Adsorption Of H2 Molecules Research Articles

A density functional study on the adsorption of hydrogen molecule onto small copper clusters

An all-electron scalar relativistic calculation on the adsorption of hydrogen molecule onto small copper clusters has been performed by using density functional theory with the generalized gradient approximation (GGA) at PW91 level. Our results reveal that after adsorption of H2 molecule, the Cu–Cu interaction is strengthened and the H–H interaction is weakened, the reactivity enhancement of H2 molecule is obvious. The VIPs, HLGs and VEAs of Cu n H2 clusters show an obvious odd–even oscillation. It is suggested that the H2 molecule is more favourable to be adsorbed by the even-numbered small copper clusters. Meanwhile, the odd–even alteration of magnetic moments is also observed and may be served as the material with tunable code capacity of ‘0’ and ‘1’ by adsorbing hydrogen molecule onto odd or even-numbered small copper clusters. Some discrepancies of dissociative adsorption between our work and previous works are found and may be understood in terms of the electron pairing effect and the scalar relativistic effect.

Calcium-Decorated Carbyne Networks as Hydrogen Storage Media

Among the carbon allotropes, carbyne chains appear outstandingly accessible for sorption and very light. Hydrogen adsorption on calcium-decorated carbyne chain was studied using ab initio density functional calculations. The estimation of surface area of carbyne gives the value four times larger than that of graphene, which makes carbyne attractive as a storage scaffold medium. Furthermore, calculations show that a Ca-decorated carbyne can adsorb up to 6 H(2) molecules per Ca atom with a binding energy of ∼0.2 eV, desirable for reversible storage, and the hydrogen storage capacity can exceed ∼8 wt %. Unlike recently reported transition metal-decorated carbon nanostructures, which suffer from the metal clustering diminishing the storage capacity, the clustering of Ca atoms on carbyne is energetically unfavorable. Thermodynamics of adsorption of H(2) molecules on the Ca atom was also investigated using equilibrium grand partition function.

Reactivity of Transition Metals (Pd, Pt, Cu, Ag, Au) toward Molecular Hydrogen Dissociation: Extended Surfaces versus Particles Supported on TiC(001) or Small Is Not Always Better and Large Is Not Always Bad

The reactivity of Pd4, Pt4, Cu4, Ag4, and Au4 clusters supported on TiC(001) toward molecular hydrogen dissociation has been studied by means of density functional based theory and periodic models and compared to that of the (111) and (001) surfaces. Pd4 and Pt4 interact rather strongly with the TiC(001) substrate, but the interaction of molecular hydrogen with the Pd4/TiC and Pt4/TiC systems is also very strong. As a consequence of the substantial admetal↔carbide interactions, the adsorbed H2 molecule becomes more difficult to dissociate than on the corresponding extended (111) and (001) surfaces. Here, having a small supported particle does not lead to an enhanced chemical activity. On the contrary, for the Cu4/TiC, Ag4/TiC, and Au4/TiC systems the combination of the small size of the particle and the polarization induced by the underlying carbide facilitates the dissociation of the hydrogen molecule with respect to the case of the extended surfaces. Here, the reduced size effectively enhances the activ...

Single walled carbon nanotubes functionalized with hydrides as potential hydrogen storage media: A survey of intermolecular interactions

AbstractIn this paper, we have analyzed the intermolecular interactions between H2and single walled carbon nanotube (SWCNT)–hydride complexes and project their capability as a practicable hydrogen storage medium (HSM). In this respect, we have investigated the type of interactions namely van der Waals, electrostatic, and orbital interactions to understand the molecular hydrogen binding affinity of various systems. We found that the charge transfer effects coupled with induced electrostatic interactions are responsible for synergetic action of SWCNT and hydrides on adsorption of H2molecules at ambient conditions. Also we have calculated the thermodynamically usable capacity of hydrogen in all the systems. This study enables one to identify and design potential hydrogen storage materials.

