Luis Humberto Mendoza-Huizar

Nucleation and Growth Kinetics of Electrodeposited Sulfate-Doped Polypyrrole: Determination of the Diffusion Coefficient of SO42― in the Polymeric Membrane

A kinetic study for the electrosynthesis of polypyrrole (Ppy) doped with SO(4)(2-) ions is presented. Ppy films were electrochemically polymerized onto a graphite-epoxy resin electrode. Experimental current density transients (j-t) were obtained for three different potentiometric behaviors: anionic, cationic, and a combination. Theoretical models were used to fit the experimental j-t data to determine the nucleation and growth processes controlling the polymer synthesis. It was encountered that, in all cases, pyrrole electropolimerization involves two concomitant processes, namely, a Ppy diffusion limited multiple 3D nucleation and growth and pyrrole electro-oxidation on the growing surface of the Ppy nuclei. SEM analysis of the electrodes surfaces reveals that Ppy deposition occurred over most of the electrode surface by multiple nucleation of hemispheres, as the theoretical model used for the analysis of the current transients required. Hemispherical particles formed the polymeric film displaying different sizes. The order for the particle size was as follows: anionic > anionic-cationic > cationic. These results are congruent with those obtained by theoretical analysis of the corresponding current transients. Analysis of the impedance measurements recorded on the anionic Ppy film, immersed in an aqueous solution with different sulfate ion concentrations evidenced that SO(4)(2-) ions diffuse through the Ppy film provoking a decrease of its electrical resistance and an increase of its dielectric constant. From the Warburg impedance coefficient, the sulfate coefficient of diffusion in the Ppy film was 1.38 x 10(-9) cm(2) s(-1).

Kinetic study of the cobalt electrodeposition onto glassy carbon electrode from ammonium sulfate solutions

We carried out an electrochemical study of the cobalt electrodeposition onto glassy carbon electrode from an aqueous solution containing 10-2 M of CoSO4 + 1 M (NH4)2SO4 at natural pH 4.5. The potentiostatic study indicated a progressive 3D nucleation and growth during the deposition process. The average diffusion coefficient calculated for this system was 2.65 X 10-6 cm2 s-1 while the ΔG for the formation of stable nucleus was 6.50 X 10-20 J/nuclei. The scanning electron microscopy images indicated the formation of small and homogeneous nucleus onto GCE of approximately 300 nm.

Theoretical and Experimental Study of Cobalt Nucleation and Growth onto Gold Substrate with Different Crystallinity

Cobalt electrodeposition onto both polycrystalline and monocrystalline (111) gold electrodes was studied using electrochemical techniques. Current density transients were recorded in these systems starting with the applied potential in the underpotential region and then jumping to different potential values in the overpotential zone. From the analysis of the experimental J-t curves, it was found that the process of cobalt deposition involves a 2D-3D nucleation growth transition in both cases. We also found that a mechanism comprising the simultaneous presence of three different contributions namely, Langmuir-type adsorption processes and 2D and 3D nucleation, both mass-transfer-controlled nucleation processes, can adequately describe the experimental evidence. This model predicts a greater nucleation rate and larger number of active nucleation sites for polycrystalline gold substrate compared with the Au(lll) substrate for each applied potential. Moreover, from a theoretical quantum study, we analyzed the reactivity of the bare Au( 111) surface determining that there exists a distribution of active reduction sites inherent to this substrate. The existence of these sites can be considered as one of several previous steps required for the formation of active nucleation sites. We found a ratio of 2.5 X 10 4 active reduction sites for each active nucleation site obtained from experimental results.

DETERMINATION OF GLYPHOSATE AND AMINOMETHYLPHOSPHONIC ACID IN SOILS BY HPLC WITH PRE-COLUMN DERIVATIZATION USING 1,2-NAPHTHOQUINONE-4-SULFONATE

A simple selective high-performance liquid chromatography (HPLC) method has been developed for the determination of glyphosate and its main metabolite aminomethylphosphonic acid residues in soil samples based on a pre-column derivatization with 1,2-naphthoquinone-4-sulfonate (NQS). The reaction is performed at pH 9.4 and 60°C for 5 min. The separation of glyphosate was carried out in a C18 column using a 1% acetic acid–methanol (40:60, v/v) mobile phase and UV detection. An experimental design approach based on the central composite design is used to investigate the dependence of the derivatization variables of pesticide. The concentration of NQS and reaction temperature were found to be statistically significant. The limits of detection obtained were 0.064 mg kg−1 for glyphosate and 0.098 mg kg−1 for its metabolite. The proposed method was applied successfully to the analysis of real soil samples from corn crops. The proposed method is easier and faster compared to other ones.

