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.     

Theoretical study of hydrogen dissociation and diffusion on Nb and Ni co-doped Mg(0001): A synergistic effect

The interaction of H₂ with clean, Ni and Nb doped Mg(0001) surface are investigated by first-principles calculations. Individual Ni and Nb atoms within the outermost surface can reduce the dissociation barrier of the hydrogen molecule. They, however, prefers to substitute for the Mg atoms within the second layer, leading to a weaker catalytic effect for the dissociation of H₂, a bottleneck for the hydriding of MgH₂. Interestingly, co-doping of Ni and Nb stabilizes Ni at the first layer, and results in a significant reduction of the dissociation barrier of H₂ on the Mg surface, coupled with an increase of the diffusion barrier of H. Although codoped Ni and Nb shows no remarkable advantage over single Nb here, it implies that the catalytic effect could be optimized by co-doping of “modest” transition metals with balanced barriers for dissociation of H₂ and diffusion of H on Mg surfaces.     

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.     

Hydrogen adsorption,absorption and diffusion on and in transition metal surfaces: A DFT study

Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals—the (100) and (111) facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the (100) and (110) facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H₂ on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.     

The linewidth of surface states of simple metals

The linewidths (inverse lifetimes) Γ_e-e of Be(0001) and Mg(0001) surface electronic states are calculated as the projection of the imaginary part of the self-energy operator of a quasiparticle onto the state. The screened Coulomb interaction is calculated using a model potential, which takes into account the energy gap in the band structure and a surface state located in this gap. The wave functions and energies of electron states are calculated by a self-consistent film pseudopotential method. It is shown that Γ_e-e essentially depends on the position of the surface state in the Brillouin zone. The difference between the calculated values of Γ_e-e and those obtained in a homogeneous electron gas model is shown to be basically due to transitions from surface bands.     

Studies on interdiffusion in Pd/Mg/Si films: Towards improved cyclic stability in hydrogen storage

The paper reports the diffusion coefficients of grain boundary diffusion and grain boundary assisted lattice diffusion of Pd in Mg in Pd/Mg/Si system, a useful material for hydrogen storage, at 473 K in vacuum. The grain boundary diffusivity is measured by Whipple model and grain boundary assisted lattice diffusivity by plateau rise method using Pd depth profiles constructed by Rutherford backscattering spectrometry. It is established that grain boundary diffusivities are about six orders of magnitude faster than lattice diffusivities. Fine grained microstructure of Pd film, high abundance of defects in Mg film and higher stability associated with Pd-Mg intermetallics are responsible for the diffusion of Pd into grain boundaries and subsequently in the interiors of Mg. Besides the indiffusion of Pd, annealing also brings about an outdiffusion of Mg into Pd film. Examination by nuclear reaction analysis involving ²⁴Mg(p,p′γ)²⁴Mg resonance reaction shows the occurrence of Mg outdiffusion. Minimization of surface energy is presumably the driving force of the process. In addition to Pd/Mg interface, diffusion occurs across Mg/Si (substrate) interface as well on increasing the annealing temperature above 473 K. These studies show that dehydrogenation of films accomplished by vacuum annealing should be limited to temperatures less than 473 K to minimize the loss of surface Pd, the catalyst of the hydrogen absorption-desorption process and Mg, the hydrogen storing element, by way of interfacial reactions.     

铁电阴极前电极表面等离子体横向扩散速度估算

分析了激励脉冲电压作用下铁电阴极电容的变化,即等离子体沿着铁电阴极前电极表面扩散而引起电容变化;建立了激励脉冲电压作用下铁电阴极等效电容模型并推导铁电阴极前电极表面等离子横向扩散速度表达式。采用传统固相烧结工艺制备的掺镧锆锡钛酸铅反铁电陶瓷作为阴极材料,通过测量激励脉冲电压作用下铁电阴极两端的电压及充放电电流,计算得到掺镧锆锡钛酸铅陶瓷表面等离子体横向扩散速度为1.89×106 cm/s。     

Highly regular nanometer-sized hexagonal pipes in 6H-SiC(0001)