Li-doped B2C graphene as potential hydrogen storage medium

Based on first-principles density functional theory, we show that Li-doped B2C graphene can serve as a high-capacity hydrogen storage medium with the gravimetric density of 7.54 wt %. The present results indicate that the strong binding of Li onto the substrate comes from the hybridizations of B 2p and C 2p orbitals with the partial occupancy of Li 2p orbitals. Both the polarization mechanism and the orbital hybridizations contribute to the adsorption of H2 molecules and the resulting adsorption energy is in the range of 0.12–0.22 eV/H2. The system reported here is favorable for the reversible hydrogen adsorption/desorption at the room temperature.

Hydrogen Storage Capacity of C120 Nanocapsules: Density Functional Theory Based Treatments

The adsorption of hydrogen molecule on the external surface of pure C120 nanocapsule and endohedrally H2@C120 complex has been examined using the density functional theory calculations. Several different bonding configurations are considered for the hydrogen molecule approaching the outer surface of the considered nanocages. It has been found that the adsorbed H2 molecule bound weakly to the outer surface of the pure C120 nanocapsules in agreement with the recent experimental and theoretical results while, it prefers to be adsorbed rather strongly on the side wall of the endohedrally H2@C120 complex. The adsorption of a single layer and bi-layer of two H2 molecules on the most stable states of the considered H2@C120 complex appears to be feasible, although the molecules of the second layer are weakly bound. Furthermore, it is found that the formation of 100% coverage is favorable thermodynamically, which corresponds to about 20% by weight storage of H2 molecules. Thus, surprisingly, we arrive at the prediction that the C120 nanocapsules can be implemented as a novel material for energy storage.

DFT Study of Hydrogen Adsorption on Palladium Decorated Graphene

The adsorption of several molecular and dissociative dihydrogen systems on a Pd-decorated graphene monolayer was studied using the density-functional theory. Our calculations show that the most favorable graphene-supported coordination structure is similar to the PdH2 complex in vacuum, where the H−H bond is relaxed but not dissociated. We also computed overlap populations corresponding to bonds and atomic orbital interactions in order to study the evolution of the chemical bonding. During the decoration process with Pd, we detected a weakening of C−C bonds close to the adsorption site and the formation of strong C−Pd bonds, coming from interaction between C 2pz and Pd 5s, 5pz, and 4dz2 orbitals. After H2 molecule adsorption, the H−Pd bond is formed by the H 1s orbital overlap with the Pd 5s orbital, but this interaction became stronger during the atomic hydrogen adsorption. The objective of this work is to contribute to the understanding of the hydrogen uptake of Pd-doped graphene surfaces.

Theoretical realization of cluster-assembled hydrogen storage materials based on terminated carbon atomic chains

The capacity of carbon atomic chains with different terminations for hydrogen storage is studied using first-principles density functional theory calculations. Unlike the physisorption of H(2) on the H-terminated chain, we show that two Li (Na) atoms each capping one end of the odd- or even-numbered carbon chain can hold ten H(2) molecules with optimal binding energies for room temperature storage. The hybridization of the Li 2p states with the H(2)σ orbitals contributes to the H(2) adsorption. However, the binding mechanism of the H(2) molecules on Na arises only from the polarization interaction between the charged Na atom and the H(2). Interestingly, additional H(2) molecules can be bound to the carbon atoms at the chain ends due to the charge transfer between Li 2s2p (Na 3s) and C 2p states. More importantly, dimerization of these isolated metal-capped chains does not affect the hydrogen binding energy significantly. In addition, a single chain can be stabilized effectively by the C(60) fullerenes termination. With a hydrogen uptake of ∼10 wt.% on Li-coated C(60)-C(n)-C(60) (n = 5, 8), the Li(12)C(60)-C(n)-Li(12)C(60) complex, keeping the number of adsorbed H(2) molecules per Li and stabilizing the dispersion of individual Li atoms, can serve as better building blocks of polymers than the (Li(12)C(60))(2) dimer. These findings suggest a new route to design cluster-assembled hydrogen storage materials based on terminated sp carbon chains.

