Computational Lattice-Gas Modeling of the Electrosorption of Small Molecules and Ions

We present two recent applications of lattice-gas modeling techniques to electrochemical adsorption on catalytically active metal substrates: urea on Pt(100) and (bi)sulfate on Rh(111). Both involve the specific adsorption of small molecules or ions on well-characterized single-crystal electrodes, and they provide a particularly good fit between the adsorbate geometry and the substrate structure. The close geometric fit facilitates the formation of ordered submonolayer adsorbate phases in a range of electrode potential positive of the range in which an adsorbed monolayer of hydrogen is stable. In both systems the ordered-phase region is separated from the adsorbed- hydrogen region by a phase transition, signified in cyclic voltammograms by a sharp current peak. Based on data from {\it in situ\/} radiochemical surface concentration measurements, cyclic voltammetry, and scanning tunneling micro- scopy, and {\it ex situ\/} Auger electron spectroscopy and low-energy electron diffraction, we have developed specific lattice-gas models for the two systems. These models were studied by group-theoretical ground-state calcu- lations and numerical Monte Carlo simulations, and effective lattice-gas inter- action parameters were determined so as to provide agreement with experiments.Comment: 17 pp. uuencoded postscript, FSU-SCRI-94C-9

Variación espacial y temporal de material partículado PM10 en la atmosfera de la zona poniente de Villahermosa, Tabasco

Este estudio tiene como objetivo la caracterización gravimétrica del material paritculado fracción respirable (PM10) colectado en la estación de Monitoreo Atmosférico de la División Académica de Ciencias Biológicas de la UJAT. Para lograr la generación continua de datos atmosféricos confiables, se utilizaron metodologías y procedimientos estandarizados por normas nacionales (NOM-035-ECOL-1993) e internacionales (US-EPA-IO 2.1). Las PM10 fueron muestreadas de agosto 2015 a julio 2017 (n = 90), durante 24 h cada una. La máxima concentración fue de 80.59 ?g/m3 en febrero del 2017. La norma superó los límites máximos permisibles dos días durante el periodo 2016-2017. En las estaciones de primavera y principios de verano del 2016 se observaron las tendencias de concentración más alta de todo el periodo de muestreo (arriba de 50 ?g/m3). La concentración total de partículas suspendidas PM10 durante 24 h varía en un rango de 2.45 a 80.59 ?g/m3. La concentración PM10 se incrementa de dos a cuatro veces en la época de estiaje. La información generada por ésta estación permite a las autoridades estatales y municipales, conocer y evaluar la calidad del aire de una región de la ciudad de Villahermosa y vigilar el cumplimiento de la normatividad correspondiente, para de esta manera, asegurar el derecho a la salud de la población tabasqueña

Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: a theoretical and experimental study

We study the underpotential deposition of Cu on single-crystal Au(111) electrodes in sulfate-containing electrolytes by a combination of computational statistical-mechanics based lattice-gas modeling and experiments. The experimental methods are in situ cyclic voltammetry and coulometry and ex situ Auger electron spectroscopy and low-energy electron diffraction. The experimentally obtained voltammetric current and charge densities and adsorbate coverages are compared with the predictions of a two-component lattice-gas model for the coadsorption of Cu and sulfate. This model includes effective, lateral interactions out to fourth-nearest neighbors. Using group-theoretical ground-state calculations and Monte Carlo simulations, we estimate effective electrovalences and lateral adsorbate--adsorbate interactions so as to obtain overall agreement with experiments, including both our own and those of other groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode potentials, delimited by phase transitions at both higher and lower potentials. Our approach provides estimates of the effective electrovalences and lateral interaction energies, which cannot yet be calculated by first-principles methods.Comment: 36 pages, 14 Postscript figures are in uufiles for

