L. Bonnetain

Formation and characterization of catalytic carbons obtained from co disproportionation over an iron nickel catalyst—II: Characterization

The metal-carbon composites obtained by CO disproportionation (2CO→CO2 + C) over an iron-nickel (containing 75 Wt%Ni) are characterized by T.E.M., X-ray diffraction and krypton adsorption at 77 K. Besides the now well known carbon filaments, two other types of morphology are described and the experimental conditions of their formation determined: the bitubes and bifilaments as low temperature products and carbon shells as high temperature products. X-Ray diffraction studies show that the structure of the alloy is not modified by fragmentation and deposition and that the d 00.2 spacing is about 3.42 A when formed below 500°C and about 3.38 A when formed above 500°C. Krypton isotherms confirm that the mean size of the carbon particles increases with temperature of their formation. The filamentous-shell morphology transition suggests that a change in the mechanism of metal fragmentation occurs around 500°C which is also observed in the deposition kinetics. Finally we propose two semi quantitative index numbers for characterization of the crystallographic orientation of surfaces, the two numbers being derived from the shape of krypton adsorption isotherms.

Insertion des ions PF6−, AsF6− et SbF6− dans le graphite par methode electrochimique. caracterisation des produits obtenus

Abstract PF 6 − , AsF 6 − and SbF 6 − ions were intercalated into graphite from a propylene carbonate electrolyte by an electrochemical method. During the intentiostatic charging of the system Li-Al/P.C., LiX (molar)/graphite one can observe a continuous increase of potential up to 5.2 V. This value is reached for a C 24 + charge of the graphite (Fig. 3). The discharge from the intercalated compound of graphite (I.C.G.) presents three plateaus: at 4.7 V until C 48 + , 4 V between C 48 + and C 96 + and 2.5 V between C 96 + and graphite (Fig. 4). X-Ray powder studies (Fig. 5) of C 48 + X − χ P.C.I.C.G. show that such compound is a second stage one. The anion is solvated by four molecules of propylene carbonate between the layers of graphite. This solvation causes the considerable thickness of the inserted layers (10.6 A for PF 6 − 4 P.C., 10.9 A for SbF 6 − 4 P.C.). The detailed structure (Fig. 8) of these layers is clarified by intensity line calculations. The presence of four molecules of solvent was confirmed by thermogravimetric measurements (Fig. 6). The first stage compound C 24 + X − 4 P.C. could not be obtained outside of the cell. The C, P, As and Sb analysis in the I.C.G. provided a composition in good agreement with the formula C 24n + P.C., where n is the stage. Owing to their high potential of 5.2 V against lithium, these compounds are of great interest for their utilisation as cathodes in high energy density batteries.

Composés d'insertion ternaires graphite-métal alcalin Lourd-Benzène

Resume Les composes ternaires graphite-metaux alcalins (K, Rb, Cs)-benzene ont ete prepares par immersion des composes binaires dans le benzene liquide a temperature ambiante. Apres quelques heures ou quelques jours dimmersion, suivant lorigine du graphite (Ceylan ou Madagascar), aucune evolution des echantillons nest plus perceptible et ils sont alors seches et analyses. Les resultats sont tres semblables pour tous les graphites ou metaux alcalins utilises. Si les composes binaires du premier stade MC8 restent inchanges, par contre les composes de stade superieur gonflent considerablement dans le benzene. Les analyses chimiques et thermogravimetriques faites essentiellement sur les composes avec K permettent dattribuer la formule KC24(C6H6)2,25-3,1 au compose forme a partir de KC24. Les diffractogrammes des composes ternaires de meme type sont identiques quel que soit le metal alcalin K, Rb, ou Cs. Linsertion du benzene provoque un changement de stade: un compose binaire de deuxieme stade donnant un compose ternaire du premier stade, un compose de troisieme stade donnant un melange du premier et du deuxieme stade…. Les periodes didentite Ic suivant laxe c sont 9,29 A pour MC24(C6H6)x (premier stade), 12,60 A pour MC48(C6H6)x (deuxieme stade). Labsence dinfluence de la dimension de lion alcalin permet de conclure que seules les molecules de benzene sont responsables de lecartement des plans dans les composes ternaires. La largeur de lespace libre qui est de 5,94 A implique que les molecules de C6H6 sont perpendiculaires aux plans graphitiques.

