Global Vision of the Reaction and Deactivation Routes in the Ethanol Steam Reforming on a Catalyst Derived from a Ni-Al Spinel.

Ethanol steam reforming (ESR) over a Ni/Al2O3 catalyst prepared by reduction of a NiAl2O4 spinel is a promising alternative route to produce H2 from biomass. This work deepens into the effect of reaction conditions (450-650 °C, a steam/ethanol (S/E) ratio of 3-9, and a weight space time up to 1.3 h) and evaluates the time on stream evolution of the yields of H2, gaseous byproducts (CO, CO2, CH4, C2H4, C2H4O), and formed carbon/coke. The results are explained taking into consideration the thermodynamics, the extent of each individual reaction, and the catalyst deactivation. Up to 600 °C, the predominant intermediate in the H2 formation is C2H4 (formed by ethanol dehydration) with the preferential formation of nanostructured carbon (nanotubes/filaments) by C2H4 decomposition. The deposition of this type of carbon partially deactivates the catalyst, mainly affecting the extent of the C2H4 decomposition causing a sharp decrease in the H2 and carbon yields. Nevertheless, the catalyst reaches a pseudosteady state with an apparent constant activity for other reactions in the kinetic scheme. At 650 °C, C2H4O (formed by the ethanol dehydrogenation) is the main intermediate in the H2 formation, which is the precursor of an amorphous/turbostratic carbon (coke) formation that initially causes a rapid deactivation of the catalyst, affecting the ethanol dehydration and, to a lower extent, the reforming and water gas shift reactions. The increase in the S/E ratio favors the H2 formation, attenuates the catalyst deactivation due to the suppression of the ethanol dehydration to C2H4, and promotes the reforming, water gas shift, and carbon/coke gasification reactions. A H2 yield of 85% stable for 48 h on stream is achieved at 600 °C, with a space time of 0.1 h and an S/E ratio of 9.

Insights into the Reaction Routes for H2 Formation in the Ethanol Steam Reforming on a Catalyst Derived from NiAl2O4 Spinel.

This work describes the satisfactory performance of a Ni/Al2O3 catalyst derived from NiAl2O4 spinel in ethanol steam reforming and focuses on studying the prevailing reaction routes for H2 formation in this system. NiAl2O4 spinel was synthesized using a coprecipitation method and reduced at 850 °C to obtain a Ni/Al2O3 catalyst. The spinel structure and catalyst were characterized using XRD, TPR, N2 physisorption, NH3 adsorption and TPD, TPO, SEM, and TEM. The experiments were carried out in a fluidized-bed reactor at 500 or 600 °C and different space-time values, using pure ethanol, ethanol-water, pure ethylene, or ethylene-water feeds. The reaction takes place through two paired routes activated by each catalyst function (metal and acid sites) whose extent is limited by the selective catalyst deactivation. The results evidence that at the beginning of the reaction the main route for the formation of H2 and carbon (nanotubes) is the dehydration of ethanol on acid sites followed by decomposition of ethylene on the Ni-Al2O3 interface. This route is favored at 500 °C. After the rapid deactivation of the catalyst for ethylene decomposition, the route of H2 formation by steam reforming of ethanol and water gas shift reactions over Ni sites is favored. The morphology of the carbon deposits (nanotubes) allows the catalyst to maintain a notable activity for the latter pathways, with stable formation of H2 (during 48 h in the experiments carried out). At 600 °C, the extent of the gasification reaction of carbon species lowers the carbon material formation. The high formation of carbon material is interesting for the coproduction of H2 and carbon nanotubes with low CO2 emissions.

Combined Ex and In Situ Measurements Elucidate the Dynamics of Retained Species in ZSM-5 and SAPO-18 Catalysts Used in the Methanol-to-Olefins Reaction.

The dynamics of the retained species on ZSM-5 and SAPO-18 catalysts are studied by using a combination of temperature-programmed desorption/oxidation, ex situ analysis, and in situ FTIR spectroscopic measurements over the entire conversion range, using fixed-bed and spectroscopic cell reactors, in continuous and discontinuous mode. The results point to the appropriateness of the combined methodologies to track the interconversion of active into deactivating species. A statistically relevant (supported by linear regression and multivariate analysis) association of the observations is found by using the different complementary methodologies. The kinetics of this interconversion depends on the initial conversion (tuned by acidity and space time) and microporous topology, and involve: (i) in the ZSM-5 catalysts, the diffusion of monocyclic aromatics toward the exterior of the zeolite to form coke, and (ii) in the SAPO-18 catalysts, the obstruction of the cavities by aromatics that grow into tetracyclic aromatic islands.

