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This work explores the preparation of graphene-related materials (GRMs) grown on stainless steel foams via catalytic decomposition of methane (CDM). The main active phases for the reaction are the Fe nanoparticles segregated from the stainless-steel after the activation stage of the foam. The effect of the feed composition and reaction temperature has been studied in order to maximize the productivity, stability and selectivity to GRMs. The maximum productivity attained was 0.116 gC/gfoam h operating at 950 °C with a feed ratio of CH4/H2 = 3 (42.9 %CH4:14.3 %H2). The carbonaceous nanomaterials (CNMs) obtained were characterized by X-Ray diffraction, Raman spectroscopy and by transmission and scanning electron microscopy. The parameters of the kinetic model developed are directly related to the relevant stages of the process, including carburization, diffusion-precipitation and deactivation-regeneration. The balance among these sequential stages determines the overall performance of the activated foam. In conditions of rapid carburization of the Fe NPs (pCH4 > 14 %), the productivity to CNMs is favoured, avoiding an initial deactivation of the active sites by fouling with amorphous carbon. After a rapid carburization, the selectivity to the different CNMs is governed by the ratio CH4/H2, and mainly by the temperature. Thus, the formation of GRMs, mainly Few Layer Graphene (FLG) and even graphene, is favoured at temperatures above 900 °C. At lower temperatures, carbon nanotubes are formed., The authors acknowledge financial support from MICINN (Madrid, Spain) FEDER, Project ENE2017-82451-C3. P. Tarifa acknowledges the grant of MICINN (Ref.: BES-2014-069010).

In this work, we present the results of production of carbonaceous nanomaterials by decomposition of methane on a catalyst of Ni supported on a Biomorphic Carbon. The catalyst was prepared by thermal decomposition in a reductive atmosphere of vine shoots previously impregnated with the Ni precursor. In order to optimize the reaction productivity and selectivity, the effect of the main operational conditions (reaction temperature and feed composition) has been studied in a thermobalance. The main textural properties, BET area of 63 m2/g and 56% of microporosity, of the catalyst synthesized indicates that these materials are suitable for gas-phase reactions even in harsh conditions. Thus, the catalyst has proved to be active in the synthesis of carbon nanofibers and graphene related materials at elevated temperatures. The productivity, type, and quality of the carbonaceous nanomaterials obtained are deeply dependent on the operating conditions during the reaction. As an important fact, is has been obtained that the reaction temperature strongly affects the type of the nanomaterial produced. Thus, it is produced CNFs of bamboo type at temperatures until 850°C. Above this critical temperature, it is mainly obtained nanolayers of graphitic nature. The characterization results indicate that the highest quality graphenic materials were obtained operating at 950°C with 14.3% of CH4 and 14.3% of H2. The kinetic model used to analyze the experimental data is based on the more relevant stages of the mechanism of reaction., We acknowledge financial support from MINECO (Madrid, Spain) FEDER, Projects ENE2017-82451-C3 y ENE2013-47880-C3.

Determination of the optimal operating conditions for large-scale production of carbonaceous nanomaterials (CNTs), mainly carbon nanotubes (CNTs) but also graphene-related materials (GRMs), remains as a major industrial and scientific challenge. For this purpose, our group is conducting an extensive study to evaluate the effect of the reaction conditions and catalyst compositions on the productivity and selectivity to different carbonaceous nanomaterials. In the case of Co-based catalysts, we have found that the promotion with Cu or Mn has a dramatic effect on the type of carbon nanomaterials synthesized. Thus, the presence of Cu promotes the growth of CNTs, while Mn favors the formation of two-dimensional GRMs. In this context, we present here a comprehensive study of the CNTs growth via catalytic decomposition of methane using a Co-Cu/cellulose derived carbon catalyst. The influence of the reaction temperature (650–950 °C) and feed composition (7.1%–42.9% of CH4 and H2) on the yield and CNTs quality was evaluated. A transition in the characteristics of the carbonaceous nanomaterial growth was observed at about 800 °C. Below this temperature, the reaction was selective towards the formation of CNTs, while above 800 °C, the obtained nanomaterial exhibited a graphite-like morphology. In addition, the catalyst deactivation was quite low in the CNTs growth regime, attaining high productivity under 11 h of operation. The model used to study the kinetics of carbon formation allowed understanding the most influential variables in the growth process, revealing the existence of a transition temperature at which there is a change in the preferential path for the formation of CNTs or GRMs. After exploring a large set of reaction conditions, the best operating parameters for growing CNTs with high productivity (0.29 gC/gcat∙h) and quality (ID/IG = 1.10, I2D/IG = 0.13) were found at 750 °C under 28.6% CH4:14.3% H2:57.1% N2., The authors acknowledge financial support from the Ministry of Science and Innovation (Madrid, Spain), Projects ENE2017-82451-C3-1-R and ENE2017-82451-C3-2-R. W. Henao acknowledges the financial support from Fundación Carolina (Madrid, Spain) and the University of Zaragoza.

This paper reports the chemical identity and mechanism of action and formation of a cell growth inhibitory compound leached from some single-use Erlenmeyer polycarbonate shaker flasks under routine cell culture conditions. Single-use cell culture vessels have been increasingly used for the production of biopharmaceuticals; however, they often suffer from issues associated with leachables that may interfere with cell growth and protein stability. Here, high-performance liquid-chromatography preparations and cell proliferation assays led to identification of a compound from the water extracts of some polycarbonate flasks, which exhibited subline- and seeding density-dependent growth inhibition of CHO cells in suspension culture. Mass spectroscopy, nuclear magnetic resonance spectroscopy, and chemical synthesis confirmed that this compound is 3,5-dinitro-bisphenol A. Cell cycle analysis suggests that 3,5-dinitro-bisphenol A arrests CHO-S cells at the G1/Go phase. Dynamic mass redistribution assays showed tha...

