Adsorption Of Molecular Hydrogen Research Articles

Density Functional Study of Molecular Hydrogen Adsorption on Small Gold–Copper Binary Clusters

Density functional theory calculations were performed to investigate the molecular adsorption behaviors of hydrogen on small bimetallic Au n Cu m clusters with n + m ≤ 5. H2 prefers to bind to a copper atom in Au n Cu m H2 complexes when both Au and Cu sites co-exist. The adsorption energies of Au n H2 are larger than Cu n H2 clusters with the same n and the adsorption energies of bimetallic cluster hydrides are between those of mono Au n H2 and Cu n H2. The adsorption of H2 on Au n Cu m can enhance the stability of the whole cluster. The vertical ionization potentials of the cluster hydrides generally decrease as the Cu content increases for the given cluster size. The H–H and M–H stretching frequencies (M = Au or Cu) are highly correlated to the atoms to which the adsorbate is attached. The reaction paths for H2 dissociation on AuCu, Au2Cu and AuCu2 clusters were also investigated and discussed.

Ti-decorated zigzag graphene nanoribbons for hydrogen storage. A van der Waals-corrected density-functional study

We perform density functional calculations to investigate the adsorption of molecular hydrogen on Ti-doped zigzag graphene nanoribbons using a nonlocal van der Waals functional that has recently been proposed for accurate description of exchange and correlation effects in weakly bound systems. Our results show that the adsorption of a single H2 molecule is dissociative in purely energetic terms, but there exists an energy barrier that prevents dissociation when the molecule is deposited on the Ti-doped graphene nanoribbon. When the Ti atom is adsorbed at a central or lateral hole site, each atom can bind up to four H2 molecules, in each case satisfying the binding energy criterion specified by the U.S. Department of Energy for novel hydrogen-storage materials. On this basis, one can consider an effective hydrogen coverage on Ti-coated graphene nanoribbons with gravimetric density beyond the target of 6 %.

Competition between molecular and dissociative adsorption of hydrogen on palladium clusters deposited on defective graphene

The contribution of Pd doping to enhance the hydrogen storage capacity of porous carbon materials is investigated.

Theoretical Study of Hydrogen Adsorption on Ru-Decorated (8,0) Single-Walled Carbon Nanotube

Hydrogen adsorption on Ru-decorated (8,0) zigzag single-walled carbon nanotube (SWCNT) was studied using density functional theory (DFT). Several decoration sites on the CNT surface were investigated before atomic or molecular hydrogen adsorption. The most stable location for a single Ru atom is above the hollow site, with an adsorption energy of Eads(Ru) = −2.133 eV. Ru decoration increases hydrogen adsorption energy nearly 46% compared to pristine CNT. When a hydrogen molecule is considered on Ru/SWCNT its adsorption is dissociative with an Eads(H2) = −0.697 eV. The Ru-decorated SWCNT systems exhibit magnetic properties. Density of states (DOS) and overlap population density of states (OPDOS) were computed in order to study the evolution of the chemical bonding. C–C bonds interact with Ru and are weakened after adsorption. Strong Ru–H bonds are formed during hydrogen adsorption process at expenses of C–Ru bonds. The mains interactions include the Ru 5pz and 4dz2 and C 2pz bands.

Core-shell Prussian blue analogue molecular magnet Mn(1.5)[Cr(CN)6]·mH2O@Ni(1.5)[Cr(CN)6]·nH2O for hydrogen storage.