Disproportionation of CO on iron-cobalt alloys—II

Catalyst poisoning observed during carbon formation by CO disproportionation can be related to competing carbiding and oxidizing reactions. Hence the thermodynamic stability of the catalytic metal (Fe, Co or Ni) under the conditions of the disproportionation is essential to prevent the formation of oxides or carbides catalytically inactive. We show that alloying the metal, which decreases its thermodynamic activity, increases the (T, pco, pco2) domain of its stability. A thermodynamic approach of the carbon-oxygen-iron cobalt alloy system predicts that a maximum expansion of the domain of stability of the metallic phase would be obtained with an alloy containing 50% cobalt, 50% iron. These results are discussed by comparison with some previous experimental results.

Disproportionation of CO on iron-cobalt alloys—III: Kinetic laws of the carbon growth and catalyst fragmentation

Abstract The rate of CO disproportionate catalysed by filings of an iron-cobalt alloy (49wt% Co, 49wt% Fe, 2wt% V) has been studied between 400°C and 650°C using C0 C0 2 mixtures ranging from 95% CO, 5% CO 2 to 35% CO, 65% CO 2 . Consistent with a previous thermodynamic study, poisoning of the reaction is observed when the temperature and the gas composition are such that carburizing or oxidizing of the catalyst may occur. When the metal is thermodynamically stable, the only processes to be taken into account are the fragmentation of the catalyst and the carbon growth on the metal fragments. The dependence of the rate of these two processes on the thermodynamic activity of the carbon in the gas phase has been determined. The influence of this thermodynamic activity and of the temperature on the size of the metal fragment has been studied. Our results support a mechanism of carbon deposition controlled by a bulk diffusion of carbon atoms through the metal fragment driven by a concentration gradient.

Hydrogenation of catalytic carbons obtained by CO disproportionation or CH4 decomposition on nickel

Abstract The hydrogenation of catalytic carbons has been studied in the temperature range of their deposition (300–700°C) by CO disproportionation or by CH 4 decomposition on nickel powders. When obtained under non carbiding conditions, the catalytic carbons are very reactive between 350 and 600°C where uncatalysed carbons are inert. The reactivity does not depend on the temperature of deposition and on preliminary heat treatment, but depends on the degree of gasification. This reactivity is imputed to the quality of the metal carbon interface which allows a good deposition-gasification reversibility. When deposition occurs under carbiding conditions, both deposition and subsequent hydrogenation are poisoned by the carbon formed by thermal decomposition of the carbide.

Disproportionation of CO on iron-cobalt alloys—II: Kinetic study on iron-cobalt alloys of different compositions

Abstract The catalytic activities in CO disproportionate of iron-cobalt alloys of various composition have been compared to the activities of the pure metals at 600°C using a C0 C0 2 gas mixture (75% CO). The catalytic activities of the alloys are all greater than those of the pure metals. The synergetic effect is maximum for cobalt content between 37.5 and 50 wt %. These findings are consistent with a previous thermodynamic study which predicts that such compositions are the least liable to be poisoned by side reactions of carburization and oxidization. Furthermore X-ray diffraction studies show that contrary to pure metals the structures of the alloys are not modified by carbon deposition. The filamentous carbon deposit induces an important fragmentation of the metal catalyst originally under the form of coarse filings. The smallest particle sizes are obtained from the most active alloy composition.