In situ studies of adsorption of inorganic anions on metallic electrodes

Typescript (photocopy).The adsorption of HSO4-, Cl- and I- from aqueous solutions was examined using radiotracer, FTIR spectroscopy and ellipsometry techniques. Adsorption dependence was measured as a function of time, concentration and potential. For HSO4-, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the "ln Γ[t] - t" nor "Γ[t] - ln t" relations expected from previously deduced theoretical models. Up to -1/4r, θ[t] was proportional to the adsorption time. The dependence of adsorption on concentration was done over an extended range: 10^-9 to 10^-2M. Up to 10^-5M it followed the Henry isotherm and at concentrations up to 10^-3M, the results followed a logarithmic law. ([partial]θ/[partial]log c)[V,T] was about the same for HSO4- and Cl- but around twice higher for I-. θ[max] of HSO4- and Cl- were reached at ~0.25 and ~1 for I-. The dependence on potential at constant concentration was essentially linear until oxide formation for HSO4- and Cl-. I- results showed little variation with potential before oxide formation. The dependence of (θ)[v,c] as a function of temperature for HSO4- cannot be expressed as a simple law because of the low number of experiments successfully performed; nevertheless, approximate values of ΔG°, ΔH° and ΔS° were determined. An isotherm was derived such that the heterogeneity of the surface, ion-ion repulsion through a simple imaging model, and ion-ion dispersive forces were considered. Electrostriction effects on ion size were computed. Evidence for partial charge transfer was found: 0.2 (HSO4-), 0.6 (Cl-) and 0.9 (I-). Evidence of the displacement of solvent molecules from the surface was consistent with the desorption of an entity represented by a dimer. For HSO4-, ΔG°=-32 kJ/mol (25°C), ΔH°=-57 kJ/mol and ΔS°=-78 J/mo l°K (θ[ss]=0.22). From these values, the Pt-0 bond energy was estimated as ~1/2 of that expected for isolated Pt-O bonds in the gas phase. The entropy was consistent with immobile adsorbed ions with hindered rotation. The concentration dependence of SO4= adsorption on bare iron electrodes followed a logarithmic trend, ΔG°=-28 kJ mol^-1. The values of θ>>1 observed on iron-oxide were explained in terms of formation of iron-sulfate complexes

In situ studies of adsorption of inorganic anions on metallic electrodes

Typescript (photocopy).The adsorption of HSO4-, Cl- and I- from aqueous solutions was examined using radiotracer, FTIR spectroscopy and ellipsometry techniques. Adsorption dependence was measured as a function of time, concentration and potential. For HSO4-, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the "ln Γ[t] - t" nor "Γ[t] - ln t" relations expected from previously deduced theoretical models. Up to -1/4r, θ[t] was proportional to the adsorption time. The dependence of adsorption on concentration was done over an extended range: 10^-9 to 10^-2M. Up to 10^-5M it followed the Henry isotherm and at concentrations up to 10^-3M, the results followed a logarithmic law. ([partial]θ/[partial]log c)[V,T] was about the same for HSO4- and Cl- but around twice higher for I-. θ[max] of HSO4- and Cl- were reached at ~0.25 and ~1 for I-. The dependence on potential at constant concentration was essentially linear until oxide formation for HSO4- and Cl-. I- results showed little variation with potential before oxide formation. The dependence of (θ)[v,c] as a function of temperature for HSO4- cannot be expressed as a simple law because of the low number of experiments successfully performed; nevertheless, approximate values of ΔG°, ΔH° and ΔS° were determined. An isotherm was derived such that the heterogeneity of the surface, ion-ion repulsion through a simple imaging model, and ion-ion dispersive forces were considered. Electrostriction effects on ion size were computed. Evidence for partial charge transfer was found: 0.2 (HSO4-), 0.6 (Cl-) and 0.9 (I-). Evidence of the displacement of solvent molecules from the surface was consistent with the desorption of an entity represented by a dimer. For HSO4-, ΔG°=-32 kJ/mol (25°C), ΔH°=-57 kJ/mol and ΔS°=-78 J/mo l°K (θ[ss]=0.22). From these values, the Pt-0 bond energy was estimated as ~1/2 of that expected for isolated Pt-O bonds in the gas phase. The entropy was consistent with immobile adsorbed ions with hindered rotation. The concentration dependence of SO4= adsorption on bare iron electrodes followed a logarithmic trend, ΔG°=-28 kJ mol^-1. The values of θ>>1 observed on iron-oxide were explained in terms of formation of iron-sulfate complexes

Modern Electrochemistry 2A

xxviii;i.;1534hal.;23c

Genotoxicity in child populations exposed to Polycyclic Aromatic Hydrocarbons (PAHs) in the air from Tabasco, Mexico

The economy of the state of Tabasco is based on oil extraction. However, this imposes major effects to the environment and communities. Examples are the Polycyclic Aromatic Hydrocarbons (PAHs) that may be found in the soil, water and sediment of the region. Their volatility makes them available to living beings and results in genotoxic activity. The purpose of this study was to quantify the levels of PAHs in the air at several points in the state, and to analyze their relationship with possible damage to DNA on local inhabitants. Single Cell Gel Electrophoresis Assay (Comet Assay) was applied to peripheral blood lymphocytes of five groups of children between six and 15 years of age. PAH samples were analyzed following US/EPA TO-13-A method. Results indicated the presence in the air of most of the 16 PAHs considered as high priority by EPA, some of which have been reported with carcinogenic activity. Differences (