Aplicabilidad de la ecuación de BET a sólidos de textura diversa

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Assessment of the Role of Micropore Size and N-Doping in CO2 Capture by Porous Carbons

The role of micropore size and N-doping in CO2 capture by microporous carbons has been investigated by analyzing the CO2 adsorption properties of two types of activated carbons with analogous textural properties: (a) N-free carbon microspheres and (b) N-doped carbon microspheres. Both materials exhibit a porosity made up exclusively of micropores ranging in size between <0.6 nm in the case of the pristine materials and up to 1.6 nm for the highly activated carbons (47% burnoff). The N-doped carbons possess ∼3 wt % of N heteroatoms that are incorporated into several types of functional groups (i.e., pyrrole/pyridone, pyridine, quaternary, and pyridine-N-oxide). Under conventional operation conditions (i.e., T ∼ 0–25 °C and PCO2 ∼ 0–1 bar), CO2 adsorption proceeds via a volume-filling mechanism, the size limit for volume-filling being ∼0.7–0.8 nm. Under these circumstances, the adsorption of CO2 by nonfunctionalized porous carbons is mainly determined by the volume of the micropores with a size below 0.8 nm. It was also observed that the CO2 capture capacities of undoped and N-doped carbons are analogous which shows that the nitrogen functionalities present in these N-doped samples do not influence CO2 adsorption. Taking into account the temperature invariance of the characteristic curve postulated by the Dubinin theory, we show that CO2 uptakes can be accurately predicted by using the adsorption data measured at just one temperature.The financial support for this research work provided by the Spanish MINECO (MAT2012-31651) is gratefully acknowledgedPeer reviewe

Role of activated carbon features on the photocatalytic degradation of phenol

[EN] In this work we have investigated the role of porous carbon material used as a photocatalyst and a catalyst support in the carbon/titania composite in the photodegradation of phenol, and compared the results to those of bare titanium oxide. The immobilization of titania on an activated carbon provoked acceleration of the degradation rate under UV irradiation, which is likely to be attributed to the porosity of the carbon support. The identification of the degradation intermediates detected in the solution showed that the presence of the carbon support affects the nature of phenol degradation mechanism through the formation of different intermediates. Additionally, phenol photodecomposition rate over the carbon support outperformed that attained in the carbon/titania composite, suggesting an important self-photoactivity of the carbon support.This work was supported by the Spanish MICINN (CTM2008-01956). COA is grateful to the Spanish MEC for the Ramon y Cajal Research Contract. LFV is grateful to CSIC for the JAE predoctoral fellowship.Peer reviewe

Effect of microwave and conventional regeneration on the microporous and mesoporous network and on the adsorptive capacity of activated carbons

The effect of different heating mechanisms on the porous network of activated carbons (AC) previously exhausted with phenol has been studied. To this end, thermal treatment of the exhausted AC was performed using two experimental devices: a single mode microwave device operating at 2450 MHz and a conventional electric furnace. By employing microwave energy, the regeneration time was considerably shortened compared to conventional thermal heating. Moreover, microwave heating preserved the porous structure of the regenerated AC more efficiently than treatment in a conventional device. In both cases successive regeneration cycles reduced the microporosity of the activated carbons. However, conventional heating shifted the micropore size distribution to pores of narrower sizes. The apparent BET surface areas were also reduced significantly over the regeneration cycles. A loss of the adsorptive capacity of the carbon material was observed after six adsorption–desorption cycles in both systems. The phenol adsorption capacities decreased to a greater extent in the samples regenerated in the electric furnace.Work carried out with a financial grant from the European Coal and Steel Community (Project 7220-PR-139). The authors wish to thank Dr. Martín-Gullón for his considerate and helpful assistance with the software tool used for the DRS calculation.Peer reviewe

Microwave-assisted regeneration of activated carbons loaded with pharmaceuticals

8 pages, 4 figures, 2 tables.-- PMID: 17572468 [PubMed].-- Printed version published Aug 2007.The purpose of this work was to explore the application of microwaves for the regeneration of activated carbons spent with salicylic acid, a metabolite of a common analgesic frequently found in wastewater from the pharmaceutical industry. The exhausted carbon was treated in a quartz reactor by microwave irradiation at 2450 MHz at different temperatures and atmospheres, the regeneration efficiency being highly dependent on the operating conditions. Quantitative desorption of the pollutant was achieved at high temperature and oxidizing atmosphere, with regeneration efficiencies as high as 99% after six cycles. The stripping efficiency was superior to 95% at high temperatures and decreased at 450ºC. The incomplete desorption of the adsorbate at low temperature was further confirmed by the changes in the porosity observed by N2 and CO2 adsorption isotherms. Hence, micropores remain blocked which results in a reduction in loading capacities in successive cycles.The authors thank Química Farmacéutica Bayer for kindly providing the adsorbent and for financial support. COA thanks the CSIC I3P Program, co-financed by the European Social Fund, for a postdoctoral contract.Peer reviewe

