D. H. Galván

Structure and catalytic properties of hexagonal molybdenum disulfide nanoplates

Molybdenum disulfide (MoS2) is a compound very useful for its properties; it is used as a lubricant, catalyst in hydrodesulfurization, in hydrogen fuel storage, etc. In this work MoS2 hexagonal nanoplates were synthesized at different temperatures and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). These nanoplates have a size of 25–35 nm as revealed by SEM. With aberration corrected STEM it was possible to measure the interatomic distance of Mo–Mo, which was found to be 2.8 A. The catalytic properties of the nanoplates were measured in the hydrodesulfurization of dibenzothiophene, showing high activity and high direct desulfurization pathway (DDS) selectivity for an unpromoted MoS2 catalyst. Theoretical calculations were performed in 2H-MoS2 as well as 2H-MoS2 with a rotation of 16° and 19° which was applied to the two S planes in the crystalline structure. The results obtained on the rotated 2H-MoS2 yielded indication of a small gap semiconductor of Eg = 0.38 eV when compared to the unrotated 2H-MoS2, which is a semiconductor of Eg = 2.00 eV. This tendency of the rotated crystalline 2H-MoS2 toward metallicity could be responsible for the enhancement of the catalytic properties observed in the material in question, compared to other MoS2-based catalysts.

STRUCTURE AND CATALYTIC ACTIVITY CHARACTERIZATION OF BISMUTH MOLYBDATE CATALYSTS

We present a study of Bi-Mo catalysts prepared from pure oxides (MoO3 and Bi2O3) by solid state reaction methods. The structure characterization by X-ray diffraction shows only the low temperature (koechlinite) and high temperature (γ(H)) phases in varying proportions depending on the calcination temperature. The carbon monoxide oxidation shows a synergetic effect for mixed oxides as compared to pure oxides. Significant differences in catalytic activity, surface morphology and surface concentration were observed when the molybdenum precursor was changed from molybdenum trioxide to ammonium heptamolybdate in the catalyst preparation.

Comparison Between γ-Bi2MoO6 and Bi2WO6 Catalysts in the CO Oxidation

Bismuth molybdate (γ-Bi2MoO6) and bismuth tungstate (Bi2WO6) catalysts were prepared by solid-state reaction and their catalytic properties evaluated in the CO oxidation reaction. We characterize their structure by X-ray diffraction, scanning electron microscopy, Auger electron spectroscopy, and BET nitrogen absorption. X-ray diffraction analysis shows that both γ-Bi2MoO6 and Bi2WO6 are structural analogs (SG P21ab). Auger analysis shows that Bi2WO6 catalysts have more bismuth on the surface than γ-Bi2MoO6, although both samples are bismuth deficient as compared to the stoichiometric compound. The results regarding catalytic activity show that Bi2WO6 prepared at 1073 K reaches total conversion of CO (100%) at a lower temperature when compared to γ-Bi2MoO6. It indicates that Bi2WO6 is a potential candidate to be used as catalyst in the CO to CO2 oxidation.

Experimental and Theoretical Studies of Microstructural Instabilities in WS2 Undergoing Electron Irradiation

Abstract In this article, we report the study of WS2 crystals after being irradiated with a high dose of electrons. A 2 MeV Van de Graaff acelerator was used to irradiate the sample at the following conditions: 1.3 MeV voltage, 5μA current, dose rate 25 kGy/min, total dosage 1000 kGy. The samples were examined in a high resolution transmission electron microscope. Three distinct structures were observed: onions layers with fullerene like structure, randomly oriented worm-like structures with their planes rotated with respect of each other by well-defined angles, and nanotube like structures. We present calculations showing that relative rotations of the sulphur layers by 2°, 4°, 5°, 8° and 16° basically have the same average energy and average energy/atom as the unrotated structure. Therefore, rotations of the layers through those angles will be favorable. Instabilities on the structure produced by the irradiation will induce rotations as well as other capricious structures resembling a knot formation.

Structural Changes and Electronic Properties of Gamma Irradiated Graphite: An Experimental and Theoretical Study

Abstract In this article, we report the study of commercial carbon soots after being irradiated with a high dose of gamma rays. An industrial gamma beam was used to irradiate the samples at 1 Mgy and a dose of 8.4 kGy/hr. Samples were examined in a high resolution electron microscope. Several structures were observed but the two most important were onion layers with fullerene like structure and graphitic like structure with rotational and traslational displacements. We present Extended Huckel calculations showing that relative rotation of the carbon layers by 10 and 12 degrees basically have the same total energy as the unrotated structure. Therefore, rotation of the layers through those angles will be favored. At the same time, we calculate cohesive energy (Ecoh), energy bands and projected density of states (PDOS) showing that as long as the rotation is induced, the system becomes metallic. At the same time, energy gaps are open in some bands, at certain points of the Brillouin zone, a clear indication ...

