Mimoun Aouine

Crystal chemistry and phase composition of the MoVTeNbO catalysts for the ammoxidation of propane

Unit cell parameters and space group symmetry have been ndetermined for the two main phases of the MoVTeNbO catalysts; models of ntheir structures are proposed based on electron micrographes, EDX data and ncomparison with other crystal structures.

Preparation, structural and hydrotreating catalytic properties of unsupported NiRh2S4

NiRh2S4 unsupported catalysts were prepared by a coprecipitation method and characterized by X-ray diffraction (XRD), chemical analysis, specific surface area measurements and high-resolution transmission electron microscopy (HRTEM). It has been shown by XRD and HRTEM that the main phase of the prepared solids has a cubic spinel type structure. The catalytic properties were evaluated in the hydrodesulfurization (HDS) of thiophene and the hydrogenation (HYD) of tetralin. The intrinsic HDS and HYD activities of the NiRh2S4 catalyst were compared with dispersed Rh, Mo binary sulfides and ternary NiMoS and NiCr2S4 sulfides. The intrinsic HDS activity of NiRh2S4 is higher than the sum of those of the individual Ni3S2 and Rh2S3 compounds, suggesting the existence of a synergistic effect between Ni and Rh.

HRTEM study of the morphology of RuS2 supported particles

Abstract Nanoparticles of ruthenium disulfide supported on silica have been characterized by high resolution transmission electron microscopy. Lattice resolution images were obtained and provided the morphology of the RuS2 nanoparticles. The interpretation of the atomic resolution images was performed with the help of image simulations, which enabled the atomic columns to be correctly positioned and the thickness of the nanoparticles to be estimated. Several surface planes, such as (2xa00xa00) or (1xa01xa01), which are supposed to present different catalytic activities, have been identified.

Spatial Distribution of the Vanadium Atomic Species in MoVTeO and MoVTeNbO Oxide Catalysts as Revealed by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy

High‐angle annular dark‐field scanning transmission electron microscopy (STEM–HAADF) was used to characterize a light‐alkane mild‐oxidation catalyst corresponding to two different MoVTe(Nb)O M1 phases. For that purpose a semiautomatic integration routine was developed, which allows the analysis of several hundred atomic columns with a normalization procedure to quantitatively determine the vanadium content of mixed (V, Mo) crystallographic sites with a low statistical variability. The obtained results were compared with data previously obtained on other M1 phases with different compositions. The comparison showed that the V distribution over the sites was not strongly influenced by the presence or absence of niobium, the method of synthesis used, and the nature of the Te or Sb cations in the hexagonal channels, but depended mainly on the total V content of the phases. Extrapolation to low V content enabled a discussion on the possible location of the vanadium active sites.

Simulation of EDS Spectra Using X-RES Software

This work is related to the general framework of activities concerning microanalysis in electron microscopy. Especially, X-ray spectroscopy is a very powerful tool used by physicists and metallurgists. Its application field is commonly the study of new materials; since they are more and more complex, their development requires a detailed microstructural characterization. Our work brings a contribution to the methodology of analyzing X-ray spectra acquired in a transmission electron microscope. We have proposed a method which allows complex cases, frequently encountered during microscopic observations of real materials, to be evaluated. Such cases define a common class of problems, that is the very localized chemical analysis near an interface, or, more generally, chemical segregations. The problem has been approached by means of simulation of the X-ray emission, coupled with a mathematical deconvolution method of spreading effects of the electron beam. A specific software program has been developed on PC-type computers. The use of the Visual-Basic tool kit has allowed a friendly user interface to be written. The method has been applied to several cases of microanalysis of complex microstructures [1]; a test case will be presented in this contribution. All these applications show the interest of our approach, and point out the advantages offered by the software, which might thus be of a great help for microscopists.

Promoting effect of ceria on the performance of NiPd/CeO2–Al2O3 catalysts for the selective hydrogenation of 1,3-butadiene in the presence of 1-butene

The removal of traces of highly unsaturated compounds, such as 1,3-butadiene, from olefin feedstocks via selective hydrogenation is of particular importance because the oligomerization of these impurities produces deactivation and an increased pressure drop across the catalytic beds used in the polymerization processes. The present work focuses on the selective hydrogenation of 1,3-butadiene in the presence of 1-butene using (xCeO2-)Al2O3-supported NiPd catalysts (x = 0, 1, 2, and 3 wt% CeO2) in both liquid and gas conditions. The samples were characterized by XRD, N2 physisorption, Zeta potential, H2-TPR, NH3-TPD, HRTEM and STEM-HAADF. Modifying the catalysts with CeO2 resulted in an increase of 1,3-butadiene conversion, an enhancement of 1-butene selectivity and a decrease of butane production under liquid phase conditions. In contrast, in the gas phase both the Ce-modified and unmodified catalysts behaved similarly: while the fresh catalysts showed similar reactivity to that found in liquid phase conditions, the reacted and thermally pretreated catalysts showed increased reactivity, leading to full hydrogenation up to butane. The improvements observed under liquid phase conditions may be related to modification of the acid strength with increasing CeO2 loading, which would increase the adsorption of 1,3-butadiene and diminish the further hydrogenation of 1-butene. Optimal activity and selectivity were observed for the catalyst with an Ni/Pd atomic ratio of 1, and loadings of 0.5 wt% Pd and 3.0 wt% CeO2.

Uncovering the three-dimensional structure of combustion-synthesized noble metal-ceria nanocatalysts

With its unique redox properties, ceria is an oxide with a range of applications, including automotive catalytic converters, which consist of platinum‐group metal nanoparticles on ceria‐containing supports. In this work, the 3u2009D architecture of a ceria‐based material synthesized by the widely employed glycine‐nitrate solution combustion method is revealed for the first time. Together with N2 adsorption volumetry, scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM), STEM tomography provides a comprehensive picture of the multimodal porous network of a pre‐reduced Pt‐CeO2 catalyst, from the nanometer to the micrometer scale. This material consists of ceria nanocrystallites forming 3u2009D aggregates and puzzle‐like 2u2009D walls separating large roundish mesopores and macropores. The small voids between imperfectly assembled crystallites give rise to some microporosity. In addition, it is demonstrated that a significant proportion of platinum nanoparticles (3–4u2005nm) are not located at the ceria surface following the one‐step synthesis process, about half of them are buried within ceria. This result is valid for another metal (Rh) and another fuel (oxalyl dihydrazide), and has important implications for heterogeneous catalysis.

Silica grain labelling and EDX spectroscopy; evidencefor inter-grain diffusion of Pt(NH3)42+species during Pt/SiO2 catalyst preparation by ionicexchange

Labelling of silica grains and energy dispersive X-ray nspectroscopy (EDX) in a TEM-FEG (field emission gun) were used to ndemonstrate the migration of Pt(NH3)42+ nspecies from one grain to another during Pt/SiO2 catalyst npreparation by the ion-exchange procedure.