₂分子逸出表面.H₂分子中H-H键为明显的共价键.     

H2O分子在Fe（100）, Fe（110）, Fe（111）表面吸附的第一性原理研究

采用第一性原理研究了H₂O分子在Fe（100）,Fe（110）,Fe（111）三个高对称晶面上的表面吸附.结果表明,H₂O分子在三个晶面上的最稳定结构皆为平行于基底表面的顶位吸附结构.H₂O分子与三个晶面相互作用的吸附能及几何结构计算结果表明H₂O分子与三个晶面的相互作用程度不同,H₂O分子与Fe（111）晶面的相互作用最强,其次是Fe（100）,相互作用最弱的是Fe（110）表面,而这与晶面原子 关键词： 第一性原理 Fe单晶表面 2O分子')" href="#">H₂O分子 分子吸附     

五角石墨烯吸附H2S的第一性原理计算

硫化氢作为一种神经毒物和腐蚀性气体污染物,严重威胁人体健康并限制工业的发展.本文采用第一性原理计算的方法,研究了H₂S气体分子在原始五角石墨烯、非金属掺杂及金属掺杂五角石墨烯上的吸附行为.详细计算了H₂S气体分子与五角石墨烯之间的吸附构型、吸附能及电荷转移.结果表明:1)原始五角石墨烯和非金属元素掺杂五角石墨烯与H₂S气体分子间仅为微弱的物理吸附,无法直接用于吸附H₂S气体;2) Co、Ni、Cu及Ti掺杂的五角石墨烯对H₂S气体的吸附作用显著增强.当金属掺杂在sp~2C位置时,掺杂五角石墨烯对H₂S气体的吸附效果较好,此时的吸附均为化学吸附;3) Co、Ni、Cu及Ti掺杂的五角石墨烯均可作为H₂S气体的传感/捕获材料.其中Cu掺杂五角石墨烯吸附H₂S气体的效果最好.本文的研究结果对设计开发基于五角石墨烯的H₂S气体传感/捕获材料提供了理论指导.     

石墨烯吸附Al2Cl6分子的第一性原理与Monte Carlo研究

采用第一性原理与蒙特卡罗方法研究Al₂Cl₆气体分子在石墨烯表面的吸附性能与光电性质，结果表明：(1)石墨烯对Al₂Cl₆气体分子具有较强的物理吸附作用，两个Al原子的连线与石墨烯平面近乎平行且两个Al原子处于紧靠顶位的桥位位置时最稳定；(2)温度升高不利于Al₂Cl₆气体分子吸附并存在阶跃式降低，气体逸度增加有利于吸附并存在阶跃式升高，Al₂Cl₆气体分子插入石墨/双层石墨烯/多层石墨烯宜将温度维持在AlCl₃沸点附近，并增加气体的压力；(3)Al₂Cl₆的吸附对石墨烯的电子结构进行了调控，但没有明显改变石墨烯费米能级附近的态密度以及“赝能隙”;(4)Al₂Cl₆的吸附对体系光学参数的影响十分明显，静态介电常数提高近5倍，使体系屏蔽效应有较大增强，在长波波段的吸收性能、反射性能及光电导也有了明显提升.     

NaAlH4 表面Ti催化空间构型和X射线吸收光谱: Car-Parrinello分子动力学和密度泛函理论研究

通过采用Car-Parrinello分子动力学方法对掺杂Ti前后的NaAlH₄(001)2×2×1超晶胞表面晶体在333 K(60 ℃)温度条件催化脱氢的空间构型做了理论研究,发现掺杂Ti的合金中AlH₄团的其中两个Al—H键长分别从约1.64 (1 =0.1 nm)增大至1.74和1.93 ,而未掺杂合金表面中AlH₄团的4个Al—H键长基本不变,这意味着掺杂Ti相对未掺杂的合金更易于放氢.但在模拟温度条件下并未发现Ti-Al成键趋     

