Norberto J. Castellani

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.

Photoemission study of Pt adlayers on Ni(111)

Abstract The growth of Pt on Ni(111) was followed up to 1.7 monolayers by photoelectron spectroscopy. During the growth, the binding energy of the Pt 4f peaks remained almost constant and close to that of platinum bulk. This reveals that a high Pt-Ni interaction is attained at least in the submonolayer range. The direct consequence of this observation is that the Pt-Pt bond is weakened and that a distorted layer is formed. We show that this is consistent with results from other authors concerning Pt/Ni interfaces as well as Pt-Ni alloys.

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.

The location of a hydrogen atom and hydrogen molecules in BCC Fe: an ASED-MO approach

The semiempirical atom superposition and electron delocalization molecular orbital (ASED-MO) theory was used to study the H-Fe interaction. Calculations were carried out using clusters of the type Fe35 simulating both the adsorption and the absorption of an H atom on/in the (100) face and in the (100) dislocation. The results indicate that in general the H-Fe interaction is stronger for the dislocated structure and that several H atoms can be associated in the Fe void due to a strong indirect interaction mediated by the Fe matrix. The relative stability of this situation was compared with the H-H bond in vacuum.

A theoretical study of dehydrogenation of cyclopentene on pt(111)

Abstract The dehydrogenation of cyclopentene (c-C 5 H 8 ) to cyclopentadienyl (c-C 5 H 5 ) on Pt(111) is studied using a semiempirical molecular orbital method. Two possible mechanisms of reaction are considered. Results show that a sequential mechanism is favored over a simultaneous one. We also find a strong parallelism with previous theoretical results for c-C 6 H 12 dehydrogenation to benzene. A bonding mechanism dominated by electron donation from π orbitals of c-C 5 H 5 into metal orbitals is found. Similar results have been reported for benzene adsorbed on Pt(111).

C5Hn rings adsorbed on Pt(111): a theoretical study

Abstract The adsorption of cyclopentane, cyclopentene and cyclopentadienyl on Pt(111) has been studied using a semiempirical molecular orbital method. Results show an increasing molecule–surface interaction according to the ring insaturation. Cyclopentadienyl undergoes an extended rehybridization, with frequency vibrations similar to those of metallocenes.

The Co-adsorption of tetramethylpiperidine and TiCl4 on β-MgCl2. A theoretical study of a Ziegler-Natta pre-catalyst

Abstract An extended Huckel method (ICONC), which includes core-core repulsion energy terms, was used to characterise the MgCl 2 2,2,6,6-tetramethylpiperidine (TMPiP) TiCl 4 system by modelling the TMPiP and TiCl4 adsorption on the (100)–(101) and (110) faces of MgCl2. The results show how the amine modifies the chemical and electronic states of adsorbed TiCl4. The appearance of complexes at the surface, previously studied by infrared spectroscopy is supported by our calculation. The geometrical and structural factors governing the formation of precursors of active sites for propylene polymerisation were also studied.

Hydrogen molecule dissociation on PtNi(111) systems

Abstract In the present work we consider with a molecular orbital model the H2 dissociation mechanism on a Pt layer grown on Ni(111), and comparatively, on the pure metal components. The metallic substrate was represented by a four layers cluster. In the bimetallic case, the overall adiabatic energy profile is less stable in comparison with Pt(111). All these results are in good agreement with experimental evidence and can be explained using electronic structure arguments.

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.