Modeling of Current Distribution on Smooth and Columnar Platinum Structures

Studying the growth and stability of anisotropic or isotropic disordered surfaces in electrodeposition is of importance in catalytic electrochemistry. In some cases, the metallic nature of the electrode defines the topography and roughness, which are also controlled by the experimental time and applied external potential. Because of the experimental restrictions in conventional electrochemical techniques and ex situ electron microscopies, a theoretical model of the surface geometry could aid in understanding the electrodeposition process and current distributions. In spite of applying a complex theory such as dynamic scaling method or perturbation theories, the resolution of mixed mass‐/charge‐transfer equations (tertiary distribution) for the electrodeposition process would give reliable information. One of the main problems with this type of distribution is the mathematics when solving the spatial n‐dimensional differential equations. Use of a primary current distribution is proposed here to simplify the differential equations; however it limits wide application of the first assumption. Distributions of concentration profile, current density, and electrode potential are presented here as a function of the distance normal to the surface for the cases of smooth and rough platinum growth. In the particular case of columnar surfaces, cycloid curves are used to model the electrode, from which the concentration profile is presented in a parameterized form after solving a first‐type curvilinear integral. The concentration contour results in a combination of a trigonometric inverse function and a linear distribution leading to a negative concavity curve. The calculation of the current density and electrode potential contours also show trigonometric shapes exhibiting forbidden imaginary values only at the minimal values of the trochoid curve.     

Catalytic effects on methanol oxidation produced by cathodization of platinum electrodes

A catalytic effect is found for methanol oxidation after new active surface states are produced on polycrystalline platinum by potentiostatic cathodization in acid media at room temperature. This procedure originates surface states not available on the original polycrystalline electrodes with unexpected cyclic voltammetric responses; i.e., at least four new peaks below 0.9 V are observed. The cathodization process also induces a rearrangement of the bulk platinum oxide, showing a defined peak at 1.2 V. The appearance of these new states is also proven by open-circuit potential decays. The electrocatalytic activity of these new surfaces in methanol oxidation is compared with that of the untreated electrodes by electrochemical impedance spectroscopy, chronoamperometry, and cyclic voltammetry. The cathodic procedure enhances the methanol oxidation voltammetric current peaks with charge density values higher than those on untreated platinum. The integration of chronoamperometric plots over 10 min in methanol acid media presents the largest difference between 0.6 and 0.7 V with respect to the original surface. Analysis of the impedance data shows that the values of polarization resistance for methanol oxidation on the cathodically treated platinum are lower than those of the original surface. According to the time constant values for methanol oxidation, the original surface can be considered less tolerant of the formation of catalytic poisons. A discussion of the most likely mechanism for the formation of the new active sites on platinum is presented here, assuming the presence of hydrogen subsurface states, ordered water clusters, and low-coordinated platinum atoms.     

Catalytic effects of ruthenium and osmium spontaneous deposition on platinum surfaces toward methanol oxidation

The influence of ruthenium and osmium spontaneous deposition on polycrystalline platinum in sulfuric acid was studied by conventional electrochemical techniques. The inhibition of the hydrogen adatom voltammetric profile by the foreign adatoms was used to calculate the degree of surface coverage of ruthenium, osmium, and a mixture of both metal ions from solutions of different composition. Methanol adsorption and oxidation were compared on bare platinum, platinum/ruthenium, platinum/osmium, and ternary compounds, considering the efficiency of methanol oxidation per hydrogen adatom displaced by the foreign metal on platinum.     

Topography changes of rhodium electrodes induced by the application of fast periodic potential routines

The surface structure of polycrystalline rhodium electrodes in contact with aqueous sulfuric acid was modified by chemical etching with hot concentrated acid or by applying fast square waves with an upper potential equal to 1.55 V and a lower potential within the –0.75 V to –0.35 V range. Polycrystalline rhodium and chemical etched electrodes were characterized by voltammetry, Cu underpotential deposition (upd) and X-ray diffraction. For electrofaceted surfaces were used voltammetry, Cu upd and SEM, revealing that two modified rhodium electrodes exhibit similar voltammetric characteristics as those found for Rh(111) and Rh(110) single crystals, and a third surface with an equal distribution of (110) and (111) planes. In addition, the upd of Cu on those surfaces corroborated the existence of those crystallographic planes. SEM micrographs show surface structures with a high density of terraces and steps. A mechanism of faceting is proposed. Electronic Publication     

Carbon monoxide oxidation assisted by interfacial oxygen-water layers

Journal of Solid State Electrochemistry - Based on the fact that oxygenated species promotes the oxidative desorption of carbon monoxide adsorbates, a simple methodology is described here to...     

