Matías Rafti

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.

Heterogeneous Catalytic Activity of Platinum Nanoparticles Hosted in Mesoporous Silica Thin Films Modified with Polyelectrolyte Brushes

Platinum nanoparticles of 3 nm diameter were included in mesoporous silica thin films by controlling the mesopore surface charge with a short polymer brush. This metal-polymer-mesopore nanocomposite presents high catalytic activity toward ammonia oxidation at low temperatures with 4.5% weight platinum loading. An anomalous partial selectivity toward nitrous oxide is observed for the first time, which can be traced back to the synergy of the particles and modified surface. This effect opens a path toward the design of nanocomposite catalysts with highly controlled environments, in which the size- and function-controlled cavities can be tuned in order to lower the reaction barriers.

Traveling interface modulations in the NH3 + O2 reaction on a Rh(110) surface

A new type of traveling interface modulation has been observed in the NH(3) + O(2) reaction on a Rh(110) surface. A model is set up which reproduces the effect, which is attributed to diffusional mixing of two spatially separated adsorbates causing an excitability which is strictly localized to the vicinity of the interface of the adsorbate domains.

Photophysical and Photochemical Properties of Naturally Occurring normelinonine F and Melinonine F Alkaloids and Structurally Related N(2)- and/or N(9)-methyl-β-carboline Derivatives

In the present work, we have synthesized and fully characterized the photophysical and photochemical properties of a selected group of N‐methyl‐β‐carboline derivatives (9‐methyl‐β‐carbolines and iodine salts of 2‐methyl‐ and 2,9‐dimethyl‐β‐carbolinium) in aqueous solutions, in the pH range 4.0–14.5. Moreover, despite the quite extensive studies reported in the literature regarding the overall photophysical behavior of N‐unsubstituted βCs, this work constitutes the first full and unambiguous characterization of anionic species of N‐unsubstituted βCs (norharmane, harmane and harmine), present in aqueous solution under highly alkaline conditions (pH > 13.0). Acid dissociation constants (Ka), thermal stabilities, room temperature UV–visible absorption and fluorescence emission and excitation spectra, fluorescence quantum yields (ФF) and fluorescence lifetimes (τF), as well as quantum yields of singlet oxygen production (ФΔ) have been measured for all the studied compounds. Furthermore, for the first time to our knowledge, chemometric techniques (MCR‐ALS and PARAFAC) were applied on these systems, providing relevant information about the equilibria and species involved. The impact of all the foregoing observations on the biological role, as well as the potential biotechnological applications of these compounds, is discussed.

Early stages of ZIF-8 film growth: the enhancement effect of primers exposing sulfonate groups as surface-confined nucleation agents

We describe the use of sulfonate-terminated self-assembled monolayers as very efficient surface-confined nucleation agents for rapid growth of dense, thick and well-percolated ZIF-8 films.

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.

Traveling interface modulations and anisotropic front propagation in ammonia oxidation over Rh(110).

The bistable NH3 + O2 reaction over a Rh(110) surface was explored in the pressure range 10(-6)-10(-3) mbar and in the temperature range 300-900 K using photoemission electron microscopy and low energy electron microscopy as spatially resolving methods. We observed a history dependent anisotropy in front propagation, traveling interface modulations, transitions with secondary reaction fronts, and stationary island structures.

EXTENDED MEAN FIELD APPROACH TO ANALYZING PATTERN FORMATION IN SURFACE CHEMICAL REACTIONS

In this paper we apply a model of surface chemical reactions to describe pattern formation in the CO oxidation on Pt(100). The model is an extended mean field approximation (EMFA), where two coupling processes are included: gas global coupling (to take into account the mass chemical reaction balance) and CO diffusion. The surface is divided into M×M cells; inside each of them the mean field approximation (MFA) is fulfilled. Anisotropic diffusion between nearest neighbor cells and gas coupling among all cells are also allowed. The EMFA goes beyond the traditional reaction–diffusion approaches with Fickian terms, in favor of a more general mass balance equation that consistently incorporates the coupling between CO diffusion and the nonlinear phase transition of the substrate. In this paper we study three different cases: (a) homogeneous coverage oscillations — when CO and O2 partial pressures are kept constant and the whole set of M×M cells have the same value of partial coverages and phase fractions; in this case the system exhibits self-oscillations, such as those experimentally observed; (b) inhomogeneous coverage oscillations — when CO and O2 partial pressures are kept constant and inhomogeneities of coverage are present; the system evolves by CO diffusion between nearest neighbor cells; (c) gas global coupling — when gas global coupling is allowed and inhomogeneities of coverage are present, there are oscillations in the gas phase that follow the surface evolution.

Polyelectrolyte Capping As Straightforward Approach toward Manipulation of Diffusive Transport in MOF Films

We present experimental results demonstrating the suitability of polyelectrolyte capping as a simple and straightforward procedure to modify hydrophilic/hydrophobic character of porous films, thus allowing additional control on transport properties. In particular, we synthesized ZIF-8 metal organic framework (MOF) films, an archetypal hydrophobic zeolitic imidazolate framework, constituted by Zn2+ ions tetrahedrally coordinated with bidentate 2-methylimidazolate organic linker, and poly(4-styrenesulfonic acid) as capping agent (PSS). MOF films were synthesized via sequential one pot (SOP) steps over conductive substrates conveniently modified with primer agents known to enhance heterogeneous nucleation, followed by dip-coating with PSS aqueous solutions. Crystallinity, morphology, and chemical composition of ZIF-8 films were confirmed with traditional methods. Continuous electron density depth profile obtained with synchrotron light X-ray reflectivity (XRR) technique, suggest that PSS capped-films do not adopt segregated configurations in which PSS remains surface-confined. This affects functional properties conferred by PSS capping, which were assessed using cyclic voltammetry with both positively and negatively charged redox probe molecules. Furthermore, taking advantage of the control attained, we successfully carried in situ synthesis of film-hosted d-block metal nanoparticles (Au and Pt-NPT@5x-ZIF-8+PSS) via direct aqueous chemical reduction of precursors (diffusion-reaction approach).