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.
Hydrogen spillover on the MoO3 (010) surface in the presence of a platinum catalyst was modeled using periodic density functional theory (DFT). The migration of H from a saturated Pt6 cluster to the ...MoO3 (010) surface was found to undergo a transition from repulsive electrostatic to attractive proton−oxygen interactions. The hydrogen is able to move nearly freely on the surface and diffuse into the bulk lattice at ambient temperatures, leading to the formation of hydrogen molybdenum bronze. We show that the high proton mobility is largely attributed to the massive H-bonding network in the MoO3 lattice.
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 H2 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 H2 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 H2 loading and nanotube type, what is the H2 distribution in the nanotube bundle? 2. For a given nanotube type, what is the maximal loading (H2 coverage)? 3. What is the diameter range and chirality for which H2 adsorption is most energetically favorable? Our simulation results suggest strong dependence of H2 adsorption energies on the nanotube diameter but less dependence on the chirality. Substantial lattice expansion upon H2 adsorption was found. The average adsorption energy increases with the lowering of nanotube diameter (higher curvature) and decreases with higher H2 loading. The calculated H2 vibrational power spectra and radial distribution functions indicate a strong attractive interaction between H2 and nanotube walls. The calculated diffusion coefficients are much higher than what has been reported for H2 in microporous materials such as zeolites, indicating that diffusivity does not present a problem for hydrogen storage in carbon nanotubes.
The dissociative chemisorption of dihydrogen on a Pt6 cluster with an octahedral configuration was studied with density functional theory. The on-top site was identified to be the most favorable ...chemisorption site for dihydrogen followed by the bridge site and the 3-fold hollow site. It was found that the dissociative chemisorption process is dictated by the charge transfer from the Pt cluster to H2 molecules via the overlap of 5d orbitals of Pt6 and 1σ* orbital of H2 and that the cluster achieves its full saturation with H atoms at the Pt/H ratio of 1:4. The effect of H coverage on the Pt cluster catalytic activities was discussed.
We investigate the dynamics of physisorbed atomic hydrogen on several carbon based materials (various fullerenes and a graphene sheet) using first principles molecular dynamics simulations. The ...physisorbed H atoms, generated upon H2 dissociative chemisorption on metal catalysts and interaction with carbonized “bridge” materials and substrates ( Chen L. ; et al. J. Phys. Chem. C 2007, 111, 18995 ), can diffuse freely on carbon surfaces with high mobility. Our results indicate that physisorption of H atoms is a metastable state and the atoms will readily recombine to form H2 molecules, which can be recycled to generate more H atoms, or attack the substrates to form C−H bonds. The strength of the resulting C−H bonds exhibits a strong dependency on the carbon material surface curvature. The implication of C−H bond strength on the dehydrogenation of hydrogenated carbon materials to form molecular H2 is discussed.
Bis(2-methoxyethyl)aminosulfur trifluoride, (CH3OCH2CH2)2NSF3 (Deoxo-Fluor reagent), is a new deoxofluorinating agent that is much more thermally stable than DAST (C2H5)2NSF3 and its congeners. It is ...effective for the conversion of alcohols to alkyl fluorides, aldehydes/ketones to the corresponding gem-difluorides, and carboxylic acids to the trifluoromethyl derivatives with, in some cases, superior performance compared to DAST. The enhanced stability is rationalized on the basis of conformational rigidity imposed by a coordination of the alkoxy groups with the electron-deficient sulfur atom of the trifluoride.
We present a combined theoretical and experimental study on H2 physisorption in partially fluorinated graphite. This material, first predicted computationally using ab initio molecular dynamics ...simulation and subsequently synthesized and characterized experimentally, represents a novel class of “acceptor type” graphite intercalated compounds that exhibit significantly higher isosteric heat of adsorption for H2 at near ambient temperatures than previously demonstrated for commonly available porous carbon-based materials. The unusually strong interaction arises from the semi-ionic nature of the C−F bonds. Although a high H2 storage capacity (>4 wt %) at room temperature is predicted not to be feasible due to the low heat of adsorption, enhanced storage properties can be envisaged by doping the graphitic host with appropriate species to promote higher levels of charge transfer from graphene to F− anions.