Insights on the Anomalous Adsorption of Carbon Dioxide in LTA Zeolites

We use a combination of experiments and molecular simulations to address the discrepancies of the force fields available in the literature to accurately reproduce CO2 adsorption in zeolites with high density of aluminum atoms and extra framework cations. We attribute these discrepancies to the fact that previous force fields are not parametrized to take into account the formation of carbonate-like complexes in these zeolites during CO2 adsorption. Our data show that the formation of carbonate-like complexes has a marked effect on the accessible porous structure of the zeolite, and the strength is controlled by the density and nature of extra framework cations. Strong carbonate-like complexes are formed in zeolite topologies containing high density of sodium, whereas bivalent cations give rise to more labile complexes. We provide a new set of parameters capable to reproduce the experimental adsorption in these systems. Our approach consists of the modification of the partial charges of the atoms of the zeolite that are directly involved in the formation of the surface complexes (oxygen atoms and cations). The new set of charges combined with our previous transferable force field reproduces the experimental adsorption in structures containing carbon dioxide–cation complexes.This work was supported by the Spanish “Ministerio de Educacion, Cultura y Deporte” (CTQ2010-16077/BQU), and the European Research Council through an ERC Staring Grant (ERC-StG-279520-RASPA). A. Martín-Calvo thanks the Spanish “Ministerio de Educación” for her predoctoral fellowship.Peer reviewe

Low temperature regeneration of activated carbons using microwaves: Revising conventional wisdom

[EN] The purpose of this work was to explore the application of microwaves for the low temperature regeneration of activated carbons saturated with a pharmaceutical compound (promethazine). Contrary to expectations, microwave-assisted regeneration did not lead to better results than those obtained under conventional electric heating. At low temperatures the regeneration was incomplete either under microwave and conventional heating, being this attributed to the insufficient input energy. At mild temperatures, a fall in the adsorption capacity upon cycling was obtained in both devices, although this was much more pronounced for the microwave. These results contrast with previous studies on the benefits of microwaves for the regeneration of carbon materials. The fall in the adsorption capacity after regeneration was due to the thermal cracking of the adsorbed molecules inside the carbon porous network, although this effect applies to both devices. When microwaves are used, along with the thermal heating of the carbon bed, a fraction of the microwave energy seemed to be directly used in the decomposition of promethazine through the excitation of the molecular bonds by microwaves (microwave-lysis). These results point out that the nature of the adsorbate and its ability to interact with microwave are key factors that control the application of microwaves for regeneration of exhausted activated carbons.EÇ thanks The Council of Higher Education of Turkey (YÖK) for supporting her stay at INCAR (Oviedo) and the financial support of Istanbul University Research Fund (Project 3991) for her PhD thesis. JMB acknowledges CSIC for a JAE predoctoral fellowship. COA thanks the financial support of the projects CTM2008-01956 and CSIC-200980I131.Peer reviewe

Role of crystal size on swing-effect and adsorption induced structure transition of ZIF-8

The flexibility and structure transition behaviour of ZIF-8 in a series of samples with different particle size has been studied using a combination of high-resolution N2 gas adsorption isotherms and, for the first time, a broad in situ PXRD and Rietveld analysis. During the stepped adsorption process, large particles showed a narrow adsorption/desorption pressure range with a shorter equilibrium time due to lower kinetic hindrance, deriving from higher amount of active sites. In situ PXRD showed that both the rotation of imidazole ring and a bend in the methyl group led to the gate opening of ZIF-8.This work was funded by the EPSRC IAA Partnership Development Award (RG/75759). D. F.-J. thanks the Royal Society for funding through a University Research Fellowship. We thank Diamond Light Source for beamtime at beamline I11 (visit EE9750).Peer reviewe

Transferable force fields for adsorption of small gases in zeolites

We provide transferable force fields for oxygen, nitrogen, and carbon monoxide that are able to reproduce experimental adsorption in both pure silica and alumino-substituted zeolites at cryogenic and high temperatures. The force field parameters can be combined with those previously reported for carbon dioxide, methane, and argon, opening the possibility for studying mixtures of interest containing the six components. Using these force field parameters we obtained some adsorption isotherms at cryogenic temperatures that at first sight were in discrepancies with experimental values for certain molecules and structures. We attribute these discrepancies to the sensitiveness of the equipment and to kinetic impedimenta that can lead to erratic results. Additional problems can be found during simulations when extra-framework cations are present in the system as their lack of mobility at low temperatures could lead to kinetic effects that hinder experimental adsorption.This work was supported by the European Research Council through an ERC Staring Grant (ERC-StG-279520-RASPA). A. Martin-Calvo thanks the Spanish “Ministerio de Educación Cultura y Deporte” for her predoctoral fellowship. The authors want to thank the “Instituto de Tecnología Química” (ITQ-CSIC) from Valencia, for providing the pure silica zeolites (RSIL and ITQ-29).Peer reviewe

Dual role of copper on the reactivity of activated carbons from coal and lignocellulosic precursors

The synthesis of copper-doped activated carbons from different origin (i.e., lignocellulosic and bituminous coal) by a wet impregnation and low temperature calcination procedure has been explored, as well as the role of copper particles on the physicochemical and structural features of the resulting materials. The textural characterization and isothermal reactivity analysis of the pristine and doped activated carbons have shown that the role of copper during the calcination step strongly depended on the nature of the carbon matrix. Copper impregnation of a coal-derived activated carbon catalyzed the air gasification of the material at a very low temperature (i.e., 325 °C), bringing about the development of microporosity on the doped carbon. In contrast, when copper was immobilized on a lignocellulose-derived activated carbon, the metallic species act as combustion retardant during the calcination step, protecting the carbon matrix during the catalytic gasification. In both cases, the resulting materials displayed a homogenous distribution of copper within the carbon matrix, while preserving large textural properties.The authors thank the Spanish MICINN for financial support (projects CTM2008-01956/TECNO and Acción Integrada AIB2010PT-00209). MH thanks CSIC for a postdoctoral contract. MA thanks FCT for her PhD fellowship (SFRH/BD/71673/2010). The authors also thank Cordex for kindly providing the sisal residues.Peer reviewe