Adsorption of hydrogen on normal and pentaheptite single wall carbon nanotubes

Density functional calculations of the physisorption of molecular hydrogen and the dissociative atomic chemisorption on the external surface of hexagonal and pentaheptite carbon nanotubes, have been carried out. Physisorption binding energies are near 100 meV/molecule and are similar on metallic and semiconducting nanotubes. Full coverage of the nanotube with one molecule per graphitic hexagon decreases the binding energy per molecule. Chemisorption binding energies per H atom are larger on pentaheptites than on hexagonal carbon nanotubes. The molecular physisorption and dissociative chemisorption states on pentaheptites have very similar total energies (some chemisorbed states are even slightly more stable than the physisorbed states), while on hexagonal carbon nanotubes molecular physisorption is more stable than dissociative chemisorption. However, a substantial energy barrier has to be overcome to go from physisorption to dissociative chemisorption in both types of nanotubes.     

Challenges in hydrogen adsorptions: from physisorption to chemisorption

In this short review, we will briefly discuss the story of hydrogen storage, its impact on clean energy application, especially the challenges of using hydrogen adsorption for onboard application. After a short comparison of the main methods of hydrogen storage (high pressure tank, metal hydride and adsorption), we will focus our discussion on adsorption of hydrogen in graphitic carbon based large surface area adsorbents including carbon nanotubes, graphene and metal organic frameworks. The mechanisms, advantages, disadvantages and recent progresses will be discussed and reviewed for physisorption, metal-assisted storage and chemisorption. In the last section, we will discuss hydrogen spillover chemisorption in detail for the mechanism, status, challenges and perspectives. We hope to present a clear picture of the present technologies, challenges and the perspectives of hydrogen storage for the future studies.     

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     

Simulation of chemical adsorption of hydrogen by carbon nanotubes

The hydrogen chemical adsorption on a single-walled carbon nanotube (6, 6) has been studied by quantum-chemical computer simulation. Different variants of hydrogen coverage of the nanotube have been considered, and the dependences of the adsorption energy and the nanotube strain energy on the coverage density have been found. In addition, the adsorption has been considered on both the outer and inner surfaces of the nanotube wall. It has been established that some adsorption conformations are unstable, which leads to fracture of the nanotubes.     

Chemisorption of hydrogen by carbon nanotubes

Chemisorption of hydrogen by carbon nanotubes (CNTs) is studied by thermodynamics and kinetics methods. Expressions are derived for the adsorption isotherm and desorption kinetics. Methods for determining chemisorption parameters are developed. The partial free energy of binding of hydrogen with a CNT (3.6 eV) is determined. It is shown that residual products of synthesis are removed from CNTs as a result of prolonged annealing at high temperatures. The capacity of a CNT relative to chemisorbed hydrogen is esti mated at 4 mass %.     

Sidewall fluorination and hydrogenation of single-walled carbon nanotubes: a density functional theory study

The fluorination and hydrogenation reactions on (6, 6) and (10, 0) single-walled carbon nanotubes (SWCNTs) have been examined via computing the reaction energy for the chemisorption. The examined nanotubes have comparable lengths and diameters, with or without Stone-Wales defects on the sidewall. The two fluorine or hydrogen atoms are anchored to the external walls of the SWCNTs. The computed chemisorption energies of these virtual reactions reveal that the fluorination and hydrogenation of the nanotubes are moderately sensitive to the nanotube chirality and the sidewall topology, and the (10, 0) SWCNT with Stone-Wales defect can be easily fluorinated and hydrogenated.      

