José Luis Vicente

Comparative study of methane adsorption on single-walled carbon nanotubes.

We present the combined results of ab initio and molecular mechanical calculations, computer simulations, and adsorption isotherms investigations of CH(4) adsorbed on HiPco single-walled carbon nanotubes. Isotherms and adsorption energies obtained in our model and simulations are in good agreement with ours and others experimental results. The theoretical analysis conducted for various homogeneous bundles of close-ended and open-ended tubes confirm not only the adsorption in at least two different stages but also the role played by each of the different adsorption sites on the nanotube bundles. The study of different site and nanotube sizes allows us to establish the presence of open tubes in the as-produced HiPco bundles, without regarding the role that adsorption in large interstitial channels may play. Our results also show that predicted scenarios, for the mechanism and the preferential adsorption sites depend on the size of the nanotubes and those of the bundles.

Ethane/Ethylene Adsorption on Carbon Nanotubes: Temperature and Size Effects on Separation Capacity

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.

THERMODYNAMIC ANALYSIS OF ADSORPTION MODELS OF PHENOL IN LIQUID PHASE ON DIFFERENT ACTIVATED CARBONS

ABSTRACT This paper studies the thermodynamic aspects of the processes of adsorption of phenol from dilute aqueous solutions on different commercial carbons, evaluating how to optimize the removal of this persistent contaminant. Two powdered activated carbons from two different companies were used: Tetrahedron Carbon (Andes Chemistry Lab., Mendoza, Argentina), and Norit (Norit Americas Inc., USA). Both specific surface areas were measured by means of the BET method. The adsorbate was high purity solid phenol (Fluka ® ≥ 99.5%). Experimental isotherms were determined at 293 K, 303 K and 313 K. The Freundlich and Sips theoretical models were used to fit the experimental data. Freundlich isotherm slightly diverges with the experimental results for higher equilibrium concentrations. Thermodynamic parameters were calculated and correlated with the adsorption behaviours. The values of the thermodynamic parameters obtained indicate an exothermic and spontaneous process for both carbons, and mainly for Norit. This is due to the fact that there might be chemically activated regions on the surface of the Norit carbon, which give rise to combined mechanisms of physisorption and chemisorption.

Dielectric breakdown in solids modeled by DBM and DLA

Abstract Using numerical simulation, two stochastic models of electrical treeing in solid dielectrics are compared. These are the diffusion-limited aggregation (DLA) model and the dielectric breakdown model (DBM or η-model). On a linear two-dimensional geometry, the relationship between both models, when the size of the structures is of the order of the experimental samples (the electrode gap is 100 times the length of the discharge channel), is explored by statistical methods. Although there is a one-to-one correspondence between DBM with η=1 and the DLA model when the structure size is very large, the case of rather smaller structures is not well known. From a fractal analysis, employing the method of the correlation function C(r), it follows that average fractal dimension of electrical trees, generated with the DLA or with the DBM (η=1), collapse (up to the numerical uncertainty), on a single curve that “universally” accounts for finite size effects. Even more, from this analysis we conclude that the two curves obtained for DLA and DBM (η=1) cannot be distinguished if one takes into account the error bars. This means that finite size effects in the fractal analysis of DLA and DBM (η=1) are quite the same (despite the differences in the algorithms respectively used to generate the electrical trees). To our knowledge no comparison has ever been made between the similarities and differences of the DBM and DLA approach on a geometry other than the open-planar geometry.

Comparative study of methane adsorption on graphite.

To study methane adsorption on graphite in a wide range of coverages and temperatures, we compare experimental results with Monte Carlo simulations (MCSs) of the grand canonical ensemble (GCE) and mean-field approximation (MFA) of the lattice gas model (LGM). MCSs were performed by employing two models for the substrate description; we utilized Steeles 10-4-3 analytical potential, and as a second approach, we represented the graphite surface as composed of several graphene layers (at the atomic level). We obtained adsorption isotherms and density profiles that confirm a layer-by-layer mechanism at low temperatures; the later results in the analytical model having a denser condensed phase than the atomistic one. LGM calculations show a close-packed lattice configuration and also allow us to describe the adsorption mechanism changes with temperature. The isosteric heat of adsorption that was found was approximately 13 kJ/mol. We can also conclude that, in spite of the greater computational cost, the atomistic model could be employed for surfaces that are not necessarily homogeneous and beyond the low-pressure range that are not covered by the simple, fast description given by the analytical model.

Adsorption of Phenols from Different Solvents on Graphene: Semi-Empirical Quantum Mechanical Calculations:

The adsorption of phenol from aqueous solutions on carbon surfaces is discussed from different theoretical points of view, such as Monte–Carlo simulations, semi-empirical calculations, density functional theory and molecular dynamics. We performed a quantitative analysis of the adsorption of aromatics in general, and phenolic compounds in particular, through semi-empirical quantum mechanical calculations using different approaches. Our results raise doubts that phenol is primarily adsorbed in flat position on the graphene layers, and consequently whether the adsorption forces are controlled by π–π dispersion interactions between the aromatic ring of phenol and the graphene layer structure. Based on the results of quantum mechanical calculations (carried out through various approaches), we conclude that neither surface oxidation nor the presence of a polarizable solvent is consistent with the claim that π–π interactions are dominant in the adsorption of phenolic compounds on graphite.