On the existence of conically self-similar free-vortex solutions to the Navier-Stokes equations

In this paper a proof of existence and non-existence of theconically self-similar free-vortex solutions to the Navier-Stokesequations, originally found by Yih et al. (1982, Phys. Fluids.25, 2147-2158), is presented. This proof clearly establishesthat these solutions do not have any kind of singularity atthe symmetry axis. This analysis gives considerably improvedexistence and non-existence bounds and it is shown that thesebounds are close to optimal in the low-swirling limit. Thisapproach links the questions of existence and non-existencefor the swirling case and for the non-swirling case. The proof,which is an extension of techniques developed by Serrin (1972,Phil. Trans. R. Soc. Lond. 271, 325-360), is based on Schauder'sFixed Point Theorem and is, therefore, non-constructive. Therefore,the paper ends with a brief discussion of the question of howto compute the conically self-similar free-vortex solutionsto the Navier-Stokes equations.     

Inhibition of steel corrosion by calcium benzoate adsorption in nitrate solutions: theoretical and experimental approaches

The inhibitive effects of calcium benzoate on steel corrosion were studied in sodium nitrate solutions at room temperature. Corrosion parameters of the steel/nitrate and steel/benzoate + nitrate interfaces were obtained from polarization curves. Adsorption parameters of benzoate on steel in sodium nitrate solutions were determined through changes in the degree of surface coverage by the inhibitor, as a function of concentration, time, and adsorption potential. The most likely adsorption configuration of benzoate on iron was envisaged with the help of semiempirical calculations such as extended Hückel calculations. A two-dimensional flat configuration was involving at least two metal atoms, one interacting with the phenolic group and the other with the carboxylate moiety. The effect of chloride on the corrosion inhibition of benzoate was analyzed by exposing the metal to different chloride solution concentrations, from which corrosion parameters were calculated and compared with those in nitrate solutions.     

Surface metal modifiers for methanol electrooxidation on platinum; silver and mercury

The catalytic activity of platinum surfaces towards methanol electrooxidation can be modified by the deposition of a second metal using different methodologies. There is little information about the catalytic performance of polycrystalline platinum modified by silver and mercury adatoms using spontaneous and electrochemical deposition methods. Cyclic voltammetrics have been performed to compare the current vs potential profiles of modified platinum surfaces in acid solution at room temperature. The inhibition of the hydrogen adatom voltammetric profile by foreign metal adatoms on platinum was used to calculate the degree of surface coverage by the metal. Poisoning effects were checked by anodic stripping experiments of methanol residues on the modified platinum surfaces at adsorption potentials in the hydrogen electrosorption region using a micro flux cell. Methanol solution oxidation was also evaluated at slow scan rates of up to 0.8 vs reversible hydrogen electrode (RHE) on the platinum-modified surfaces. The comparison between the amounts of carbon-monoxide-type residues and the solution oxidation of methanol was analysed to check for their utility as catalytic surfaces for direct methanol fuel cells. Dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

Voltammetric Characterization of trans-Dioxo Ethylenediamine Complexes of Re(V) in Aqueous Solutions

The electrochemical behavior of trans-[Re((V))O(2)(en)(2)]I and trans-[Re((V))O(2) (en)(2)]ClO(4) (en=ethylenediamine) complexes was studied by cyclic voltammetry on Au electrodes. Experiments were performed in aqueous solutions at pH 7.0 and at room temperature. The complex voltammogram was characterized by Re-containing species, assigned to the [Re((V))O(2)(en)(2)](+)/[Re((IV))O(2)(en)(2)] couple, and I-containing species. To overcome I interference, the electrochemical response of Re complexes was segregated by performing a reductive desorption of adsorbed I from Au. Copyright 2001 Academic Press.