Christian Minot

Density functional theory analysis of the structural and electronic properties of TiO2 rutile and anatase polytypes: performances of different exchange-correlation functionals.

The two polymorphs of TiO2, rutile and anatase, have been investigated at the ab initio level using different Hamiltonians with all-electron Gaussian and projector augmented plane wave basis sets. Their equilibrium lattice parameters, relative stabilities, binding energies, and band structures have been evaluated. The calculations have been performed at the Hartree-Fock, density functional theory (DFT), and hybrid (B3LYP and PBE0) levels. As regards DFT, the local density and generalized gradient (PBE) approximations have been used. Our results show an excellent agreement with the experimental band structures and binding energies for the B3LYP and PBE0 functionals, while the best structural descriptions are obtained at the PBE0 level. However, no matter which Hamiltonian and method are used, anatase is found more stable than rutile, in contrast with recent experimental reports, although the relative stabilities of the two phases are very close to each other. Nevertheless, based on the overall results, the hybrid PBE0 functional appears as a good compromise to obtain an accurate description of both structural and electronic properties of solids.

A theoretical investigation of water adsorption on titanium dioxide surfaces

Water adsorption on various crystallographic faces of TiO2 (anatase and rutile) are calculated using a periodic Hartree-Fock method. Titanium oxide is an amphoteric compound. Water adsorbs on the acidic site, the titanium atom, and then dissociates to give hydroxyl groups. The adsorption energy is larger on the (110) face of the rutile structure than on other faces and is correlated with its very acidic sites. The OH groups are oriented to maximize hydrogen bonding. Hydrogen bonding is particularly important for molecular adsorption on the (100) face of the rutile structure; in this case, the molecular adsorption becomes competitive with the dissociative one. The thermodynamics of water adsorption strongly favor dissociation when singly-coordinated oxygen atoms are present on the surface as it is in a perfectly truncated anatase surface.

Reactivity of a reduced metal oxide surface: hydrogen, water and carbon monoxide adsorption on oxygen defective rutile TiO2(110)

The reactivity at reduced surface differs from that on the stoichiometric perfect surfaces. This does not originate uniquely from the modification of the coordination; electron count also is determining. The general trend is a decrease of the heat of adsorption on the metal cations. The reactivity decreases at sites in the vicinity of the defects due to the reduction induced by the O vacancies. At the defect site the decrease is less pronounced for H, H2, CO and molecular H2O. In the case of H2O dissociative adsorption, the defect site is more reactive than the perfect surface. Thus, a hydration converting the defective-reduced TiO2 to the hydrogenated non-defective-reduced surface is easy. The resulting structure possesses surface hydroxyl groups. It is probably the easiest way to form the hydrogenated non-defective surface. On TiO2, the defective surface requires very anhydrous conditions.

Molecular surface structure of ice(0001): dynamical low-energy electron diffraction, total-energy calculations and molecular dynamics simulations

Abstract A structural study of the surface of an ultrathin ice film grown on a Pt(111) substrate was performed using dynamical low-energy electron diffraction (LEED) at 90 K, together with total-energy calculations and molecular dynamics (MD) simulations. This ice film exhibits the common hexagonal phase Ih and exposes the (0001) surface without reconstruction. The surface is terminated as a full-bilayer that maximizes the number of surface hydrogen bonds as confirmed by our total-energy calculations. Both LEED and MD simulations find that the outermost water molecules have enhanced vibrational amplitudes making them practically undetectable by LEED even at 90 K. MD simulations of the half-bilayer terminated surface yield results inconsistent with the LEED findings, thus excluding this model.

Adsorption on perfect and reduced surfaces of metal oxides

Two major chemical processes, acidobasic and redox, track the adsorption mechanism on metal oxides. Molecular and dissociative adsorption on stoichiometric surfaces can be understood as acid–base processes. Clean and anhydrous surfaces of metal oxides have two different active sites: cations and anions. Electron-rich molecules or fragments arising from a heterolytic bond cleavage (Lewis bases) react with Mn+, while electron poor ones (Lewis acids) react with O2−. The MgO and TiO2 surfaces clearly appear to be predominantly acidic and molecules that do not dissociate generally bind to the metal cation. The electronic structure, insulating character for the stoichiometric surface, is preserved upon adsorption. When the initial system does not favor an energy gap (open-shell adsorbates, defective surfaces), the best adsorption mode is via a redox mechanism that restores the situation of an insulator and the highest oxidation states for all the atoms. For radical adsorption a first solution occurring on irreducible oxides is to couple the electrons and form two opposite ions adsorbed on the two surface sites, as for H2/MgO, involving an acid–base mechanism. Another possibility occurring on reducible oxide is via an electron transfer to or from the oxide (redox mechanism). The electron transfer occurs from the substrate to the adsorbate for electronegative group (Cl adsorption on O) or the other way around for an electropositive one (NO adsorption on M). The reactivity at surfaces deviating from stoichiometry differs from that on the perfect ones. The difference does not only originate from the modification of the coordination number but also from the electron counting.