This work presents the results obtained with novel Pd and Pd-Al catalysts supported on carbon, which have been prepared using a biomorphic mineralization technique. The catalyst synthesis procedure includes a stage of thermal decomposition under reductive atmosphere of cellulose previously impregnated with the metallic precursors. We have studied the influence of the temperature and time of decomposition, and of the Al precursor addition, on the textural and catalytic properties. The characterisation results indicate that the preparation method used leads to the formation of carbonaceous supports with a high microporosity (up to 97% micropore volume) and values of the BET surface up to 470 m/g while maintaining the original external structure. The use of low temperatures (ca. 600 °C) during the decomposition step allows the preparation of highly dispersed catalysts with narrow Pd particle size distributions. However, the thermal decomposition at elevated temperatures (ca. 800 °C) increases the Pd particle size due to the sintering of the metallic phase. This phenomenon is augmented with the decomposition time and is not affected by the presence of Al. Consequently, the catalytic activity of these materials in cyclohexene hydrogenation is strongly affected by the operational conditions used during the thermal decomposition step. Unexpectedly, the more sintered catalysts, i.e. those prepared at 800 °C, show the highest activity. According to the characterization results, this fact can be explained considering that the smaller Pd particles obtained after preparation at e.g. 600 °C are quite inactive because they are confined in the internal structure of the micropores of the support and/or embedded inside the carbon matrix. In contrast, after decomposition at 800 °C, the larger Pd particles formed are placed at the external surface of the catalyst, being accessible to the reactants. In addition, for the specific conditions under which the Pd is accessible, the presence of Al favours the cyclohexene conversion due to the enhancement of the adsorption on the Pd surface as a consequence of a charge transfer phenomenon. These results can serve as a guideline for the preparation of these catalysts based on raw lignocellulosic materials in order to maximize their catalytic performance., The authors acknowledge financial support from MINECO (Madrid, Spain)-FEDER, Projects CTQ2010-16132 and ENE2013-47880-C3-1-R.

This work addresses the preparation and application of the synthesis of graphene in Ni-Cu catalysts supported on carbonaceous materials. The catalysts have been prepared by a biomorphic mineralization technique which involves the thermal decomposition, under reductive atmosphere, of commercial cellulose previously impregnated with the metallic precursors. The characterization results indicate that the preparation method leads to the formation of carbonaceous supports with a moderate microporosity (ca. 33% pore volume) and adequate surface area (343 m2/g), maintaining the original external texture. The catalytic performance of these materials was previously tested in liquid phase reactions (Zampieri et al., 2007 [7]). In order to extend the use of these catalysts, in this work we present a study corresponding to a gas phase reaction: the synthesis of graphenic nanomaterials by catalytic decomposition of methane (CDM). The influence of the reaction temperature and of the feed composition (i.e.%CH4 and%H2) has been studied. The graphenic nanomaterials obtained after reaction were characterized by nitrogen adsorption-desorption isotherms, Raman spectroscopy and transmission electron microscopy (TEM). The results indicate that the carbonaceous nanomaterial with the highest quality is obtained operating at 950 °C and feeding 28.6% of CH4 and 14.3% of H2. The evolution of the carbon mass during the reaction time was analysed using a phenomenological kinetic model that takes into account the main stages involved during the formation of carbonaceous nanomaterials (NCMs). The results obtained from the kinetic model along with the characterization results enable the influence of the operating variables on each stage of the carbonaceous nanomaterial formation to be discerned.

The biomorphic mineralization technique is a powerful tool that allows to synthetize inorganic materials using as templates some structures of biological origin. This work presents kinetics results obtained during the liquid-phase hydrogenation of cyclohexene using a novel Pd–Al catalyst supported on biomorphic carbon. The catalysts have been prepared by a reactive method, using cellulose as carbonaceous precursor of the biomorphic support. The influence of the time and decomposition temperature (600–800 °C) of cellulose under hydrogen atmosphere has been studied. The characterization results indicate that the synthesis of these biomorphic materials produce solids with high microporosity. The metallic dispersion of the Pd nanoparticles is strongly dependent on the preparation conditions (i.e. heating rate, thermal decomposition temperature and time) of the carbonaceous precursor. The kinetic study has covered the effect of the hydrogen pressure and temperature, and was carried out using a real-time in situ Raman probe. The developed kinetic model considers the non-competitive adsorption between hydrogen and cyclohexene in two different types of Pd metallic sites. The model explains all the experimental results obtained, providing a rational explanation for the variation of the kinetic orders and apparent activation energy obtained using the empirical power-law model.

In recent years, catalytic decomposition of light hydrocarbons has been explored as an interesting alternative to produce COx-free hydrogen along with valuable carbonaceous nanomaterials. In this contribution, we report on the effect of B on the catalytic performance of Co/Al2O3 catalysts during propane decomposition. Unpromoted and B-promoted (0.5–5 wt.% B) Co catalysts were prepared by incipient wetness impregnation and thoroughly characterized using XRD, XPS, TPR, H2 chemisorption, CO-FTIR, Raman, TPO and TEM. The kinetic data were analysed using a phenomenological kinetic model. The kinetic results indicate the existence of an optimal amount of boron (ca. 2%) that maximizes both activity and stability of the catalyst. Based on the analysis of the kinetic parameters, it can be concluded that the presence of boron slows down the carburization and carbon diffusion steps, decreasing the amount of CNTs formed. However, the addition of Boron simultaneously decreases the formation of encapsulating coke, which is the cause of the catalyst deactivation.

We have studied the coupling of mass transport and chemical reaction rates in a biphasic emulsion system. The emulsion is stabilized by the presence of solid nanoparticles that, in addition, act as a catalyst for the reaction. These solid nanoparticles are composed of carbon nanotubes (CNTs) attached to SiO 2 nanoparticles. Both components, CNTs and silica, can be selectively impregnated by a catalytic active phase (e.g. Pd) forming the so-called nanohybrids . We have studied the application of the catalytic nanohybrids in emulsions of type ( w / o ), during the reaction of hydrodeoxygenation of vanillin (4-hydroxy-3-methoxy-benzaldehyde). This is a model reaction that occurs in the aqueous side of the interface of the emulsion. The main target for this work is to analyze the impact of mass transport of the main compounds between the aqueous and organic phases on the global performance of the reaction.

