Towards a structure-performance relationship for hydrogen storage in Ti-doped NaAlH4 nanoparticles

Hydrogen storage properties of Ti-doped nanosized (~20 nm) NaAlH(4) supported on carbon nanofibers were affected by the stage at which Ti was introduced. When Ti was deposited first followed by NaAlH(4), sorption properties were superior to the case where NaAlH(4) was deposited first followed by NaAlH(4). This was the result of both a smaller NaAlH(4) particle size and the more extensive catalytic action of Ti in the former material.     

A reality check on using NaAlH4 as a hydrogen storage material

Sodium aluminum hydride or sodium alanate (NaAlH₄) has been considered as a potential material for hydrogen storage. Although its theoretical hydrogen storage capacity is 5.5 wt.% at 250 °C, the material still has its drawback in the regeneration issue. With the use of certain catalysts, the regeneration problem can somewhat be alleviated with added benefits in the decrease in the hydrogen decomposition temperature and the increase in the decomposition rate. This work summarizes what we have learned from the decomposition of NaAlH₄ with/without catalysts and co-dopants. The decomposition was carried out using a thermovolumetric apparatus. For the tested catalysts—HfCl₄, VCl₃, TiO₂, TiCl₃, and Ti—the decomposition temperature of the hydride decreases; however, they affect the temperature in the subsequent cycles differently and TiO₂ appears to have the most positive effect on the temperature. Sample segregation and the morphological change are postulated to hinder the reversibility of the hydride. To prevent the problems, co-dopants—activated carbon, graphite, and MCM-41—were loaded. Results show that the hydrogen reabsorption capacity of HfCl₄- and TiO₂-doped NaAlH₄ added with the co-dopants increases 10–50% compared with that without a co-dopant, and graphite is the best co-dopant in terms of reabsorption capacity. In addition, the decomposition temperature in the subsequent cycles of the co-dopant doped samples decreases about 10–15 °C as compared to the sample without a co-dopant. Porosity and large surface area of the co-dopant may decrease the segregation of bulk aluminum after the desorption and improve hydrogen diffusion in/out bulk of desorbed/reabsorbed samples.     

Theoretical study of the hydrogen chemisorption on a ZnO surface

Reactions of a hydrogen molecule with a ZnO surface are studied by an ab initio method. For simulating the ZnO (10 1 0) surface, one ZnO molecule both with and without a Madelung potential is used. Since the electrostatic potential due to the ionic layer decreases exponentially, the effect of the layers deeper than the second one can be neglected. The Madelung potential is, therefore, expressed by the 32 point charges of ±0.5 situated on the first and second layers. Several low-lying states of ZnO and the ZnO + H₂ system have been calculated by the symmetry-adapted cluster (SAC ) and SAC –CI methods. It is found that the ¹Σ⁺ state of ZnO is the ground state and catalytic active and the other states are inactive. ZnO (¹Σ⁺) reacts with H₂ and dissociatively adsorbs it with making Zn? H and O? H bonds. This occurs both with and without the Madelung potential. Without the Madelung potential, the heat of reaction is 81.3 kcal/mol and the reaction barrier is 14.0 kcal/mol. With the Madelung potential, the heat of reaction decreases to 73.5 kcal/mol and the barrier decreases to 11.5 kcal/mol. The mechanism of this reaction is the electron donation from the 2pπ orbital of O to the antibonding σ_u MO of H₂ and the back-donation from the bonding σ_g MO of H₂ to the LUMO of ZnO. In the intermediate stage of the reaction, the dipole of ZnO works to increase the overlap of the active MOS to make the reaction easier. Throughout the reaction, the in-plane 2pπ orbital of O and the HOMO of ZnO are inactive and work to keep the ZnO bond stable during the catalytic process.     

