N° 23. — Étude cinétique de la synthèse du tungstate de cobalt CoWO

La synthèse du tungstate de cobalt CoWO4 par réaction à l’état solide d’un mélange d’oxyde cobalteux CoO et d’anhydride tungstique, a fait l’objet d’une étude cinétique, en utilisant des produits divisés et en travaillant sous atmosphère neutre. Deux mélanges réactionnels ont été préparés par broyage prolongé des réactifs. Ce broyage a pour effet de fournir des mélanges d’oxydes où une première germination a déjà eu lieu. Dans ces conditions, pour des températures peu élevées, les courbes cinétiques ont une forme sigmoïde suggérant un processus autoaccéléré. Par contre, pour des températures supérieures, comme à toute température dans le cas d’un mélange initialement exempt de tungstate, l’allure de la courbe isotherme indique une réaction autoinhibée. Enfin, certaines précisions sont apportées sur la succession des diffusions intergranulaire et en volume dans de telles réactions

Heats of Adsorption of CO on a Cu/Al2O3Catalyst Using FTIR Spectroscopy at High Temperatures and under Adsorption Equilibrium Conditions

International audienceThe adsorption of CO on a 4.7% Cu/Al2O3 catalyst either reduced or oxidized is studied using the FTIR spectra recorded at adsorption temperatures Ta in the range 298-740 K and with two constant partial pressures Pa (10 3 and 10 4 Pa). On the reduced solid and at Ta = 300 K a single IR band is detected at 2092 cm-1 ascribed to a linear CO species (denoted by L0) on Cu particles. The FTIR spectra lead to the determination of the evolution of the coverage of the L0 species with Ta for the two CO pressures. In the temperature range 380 K > Ta > 480 K, the curves = f(Ta) for the two CO pressures are in very good agreement with Temkin's adsorption model. Moreover, it is shown that, in the range 300-600 K, the curves are in agreement with an adsorption model considering (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the coverage in the range 0-1. This leads to the determination of the heat of adsorption E , at various coverages from E0 = 82 kJ/mol at = 0 to E1 = 57 kJ/mol at = 1. It is shown that these values are similar to those obtained using the Clausius-Clapeyron equation. On the oxidized Cu/Al2O3 catalyst, an IR band is detected at 2120 cm-1 at 300 K. Its intensity increases either with time on stream in 1% CO/He or with increase in Ta. This IR band is ascribed to a linear CO species (denoted by L1) on Cu + sites and the increase in its intensity is assigned to the reduction of the copper oxide surface. The maximum in the superficial concentration of the Cu + sites is obtained after reduction in 1% CO/He at 473 K. This allows the determination of the heats of adsorption of L1 species at various coverages which linearly vary with from E'0 = 115 kJ/mol at = 0 to E'1 = 58 kJ/mol at = 1. The two L species are present simultaneously on the surface for an incomplete reduction of the solid, and it is shown that this does not significantly affect the heats of adsorption of the two adsorbed species. Moreover, the aging of the reduced catalyst has no effect on the heats of adsorption of the L0 and L1 species but leads to the detection of a new IR band at 2003 cm-1 ascribed to a bridged CO species on Cu sites. The heat of adsorption of this species is strongly higher than those of the two L species varying with the coverage from 130 kJ/mol at = 0 to 78 kJ/mol at = 1. Finally, it is shown that the heats of adsorption of the L0 and L1 species are in accord with the literature data on the stability of the two adsorbed species in the course of a desorption at room temperature

Diversification écologique des premiers ostracodes planctoniques au Silurien Supérieur

International audienc

Diversification écologique des premiers ostracodes planctoniques au Silurien Supérieur

International audienc

Des faunes d'ostracodes myodocopes écologiquement distinctes provenant d'un même horizon du Silurien supérieur d’Espagne