Cobalt electrodeposition onto polycrystalline gold from ammoniacal solutions

AbstractIn the present work, the cobalt electrodeposition onto polycrystalline gold electrodes from aqueous solutions containing 0.01M CoSO4 + 1 M (NH4)2SO4 at pH=7 was analyzed. Linear voltammetry results suggested a change in the kinetic of the cobalt electrodeposition. In all cases, the nucleation rate (A), the number of active nucleation sites (N0) and the saturation number of nuclei (Ns) values were potential dependent. The calculated Gibbs free energy (ΔG) for this system was 1.88×10−20 J nuclei−1 and the transfer coefficient for the Hydrogen Electroreduction Reaction (HER) was 0.47.

Cobalt electrodeposition onto highly oriented pyrolytic graphite (HOPG) electrode from ammonium sulfate solutions

-2 M of CoSO 4 + 1M (NH 4 ) 2 SO 4 at pH 4.5 (natural pH). All solutions were prepared by using analytic grade reagents with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N 2 for 15 min before each experiment. Once the solution was deoxygenated a nitrogen atmosphere was maintained over the solutions. During the experiment the bubbling was stopped to avoid the presence of additional diffusional variables caused by the nitrogen bubbles on the electrode surface. The working electrodes were freshly cleaved HOPG surfaces. A graphite bar with an exposed area greater than the working electrode was used as counter electrode. A saturated silver electrode (Ag/AgCl) was used as the reference electrode, and all measured potentials are referred to this scale. The electrochemical experiments were carried out in a BAS potentiostat connected to a personal computer running the BAS100W software to allow the control of experiments and data acquisition. In order to verify the electrochemical behavior of the electrode in the electrodeposition bath, cyclic voltammetry was performed in the 0.600 to -1.300 V potential range. The kinetic mechanism of Co deposit onto HOPG was studied under potentiostatic conditions by the analysis of the experimental potentiostatic current density transients obtained with the potential step technique. In all cases, the perturbation of the potential electrode always started at 0.600 V because this is the null current zone in where the Co deposition had not still begun. Thus, the application of this potential guarantees that the electrochemical signals recorded during the cathodic polarization are caused by the cobalt electroreduction on the electrode. 20

Copper electrodeposition on glassy carbon and highly oriented pyrolytic graphite substrates from perchlorate solutions

In present work, we analyzed the copper electrodeposition onto GCE (System I) and HOPGE (System II) from perchlorate solutions. The current density transients obtained from system I and II were well described through a kinetic mechanism that involves four different contributions: (a) a Langmuir type adsorption process, b) an electron transfer from Cu2+→Cu+, (c) a 3D nucleation limited by a mass transfer reaction and (d) a proton reduction process. It was observed that the values of the nucleation rate, the number of active nucleation sites were increased with the overpotential and they are bigger onto GCE in comparison with HOPGE.

Cobalt electrodeposition on polycrystalline palladium. Influence of temperature on kinetic parameters

Cobalt electrodeposition on polycrystalline palladium was studied at different temperatures using potentiostatic and voltammetric techniques. Temperature effect on kinetics parameters, diffusion coefficient, and charge transfer coefficient was analyzed. The values of nucleation rate and rate constant of the proton reduction reaction (kPR) increased with the temperature increment and the applied overpotential. The number of active nucleation sites was slightly affected with temperature increase. At higher temperatures, the larger kPR values suggested the proton reduction process is favored. The temperature effect on the values of the transfer coefficient was analyzed and a decrease in its value with the temperature increase was observed. From a Conway plot, it was observed that entropy change is the main factor that controls the kinetics of the reaction in this system.

Chemical Reactivity of Isoproturon, Diuron, Linuron, and Chlorotoluron Herbicides in Aqueous Phase: A Theoretical Quantum Study Employing Global and Local Reactivity Descriptors

We have calculated global and local DFT reactivity descriptors for isoproturon, diuron, linuron, and chlorotoluron herbicides at the MP2/6-311

GLOBAL AND LOCAL REACTIVITY DESCRIPTORS FOR PICLORAM HERBICIDE: A THEORETICAL QUANTUM STUDY

In this work, we studied the reactivity of picloram in the aqueous phase at the B3LYP/6-311++G(2d,2p) and MP2/6-311++G(2d,2p) levels of theory through global and local reactivity descriptors. The results obtained at the MP2 level indicate that the cationic form of picloram exhibits the highest hardness while the anionic form is the most nucleophilic. From the Fukui function values, the most reactive site for electrophilic and free radical attacks are on the nitrogen in the pyridine ring. The more reactive sites for nucleophilic attacks are located on the nitrogen atom of the amide group and on the carbon atoms located at positions 2 and 3 in the pyridine ring.