Dynamic study of carbon nanotube growth and catalyst morphology evolution during acetylene decomposition on Co/SBA-15 in an environmental TEM

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Bimetallic PdAg nanoparticle arrays from monolayer films of diblock copolymer micelles

International audienceThe self-assembly technique provides a highly efficient route to generate well-ordered structures on a nanometer scale. In this paper, well-ordered arrays of PdAg alloy nanoparticles on flat substrates with narrow distributions of particle size (6-7 nm) and interparticle spacing (about 60 nm) were synthesized by the block copolymer micelle approach. A home-made PS-b-P4VP diblock copolymer was prepared to obtain a micellar structure in toluene. Pd and Ag salts were then successfully loaded in the micellar core of the PS-b-P4VP copolymer. A self-assembled monolayer of the loaded micelles was obtained by dipping the flat substrate in the solution. At this stage, the core of the micelles was still loaded with the metal precursor rather than with a metal. Physical and chemical reducing methods were used to reduce the metal salts embedded in the P4VP core into PdAg nanoparticles. HRTEM and EDX indicated that Pd-rich PdAg alloy nanoparticles were synthesized by chemical or physical reduction; UV-visible spectroscopy observations confirmed that metallic PdAg nanoparticles were quickly formed after chemical reduction; XPS measurements revealed that the PdAg alloy nanoparticles were in a metallic state after a short time of exposure to O-2 plasma and after hydrazine reduction

Chemical, Structural, and Morphological Changes of a MoVTeNb Catalyst during Oxidative Dehydrogenation of Ethane

MoVTeNb mixed oxide, a highly active and selective catalyst for the oxidative dehydrogenation of ethane to produce ethylene, exhibits the so-called M1 and M2 crystalline phases. The thermal stability of the MoVTeNb catalytic system was assessed under varying reaction conditions; to this end, the catalyst was exposed to several reaction temperatures spanning from 440 to 550 °C. Both the pristine and spent materials were analyzed by several characterization techniques. The catalyst was stable below 500 °C; a reaction temperature of ≥500 °C brings about the removal of tellurium from the intercalated framework channels of the M1 crystalline phase. Rietveld refinement of X-ray diffraction patterns and microscopy results showed that the tellurium loss causes the progressive partial destruction of the M1 phase, thus decreasing the number of active sites and forming a MoO2 crystalline phase, which is inactive for this reaction. Raman spectroscopy confirmed the MoO2 phase development as a function of reaction temperature. From highresolution transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses it was noticed that tellurium departure occurs preferentially from the end sides of the needlelike M1 crystals, across the [001] plane. Detailed analysis of a solid deposited at the reactor outlet showrf that it consisted mainly of metallic tellurium, suggesting that the tellurium detachment occurs via reduction of Te4+ to Te0 due to a combination of reaction temperature and feed composition. Thus, in order to sustain the catalytic performance exhibited by MoVTeNb mixed oxide, hot spots along the reactor bed should be avoided or controlled, maintaining the catalytic bed temperature below 500 °C.This work was financially supported by the Instituto Mexicano del Petroleo.Valente, JS.; Armendariz-Herrera, H.; Quintana-Solorzano, R.; Del Angel, P.; Nava, N.; Masso Ramírez, A.; López Nieto, JM. (2014). Chemical, Structural, and Morphological Changes of a MoVTeNb Catalyst during Oxidative Dehydrogenation of Ethane. ACS Catalysis. 4:1292-1301. doi:10.1021/cs500143jS12921301

In Situ Environmental STEM Study of the MoVTe Oxide M1 Phase Catalysts for Ethane Oxidative Dehydrogenation

