F. Marinelli

Confined water dissociation in microporous defective silicates: mechanism, dipole distribution, and impact on substrate properties.

Interest in microporous materials has risen in recent years, as they offer a confined environment that is optimal to enhance chemical reactions. Calcium silicate hydrate (C-S-H) gel, the main component of cement, presents a layered structure with sub-nanometer-size disordered pores filled with water and cations. The size of the pores and the hydrophilicity of the environment make C-S-H gel an excellent system to study the possibility of confined water reactions. To investigate it, we have performed molecular dynamics simulations using the ReaxFF force field. The results show that water does dissociate to form hydroxyl groups. We have analyzed the water dissociation mechanism, as well as the changes in the structure and water affinity of the C-S-H matrix and water polarization, comparing the results with the behavior of water in a defective zeolite. Finally, we establish a relationship between water dissociation in C-S-H gel and the increase of hardness due to a transformation from a two- to a three-dimensional structure.

Density functional theory investigation of H adsorption and H2 recombination on the basal plane and in the bulk of graphite: Connection between slab and cluster model

The scope of this work is the study of hydrogen atom interaction with the graphite surface taken as a model of the interactions that occur in the tokamaks (magnetic confinement fusion devices) between the carbon covered wall and the hydrogen ions (H+ or D+ or T+) coming out of the plasma. This study is performed at the atomic scale in the framework of the density functional theory. The graphite surface is modeled by the (0001) layer in either a periodic or a molecular approach. The clusters best reproducing the periodic two-dimensional results were selected to investigate hydrogen–graphite interaction. One- and two-layer clusters were used to model the basal plane and the bulk of graphite. It was found that hydrogen atoms could be bonded to the surface and in the bulk with an exothermic energy. The potential-energy barriers corresponding to the over crossing of the first surface layer by an atomic hydrogen have been determined. The H+H recombination (Eley–Rideal mechanism) was investigated on the surface ...

Adsorption, diffusion, and recombination of hydrogen on pure and boron-doped graphite surfaces

Boron inserted as impurity by substitution of carbon atoms in graphite is known to modify the reactivity of the surface in interaction with hydrogen. Boron induces a better H retention capability in graphite while it makes easier the recombination into molecular hydrogen under heating in thermal-desorption experimental conditions. It has already been calculated that boron modifies the electronic structure of the surface, which results in an increase of the adsorption energy for H. This result seems in good agreement with the better retention for H in doped graphite, but contradictory with the easier recombination observed. The aim of this work is to dismiss this contradiction by elucidating the modifications induced by boron in the recombination mechanism. We studied the diffusion of H on pure and boron-doped graphite in the density functional theory framework. We determined a diffusionlike mechanism leading to molecular hydrogen formation. Finally, we have shown the fundamental modifications induced by boron on the [0001] graphite surface reactivity. From these calculations it stands out that recombination is the result of desorption on pure graphite and diffusion on B-doped surfaces, while the activation energy for the rate limiting step is half reduced by boron. The results are compared to experimental observations. The connection between the cluster and periodic quantum modes for graphite is also discussed.

Thermal reactivity of HNCO with water ice: an infrared and theoretical study

Abstract The structure and energy of the 1:1 complexes between isocyanic acid (HNCO) and H 2 O are investigated using FTIR matrix isolation spectroscopy and quantum calculations at the MP2/6-31G(d,p) level. Calculations yield two stable complexes. The first and most stable one (Δ E =23.3 kJ/mol) corresponds a form which involves a hydrogen bond between the acid hydrogen of HNCO and the oxygen of water. The second form involves a hydrogen bond between the terminal oxygen of HNCO and hydrogen of water. In an argon matrix at 10 K, only the first form is observed. Adsorption on amorphous ice water at 10 K shows the formation of only one adsorption site between HNCO and ice. It is comparable to the complex observed in matrix and involves an interaction with the dangling oxygen site of ice. Modeling using computer code indicates the formation of polymeric structure on ice surface. Warming of HNCO, adsorbed on H 2 O ice film or co-deposited with H 2 O samples above 110 K, induces the formation of isocyanate ion (OCN − ) characterized by its ν as NCO infrared absorption band near 2170 cm −1 . OCN − can be produced by purely solvation-induced HNCO dissociative ionization. The transition state of this process is calculated 42 kJ/mol above the initial state, using the ONIOM model in B3LYP/6-31g(d,p).

