CRYSTALLITE MORPHOLOGY IN GOLD CATALYSTS OBTAINED BY RIETVELD REFINEMENT

ABSTRACT The microstructure of gold catalysts supported on rutile was obtained by Rietveld refinement. Three-dimensional images of the crystallites were generated in real space, as well as 3-D images of the microstrain distribution in reciprocal space. These images were used to analyze the effect of the different parameters of the refinement models on the interpretation of the microstructure of the samples. The 3-D representations can be used to obtain the crystallite volume, mass, number of atoms, and specific area of the crystallites, which can be compared with the BET specific surface area of the sample measured experimentally

Distribución atómica y morfología de las micelas de soles precursores de boehmita, bismutinita, hidrotalcita y rutilo

Se prepararon soles precursores de boehmita, bismutinita, hidrotalcita y rutilo. Las micelas de estos soles se analizaron mediante difracción de rayos X y microscopía electrónica de transmisión. En todos los casos las micelas fueron nanocápsulas con diámetros entre 20 y 100 nm y paredes entre 3 y 4 nm de espesor, con sus átomos formando cúmulos arreglados sin simetría traslacional. Cuando las micelas interactúan entre sí, en la región en contacto, los átomos, inicialmente ordenados en cúmulos, se reordenan formando la estructura cristalina de la fase de la cual el sol es precursor. Esta interacción entre micelas produjo su agregación para dar origen a estructuras tridimensionales porosas o a estructuras unidimensionales, precursoras de nanotubos y nanobarras, o de redes tridimensionales que finalmente pueden generar un gel

Dye-Sensitized Solar Cells Prepared with Mexican Pre-Hispanic Dyes

A dye-sensitized solar cell (DSSC) is a photovoltaic device capable of generating electrical power from the absorption of solar radiation. These cells use a SnO2 : F/TiO2/dye heterojunction as the active electrode (working electrode). Active electrodes containing TiO2 in the anatase crystalline phase and synthetic dyes are used to achieve high conversion efficiencies. Synthetic dyes, whether organic or organometallic compounds, have the disadvantage of being expensive. For this reason, many efforts are made worldwide to find natural dyes with lower production costs that can be used in the fabrication of DSSCs. Nocheztli is a natural red dye obtained from the cochineal insect Dactylopius coccus; the dye dates from pre-Hispanic times and contains high levels of carminic acid (CA). Nocheztli has been used in Mexico in textile dyeing from pre-Hispanic times to the present. Carmine is an organometallic dye with two molecules of carminic acid and one atom of aluminum in its structure; it is obtained by the interaction of the carminic acid from Nocheztli with aluminum salts. Carminic acid and carmine molecules contain a carboxyl group in their structure, allowing them to anchor to TiO2, creating a suitable heterojunction to prepare DSSCs. In this study, both dyes are used to sensitize the mesoporous TiO2m semiconductor to prepare DSSCs

Pressure-Induced Metallization of Li⁺‑Doped Hydrogen Clusters

Endohedrally encapsulated hydrogen clusters doped with inert helium (H₂₄He) and ionic lithium (H₂₄Li⁺) are investigated. The confinement model is a nanoscopic analogue of the experimental compression of solid hydrogen. The structural and electronic properties of the doped hydrogen clusters are determined under the effects of pressure. The results are compared with these of the isoelectronic (pure) hydrogen counterpart H₂₆ under similar physical conditions. Pressure increase rates with respect to H₂₆ of approximately 1.1 are observed with the insertion of helium or lithium. The changes of geometrical structures and HOMO–LUMO gap energies with the pressure point out the pressure-induced metallization of the Li⁺-doped cluster. The computations are done using density functional theory in the form implemented for molecules; they include zero-point energy effects and, to our best knowledge, are the first of their kind

Dibenzothiophene hydrodesulfurization with NiMo and CoMo catalysts supported on niobium-modified MCM-41

In the present work, we studied the effect of the addition of niobium to CoMo/MCM-41 and NiMo/MCM- 41 catalysts on their characteristics and performance in the hydrodesulfurization of dibenzothiophene. The MCM-41 support was synthesized at room temperature according to a well-known procedure. The metal species (Nb, Mo, Ni or Co) were deposited by successive impregnation using aqueous solutions of the corresponding metal salts. MCM-41 and Nb-containing MCM-41 supports were characterized by X-ray diffraction, N2 physisorption, FT-Raman spectroscopy, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy and scanning electron microscopy. The CoMo and NiMo catalysts in oxide state, in addition to the above techniques, were characterized by temperature-programmed reduction and the sulfided catalysts were characterized by high resolution transmission electron microscopy. The results show that the incorporation of small amounts of Nb (3–5 wt.%) increased the catalytic activity of both NiMo and CoMo catalysts in dibenzothiophene hydrodesulfurization and affected their selectivity. The effect of Nb on the selectivities of the NiMo and CoMo catalysts was different: for the NiMo catalysts, Nb increased selectivity towards the HYD route, whereas for the CoMo catalysts, it increased selectivity towards the DDS route. The analysis of the effect of Nb on the catalysts’ selectivity was performed based on the kinetic constants calculated for the different steps of the DBT HDS network.F.J. Méndez acknowledges DGAPA-UNAM for the postdoctoral grant. This work was supported by DGAPA-UNAM, México (grant number IN-113715) and Consejo Nacional de Ciencia y Tecnología (CONACYT), México (grant number CB-220175).

On the role of Fe3+ions in FexOy/C catalysts for hydrogen productionfrom the photodehydrogenation of ethanol

FexOy/C photocatalysts at different iron content were prepared by the incipient wet impregnation methodand calcined at 773 K. The photocatalysts were characterized by means of nitrogen adsorption–desorptionisotherms, surface fractal dimension, non-local density functional theory, X-ray diffraction, Rietveldrefinement and UV–vis spectroscopy. The photocatalytic activity was evaluated using the photodehy-drogenation of ethanol as a model reaction for the production of hydrogen. The specific surface areasof FexOy/C substrates, with 15, 20 and 30 wt% iron content, diminished from 638 to 490 m2/g, as theiron content increased. X-ray diffraction analysis showed that iron oxides coexist as wustite and mag-netite in samples with Fe contents of 15 and 20 wt%; for sample with 30 wt% Fe, wustite, magnetite andhematite phases were observed. The photophysical, textural and structural properties were modified bythe hematite phase formed by thermal treatment. The Rietveld refinements denoted changes in occu-pancy of Fe3+and Fe2+in FexOycrystallites. A relationship between the Fe3+ions content and the reactivityfor the hydrogen production from the photodehydrogenation of ethanol (from 1360 to 2125 mol h−1),was evidenced