Core-shell Prussian blue analogue molecular magnet Mn1.5[Cr(CN)6]·mH2O@Ni1.5[Cr(CN)6]·nH2O has been synthesized using a core of Mn1.5[Cr(CN)6]·7.5H2O, surrounded by a shell of Ni1.5[Cr(CN)6]·7.5H2O compound. A transmission electron microscopy (TEM) study confirms the core-shell nature of the nanoparticles with an average size of ∼25 nm. The core-shell nanoparticles are investigated by using x-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and elemental mapping, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and infrared (IR) spectroscopy. The Rietveld refinement of the XRD pattern reveals that the core-shell compound has a face-centered cubic crystal structure with space group Fm3m. The observation of characteristic absorption bands in the range of 2000-2300 cm(-1) in IR spectra corresponds to the CN stretching frequency of Mn(II)/Ni(II)-N≡C-Cr(III) sequence, confirming the formation of Prussian blue analogues. Hydrogen absorption isotherm measurements have been used to investigate the kinetics of molecular hydrogen adsorption into core-shell compounds of the Prussian blue analogue at low temperature conditions. Interestingly, the core-shell compound shows an enhancement in the hydrogen capacity (2.0 wt % at 123 K) as compared to bare-core and bare-shell compounds. The hydrogen adsorption capacity has been correlated with the specific surface area and TGA analysis of the core-shell compound. To the best of our knowledge, this is the first report on the hydrogen storage properties of core-shell Prussian blue analogue molecular magnet that could be useful for hydrogen storage applications.

Molecular hydrogen physisorption on boron-nitride nanotubes probed by second harmonic generation

We present ab initio calculations to investigate second harmonic generation (SHG) response of single wall zigzag pristine boron-nitride nanotubes (BNNTs) and BNNTs modified by the molecular hydrogen adsorption. Calculations have been performed using density functional theory (DFT) within the local-density approximation (LDA) together with the GW Green function method to determine the band gap. A length gauge approach has been used to calculate the nonlinear optical response with the scissors correction to obtain the nonlinear susceptibility ${\ensuremath{\chi}}^{zzz}(\ensuremath{-}2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega})$ of the zigzag BNNTs. We have found that, contrary to reports in the literature, the (5,0) and (9,0) BNNTs have a nonvanishing SHG response. We have also found that SHG intensity decreases with the increase of the molecular hydrogen coverage.

A first-principles study of alkali-metal-decorated graphyne as oxygen-tolerant hydrogen storage media

One serious problem encountered in hydrogen storage based on metal-decorated porous materials is the oxidation of metal atoms as it is irreversible due to strong oxygen-binding and blocks the adsorption of molecular hydrogen. We study the adsorption of molecular oxygen on graphyne decorated with alkali (AM) and alkali earth metals (AEM) using first-principles calculations. For comparison, we also calculate the adsorption characteristics of metal atoms and subsequent molecular oxygen in pristine and boron-doped graphene. We find that the binding energy of molecular oxygen on AM–graphyne complexes, especially Li–graphyne complex, is much smaller than that on AM in graphene or comparable to that in boron-doped graphene. We show that the binding strength of molecular oxygen is mainly affected by the center of empty p- or d-band of AM or AEM on adsorbents and by the work-function of metal-adsorbent complexes. We investigate the effect of biaxial tensile strain as a means of controlling the binding strength of molecular oxygen.

Hydrogen adsorption in metal-organic frameworks: the role of nuclear quantum effects.

The role of nuclear quantum effects on the adsorption of molecular hydrogen in metal-organic frameworks (MOFs) has been investigated on grounds of Grand-Canonical Quantized Liquid Density-Functional Theory (GC-QLDFT) calculations. For this purpose, we have carefully validated classical H2-host interaction potentials that are obtained by fitting Born-Oppenheimer ab initio reference data. The hydrogen adsorption has first been assessed classically using Liquid Density-Functional Theory and the Grand-Canonical Monte Carlo methods. The results have been compared against the semi-classical treatment of quantum effects by applying the Feynman-Hibbs correction to the Born-Oppenheimer-derived potentials, and by explicit treatment within the GC-QLDFT. The results are compared with experimental data and indicate pronounced quantum and possibly many-particle effects. After validation calculations have been carried out for IRMOF-1 (MOF-5), GC-QLDFT is applied to study the adsorption of H2 in a series of MOFs, including IRMOF-4, -6, -8, -9, -10, -12, -14, -16, -18, and MOF-177. Finally, we discuss the evolution of the H2 quantum fluid with increasing pressure and lowering temperature.

