Hydrogenation mechanism in lanthanum-activated magnesium films

With density-functional theory, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and La-doped Mg(0001) surfaces are studied. Calculation results show that the energy barrier obtained for hydrogen dissociation on the La-doped Mg(0001) surface is smaller due to back-donated bonding between molecular H₂ and doped La atom. The obtained diffusion barriers (0.8–0.22 eV) imply a fast motion of atomic H on La-doped Mg(0001) surface.     

Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: a first-principles study

Using first-principles calculations,we systematically study the potential energy surfaces and dissociation processes of the hydrogen molecule on the Mg(0001) surface.It is found that during the dissociative adsorption process with the minimum energy barrier,the hydrogen molecule first orients perpendicularly,and then rotates tobecome parallel to the surface.It is also found that the orientation of the hydrogen molecule in the transition state is neither perpendicular nor parallel to the surface.Most importantly,we find that the rotation causes a reduction of the calculated dissociation energy barrier for the hydrogen molecule.The underlying electronic mechanism for the rotation of the hydrogen molecule is also discussed in the paper.     

Influences of Al doping on the electronic structure of Mg(0001) and dissociation properties of H2

By using the density functional theory method, we systematically study the effects of the doping of an Al atom on the electronic structures of the Mg(0001) surface and on the dissociation behaviors of H₂ molecules. We find that for the Al-doped surfaces, the surface relaxation around the doping layer changes from expansion of a clean Mg(0001) surface to contraction, due to the redistribution of electrons. After doping, the work function is enlarged, and the electronic states around the Fermi energy have a major distribution around the doping layer. For the dissociation of H₂ molecules, we find that the energy barrier is enlarged for the doped surfaces. In particular, when the Al atom is doped at the first layer, the energy barrier is enlarged by 0.30 eV. For different doping lengths, however, the dissociation energy barrier decreases slowly to the value on a clean Mg(0001) surface when the doping layer is far away from the top surface. Our results well describe the electronic changes after Al doping for the Mg(0001) surface, and reveal some possible mechanisms for improving the resistance to corrosion of the Mg(0001) surface by doping of Al atoms.     

Adsorption and diffusion of H2O molecule on the Be(0001) surface: A density-functional theory study

Using first-principles calculations, we systematically study the adsorption behavior of a single molecular H₂O on the Be(0001) surface. We find that the favored molecular adsorption site is the top site with an adsorption energy of about 0.3 eV, together with the detailed electronic structure analysis, suggesting a weak binding strength of the H₂O/Be(0001) surface. The adsorption interaction is mainly contributed by the overlapping between the s and pz states of the top-layer Be atom and the molecular orbitals 1b₁ and 3a₁ of H₂O. The activation energy for H₂O diffusion on the surface is about 0.3 eV. Meanwhile, our study indicates that no dissociation state exists for the H₂O/Be(0001) surface.     

Be(0001)薄膜中的量子尺寸效应与吸附氢的第一性原理计算

通过基于密度泛函理论的第一性原理方法对铍自由表面的电子结构及表面原子氢的吸附能作了详细计算，给出Be(0001)薄膜的电子结构、表面能、电子功函数、层间弛豫等物理量随厚度变化关系，同时讨论了原子氢在Be(0001) 表面的吸附性质，给出了吸附能及局域电子态密度随铍薄膜层厚的变化关系，体现了铍薄膜的量子尺寸效应.     

氢分子在Mg2Ni(010)面的吸附与分解

本文采用基于密度泛函理论(DFT)的第一原理赝势平面波(PW-PP)方法,对氢分子在Mg2Ni(010)面的吸附与分解进行了研究,我们发现氢分子以Hor1的方式吸附在表面层Ni原子的顶位时吸附能最高,为0.6769eV,这表明氢分子最可能以Hor1的方式吸附在表面层Ni原子的顶位,此时氢分子跟表面的距离( )和氢分子的键长( )分别为1.6286Å和0.9174Å. 在分子吸附的基础上计算了氢分子沿着选取的反应路径分解时的反应势垒,发现要使氢分子分解需要0.2778eV的活化能,而氢分子分解时的吸附能为0.8390eV,分解后两个氢原子的距离为3.1712Å. 在分子吸附和分解吸附时氢原子跟正下方的Ni原子都有较强的相互作用,氢原子所得到的电子主要来自氢分子正下方的Ni原子.     

