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In the field of hydrotreating (HDT) catalysis, density functional theory (DFT) calculations are of great help to explore new active phases on the basis of volcano curve relationships correlating HDT activities and the sulfur–metal (S–M) bond energy chemical descriptor, calculated in transition metal sulfides catalysts. In the present study, we synthesize Mo 1− x W x S 2 solid solutions supported on γ-alumina. For non-promoted systems, the catalytic tests reveal that a continuous and linear evolution of the catalytic activity is obtained for solid solutions for x varying between 0 and 1. As expected from the calculated S–M bond energy values, no synergy effect is observed in that case. For Ni and Co promoted Mo 1− x W x S 2 active phases (ternary metal sulfides), the S–M bond energy values determined with an interpolation model of the binary sulfides predict that NiMo 1− x W x S 2 phases should be more active than NiMoS and NiWS ones. In contrast, CoMo 1− x W x S 2 phases are expected to develop weak synergetical effect with respect to CoMoS and CoWS ones. Experiments performed on Co and Ni promoted Mo 1− x W x S 2 active phases confirmed the DFT-volcano curve prediction and an increment of about 30% in HDS catalytic activity is obtained for NiMo 0.5 W 0.5 S catalysts in both model molecule conversion and gas oil treatment.

Electrochemical reduction of a SnO 2 electrode for a lithium ion cell is known to result in formation of Li 4.4 Sn alloy+2Li 2 O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO 2 electrode, using in situ 119 Sn Mossbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li 4.4 Sn for β -Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li 1.4 Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li 2 O). Then (third step), red SnO-like Sn(II) species are formed, along with some Sn(IV). Especially during the second and third steps, the working electrode is far from thermodynamic equilibrium despite the low oxidation rate. This non-equilibrium behavior is probably related to the ultrafine particle size resulting from electrochemical grinding.

International audience; Synchrotron and Neutron Solutions to Oil Industry ProblemBulk and alumina-supported Mo0.5W0.5S2 compounds were prepared. The presence of both molybdenum and tungsten in every [MS2] layer (intralayer solid solution) was evidenced using EXAFS spectroscopy, the only local method which can provide such relevant structural information. The catalytic interest of such solid solution is to provide a continuous variation of M-S bond strength which is considered as the key parameter of the catalytic activity transition metal sulfides. The solid solution is maintained after promotion by Ni either on unsupported or supported state.

In situ 119Sn Mossbauer study of an SnO2 electrode was performed during discharge of a lithium ion cell. The first step is lithium intercalation into the SnO2 host structure. This lithium intercalation results in reinforcement of the SnO2 lattice instead of direct decomposition of the oxide upon reduction. This first step is followed by the reduction of tin dioxide into unusual tin species (possibly “exotic” forms of Sn(II) or Sn(0)). The last step of the discharge consists in Li–Sn alloy formation. However, non-reduced SnO2 is present nearly up to the end of the discharge despite a very low discharge regime. It seems highly probable that this fact is related both to slow Li diffusion and disconnection of SnO2 particles due to Li2O formation. The working electrode appears to be rather far from equilibrium during continuous discharge, which means that ideal succession of well-defined stages cannot describe the real phenomena involved in the operating battery.

A series of highly dispersed single phase pentlandites (Co9S8, NiCo8S8, FeCo8S8, Ni3Co6S8, Ni4.5Fe4.5S8) was prepared by coprecipitation followed by sulfidation and reducing treatment at 300 °C. These mixed sulfides consist of crystalline particles with homogeneous distribution of the elements as shown by high-resolution microscopy and EDX analysis. The distribution of the cations in tetrahedral and octahedral sites of the pentlandite structure was characterized by EXAFS and Mossbauer spectroscopy. The catalytic activity of the sulfides in the model thiophene hydrodesulfurization reaction increases in the following order: Ni4.5Fe4.5S8 ≪ FeCo8S8 < Co9S8 < NiCo8S8 < Ni3Co6S8. Activity−properties correlations are discussed, on the basis of extended Huckel electronic structure calculations and temperature programmed reduction measurements.

Niobium-molybdenum disulfide solid solution (Nb x Mo 1− x S 2 ) has been prepared in a dispersed state on gamma alumina. The existence of this solid solution supported on alumina carrier has been proven with the help of EXAFS technique. The catalytic properties of these materials have been studied in hydrogenation and hydrodesulfurization reactions. Interestingly, as already observed for niobium sulfide, the activity of the Nb x Mo 1− x S 2 solid solution (HDS of DBT, P tot =33 bar) is not decreased in the presence of H 2 S up to p (H 2 S)=200 Torr, at least up to x =0.4.

