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Alkali and alkaline earth metal cations were introduced into zeolite to explore their influence on the activity of supported palladium in the total combustion of methane and to study how the acido-basic properties of the support affected the structure and the oxidation state of Pd. The results show that all samples exhibit a slight activation with time. Moreover, at temperatures below 430°C, PdLiNaY seems to be the most active catalyst, while PdHY is the most active sample at higher temperatures. Except for PdHY, all Pd catalysts lead to an activation energy (Ea) value in the range of 79-97 kJ [mol.sup.-1]. Key words: NaY zeolite, basicity, palladium, methane combustion. Resume : Les cations compensateurs alcalins et alcalino-terreux sont introduits dans les supports a base de zeolithe pour etudier l'influence de leurs basicites sur l'activite du palladium supporte dans la reaction de combustion totale du methane. Cette etude a aussi pour objectif de montrer que les proprietes acido-base du support peuvent affecter la structure et l'etat d'oxydation du palladium. Les resultats montrent que tous les echantillons s'activent legerement avec le temps. De plus, a une temperature inferieure a 430°C, PdLiNaY semble etre le catalyseur le plus actif, tandis qu'a des temperatures plus elevees, PdHY est l'echantillon le plus actif. A l'exception de PdHY, tous les catalyseurs au palladium ont une energie d'activation (Ea) dans la gamme entre 79 et 97 kJ [mol.sup.-1]. Mots-cles : zeolithe NaY, basicite, palladium, combustion de methane., Introduction La reaction de combustion du methane etudiee sur les catalyseurs a base de palladium ou de platine a ete consideree sensible a la structure. (1-4) La nature de la [...]

The synthesis of 1,2-naphtho-annelated barrelene derivatives, namely dimethyl-7,12-dihydro-7, 12-ethenobenzo[a]anthracene-13,14-dicarboxylate (4a) and dimethyl-7,14-dihydro-7,14-ethenodibenzo [a,j]-anthracene-15,16-dicarboxylate (4b), and the investigation of their photoreactivity in solution and in the solid state is reported. The irradiation of 4a and 4b resulted in regioselective di-π-methane rearrangements to give semibullvalene products; however, the product distribution is inverted upon changing the reaction medium from solution to the solid state. In solution, an [alpha-naphtho-vinyl bridging predominates as the initial photochemical step because of the selective stabilization of the corresponding transition state, as discussed on the basis of the structures of the related biradical intermediates, whereas the solid-state photoreaction is significantly influenced by the steric constraints within the crystal lattice that only allow a β-naphtho-vinyl bridging. Key words: di-π-methane rearrangement, barrelene derivatives, solid-state photochemistry. On a reahse la synthese de derives condenses 1,2-naphtobarrelenes, a savoir le 7,12-dihydro-7,12-ethenobenzo[a]anthracene-13,14-dicarboxylate de dimethyle (4a) et le 7,14-dihydro-7,14-ethenobenzo[a,j]anthracene-15,16-dicarboxylate de dimethyle (4b), et on a etudie leur photoreactivity en solution et a l'etat solide. L'irradiation des composes 4a et 4b conduit a des rearrangements di-π-methane regioselectifs et a la formation de produits semibullvalenes; toutefois, la distribution des produits est inversee lorsque le milieu reactionnel passe de la solution a 1'etat solide. En solution, un pont α-naphtovinyle predomine comme etape photochimique initiale en raison de la stabilisation selective de l'etat de transition correspondant, tel que discute sur la base des structures des intermediaires biradicalaires apparentes, alors que la photoreaction a l'etat solide est fortement influencee par des contraintes steriques dans le reseau cristallin qui ne permettent que la formation d'un pont β-naphtovinyle. Mots-cles : rearrangement di-π-methane, derives du barrelene, photochimie a l'etat solide. [Traduit par la Redaction], Introduction The di-π-methane rearrangement (1) of benzo-annelated derivatives of barrelene (bicyclo[2.2.2]octatriene) (2) and norbornadiene (bicyclo[2.2.1]heptadiene) (3) has been shown to be an appropriate model reaction for the assessment of the [...]

The production of multi-walled carbon nanotubes (MWNTs) by the catalytic chemical vapour deposition (CCVD) method was examined over a series of Ni-Co/[La.sub.2][O.sub.3] catalysts with methane as the carbon source. The catalyst composition and the reaction conditions were optimized by analyzing the effluent gas with gas chromatography. Various techniques, such as X-ray diffraction (XRD), Transmission electron microscopy (TEM), Raman spectra, and Thermal gravimetric analysis (TGA) were used to characterize the catalysts and products. The results indicate that the unreduced catalyst 30Ni-10Co/[La.sub.2][O.sub.3] showed the highest activity for methane decomposition, and the highest carbon yield, being 624.2%, was obtained over this catalyst at 700 °C after 60 min of reaction with hydrogen as carrier gas. TGA results suggest that the purity of carbon nanotubes (CNTs) can reach up to 98 wt% after a single-step purification by hydrochloric acid. Key words: carbon nanotubes, catalytic chemical vapour deposition, methane decomposition, Ni-Co/[La.sub.2][O.sub.3] catalyst. On a examine la production de nanotubes de carbones a parois multiples (NTCPM) par la methode de deposition de vapeur chimique catalytique (DVCC) sur une serie de catalyseurs de Ni-Co/[La.sub.2][O.sub.3], en n'utilisant que du methane comme source de carbone. On a optimise les conditions reactionnelles et la decomposition du catalyseur en procedant a l'analyse des gaz effluents par chromatographie gazeuse. La caracterisation des catalyseurs et des produits a ete effectuee en utilisant diverses techniques, tels les patrons de diffusion des rayons X (DRX), la microscopie de transmission electronique (MTE), la spectrometrie Raman et l'analyse par thermogravimetrie (ATG). Les resultats indiquent que les catalyseurs non reduits, 30Ni-10Co/[La.sub.2][O.sub.3] presentent la plus grande activite pour la decomposition du methane et le rendement le plus eleve en carbone, 624,2%, a ete obtenu sur ce catalyseur, a 700 °C, apres 60 minutes de reaction avec de l'hydrogene comme gaz porteur. Les resultats de ATG suggerent que la purete des nanotubes de carbone (NTC) peut atteindre 98% en poids apres une seule purification avec de l'acide chlorhydrique. Mots-cles : nanotubes de carbone, deposition de vapeur chimique catalytique, decomposition du methane, catalyseur de Ni-Co/[La.sub.2][O.sub.3]. [Traduit par la Redaction], Introduction Carbon nanotubes have been widely studied not only for their unique structural, electronic, and mechanical properties but also for their potential applications in electronic devices, displays, and in the [...]