MICROSCOPIE+ECI2D+MAO:TEP:JMIInternational audienceIn situ environmental STEM-(HA)ADF has been used to characterize a light alkane mild oxidation catalyst corresponding to a MoVTeO M1 phase. The results obtained show that there is almost no structure disordering under reaction conditions closed to ethane oxidative dehydrogenation catalytic reactions: i.e., 350 degrees C under 1 mbar of a 30/15/55 O-2/C2H6/N-2, gas mixture. They further demonstrate that {Te-O-x} chains present in the hexagonal channels of the structure participate in the redox process of the catalyst and constitute a preferential pathway for the reoxidation of the surface catalytic active sites. Upon reduction it is proposed that labile oxygens are removed from the chains, leading to a high content of Te4+-O3E and Te2+O2 species and a displacement of the tellurium position toward the atomic columns between the two hexagonal channels. This process is proposed to involve the reduction of tellurium only under the strongest reducing conditions. The study also shows a constant ending of the [001] zone by {Mo(Mo)(5)} structural units and the bulk catalytic sites generally proposed should not appear on the facets of this type of rod

In-situ observations of low temperature soot combustion on ceria-zirconia by environmental transmission electron microscopy (ETEM)

MICROSCOPIE+CARE+PVE:MAOInternational audienceEnvironmental transmission electron microscopy (ETEM) equipped with an aberration corrector was used to get further insights into the oxidation soot on ceria-zirconia. In-situ ETEM observations in the presence of a few mbar of oxygen have shown that soot in intimate contact with the catalyst crystallites, achieved by a high-energy milling, can be oxidized at room temperature. This study confirms the importance of the soot/catalyst interface at nanoscale in the soot oxidation mechanism

Optimizing the efficiency of MoVTeNbO catalysts for ethane oxidative dehydrogenation to ethylene

+MAOMoVTe(Sb)NbO catalysts have been prepared using the slurry method and tested as catalysts for the oxidative dehydrogenation of ethane. The influence of both the addition of silica to the starting slurry during the preparation and of a final heat treatment under nitrogen at 873 K after the dissolution of the M2 phase formed concomitantly to the active M1 active phase has been studied. While the parameters change the texture of the catalysts without modification of the catalytic sites, they have opposite effects on catalytic properties. The addition of silica increases the conversion without modifying the selectivity to ethylene and the final heat treatment decreases the conversion but increases the selectivity. These effects have been explained respectively by a decrease of the agglomeration and sintering of the M1 phase crystallites and a decrease of the number of small pores, where total oxidation takes place. These effects could be leveraged to optimize the preparation protocol and obtain more efficient catalysts for the oxidative dehydrogenation of ethane. (C) 2012 Elsevier B.V. All rights reserved

Apport de la microscopie électronique en transmission à l'étude de catalyseurs

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In-situ observations of low temperature soot combustion on ceria-zirconia by environmental transmission electron microscopy (ETEM)

MICROSCOPIE+CARE+PVE:MAOInternational audienceEnvironmental transmission electron microscopy (ETEM) equipped with an aberration corrector was used to get further insights into the oxidation soot on ceria-zirconia. In-situ ETEM observations in the presence of a few mbar of oxygen have shown that soot in intimate contact with the catalyst crystallites, achieved by a high-energy milling, can be oxidized at room temperature. This study confirms the importance of the soot/catalyst interface at nanoscale in the soot oxidation mechanism

Ordered arrays of nanorods obtained by solid-liquid reactions of LaOCl crystals

cited By 12International audienceA new top-bottom approach is developed to prepare ordered arrays of nanorods, based on controlled etching, as exemplified by the case of LaOCl crystals. Molten-salt issued LaOCl was applied as a precursor, with highly anisotropic crystals oriented along the [001] axis. These LaOCl flat crystals are reactive toward aqueous solutions at room temperature, yielding ordered assemblies of rod-like crystals. Depending on the reaction mixture composition, nanorods of LaOCl itself or other La compounds can be obtained. The morphology of the products obtained in the solutions can be finely controlled by adding extraneous chemical species which modify the etching kinetics. Thus, LaOCl reacts with the acidic phosphate solutions, leading to the arrays of LaPO 4 nanorods, oriented along the [001] axis of the initial solid. By contrast, reaction with aqueous HCl produced assemblies of perpendicular rods, oriented along the [100] and [010] axes. The catalytic properties of the nanorods as obtained were tested in the decomposition of isopropanol. Strong differences of acid-base properties of differently oriented LaOCl particles have been observed. © 2010 American Chemical Society