Nicolas Keller

Carbon nanofiber supported palladium catalyst for liquid-phase reactions: An active and selective catalyst for hydrogenation of cinnamaldehyde into hydrocinnamaldehyde

Abstract Carbon nanofibers (CNFs) prepared by decomposition of ethane over a Ni/alumina catalyst, are used as support for palladium clusters. The carbon support displays a mean diameter of 40–50xa0nm, lengths up to several tens of micrometers, as highlighted by transmission electron microscopy (TEM) observations and a specific surface area of about 50xa0m 2 /g. The spheroidal palladium particles have a relatively homogeneous and sharp size distribution, centered at around 4xa0nm. This novel Pd/carbon nanofiber catalyst displays unusual catalytic properties and is successfully used in the selective hydrogenation of the C C bond in cinnamaldehyde at a reaction temperature of around 80°C, under continuous hydrogen flowing at atmospheric pressure. The high performances of this novel catalyst in terms of efficiency and selectivity are, respectively, related to the inhibition of the mass-transfer processes over this non-porous material and to peculiar palladium–carbon interactions. It is concluded that the absence of microporosity in the carbon nanofibers favours both the high activity and selectivity which is confirmed by comparison with the commercially available high surface area charcoal supported palladium catalyst.

Synthesis and catalytic uses of carbon and silicon carbide nanostructures

Abstract Carbon nanofibers and nanotubes with controlled diameters were synthesized by catalytic decomposition of an ethane/hydrogen mixture over nickel and iron supported catalysts. The synthesis of the first silicon carbide (SiC) nanotubes was performed according to the shape memory synthesis (SMS) method. The benefit of using carbon and SiC nanotubes as catalyst supports was evidenced, respectively in the case of the selective Cue605C bond hydrogenation in the α,β-unsaturated cinnamaldehyde and the low temperature selective oxidation of H 2 S into elemental sulfur (60xa0°C). Carbon nanotubes as support allowed to increase both cinnamaldehyde conversion and selectivity toward Cue605C bond hydrogenation. Supporting a nickel-based catalyst on SiC nanotubes allowed to increase both desulfurization activity of the catalyst and its solid sulfur storage capacity. The inner partial pressure concept, or confinement effect, was developed to explain the high performances of this new SiC-based catalyst. The last section is devoted to further objectives for developing such highly performing new support materials.

Synthesis and characterisation of medium surface area silicon carbide nanotubes

Abstract Silicon carbide nanotubes with medium surface area (30–60 m2/g) were successfully prepared by reaction between carbon nanotubes and SiO vapor according to the shape memory synthesis (SMS). The gross morphology of the carbon nanotubes was maintained during the carburization process. A calcination in air at 600xa0°C was performed to remove unreacted carbon domains in order to obtain pure carbon-free SiC nanotubes. The synthesized SiC nanotubes had a mean outer diameter of 100 nm and lengths up to several tens of micrometres.

Continuous process for selective oxidation of H2S over SiC-supported iron catalysts into elemental sulfur above its dewpoint

Abstract Iron supported on silicon carbide (SiC) is shown to be a high conversion catalyst for the oxidation of H2S with a high selectivity into elemental sulfur above sulfur dewpoint in the presence of large amounts of oxygen and steam in the feed. The active phase is probably an iron oxysulfide or a non-stoichiometric sulfate phase. It is stable for several weeks in an industrial micropilot plant, and no deactivation is observed even in the presence of a large amount of steam. These performances are due to the intrinsic physical properties of the SiC carrier but also because of the optimal dispersion of the active phase.

Direct oxidation of H2S into S. New catalysts and processes based on SiC support

Abstract Nickel sulphide, supported on SiC, exhibits a very high activity and selectivity for the direct oxidation of H 2 S into S at medium temperatures (100–120°C) or at room temperature (20–40°C). Iron oxide, also supported on SiC, is highly reactive, and selective, for the same reaction at higher temperatures (210–240°C). This support is very stable, insensitive to steam and to any sulphur compounds in this range of temperature.

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.

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.

Low temperature use of SiC-supported NiS2-based catalysts for selective H2S oxidation: Role of SiC surface heterogeneity and nature of the active phase

Abstract High activity for the direct oxidation of H2S into elemental sulfur at low reaction temperature (40–60xa0°C) on medium surface area silicon carbide-supported nickel sulfide (NiS2/SiC) catalyst was attributed to the formation of a highly active superficial nickel oxysulfide. The hypothesis of the superficial formation of either nickel oxide or nickel sulfate was rejected. The superiority of the SiC support in terms of performance as compared to silica, high surface area alumina and activated charcoal was evidenced. The high stability of the SiC-supported catalyst as a function of time and solid sulfur loading in the presence of water was explained by a peculiar mode of sulfur deposition, involving the role of water and the hydrophilic/hydrophobic duality of the SiC support surface. Water acts as a conveyor belt, continuously cleaning the active phase particles, located on the hydrophilic oxygen-containing surfaces of the support. Hydrophobic pure SiC, located outside the mesoporosity and exempt of active phase, remains as an available surface for the storage of high amounts of solid sulfur.

Synthesis and characterization of a new medium surface area TiO2–β-SiC material for use as photocatalyst

A new medium surface area and non-microporous TiO2–β-SiC photocatalytic material was successfully obtained according to an extension of the Shape Memory Synthesis (SMS) method, and following a three step procedure. A high surface area TiO2 supported on an activated charcoal precursor was prepared in a first step by the sol–gel precipitation of titanium tetraisopropoxide in the presence of activated charcoal powder. The second step consisted of a gas–solid reaction between this precursor and SiO vapors at 1300 °C under dynamic vacuum which formed TiC–β-SiC, by carboreduction of SiO vapors and TiO2 into β-SiC and TiC respectively. The final step was oxidation in air at temperatures ranging from 420 °C to 600 °C which produced the TiO2–β-SiC material, using the high resistance of β-SiC towards oxidation to selectively form TiO2. The gross morphology of the activated charcoal precursor was maintained during the carburization and the subsequent oxidation processes according to the SMS concept. Both TiO2 and the β-SiC semiconductor phases were in intimate contact in the same grain of catalyst, without the usually problematic post-synthesis shaping of high mechanical strength SiC-based materials and using no binder. The TiO2-to-SiC ratio could be directly tuned by adjusting the TiO2 content of the precursor, whereas the anatase-to-rutile ratio could be modified by changing the post-synthesis calcination temperature, without altering the β-SiC properties or significantly increasing the average TiO2 particle size.