A. Allouche

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 ...

Quantum study of the active sites of the γ-alumina surface: chemisorption and adsorption of water, hydrogen sulfide and carbon monoxide on aluminum and oxygen sites

Abstract In this work, we have performed quantum calculations, using cluster models, in order to study the reactivity of the aluminum and oxygen sites of the γ-alumina surface. Water, hydrogen sulfide and carbon monoxide molecules are employed as the test molecules for our cluster models. It is found that the tricoordinated aluminum sites of the surface are the dissociative sites for the water and the hydrogen sulfide molecules. The tetracoordinated aluminum sites are the non-dissociative adsorption sites for the water molecule, whereas they are the dissociative sites for the hydrogen sulfide molecule. The pentacoordinated aluminum sites are the non-dissociative sites for the water and the hydrogen sulfide molecules. On the aluminum sites, carbon monoxide is preferentially adsorbed via its carbon atom. In contrast to the aluminum sites, the oxygen sites of the alumina surface are not adsorption sites for water, hydrogen sulfide and carbon monoxide molecules.

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.

Sodium hydroxide formation in water clusters: The role of hydrated electrons and the influence of electric field

The stability, structure and reactivity of Na(H2O)7, Na2(H2O)7, and Na2(H2O)10 clusters have been investigated by means of the density functional theory (DFT) method. In all cases, the 3s Na electrons are located far from their nuclei and hydrated. Particular emphasis has been placed on the influence of the water-generated electric field on sodium dimer polarization. The metal atoms hydrolysis reaction has been studied for the lone sodium atom as well as for the sodium pair; the calculated activation energies are found to be very similar in terms of magnitude. Reaction mechanisms are proposed that exhibit the role of the hydrogen bond cooperative effects in combination with proton tunneling.

Hydrogenation and dehydrogenation of graphite (0001) surface: a density functional theory study

The adsorption of hydrogen atoms on the (0001) basal plane of graphite is studied using periodic first principle density functional formalism. A model structure for the hydrogen-saturated surface is proposed according to the periodic boundary conditions. Quantum molecular dynamics using the Nos??Hoover thermostat is carried on to study the saturated surface dehydrogenation by atomic oxygen at a temperature of 300?K.

Electron solvation by highly polar molecules: Density functional theory study of atomic sodium interaction with water, ammonia, and methanol

This study further extends the scope of a previous paper [Y. Ferro and A. Allouche, J. Chem. Phys. 118, 10461 (2003)] on the reactivity of atomic Na with water to some other highly polar molecules known for their solvation properties connected to efficient hydrogen bonding. The solvation mechanisms of ammonia and methanol are compared to the hydration mechanism. It is shown that in the case of ammonia, the stability of the solvated system is only ensured by electrostatic interactions, whereas the methanol action is more similar to that of water. More specific attention is given to the solvation process of the valence 3s Na electron. The consequences on the chemical reactivity are analyzed: Whereas ammonia is nonreactive when interacting with atomic sodium, two chemical reactions are proposed for methanol. The first process is dehydrogenation and yields methoxy species and hydrogen. The other one is dehydration and the final products are methoxy species, but also methyl radical and water. The respective roles of electron solvation and hydrogen bonds network are analyzed in detail in view of the density of states of the reactive systems.