Crystal studies, vibrational spectra and non-linear optical properties of L-histidine chloride monohydrate

This paper presents the results of our calculations on the geometric parameters, vibrational spectra and hyperpolarizability of a non-linear optical material L-histidine chloride monohydrate. Due to the lack of sufficiently precise information on geometric parameters available in literature, theoretical calculations were preceded by re-determination of the crystal X-ray structure. Single crystal of L-histidine chloride monohydrate has been growing by slow evaporation of an aqueous solution at room temperature. The compound crystallizes in the non-Centro-symmetric space group P2(1)2(1)2(1) of orthorhombic system. IR spectrum has been recorded in the range [400-4000 cm(-1)]. All the experimental vibrational bands have been discussed and assigned to normal mode or to combinations on the basis of our calculations. The optimized geometric bond lengths and bond angles obtained by using DFT//B3LYP/6-31G (d) method show a good agreement with the experimental data. The calculated vibrational spectra are in well agreement with the experimental one. To investigate microscopic second-order non-linear optical NLO behavior of the examined complex, the electric dipole mu, the polarizability alpha and the hyperpolarizability beta were computed using DFT//B3LYP/6-31G (d) method. The time-dependent density functional theory (TD-DFT) was employed to descript the molecular electron structure of the title compound using the B3LYP/6-31G (d) method. According to our calculations, L-histidine chloride monohydrate exhibits non-zero beta value revealing microscopic second-order NLO behavior.

Competitive hydrogenation of benzene and toluene: Theoretical study of their adsorption on ruthenium, rhodium, and palladium

Abstract Benzene and toluene adsorption on hexagonally close packed Pd, Rh, and Ru surfaces has been studied by semiempirical method. Simple MO arguments, supported by extended Mickel calculations, are used to show that toluene is more strongly adsorbed than benzene (the energy gap between the frontier orbitals is smaller and the bending of the methyl group removes the steric repulsion). The differences in the ease of adsorption on the different metals induce differences in the competitive hydrogenation reactions. These differences are increased by the presence of acceptor coadsorbates which induce a shift of the surface work function to large values.

A theoretical study of CO2 adsorption on TiO2

This paper presents a theoretical study of the interaction of CO2 with the rutile TiO2 surface. The calculations are performed using the periodic Hartree-Fock crystal program. Several modes for the adsorption are investigated. On the bare surface at θ = 12, the best adsorption mode is obtained when the CO2 molecule is vertically adsorbed over a titanium atom; at saturation, the CO2 molecules are tilted toward the surface; then, the adsorption mode is also controlled by the adsorbate-adsorbate interactions. Another adsorption mode, competitive with the first one, corresponds to a parallel CO2 molecule over two titanium atoms. The adsorption over an oxygen atom of the oxide is weak. On the hydrated surface, the presence of hydroxyl groups favors the CO2 adsorption and leads to the formation of adsorbed bicarbonate ions.

A theoretical analysis of NH3 adsorption on TiO2

Abstract From periodic Hartree-Fock calculations NH3 chemisorption is shown to occur on TiO2 without dissociation. It is bound as a base on the vacant titanium sites from the surface. The coadsorption with water (dissociated on the titanium oxide) is favorable since it allows the formation of hydrogen bonds. On the hydrated surface, when all the Ti(IV) sites are hydroxylated, NH3 binds to the proton of one hydroxyl group. This leads to a weaker adsorption mode. The formation of ammonium ions is very endothermic unless they are bonded by three hydrogen bonds to O(II) sites from the surface or oxygen atoms from the adsorbed hydroxyl groups.

Benzene adsorption on the Rh(111) metal surface: A theoretical study

Abstract The chemisorption of benzene on the Rh(111) surface is studied in the general framework of the extended Huckel theory, with optimized Rh parameters. MO arguments are used to discuss the main features of the electronic interaction of the aromatic system with the surface. Binding energy curves for adsorption on the most likely surface sites are computed, and the effect of the CH bond back-bending and of tilting of the benzene plane are examined. The most favorable chemisorption geometry is found when the C-atom ring is parallel to the surface and the center of the ring is above a threefold hollow site, but H atoms are farther away from the surface than C atoms. The optimum CRh distance compares well with those found in metal complex molecules. The on-top site is unfavorable for benzene chemisorption. The results of tight-binding calculations on infinite slabs agree with, and support, those of simple cluster calculations. A slight “Kekule distortion” and a small activation barrier to chemisorption are predicted.