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.     

Hydrogen chemisorption on <Emphasis Type="BoldItalic">sp</Emphasis><Superscript><Emphasis Type="Bold">2</Emphasis></Superscript>-bonded carbon: Influence of the local curvature and local electronic effects

We report on the interaction of hydrogen with sp²-bonded carbon which has been investigated on graphite (0001), single-walled carbon nanotubes and C₆₀ multilayer films. These substrates have been chosen to represent a large range of curvature in the carbon network. The photoelectron spectroscopy study of the samples treated with atomic hydrogen and low-energy hydrogen ions reveals that hydrogen is chemisorbed on the basal plane of the sp²-bonded carbon networks, as evidenced by the lowered emission from -derived states and a lowering of the electron work function of up to 1.3 eV. The hydrogen adsorption energy barrier is found to strongly depend on the local curvature of the carbon network whereby the barrier is lowered with increasing curvatures. Whereas in the case of C₆₀ and single-walled carbon nanotubes, hydrogen chemisorption can be achieved by exposure to atomic hydrogen, the chemisorption on graphite (0001) requires hydrogen ions of low kinetic energy (1 eV). Furthermore, the adsorption energy barrier is found to increase with hydrogen coverage.The scanning tunnelling microscopy study of individual adsorption sites on the graphite (0001) surface reveals long-ranged (5 nm) electronic effects observed as a (sqrt(3)×sqrt(3))R30° superstructure in the local density of states. It is shown that this superstructure is due to the scattering of delocalized electron wavefunctions at the point defects. The resulting standing waves induce a redistribution of the local density of states which is directly related to the point-like Fermi surface of graphite. PACS 68.43.-h; 71.20.Tx; 68.37.Ef     

Effects of the Be_(22)W phase formation on hydrogen retention and blistering in mixed Be/W systems

We have performed first-principles density functional theory calculations to investigate the retention and migration of hydrogen in Be_(22) W, a stable low-W intermetallic compound. The solution energy of interstitial H in Be_(22) W is found to be 0.49 eV lower, while the diffusion barrier, on the other hand, is higher by 0.13 eV compared to those in pure hcp-Be. The higher solubility and lower diffusivity for H atoms make Be_(22) W a potential beneficial secondary phase in hcp-Be to impede the accumulation of H atoms, and hence better resist H blistering. We also find that in Be_(22) W, the attraction between an interstitial H and a beryllium vacancy ranges from 0.34 eV to 1.08 eV, which indicates a weaker trapping for hydrogen than in pure Be. Our calculated results suggest that small size Be_(22) W particles in hcp-Be might serve as the hydrogen trapping centers, hinder hydrogen bubble growth, and improve the resistance to irradiation void swelling, just as dispersed oxide particles in steel do.     

Density functional theory study of hydrogenation mechanism in Fe-doped Mg(0 0 0 1) surface

Using density functional theory (DFT) in combination with nudged elastic band (NEB) method, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and Fe-doped Mg(0 0 0 1) surfaces are studied. Firstly, the dissociation pathway of H₂ and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0 0 0 1) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0 0 0 1) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe. Then, the diffusion processes of atomic hydrogen on pure and Fe-doped Mg(0 0 0 1) are presented. The obtained diffusion barrier to the first subsurface is 0.45 eV and 0.98 eV, respectively. Finally, Chou method was used to investigate the hydrogen sorption kinetic mechanism of pure MgH₂ and Mg mixed with 5 at.% Fe atoms composites. The obtained activation energies are 0.87 ± 0.02 and 0.31 ± 0.01 eV for H₂ dissociation on the pure surface and H atom diffusion in Fe-doped Mg surfaces, respectively. It suggests that the rate-controlling step is dissociation of H₂ on the pure Mg surface while it is diffusion of H atom in the Fe-doped Mg surface. And both of fitting data are matching well with our calculation results.     

