Search

SSCI-VIDE+ING+DLP:CTH:ATU:DFA:FRM; International audience; The steam reforming of methane was studied over Rh and Ni-based catalysts exposed to naphthalene, which was used as a representative of polyaromatic tars found in biomass-derived biogas. In particular, two Ni-based samples in which part of the metal was encapsulated within silicalite-1 nanoboxes were tested. The reforming reaction was carried at 700, 800 and 900 degrees C using a model feed and high space velocities to limit methane full conversion and better evidence any deactivation. A strong decrease of methane conversion was observed in the presence of 1400 ppm of naphthalene, stressing the marked deleterious effect of this molecule in the present conditions. The effect of naphthalene was partly reversible, especially at higher temperatures. The silicalite-1 membrane could not prevent the deactivation of embedded nickel particles, probably because naphthalene (kinetic diameter = 0.62 nm) could diffuse throughout the MFI-type (pore diameter ca. 0.56 nm) porous layer at the high reaction temperatures used. The effect of 5 ppm of the bulkier pyrene (kinetic diameter = 0.74 nm) was investigated at 700 degrees C and also led to a rapid deactivation of the Ni@silicalite-1, likely because pyrene was cracked into naphthalene, which could then enter the silicalite-1 nanoboxes. The poisoning effect of toluene on the Ni-based catalysts was minor in comparison to that induced by the polyaromatics. A marked sintering of the embedded Ni was also observed. (C) 2016 Elsevier B.V. All rights reserved.

A series of Rh and Ni-based catalysts were tested for the reforming of methane and phenanthrene using a feed composition similar in terms of main components to that of the producer gas obtained from wood gasification. The objective was to identify formulations that would enable reforming methane and tars without the need to purify the producer gas. Phenanthrene was chosen as a representative tar because this compound is produced in significant concentrations during low-temperature gasification of woody biomasses and its three-ring structure favors coke formation. The concentration of H2S used was set at the higher range of typical values, i.e. 200 ppm, to make these tests more challenging. At 900 °C, 200 ppm of H2S induced a significantly higher activity loss for methane reforming than that induced by 200 ppm of phenanthrene. The rate of methane consumption in the presence of both poisons varied linearly with the Rh metal surface area. The slight deviation observed at higher loading was in part due to heat and mass transport limitations. The rate of methane consumption per unit of metal surface area was about 5-fold higher on Rh than that on Ni. No information could be derived from the measure of apparent activation energies, due to changes in adsorption coverages of reactants and poisons with temperature. Incomplete conversion of polycyclic aromatic hydrocarbons, well-known coke precursors, was obtained at 875 °C and 850 °C.

SSCI-VIDE+ING+DLP:ATU:DFA:FRM; International audience; Multi-hollow silicalite-1 single crystals (MH) were prepared for the first time by an original synthesis pathway by using tetrabutylphosphonium hydroxide (TBPOH) as a mild desilicating agent. This new generation of hierarchical zeolite allowed the encapsulation of nanoparticles (NPs) featuring an enhanced confinement of the metallic guest and a thin wall thickness. The MH catalyst exhibited a better stability for methane steam reforming at 700 degrees C than a single-hollow counterpart (SH). Ni average particle size could be kept lower than 4 nm after 20 h on stream for the MH sample. However, a detailed analysis of kinetic data of the structure-insensitive CO methanation used as a model reaction revealed that the sample activity was adversely affected by two main factors deriving from the preparation steps. First, a siliceous over-layer derived from the decomposition of intermediate Ni phyllosilicates, which partly covered the resulting Ni nanoparticles. Second, phosphorus from the templates remained in the samples, probably forming a Ni-P compound upon reduction. The overall catalytic activities observed here were therefore a complex interplay of improved dispersion and poisonous effects.