Scanning tunneling microscopy and electrochemistry

The experimental gap existing between surface science and heterogeneous chemical kinetics applies also to electrochemistry. Difficulties for modelling the electrode surface topography under equilibrium and non-equilibrium conditions, particularly for solid electrodes, are presented. Attention is focussed on structural problems of metal/solution interfaces encountered in corrosion and passivation of metals and in electrocatalysis, although the analysis also extends to metal electrodeposition and to semiconductor/solution interfaces. STM has already provided important imaging of preferred oriented platinum electrode surfaces as well as large surface area platinum electrodes exhibiting practically no diffusional and ohmic polarization effects. Possible applications of STM to underpotential deposition of metals and to chemically modified electrodes are envisaged among others. Some future perspectives of STM for electrochemistry are advanced.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Kinetics and mechanism of the nickel electrode—II. Acid solutions containing a high concentration of sulphate and nickel ions

The kinetics of the Ni electrode in acid solutions with a high sulphate and nickel ion concentration has been investigated in the range of 25–75°C. The active dissolution and the passive regions and the active-passive transition phenomena have been studied employing different potential perturbation techniques using both still and stirred solutions. Two electrochemical processes are competing within the prepassive film potential region, namely, the active Ni dissolution to Ni(II) ions and the Ni(OH)2 film formation. Nickel passivation is explained by reaction pathways involving successive electrochemical and chemical steps implying the occurrence of hydroxo- and oxo-species as reactions intermediates. The advanced mechanism is based upon a positively charged Ni surface structure and attributes the onset of the complete passivity to a particular surface oxide species.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

Kinetics and mechanism of the nickel electrode : I. Acid solutions containing a high concentration of chloride and nickel ions

The kinetics of the Ni electrode in acid solutions with a high chloride ion concentration has been investigated in the range of 25–75°C. Dissolution occurs uniformly only at low anodic potentials. When the latter exceeds a critical value, Ecrit, net localized metal corrosion takes place. The Eln ia plot at E 2RT3F at (E − Erest) → 0, and 2RTF at E → Ecrit. The cathodic Tafel slope is 2RTF. These results, including their pH dependence, are explained with a reaction mechanism involving the participation of adsorbed hydroxo-species.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Kinetics and mechanism of the nickel electrode—II. Acid solutions containing a high concentration of sulphate and nickel ions

The kinetics of the Ni electrode in acid solutions with a high sulphate and nickel ion concentration has been investigated in the range of 25–75°C. The active dissolution and the passive regions and the active-passive transition phenomena have been studied employing different potential perturbation techniques using both still and stirred solutions. Two electrochemical processes are competing within the prepassive film potential region, namely, the active Ni dissolution to Ni(II) ions and the Ni(OH)2 film formation. Nickel passivation is explained by reaction pathways involving successive electrochemical and chemical steps implying the occurrence of hydroxo- and oxo-species as reactions intermediates. The advanced mechanism is based upon a positively charged Ni surface structure and attributes the onset of the complete passivity to a particular surface oxide species.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

Kinetics and mechanism of the hydrogen-evolution reaction on mercury in HCl-dimethylsulphoxide solutions

Rapid galvanostatic and potentiostatic stationary E/I curves, E/time characteristics at current interruption and electrode capacitance measurements were made to investigate hydrogen-ion discharge on mercury in HCl-DMSO solutions, with and without the addition of LiCl as supporting electrolyte. Kinetic parameters and their temperature and concentration dependences are discussed in terms of the initial electron-trnsfer reaction as rate-determining.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Thermodynamic diagrams of the copper/ molten potassium nitrate system

Thermodynamic diagrams for the copper/molten potassium nitrate system have been calculated and expressed either as potential/pO2−, potential/pO2− or potential/pO22− diagrams. They exhibit the corresponding immunity, active dissolution, passivity and transpassivity regions.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Complex processes related to the early stages of mercury electrodeposition on Pt electrodes

