María M. Branda

On the difficulties of present theoretical models to predict the oxidation state of atomic Au adsorbed on regular sites of CeO2(111).

The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO(2)(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a(0) and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO(2)(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference between the two states is typically within the limits of the accuracy of the present exchange-correlation potentials, and therefore, a clear lowest-energy state cannot be identified. These results suggest the possibility of a dynamic distribution of Au(0) and Au(+) atomic species at the regular sites of the CeO(2)(111) surface.

Methanol adsorption on magnesium oxide surface with defects: a DFT study

Abstract The methanol adsorption on several defects of the magnesium oxide surface were studied. Structural and electronic study with geometrical optimization and natural bond orbital (NBO) analysis were performed using a density functional theory (DFT) method. Oxygen and magnesium with different coordination numbers have very different reactivity in this surface producing dissociated and non-dissociated species. These results are in agreement with infrared spectroscopy observations where CH 3 OH, OCH 3 and OH species were found in defective MgO surfaces.

Role of step sites on water dissociation on stoichiometric ceria surfaces

The adsorption and dissociation of water on CeO2(111), CeO2(221), CeO2(331), and CeO2(110) has been studied by means of periodic density functional theory using slab models. The presence of step sites moderately affects the adsorption energy of the water molecule but in some cases as in CeO2(331) is able to change the sign of the energy reaction from endo- to exothermic which has important consequences for the catalytic activity of this surface. Finally, no stable molecular state has been found for water on CeO2(110) where the reaction products lead to a very stable hydroxylated surface which will rapidly become inactive.

Proton abstraction ability of MgO: a DFT cluster model study of the role of surface geometry

Methanol and isocyanic acid adsorptions on a defective MgO surface have been studied. Equilibrium geometries, adsorption energies, atomic and molecular charges and electronic densities were obtained using a density functional theory method. Oxide surface atoms with different coordination numbers show very different reactivity giving both molecular and dissociated adsorbed species. The methanol molecule requires lower coordination numbers of the active site than the isocyanic acid molecule for dissociative adsorption. The role of the acidic and basic sites has been considered and analyzed in terms of natural bond orbital charges. The main vibration frequencies have been compared with available infrared spectroscopic data.

DFT studies of zirconocene/MAO interaction

Abstract A theoretical study of Cp 2 ZrCl 2 /MAO system is presented based on Density Functional Theory (DFT) calculations performed with gaussian basis-sets. The MAO cocatalyst was modeled as a small finite section of the most probably species in solution. The geometry of the cation–counterion forming the ionic-pair active site was fully optimized and its electronic structure was examined in terms of the Natural Bond Orbital (NBO) population analysis. In this way the ionic-pair interaction can be related to the electronic charge changes around the zirconium cation. The results indicate the formation of a stable ionic-pair and the redistribution of charge in the cation complex and counterion. The latter was concluded by mapping the charge density difference. The Laplacian of charge density confirm the ionic interaction of the metallocene/MAO system.

The distribution of silanols on the amorphous silica surface: a Monte Carlo simulation

A study of the heterogeneity of surface silanols over partially dehydrated amorphous silica has been carried out by a Monte Carlo simulation. For this purpose the presence of (100) and (111) face patches on the surface is considered. Before the dehydration, the surface is composed only of single and geminal silanols. After this process begins, the surface sites are also vicinal and empty. Two silanols have a hydrogen bridge linking each other depending on the local geometric structure. The simulation shows that the distribution of the silanols over the silica surface is random at all the temperatures considered.

A molecular orbital model of dehydration reactions on an amorphous silica surface

A theoretical study at the molecular level is presented for the most important dehydration reactions taking place on amorphous silica surfaces. The influence of different degrees of rigidity has also been considered. Results are contrasted with previous theoretical data based on a Monte Carlo simulation for this kind of material.

Ab initio study of the isocyanate surface complex over silica and alumina

Abstract Ab initio calculations have been performed to study the stability of isocyanate complex (NCO) over silica and alumina surfaces. Mulliken and natural bond population analysis methods have been used in order to analyze charge distributions and the direction of charge transfer processes. The results indicate that the NCO group adsorbs linearly and perpendicularly to both surfaces; namely, on top site over silica and on bridge site over alumina. It was observed that the charge transfer from the oxygen lone pairs of NCO to the NC antibonds produces a weakening of NC link. This phenomenon is more important over alumina, yielding to an easier NCO decomposition over the surface of this oxide.