Hydrogen adsorption on M-ZSM-12 zeolite clusters (M = K, Na and Li): a density functional theory study

The molecular adsorption of hydrogen has been studied theoretically via DFT on additional framework with alkali metal atoms (K, Na and Li) in ZSM-12 zeolite. A 14T channel zeolite cluster model was used. Lewis acidity of alkali metals decreases with increasing atomic radius of alkali metal and H2 adsorption. Adsorption enthalpy values were computed to be −7.4 and −5.1 kJ/mol on Li- and Na-ZSM-12 clusters, respectively. Hydrogen adsorption enthalpy values for Li- and Na-cases are meaningfully larger than the liquefaction enthalpy of hydrogen molecule. This designates that Li- and Na-ZSM-12 zeolites are potential cryoadsorbent materials for hydrogen storage.

Unprecedented adsorption of molecular hydrogen in the porous hydride framework

"Recently the first porous hydride, gamma-Mg(BH4)2, featuring so-called ""borohydride framework"" and capable to store reversibly guest species was discovered [1]. This example clearly shows that the covalently bound hydride anions, such as borohydride, can act as directional ligands, capable to form molecular and polynuclear complexes, as well as framework structures typically occurring in classical coordination chemistry. Various small molecules are reversibly absorbed in gamma-Mg(BH4)2. In this work we show that molecular hydrogen and nitrogen have different adsorption sites in gamma-Mg(BH4)2, leading to different capacities on saturation and to different H2 and N2 BET areas. Only up to 0.66 N2 molecules are adsorbed per Mg atom, but the saturation capacity is double for the smaller hydrogen molecule. Moreover, at higher pressures, the second hydride phase forms with unprecedented hydrogen content of ~22 weight % (!). The density of hydrogen adsorbed into the pores is much higher than in liquid hydrogen, having no analogues among other porous systems. On the technical side, we will illustrate how in-situ diffraction at neutron and synchrotron sources allows to follow adsorption isobars, aiming for extraction of isosteric heats of adsorption directly from diffraction data, as well as for clarifying the microscopic mechanisms in terms of guest-host and guest-guest interactions."

Theoretical modelling of adsorption of hydrogen onto graphene, MOFs and other carbon-based substrates

Adsorption of molecular hydrogen () onto graphene and other carbon-based substrates is currently a research area of interest, where molecular-based approaches are required to describe thermodynamic properties of this and other related systems. We present a semiclassical theoretical framework to model adsorption isotherms of quantum fluids such as , based on the statistical associating fluid theory for chain molecules interacting with potentials of variable range for classical and quantum bulk fluids (SAFT-VRQ), and its extension to describe adsorbed systems (SAFT-VR-2D). Although the application of the theory relies on the determination of eight molecular parameters, seven of them can be obtained from bulk thermodynamic properties, the ratio of the critical temperatures of the adsorbed and bulk phases, and theoretical estimations about the range of the surface-particles potential and the energy depth of the particle–particle potential of the adsorbed fluid. The energy depth of the surface-particle potential, εw, is the free molecular parameter that can be obtained by fitting to experimental data of adsorption isotherms. Results obtained for εw according to this procedure are consistent with experimental values of the isosteric heat and the prediction of adsorption isotherms is in very good agreement with the experimental data.

ADSORPTION OF H2 ON FRAGMENTS OF MOF-210: A DFT INVESTIGATION

Molecular hydrogen adsorption on MOF-210 was evaluated at the density functional theory level. The most stable H 2 adsorption occurs near the acetenyls in the organic linker, but its binding energy (0.113 eV) is not sufficient to satisfy the minimum value (0.24 eV) required for practical applications. Meanwhile, Li cation-decorated MOF-210 has the average hydrogen adsorption energies of 0.28 eV, and its saturated hydrogen storage capacity reaches 5.35 wt.%.