We have determined the single phase domain of LiGd 1− x Yb x (WO 4 ) 2 . The lattices parameters decrease as a function of Yb 3+ substitution in Gd 3+ sites. Transparent LiGd 1− x Yb x (WO 4 ) 2 fibers single crystals were successfully grown by the micro-pulling down technique (μ-PD). The Yb 3+ -doped LiGd(WO 4 ) 2 fibers single crystals have been pulled under stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to 120 μm. The fibers diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Fibers single crystals free of defects are observed for Ytterbium concentration in the melt up to 5 at%. Above this limit, inclusions and cracks appear and the optical quality of the fibers were deteriorated. The emission spectra of Yb 3+ -doped LiGd(WO 4 ) 2 were investigated.

This paper reports the characterization and catalytic results of a bimetallic Ni-Co/Mg-Al catalyst during the methane decomposition reaction for the co-production of H2 and nanocarbonaceous materials (e.g. carbon nanotubes-CNTs). The catalyst has been characterized before and after reaction with the aim of observing the changes in its structure and also the type of nanocarbonaceous materials formed. The influence on CNT growth rate, of the temperature during the calcination and reaction stages has been studied. The activity, stability and selectivity to the production of hydrogen and carbon nanotubes greatly depend on the catalyst composition and on the activation and reaction conditions used. The bimetallic Ni-Co/Mg-Al catalyst, prepared by controlled coprecipitation, shows higher stability that the monometallic catalysts Ni/Mg-Al and Co/Mg-Al. TEM and TPO observations indicate that the type of nanocarbonaceous material obtained when the catalyst does not suffer deactivation, corresponds to multi-wall carbon nanotubes (MWNT), with average diameter of ca. 20 nm. The kinetic results of CNT formation, obtained in a thermobalance, were evaluated using a Phenomenological Kinetic Model that includes the main relevant steps involved in CNT growth by catalytic decomposition of hydrocarbons (CCVD): (i) hydrocarbon decomposition at the gas side; (ii) carburization of the metallic nanoparticle surface; (iii) atom carbon diffusion (bulk and/or surface) (iv) nucleation and CNT growth; and finally (v) growth termination by catalyst deactivation or by the effect of steric hindrance of the CNTs formed. In this paper we have applied the Phenomenological Kinetic Model to study the influence of the calcination and reaction temperature. The obtained values for the kinetic parameters have realistic physical meaning in good agreement with the mechanism considered during the formation and growth of carbon nanotubes.

In this work, we present the application of a phenomenological kinetic model to the analysis of experimental results obtained with a LDH-based Ni–Mg–Al catalyst during the CCVD of methane. This model takes into account all the main stages implicated in CNT growth: methane decomposition, metal surface carburization, carbon diffusion, CNT nucleation and growth. The main causes of growth cessation, catalyst deactivation and steric hindrance have also been considered. The effect of the main operational variables, reduction (activation) and reaction temperatures and feed composition (hydrogen and methane composition), have been studied. The model fits very well the data obtained with the studied catalyst at the different operating conditions. As a consequence of the application of the model to the kinetic results, we can describe the influence of the operating conditions on each of the individual stages involved in the CNT formation.

A Phenomenological Kinetic Model has been developed that includes all the relevant steps involved in CNT growth by CCVD,that is, carbon source decomposition, nanoparticle surface carburization, carbon diffusion, nucleation, CNT growth, and growth termination by catalyst deactivation or by the effect of steric hindrance. Here we emphasize the importance of using a proper kinetic description of all the stages, in particular the initial carburization-nucleation and the growth cessation. We have discussed the different mechanisms proposed to explain the critical step of carburization−nucleation and have used an autocatalytic kinetic model to describe it. The two parameters involved in this autocatalytic equation allow a very good fit of the initial induction period usually observed during the growth of CNTs. In addition, rigorous formulations of the main causes of CNT growth cessation (catalyst deactivation by several causes and steric hindrance) have been proposed. The developed model is a versatile tool of ...

Hydrogenation of (-)-menthone and (+)-isomenthone was studied at 2.7 MPa and 100 oC. The objective was to produce a liquid menthol mixture rich in (-)-menthol from dementholized peppermint oil. Ni-based catalysts were tested and compared for this reaction: a) 6 and 12% Ni dispersed into a nonstoichiometric magnesium aluminate (Ni-Mg-Al) with spinel structure; b) Ni-Raney catalyst. Both types of catalysts were active for (-)-menthone and (+)-isomenthone hydrogenation. Lower conversion but higher selectivity to (-)-menthol was obtained with Ni-Mg-Al catalysts. However, they rapidly lost their activity. Instead Ni-Raney catalysts kept its original activity even after several hydrogenation runs.

We have studied the growth of layers of aligned nanocarbonaceous materials (NCM), in particular carbon nanotubes (CNTs) and carbon nanofibres (CNFs), synthesized via catalytic chemical vapour deposition (CCVD) of ethane over the surface of stainless steel mesh monoliths. The NCM coating properties have been optimized studying the effect of pretreatment of the substrate (acid attack, oxidation and reduction), reaction temperature, and C2H6 and H2 feed concentrations. The experimental results indicate that it is possible to obtain aligned bundles of carbon nanotubes and nanofibres uniformly distributed over all the surface of the mesh. These experiments also allow us to find the kinetics of growth of the aligned bundles of CNTs. The preparation of a catalytic coating with good properties such as good adhesion to substrate, open porosity (mesoporosity) to enhance the diffusion of reactants, uniform thickness of NCM layer, good mechanical strength and control over the microstructure of CNTs ensures good activity and durability of the CNT-based structured catalytic reactor.

The involvement of the cervical sympathetic ganglia (SCG) on body temperature and during the occurrence of the induced febrile response was investigated in rats. Bilateral superior cervical gaglionectomy (SCGx) attenuated the daily dark-phase temperature compared to that of the sham-operated rats during the first 2 days post surgery. Body temperatures returned to pre-surgery levels by Day-3. Ten days after surgery, a febrile response was induced by lipopolysaccharide (LPS) immune challenge. SCGx significantly blunted the LPS-induced febrile response. These data suggest that obliteration of the cervical sympathetic peripheral innervation impairs the capability to produce an induced febrile response.

