Oscar Olvera-Neria

Nucleation mechanisms of aromatic polyesters, PET, PBT, and PEN, on single-wall carbon nanotubes: early nucleation stages

Nucleation mechanisms of poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT), and poly (ethylene naphthalate) (PEN) on single-wall carbon nanotubes (SWNTs) are proposed, based on experimental evidence, theoretical epitaxy analysis, and semiempirical quantum chemical calculations. In order to elucidate early nucleation stages polyester-coated nanotubes were obtained from highly diluted solutions. High-resolution transmission electron microscopy (HRTEM) revealed helical morphologies for PET/SWNTs and PEN/SWNTs and the formation of lobules with different orientations for PBT/SWNTs. To explain the morphological behavior one model was proposed based on crystallographic interactions, that is, epitaxy. Theoretical epitaxy calculations indicated that epitaxy is not possible from the strict epitaxy point of view. Instead, aromatic self-assembly mechanism was proposed based on p-p interactions and the chirality of the nanotube. It was proposed that themechanism implies two steps to produce helical or lobular morphologies with different orientations. In the first step polymer chains were approached, aligned parallel to the nanotube axis and adsorbed due to electrostatic interactions and the flexibility of the molecule. However, due to p-p interactions between the aromatic rings of the polymer and the nanotube, in the second step chains reoriented on the nanotube surface depending on the chirality of the nanotube. The mechanism was supported by semi-empirical calculations.

Density Functional Theory and Electrochemical Studies: Structure–Efficiency Relationship on Corrosion Inhibition

The relationship between structure and corrosion inhibition of a series of 30 imidazol, benzimidazol, and pyridine derivatives has been established through the investigation of quantum descriptors calculated with PBE/6-311++G**. A quantitative structure-property relationship model was obtained by examination of these descriptors using a genetic functional approximation method based on a multiple linear regression analysis. Our results indicate that the efficiency of corrosion inhibitors is strongly associated with aromaticity, electron donor ability, and molecular volume descriptors. In order to calibrate and validate the proposed model, we performed electrochemical impedance spectroscopy (EIS) studies on imidazole, 2-methylimidazole, benzimidazole, 2-chloromethylbenzimidazole, pyridine, and 2-aminopyridine compounds. The experimental values for efficiency of corrosion inhibition are in good agreement with the estimated values obtained by our model, thus confirming that our approach represents a promising and suitable tool to predict the inhibition of corrosion attributes of nitrogen containing heterocyclic compounds. The adsorption behavior of imidazole or benzimidazole heterocyclic molecules on the Fe(110) surface was also studied to elucidate the inhibition mechanism; the aromaticity played an important role in the adsorbate-surface complex.

Effect of Spin Multiplicity in O2 Adsorption and Dissociation on Small Bimetallic AuAg Clusters

To dispose of atomic oxygen, it is necessary the O2 activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O2 adsorption and dissociation on small pure Aun and Agm and bimetallic AunAgm (n + m ≤ 6) clusters using the density functional theory (DFT) and the zeroth-order regular approximation (ZORA) to explicitly include scalar relativistic effects. The most stable AunAgm clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O2 adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O2 is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O2 occupies the σ*O-O antibonding natural bond orbital, which weakens the oxygen bond. The Au3 (2A) cluster presents the lowest activation energy to dissociate O2, whereas the opposite applies to the AuAg (3A) system. In the O2 activation, bimetallic clusters are not as active as pure Aun clusters due to the charge donated by Ag atoms being shared between O2 and Au atoms.

C-, N-, S-, and F-Doped Anatase TiO2 (101) with Oxygen Vacancies: Photocatalysts Active in the Visible Region

Anatase TiO2 presents a large bandgap of 3.2 eV, which inhibits the use of visible light radiation (  > 387 nm) for generating charge carriers. We studied the activation of TiO2 (101) anatase with visible light by doping with C, N, S, and F atoms. For this purpose, density functional theory and the Hubbard approach are used. We identify two ways for activating the TiO2 with visible light. The first mechanism is broadening the valence or conduction band; for example, in the S-doped TiO2 (101) system, the valence band is broadened. A similar process can occur in the conduction band when the undercoordinated Ti atoms are exposed on the TiO2 (101) surface. The second mechanism, and more efficient for activating the anatase, is to generate localized states in the gap: N-doping creates localized empty states in the bandgap. For C-doping, the surface TiO2 (101) presents a “cleaner” gap than the bulk TiO2, resulting in fewer recombination centers. The dopant valence electrons determine the number and position of the localized states in the bandgap. The formation of charge carriers with visible light is highly favored by the oxygen vacancies on TiO2 (101). The catalytic activity of C-doping using visible radiation can be explained by its high absorption intensity generated by oxygen vacancies on the surface. The intensity of the visible absorption spectrum of doped TiO2 (101) follows the order: C > N > F > S dopant.

The CO oxidation mechanism on small Pd clusters. A theoretical study.

AbstractCO is a pollutant that is removed by oxidation using Pd, Pt or Rh as catalysts in the exhaust pipes of vehicles. Here, a quantum chemistry study on the CO + O2 reaction catalyzed by small Pdn clusters (n ≤ 5) using the PBE/TZ2P/ZORA method is performed. The limiting step in this reaction at low temperature and coverage is the O2 dissociation. Pdn clusters catalyze the O=O bond breaking, reducing the energy barrier from 119 kcal mol-1 without catalyst to ∼35 kcal mol-1. The charge transfer from Pd to the O2,ad antibonding orbital weakens, and finally breaks the O─O bond. The CO oxidation takes place by the Eley-Rideal (ER) mechanism or the Langmuir-Hinshelwood (LH) mechanism. The ER mechanism presents an energy barrier of 4.10-7.05 kcal mol-1 and the formed CO2 is released after the reaction. The LH mechanism also shows barrier energies to produce CO2 (7-15 kcal mol-1) but it remains adsorbed on Pd clusters. An additional energy (7-25 kcal mol-1) is necessary to desorb CO2 and release the metal site. The triplet multiplicity is the ground states of studied Pdn clusters, with the following order of stability: triplet > singlet > quintet state. Graphical AbstractCO oxidation mechanism on small Pd clusters