First-principles study of hydrogen storage on Li12C60

Solid state materials capable of storing hydrogen with high gravimetric (9 wt %) and volumetric density (70 g/L) are critical for the success of a new hydrogen economy. In addition, an ideal storage system should be able to operate under ambient thermodynamic conditions and exhibit fast hydrogen sorption kinetics. No materials are known that meet all these requirements. While recent theoretical efforts showed some promise for transition-metal-coated carbon fullerenes, later studies demonstrated that these metal atoms prefer to cluster on the fullerene surface, thus reducing greatly the weight percentage of stored hydrogen. Using density functional theory we show that Li-coated fullerenes do not suffer from this constraint. In particular, we find that an isolated Li(12)C(60) cluster where Li atoms are capped onto the pentagonal faces of the fullerene not only is very stable but also can store up to 120 hydrogen atoms in molecular form with a binding energy of 0.075 eV/H(2). In addition, the structural integrity of Li(12)C(60) clusters is maintained when they are allowed to interact with each other. The lowest energy structure of the dimer is one where the Li atom capped on the five-member ring of one fullerene binds to the six-member ring of the other. The binding of hydrogen to the linking Li atom and the potential of materials composed of Li(12)C(60) building blocks for hydrogen storage are discussed.     

Catalytic effects of subsurface carbon in the chemisorption of hydrogen on a Mg(0001) surface: an ab-initio study

Ab initio density functional theory (DFT) calculations are performed to explore possible catalytic effects on the dissociative chemisorption of hydrogen on a Mg(0001) surface when carbon is incorporated into Mg materials. The computational results imply that a C atom located initially on a Mg(0001) surface can migrate into the subsurface and occupy an fcc interstitial site, with charge transfer to the C atom from neighboring Mg atoms. The effect of subsurface C on the dissociation of H2 on the Mg(0001) surface is found to be relatively marginal: a perfect sublayer of interstitial C is calculated to lower the barrier by 0.16 eV compared with that on a pure Mg(0001) surface. Further calculations reveal, however, that sublayer C may have a significant effect in enhancing the diffusion of atomic hydrogen into the sublayers through fcc channels. This contributes new physical understanding toward rationalizing the experimentally observed improvement in absorption kinetics of H2 when graphite or single walled carbon nanotubes (SWCNT) are introduced into the Mg powder during ball milling.     

A precursor state for formation of TiAl3 complex in reversible hydrogen desorption/adsorption from Ti-doped NaAlH4

The structure of a TiAl3Hx complex for the formation of a TiAl3 binary phase that could play important roles in the reversible de-/hydrogenation of Ti-doped NaAlH4 has been identified on the basis of first principles density functional theory studies.     

A novel catalyst precursor K2TiF6 with remarkable synergetic effects of K, Ti and F together on reversible hydrogen storage of NaAlH4

A synergetic effect of K, Ti and F together on improving the reversible hydrogen storage properties of NaAlH(4) is found by intruding K(2)TiF(6) as catalyst precursor. Around 4.4 wt% of hydrogen can be released from the NaAlH(4)-0.025 K(2)TiF(6) sample within 40 min at 140 °C.     

Quasiclassical trajectory calculations of hydrogen absorption in the (NaAlH4)2Ti system on a model analytical potential energy surface

We performed a quasiclassical trajectory dynamics study on a model analytical 21-dimensional (7 active atoms) potential energy surface (PES) to examine in detail the mechanism of the hydrogen absorption in a simple (NaAlH(4))(2)Ti model system. The reaction involves a capture of H(2) by the Ti centre and formation of the (η(2)-H(2))Ti(NaAlH(3))(2) coordination complex containing the side-on bonded dihydrogen ligand. The calculated rate constant corresponds to a very fast capture of H(2) by the Ti coordination sphere without a demonstrable barrier. This implies that this step is not the rate-determining step in the complex multi-step process of the NaAlH(4) recovery. The model analytical PES captures the essence of this reaction well and the corresponding energy contours compare favourably to those based on the all-atom hybrid density functional theory calculations.     