Catalytic decomposition of methane is a potential alternative route for the production of hydrogen and nanocarbonaceous materials from natural gas and other hydrocarbon feedstocks. In the present paper, we report the results of characterization and catalytic behaviour during the methane decomposition reaction of a spinel-like Ni–Mg–Al catalyst prepared by coprecipitation. The influence of reaction temperature and feed composition on carbon content, carbon formation rate and carbon morphology has also been studied. The main consequence of MgO addition to the support is the increase in the activity and stability of the Ni–Al catalysts. The better performance of Ni–Mg–Al catalysts is due to the higher interaction generated between Ni particles and the support in this catalyst, which prevents the formation of large metallic particles. The carbonaceous products are carbon nanofibres (diameters ~10–35 nm) and amorphous carbon, which causes the catalyst deactivation by encapsulation. The amount of each type of carbonaceous material depends on the different operating conditions used. The reduction–reaction–regeneration cycles lead to a remarkable sintering of the Ni crystallites due to weakening of the metal-support interaction.

The growth of carbon nanofibers (CNF) was studied over a cordierite monolith previously coated with mordenite synthesized by a hydrothermal route. The zeolitic layer was deposited in order to have the sufficient number of cationic exchange sites to anchor the metallic particles responsible for the CNF formation. To produce the carbon nanofibers, cobalt was the active metal and acetylene, the source of carbon. The amount and type of nanocarbonaceous material obtained were studied as a function of the following operating conditions: (i) the reduction temperature of the metallic phase, (ii) the presence or absence of H 2 in the feed and (iii) the orientation of the monolith channels with respect to the flow direction. The results obtained indicate that under specific operating conditions, large amounts of fibers grow over the whole internal surface of the channels. These fibers provide a large specific surface, which indicates that this type of structured catalysts could be used as three-phase reactors.

cited By 57; International audience; Catalytic decomposition of methane is a potential alternative route for the production of hydrogen and carbon nanofilaments from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of Ni-Cu-Mg-Al catalysts. The effect of the Cu addition in the catalyst composition on activity and stability has been investigated. The influence of operating temperature and feed composition on carbon content and carbon formation rate has also been studied. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. A higher reaction temperature increases both the deactivation rate and the growth rate of filaments. An increase in the methane concentration generates a rise of the rate of carbon filament formation. The size of metal particles and the carbon filaments as well as the nanofilament texture depends on the copper content of the catalyst. When the Cu content is 7.6 wt.%, the carbon nanofilaments are nanofibers with a platelet texture, and the particles and CNFs sizes are widely distributed (50-400 nm). © 2006 Elsevier B.V. All rights reserved.

A kinetic study of the synthesis of carbon nanotubes (CNTs) by catalytic decomposition of acetylene over an iron-supported catalyst was carried out using a thermobalance. The CNTs were mainly characterized by transmission electron microscopy (TEM and HRTEM). It was found that there is a high influence of the reaction conditions (feed mixture, reaction temperature, etc.) on the final reaction yield and on the structural and morphological characteristics of the carbon products obtained. A complete discussion of the results by comparison of the carbon deposition curves with the TEM images is given. Finally, a kinetic model for explaining the growing mechanism of carbon nanotubes is developed and the displayed conclusions are clearly shown.

The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al2O3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.

This paper presents a mathematical relationship between the parameters of Levenspiel’s Deactivation Kinetic Model (LDKM) and those of the Deactivation Models with Residual Activity (DMRA) and their evolution over time. This correlation provides an explanation for the erroneous variation obtained in the kinetic parameters (deactivation order and deactivation function) over time when LDKM is used to fit deactivation data having a certain level of residual activity, which frequently leads to systematic errors in estimating intrinsic parameters, such as activation energies. The variations of the LDKM parameters cannot in fact be related to a physical phenomenon, but are only a consequence of a mathematical artifact. The methodology developed in this work provides a valuable tool for the comparison and discrimination between different models used in kinetic studies. The equations here presented are applied to analyze the deactivation by fouling of Pt/Al2O3 reforming catalysts during methyl cyclohexane dehydrogenation.

Ni−Al coprecipitated catalysts promoted with magnesium have been prepared using the rising and the constant pH techniques, two precipitant agents [(1) KOH and K2CO3, and (2) NH4OH)] and different metal contents. Catalyst characterization by temperature-programmed reduction and CO2 reforming of methane as a test reaction served to select the appropriate catalysts for use in the steam gasification of biomass. The catalysts selected were NiMgAl2O5, prepared at constant pH and precipitated with KOH and K2CO3; NiMgAl4O8 and NiMgAl1.24O3.86, both prepared at increasing pH with NH4OH. Biomass steam gasification experiments were carried out at 700 °C and at atmospheric pressure using different steam/biomass (S/B) and catalyst weight/biomass flow rate (W/B) ratios. From an analysis of the results obtained, the initial activity and stability of the catalysts have been studied. The NiMgAl2O5 catalyst presents the best performance showing the highest initial activity and stability. This work evidences an improvement ...

International audience; The effect of sulfur (S) content on the purity, stability and crystallization of Cu2ZnSnS4 material has been investigated. It was found that, for an excess sulfur starting concentration from 3 wt% to 12 wt%, high purity Cu2ZnSnS4 single phase compound could be obtained. In this sulfur range composition, Cu2ZnSnS4 alloy has been synthetized and vertically well textured along melt flow. Depending on the sulfur starting composition, binary SnS and ternary Cu4Sn7S16 phases can be precipitated and their concentration is connected to the initial sulfur quantity in the starting charge.

Glossopharyngeal afferents may be the neural channel by which immune challenge of the posterior oral cavity conveys information to the brain. If this is the case, then bilateral transection of the glossopharyngeal nerves (GLOx) should disrupt this communication. Injection of lipopolysaccharide (LPS) or interleukin (IL)-1beta into the soft palate (ISP) of sham-operated rats induced a dose-related febrile response. GLOx significantly attenuated the febrile response induced by ISP injection of both LPS and IL-1beta. In contrast, GLOx did not affect the febrile response when LPS or IL-1beta were injected intraperitoneally, indicating that the effect of GLOx is not systemic. These results provide experimental evidence for a novel neural pathway for immune-to-brain communication.

Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

The reforming of CH4with CO2(dry reforming) was studied at 800°C over SiO2and ZrO2supported Pt–Sn catalysts. Several preparation methods were investigated. It was found that the Pt/ZrO2catalyst had much higher activity and stability than the Pt/SiO2catalyst due to the ability of the ZrO2to promote CO2dissociation. On this catalyst, the decomposition of CH4and the dissociation of CO2occur via two independent pathways. The long-term activity of the catalyst is dependent upon the balance between the rate of CH4decomposition and the rate of cleaning of carbonaceous deposits. The co-impregnation of Sn and Pt on the ZrO2results in lower activity and stability than the monometallic catalysts. Depending on the reaction conditions, disruption of the Pt–Sn alloys may occur, causing deposition of tin oxide that inhibits the role of the ZrO2. Special preparation methods can result in the controlled placement of Sn on the Pt particle, minimizing the promoter–support interaction. These catalysts exhibit higher activity and stability than the monometallic catalyst under severely deactivating conditions, 800°C, and a 3 : 1 ratio of CH4: CO2. It is possible to deposit Sn onto Pt/ZrO2catalysts in a manner which reduces carbon deposition without inhibiting the beneficial role of the support.

Human mesenchymal stem cells (hMSCs) possess three properties of great interest for the development of cell therapies and tissue engineering: multilineage differentiation, immunomodulation, and production of trophic factors. Efficient ex vivo expansion of hMSCs is a challenging requirement for large scale production of clinical grade cells. Low-cost, robust, scalable culture methods using chemically defined materials need to be developed to address this need. This study describes the use of a xeno-free synthetic peptide acrylate surface, the Corning® Synthemax® Surface, for culture of hMSCs in serum-free, defined medium. Cell performance on the Corning Synthemax Surface was compared to cells cultured on biological extracellular matrix (ECM) coatings in xeno-free defined medium and in traditional conditions on tissue culture treated (TCT) plastic in fetal bovine serum (FBS) supplemented medium. Our results show successful maintenance of hMSCs on Corning Synthemax Surface for eight passages, with cell expansion rate comparable to cells cultured on ECM and significantly higher than for cells in TCT/FBS condition. Importantly, on the Corning Synthemax Surface, cells maintained elongated, spindle-like morphology, typical hMSC marker profile and in vitro multilineage differentiation potential. We believe the Corning Synthemax Surface, in combination with defined media, provides a complete synthetic, xeno-free, cell culture system for scalable production of hMSCs.

In adrenal cortex and other steroidogenic tissues including glial cells, the conversion of cholesterol into pregnenolone is catalyzed by the cytochrome P450scc located in the inner mitochondrial membrane. A complex mechanism operative in regulating cholesterol access to P450scc limits the rate of pregnenolone biosynthesis. Participating in this mechanism are DBI (diazepam binding inhibitor), an endogenous peptide that is highly expressed in steroidogenic cells and some of the DBI processing products including DBI 17-50 (TTN). DBI and TTN activate steroidogenesis by binding to a specific receptor located in the outer mitochondrial membrane, termed mitochondrial DBI receptor complex (MDRC). MDRC is a hetero-oligomeric protein: only the subunit that includes the DBI and benzodiazepine (BZD) recognition sites has been cloned. Several 2-aryl-3-indoleacetamide derivatives (FGIN-1-X) with highly selective affinity (nM) for MDRC were synthesized which can stimulate steroidogenesis in mitochondrial preparations. These compounds stimulate adrenal cortex steroidogenesis in hypophysectomized rats but not in intact animals. Moreover, this steroidogenesis is inhibited by the isoquinoline carboxamide derivative PK 11195, a specific high affinity ligand for MDRC with a low intrinsic steroidogenic activity. Some of the FGIN-1-X derivatives stimulate brain pregnenolone accumulation in adrenalectomized-castrated rats. The FGIN-1-X derivatives that increase brain pregnenolone content, elicit antineophobic activity and antagonize punished behavior in the Vogel conflict test in rats. These actions of FGIN-1-X are resistant to inhibition by flumazenil, a specific inhibitor of BZD action in GABAA receptors but are antagonized by PK 11195, a specific blocker of the steroidogenesis activation via MDRC stimulation. It is postulated that the pharmacological action of FGIN-1-X depends on a positive modulation of the GABA action on GABAA receptors mediated by the stimulation of brain neurosteroid production.

Acute superior cervical ganglionectomy (SCGx) induces in the rat a supraliminal release of neurotransmitter in the innervated tissues (i.e., thyroid gland). This temporary adrenergic hyperactivity is correlated with a significant depression of the thyroid economy resembling the nonthyroidal illness (NTI) syndrome in the rat, and suggest that the sympathetic nervous system may mediate thyroidal changes in NTI. In order to gain further insight into the thyroidal depression in the NTI syndrome, we studied the thyroidal norepinephrine (NE) turnover in turpentine oil (TURP)-induced NTI syndrome and the role of the cervical ganglia (SCG) in the development of NTI in the rat. TURP administration to sham operated rats induced a rapid and significant fall in plasma T4 and TSH levels, in the thyroidal response to exogenous TSH (TIU) and in the thyroidal NE content compared to controls (sham + saline) (T4: 3.1 +/- 0.3 vs. 5.1 +/- 0.6 micrograms/dl, respectively, mean +/- SE, p less than 0.02; TSH: 1.4 +/- 0.4 vs. 4.7 +/- 1.4 ng/ml, respectively, p less than 0.05; TIU: 92 +/- 14 vs. 201 +/- 20 cpm.microliter thyroid/cpm.mg plasma (T/P ratio), respectively, p less than 0.01; thyroidal NE: 680 +/- 20 vs. 761 +/- 29 pg/mg thyroid, respectively, p less than 0.05). The thyroidal turnover rate of NE, however, was significantly increased in TURP-injected rats compared to controls (122 +/- 13 vs. 86 +/- 10 pg/mg/h, respectively, p less than 0.05). TURP injection to chronic SCGx rats induced a similar fall in plasma TSH compared to controls (SCGx + saline) (1.3 +/- 0.2 vs. 4.3 +/- 1.1 ng/ml, respectively, p less than 0.02); plasma T4 and TIU, however, did not change significantly (T4: 3.4 +/- 0.4 vs. 3.7 +/- 0.3 micrograms/dl, respectively, NS; TIU: 172 +/- 8 vs. 226 +/- 27 T/P ratio, respectively, NS).(ABSTRACT TRUNCATED AT 250 WORDS)