氧气在碳化钨负载的铜-金合金单层上吸附与解离的第一性原理研究

采用基于密度泛函理论的第一性原理方法研究了碳化钨负载的铜-金合金单层催化剂对氧气吸附和解离性质的影响.通过比较碳化钨负载的纯金单层与铜掺杂的合金单层的电荷布局,发现掺杂铜可以创造新的活性位点,改变氧气的吸附构型,改善氧气的吸附和解离性质.进一步探究了氧气在Cu₁Au₈/WC(0001),Cu₂Au₇/WC(0001)上的解离过程,发现:氧气在Cu₂Au₇/WC(0001)上解离势垒仅为0.64 eV,远低于在纯金单层上的1.56 eV,表明铜掺杂有效促进了氧气的解离.铜掺杂后的单层结构变化预示了在单层产生了配位效应和局部应力效应.电荷布局和态密度结果表明铜掺杂促进了单层上电荷的重新分布,加强了氧气分子与基底的电荷转移和轨道杂化,从而增强了对氧气的吸附.当前的研究结果阐明了碳化钨负载合金单层对氧气分解的促进作用和机理,为设计更加高效廉价的氧还原反应催化剂奠定理论基础.     

Effect of van der Waals interactions on H<Subscript>2</Subscript> dissociation on clean and defected Ru(0001) surface

Ab initio computational methods are used to study the relevance of van der Waals interactions in the case of a hydrogen molecule adsorption on the Ru(0001) surface. In addition to the clean surface, the effects of ruthenium adatom and vacancy on the process are studied. The adsorption characteristics are analyzed in terms of two dimensional cuts of the potential energy surface (PES). Based on the earlier studies for such systems, we mostly concentrate on the trajectories where the hydrogen molecule approaches the surface in parallel orientation. The results indicate that for a clean Ru(0001) the calculations applying the non-local van der Waals potentials yield higher barriers for the dissociation of the H₂ molecule. Of the high symmetry sites on Ru(0001), the top site is found to be the most reactive one. The vacancy and ruthenium adatom sites exhibit high dissociation barriers compared with the clean surface.     

Dissociation of H_2 on Mg-coated B_(12)C_6N_6

The dissociation of H_2 molecule is the first step for chemical storage of hydrogen, and the energy barrier of the dissociation is the key factor to determine the kinetics of the regeneration of the storage material. In this paper, we investigate the hydrogen adsorption and dissociation on Mg-coated B_(12)C_6N_6. The B_(12)C_6N_6 is an electron deficient fullerene, and Mg atoms can be strongly bound to this cage by donating their valance electrons to the virtual 2p orbitals of carbon in the cluster. The preferred binding sites for Mg atoms are the B_2C_2 tetragonal rings. The positive charge quantity on the Mg atom is 1.50 when a single Mg atom is coated on a B_2C_2 ring. The stable dissociation products are determined and the dissociation processes are traced. Strong orbital interaction between the hydrogen and the cluster occurs in the process of dissociation, and H_2 molecule can be easily dissociated. We present four dissociation paths, and the lowest energy barrier is only 0.11 eV, which means that the dissociation can take place at ambient temperature.     

Hydrogen depassivation of the magnesium acceptor by beryllium in p-type GaN

Under nitrogen-rich growth conditions, the present ab initio study predicts that hydrogen passivation is more effective on the acceptor Be instead of Mg in a co-doped p-type GaN. The formation energy is 0.24 eV for (H-Be_Ga) complex, and 0.46 eV for (H-Mg_Ga) complex. Congruently, the binding energy is 1.40 eV for (H-Be_Ga), and 0.60 eV for (H-Mg_Ga). Owing to the lower binding energy, (H-Mg_Ga) is not thermally stable. As Be is incorporated in Mg-doped GaN, a (H-Mg_Ga) may release a H⁺ cation at relatively elevated temperatures. Consequently, the H⁺ diffuses swiftly away from a Mg⁻_Ga, across a barrier of 1.17 eV, towards a Be⁻_Ga and forms a stable (H-Be_Ga) with it. The activation of Mg acceptors can be thus facilitated. In this view, the process of hydrogen depassivation of the Mg acceptor by Be can convert the as-grown high-resistivity Mg-doped GaN into a p-conducting material, as observed in the experiments.     