Beyond carbon nanotubes: adsorptions on and electronic structures of silicon nanotubes.

In this paper, we have reviewed some of the recent theoretical studies on the electronic and structural properties of silicon nanotubes from single-walled to double-walled nanostructures, primarily focusing on the studies performed by the present authors. The studies so far have not indicated any metallic behavior in both single-walled and double-walled silicon nanotubes. Atomic and molecular adsorptions of elements including hydrogen, oxygen and alkali metals on single-walled silicon nanotubes are also reviewed and new results presented in detail. A systematic study of molecular adsorption and co-adsorptions of hydrogen and oxygen molecules in zigzag silicon nanotube (SiNT) has been performed using hybrid density functional theory. For adsorption of two hydrogen molecules in SiNT (10, 0), the original diatomic molecular structure was maintained after adsorption. The most preferred final site for hydrogen molecules is the on-top site. For adsorption of two oxygen molecules, the most preferred sites are bridge sites, which are the parallel or zigzag bridge sites. Complete dissociation, partial dissociation and non-dissociation were observed for adsorption of two oxygen molecules. Peroxide structure and Si-O-O structures have also been observed in adsorption of two oxygen molecules with smaller adsorption energies rather than complete dissociation. For the co-adsorption of one hydrogen molecule and one oxygen molecule, the hydrogen molecule is slightly polarized, and a suppression effect on HOMO-LUMO gap was observed.

Multi-scale theoretical investigation of molecular hydrogen adsorption over graphene: coronene as a case study

Multiscale modeling and simulation (MMS) combining B97-D/TZV2P DFT calculations and molecular dynamics simulations are performed to investigate the adsorption of hydrogen over coronene as a model of graphene.

Computational investigation of the adsorption of molecular hydrogen on the nitrogen-doped corannulene as a carbon nano-structure

The effect of the nitrogen substitution in corannulene as a carbon nanostructure on the molecular hydrogen adsorption was evaluated at MP2/6-31G(d) level of theory. Two orientations of the hydrogen were used on the concave and convex sides of corannulene. The average binding energy was calculated and corrected for the basis set superposition error (BSSE) using counterpoise method. Results showed that 4N-substituted, 2N-substituted and intact corannulene have the most adsorption energy, respectively. The increment of the binding energy was dependent on the number and position of the nitrogen atoms. Doping with N atoms leads to decrease the gap and the kinetic stability. The isomers with two N atoms in alternate bridge and rim positions and two N atoms in alternate hub position are the best and the worst for the hydrogen storage, respectively. For 4N substituted corannulene, the isomer with four N atoms in hub position is the best for hydrogen adsorption.

Density functional theory calculations of hydrogen molecule adsorption on monolayer molybdenum and tungsten disulfide

In this work, we report a theoretical study on the adsorption of molecular hydrogen (H2) on monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) using a first-principles van der Waals functional (vdW-DF) method. Our calculations show that the most favorable configuration for the adsorption of a H2 molecule on monolayer MoS2 is similar to that for the monolayer WS2 surface. The H2 molecule is physisorbed on the surface of the monolayers MoS2 and WS2 with adsorption energies of −131.61 and −169.44meV, respectively. Analysis of the electronic structures and charge confirmed that no significant hybridization between the respective orbitals occurs, and quantitative analysis revealed that a small interaction was obtained in terms of binding energies. Furthermore, adsorption of two H2 molecules on one and both sides of the monolayer MoS2 and WS2 was examined. The obtained results indicated that, with the adsorption of the second H2 molecule on one- and both-side of WS2, the binding energies were decreased to −155.2 and −153.6meV, respectively. However, the values for adsorption of the second H2 on monolayer MoS2 remained nearly constant compared to the adsorption of the first H2 molecule. In addition, we calculated the binding energy between the H2 molecule and the substrate (MoS2 and WS2) under axial strain of −10% to 10%, which revealed that the binding energy between the H2 molecule and the substrate was enhanced by axial strain. Our results also indicated that applying compression to monolayer WS2 yields higher reactivity with the hydrogen molecule than applying compression to monolayer MoS2.