Microstructural study of samples irradiated with rays at high doses

We have investigated the damage induced by irradiation in silver added samples. The samples were prepared with 0 and 6.5 wt% of silver and irradiated by high-energy irradiation (50-150 Mrad). The roles of silver and dosage irradiation are discussed in terms of their effects on microstructure, crystallinity, critical temperature and zero-resistance temperature . After irradiation, decreased while the room-temperature electrical resistance increased by a factor of 8 for some of the samples. The difference in between irradiated and non-irradiated YBCO samples was of the order of 10 K. We have found that the difference is bigger for silver-added samples. We have also observed several changes in diffraction patterns of YBCO and YBCO-silver samples. SEM images, EDS and XPS analysis showed that silver resided inside the grains as single atoms and as metallic clusters. The relative concentrations of the elements in samples were quantified by Auger electron spectroscopy. The values showed a gradual increase for radiation doses ranging between 0 and 100 Mrad. For doses up to 100 Mrad, decreased because of the weak-link breakage induced by high doses of rays.

Experimental study of microstructure and critical current density of YBCO/Ag thick films under silver addition and electron irradiation

Thick polycrystalline films of silver doped YBCO were produced by the standard screen printing method and irradiated with a source of electrons at 1000 kGy. The critical temperature , the zero resistance temperature , the broadening of the superconductor transition and normal resistance, measured at 298 K, changed substantially in the irradiated sample when compared to the non-irradiated specimen. A detailed study using Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) on the cross sectional surface of the films revealed that silver was distributed in small amounts at the grain boundaries while some regions showed silver-rich-like clumps on the surface. In both cases, silver was found in a metallic state only. Such distribution was also confirmed by SEM images. We found by x-ray diffraction analysis important microstructural changes on the irradiated sample. The critical current density showed a sixfold increase after the sample was irradiated. The enhancing is directly associated with the increase in the flux pinning generated by the irradiation induced defects.

Formation of MoTe2Nanotubes by Electron Irradiation

Abstract In this work, we report the production of NbSe2, (niobium diselenide) nanotubes by high doses of electron irradiation. The apparatus used for irradiation was a 2 MeV Van de Graaff accelerator, while the irradiation conditions were the following: voltage 1.3 MeV, current 5μA, dose rate 25 kGy/min, and total dosage 1000 kGy. Samples were analyzed with a high-resolution transmission electron microscope. The main characteristics observed were huge and very well defined nanotubes of several nm long and few nm wide, which are presumably hollow, although they are capped at one end. At this level of irradiation, we were able to find neither onion-like structure nor nanoparticles through out other areas, as it is usual in similar hexagonal structures of the dichalcogenide family.

Catalytic activity and X-ray photoelectron spectroscopy performance of Bi2MoxW(1−x)O6 solid-solutions

The present paper describes the results of the study of solid solutions of Bi2MoxW(1−x)O6, which were prepared by the route of solid-state reaction and tested for catalytic activity in the oxidation of CO into CO2. X-Ray photoelectron spectroscopy (XPS) was conducted to obtain information on the chemical composition, and with the use of Bi2MoO6 and Bi2WO6 synthesized standards, quantitative measures were obtained. Catalytic studies of the samples show that they possess high activity, as a change in the temperature of activation as a function of the sintering temperature with the Mo/W ratio was detected. Bi–Mo mixed oxides have synergistic effects on the oxidation of CO, making possible the full conversion at lower temperatures than compounds that contain only single phases. The work described here demonstrates that a solid solution can be more efficient in oxidation, compared with single oxides studied separately.

WSe2 Nanotubes: Their Formation by Electron Irradiation

Abstract The discovery of multi-walled carbon nanotubes1 and single-walled carbon nanotubes2 has prompted numerous studies of the structure, properties3,4, and potential applications5,6of these materials. For example, nanotubes are expected to have a high strength-to-weight ratio6, which is advantageous in advanced composites to be used in high performance materials such as aircraft frames. The small dimensions of the tubes show promise for use as a gas absorption medium7, a field emitter for use in flat-panel displays8, and nanoscale electronic devices9.