Triassic basalt of the Middle Atlas has been subject to metamorphic transformation then weathering. Occurrence in both metabasalt and saprolite of ubiquitous clay minerals, such as smectite and mixed layers chlorite–smectite, makes it difficult to distinguish between the two alteration facies and explains the interest of complementary sources of information. In the Bhallil weathering profile, petrographical and mineralogical analyses of primary igneous minerals and their alteration products coupled with Fe oxidation state determination in clay fractions allow to identify three alteration facies: ( i ) metamorphic basalt, where iron occurs mainly as the ferrous form; ( ii) the lower part of saprolite, where iron is partially oxidized to its ferric form; ( iii ) the upper part of saprolite, where iron is completely oxidized. To cite this article: A. Dekayir et al., C. R. Geoscience 334 (2002) 877–884.

In this paper we report our structural and electrochemical investigations of tin dioxide and lead dioxide electrodes in order to highlight the difference observed between them. The electrochemical reactions of these two oxides are known: the reduction of the metal oxide and the reversible formation/decomposition of the lithium-metal alloys. The reversible capacity of these systems is based on the alloy formation. The first reaction is supposedly irreversible (formation of Li2O), but the X-ray diffraction analysis and especially119Sn Mossbauer spectrometry show a possible re-oxidation of the metal particles in the case of tin dioxide electrodes. However, this reaction is not fully reversible and occurs at a high potential vs. Li. For lead dioxide electrodes, the re-oxidation of the metal particles seems more difficult in spite of the similar structure of both oxides.

The catalytic properties of niobium sulphides, i.e. NbS 3 and Nb 1.12 S 2 have been examined in the conversion of nitrogen-containing molecules (n-pentylamine and pyridine), and in the hydrogenation of biphenyl. Compared to MoS 2 , NbS 3 and Nb 1.12 S 2 show higher activities for all these reactions and a peculiar ability to perform CN and CC bond cleavage. The activities for these last reactions have been related to the acid-base properties of niobium compounds. Moreover, the trisulphide is more active than Nb 1.12 S 2 . These differences in activities are discussed in relation to the structural features of both sulphides and the existence in NbS 3 of S 2 2− groups and metal-metal pairing.

Intercalation compounds of MoS2 with monomeric or OH-bridged dimeric ruthenium arene cationic complexes have been obtained by interaction of MoS2 single-layer dispersions with aqueous solutions of [(arene)Ru(H2O)3 ]SO4 [arene=C6H6 , C6H2Me4-1,2,4,5, C6H4Me-1-Pri-4]. The structural arrangement of these intercalation compounds is discussed on the basis of XRD and EXAFS data. The arene ligands are orientated roughly perpendicular to the MoS2 slabs. Their nature determines the interlayer spacing as well as the intercalated cation content. Heating removes the organic ligands, while ruthenium-containing species remain in the van der Waals space of MoS2.

Precipitation of sulfur from acetone solution after heating at 50 °C in the presence of (NH4)2Mo2S12 leads to the formation of hollow microtubules (L = 5−100 μm, d = 100−400 nm). The walls of the tubes (ca. 10 nm) consist of S8 molecules, the surface layer being enriched with molybdenum sulfide species

A series of simple (CoS(2), Co(9)S(8), NiS(2), NiS, Ni(3)S(2)) and mixed sulfides (NiCo(2)S(4), Ni(0.33)Co(0.67)S(2), Ni(3)Co(6)S(8), CuCo(2)S(4), Cu(0.33)Co(0.67)S(2)) was prepared using low-temperature procedures. To obtain the mixed sulfides, the mixtures of the solutions of the corresponding salts were precipitated by Na(2)S and then heated in a sulfiding atmosphere at 300 degrees C. It has been found that the product phase composition depends on the sulfiding atmosphere. Using a H(2)S/Ar mixture leads to pyrite type sulfides, whereas treatment in H(2)S/H(2) flow allowed the preparation of Ni-Co and Cu-Co thiospinels. The as prepared highly dispersed single-phase materials were characterized by X-ray powder diffraction, scanning electron microscopy, temperature-programmed reduction (TPR), elemental analysis, and BET surface area measurements.

W x Mo (1− x ) S 2 compounds with both molybdenum and tungsten present in every disulfide layer, i.e., as an intralayer solid solution, can be obtained. Several synthesis routes have been used but all of them do not allow this solid solution, which evidences the decisive role of the nature of the precursor, to be obtained. We never observed formation of a W x Mo (1− x ) S 2 solid solution resulting from stacking of homocationic [WS 2 ] and [MoS 2 ] layers (interlayer solid solution).

