Eugenio Furtado de Souza

The molecular basis for the behaviour of niobia species in oxidation reaction probed by theoretical calculations and experimental techniques

This paper describes the preparation and use of a new class of materials based on synthetic niobia as catalysts in the oxidation of organic compounds in aqueous medium. The chemical reactions were carried out in the presence of hydrogen peroxide (H2O2). The material was characterized with X-ray diffraction, XPS and H2-TPR (temperature-programmed reduction) measurements. The organic molecule methylene-blue was used in the decomposition study as a probe contaminant. The analysis using the ESI-MS technique showed complete oxidation observed through different intermediates. This suggests the use of niobia species as an efficient Fenton-like catalyst in degradation reactions. Theoretical quantum DFT calculations were carried out in order to understand the degradation mechanism.

99Tc NMR as a promising technique for structural investigation of biomolecules: theoretical studies on the solvent and thermal effects of phenylbenzothiazole complex

The phenylbenzothiazole compounds show antitumor properties and are highly selective. In this paper, the 99Tc chemical shifts based on the (99mTc)(CO)3(NNO) complex conjugated to the antitumor agent 2‐(4′‐aminophenyl)benzothiazole are reported. Thermal and solvent effects were studied computationally by quantum‐chemical methods, using the density functional theory (DFT) (DFT level BPW91/aug‐cc‐pVTZ for the Tc and BPW91/IGLO‐II for the other atoms) to compute the NMR parameters for the complex. We have calculated the 99Tc NMR chemical shifts of the complex in gas phase and solution using different solvation models (polarizable continuum model and explicit solvation). To evaluate the thermal effect, molecular dynamics simulations were carried, using the atom‐centered density matrix propagation method at the DFT level (BP86/LanL2dz). The results highlight that the 99Tc NMR spectroscopy can be a promising technique for structural investigation of biomolecules, at the molecular level, in different environments. Copyright

Adsorption and desulfurization reaction mechanism of thiophene and its hydrogenated derivatives over NbC(001) and NbN(001): an ab initio DFT study

Herein, we present periodic DFT-based calculations on the thiophene adsorption and reaction pathways over niobium carbide and nitride cubic face-centered (001) surfaces by considering both direct (DDS) and hydrogenating (HYD) routes for further desulfurization reactions and evaluate the implications for the mechanism of hydrodesulfurization (HDS). The theoretical studies were based on ultrasoft pseudopotentials and a plane-wave basis set and were performed with the help of the Quantum-ESPRESSO package. To understand the roles of both surfaces in the adsorption and desulfurization processes, various starting configurations for the adsorbed thiophene were tested and the energetically most stable ones were used in bond breaking studies. It was observed that thiophene adsorbs preferentially in a η-5 configuration by interacting with the nitride surface through its π-structure, whereas, on the carbide surface, thiophene was found to prefer a tilted η-1 configuration. Based on nudged elastic band (NEB) studies, our results suggest that the ring hydrogenation does not necessarily lead to a preference for the HYD pathway of thiophene desulfurization. Furthermore, surface and electronic effects were also evaluated. We have found that under ideal conditions the niobium nitride surface should present better performance for the desulfurization of thiophene than the carbide surface.

Combined DFT and experimental study of the dispersion and interaction of copper species in Ni-CeO2 nanosized solid solutions

Nanosized nickel (Ni) and copper (Cu) doped ceria (CeO2) have attracted attention as solid solutions for energy- and environment-related applications. Furthermore they present an interesting combination of thermal and chemical stability and catalytic activity in technologically important reactions like water gas shift, ethanol reforming, hydrogenation, among others. In contrast, not much is known about the key-factors that govern the formation and the nature of the atomic structure of these materials. This study investigated with the help of the density functional theory (DFT) and experimental methodologies the formation of ceria-based solid solutions in the presence of Ni and Cu species. The materials were prepared by the incipient wetness impregnation and subsequently characterized by various experimental techniques (XPS, Raman, XRD, XRF, HR-TEM), while the electronic structures have been investigated by using DFT calculations with Hubbard corrections (DFT+U method). Theoretical calculations and experimental studies suggest that Ni species are able to form a solid solution by isomorphic substitution of bulk Ce atoms, however it was found a limit after which saturation is reached and therefore the addition of extra Ni atoms do not affect the crystal structure of the solid solution. Consequently, the formation of surface domains of nickel oxide (NiO) phases is expected. According to our findings the addition of small amounts of Cu can neither disturb the bulk structure nor force the incorporation of Cu atoms and therefore Cu species are also expected to segregated oxide (CuO). Our theoretical approach is consistent with the experimental data and we could identify an idealized solid solution structure that presents a close similarity with the experimental findings. From this theoretical structure, an interaction of the type Ni–Ni pair was identified. Our theoretical studies have predicted lattice contraction as a function of the Ni loading. From an energetic point of view we show that small amounts of Ni are easily incorporated whereas by raising Ni concentration and by adding Cu a sharp increase of the formation energy is observed. High formation energies along with strong lattice contractions was associated as plausible causes for the segregation of both Ni and Cu oxides and have been suggested as simple indicators of key factors for tailoring doped oxides containing controlled dopant concentrations.