The voltammetric response of vitreous carbon electrodes in nitrate solution in the presence of Tl3+ shows the reduction of Tl3+ in two stages, to Tl+ and to metallic thallium, respectively. Nitrate ions are reduced at high rates during the second stage, concurrently with Tl deposition. The catalytic current varies with the concentration of nitrate, but is virtually independent of the Tl3+ concentration. No nitrate reduction occurred when Tl deposition was carried out in a single stage from Tl+ solution, nor during reduction of Tl3+ to Tl+. The results obtained indicate that Tl2+, arising from disproportionation of Tl3+ and Tl0 to yield Tl+, mediates the catalytic reduction of nitrate to ammonia during Tl electrodeposition from Tl3+ solutions.

La etapa inicial de los procesos donde se desarro-llan transformaciones de fases, como la condensacion de un gas o la aparicion de un precipitado en solucion, corresponde a la formacion de pequenas estructuras de-nominadas nucleos. Este proceso se conoce como nucleacion y se presenta en todas las transformaciones de fases de primer orden. La nucleacion de una fase nueva puede producir-se por variacion ya sea en la composicion, la presion o la temperatura y en sistemas electroquimicos por variacio-nes en el potencial electrico. Experimentalmente los pro-cesos de nucleacion electroquimica se estudian median-te tecnicas potenciostaticas. La respuesta del sistema a un pulso o escalon de potencial es un transitorio de co-rriente que presenta una forma caracteristica, mostran-do, despues del decaimiento de la corriente debido ala carga de la doble capa electroquimica, un aumento de la corriente producido por la formacion y crecimiento de nu-cleos y una caida posterior originada por la disminucion de la concentracion de la especie electroactiva en las cercanias de la superficie del electrodo

The early stages of the electrolytic deposition of silver onto vitreous carbon electrodes from ammonium hydroxide solutions have been investigated by the potential step technique. The analysis of the experimental current transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed by diffusion controlled growth of nuclei. It is shown that treatments that involve classifying the process as either instantaneous or progressive nucleation are not always adequate for the quantitative analysis of electrochemical nucleation phenomena. The nucleation kinetics parameters A (nucleation rate constant per site) and No (number density of active sites on the substrate surface) were estimated separately from the current transient maxima by two different approaches. Both quantities were found to vary with the potential and with the concentration of silver ions, except at very high overpotentials for silver deposition. The potential dependence of the nucleation rate A was interpreted according to the atomistic theory and in all cases it was found that the number of atoms in the critical nucleus (n(k)) was one over the entire potential range analyzed.

Silver electrocrystallization from aqueous solutions at pH11, pC10 and pNH3 − 0.2, where Ag(NH3)2− is the dominant Ag(i) species, has been studied. In spite of the complexities of this medium, the experimental results can be satisfactorily described in terms of multiple nucleation and diffusion-controlled growth of hemispherical nuclei. Nucleation rates, A, and number densities of active sites on the electrode surface, N0, were determined from potentiostatic current transients as a function of overpotential. Saturation number densities of silver nuclei on the electrode surface obtained from the A and N0 values were found to be in excellent agreement with those obtained from the direct, microscopic observation of the electrode surface. Spatial distributions of nuclei were also analysed for silver electrodeposited at different potentials. It was found that nuclei were uniformly distributed when electrodeposited at low overpotentials, whereas inhibition of nucleation close to already established nuclei occurred at higher overpotentials. From the change of the true nucleation rate with overpotential, it was found that the critical nucleus is formed by a single atom within the −100 to −300 mV over-potential range.

Distributions of distances between nearest and subsequent neighbouring lead nuclei electrodeposited on to vitreous carbon electrodes were obtained. These were compared with both simulations taking into account nucleation exclusion around growing nuclei and with an assumed uniform distribution of particles. Deviation from the latter was observed for all neighbours considered. From analysis of the pair correlation functions derived from the same data, it was concluded that the spatial distribution arising in the presence of nucleation exclusion effects differs from the uniform distribution, albeit without inducing spatial correlation among the nuclei.

The nucleation of lead onto vitreous carbon electrodes has been investigated. Diffusion-controlled growth of nuclei inhibits nucleation, giving rise to saturation with nuclei and introducing correlations in their location on the electrode surface. Similar nuclear number densities were obtained from direct, microscopic observation of the surface and from analysis of potentiostatic current transients considering the development of nucleation exclusion zones around already established nuclei. At low overpotentials nuclei were found to be uniformly distributed on the surface, whereas at high overpotentials the results were found to be consistent with simulations of the process that allowed for the existence of exclusion zones around nuclei, with radii proportional to the square root of their age.

Interactions among growing nuclei diminish the nucleation rate, giving rise to saturation nuclear number densities and introducting correlations in their location on the electrode surface. These have been investigated considering the existence of nucleation exclusion zones around each nucleus, with radii r(e) growing with the age u of nuclei according to laws of the type r(e) = ku(x), with 0 less-than-or-equal-to x less-than-or-equal-to infinity digital simulations of the process, the nearest-neighbour distributions corresponding to the cases of x = 0, 1/2, 1 or infinity were obtained and compared with expressions available for x = infinity and x = 0. The results show that the propagation rates of the exclusion zones can be obtained from microscopic studies of the electrode surfaces in single-step potentiostatic experiments. It is also shown that the analysis of spatial distributions of nuclei on the surface also provides additional criteria for the identification of nucleation rate laws.