Marc J. Ledoux

The influence of sulfide structures on the hydrodesulfurization activity of carbon-supported catalysts

Abstract All transition metals in the first, second, third rows and some in the fourth row, supported on carbon at low concentration, have been tested in their sulfided form to evaluate their activity toward the hydrodesulfurization (HDS) reaction at four different temperatures. The first row shows two maxima on V Cr and Co whatever the temperature with two orders of magnitude between the lowest and the highest activities (one order between Ni and Co). The second and third rows present a single maximum on Rh and Ir, respectively, with three orders of magnitude between the lowest and the highest activities. Electronic considerations have been proposed (S. Harris and R.R. Chianelli, J. Catal. , 86 , 400, 1984) to account for these different activities. A new complementary approach is developed by introducing the structural properties of the Sulfides.

Compared activities of platinum and high specific surface area Mo2C and WC catalysts for reforming reactions. I: Catalyst activation and stabilization : reaction of n-hexane

The catalytic properties of high specific surface area (> 150 m[sup 2]/g) molybdenum and tungsten carbides are studied in hydrocarbon-reforming reactions and compared to conventional platinum supported on alumina. The isomerization of n-hexane is used as a test reaction. TPR and XPS analyses show that the raw materials are contaminated by oxygen and need an activation process to become reactive. These analyses also show that Mo[sub 2]C is decomposed by pure hydrogen at 800 C to form methane and metallic molybdenum. Different methods of reductive activation are tested: high-temperature reduction (800 C) by hydrogen leads to metallic Mo and W on the surface (catalytically unreactive for reforming); coreduction by a mixture of pentane and hydrogen (700 C) gives active catalysts but less so than conventional platinum, probably because of the presence of carboneous residues formed by decomposition of the n-pentane. Trace amounts of different group VIII transition metals ([le] 500 ppm) can catalyze the activation process, probably by preventing the formation of the carboneous residues. Mo[sub 2]C activated by 500 ppm of Pt, Ir, or Ru can reach total specific activities 6 to 7 times higher than the conventional Pt catalyst. However in terms of yield, the best carbide, activatedmorexa0» by Ir, only doubles the performance of conventional platinum, with a high amount of cracked molecules formed in parallel. Clean surfaces of Mo[sub 2]C or WC can be much more reactive than conventional Pt catalysts in terms of specific activity, isomerization, plus cracking; however, the best selectivity in isomers never exceeds 30% while selectivity on Pt is usually in the range 75 to 85%.«xa0less

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.

Large scale synthesis of carbon nanofibers by catalytic decomposition of ethane on nickel nanoclusters decorating carbon nanotubes

A large scale synthesis of carbon nanofibers with a controlled diameter of about 50 nm has been achieved at relatively low temperatures (550–650u2006°C) by the decomposition of ethane on a carbon nanotube supported nickel catalyst. The carbon nanofibers can be used as a catalyst or a catalyst support without subsequent purification, due to the use of carbon nanotubes as support, the high nanofiber yields, and the purity obtained.

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.

CoMo sulfide catalysts studies by metal solid NMR: The question of the existence of the chemical synergy

For the first time the 59Co NMR has been used for the study of the cobalt sulfides in the Co-based HDS catalysts. Four different cobalt sites were observed on these catalysts, two of them corresponding to the regular octahedral and tetrahedral Co9S8 sites, the two other sites, one distorted tetrahedral and one octahedral, were only found in highly dispersed catalysts. A good correlation has been found between the HDS activity on CoSiO2 and CoC catalysts and the amount of these new tetrahedral Co which are found very active. A parallel study of the HDS activity on mixed CoMoSiO2 has shown the classical so-called promoting effect of Co on Mo, but this effect can be simply described in terms of additivity of the Mo activity and of the new very active tetrahedral Co which are also observed to be dominant at the maximum of the volcano curve.

Numeration methods for targeting photoactive materials in the UV-A photocatalytic removal of microorganisms

This tutorial review reports on the different numeration methods for evaluating the efficiency of the photocatalytic action on microorganisms. Here we put forward the advantages and drawbacks of the standard methods such as the plate count, the fluorescence techniques and the Most Probable Number method for determining the biocidal photocatalytic activity and thus selecting efficient photocatalytic materials among complex systems. We highlight that bacterial spores are a representative and suitable tool for meeting the restrictive requirements resulting from the complex use of living matter instead of chemical targets.

Hydrodenitrogenation activity and selectivity of well-dispersed transition metal sulfides of the second row on activated carbon

The study of the second row of transition metal sulfided catalysts well-dispersed on activated carbon and of a conventional NiMo/alumina sulfided catalyst shows that these sulfides can be classified into three families following their pyridine hydrodenitrogenation activity: the very poor catalysts (order 1) such as MiMo, Zr, Ag, and Nb; the active catalysts (order 10) such as Mo, Rh, and Pd; and the very active catalyst Ru. The selectivity of products differs greatly from one catalyst to another: the higher the concentration in saturated C{sub 5} hydrocarbons, the higher the activity; the higher the concentration in cracked hydrocarbons (C{sub 4}, C{sub 3}, C{sub 2}, and C{sub 1}) the lower the activity. In addition, there is no simple correlation between the concentrations of the different intermediate amines (piperidine and pentylamines mainly) and activity, which excludes any simple kinetical explanation.