Model Of Hydrogen Adsorption Research Articles

“Plug-and-Play” potentials: Investigating quantum effects in (H2)2–Li+–benzene

Quantum and anharmonic effects are investigated in (H2)2-Li(+)-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials, using rigid-body diffusion Monte Carlo (RBDMC) simulations. The potential-energy surface (PES) is calculated as a modified Shepard interpolation of M05-2X/6-311+G(2df,p) electronic structure data. The RBDMC simulations yield zero-point energies (ZPE) and probability density histograms that describe the ground-state nuclear wavefunction. Binding a second H2 molecule to the H2-Li(+)-benzene complex increases the ZPE of the system by 5.6 kJ mol(-1) to 17.6 kJ mol(-1). This ZPE is 42% of the total electronic binding energy of (H2)2-Li(+)-benzene and cannot be neglected. Our best estimate of the 0 K binding enthalpy of the second H2 to H2-Li(+)-benzene is 7.7 kJ mol(-1), compared to 12.4 kJ mol(-1) for the first H2 molecule. Anharmonicity is found to be even more important when a second (and subsequent) H2 molecule is adsorbed; use of harmonic ZPEs results in significant error in the 0 K binding enthalpy. Probability density histograms reveal that the two H2 molecules are found at larger distance from the Li(+) ion and are more confined in the θ coordinate than in H2-Li(+)-benzene. They also show that both H2 molecules are delocalized in the azimuthal coordinate, ϕ. That is, adding a second H2 molecule is insufficient to localize the wavefunction in ϕ. Two fragment-based (H2)2-Li(+)-benzene PESs are developed. These use a modified Shepard interpolation for the Li(+)-benzene and H2-Li(+)-benzene fragments, and either modified Shepard interpolation or a cubic spline to model the H2-H2 interaction. Because of the neglect of three-body H2, H2, Li(+) terms, both fragment PESs lead to overbinding of the second H2 molecule by 1.5 kJ mol(-1). Probability density histograms, however, indicate that the wavefunctions for the two H2 molecules are effectively identical on the "full" and fragment PESs. This suggests that the 1.5 kJ mol(-1) error is systematic over the regions of configuration space explored by our simulations. Notwithstanding this, modified Shepard interpolation of the weak H2-H2 interaction is problematic and we obtain more accurate results, at considerably lower computational cost, using a cubic spline interpolation. Indeed, the ZPE of the fragment-with-spline PES is identical, within error, to the ZPE of the full PES. This fragmentation scheme therefore provides an accurate and inexpensive method to study higher hydrogen loading in this and similar systems.

Quantum effects and anharmonicity in the H2-Li+-benzene complex: A model for hydrogen storage materials

Quantum and anharmonic effects are investigated in H2-Li(+)-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials. Three- and 8-dimensional quantum diffusion Monte Carlo (QDMC) and rigid-body diffusion Monte Carlo (RBDMC) simulations are performed on potential energy surfaces interpolated from electronic structure calculations at the M05-2X/6-31+G(d,p) and M05-2X/6-311+G(2df,p) levels of theory using a three-dimensional spline or a modified Shepard interpolation. These calculations investigate the intermolecular interactions in this system, with three- and 8-dimensional 0 K H2 binding enthalpy estimates, ΔH(bind) (0 K), being 16.5 kJ mol(-1) and 12.4 kJ mol(-1), respectively: 0.1 and 0.6 kJ mol(-1) higher than harmonic values. Zero-point energy effects are 35% of the value of ΔH(bind) (0 K) at M05-2X/6-311+G(2df,p) and cannot be neglected; uncorrected electronic binding energies overestimate ΔHbind (0 K) by at least 6 kJ mol(-1). Harmonic intermolecular binding enthalpies can be corrected by treating the H2 "helicopter" and "ferris wheel" rotations as free and hindered rotations, respectively. These simple corrections yield results within 2% of the 8-dimensional anharmonic calculations. Nuclear ground state probability density histograms obtained from the QDMC and RBDMC simulations indicate the H2 molecule is delocalized above the Li(+)-benzene system at 0 K.

The states of hydrogen adsorbed on the surface of skeleton nickel

The experimental data on hydrogen adsorption on skeleton nickel were used to construct the pK spectrum of atomic hydrogen on this surface. The pK spectrum obtained corresponded to the presence of three adsorption centers on the surface of skeleton nickel. A chemical model of hydrogen adsorption on the surface of skeleton nickel was suggested.

Behavior of Adsorbed Hydrogen Molecules on Zeolites

A general model for hydrogen adsorption was derived using Ono-Kondo lattice theory. The maximum monolayer adsorption capacities of hydrogen molecules on zeolites of NaX, CaA, NaA, and ZSM-5 were determined by fitting experimental adsorption data at different temperatures to the general model. The interaction potential between hydrogen molecules and pore surface atoms in the zeolite was calculated using gas-surface Virial coefficients and the Lennard-Jones (12-6) potential model for cylindrical pores. Results show that the general model can correctly describe supercritical experimental adsorption data of hydrogen on zeolites. Maximum monolayer adsorption capacities of hydrogen molecules on zeolites are dependent on the type of zeolite but independent of temperature. The adsorption interaction potential of the hydrogen-zeolite obtained from the above cylindrical pore model and gas-surface Virial coefficients agrees with a previously reported isosteric heat of hydrogen on zeolites. Our results indicate that the adsorption of hydrogen into zeolite pores is predominately caused by physisorption and a hydrogen-hydrogen attractive interaction.

Mechanism and Modeling of Hydrogen Adsorption, Absorption, and Evolution

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Model of atomic hydrogen adsorption on thin gold film surface

A model for hydrogen adsorption on surface of thin gold film was proposed on the basis of measured parameters: (i) initial heat of adsorption; (ii) activation energy for desorption at low coverages (θ ≊ 0.01); (iii) heat of hydrogen solution in the bulk of thin gold film; and (iv) calculated activation energy for dissociative H 2 adsorption.

Chemisorption properties of thin Pd overlayers on Nb(110) and Ag(111)

Abstract A simple model of hydrogen adsorption on a Pd covered Nb(110) substrate is presented. In agreement with recent experiments, it is found that for the commensurate bcc (110) phase, dissociative H 2 adsorption is not favoured in contrast to the incommensurate fcc (111) phase. The major reason for the dramatic reduction in surface reactivity for the bcc (110) phase, is due to the increase in Pd-Pd distances which results in a narrowing of the d band and also a lowering of the antibonding resonance made up of the H 1s orbital and the substrate d states, to an energy position where it is now occupied. A similar behaviour is expected for H adsorbed on a Pd/Ag(111) composite substrate, where at low Pd coverages (less than one monolayer) the expanded Pd layer is expected to act against the H 2 dissociation. Calculations of the one-electron energies due to the metal d states for H on Pd and Nb surfaces, indicate a larger chemisorption energy for Nb than Pd. The H-surface bond is thus more easily broken for a Pd covered Nb surface, than for a clean Nb surface, thereby resulting in a faster H penetration into the bulk of the composite substrate.