Titanium-capped carbon chains as promising new hydrogenstorage media

The capacity of Ti-capped sp carbon atomic chains for use as hydrogen storage media is studied using first-principles density functional theory. The Ti atom is strongly attached at one end of the carbon chains via d-p hybridization, forming stable TiCn complexes. We demonstrate that the number of adsorbed H2 molecules on Ti through Kubas interactions depends upon the chain types. For polyyne (n even) or cumulene (n odd) structures, each Ti atom can hold up to five or six H2 molecules, respectively. Furthermore, the TiC5 chain effectively terminated on a C20 fullerene can store hydrogen with an optimal binding energy of 0.52 eV per H2 molecule. Our results reveal a possible way to explore high-capacity hydrogen storage materials in truly one-dimensional carbon structures.

Development of new material for the environmental-friendly energy generation processes

The aim of this study was to develop a new material which has high hydrogen storage capacity and hydrogen absorption/desorption rate. This material, which is based on cobalt powder onto which palladium was chemically deposited can be used in environmentally friendly energy generation processes, thus ensuring preventive protection of human health. Hydrogen absorption was investigated using differential scanning calorimetry (DSC) and by measuring hydrogen pressure under isothermal and non-isothermal conditions in a chamber containing either pure cobalt powder either cobalt powder onto which palladium was chemically deposited (Co-0.003 Pd). The mechanism of hydrogen absorption has been described. It was noticed that palladium catalyses hydrogen absorption by dissociating the adsorbed H2 molecules into H atoms more rapidly on its own atoms than on cobalt ones. The catalysis of dissociation of H2 molecules enabled hydrogen absorption into Co-0.003 Pd powder to occur at lower temperatures. The results of this study enable integration of technological and public health investigation in order to ensure preventive protection of human health through enviromental protection.

First principles study of H2 molecule adsorption on Li3 N(110) surfaces

The adsorption of H2 on a Li3N(110) crystal surface is studied by first principles. Preferred adsorption sites, adsorption energy, dissociation energy and electronic structure of the H2/Li3N(110) systems are calculated separately. It is found that H2 is adsorbed on the N bridge site more favorably than on the other sites, while two NH radicles are formed on the Li3N(110) crystal surface. The calculated adsorption energy on the N bridge site is 1.909 eV, belonging to a strong chemical adsorption. The interaction between H2 and Li3N(110) surface is due mainly to the overlapping among H 1s, N 2s and N 2p states, through which covalent bonds are formed between N and H atoms. An activation barrier of 1.63 eV is found for the dissociation of H2 molecule in N bridge configuration, which indicates that the dissociative adsorption of H2 on Li3N(110) surface is favorable under the certain heat activation condition; NH2 radicle is formed after the optimization of H2 adsorbed on the N top site. The adsorption energy on the N top site is negative. In other words, this adsorption is unstable. So it is concluded that it is not easy to produce the LiNH2 between Li3N(110) face and H2 directly.

Computational Study of Silica-Supported Transition Metal Fragments for Kubas-type Hydrogen Storage

To verify the role of the Kubas interaction in transition metal grafted mesoporous silicas, and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems, computational studies have been performed. Thus, the interaction of H2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H2 molecules is found to be of a synergic, Kubas type, and a maximum of four H2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H2 units increases from two to four, in agreement with the experimental observation that the H2 adsorption enthalpy increases as the number of adsorbed H2 molecules increases. The Ti-H2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor π-acceptors, and when the ancillary ligand causes the least steric hindrance to the metal. Extension of the target systems to vanadium and chromium shows that, for molecules containing hydride ancillary ligands, a good relationship is found between the energies of the frontier molecular orbitals of the molecular fragments, which interact with incoming H2 molecules, and the strength of the M-H2 interaction. For the benzyl systems, both the differences in M-H2 interaction energies and the energy differences in frontier orbital energies are smaller than those in the hydrides, such that conclusions based on frontier orbital energies are less robust than for the hydride systems. Because of the high enthalpies predicted for organometallic fragments containing hydride ligands, and the low affinity of Cr(III) for hydrogen in this study, these features may not be ideal for a practical hydrogen storage system.