FIRST-PRINCIPLES STUDY OF Ti-CATALYZED HYDROGEN ADSORPTION ON LiB (001) SURFACE

Ti -doped LiB (001) is a promising material for hydrogen storage. The adsorption of H 2 is greatly enhanced by doping Ti into LiB (001), change the electronic structures of the surface Li , B atoms. After H 2 is adsorbed on the surface, the E ad of the ( H 2)n@ Ti / LiB (001) system is considered. It is around -0.22 eV/ H 2 to -0.31 eV/ H 2, which is close to the target specified by U.S. Department of Energy. The nature of the bonding between Ti and H 2 is due to the H 1s, Ti 4s and B 2s orbital hybridization. In addition, Ti 3d orbital is hybridized strongly with B -2p orbital, resulting in more stable Ti / LiB (001) system. These results are verified by the electron density distribution intuitively. It is found that the system can adsorb up to four H 2 at ambient temperature and pressure. Therefore, the Ti -doped LiB (001) would be a promising hydrogen storage material. Such optimal molecular hydrogen adsorption system makes H 2 adsorption feasible at ambient conditions, which is critical for practical applications.

Volumetric apparatus for hydrogen adsorption and diffusion measurements: Sources of systematic error and impact of their experimental resolutions

The development of a volumetric apparatus (also known as a Sieverts' apparatus) for accurate and reliable hydrogen adsorption measurement is shown. The instrument minimizes the sources of systematic errors which are mainly due to inner volume calibration, stability and uniformity of the temperatures, precise evaluation of the skeletal volume of the measured samples, and thermodynamical properties of the gas species. A series of hardware and software solutions were designed and introduced in the apparatus, which we will indicate as f-PcT, in order to deal with these aspects. The results are represented in terms of an accurate evaluation of the equilibrium and dynamical characteristics of the molecular hydrogen adsorption on two well-known porous media. The contribution of each experimental solution to the error propagation of the adsorbed moles is assessed. The developed volumetric apparatus for gas storage capacity measurements allows an accurate evaluation over a 4 order-of-magnitude pressure range (from 1 kPa to 8 MPa) and in temperatures ranging between 77 K and 470 K. The acquired results are in good agreement with the values reported in the literature.

Adsorption and desorption of hydrogen on graphene with dimer conversion

We apply non-equilibrium thermodynamics to describe the adsorption and desorption of molecular hydrogen on graphene. Lateral interactions, precursor states in both adsorption and desorption, and limited dimer conversion are important to explain semi-quantitatively the main features of temperature-programmed desorption spectra. All energy and vibrational parameters are taken from density functional calculations. Deficiencies in previous attempts are discussed. We also point out the need for a multi-dimensional dynamic theory.

Diffusion, adsorption, and desorption of molecular hydrogen on graphene and in graphite

The diffusion of molecular hydrogen (H2) on a layer of graphene and in the interlayer space between the layers of graphite is studied using molecular dynamics computer simulations. The interatomic interactions were modeled by an Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. Molecular statics calculations of H2 on graphene indicate binding energies ranging from 41 meV to 54 meV and migration barriers ranging from 3 meV to 12 meV. The potential energy surface of an H2 molecule on graphene, with the full relaxations of molecular hydrogen and carbon atoms is calculated. Barriers for the formation of H2 through the Langmuir-Hinshelwood mechanism are calculated. Molecular dynamics calculations of mean square displacements and average surface lifetimes of H2 on graphene at various temperatures indicate a diffusion barrier of 9.8 meV and a desorption barrier of 28.7 meV. Similar calculations for the diffusion of H2 in the interlayer space between the graphite sheets indicate high and low temperature regimes for the diffusion with barriers of 51.2 meV and 11.5 meV. Our results are compared with those of first principles.