We present a systematic study on the possible mechanisms of hydrogen spillover onto several carbon-based materials using density functional theory (DFT). Adsorption and diffusion of atomic hydrogen on a graphene sheet, single-walled carbon nanotubes, and a polyaromatic compound, hexabenzocoronene, were calculated, and the potential energies along the selected adsorption and diffusion minimum energy pathways were mapped out. We show that the migration of H atoms from a Pt cluster catalyst to the substrates is facile at ambient conditions with a small energy barrier, although the process is slightly endothermic, and that the H atoms can be either physisorbed or chemisorbed on carbon surfaces. Our results indicate that diffusion of H atoms in a chemisorbed state is energetically difficult since it requires C-H bond breaking and hydrogen spillover would occur likely via physisorption of H atoms. The curvature of the carbon materials is found to have a pronounced influence on the mobility of H atoms. The role of the "bridge" materials used in experiments is also discussed.

The absorption, diffusion, and desorption of atomic hydrogen in layered orthorhombic molybdenum trioxide (α-MoO3) was investigated using density functional theory. Hydrogen atoms are absorbed in bulk α-MoO3 to form the hydrogen molybdenum bronze H x MoO3 (x = 0.25, 0.5, 0.75, 1, 1.25, and 1.5). The semiconductor band gap of bulk α-MoO3 shifts to metallic upon hydrogen bronze formation at the H atom loadings selected in the present study. The hydrogen atoms become protonic when coordinated to oxygen, which gives rise to a charge reduction on the Mo atoms adjacent to the absorption sites. Hydrogen migration along a prescribed diffusion pathway in the lattice was found to be facile due to small energy barriers for H atom transfer between O atoms, facilitated by a hydrogen bonding network. The sequential hydrogen desorption from the bronze and the mechanisms of hydrogen spillover in α-MoO3 are also discussed.

No abstract

Hydrogen spillover has emerged as a possible technique for achieving high-density hydrogen storage at near-ambient conditions in lightweight, solid-state materials. We present a brief review of our combined theoretical and experimental studies on hydrogen spillover mechanisms in solid-state materials where, for the first time, the complete mechanisms that dictate hydrogen spillover processes in transition metal oxides and nanostructured graphitic carbon-based materials have been revealed. The spillover process is broken into three primary steps: (1) dissociative chemisorption of gaseous H 2 on a transition metal catalyst; (2) migration of H atoms from the catalyst to the substrate and (3) diffusion of H atoms on substrate surfaces and/or in the bulk materials. In our theoretical studies, the platinum catalyst is modeled with a small Pt cluster and the catalytic activity of the cluster is examined at full H atom saturation to account for the essentially constant, high H 2 pressures used in experimental studies of hydrogen spillover. Subsequently, the energetic profiles associated with H atom migrations from the catalyst to the substrates and H atom diffusion in the substrates are mapped out by calculating the minimum energy pathways. It is observed that the spillover mechanisms for the transition metal oxides and graphitic carbon-based materials are very different. Hydrogen spillover in the transition metal oxides is moderated by massive, nascent hydrogen bonding networks in the crystalline lattice, while H atom diffusion on the nanostructured graphitic carbon materials is governed mostly by physisorption of H atoms. The effects of carbon material surface curvature on the hydrogen spillover as well as on hydrogen desorption dynamics are also discussed. The proposed hydrogen spillover mechanism in carbon-based materials is consistent with our experimental observations of the solid-state catalytic hydrogenation/dehydrogenation of coronene.

We present systematic molecular dynamics simulation studies of hydrogen storage in single walled carbon nanotubes of various diameters and chiralities using a recently developed curvature-dependent force field. Our main objective is to address the following fundamental issues: 1. For a given H 2 loading and nanotube type, what is the H 2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H 2 coverage)? 3. What is the diameter range and chirality for which H 2 adsorption is most energetically favorable? Our simulation results suggest strong dependence of H 2 adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H 2 adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H 2 loading. The calculated H 2 vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H 2 and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H 2 in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.

No abstract

While Ir(H)2(PtBu2Ph)2 + has been shown experimentally to have two agostic tBu groups, ab initio B3LYP calculations on IrH2[P(Et)H2]2 + show that the CH3 group of the phosphine ligand does not form any agostic bond with the strongly electron-deficient (14-valence electron) metal. In contrast, integrated molecular mechanics/molecular orbital (IMOMM) calculations on the full complex Ir(H)2(PtBu2Ph)2 + duplicate the experimentally observed agostic interaction. Thus, at least in this case, the agostic interaction is due in part to the trapping of a C−H bond in the vicinity of the metal by the steric effects of the other groups of the bulky phosphine. This necessity of steric “constraint” identifies an additional influential factor for the agostic interaction.

A general force field methodology is developed for description of molecular interactions in carbon-based materials. The method makes use of existing parameters of potential functions developed for sp(2) and sp(3) carbons and allows accurate representation of molecular forces in curved carbon environment. The potential parameters are explicitly curvature and site dependent. The proposed force field approach was used in molecular dynamics (MD) simulations for hydrogen adsorption in single-walled carbon nanotubes (SWNTs). The results reveal significant nanotube deformations and the calculated energies of adsorption are comparable to the reported experimental heat of adsorption for H2 in SWNTs.