M.A. Cortés-Jácome

Relationship between the catalytic activity and Mo–V surface species in bimetallic catalysts for the oxidative desulfurization of dibenzothiophenic compounds

Abstract This work shows the performance of MoOx–VOx based bimetallic catalysts tested on the oxidative desulfurization (ODS) process of refractory dibenzothiophenic compounds using H2O2 as an oxidant. The catalytic activity was related with the oxidation state of molybdenum and vanadium surface species and with the interaction of both metals. The prepared molybdenum–vanadium oxides supported on alumina were subjected to reduction treatments at different temperature to obtain molybdenum and vanadium species with different oxidation state. Catalysts were characterized by their textural properties, scanning electron microscopy–energy dispersive X-ray, X-ray diffraction, temperature programed reduction and X-ray photoelectron spectroscopy. The characterization results showed that metal interactions promote the generation of highly active tetrahedral molybdenum species and isolated vanadium species, which increase the ODS performance of Mo–V based catalysts compared with their respective monometallic catalysts. Also, it was observed that combination of Mo6+, Mo4+ and V4+ superficial species promoted the ODS catalytic activity.

Chemical Quantification of Mo-S, W-Si and Ti-V by Energy Dispersive X-Ray Spectroscopy

Elemental chemical identification of a specimen and its quantification is fundamental to obtain information in the characterization of the materials (Angeles et al., 2000; CortesJacome et al., 2005). Energy dispersive X-ray spectroscopy (EDXS) is the technique that allows obtaining information concerning the elemental chemical composition using the EDX spectrometer. Generally is attached to a scanning electron microscope (SEM) (Goldstein & Newbury, 2003) and/or in a transmission electron microscope (Williams & Barry-Carter, 1996). The technique is very versatile because the spectrometer gives results in few minutes. The instrument is compact, stable, robust and easy to use and its results can be quickly interpreted. The analysis is based in the detection of the characteristic X-rays produced by the electron beam-specimen interaction. The information can be collected in very specific local points or on the whole sample. So, both electron microscopy and EDXS, give valuable information about the morphology and chemical composition of the sample. In order to give an accurate interpretation of the data collected by the instrument is important to know the fundaments of the technique. The characteristic X-rays are produced by the atoms of the sample in a process called inner-shell ionization (Jenkins & De Dries, 1967). This process is carried out when an electron of inner-shell is removed by an electron of the beam generating a vacancy in the shell. At this moment the atom remain ionized during 10-14 second and then an electron of outer-shell fills the vacancy of the inner-shell. During this transition a photon is emitted with a characteristic energy of the chemical element and its shell ionized. The emitted photons are named by the shell-ionized type as K, L, M lines.... and ┙, ┚, ┛... by the outer-shell corresponding to the electron that filled the inner-shell-ionized. For atoms with high atomic numbers, is important to note that some transitions are forbidden. Permissible transitions can be followed by the quantum selection rules and the notation can be followed by Manne Siegbahn and/or, IUPAC rules (Herglof & Birks, 1978). During the beam-sample interaction, another X-ray source is produced and it is known as Bremsstrahlung radiation or continuum X-rays which are generated for the deceleration of the electron beam in the Coulombic field of the specimen atoms. When the electrons are braked, they emit photons with any energy value giving rise to a continuous electromagnetic spectrum appearing in the EDX spectrum as

Structural Characterization by HRTEM of WOx Nanocluster Obtained from W-ZrO2 Solid Solution

Tungsten oxide dispersed on zirconia in WO3-ZrO2 system seems to be the more stable catalyst with strongly acidic properties. The activation of the catalytic sites depends on synthesis method. It has been assumed that for impregnated catalysts all tungsten is on crystallite surface, whereas in coprecipitated and sol gel synthesis methods WO3 crystallites remain in ZrO2 bulk, stabilizing the tetragonal structure [1,2]. Recently we reported the formation of the solid solution W-ZrO2 occurring below 800 °C, which stabilizes the tetragonal structure in a highly symmetric state producing crystallites with flat surfaces[3]. Upon 800 °C, the tungsten atoms segregate from the tetragonal solid solution, producing the monoclinic and tetragonal phases of zirconia. It seems that the segregation process of tungsten atoms from the solid solution controls the aggregation of the WOx species on the zirconia crystallites surface. In this work experimental evidence of the segregation and well dispersed WO3 on ZrO2 surface was obtained by using high-resolution transmission electron microscopy HRTEM (JEOL 2010F).