[Chromosome alterations in workers exposed to ionizing radiation]. / Alteraciones cromosómicas en trabajadores expuestos a radiaciones ionizantes.

With the purpose of determining and characterizing chromosomal alterations and their relation to the radiation dose, time of employment and weekly exposure time, a transversal cut-descriptive study was performed on 18 workers, exposed to ionizing radiation, from a specialized company in the Venezuelan oil industry. These workers, male and females, constituted all the population studied, aged between 32 and 59 years, with at least one year on the job. A random sample of a non-exposed group of workers was used as a control. An occupational medical report was applied and personal dosimetry, environmental monitoring and a chromosomal analysis using two chromosomic culture techniques, were performed. The results show, in the exposed groups, an age average of 46.10 +/- 7.69 years, an average 17.5 +/- 5.00 years of employment and a weekly exposure of 4.30 +/- 1.33 hours. In the exposed population, 444 chromosomal abnormalities were evidenced in 700 metaphases studied; these abnormalities consisted of 66.6% single fragilities, 22% of combined fragilities, with chromosomic ruptures, deletions and poliploids, and 11% presented a normal kariotype. The control group presented chromosomic alterations as single fragilities in 55% of the cases. Radiologists presented 88.8% of chromosomic alterations, with below permissible doses detected, and 11.2% of them with exceed doses, presented the greatest number of fragilities and multiple chromosomic ruptures. The radiologists with weekly exposures of 8 hours presented the highest number of chromosomic alterations. 88% of radiologists with chromosomal abnormalities had more than 10 years of exposure. In conclusion, chronic exposure to low levels of ionizing radiation can induce chromosomic alterations, depending on the years of employment and the weekly time of exposure.

Alteraciones cromosómicas en trabajadores expuestos a radiaciones ionizantes / Chromosome alterations in workers exposed to ionizing radiation

Con el objeto de determinar y caracterizar las alteraciones cromosómicas, y el tiempo de exposición semanal a la radiación ionizante, se realizó un estudio descriptivo, de corte transversal, en 18 trabajadores de ambos sexos, que constituyeron la población estudio, con edades entre 32 y 59 años, con un año mínimo de exposición en una empresa especializada en el negocio petrolero venezolano. El grupo control seleccionado al azar simple, no estuvo expuesto a radiación ionizante. Se practicó una historia médico ocupacional, análisis cromosómico utilizando dos técnicas de cultivo cromosómico, dosimetría personal y monitoreo ambiental al grupo expuesto a radiación ionizante. Los resultados mostraron una edad promedio de 46,10 ± 7,69 años en el grupo expuesto, con antigüedad en el puesto de trabajo de 17,5 ± 5,0 años y tiempo de exposición semanal de 4,30 ± 1,33 horas. Se evidenciaron 444 alteraciones cromosómicas en 700 metafases estudiadas en la población expuesta, representadas por fragilidades simples 66,6 por ciento fragilidades combinadas, con rupturas cromosómicas, delecciones y poliploidías en 22,2 por ciento, el 11,1 por ciento presentó cariotipo normal. El grupo control presentó alteraciones cromosómicas tipo fragilidades simples en el 55,5 por ciento. En los radiólogos se observó el 88,8 por ciento de alteraciones cromosómicas, con dosis detectada por debajo de lo permisible, y el 11,2 de ellos, con dosis excedidas, presentó el mayor número de fragilidades y rupturas cromosómicas múltiples. Los radiólogos con exposición semanal de 8 horas mostraron el mayor número de rupturas cromosómicas. Ochenta y ocho por ciento de los radiólogos con alteraciones cromosómicas tenían antigüedades mayor de 10 años de exposición. En conclusión, la exposición crónica a bajas dosis de radiación ionizante puede inducir alteraciones cromosómicas, dependiendo de la antigüedad en la ocupación y la exposición semanal