Density functional theory investigation of H adsorption on the basal plane of boron-doped graphite

The scope of this paper is the theoretical study of hydrogen atom interaction with the boron-doped graphite surface taken as a model for the interactions that occur in controlled thermonuclear fusion devices. This work is carried out in the framework of the density functional theory. The boron-doped graphite surfaces are modeled using a small modified C16H10 cluster, in which one or two carbon atoms are substituted by boron. The efficiency of the C16H10 cluster in modeling the H-graphite interaction has already been established in a previous paper [J. Chem. Phys. 116, 8124 (2002)]. In this study, we show that the boron atom: (i) is not a stable adsorption site for H, that it induces (ii) an increase in the H binding energy, (iii) an increase in the permeability to H of the boron-doped graphite layer, and (iv) a long range electronic perturbation in its graphitic environment. A good agreement is found between our results and experimental studies dealing with erosion mechanisms of boron-doped graphite expos...

An ab initio study of acetone and formaldehyde monolayers adsorbed on ice

Abstract The periodic-Hartree–Fock method is used in order to determine adsorption sites of formaldehyde and acetone on ice surfaces. For both molecules, at monolayer coverage, one minimum energy configuration is found: it corresponds to hydrogen bonding of the CO group with a ice surface dangling OH. In the particular case of formaldehyde, a second stable configuration is obtained: this minimum energy is due to adsorbate–substrate electrostatic interactions. These results are in good agreement with experimental data on acetone. Tests are made to explore the role of defects on the ice surface.

Reactivity of HNCO with NH3 at low temperature monitored by FTIR spectroscopy: formation of NH4+OCN−

Abstract The reactivity of isocyanic acid (HNCO) with solid ammonia (NH3) was first studied at 10 K, using FTIR spectroscopy. The ammonium isocyanate (NH4+OCN−) is formed from a reaction between HNCO and NH3. Vibrational band assignments for NH4+OCN− have been given. On the other hand, when HNCO is adsorbed on amorphous NH3 film, the reaction does not occur. Warming up of this sample at 90 K induces the NH4+OCN− formation. Quantum calculations showed that the solvation of NH3 directly bonded to HNCO by at least three NH3 molecules plays a major role in the NH4+OCN− formation process and confirmed the spontaneous character of this reaction.

Strong physisorption site for H2 in K-and Li-doped porous carbons

Molecular hydrogen adsorption between two Li, K-doped coronene molecules (taken as local environment of carbon microporous materials) is studied by first-principles DFT-B3LYP calculations. These cluster calculations are complemented with periodic DFT-LDA/GGA calculations on extended Li- and K-doped structures. In all cases, energy minimization calculations unravel that there is a stable adsorption site for molecular hydrogen in these Li- and K-doped sp(2) carbon structures with large adsorption energies. This is the direct consequence of the significant charge transfer from the doping agents on neighboring slab carbon atoms, which allows the coupling of the molecular H(2) polarizability with the resulting substrate electric field (polarization interaction) that in turn induces the stabilization of molecular hydrogen. These calculations also give an insight on the atomic configurations of interlayer species (H(2) and LiK) as the interlayer spacing increases. It can be shown that large positional changes correlate with electronic properties of interlayer species. The confined hydrogen molecule does not show any tendency for dissociation and adopts a position in the interlayer void that is deeply related to that of doping ions.

Inelastic scattering of fast electrons from molecular systems. I. Hydrogen molecule

Abstract Inelastic scattering of fast electron from the electronic ground state of the H 2 molecule is investigated within the first Born approximation. The relevant transition properties have been obtained in some approximation corresponding so ??? order solutions of the equations-of-motion formation put forward by Rowe-Tamm-???approximation, random phase approximation. Generalized oscillator strengths and ??? of different symmetry are evaluated and discussed.

Evaluation of fourier transform of two-center charge distribution for arbitrary slater-type orbitals

Fourier transform of two-center charge distributions corresponding to arbitrary Slater-type orbitals are evaluated by a Gaussian quadrature procedure without any preliminary series expansion of the integrand. Convergence and accuracy of the method are discussed and illustrated.