A First Principles Study on Dissociation and Adsorption Processes of H2 on Pd3Ag(111) Surface

We investigated dissociative adsorption of H2 molecule on Pd3Ag(111) surface based on the constructed potential energy surfaces (PESs) from the results of first principles calculations. This study is performed to understand H2 dissociative adsorption mechanism on Pd3Ag(111) surface which acts as permeable film for H2 which is a product of biomass gasification. The PES results indicate that when the H2 molecule approaches the Ag atom of the 1st atomic layer, the activation barriers for dissociation start to increase. The dissociation of H2 on the surface has negligible activation barrier when the H2 center of mass (CM) is directly above the bridge site of Pd atoms while the hydrogen atoms are directed towards the hcp and fcc hollow sites. The average local density of states (LDOS) of the d-orbital of surface Pd atoms show peak in the region around the Fermi level which is not observed from the LDOS of the Ag atom in Pd3Ag(111) surface. This strongly supports the results of the constructed PES for H2 dissociative adsorption mechanism towards Pd3Ag(111) surface. This study will be significant for the design of hydrogen-permeable films which has applications on biomass-operated fuel cells.

Effect of the benzyl groups on the binding ofH2by three-coordinated Ti complexes

Using first-principles calculations, we investigate the adsorption of H2 molecules on a three-coordinated benzyl-decorated titanium complex suggested in a recent experiment [Hamaed et al., J. Am. Chem. Soc. 130, 6992 (2008)]. Unlike the interpretation of the experimental results that the Ti(III) complex can bind five H2 molecules via the Kubas interaction, the Ti(III) complex cannot adsorb H2 molecules via the Kubas interaction. In contrast, a benzyl-released Ti(III) complex can adsorb up to two H2 molecules with a binding energy of ~0.25 eV/H2 via the Kubas interaction, in good agreement with the measurement of ~0.2 eV. The calculated occupation number of H2 molecules at 25 oC and -78 oC under 60 atm is 0.9 and 1.9, respectively, in good agreement with the measurement of 1.1 and 2.4 near the conditions, respectively. Our results suggest that the Ti complex in experiment might be a benzyl-released form.

A density functional theory study on the interaction of hydrogen molecule with MOF-177

The binding energies of H2 molecule with metal-organic framework MOF-177 clusters at various possible interaction sites have been calculated using density functional theory. The binding energy of adsorbed H2 molecule in MOF-177 was investigated, with the consideration of the favourable adsorption sites and the orientations at the inorganic cluster Zn4O and organic linker (1,3,5-benzenetribenzoate) in order to evaluate the role of these two principal components in MOF for H2 adsorption. Our results showed that both the inorganic connector and the organic linker play an important role in the H2 adsorption. The binding energy calculated for the inorganic cluster is 2.96–4.50 kJ mol− 1 and for the organic linker is 2.6–3.8 kJ mol− 1.

Li12Si60H60 Fullerene Composite: A Promising Hydrogen Storage Medium

By using the first-principles DFT calculations, we design a novel hydrogen storage material, Li(12)Si(60)H(60) composite, and validate its geometric stability. It is found that the adsorbed Li atoms do not cluster on the Si(60)H(60) fullerene unlike other metals such as Ti, owing to the relatively low Li-Li binding energy and the inhibition of Si-H bonds. Our results show that the Li-doping enhances the hydrogen adsorption ability of Si(60)H(60) significantly, owing to the charge transfer from the doped Li atoms to the host material and the polarization of the adsorbed H(2) molecules. By combining the first-principles calculation and grand canonical Monte Carlo simulation, we further investigate the hydrogen storage capacity of the simulation-synthesized exohedral Li(12)Si(60)H(60) composite at T = 77 K. As the vdW gap (i.e., the separation between the surfaces of two Li(12)Si(60)H(60) fullerenes) is equal to 8.2 A, the total hydrogen uptake of the square-arranged Li(12)Si(60)H(60) array reaches 12.83 wt % at p = 10 MPa, while the excess hydrogen uptake shows a maximum of 7.46 wt % at p = 6 MPa. Impressively, at T = 298 K and p = 10 MPa, the Li(12)Si(60)H(60) array still exhibits a total hydrogen uptake of 3.88 wt % at the vdW gap of 8.2 A. These results clearly indicate that the composite, Li(12)Si(60)H(60) fullerene, is a promising candidate for hydrogen storage.

