Binding of hydrogen molecules by a transition-metal ion

Abstract

Using self-consistent-field molecular-orbital theory, we show that the interaction of hydrogen molecules with a Ni + ion is characteristically different from that with a neutral Ni atom. While hydrogen chemisorbs dissociatively on the neutral metal atom, it is bound to the cation in its molecular form. The atomic bonding is a consequence of the Pauli exclusion principle whereas the bonding of the molecular hydrogen results from an electrostatic interaction. We predict that a Ni + ion can bind at least six hydrogen molecules. PACS numbers: 3l.20.Ej, 35.20.Gs The interaction of hydrogen with metals and metal surfaces has been studied for many years. It is commonly understood that a H2 molecule breaks up into individual atoms at about 0.5 A from the metal surface and atomic chemisorption ensues [1]. No evidence exists, to our knowledge, of molecular chemisorption of hydrogen on the metal surface or inside the bulk metal. However, pairing of hydrogen mediated by a metal atom in rareearth systems [2] and molecular hydrogen in Si [3] have recently been observed.Little is known about the interaction of hydrogen with small metal particles consisting of a few atoms. The first evidence that hydrogen interaction with metal clusters may be fundamentally different from bulk came from recent experiments [4] on hydrogen reactivity and absorption. The reactivity was found to change by orders of magnitude by changing only a few atoms in the cluster. Equally interesting was the observation [5] that metal clusters might be able to absorb as many as eight hydrogen atoms per metal atom even though no corresponding bulk metal hydride exists. Not much is known regarding why clusters behave so differently towards hydrogen than bulk or whether hydrogen is bonded to the cluster in atomic or in molecular form.In this Letter we show that hydrogen interacts the same way with a neutral atom as it does with the atoms on surfaces and in the bulk. However, the interaction of hydrogen with a metal ion is fundamentally different. In the case of the neutral atom, an electron is transferred from the metal atom to the approaching H2 molecule and, in keeping with the Pauli exclusion principle, occupies the antibonding orbital of the molecule. This, in turn, breaks the molecular bond in favor of atomic bonding between the dissociated hydrogen atoms and the metal atom. However, when a H2 molecule approaches a transition-metal ion, it becomes energetically inefficient for the metal atom to donate an electron to H2 since the second ionization potential of the metal atom is rather high. Instead, the ion polarizes the H2 molecule and the bonding between the ion and the H2 molecule is governed by a dipole mechanism. We predict that a single Ni + ion can trap up to ten hydrogen molecules.Our results are based on first-principles calculations of

Combinatorial Search for Optimal Hydrogen-Storage Nanomaterials Based on Polymers

“…In cispolyacetylene, for example, they are 0.55, 0.58, 0.48, 0.42, and 0.46 eV/H 2 for l =1, 2, 3, 4, and 5, respectively. As pointed out in previous works, the adsorption of a large number of H 2 molecules presumably occurs through the Dewar-Chatt-Duncanson coordination or Kubas interaction [18,19,20]. We found the elongation of H 2 molecules by ∼10 % through electron back donation from metal d orbitals to the antibonding hydrogen s orbitals, which supports these theories.…”

Adsorption of hydrogen molecules on the platinum-doped boron nitride nanotubes

“…The charge transfer leaves the metal dopant in cationic form so that the hydrogen molecule can be trapped by the metal cation via the charge polarization mechanism. 23 Assuming the same mechanism is applicable to the case of BN nanotube, the decrease of the adsorption energy with increasing the number of H 2 can be understood: When more than one hydrogen molecule are adsorbed on the Pt-doped BN tube, there is effectively less charge transfer from the Pt-doped BN nanotube to every hydrogen molecules. Besides the charge-transfer mechanism, chemical interaction may also play an important role in the adsorption process due to the charge overlap.…”

“…More recently, hydrogen adsorption on metal-doped carbon nanotubes and fullerenes has been investigated. [19][20][21][22][23] The doping of SWCNT and fullerenes can generally promote more hydrogen uptake. The aim of this article is to assess to what extent the Pt-doped BN nanotubes can enhance the hydrogen uptake by using DFT methods.…”

Progress on first-principles-based materials design for hydrogen storage

“…In another view, it is interpreted as the electron donation from the H 2 σ-bonding orbital to empty metal 3d orbitals and the simultaneous back-donation from the filled metal d orbital to the H 2 σ-antibonding orbital. Upon coordination, the H-H bond length increases by approximately 10% of the free H 2 bond length (45,46), mainly owing to some occupation of the H 2 σ-antibonding orbital. Several H 2 molecules can cluster onto a single metal atom through this type of orbital interactions.…”