Jean Guille

New synthesis and uses of high-specific-surface SiC as a catalytic support that is chemically inert and has high thermal resistance

A new method for preparation of high-specific-surface (60–400 m2/g) SiC is described. This method consists of the attack of high-specific-surface activated carbon by SiO vapor generated by the high-temperature reaction of Si and SiO2. The high specific surface is probably due both to the low temperature of the reaction between SiO and C and to the presence of a stable carbon skeleton. Before reaction, the carbon can be doped with different additives to improve the surface interaction between SiC and the impregnated active phase. The action of uranium doping is also shown. SEM, XRD, thermogravimetry, TPR, XPS, BET, and porosimetry have been used to characterize the supports and the catalysts. Test reactions (hydrodesulfurization by CoMo) are also reported.

Physical characterization of molybdenum oxycarbide catalyst; TEM, XRD and XPS

Abstract Controlled reduction of MoO 3 can produce different phases of catalytic interest. One of these phases has been considered as being an oxycarbide of molybdenum. Various techniques mainly TEM but also XRD and others have been extensively used to understand the mechanism of formation and the structure of this oxycarbide. Its structure is reminiscent of the MoO 3 structure as shown by XRD, but reconstructed by shear planes and the introduction of carbon atoms to fill oxygen vacancies, both blocking the formation of MoO 2 the normal product of the slow reduction of MoO 3 . The HRTEM pictures showed a ‘chevron-like’ arrangement and the electronic microdiffraction a square lattice reminiscent of the (0 k 0) planes of MoO 3 .

Influence of the preparation conditions on the synthesis of high surface area SiC for use as a heterogeneous catalyst support

The influence of different parameters (temperature, duration and SiO source) on the synthesis of silicon carbide SiC according to the gas-solid reaction between SiO vapors and activated charcoal was investigated. The material obtained retained the general shape of the activated charcoal, which is an advantage because of the difficulty in post shaping SiC, due to the high strength of the material. High temperature (>1250 °C) and long reaction duration led to a high C* → SiC conversion but with a relatively low surface area (20–25 m2 · g−1) due to sintering via the surface diffusion phenomenon. The combination of a lower reaction temperature (1200 °C), longer reaction duration (15 h) and high (Si + SiO2)/C* weight ratio allowed SiC to be obtained with a surface area of around 50 m2 · g−1, which can be used as a support material for heterogeneous catalysis.

Structural studies of active carbon used in the growth of silicon carbide catalyst support

The microstructure of active carbon used in the growth of a silicon carbide catalyst support has been studied by BET area measurements, electron diffraction and high-resolution electron microscopy. The microstructure of the active carbon is consistent with different distributions of basic structural units (BSUs). These BSUs increase in size under heat treatment by forming adjacent distorted columns. Ordering processes are discussed from geometrical considerations based on reciprocal patterns. The reactivity of the interface between the active carbon and SiO vapour is controlled by the proportion of the highly reactive sites in the active carbon surface structure.

Nickel nano-particles in silica gel monoliths: control of the size and magnetic properties

Abstract Monolithic samples containing 1 and 5 mol% of nickel(II) have been prepared. After thermal treatment Ni(II) is reduced to metallic Ni particles, the size of the particles and the inter-particles distances have been controlled by the nickel content and treatment temperatures. The magnetic susceptibility exhibits a maximum (corresponding to blocking temperature TB) below the bifurcation point in the ZFC/FC data, consistent with the description of super-paramagnetism. The maximum shifts toward higher temperature with increasing Ni concentration. In the super-paramagnetic state (T>TB), the low field susceptibility does not follow a Curie–Weiss law. These phenomena could result to inter-particles dipolar interactions.

Synthesis and characterization of platinum-rhodium supported on SiC and SiC doped with cerium : catalytic activity for the automobile exhaust reactions

Abstract The synthesis, characterization and catalytic use of SiC and SiC doped with cerium as Pt-Rh support for the automobile exhaust reaction are described. Their resistance during two accelerated deactivation processes in the presence of water vapour at high temperature was also tested. The supports and the catalysts were characterized by mercury porosimetry, single point B.E.T., X-ray fluorescence, X-ray diffraction (XRD), transmission electronic microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS) and their catalytic activities were measured in a synthetic exhaust containing carbon monoxide, nitric oxide, hydrocarbons and nitrogen. From the results it can be concluded that SiC can replace the wash-coat of alumina on cordierite as support. The presence of cerium in the SiC support allows a good dispersion of the metal particles and confers high resistance towards sintering. In addition, the chemical inertness of the SiC support allows easy recovery of the active phase and the support by acid washing.

On the nature of metallic nanoparticles obtained from molecular Co3Ru–carbonyl clusters in mesoporous silica matrices

We report on the impregnation of THF solutions of the low-valent heterometallic cluster NEt(4)[Co(3)Ru(CO)(12)] into two mesoporous silica matrices, amorphous xerogels and ordered MCM-41, and a study of its thermal decomposition into metallic nanoparticles by X-ray diffraction, transmission electron microscopy and in situ magnetic measurements under controlled atmospheres. The decomposition of the cluster was monitored as a function of temperature by examining the chemical composition of the particles, their size distributions and their structures as well as their magnetic properties. Treatment under inert atmosphere (i.e. argon) at temperatures below 200 degrees C resulted in the formation of segregated spherical particles of hcp-ruthenium (2.3 +/- 1.0 nm) and hcp-cobalt (3.1 +/- 0.9 nm). The latter is transformed to fcc-cobalt (3.2 +/- 1.0 nm) above 270 degrees C. At higher temperatures, Co-Ru alloying takes place and the Ru content of the particles increases with increasing temperature to reach the nominal composition of the molecular precursor, Co(3)Ru. The particles are more evenly distributed in the MCM-41 framework compared to the disordered xerogel and also show a narrower size distribution. Owing to the different magnetic anisotropy of hcp- and fcc-cobalt, which results in different blocking temperatures, we were able to clearly identify the products formed at the early stages of the thermal decomposition procedure.

High-resolution transmission electron microscopy study on SiC grown from SiO and C*: crystal growth and structural characterization

Abstract The results of a high resolution transmission electron microscopy study on the growth of the catalytic support silicon carbide, obtained by reacting active carbon with silicon monoxide, are presented. On the basis of this study, it is concluded that the reaction results first in the formation of microareas of SiC randomly distributed where the carbon has been converted into carbide. The growth was found to he preferentially favourable along the 〈111〉 direction of a fcc crystal structure resulting in the formation of plates parallel to a {110}-type plane. The stacking of the Si-C elementary layers along 〈111〉 is random, which enhances the probability of occurrence of stacking faults and twins. These defects were the most striking feature in the carbide obtained. Heavily faulted 3C-SiC or one-dimensionally disordered SiC polytype was obtained.

Metallic nanoparticles from heterometallic Co–Rucarbonyl clusters in mesoporous silica xerogels and MCM-41-typematerials

Impregnation of a mesoporous xerogel or of MCM-41 with an organic solution of the heterometallic cluster [NEt4][Co3Ru(CO)12], followed by thermal treatment under an inert atmosphere, leads to highly dispersed magnetic nanoparticles under milder conditions than when conventional metal salts are used as precursors.

High-resolution transmission electron microscopy on silicon carbide whiskers

SiC whiskers were grown from the reaction of silicon monoxide (SiO) with activated carbon containing iron impurities. Growth proceeds through a VLS growth mechanism with SiO and CO as reacting gases. HRTEM combined with EDS shows that the SiC whisker is topped by a Fe3Si catalyst droplet. The SiC whisker is found to be one-dimensionally disordered along the [111] growth direction of an fcc crystal structure. Although the catalyst droplet is usually larger than the top face of the whisker, we observed a number of situations where the diameter of the droplet was smaller. The study of the SiC-Fe3Si interface showed that the growth is nucleated from the edges.