An array of troughs was prepared on a 6H-SiC(0001) surface using focused ion beam (FIB) patterning. Troughs were etched with various ion doses and close-to-circular voids of increasing depths for larger ion doses were obtained. The samples were then etched in a hot-wall reactor at a hydrogen partial pressure of 13 mbar at 1800 °C. The resulting morphological reorganizations have been studied by scanning electron and atomic force microscopy. Very regular hexagonal voids with facets oriented perpendicular to the surface were obtained after hydrogen etching. The voids were surrounded by regular secondary facets of lower inclination. Whereas the depth of the voids increases with ion dose, the void diameter and facet sizes stay constant. This effect is explained by surface diffusion during hydrogen etching. The FIB technique in combination with hydrogen etching allows the preparation of very regular surface patterns and highly ordered wells and tubes for nanometer-sized sieves and photonic crystals. PACS 47.70.Fw; 68.37.-d; 68.37.Hk; 68.37.Ps; 81.65.Cf     

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.     

Preparation and electrochemical characterisation of supporting SOFC–Ni–YZT anodes

Symmetrical cells consisting of Ni–Y_0.20Ti_0.18Zr_0.62O_1.90 (Ni–YZT) cermet electrodes on a Ni–YSZ support have been investigated with respect to the hydrogen/water partial pressures. Impedance spectra at open circuit potential were obtained as function of temperature and analysed in terms of a fractal finite length Gerischer Impedance. For fine and coarse microstructures of the Ni–YZT electrodes, a consistent set of model parameters could be obtained. The results indicate that surface diffusion rather than bulk diffusion plays a role in the hydrogen/water reaction but also that a fine-grained fraction in the mixed conducting YZT phase is advantageous for the overall electrode performance and the surface exchange reaction.     

Diffusion of Al dimers on the surface of Mg clusters

The surface diffusion of Al dimmers on Mg clusters with hexahedral structure was studied using the combination of quenched molecular dynamics and the embedded atom method. The system energy barriers of typical minimum energy diffusion paths for Al dimers on the Mg clusters were calculated using the Nudged Elastic Band method. In our study range (153–4061 atoms), the binding energies on the (0001) facets and the (1\(\hbox{$\overline 1 $}\)01) facets differed, the binding energy on the former was lower than that on the latter. Moreover, cluster size only slightly influenced the binding energy values. Two possible diffusion paths were studied. Results showed that the diffusion of the dimer on the (0001) facet easily occurred at low temperatures. Furthermore, the interaction between the two atoms of the dimer facilitated the dimer crossing of the step edge between the (1\(\hbox{$\overline 1 $}\)01) facets by hopping mechanism.     

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.     

MgF2作为镁离子内扩散LiNbO3单晶扩散源的研究

利用ＭｇＦ₂作为扩散源进行镁离子内扩散ＬｉＮＯ₃单晶基片的研究．经电子探针显微镜分析表明，镁的扩散层镁离子浓度分布具有近似半抛物形和阶跃形分布，并得到镁的激活能为１０４ＫＪ／mol.X射线衍射分析发现，当镁离子内扩散超过一定程度时，在镁的扩散层出现具有ＬｉＮｂ₃Ｏ₈的物相结构，并发现该物相结构的出现与ＬｉＮＯ₃体内锂离子的外扩散和扩散表面氟离子的存在有关．由扫描电子显微镜观察到氟离子以ＬｉＦ的形式出现，并随着扩散程度的增大，挥发和脱离开扩散层表面． 关键词：     

The effect of defects on the hydrogenation in Mg (0 0 0 1) surface

Density-functional theory was presented to investigate the hydrogen dissociation on a pure, Pt-doped, vacancy and oxide Mg(0 0 0 1) surface. Our results show that the energy barriers are 1.05, 0.39, 0.93 and 1.33 eV for H₂ dissociation on the pure, Pt-doped, vacancy and oxide Mg surface, respectively. The calculation results imply that the initial dissociation of H₂ is enhanced significantly for the Pt-doped Mg(0 0 0 1) surface, negligible for the vacancy model and weekend for the oxide model. The density of state results shows that, following the dissociation reaction coordinate, the H–H interactions are weeker for the Pt-doped model while interactions become stronger for the oxide model. It is suggested that the dissociation process is facilitated when Pt atom acts as catalyst and oxide overlayers delay hydrogen adsorption on the Mg layer. The present study will help us understand the defect role being played for the improvement or opposition effect in absorption kinetics of H₂ on the Mg(0 0 0 1) surface.     