Zr催化剂对NaAlH4和Na3AlH6可逆储氢性能的影响

采用基于密度泛函理论的平面波赝势方法,分别计算纯净的以及掺杂Zr的NaAlH₄和Na₃AlH₆的晶格结构常数、能量、电子局域函数和电子态密度.结果表明：NaAlH₄和Na₃AlH₆分别是带隙为46和31 eV的绝缘体;NaAlH₄和Na₃AlH₆中Al—H键是共价键,Na—H键是离子键;Zr原子替代Na原子 关键词： 储氢 4')" href="#">NaAlH₄ 3AlH₆')" href="#">Na₃AlH₆ Zr掺杂     

Structure of ultra-thin Ti film on the Al(001) surface

The atomic structure of sub-monolayer amounts of Ti deposited on the Al(001) surface at room temperature has been investigated using low-energy electron diffraction (LEED) and low-energy ion scattering spectroscopy (LEIS). The Ti coverage was determined using Rutherford backscattering spectroscopy (RBS). Though a crisp LEED image is inherently difficult to obtain, the symmetry of the observed c(2 × 2) LEED images allows us to infer a structure which places Ti atoms in every other Al lattice site. Analysis of the LEIS azimuth- and polar-angle scan spectra has been done to determine the best structural model which supports the c(2 × 2) symmetry of the LEED image as well as LEIS experimental data. It was concluded that the best model consistent with the experimental data, puts Ti preferentially below the surface of the Al substrate at every other lattice site for sub-monolayer coverage of Ti on Al(001). As Ti coverage increases, the presence if Ti atoms in the surface layer also increases. Results of this study are relevant to research pertaining to the possible use of Ti as a catalyst in sodium alanate (NaAlH₄) in hydrogen storage applications.     

Observations of nanoscopic,face centered cubic Ti and TiH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Transmission electron microscopy has been used to study ball milled and H cycled NaAlH₄ with 10 mol% TiCl₃. Isolated from the main phases in this hydrogen storage system, nanocrystalline aggregates of fcc TiH _x (0≤x<0.67) were found. The value of x was determined based on the assumption of a linear increase of the TiH _x lattice parameter by increasing H content. The size of the TiH _x crystallites was in the range 10 to 20 nm, and the lattice parameter decreased from 4.22 Å in TiH_0.67 to 4.10 Å in pure fcc Ti. Non-equilibrium ball milling and subsequent H cycling in combination with a small crystallite size are believed to make the TiH _x phase stable. The present results are the first observations of fcc TiH _x with low hydrogen content, and the measured fcc lattice parameter of Ti matches first-principles calculations.     

Role of hydrogen in the absorption of ionizing radiation energy by a metal-hydrogen system

Ab initio calculations of the electronic structure of pure Pd, pure Ti, and PdH_x and TiH_x (x = 1, 2, 3) systems are performed within the local density approximation. It is found that the electronic subsystem of metals containing dissolved hydrogen increases their capacity to absorb the energy of electromagnetic radiation and accumulate it for a longer time than pure metals. These two factors promote the nonequilibrium migration of hydrogen atoms and their release from metals upon exposure to ionizing radiation.     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Density functional theory study of the interaction of H2 with pure and Ti-doped WO3 (002) surfaces

Density functional theory (DFT) calculations are conducted to explore the interaction of H₂ with pure and Ti-doped WO₃ (002) surfaces. Four top adsorption models of H₂ on pure and Ti-doped WO₃ (002) surfaces are investigated respectively, they are adsorption on bridging oxygen O_1c, absorption on plane oxygen O_2c, absorption on 5-fold W_5c (Ti), and absorption on 6-fold W_6c. The most stable and H₂ possible adsorption structure in the pure surface is H-end oriented to the surface plane oxygen O_2c site, while the favourable adsorption sites for H₂ in a Ti-doped surface is not only an O_2c site but also a W_6c site. The adsorption energy, the Fermi energy level E_F, and the electronic population are investigated and the H₂-sensing mechanism of a pure-doped WO₃ (002) surface is revealed theoretically: the theoretical results are in good accordance with our existing experimental results. By comparing the above three terms, it is found that Ti doping can obviously enhance the adsorption of H₂. It can be predicted that the method of Ti-doped into a WO₃ thin film is an effective way to improve WO₃ sensor sensitivity to H₂ gas.     