A series of polyamidoamine dendrimers (generations 1.5–4.0, with ethylenediamine as the central core) has been investigated in methanol solution by means of the small-angle X-ray scattering technique. The comparative analysis of the structural properties of integer and half-integer generations of the dendrimers seems to be dominated by rather different trends.

cited By 58; International audience; This work presents the kinetics and characterization results of Ni-Cu-Mg-Al catalysts used for the synthesis of carbon nanofibers (CNFs) by catalytic methane decomposition (CCVD). Ni-Cu-Mg-Al-based lamellar double hydroxides (LDHs) were synthesized by the coprecipitation method using either a regular maturation stage or a colloid mill process. After calcination at 1073 K and reduction at 1023 K, the formation of alloyed Ni-Cu nanoparticles over a mixed MgO-MgAl2O4-NiAl2O4 support was observed. The kinetic study of CNF synthesis by CCVD of methane was carried out in a thermobalance operated as a differential reactor. Small proportions of Cu (≤ 10 wt%) were found to improve the activity and mainly the stability over time of Ni-Mg-Al catalysts. On the other hand, large proportions of Cu dramatically decrease the activity of the catalysts in terms of CNF yield. The catalysts were able to produce high-quality CNFs with a graphitic or turbostratic structure. The size of metal particles and the diameter of carbon filaments obtained depend on the catalyst composition (i.e., Ni:Cu ratio). The application of a kinetic model based on the growing mechanism of CNFs allows us to determine the influence of the operating conditions on the kinetic parameters. © 2007 Elsevier Inc. All rights reserved.

cited By 26; International audience; The question of how the texture and structure of carbon nanofilaments (CNTs) are determined during growth is addressed via their preparation using the vapour phase method over Ni-Cu-Mg-Al catalysts. The CNTs formed and the related catalyst particles were investigated by high resolution transmission electron microscopy, electron diffraction, and X-ray energy dispersive spectroscopy. The nanofilament features were found to directly relate to the catalyst particle size and morphologies, which in turn depend on both the Ni/Cu ratio in the Ni-Cu alloy that forms the catalyst particles and the route by which they were prepared. The extent and orientation of graphenes within the carbon nanofilaments were found to be controlled by the extent of the related catalyst crystal faces and the angle value between the latter. It is proposed that energetics of graphenes, basically involving the ratio of the edge over the core carbon atoms, the energetic cost of heterocycles (pentagon), and that of the stress induced by the strain at graphene bending sites, determine whether the carbon nanofilaments would actually grow as nanotubes (i.e., hollow) with the dual "herringbone-bamboo" texture, or as nanofibres (i.e., not hollow) with either the "herringbone" texture or the platelet texture. Only the latter allowed the genuine graphite (3D periodicity) structure to develop, while the other nanofilament types could merely adopt the turbostratic structure. Meanwhile, it was demonstrated that the herringbone nanotubes and nanofibres here prepared are of "cup-stack" rather than "single helix" type. © The Royal Society of Chemistry.

cited By 13; International audience; Catalytic decomposition of methane to produce carbon nanofilaments, CNFs (nanofibers and/or nanotubes), and hydrogen was carried out on Ni-Cu-Mg-Al catalysts. The catalysts used as mixed oxides were obtained from the calcination at 1073 K of the corresponding lamellar double hydroxides (LDHs). The subsequent reduction at 1023 K of the calcined LDHs gave rise to Ni0 and Cu0 (and possibly Ni-Cu alloy) associated to an inorganic substrate with a MgO-MgAl2O4-NiAl2O4 mixed composition that were able to produce quality CNFs at 923 K. The size of metal particles and the carbon filaments as well as the nanofilament texture depends on the copper content of the catalyst. At a Cu content of 7.6 wt%, the carbon nanofilaments are nanofibers with a platelet texture, and the particles and CNFs sizes are widely distributed (50-400 nm). On the other hand, for a Cu content of 3.8 wt%, the size distribution is narrower and the CNFs are thinner (20-50 nm), nanofibers and nanotubes showing a 'herringbone' texture with twice the angle between the graphene layers and fiber axis, exhibiting values mainly in the range of 40°-60°. © 2006 Elsevier Ltd. All rights reserved.

The acute-phase febrile responses of the inbred Lewis (LEW) and Fischer 344 (F344) rat strains and their parent Sprague-Dawley (S-D) strain to immune challenge with lipopolysaccharide (LPS) were compared. LEW and S-D males and females displayed a biphasic febrile response with a markedly attenuated second phase in F344 rats. At 2 h after LPS (50 microg/kg), corticosterone was significantly higher in F344 than in LEW rats, but serum interleukin-1beta was higher only in F344 than LEW males. These data suggest that the greater LPS-induced corticosterone response of F344 rats mediates differences between febrile responses of F344 and LEW males and females.

The effect of the Ni/Mg molar ratio on the activity, selectivity, and coke formation of NiO·MgO·Al2O3 catalysts modified with Cr3+ for acetylene hydrogenation is studied. An optimum Ni/Mg ratio has been found, for which it is necessary to, add MgO to the support in order to modulate the catalytic properties of Ni. When the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fail to increase, but actually decrease, while coke formation simultaneously increases. The existence of the above mentioned optimum ratio is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidine species. The rate of coke growth is assumed to be a consequence of the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.