Adsorption and dissociation of molecular hydrogen on Pt/CeO2 catalyst in the hydrogen spillover process: A quantum chemical molecular dynamics study

Ultra accelerated quantum chemical molecular dynamics method (UA-QCMD) was used to study the dynamics of the hydrogen spillover process on Pt/CeO₂ catalyst surface for the first time. The direct observation of dissociative adsorption of hydrogen on Pt/CeO₂ catalyst surface as well as the diffusion of dissociative hydrogen from the Pt/CeO₂ catalyst surface was simulated. The diffusion of the hydrogen atom in the gas phase explains the high reactivity observed in the hydrogen spillover process. Chemical changes, change of adsorption states and structural changes were investigated. It was observed that parallel adsorption of hydrogen facilitates the dissociative adsorption leading to hydrogen desorption. Impact with perpendicular adsorption of hydrogen causes the molecular adsorption on the surface, which decelerates the hydrogen spillover. The present study also indicates that the CeO₂ support has strong interaction with Pt catalyst, which may cause an increase in Pt activity as well as enhancement of the metal catalyst dispersions and hence increasing the rate of hydrogen spillover reaction.     

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.     

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.     

Chemisorption of hydrogen on titanium: Embedding theory and comparisons with small clusters

The chemisorption of H₂ on Ti(0001) is treated using an ab initio CI theory for the surface region. Dissociation of H₂ occurs above the surface but more stable 3-fold coordination sites lie closer to the surface at ～ 1.3 Å. Adsorption in adjacent 3-fold sites is less stable than in separated sites sharing only one surface atom. The calculated adsorption energy of 45 kcal/mol H₂ compares favorably with experiment. Bonding involves mainly the 4s electrons of the metal leading to hydridic hydrogens and a polarized lattice electron distribution, but d bonding and correlation effects significantly increase the binding energy. Calculations on small metal clusters also show dissociative adsorption but much larger hydrogen binding energies are obtained.     

温度对ZnO/Al2O3(0001)界面的吸附、扩散及生长初期模式的影响

构建了一个ZnO沉积在α-Al₂O₃(0001)表面生长初期的模型，采用基于密度泛函理论的平面波超软赝势法进行了动力学模拟.发现在400，600和800℃的条件下界面原子有不同的扩散能力，因此温度对ZnO/α-Al₂O₃(0001)表面界面结构以及ZnO薄膜生长初期模式有决定性的影响.在整个ZnO吸附生长过程中，O原子的扩散系数大于Zn原子的扩散系数，O原子的层间扩散对薄膜的均匀生长起着重要作用.进一步从理论计算上证实了ZnO在蓝宝石(0001)上两种生长模式的存在，400℃左右生长模式主要是Zn螺旋扭曲生长，具有Zn六角平面对称特征，且有利于Zn原子位于最外表面.600℃左右呈现为比较规则的层状生长，且有利于O原子位于最外表面.模拟观察到在ZnO薄膜临近Al₂O₃基片表面处，Zn的空位缺陷明显多于O的空位缺陷. 关键词： 扩散 薄膜生长 2O₃(0001)')" href="#">α-Al₂O₃(0001) ZnO     

Local and nonlocal contributions to adhesive energies at metallic interfaces

A self-consistent computation of the adhesive binding energy versus separation is reported for the following contacts: Al(111)-Al(111), Mg(0001)- Mg(0001), Zn(0001)-Zn(0001), and Na(110)-Na(110). Results are obtained for the local density approximation to exchange and correlation as well as for a nonlocal approximation. The resultant shapes of the adhesive binding energy curves varied substantially from metal to metal. The inclusion of nonlocality has little effect on the overall shape of the binding energy curves, but improves agreement of the binding energies as measured from the energy minimum with experimental surface energies.