乙烯和乙炔基在Ni(110)表面上吸附结构的研究

采用平面波赝势方法，利用基于从头计算的软件包，对乙烯和乙炔基在Ni(110)表面上吸附的问题进行了计算. 在低覆盖度时，孤立的乙烯分子的吸附能比密集时高，乙烯分子的C-C 轴大致沿衬底的Ni原子链方向(即沿［110］晶向)，C-C轴与衬底Ni(110)表面有12°的倾斜角，乙烯分子的C—C键的键长为 0.147nm. 乙烯分子中接近顶位的C原子与衬底中距离最近的Ni原子为0.199nm. 在高覆盖度时，乙烯分子在Ni(110)表面上形成c(2×4)再构，每个表面二维元胞中有两个乙烯分子，每个乙烯分子的吸附位置与低覆盖度时相似，而C—C键长比低覆盖度时要短. 乙炔基是乙烯在Ni(110)表面上分解的产物. 关于乙炔基的计算结果表明：乙炔基的两个C原子的间距为0.131nm，比乙烯分子中C原子的间距更短. 与乙烯分子相比，乙炔基的吸附位置更靠近顶位. H原子与吸附在顶位上的C原子相连接，C—H键也大致沿衬底的Ni原子链方向，与Ni表面呈45°的倾斜角. 关键词： 乙烯和乙炔基 平面波赝势方法 吸附几何结构     

Crystalline Al1 − x Ti x phases in the hydrogen cycled NaAlH4 + 0.02TiCl3 system

The hydrogen (H) cycled planetary milled (PM) NaAlH₄?+?0.02TiCl₃ system has been studied by high resolution synchrotron X-ray diffraction and transmission electron microscopy during the first 10?H cycles. After the first H absorption, we observe the formation of four nanoscopic crystalline (c-) Ti-containing phases embedded on the NaAlH₄ surface, i.e. Al₂Ti, Al₃Ti, Al₈₂Ti₁₈ and Al₈₉Ti₁₁, with 100% of the originally added Ti atoms accounted for. Al₂Ti and Al₃Ti are observed morphologically as a mechanical couple on the NaAlH₄ surface, with a moderately strained interface. Electron diffraction shows that the Al₈₂Ti₁₈ phase retains some ordering from the L1₂ structure type, with the observation of forbidden (100) ordering reflections in the fcc Al₈₂Ti₁₈ lattice. After 2?H cycles the NaAlH₄?+?0.02TiCl₃ system displays only two crystalline Ti-containing phases, Al₃Ti and Al₈₉Ti₁₁. After 10?H cycles, the Al₈₉Ti₁₁ is completely converted to Al₈₅Ti_15. Al₈₉Ti₁₁, Al₈₅Ti₁₅ and Al₃Ti do not display any ordering reflections, and they are modeled in the A1 structure type. Quantitative phase analysis indicates that the Al₃Ti proportion continues to increase with further H cycles. The formation of Ti-poor Al_{1??? x} Ti _x (x?<?0.25) phases in later H cycles is detrimental to hydrogenation kinetics, compared to the starting Ti-richer near-surface Al₂Ti/NaAlH₄ interface present during the first absorption of hydrogen.     

Ti-doped NaAlH4 as a hydrogen-storage material – preparation by Ti-catalyzed hydrogenation of aluminum powder in conjunction with sodium hydride

Ti-doped NaAlH₄ can be prepared in a simple way by hydrogenation of aluminum powder in conjunction with sodium hydride in the presence of Ti(OBuⁿ)₄ (Buⁿ= n-C₄Hg) as a dopant. After a few hydrogenation/dehydrogenation cycles the material reaches a storage capacity of ∼4 wt %H₂ and exhibits reaction rates comparable to or exceeding those previously found for NaAlH₄ doped with Ti(OBuⁿ)₄ in organic solvents [Bogdanović et al.: J. Alloys Compd. 302, 36 (2000)]. Received: 25 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001