Composés d'insertion graphite-hexafluorures de tungstène et de molybdène

Resume Malgre leurs grandes similitudes, les fluorures WF6 et MoF6 ne reagissent pas de la meme facon avec le graphite. Par simple contact avec le graphite a temperature ordinaire, MoF6 donne un compose bleu noir du deuxieme stade apres 48 heures et un du premier stade apres une semaine. Par contre, linsertion de WF6 ne se produit seulement que si Cl2, F2 ou HF gazeux ou liquide sont ajoutes dans le ballon de reaction. Le compose le plus riche (deuxieme stade) est obtenu avec HF. Des analyses thermogravimetriques preliminaires des composes graphite-fluorure de neme stade donnent les formules C(11 ± 1)nMoF6 et C(14 ± 1)nWF6. La diffraction X de ces composes donne la meme serie de raies 00l avec une periode didentite suivant laxe c, Ic = 5,0 + 3,35nA. Ainsi, lespace entre deux couches adjacentes au fluorure insere est a peu pres de 5A, proche de la valeur 4,78 A calculee pour la plus petite epaisseur des molecules WF6 et MoF6.

Essais d'insertion dans le graphite d'halogenures de titane(IV): Structure et proprietes du compose lamellaire graphite-tetrafluorure de titane

Abstract Syntheses of lamellar compounds were attempted by heating mixtures of graphite and halides of titanium(lV), TiCl4 and TiF4 in chlorine atmosphere confined in sealed tubes. The whole liquid range of TiCl4 was explored without observing any reaction. Thus Crofts result [4] about the non-intercalation of this compound was confirmed. In case of TiF4, a synthesis was obtained at 300°C. Differential thermal analysis (DTA, see Fig. 1) and X-ray powder diffraction show clearly that TiF4 is intercalated into the lattice of graphite. DTA: Fig. 1(c, d and e) are the curves for the intercalated compound: 1(c and d) are obtained under dry argon, 1(e) under air. No indications of physical transformation of pure TiF4 (Fig. 1a) are observed. An X-ray pattern of a product obtained at 25 atm of chlorine is given in Table 1: the strong band for pure graphite is split in two lines around 3 and 3.7 A. Elemental analysis gives a certain range of non stoichiometry from C19 to C24,TiF4 for the product made under chlorine at a pressure larger than 5 atm. In all cases, no chlorine has been found in the compound. Assuming a 3rd stage layer structure and atomic configuration of TiF4 as shown in Fig. 2. the relative intensities of X-rays diffraction lines were calculated for C15 to C27TiF4. The results are shown in Fig. 3 for the 00l diffraction lines: 001 reflexion has a minimum for C21 TiF4 and as we have never seen this reflexion, we assume that this is the ideal stoichiometry. Table 2 compares the observed and calculated intensities for C21TiF4. Agreement is quite good. Synthesis carried out under pressure of chlorine lower than 5 atm gave disordered products. Table 3 compares the obtained X-ray pattern for a product made at 1 atm of chlorine and the pattern calculated on the following assumptions: 4th stage, stoichiometry of C28TiF4. The pattern of TiOF2 is also present in the table. Agreement is not very good, and moreover the pattern of products depends on the pressure of chlorine as shown on Fig. 4 for the 004 and 005 reflexions. We have had then to introduce a “pseudo-stage” n + x (formula (2) in appendix). On Fig. 5 the inverse of this pseudo-stage is plotted vs the Cl2 pressure. A stable domain of 3rd stage is observed between 5 and 19 atm. Below 5 atm higher stages and disordered products are obtained. Figures 6 and 7 relate to a thermal stability study of the C21 TiF4 product. Figure 6 is a thermolysis curve made under dry argon (a) and under vacuum (10 2torr) (b). Figure 7 is a thermolysis curve made in a diffraction high temperature chamber under vacuum (10−5 torr): 1 (n + x) is plotted vs temperature. In all cases the product gives higher stages and disordered products.

Generateurs electrochimiques lithium/composes d'insertion du graphite avec FeCl3, CuCl3, MnCl2 et CoCl2

Resume Des composes dinsertion du graphite du premier stade avec FeCl3, CuCl2, MnCl2 et CoCl2 ont ete prepares et utilises comme electrodes positives de generateurs electrochimiques au lithium. Les techniques electrochimiques de decharge ont permis de determiner les caracteristiques de ces piles, notamment les rendements en courant et en energie, ainsi que les densites denergie atteintes. Les resultats obtenus presentent un grand interet pratique.