Ricardo M. Ferullo

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.

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 study on the interaction between atomic aluminum and graphene

In the present work, molecular orbital calculations using cluster models were performed within density functional theory (DFT) in order to study the adsorption of an Al atom on regular and defective graphene. Depending on the theoretical treatment of electronic exchange and correlations effects, different bonding results for the adsorption on the perfect surface are obtained. On the other hand, they are very similar for Al adsorbed on a carbon monovacancy. On regular graphene, the adsorption is exothermic when the Perdew, Burke and Ernzerhof (PBE) functional is used and endothermic with the Becke, 3-parameter, Lee–Yang–Parr (B3LYP) functional. Regarding the defective graphene surface, it was shown that the carbon atoms of concave angles in the vacancy are the most reactive to a radical attack. The adsorption of an Al atom on the vacancy restores the trigonal symmetry lost after the extraction of the C atom from regular graphene. Complementary calculations performed at PBE level on both regular and defective surfaces imposing periodic conditions qualitatively support the results obtained with the cluster model.

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.

Quantum chemical study on surface complex structures of phosphate on gibbsite

Quantum mechanics calculations based on the density functional theory (DFT) were used to identify phosphate surface complexes on gibbsite at low and high pH. The different phosphate species were represented using the Al₆(OH)₁₈(H₂O)₆ cluster model considering four different geometries: monodentate mononuclear (Pmm), monodentate binuclear (Pmb), bidentate mononuclear (Pbm) and bidentate binuclear (Pbb). The corresponding adsorption reactions were modelled via ligand exchange between phosphate species and surface functional groups (hydroxyls and protonated hydroxyls at high and low pH, respectively). The theoretical results indicate that phosphate surface complexes are thermodynamically more favored at acid pH, in agreement with experimental evidences. The first step in these reactions, i.e., the generation of required aluminum vacant sites, was predicted to be particularly favorable when singly coordinated aquo groups are released. Stretching and bending vibrational frequencies associated with the different surface structures were calculated at both pH conditions. The corresponding values at low pH were found to be shifted to higher frequencies with respect to those ones at high pH. ATR-FTIR studies were also carried out. The resulting spectra are dominated by a strong band within the 800-840 cm(-1) interval due to P-OH stretching modes. The corresponding peak appearing around 820 cm(-1) at high pH is shifted to lower frequencies with respect to the position at low pH, a tendency well predicted by DFT calculations.