The underpotential deposition and early stages of bulk mercury electrodeposition on platinum from aqueous solution containing Hg2+2 ions were studied by using combined potentiostatic and potentiodynamic techniques. The simultaneous electroreduction of the O-containing surface species and the underpotential deposition of Hgsup>2+2 ions render an electrode surface containing an O/Hg atom ratio which depends on both the applied potential and Hgsup>2+2 ion concentration in solution. The nucleation and growth of bulk mercury is impeded by the presence of O-containing surface species, whereas it is markedly enhanced by the presence of Hg-adatoms on the electrode surface. A small number of sites are involved in the nucleation process. Apparently defective points at the UPD Hg layer act as preferred sites for nucleation. It takes potentials more negative than the reversible potential of the Hg/Hgsup>2+2 ion electrode and relatively long electrodeposition times to achieve a degree of surface coverage by mercury atoms greater than 1. It is possible to observe that a fraction of them penetrates into bulk platinum to form a skin-type Hg-Pt alloy.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Las academias nacionales y la educación de la sociedad

El mundo está cambiando rápidamente como consecuencia del desarrollo científico y tecnológico. La veloz acumulación de conocimiento científico y el desarrollo de sus aplicaciones técnicas está facilitada por la gran capacidad y velocidad de las computadoras y por la rapidez de las comunicaciones. Sin embargo, la realidad global es que muchas innovaciones no alcanzan a beneficiar a aquellos que más las necesitan, y las ventajas del desarrollo ciéntífico no son compartidas en el planeta. La comunidad internacional ha atendido inadecuadamente a las necesidades de capacitación en ciencia y técnica de los pueblos. Esta es una omisión crítica que influye en la disponibilidad de suficiente personal capacitado, de adecuada infraestructura e inversión y dificulta el establecimiento de reglas de juego claras para conducir la investigación científica y el desarrollo tecnológico. El mundo de los negocios no ha considerado como problema propio la diferencia entre las naciones en función de su desarrollo, antes bien ha usufructuado esa diferencia ayudando a establecer un círculo vicioso que acentúa la diferencia en el desarrollo científico y técnico entre las naciones. Esta disparidad continuará aumentando en la medida en que las naciones industrializadas continúen siendo dueñas del equipamiento y de las invenciones, tanto por lo que generan como porque captan un número apreciable de los científicos más destacados de las naciones en desarrollo.Facultad de Ciencias Exacta

Ionic mass transfer in the electrolysis of flowing solutions : The electrodeposition of copper under mass-transfer control on tubular electrodes

Ionic mass transfer for the electrodeposition of copper from aqueous copper sulphate-sulphuric acid solutions has been studied on tubular stainless steel electrodes. The following parameters were varied: rate of flow, copper ion concentration, viscosity, height and diameter of tubular electrodes, and temperature. The data can be correlated by the dimensionless equation Sh = 0·647 Re1/2Sc1/3. The temperature coefficient of the kinetic mass-transfer constant is 1.2, corresponding to an experimental heat of activation of 3850 ± 200 cal/mol. The solution of the momentum- and heat-transfer equations worked out by Eckert for flat plates, according to von Kármán's treatment, and recently extended by Wranglén to the analogous masstransfer problem, can be applied under certain conditions to tubular electrodes.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Scanning tunneling microscopy and electrochemistry

The experimental gap existing between surface science and heterogeneous chemical kinetics applies also to electrochemistry. Difficulties for modelling the electrode surface topography under equilibrium and non-equilibrium conditions, particularly for solid electrodes, are presented. Attention is focussed on structural problems of metal/solution interfaces encountered in corrosion and passivation of metals and in electrocatalysis, although the analysis also extends to metal electrodeposition and to semiconductor/solution interfaces. STM has already provided important imaging of preferred oriented platinum electrode surfaces as well as large surface area platinum electrodes exhibiting practically no diffusional and ohmic polarization effects. Possible applications of STM to underpotential deposition of metals and to chemically modified electrodes are envisaged among others. Some future perspectives of STM for electrochemistry are advanced.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada