Marc-Jacques Ledoux

High specific surface area carbides of silicon and transition metals for catalysis

Abstract A new synthesis of carbides based on the concept of shape memory is summarized. High specific surface area activated carbon is reacted with a vapor of the oxide to give the corresponding high specific surface area carbide and carbon monoxide. Some catalytic applications of these new materials are presented.

Synthesis of CoFe2O4 nanowire in carbon nanotubes. A new use of the confinement effect.

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside carbon nanotubes under mild reaction conditions using the confinement effect provided by the carbon tubular template.

Microstructural investigation and magnetic properties of CoFe2O4 nanowires synthesized inside carbon nanotubes

Cobalt ferrite nanowires with an average diameter of 50 nm and lengths up to several micrometers were synthesized inside multi-walled carbon nanotubes under mild reaction conditions, i.e. 100°C and atmospheric pressure, using an aqueous nitrate precursor salt filling the tubes. The concept of a confinement effect inside carbon nanotubes has been advanced to explain the formation of CoFe2O4 under such mild reaction conditions. The formation of caps near the tube tips at the beginning of the nitrate decomposition meant that each nanotube was considered as a closed nanoreactor, in which the reaction conditions could be very different to the macroscopic conditions outside the tube. A post-synthesis treatment under inert atmosphere allowed the growth of CoFe2O4 particles, from a disordered hair-like dendritic structure at 100°C to highly crystallized domains at higher temperatures. A material with high coercivity at room temperature for small particles of about 25 nm in diameter was obtained by submitting the CoFe2O4 nanowires after calcination in air at 100°C to an argon treatment at 550°C for 2 h.

Carbon nanomaterials with controlled macroscopic shapes as new catalytic materials

Carbon nanostructures, i.e. nanotubes and/or nanofibers, with macroscopic shaping were successfully employed as catalyst supports either in gas-phase or in trickle-bed mode. The macroscopic shaping allows the easy handling of these nanomaterials without altering their intrinsic physical and chemical properties and renders them able to be used in a conventional structured catalytic bed without diffusion and pressure drop problems. Several catalytic reactions were demonstrated in the present work including catalytic decomposition of hydrazine for satellite maneuvering, low-temperature selective oxidation of trace amount of H2S into elemental sulfur and a fuel cell (PEMFC).

Efficient Synthesis of Dimethyl Ether over HZSM‐5 Supported on Medium‐Surface‐Area β‐SiC Foam

In this study, we aimed to produce a highly selective and stable catalyst for the production of dimethyl ether by methanol dehydration. The activities were compared of different active phases of the employed system, zeolite HZSM-5 or gamma-alumina, supported on silicon carbide as foam, and it was found that the supported zeolite catalysts are more active than and as selective as the alumina-based catalysts. The as-prepared zeolite/SiC composites reveal good stability in long-term tests in the presence or absence of steam. The high stability is attributed to the presence of highly dispersed micrometer-sized zeolite particles, which make the active sites more accessible to the reactants and promote the quick transfer of the desired product, dimethyl ether, out of the catalyst bed, minimizing deactivation of the catalyst.

New carbon nanofiber/graphite felt composite for use as a catalyst support for hydrazine catalytic decomposition

Graphite felt supporting 40 nm diameter carbon nanofibers was synthesized and successfully used as a support for a high loaded iridium catalyst (30 wt%) in the decomposition of hydrazine; a strong mechanical resistance and a high thermal conductivity led to a very efficient and stable catalyst as compared to that used industrially, iridium supported on a high surface area alumina.

A sub-nanometer structural study of Pt-Rh catalysts supported on Ce doped SiC

In this paper, the HRTEM technique has been applied to the structural study of Pt-Rh catalysts supported on Ce doped SiC. The results on cerium doped active carbon and on cerium doped silicon carbide synthezised before its use as catalyst support are also reported. It was found that the Ce doped SiC catalyst support contains a new compound which is a Ce amorphous phase. The presence of such a compound within the support is thought to have the advantage of contributing to the improvement of the activity of the catalysts. At present, the HRTEM study on reduced Pt-Rh/SiC-Ce catalyst gives direct evidence of a selective association of Pt and Rh metals with the cerium oxide additive. This association, on the one hand, shows that cerium oxide is not inert towards noble metals, and on the other hand, it gives rise to a noble metal—cerium oxide composite to which the reduction is linked. The formation of an active interface, or a buffer oxygen layer which can be defined as a surface that serves as a common boundary between noble metals and cerium oxide, must be then considered in order to interpret the improvement in catalytic activities. After the ageing treatment, some structural changes were observed, particularly the formation a Rh suboxide phase formed by three to four monoatomic layers. Finally, it seems that the main oxidation and reduction processes occur through the active interface where the cerium oxide is easily able to change from cerium plus four (Ce4+) to cerium plus three (Ce3+) oxidation state. The metal will act only as a donor or acceptor of electrons. It must be emphasised that the SiC catalyst support showed a high chemical inertia since no structural modifications were observed on the noble metals caused by SiC.

Carbon nanofiber supported palladium catalyst for liquid-phase reactions. An active and selective catalyst for hydrogenation of CC bonds

Carbon nanofibers with a mean diameter of about 50 nm were successfully used as support for a palladium catalyst in the liquid phase selective hydrogenation of the CC bond in an α,β-unsaturated molecule: a less critical problem of mass-transfer limitation led to the obtention of a highly active and chemo-selective catalyst compared to a commercial high surface area activated charcoal supported palladium catalyst.

Part II.Dehydrogenation of n-butane over carbon modified MoO3 supported on SiC

Abstract Molybdenum oxide supported on SiC is shown to be an active and stable catalyst for the dehydrogenation of n -butane. The activity of the catalyst was studied in a fixed bed microreactor, and the reaction product was analysed by an on-line FTIR technique that enabled study of the activity of the catalyst during the first minutes on stream. At 560°C under atmospheric pressure, the conversion of n -butane and the selectivity for C 4 alkenes were stable at 20% and 70%, respectively. The n -butane feed was diluted with hydrogen and water, which were required for the formation of the active phase and for stable operation. The active phase in dehydrogenation was composed of molybdenum oxide modified with carbon and limited to the uppermost layer of the catalyst.

The use of pyridine and piperidine hdn as probe for activity of molybdenum-based hydrotreatment catalysts. The role of the nickel (part II)

Abstract The reactions of pyridine and piperidine HDN have been used as a probe for testing the activity and the selectivity of eleven hydrotreating catalysts in the absence of sulfur: these comprised pure alumina, doped alumina, MoO3/Al2O3, NiMo and CoMo/AI2O3. The piperidine HDN reaction needs only single Bronsted or Lewis acidic sites while the pyridine reaction needs at least a bifunctional site with a reduced centre and an acidic centre. The intervention of a trifunctional site, two reduced centres and an acidic one, explains the promoting effect of the nickel. The presence of this new site can be correlated with the existence of a phase where the nickel, in the reduced state, “decorates” the Mo oxide layer. This new phase will have as a precursor the xNiO molybdate phase proposed by Knozinger et. al. [11].