Niobium sulfide has been recently found to be an interesting new active phase for hydrodesulfurization. In this work, niobium was used as a dopant for a conventional hydrotreating catalyst. A NiMo hydrotreating catalyst in the oxide form was doped with various contents of Nb precursor salt (0

Hydrotreating properties of unsupported and supported niobium sulfides are compared to those of molybdenum sulfide catalysts. Niobium catalysts demonstrate higher activities than molybdenum sulfide ones and remarkable selectivity in cracking and isomerisation reactions.

New unsupported and carbon-supported niobium sulfide (pure or Ni doped) catalysts, characterized by EXAFS, demonstrate interesting properties for hydrodesulfurization.

Few studies deal with the properties of niobium sulfide as a hydrodesulfurization catalyst. In this paper, the preparation of carbon-supported niobium sulfide catalysts was optimized concerning (ii) the nature of the soluble precursor, (ii) the drying process, and (iii) the sulfurizing treatment, which was always performed under atmospheric pressure but for different H2S-based flows and reaction temperatures. The activities of the best samples prepared with niobium oxalate as the impregnation salt, drying at room temperature, and presulfurization with N2/H2S at 400°C are superior to that of a supported MoS2 reference catalyst. Alumina-supported systems can be sulfurized only under more severe conditions (CS2 under pressure). After optimization of the sulfurization treatment (400°C, 10 h) the maximum activity obtained is significantly higher than that of a molybdenum sulfide reference catalyst. The catalytic activities of the various catalysts studied are related to their morphological and chemical characteristics using TPR and EXAFS measurements. The work illustrates the importance of the support and the sulfurization method on the genesis of a niobium sulfide active phase.

The crystallographically ordered Fe x TiSe 2 compounds ( x = 0.25, 0.38, and 0.50) have been investigated as a function of temperature using Mossbauer spectroscopy, susceptibility measurements, and X-ray diffraction. It is found that the coupling between the iron- d localized and the TiSe 2 -band levels decreases as x increases, contrary to what is usually observed for lower iron contents in related disordered materials. For x = 0.50, magnetostriction is evidenced below T N . Besides, the consequences of iron intercalation in TiSe 2 are analyzed from the viewpoint of the crystal dimensionality as deduced from a thermal expansion study.

The two-dimensional or one-dimensional character of a structure is related to an important anisotropy of the chemical bonds. Such solids can be considered as built up of layers or chains, the internal cohesion of which is due to strong iono-covalent or metallic bounds. On the contrary, interlayer or interchain bonds are weak, generally of the Van der Waals type. The existence of such independant structural units involves crystallographic, chemical and physical consequences : — the weakness of the interlayer or interchain bonds allows gliding motions which lead to polytypism, — this weakness also allows the slabs or chains to be pulled apart through various intercalations (alkali or alkaline earth metals, organic molecules, ammonia), — the structural anisotropy results in an important anisotropy of the physical properties. Fermi surfaces with large parallel portions can allow charge density waves to occur. These facts are illustrated with some examples chosen among transition metal chalcogenides : two dimensionality of the TX₂ dichalcogenides, pseudo-one-dimensionality of compounds such as NbSe₃ and I[x]NbSe₄., Le caractère bidimensionnel ou unidimensionnel d'une structure provient d'une très forte anisotropie des liaisons chimiques qui y sont mises en jeu. Un tel solide est construit à partir d'individualités structurales telles que feuillets ou fibres. A l'intérieur de ces entités, les liaisons sont fortes (ionocovalentes ou métalliques). Entre feuillets ou fibres les liaisons sont faibles, généralement de type Van der Waals. L'existence d'unités structurales nettement individualisées implique des conséquences cristallographiques, chimiques et physiques : — la faiblesse des liaisons qui s'exercent entre individualités structurales voisines en autorise le glissement relatif. Ceci explique la multiplicité des formes polytypiques observées par exemple pour les composés lamellaires, — cette faiblesse permet également d'écarter feuillets ou fibres par le jeu des diverses insertions envisageables (alcalins, alcalino-terreux, molécules organiques, ammoniac), — l'anisotropie géométrique de telles structures se traduit par ailleurs par une grande anisotropic des propriétés physiques. Enfin, l'existence de surfaces de Fermi à larges portions parallèles peut déterminer l'apparition d'ondes de densité de charge. Ces faits sont illustrés à l'aide d'exemples choisis parmi les chalcogénures d'éléments de transition : bidimensionnalité des dichalcogénures TX₂, pseudo-unidimensionnalité de composés tels que NbSe₃ et I[x]NbSe₄., Danot Michel. Solides à dimensionnalité réduite. In: Bulletin de Minéralogie, volume 103, 3-4, 1980. Propriétés des solides minéraux. I. Dissolution. II. Ordre-désordre.