First-principles study of fluoroform adsorption on a hexagonal ice (0001) surface: weak hydrogen bonds-strong structural effects

For isolated fluoroform (F(3)CH) molecules adsorbed on a hexagonal ice (0001) surface the properties of blue- and red-shifting hydrogen bonds were studied using static density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (CP-MD) simulations. A systematic search by starting from many initial configurations was performed to determine the lowest-energy structures of F(3)CH on the ice surface, and for the optimized geometries the vibrational frequencies were calculated. The local minima structures are analyzed in terms of their coordination to the surface, with special focus on identifying blue-shifting hydrogen bonds via their spectroscopic signature of an increased frequency of the C-H fundamental stretching vibration. Subsequently, by CP-MD simulations the stability of the lowest-energy configurations at finite temperatures was verified and possible transformation pathways connecting the local minima structures were explored.     

The crystal structure and surface energy of NaAlH4: a comparison of DFT methodologies

This paper presents a comparison of the bulk structure, cleavage energies, and local densities of states of solid NaAlH4 calculated using several different density functional theory methodologies. Good agreement is obtained for the bulk crystal structure. Larger differences become apparent for the calculated surface energies and local densities of states. The (001) NaAlH4 surface is clearly identified as the most stable surface, followed by the (112) and (101) surfaces, with the (100) surface being the least stable. We present an analysis of the local density of states of atoms in the exposed NaAlH4 surface.     

1,3,5-trinitro-1,3,5-triazine decomposition and chemisorption on Al(111) surface: first-principles molecular dynamics study

We have investigated the decomposition and chemisorption of a 1,3,5-trinitro-1,3,5-triazine (RDX) molecule on Al(111) surface using molecular dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). The real-space DFT calculations are based on higher-order finite difference and norm-conserving pseudopotential methods. Strong attractive forces between oxygen and aluminum atoms break N-O and N-N bonds in the RDX and, subsequently, the dissociated oxygen atoms and NO molecules oxidize the Al surface. In addition to these Al surface-assisted decompositions, ring cleavage of the RDX molecule is also observed. These reactions occur spontaneously without potential barriers and result in the attachment of the rest of the RDX molecule to the surface. This opens up the possibility of coating Al nanoparticles with RDX molecules to avoid the detrimental effect of oxidation in high energy density material applications.     

氢原子在金属锂表面台阶附近吸附和表面扩散行为的ab initio研究

取Li7H和Li9H两个原子簇模拟氢原子与含台阶的金属锂表面的相互作用, 以小基组用ab initip方法计算了体系的吸附和表面扩散势能面(或势能曲线)。结果表明: (1)对Li7H体系, 台阶面附近沿垂直边棱方向存在三种不同的桥位吸附位, 最稳定的吸附位在上台面接近台阶边棱处, 台阶面显著地改变了表面扩散活化能, 台阶边棱处有一个较高的势垒。于是, 迁移原子将会在台阶边棱处受到反射, 并可被捕获于台阶面上及其附近。由势能面确定了最低能量表面扩散途径。(2)对Li9H体系, 在Li7H原子簇基础上增加次表面层两个锂原子后, 表面扩散活化能略有减小, 氢原子在上台面的桥位吸附更趋稳定, 各吸附位相对稳定性及势垒内何位置几无改变, 这些结果显示了台阶面对氢原子的化学吸附和表面扩散发生扰动, 台阶边棱对表面扩散起着重要作用。     

A first-principles analysis of hydrogen interaction in Ti-doped NaAlH4 surfaces: structure and energetics