H,O原子在过渡金属表面吸附能的规则性

用Allan的简化d带模型描叙过渡金属的表面电子态,用广义相移法计算吸附原子在表面的吸附能,结果表明,不仅很好地描绘了H,O在一个周期的过渡金属表面吸附能的变化趋势,而且所算得的吸附原子感应的分离能级也同紫外光发射谱的实验符合得很好;同时还指出,简单气体在过渡金属表面的吸附能呈现规则性变化主要决定于费密能级E_F与吸附原子的有效能级ε_a之差(E_F-ε_a),其次是转移矩阵元ν和能带宽度w_b。 关键词：     

Hydrogen adsorption on graphene: a first principles study

We present a systematic ab initio study of atomic hydrogen adsorption on graphene. The characteristics of the adsorption process are discussed in relation with the hydrogenation coverage. For systems with high coverage, the resultant strain due to substrate relaxation strongly affects H atom chemisorption. This leads to local structural changes that have not been pointed out to date, namely localized surface curvature. We demonstrate that the hydrogen chemisorption energy barrier is independent of the optimization technique and system size, being associated with the relaxation and rehybridization of the sole adsorbent carbon atom. On the other hand, the H desorption barrier is very sensitive to a correct structural relaxation and is also dependent on the degree of system hydrogenation.     

The co-adsorption of oxygen and hydrogen on Rh(111)

The co-adsorption of oxygen and hydrogen on Rh(111) at temperatures below 140 K has been studied by thermal desorption mass spectrometry, Auger electron spectroscopy, and lowenergy electron diffraction. The co-adsorption phenomena observed were dependent upon the sequence of adsorption in preparing the co-adsorbed overlayer. It has been found that oxygen extensively blocks sites for subsequent hydrogen adsorption and that the interaction splits the hydrogen thermal desorption into two states. The capacity of the oxygenated Rh(111) surface for hydrogen adsorption is very sensitive to the structure of the oxygen overlayer, with a disordered oxygen layer exhibiting the lowest capacity for hydrogen chemisorption. Studies with hydrogen pre-adsorption indicate that a hydrogen layer suppresses completely the formation of ordered oxygen superstructures as well as O₂ desorption above 800 K. This occurs with only a 20% reduction in total oxygen coverage as measured by Auger spectroscopy.     

A density functional study of atomic hydrogen and oxygen chemisorption on the relaxed (0001) surface of double hexagonal close packed americium

Ab initio total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorption on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals method. Chemisorption energies were optimized with respect to the distance of the adatom from the relaxed surface for three adsorption sites, namely top, bridge, and hollow hcp sites, the adlayer structure corresponding to coverage of a 0.25 monolayer in all cases. Chemisorption energies were computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at the fully relativistic level (with spin-orbit coupling SOC). The two-fold bridge adsorption site was found to be the most stable site for O at both the NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and 8.368 eV respectively, while the three-fold hollow hcp adsorption site was found to be the most stable site for H with chemisorption energies of 3.136 eV at the NSOC level and 3.217 eV at the SOC level. The respective distances of the H and O adatoms from the surface were found to be 1.196 ?and 1.164 ?. Overall our calculations indicate that chemisorption energies in cases with SOC are slightly more stable than the cases with NSOC in the 0.049–0.238 eV range. The work functions and net magnetic moments respectively increased and decreased in all cases compared with the corresponding quantities of bare dhcp Am (0001) surface. The partial charges inside the muffin-tins, difference charge density distributions, and the local density of states have been used to analyze the Am-adatom bond interactions in detail. The implications of chemisorption on Am 5f electron localization-delocalization are also discussed.     

Effect of atomic hydrogen adsorption on the transport characteristics of semiconducting carbon nanotubes

The effect of atomic hydrogen adsorption on the conduction and diffusion properties of carbon nanotubes of zigzag type in an external electric field is considered. The model of adsorption of atomic hydrogen on the surface of single-walled carbon nanotubes of zigzag type is based on the single-impurity periodic Anderson model. The theoretical calculation of the diffusion coefficient and electrical conductivity of carbon nanotubes of zigzag type doped with hydrogen atoms is carried out in the relaxation time approximation. It has been revealed that the electrical conductivity and electron diffusion coefficient decrease with increasing concentration of adsorbed hydrogen atoms. It has been shown that the dependence of the electrical conductivity and the diffusion coefficient on the amplitude of the constant electric field at the constant concentration of hydrogen adatoms is nonlinear.     