MESOSCOPIC PATTERN FORMATION IN CATALYTIC PROCESSES BY AN EXTENSION OF THE MEAN FIELD APPROACH

For some years it has been known that a number of catalytic reactions, under specified steady operating conditions, exhibit oscillations, in the rate of product formation. These are often related to beautiful spatiotemporal patterns, including targets and spirals, on the metal surface. These examples of self-organizational phenomena have attracted considerable interest, because they are proving to be theoretically amenable. Here we review different approximations to model heterogeneous surface chemical reactions, which exhibit oscillatory behavior. A focal point is the use of a detailed knowledge of the dynamics of surface structural phase transition for modeling kinetic oscillations, which represent a severe test of our understanding of chemical processes at surfaces. Advantages and disadvantages of the Monte Carlo approach are presented to model heterogeneous oscillatory chemical reactions, with special emphasis if a Monte Carlo method is going to be applied to study the time evolution of a surface chemical reaction, as there should be a linear relationship between the time unit called the Monte Carlo step (MCS) and actual time. We conclude that special care must be taken when two or more processes are included in a simulation, because now overall MCS should be compatible with every individual process. The mean field approach (MFA) takes into account only reaction processes and completely neglects spatial correlation and fluctuations. Therefore, this approach is not adequate for describing the rich variety of spatial patterns that are experimentally observed. On the other hand, Monte Carlo approaches are severely limited by computational capabilities. To overcome MFA limitations we propose to extend the earlier work of King and coworkers [J. Chem. Phys.100, 14417 (1996)], which did not include spatial dependence, by adding diffusion processes and gas global coupling to the coupled reaction equations. The extended MFA can now be used as a new tool for the analysis of pattern formation in surface chemistry.

Trivalent cations switch the selectivity in nanopores.

In this letter, we study the effect of cation charge on anion selectivity in the pore using grand canonical Monte Carlo simulations. The mechanism of anion selectivity inside nanopores was found to be primarily a consequence of the screening of negative charges by the cations. In the case of monovalent cations, screening was not very effective and anions were rejected. We found an ‘off-state’ at high pH and an ‘on-state’ at low pH. When there are divalent cations, screening is good and there is no rejection of the anion. The concentration of anions at high pH is similar to that at low pH. The system is always in an ‘on-state’. Trivalent cations show an inverse selectivity mechanism: at high pH the concentration is higher than at low pH, i.e., the pore is in the ‘on-state’ at high pH and in the ‘off-state’ at low pH.

Influencia del Tratamiento de Datos en la Detección de Regímenes Caóticos en series Temporales

Se estudia la influencia del procedimiento de filtrado aplicado sobre series temporales para su uso en la caracterizacion y deteccion de regimenes no-lineales. Para esto, se toma como ejemplo de dichos metodos, un algoritmo ampliamente utilizado para el calculo de la dimension de correlacion de la trayectoria en el espacio de las fases (el algoritmo de Grassberger-Procaccia). El interes de este ejemplo de estudio radica en la similitud con el procedimiento que se aplica al analizar imagenes experimentales de sistemas fisicoquimicos de reaccion-difusion, provenientes de tecnicas de analisis superficial como la microscopia de emision de fotoelectrones. Los resultados de las simulaciones realizadas muestran como el uso de un parametro de filtrado inadecuado puede conducir a caracterizar erroneamente estados como no lineales o caoticos.

DESCRIPTION OF DIFFERENT SOLID ADSORBENT SURFACES ADSORPTION MECHANISMS BASED ON A SEQUENTIAL DECOMPOSITION OF ISOTHERMS

In order to analyze the adsorption capacities of different solid substrates, we present a multi-step method to separately study the isotherm at different pressure ranges (steps). The method is based on simple gas isotherm measurements (nitrogen, methane, carbon dioxide, argon, and oxygen) and is tested to describe the adsorption process and characterize a graphitized surface (GCB) and two different granular activated carbons (GAC). The GCB isotherms are described as a sum of Fowler-Guggenheim-Langmuir shifted curves; isotherm behaviors are quite similar at different temperatures, but change below a certain threshold. In GAC the first steps show the same adsorption characteristics at low pressures (Dubinins description), but this behavior changes at higher pressure regimes, which allows one to elucidate how heterogeneous the surfaces are or how strong the interactions between adsorbed molecules are for this marginal adsorption to occur. We tested different approaches (from BET multilayer to Aranovich) and found quite different features. We finally conclude that if the description of the adsorption on complex substrates, such as those presented here, is carried using only one model, e. g. Dubinin in case of GACs, the resulting characteristics of the adsorbent would be very biased.