First principles density functional theory studies have been carried out to investigate the hydrogen interactions in Ti-doped NaAlH4 (001) and (100) surfaces. In both surfaces, Ti was found to energetically favor the interstitial sites formed by three neighboring AlH4- units and interact directly with them. The resulting local structure corresponds to a formula of TiAl3Hx with x = 12 before hydrogen desorption starts. The hydrogen desorption energies from many positions of TiAl3Hx are reduced considerably as compared with that from the corresponding clean, undoped NaAlH4 surfaces. The almost invariant local environment surrounding Ti during dehydrogenation makes the TiAl3Hx complex a precursor state for the formation of experimentally observed TiAl3. The importance of the complex has been explored by analyzing the structures and energetics accompanying hydrogen desorption from the complex and from the neighboring AlH4- units. The TiAl3Hx has extended effects beyond the locally reducing hydrogen desorption energy. It facilitates low-energy hydrogen desorption by either transferring hydrogen to the TiAl3Hx complex or reducing hydrogen desorption energy in the neighboring AlH4- by linking these AlH4- units with the complex structure. The possible mechanisms for forming octahedral AlH6(3-) were also identified in the vicinity of TiAl3Hx. Desorbing hydrogen atoms between Ti and Al atoms causes a symmetrical expansion of Ti-Al bonds and leads to the formation of octahedral AlH6(3-).     

An extended Hückel study of hydrogen and oxygen chemisorption on Ru(001) surface

Extended Hückel MO theory has been applied to treat the chemisorption of hydrogen and oxygen atoms on Ru(001) surfaces. The site of chemisorption, surface-adatom distance, chemisorption energy and the vibrational frequency of the adatom on the surface have been calculated. For different sites, the chemisorption energy (E_c) results are as follows: For hydrogen, |E_c|(centre) > |E_c|(top) > |E_c|(bridge); while for oxygen, |E_c|(bridge) > |E_c|(top) > |E_c|(centre). These results are critically discussed in the light of the recent results obtained from the electron energy-loss spectroscopy (EELS) experiments.     

The chemisorption of hydrogen on Cu(111): A dynamical study

Ab initio band-structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top, bridge, and threefold sites, so that the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H₂ molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The results show that (a) when H₂ is in the ground vibrational state the dissociative adsorption probability P_a increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.     

Isotope tracer study of hydrogen spillover on carbon-based adsorbents for hydrogen storage

A composite material comprising platinum nanoparticles supported on molecular sieve templated carbon was synthesized and found to adsorb 1.35 wt % hydrogen at 298 K and 100 atm. The isosteric heat of adsorption for the material at low coverage was approximately 14 kJ/mol, and it approached a value of 10.6 kJ/mol as coverage increased for pressures at and above 1 atm. The increase in capacity is attributed to spillover, which is observed with the use of isotopic tracer TPD. IRMOF-8 bridged to Pt/C, a material known to exhibit hydrogen spillover at room temperature, was also studied with the hydrogen-deuterium scrambling reaction for comparison. The isotherms were reversible. For desorption, sequential doses of H2 and D2 at room temperature and subsequent TPD yield product distributions that are strong indicators of the surface diffusion controlled reverse spillover process.     

Nondissociative chemisorption of methanethiol on Ag(110): a critical result for self-assembled monolayers

Three definitive experiments have been performed to investigate the possibility of dissociative adsorption of methanethiol (CH3SH) on clean Ag(110). On the clean Ag(110) surface, the adsorption in the first layer occurs to 0.5 ML, producing a (2 x 1) low-energy electron diffraction (LEED) structure. The undissociated molecule desorbs starting at approximately 140 K, and only tiny quantities of other gaseous products are desorbed, and only tiny quantities of S-containing species remain. Using a 50:50% mixture of CH3SD and CD3SH, we find no evidence of S-H or S-D bond scission between these molecules upon desorption. And finally, when the CH3SH molecule is incident on the clean Ag(110) surface in the temperature range of 230-400 K, less than 1% of the incident molecules dissociate to produce adsorbed sulfur-containing species. The results influence our thinking about the surface bonding of alkanethiol-based self-assembled monolayers (SAMs) on noble metals.     

Oxygen chemisorption on the basal faces of graphite: an XPS study

XPS has revealed the first direct evidence of chemisorption of atomic oxygen on (000l) faces of graphite. Sticking coefficients of O₂ are also estimated.