等离子体诱导碳纳米管到纳米金刚石的相变

采用氢等离子体，实现了碳纳米管向金刚石的结构相变，并实现了金刚石的高密度成核，有效成核密度可达10\+\{11\}／cm\+2以上，处于目前金刚石成核密度的最高行列，为制备优质的金刚石薄膜提供了保证.高分辨透射电镜、x射线衍射和拉曼光谱都证实了金刚石的形成.同时，对纳米金刚石晶粒的生成机理进行了初步探讨. 关键词： 等离子体 碳纳米管 纳米金刚石 结构相变     

碳纳米管在储氢上的应用

氢能是一种理想的能源载体,而经济有效的储氢手段是氢能实现规模应用急需解决的关键问题之一。碳纳米管在存储氢气上表现出来的独特性质,使其最有希望成为一种新的高效的储氢材料。从实验、理论研究两个方面总结了前人在碳纳米管储氢上的研究成果,并对碳纳米管储氢吸附方式,吸附量影响因素等方面做出分析。最后指出为实现碳纳米管储氢大规模应用仍需做的一些基础性研究工作。     

Evaluation of different models for the dissociation of silane on the Si(1 1 1)7 × 7 surface using the extended Brenner empirical potential

Several models have been proposed in the literature for the initial stages of the dissociative chemisorption of silane (SiH₄) on the Si(1 1 1)7 × 7 surface. In this paper, geometry optimisation calculations using the extended Brenner empirical potential have been performed to determine which of these models yields the minimum energy structure. The lowest energy configurations are found to correspond to the dissociation of silane into SiH₂ and two hydrogen atoms. The minimum energy structure involves the adsorption of the two hydrogen atoms onto the dangling bonds of an adjacent adatom and rest atom, and the insertion of the remaining SiH₂ fragment into one of the adatom backbonds. These results are discussed in the light of the existing experimental data.     

LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.     

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。     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

Tunable adsorption on carbon nanotubes

We investigated the adsorption of a single atom, hydrogen and aluminum, on single-wall carbon nanotubes from first principles. The adsorption is exothermic, and the associated binding energy varies inversely as the radius of the zigzag tube. We found that the adsorption of a single atom and related properties can be modified continuously and reversibly by the external radial deformation. The binding energy on the high curvature site of the deformed tube increases with increasing radial deformation. The effects of curvature and radial deformation depend on the chirality of the tube.     

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.     

Theoretical insight of adsorption thermodynamics of multifunctional molecules on metal surfaces

Adsorption thermodynamics based on density functional theory (DFT) calculations are exposed for the interaction of several multifunctional molecules with Pt and Au(1 1 0)-(1 × 2) surfaces. The Gibbs free adsorption energy explicitly depends on the adsorption internal energy, which is derived from DFT adsorption energy, and the vibrational entropy change during the chemisorption process. Zero-point energy (ZPE) corrections have been systematically applied to the adsorption energy. Moreover the vibrational entropy change has been computed on the basis of DFT harmonic frequencies (gas and adsorbed phases, clean surfaces), which have been extended to all the adsorbate vibrations and the metallic surface phonons. The phase diagrams plotted in realistic conditions of temperature (from 100 to 400 K) and pressure (0.15 atm) show that the ZPE corrected adsorption energy is the main contribution. When strong chemisorption is considered on the Pt surface, the multifunctional molecules are adsorbed on the surface in the considered temperature range. In contrast for weak chemisorption on the Au surface, the thermodynamic results should be held cautiously. The systematic errors of the model (choice of the functional, configurational entropy and vibrational entropy) make difficult the prediction of the adsorption-desorption phase boundaries.