The importance of charge–quadrupole interactions for H2 adsorption and diffusion in CuBTC

We have assessed the effects of charge–quadrupole interactions (CQIs) between the framework atoms of copper(II) benzene-1,3,5-tricarboxylate (CuBTC) and adsorbed H2 molecules on equilibrium adsorption properties and self- and transport diffusivities. We have also compared charges computed from periodic density functional theory (DFT) on the fully periodic CuBTC structure with charges derived from DFT calculations on cluster models of CuBTC. Our results indicate that carboxylate group atom charges computed from plane wave periodic DFT with the Bader charge analysis formalism are not consistent with the charges computed from the ChelpG method from Gaussian-based DFT cluster calculations. The charges derived from Bader analysis seem to be too large. Adsorption isotherms computed from Monte Carlo simulations and diffusivities computed from molecular dynamics simulations indicate that CQIs have a substantial impact on equilibrium and transport properties of H2 adsorbed in CuBTC at 77 K. Conversely, both adsorption isotherms and diffusivities were shown to be essentially unaffected by CQIs at 298 K.

Can Ti2−C2H4 Complex Adsorb H2 Molecules?

Ab initio molecular orbital calculations were used to examine the adsorption of H2 molecules on a Ti2−C2H4 complex. Dependence of the adsorption energy on the computational method was elucidated, and the importance of Gibbs free energy correction on the adsorption energy was clarified. However, van der Waals interaction did not play much role in the adsorption. Molecular dynamics simulations were performed to confirm whether the complex adsorbs H2 molecules at a finite temperature.

First Principles Calculation of CO and H<SUB>2</SUB> Adsorption on Strained Pt Surface

First principles calculations are carried out to analyze adsorption of CO and H2 molecules on a Pt (111) surface and the effect of surface strain on the adsorption energy. A CO molecule is more adsorptive on the Pt (111) surface than a H2 molecule under an ordinary condition. Surface expansion enhances CO poisoning on a Pt (111) surface. On the contrary, a compressive strain reduces adsorptive strength of a CO molecule. Similar tendency is also found in adsorption of a H2 molecule on the bridge, fcc-hollow, and hcp-hollow sites. However, H2 adsorption on the top site is less affected by the strain. As a consequence, the difference of adsorption energies between CO and H2 molecules becomes smaller when compressive strain is introduced into the Pt (111) surface. Based on thermodynamics, surface coverage ratio is quantitatively evaluated with taking into account the effect of surface strain and partial pressure of gas phase. It is revealed that compressive strain improves probability of H2 adsorption on Pt surface.

Fabrication of interfaces between carbon nanotubes and catalytic palladium using dielectrophoresis and its application to hydrogen gas sensor

The authors have developed a novel fabrication method of a carbon nanotube (CNT) gas sensor using positive dielectrophoresis (DEP). One advantage of the DEP technique is that one can readily build up interfaces between electrodes and CNT, or between CNT and other nanomaterials. In this paper, the DEP technique was employed to fabricate interfaces between CNT and catalytic palladium (Pd) to realize a CNT-based hydrogen (H2) gas sensor. Two types of CNT/Pd interfaces were proposed and tested. For one type, CNTs were trapped onto a microelectrode made of Pd so that the CNT/Pd interfaces were formed at both ends of CNTs lying over the Pd electrode surface. The other type of gas sensor was fabricated by simultaneously trapping CNTs and Pd nanoparticles under action of the positive DEP. It was found that both types of the CNT/Pd gas sensors could respond to hydrogen, while the CNT sensor without the Pd functionalization could not. For the hydrogen gas diluted with dry air, the electrical resistance of the CNT/Pd sensors increased at the moment of hydrogen exposure, and then turned to decrease in the later stage. The resistance increase just after the H2 exposure was probably due to reduction of CNTs by H atoms, which were produced by dissociative adsorption of H2 molecules on the catalytic Pd surface. In the later stage, the dissociated H atoms might react with dissociated oxygen atoms to create H2O molecules accompanying heat generation.