Silurian myodocopes have been demonstrated to be the pioneer pelagic ostracods. Their ecological shift into the water column, during the middle Silurian (Wenlock-Ludlow), is now well documented from sites around the world, but the evolution of this fauna during the late Silurian (Pridoli) remains little studied. We recognise, for the first time, two ecologically distinct myodocope ostracod faunas from the same late Pridoli horizon (possibly ?Wolynograptus bouceki - Skalograptus transgrediens biozones) of southern Spain (Alcaracejos, province of Córdoba). One fauna, associated with black shales, comprises five species belonging to three myodocope families (bolbozoids, entomozoids and cypridinids). The other fauna, recovered from large dark-reddish calcareous nodules and associated with the planktonic crinoid Scyphocrinites elegans, comprises seven myodocope species belonging to the same three families and includes one new species, Calocaria callundosa sp. nov. Although the shale and nodule faunas have two species in common, they are clearly different in terms of diversity, abundance and size of the specimens. The discrepancies between these two assemblages could either be explained by sampling or taphonomic bias, or because they represent faunas with different ecologies. In the latter hypothesis, the myodocope association in the shales could represent the “background” planktonic fauna, while the fauna in the nodules could have lived in the water column in the vicinity of the Scyphocrinites “floating islands”, or scavenge around the dead crinoids on the sea floor. These two diverse assemblages also allow discussions on the temporal and palaeogeographical distributions of these late Silurian myodocope ostracods.Il est aujourd'hui communément accepté que les myodocopes du Silurien étaient les premiers ostracodes pélagiques. Si leur colonisation de la colonne d'eau, au cours du Silurien moyen (Wenlock-Ludlow), est maintenant bien documenté dans des sites du monde entier, l'évolution de cette faune à la fin du Silurien (Pridoli) reste peu étudiée. Nous décrivons ici, pour la première fois, deux faunes d'ostracodes myodocopes écologiquement distinctes provenant du même horizon du Pridoli supérieur (possiblement dans les biozones ?Wolynograptus bouceki - Skalograptus transgrediens) du sud de l'Espagne (Alcaracejos, province de Cordoue). L'une des faunes, associée aux schistes noirs, comprend cinq espèces appartenant à trois familles de myodocopes (bolbozoïdés, entomozoïdés et cypridinidés). L'autre faune, trouvée dans de grands nodules calcaires rouge foncé et associée au crinoïde planctonique Scyphocrinites elegans, comprend sept espèces de myodocopes appartenant aux trois mêmes familles et inclut une nouvelle espèce, Calocaria callundosa sp. nov. Bien que les faunes des schistes et des nodules présentent deux espèces en commun, elles sont clairement différentes en termes de diversité, d'abondance et de taille des spécimens. Les divergences entre ces deux assemblages pourraient s'expliquer soit par des biais d'échantillonnage ou taphonomiques, soit elles pourraient représenter des faunes aux écologies différentes. Dans cette dernière hypothèse, la faune de myodocope des schistes pourrait représenter la faune planctonique « classique », tandis que la faune des nodules aurait vécu dans la colonne d'eau à proximité des « îles flottantes » de Scyphocrinites, ou bien se serait nourrie de manière nécrophage de ces crinoïdes morts une fois qu'ils auraient atteint le fond de la mer. Ces deux assemblages diversifiés ont également permis de discuter les distributions temporelles et paléogéographiques de ces ostracodes myodocopes du Silurien terminal.Ministerio de Ciencia e InnovaciónDepto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

Redox behavior of CeO2-ZrO2 mixed oxides: I. Influence of redox treatments on high surface area catalysts

International audienceRedox and textural/structural properties of high surface area ceria-zirconia mixed oxides with composition ranging from 0 to 85 mol% of ZrO2, have been studied using Raman spectroscopy, X-ray diffraction, textural characterization, magnetic susceptibility measurements, temperature-programmed reduction and oxidation, oxygen storage and buffering capacity measurements. Special attention was given to the effects of aging under redox conditions. Correlation between chemical composition, textural, structural and redox properties of the oxides is reported. While for pure ceria almost no reducibility occurs below 773 K, mixed oxides, independently of their composition, exhibit an oxygen storage and buffering capacity at low temperature, which remains stable even after several oxidation/reduction cycles. This improvement with respect to CeO2 is remarkably high in the oxides with high zirconia content and is not affected by worsening of textural properties induced by redox-aging. Phase transformation and/or structural modification are detected as a consequence of the redox treatments, which could be at the origin of such effect

Neuroanatomie et pneumaticité du crocodile "cornu" éteint de Madagascar Voay robustus : implications pour la phylogénie et la paléoécologie des crocodylidés

International audienceAbstract Voay robustus , the extinct Malagasy “horned” crocodile, was originally considered to be the only crocodylian representative in Madagascar during most part of the Holocene. However, Malagasy crocodylian remains have had confused taxonomic attributions and recent studies have underlined that Crocodylus and Voay populations coexisted on the island for at least 7500 years. Here, we describe the inner braincase anatomy of Voay robustus using x‐ray computed tomography on four specimens, to provide new anatomical information that distinguishes Voay from Crocodylus , especially features of the brain endocast and the paratympanic sinuses. Geometric morphometric analyses are performed on 3D models of the internal organs to compare statistically Voay with a subset of extant Crocodylidae. Following these comparisons, we build an endocranial morphological matrix to discuss the proposed phylogenetic affinities of Voay with Osteolaeminae from an endocranial point of view. Additionally, we discuss the use of internal characters in systematic studies and find that they can have a major impact on morphological analyses. Finally, new radiocarbon data on Voay and subfossil Crocodylus specimens are recovered between 2010 and 2750 cal BP, which confirm the cohabitation of the two species in the same area for a long period of time. We thus assess several extinction scenarios, and propose a slightly different ecology of Voay compared to Crocodylus , which could have allowed habitat partitioning on the island. Our approach complements information obtained from previous molecular and morphological phylogenies, as well as previous radiocarbon dating, together revealing past diversity and faunal turnovers in Madagascar

Redox behavior of CeO₂-ZrO₂ mixed oxides: II. Influence of redox treatments on low surface area catalysts

International audienceRedox and textural/structural properties of low surface area (LS) ceria-zirconia mixed oxides with composition ranging from 0 to 85 mol% of ZrO2, have been studied using Raman spectroscopy, X-ray diffraction, textural characterization, magnetic susceptibility measurements, temperature-programmed reduction and oxidation and buffering measurements. Special attention was given to the effects of aging under redox conditions. Correlation between chemical composition, textural, structural and redox properties of the oxides is reported. Comparison with previously reported high surface area (HS) samples shows that in spite of the initial low surface area of the present samples (≈20 versus ≈100 m2g−1), the reduction occurs at fairly low temperatures when zirconium is incorporated into the CeO2 lattice. Remarkably, the favorable effects of the redox-aging on the reduction behavior are more pronounced in the present LS samples compared to the previously investigated HS ones. Raman and X-ray investigations revealed that important modifications of the structure take place as a consequence of the redox aging, leading to appreciable CeO2 segregation, which could suggest some correlation between these parameters