Publisher Summary The development of heterogeneous catalysts to promote selective organic reactions is a field of growing interest. Whereas heterogeneous acid catalysis has been applied to a good number of processes, basic catalysis is far less developed. Oxidation reactions are among the most interesting processes from the industrial point of view, and epoxidation is one of the most important oxidation reactions, given the usefulness of epoxides as synthetic intermediates. The epoxidation of electron-deficient alkenes can be carried out with hydroperoxides in the presence of a base. The catalytic activity of the hydrotalcites in the epoxidation of electron-deficient alkenes depends on the basicity of the solid, which can be controlled by the Mg/A1 ratio. The highest catalytic activity corresponds to a hydrotalcite with a ratio Mg/A1 = 3. The solid with this Mg/Al ratio is also the catalyst with the highest number of basic sites, as measured by phenol adsorption in liquid phase. The epoxide/dioxolane selectivity also increases with the basicity of the solid and the reaction time, showing the reversibility of the reaction of dioxolane formation. The hydrotalcite is recoverable and reusable at least twice without the loss of catalytic activity.

The preparation, characterisation and catalytic behaviour of a Ni-Co-Cr-Zn-Al catalyst in the reaction of acetylene hydrogenation have been investigated. The influence of the temperature and feeding composition on the activity, selectivity and coking have also been examined. An increase in the partial pressure of H 2 in the feed causes an increase in the initial conversion, and a diminution in the deactivation rate, ethylene selectivity and in the rate of coke deposition. The opposite effects are observed with respect to the influence of the partial pressure of acetylene. The temperature has a low influence on the activity and selectivity. However, an increase of the operation temperature produces an augmentation in the initial coking rate and a diminution in the final rate of coke formation. A kinetic model of coking growth, that assumes the existence of two types of active sites, has been used in order to analyse the experimental coking data obtained during the acetylene hydrogenation reaction.

The increasing interest in ammonia decomposition is due to the fact that this compound can be used advantageously as a hydrogen carrier, allowing the development of single-step hydrogen generation systems. With the aim of developing efficient reactors for ammonia decomposition, e.g. for fuel cell applications, it is imperative to investigate the kinetics and reaction mechanism in depth. The main goal of this work is to develop reliable kinetic models that are able to predict the performance obtained using integral reactors, e.g. monoliths. In this case, an almost complete NH3 conversion is obtained, with a high H2 concentration at the exit of the reactor. The operating conditions, mainly the gas composition, are very different along the reactor. In addition, the temperatures needed to attain such large conversions are usually high. The kinetic models developed in this contribution are based on the Langmuir isotherm, considering that all the adsorbed species can be kinetically relevant, that the slow step or steps can be partially reversible, and that the surface can be considered as energetically uniform, i.e. ideal. Among other conclusions, the results obtained indicate that the variable kinetic orders and apparent activation energies frequently reported in the literature can be direct consequences of the data analysis and can therefore also be explained without considering any change in the controlling step with the reaction temperature or in the hydrogen or ammonia concentration.

Evidence that neurosteroids are potent modulators of the action of GABA at GABAA receptors has prompted the investigation of the mechanism that controls brain neurosteroid synthesis by glial cell mitochondria in vivo. In vitro studies suggest that the interaction of the diazepam binding inhibitor (DBI)--a polypeptide that is abundant in steroidogenic cells--with glial mitochondrial DBI receptors (MDRs) is a crucial step in the physiological regulation of neurosteroid biosynthesis. MDRs bind 4'-chlorodiazepam (4'-CD), N,N-di-n-hexyl-2-(4-fluorophenyl)-indol-3-acetamide (FGIN-1-27), and the isoquinoline carboxamide PK 11195 with high affinity, and these ligands have been used to investigate whether the stimulation of glial MDRs increases brain pregnenolone production in vivo. Adrenalectomized and castrated (A-C) male rats (to eliminate peripheral sources of pregnenolone) were pretreated with trilostane (to prevent pregnenolone metabolism to progesterone), and the pregnenolone content in brain regions dissected after fixation with a 0.8-s exposure to microwave irradiation focused to the head was determined by HPLC followed by specific radioimmunoassay. The forebrain and cerebellum of A-C rats contained 4-7 ng of pregnenolone/g of tissue, and the olfactory bulb contained 10-14 ng/g. These concentrations of brain pregnenolone are only 30-40% lower than those of sham-operated rats. In contrast, the plasma pregnenolone content of sham-operated rats was 2-3 ng/ml, but it was only 0.15-0.20 ng/ml in the plasma of A-C rats. In A-C rats, treatment with the MDR ligands 4'-CD and FGIN-1-27 increased the pregnenolone content in the brain but failed to change the plasma or peripheral tissue content of this steroid. The effect of 4'-CD on brain pregnenolone content was maximal (70-100% increase) at the dose of 18 mumol/kg, 5-10 min after intravenous injection. The effect of oral administration of FGIN-1-27 on brain pregnenolone content was maximal (80-150% increase) at doses of 400-800 mumol/kg and peaked at approximately 1 h. That this effect of FGIN-1-27 was mediated by the MDR was documented by pretreatment with the MDR partial agonist PK 11195 (100 mumol/kg, i.p.). PK 11195 did not affect basal brain pregnenolone content but prevented the accumulation of brain pregnenolone induced by FGIN-1-27. FGIN-1-27 and 4'-CD failed to increase the brain concentration of dehydroepiandrosterone in A-C rats. These data suggest that glial cell MDRs play a role in neurosteroid biosynthesis in vivo.

