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.

Dioxygen reactivity and heme redox potential of truncated human cystathionine beta-synthase.

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine and homocysteine to cystathionine, which represents the committing step in the transsulfuration pathway. CBS is unique in being a pyridoxal phosphate-dependent enzyme that has a heme cofactor. The activity of CBS under in vitro conditions is responsive to the redox state of the heme, which is distant from the active site and has been postulated to play a regulatory role. The heme in CBS is unusual; it is six-coordinate, low spin, and contains cysteine and histidine as axial ligands. In this study, we have assessed the redox behavior of a human CBS dimeric variant lacking the C-terminal regulatory domain. Potentiometric redox titrations showed a reversible response with a reduction potential of -291 +/- 5 mV versus the normal hydrogen electrode, at pH 7.2. Stopped-flow kinetic determinations demonstrated that Fe(II)CBS reacted with dioxygen yielding Fe(III)CBS without detectable formation of an intermediate species. A linear dependence of the apparent rate constant of Fe(II)CBS decay on dioxygen concentration was observed and yielded a second-order rate constant of (1.11 +/- 0.07) x 10 (5) M (-1) s (-1) at pH 7.4 and 25 degrees C for the direct reaction of Fe(II)CBS with dioxygen. A similar reactivity was observed for full-length CBS. Heme oxidation led to superoxide radical generation, which was detected by the superoxide dismutase (SOD)-inhibitable oxidation of epinephrine. Our results show that CBS may represent a previously unrecognized source of cytosolic superoxide radical.

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.

Electrochemical behavior of aqueous acid perrhenate-containing solutions on noble metals: critical review and new experimental evidence.

The actual state of the art in the reduction of perrhenate ions on noble metals is reviewed and discussed. Also, with the aim of contributing to better knowledge of this process, results of several experiments are presented. For the first time, spectroscopic evidence on the nature of the deposited rhenium layer on Pt and Rh and the detection of an intermediate in the reduction pathway toward metallic rhenium is provided. The role of the substrate in the electroreduction of perrhenate ions in aqueous acid media is emphasized, because it is directly associated with the formation of different H-containing species as reducing agents. Thus, those metals capable of adsorbing H atoms are able to reduce ReO(4)(-) to ReO(2) by H(ad) at potentials more positive than that of the hydrogen evolution reaction. Moreover, H(ad) reacts with the ReO(2) layer previously deposited, resulting in the formation of Re(III)-soluble species, which subsequently undergo disproportionation to Re and ReO(2). For metals that are not capable of adsorbing H, i.e., Au, molecular hydrogen is the reducing agent, leading to the formation of metallic Re. In addition, ReO(4)(-) is chemically reduced to metallic Re by hydride.

Redox behavior of Re(V)-amino acid containing complexes.

Three cationic complexes containing the [Re((V))O](3+) core (general formula [ReO(dien-H)(aa)](+), dien=diethylenetriamine, aa=glycine, alanine, valine) were studied on polycrystalline Au electrodes employing cyclic voltammetry techniques. The electrochemical behavior of the amino acids (aa) was also evaluated. Experiments were performed at pH 7.0 aqueous solutions at room temperature. The voltammogram of the complex showed current contributions related to the [Re((VI))O](4+)/[Re((V))O](3+) redox couple, the counterion, and the amino acid ligand.

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.