Surface diffusion: the low activation energy path for nanotube growth

We present the temperature dependence of the growth rate of carbon nanofibers by plasma-enhanced chemical vapor deposition with Ni, Co, and Fe catalysts. We extrapolate a common low activation energy of 0.23-0.4 eV, much lower than for thermal deposition. The carbon diffusion on the catalyst surface and the stability of the precursor molecules, C2H2 or CH4, are investigated by ab initio plane wave density functional calculations. We find a low activation energy of 0.4 eV for carbon surface diffusion on Ni and Co (111) planes, much lower than for bulk diffusion. The energy barrier for C2H2 and CH4 dissociation is at least 1.3 eV and 0.9 eV, respectively, on Ni(111) planes or step edges. Hence, the rate-limiting step for plasma-enhanced growth is carbon diffusion on the catalyst surface, while an extra barrier is present for thermal growth due to gas decomposition.     

First-principles study of molecular NO dissociation on Ir(100) surface

The dissociation of NO on Ir(100) surface is investigated using density functional theory (DFT). The pathway and transition state (TS) of the dissociation of NO molecule are determined using climbing image nudge elastic band (CI-NEB). The prerequisite state of NO dissociation is determining the most stable sites of the reactant and products. We found that the most energetically stable sites are the hollow for N atom and the bridge for NO molecule as well as O atom. We found that the bending of NO is the first step of the dissociation reaction due to the increase of the back-donation from the d-band of Ir to 2π ^? orbital of NO, which causes the weakening of NO bond. The dissociation energy barrier of NO molecule on Ir(100) surface is 0.49 eV.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

Hydrogen storage of beryllium adsorbed on graphene doping with boron: First-principles calculations

Based on density-functional theory, we find that B-doped graphene significantly enhances the Be adsorption energy and prevent Be atoms from clustering. The complex of Be adsorbed on B-doped graphene can serve as a high-capacity hydrogen storage medium: the hydrogen storage capacity (HSC) can reach up to 15.1 wt% with average adsorption energy ?0.298 eV/H₂ for double-sided adsorption. It has exceeded the target specified by US Department of Energy with HSC of 9 wt% and a binding energy of ?0.2 to ?0.6 eV/H₂ at near-ambient conditions. By analyzing the projected electronic density of states of the adsorbed system, we show that the high HSC is due to the change of electron distribution of H₂ molecules and a graphene system decorated with B and Be atoms.     

石墨烯在Al_2O_3(0001)表面生长的模拟研究

基于密度泛函理论的广义梯度近似法,对用化学气相沉积法在蓝宝石(α-Al_2O_3)(0001)表面上生长石墨烯进行理论研究.研究结果表明:CH_4在α-Al_2O_3(0001)表面上的分解是吸热过程,由CH_4完全分解出C需要较高能量及反应能垒,这些因素不利于C在衬底表面的存在.在α-Al_2O_3(0001)表面,石墨烯形核的活跃因子并不是通常认为的C原子,而是CH基团.通过CH基团在α-Al_2O_3(0001)表面上的迁移聚集首先形成能量较低的(CH)_x结构.模拟研究(CH)_x对揭示后续石墨烯的形核生长机理具有重要意义.     

The adsorption and dissociation of O2 molecular precursors on Cu: the effect of steps

The adsorption and dissociation of dioxygen on Cu steps are studied using periodic self-consistent density functional theory (PW91-GGA) calculations. Cu steps are modeled with a Cu(2 1 1) surface. The results are compared with those on the flat Cu(1 1 1) surface. The adsorption of both atomic and molecular oxygen is enhanced on the stepped surface: the binding energy of atomic oxygen is −4.5 eV at its preferred site on the relaxed Cu(2 1 1) surface vs. −4.3 eV at its preferred site on the relaxed Cu(1 1 1) surface, and the binding energy of the molecular oxygen precursor is increased from ∼−0.6 to ∼−1.0 eV. Several possible O₂ dissociation paths at the edge of the Cu(2 1 1) step have been investigated. The activation energies range from 0.09 to 0.24 eV, comparable to a minimum activation energy of 0.20 eV found on Cu(1 1 1). However, compared to Cu(1 1 1) the paths on Cu(2 1 1) are stabilized in their entirety by the step by ∼0.5 eV in terms of initial state, transition state, and final state energies. The dissociation of O₂ precursors at the foot of the step is close to being barrier-less. Because of the small dissociation barrier on Cu(1 1 1), the effect of steps on O₂ dissociation is nevertheless not expected to be as pronounced as in other gas/metal systems.