Preparation of carbon molecular sieves by CVD

[ES] Se han preparado tamices moleculares de carbono (CMS) mediante el dep sito de tomos de carbono, por pir lisis de benceno, sobre la superficie de carbones activados. La pir lisis de benceno a temperaturas comprendidas entre 650-850 ¼C genera el cierre progresivo de los microporos debido a la creaci n de constricciones en la red microporosa que limitan la accesibilidad de determinadas mol culas. El uso de temperaturas superiores a la temperatura de carbonizaci n del precursor (850 ¼C) introduce complicaciones debido a la descomposici n y sinterizaci n parcial del s lido. Flujos bajos de nitr geno (30 mL min-1) con alto contenido en benceno (13 %) producen un dep sito homog neo a lo largo de las paredes y los materiales presentan distribuciones microporosas mas anchas. El dep sito de carbono sobre carbones activados por pir lisis de benceno a temperaturas inferiores a 850 ¼C, utilizando flujos relativamente altos (150 mL min-1) con baja concentraci n de benceno (1 %) genera tamices con vol menes de microporos alrededor de 0,25 cm3 g-1 y anchuras de poro distribuidas en intervalos estrechos: inferiores a 0,33 nm, entre 0,33-0,41 nm y entre 0,41-0,54 nm. El porcentaje de quemado del carb n activado de partida tiene relativamente poca influencia sobre las propiedades del tamiz molecular, cuya textura microporosa guarda una estrecha relaci n con un volumen de microporos controlado por el grado de dep sito de carbono.[EN] Carbon molecular sieves (CMS) have been prepared by chemical vapour deposition (CVD) of carbon from the pyrolysis of benzene molecules on activated carbon surfaces. The pyrolysis of benzene at temperatures in the range 650-850 ¼C restricts the accessibility of the micropores due to the creation of constrictions on the microporous network. Temperatures higher than 850 ¼C (temperature of carbonisation) add difficulties due to decomposition and sinterization processes. Low flows of nitrogen (30 mL min-1) and high benzene content (13 %) produce a more uniform carbon deposition and wider micropore size distributions. The CVD process on carbons activated to different burn-offs, using temperatures below 850 ¼C, flows of 150 mL min-1 and benzene content of 1 %, gives rise to microporous materials which exhibit micropore volumes around 0,25 cm3 g-1 and narrow micropore size distributions: below 0,33 nm, between 0,33-0,41 nm or between 0,41-0,54 nm. The burn-off of the activated carbon has a relative little influence on the textural properties of the CMS that mainly depend on the degree of filling originated by the carbon deposition.Los autores agradecen al MEC la financiaci n de este trabajo a trav s del proyecto DGICYT PB94-0012-C0301; al Ministerio de Asuntos Exteriores por la Acci n Concertada Hispano-Polaca, 99PL0025, y al Comit Polaco para la Investigaci n Cient fica (KBN, Proyecto 3T0913 09614).Peer reviewe

Adsorption of H2S or SO2 on an activated carbon cloth modified by ammonia treatment

International audienceThe aim of this research is to investigate how ammonia treatment of the surface can influence the activity of a viscose-based activated carbon cloth (ACC) for the oxidative retention of H2S and SO2 in humid air at 25 8C. Surface basic nitrogen groups were introduced either by treatment with ammonia/air at 300 8C or with ammonia/steam at 800 8C. The pore structure of the samples so prepared was examined by adsorption measurements. Changes in the surface chemistry were assessed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and temperature programmed desorption (TPD). The change of ACC activity could not be merely attributed to surface nitrogen groups but to other changes in the support. Ammonia/steam treatment improved ACC performance the most, not only by introducing nitrogen surface groups, but also by extending the microporosity and by modifying the distribution of surface oxygen groups. Successive adsorption–regeneration cycles showed important differences between oxidative retention of H2S and SO2 and the subsequent catalyst/support regeneration process