The mitochondrial DBI receptor complex (mDRC; previously called the peripheral benzodiazepine receptors) is linked to the production of neurosteroids such as pregnenolone sulfate, dehydroepiandrosterone sulfate, and others. In order to gain further information as to the function of the mDRC in the brain, we have constructed and tested both in vitro and in vivo a novel series of ligands, 2-arylindole-3-acetamides. The SAR studies detailed herein delineate some of the structural features required for high affinity binding to the mDRCs. In most cases the new ligands were prepared by use of the Fischer indole synthesis. Variations in the length and number of the alkyl groups on the amide nitrogen were probed together with the effects of halogen substituents on one or both of the aryl rings. Some ligands were also synthesized for study which represent conformationally constrained versions of the parent structure. Broad screening studies revealed these indoleacetamides to be highly selective for the mDRC, since they failed to bind with any significant affinity to other receptor systems. Some of the ligands were found to exhibit Ki values in the low nanomolar range for the mDRC as measured by the displacement of [3H]4'-chlorodiazepam. A subset of these ligands was also shown to stimulate pregnenolone formation from the mitochondria of C6-2B glioma cells with an EC50 of about 3 nM. In animal experiments ligands selected for further study were found to exhibit antineophobic effects, in spite of the fact that they exhibit no direct action on GABAA receptors. Consequently, it is postulated that these ligands owe their action to an indirect modulation of GABAA receptor function, presumably by stimulation of neurosteroid production and release from glial cells, followed by neurosteroid modulation of GABA's action on the chloride ion channel conductance of GABAA receptors.

The functional and anatomic compensatory response of the parathyroid gland was examined in hemiparathyroidectomized (HPTx) rats whose parathyroid hormone (PTH) secretion was stimulated by a low calcium diet. These responses were compared with those observed in the thyroid gland of hemithyroidectomized (HTx) rats. Rats kept on a low calcium diet for 10 days were subjected to HPTx, HTx, or sham operations. Throughout the experiment (up to 28 days after surgery), serum calcium levels of HPTx rats were lower than the basal, with delta values (mg/dl, mean +/- SEM) of -0.66 +/- 0.17 and -0.84 +/- 0.17, (P less than 0.05) 3 and 28 days after surgery, respectively. Serum PTH decreased significantly from 7 to 21 days after HPTx, reaching normality at day 28 after surgery. In HTx rats, serum thyroxine (T4) levels diminished significantly 7 days after surgery, and attained normality thereafter. The mitotic index (number of metaphases/1,000 cells) in parathyroid glands of colchicine-treated HPTx rats increased significantly in comparison to sham-operated controls, when examined 2 or 40 days after surgery. The mitotic index of thyroid follicular cells was significantly higher than that of their respective controls, 2 but not 40 days after HTx. These results indicate that after HPTx, a delayed compensatory response is found when the animals are kept under a low calcium diet. Parathyroid response is both delayed and of a minor degree compared to that found in the thyroid gland after HTx.

3 H-Flunitrazepam (FNZP) binding was examined in a crude membrane fraction obtained from rat interscapular brown adipose tissue (IBAT). A single population of binding sites was apparent with dissociation constant (K D ) = 0.47 ± 0.04 um and maximal number of binding sites (B max ) = 31 ± 5 pmol.mg prot −1 . From the activity of several benzodiazepine (BZP) analogs to compete for the binding, the peripheral nature of FNZP binding was tentatively established. Similar BZP binding sites were detectable in isolated IBAT mitochondria. Exposure of rats to 4°C for 15 days decreased B max significantly without affecting K D . Cold-induced decrease in B max of BZP binding was prevented by surgical IBAT denervation. Denervation prevented or impaired the increased activity of the mitochondrial markers succinate dehydrogenase and malate dehydrogenase in IBAT of cold-exposed rats, but did not affect monoamine oxidase activity. Hypophysectomy of rats decreased significantly both K D and B max of IBAT BZP binding. Thyroidectomy, adrenalectomy or ovari ectomy did not affect IBAT BZP binding parameters. The BZP analogs diazepam, clonazepan and Ro 5–4864 decreased significantly quanosine 5' -diphosphate binding (GDP) in IBAT mitochondria while co-incubation of Ro 5–4964 or clonazepam with the peripheral type BZP antagonist PK 11195 did not modify BZP activity on GDP binding. Our results indicate that BZP binding in rat IBAT may belong to the peripheral type, is decreased by a cold environment through activation of peripheral sympathetic nerves and is affected by hypophysectomy. BZP and GDP binding in IBAT mitochondria seem not to be functionally related.

To examine the role of thyroid parasympathetic innervation in organ's function, rats subjected to inferior laryngeal nerve (ILN) section were employed. This procedure decreased thyroid [3H]choline uptake by about half. Bilaterally ILN-sectioned rats treated with methylmercaptoimidazole for 4 days exhibited a significant impairment of the methylmercaptoimidazole-induced goitrogenic response. Unilateral ILN section resulted in further atrophy of the ipsilateral thyroid lobe in hypophysectomized rats. One week after ILN section a significant decrease of serum T4 and an increase of serum TSH were observed. Bilateral ILN section generally decreased circulating T4 for up to 28 days after surgery, while a unilateral ILN section caused a transient T4 decrease for 1 week after surgery. Compensatory thyroid growth in rats subjected to unilateral thyroidectomy (hemi Tx) and ILN section performed ipsilaterally to the remaining lobe, was significantly smaller than that of rats subjected to hemi Tx alone. Hemi Tx depressed serum T4 and increased serum TSH levels significantly. These hormonal changes were prevented by unilateral superior cervical ganglionectomy (SCGx), but were unaffected by ILN section. The combination of SCGx and ILN section negated the facilitating effect of SCGx on thyroid secretion and impaired the increase in compensatory thyroid growth brought about by thyroid sympathetic denervation. Thyroid mitotic index studies in hemi Tx rats receiving ILN section, SCGx, or a combination of both indicated that the increase in the number of follicular mitosis caused by hemi Tx was significantly impaired by ILN section and was significantly increased by SCGx. SCGx potentiation of thyroid follicular mitotic activity was partially prevented by concomitant ILN section. These results support a significant role of thyroid sympathetic and parasympathetic innervation in the control of organ's growth and secretory activity.

The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO·ZnO·Al2O3 catalysts (modified with Fe3+ or Cr3+) during acetylene hydrogenation has been studied. Coke formation is decreased significantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ratio for activity, selectivity and coke formation performance has been found. The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, if the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned optimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites covered by a monolayer of ethylidines. A kinetic model of coke growth is proposed assuming the existence of two types of coke associated with the hydrogenolitic and hydrogenating sites respectively.