M.M.E. Duarte

Electro‐Oxidation of Methanol on PtRu Nanostructured Catalysts Electrodeposited onto Electroactivated Carbon Fiber Materials

The surface of different carbon substrates, such as glassy carbon (GC), graphite cloth (GC‐10), graphite felt (GF‐S2), and carbon fiber paper (CFP) was modified by electrochemical treatment to generate high concentrations of oxygenated functional groups. These activated carbons were used as substrates for the simultaneous electrodeposition of Pt and Ru by a double potentiostatic pulse program. The different catalyst/carbon systems were evaluated as electrodes for methanol oxidation in acid solution. Comparing the results for the oxidized and nonoxidized substrates, the oxidation of the different carbon materials prior to the catalyst deposition was found to lead to an increase in the electrode activity for methanol oxidation. This enhancement could be associated with a remarkable improvement of metal dispersion, reduction of particle size, and a higher active surface area of the catalyst. The electrodes prepared with oxidized graphite felt exhibited the greatest catalytic activity.

Evaluation of anode deactivation in chlor-alkali cells

Different techniques to evaluate catalytic activity and to quantify residual service life (RSL) have been applied to DSA® anodes used in chlor-alkali industrial cells. An accelerated service life test in sulfuric acid was used as a reference method. The results obtained through EIS, cyclic voltammetry and Ru content analysis of the catalytic layer confirm the results obtained with the accelerated test. There is a direct relation between the residual service life (RSL) thus obtained and the catalytic properties of the electrode. The different mechanisms of catalytic activity loss are discussed. An evaluation methodology is proposed to reduce maintenance costs of industrial cells.

Electrooxidation of Mn(II) to MnO2 on graphite fibre electrodes

Electrooxidation of Mn2+ to MnO2 on carbon felt and carbon cloth electrodes (C-materials) in a weak sulfuric acid electrolyte was investigated using stationary and rotating electrodes. These materials exhibit better performance than more conventional materials such as lead. Scanning electron micrographs of the deposits show a uniform cylindrical growth of MnO2 encapsulating the carbon fibres. The deposits spontaneously fracture following straight-lines indicating a fibrous microstructure. X-ray diffraction reveals a γ-MnO2 structure for all deposits.

Some aspects of removal of copper and cobalt from mixed ion dilute solutions

Some aspects of the electrodeposition of copper and cobalt from aqueous sulphate solutions containing low concentrations of their ions were studied with a view to heavy metal removal via an electrochemical process. Both metals were deposited on a vitreous carbon rotating disc electrode. Deposits formed under different conditions were studied employing linear sweep voltammetry, scanning electron microscopy and EDAX surface analysis. Constant potential electrolysis was used to simulate recovery in a laboratory batch reactor. Copper can be deposited without cobalt interference at potentials as cathodic as −1.0 V despite high Co concentrations. At more negative potentials, both metals are deposited simultaneously, although the copper proportion in the binary mixture is greater than that corresponding to the solution concentration ratio. Voltammetry studies effected under conditions in which codeposition occurs show only minor changes in copper behaviour. On the other hand, cobalt behaviour exhibits significant modifications. Even though formation of an intermetallic compound is possible, ASVL and microscopy tests indicate cobalt deposition in different crystalline forms as the more probable cause. In turn, cobalt deposition depends on the polarization conditions of the electrode and on the cobalt and copper concentrations.

Synthesis and Characterization of Three-Dimensional Porous Cu@Pt and Cu@Pt-Ru Catalysts for Methanol Oxidation

Highly porous Cu foams comprised of interconnected branched dendrites were used as sacrificial templates for the fabrication of pseudo core–shell Cu@Pt and Cu@Pt‐Ru unsupported electrodes by galvanic replacement. The as‐prepared materials presented a three‐dimensional structure with pores between 17 and 45 μm made of superimposed layers of ramified dendrites. TEM analysis showed that the dendrites were composed of agglomerates of grains of about 4 nm in size and mesopores of ca. 30 nm in diameter. The as‐prepared 3D electrodes were tested for methanol oxidation in acid media at different temperatures. The results showed that the catalytic activities of Cu@Pt and Cu@Pt‐Ru foams normalized to the electrochemical surface area are almost 50 % higher than that of a commercial Pt−Ru/C material, with poisoning rates that are reduced by half in the same potential range. The enhanced behavior of the as‐prepared foams is believed to be the result of the influence of copper atoms on the reactivity of platinum sites, the highly defective structure of the electrodes, as well as the facilitated diffusion of methanol molecules and the products formed during the reaction throughout the highly porous three‐dimensional structure of the electrode. The apparent activation energies (Ea,app) for the methanol oxidation reaction (MOR) were determined by potentiostatic experiments. Ea,app values of 29.07 and 24.20 kJ mol−1 were calculated for Cu@Pt and Cu@Pt‐Ru foams at 0.3 V, respectively. The results suggested that the MOR is governed by the dissociative adsorption of methanol as a result of the multifunctional nature of the catalyst and the facilitated diffusion of the products formed during the reaction.

Efecto de aditivos orgánicos en la deposición de catalizadores Pt-Ru

The simultaneous codeposition of Pt and Ru on carbon materials was carried out potentiostatically at -0.5 V from H2PtCl6 and RuCl3 aqueous solutions in the presence of different organic additives: 1) ethylene glycol, 2) ethanol, 3) formic acid, 4) sodium citrate, 5) sodium tartrate and 6) Na2H2EDTA. The electrodeposition process was influenced thermodynamically and kinetically by the stabilizers. SEM and AFM analysis of the electrodes showed that both the particle size and the active surface area were notably affected by the presence of the stabilizing agents. The different behaviour of the stabilizing agents may be associated with the following effects: (1) the capability of the stabilizing agent to complex the metal ions, (2) the specific adsorption of organic molecules which inhibits particle growth, and (3) the reduction capability of the organic compounds. The electrodes prepared with different organic additives exhibit the following decreasing order of activities for methanol oxidation: citrate > tartrate > ethanol > Na2H2EDTA > ethylene glycol > formic acid > without stabilizer. This comportment can be related to differences in the active surface area, particle size and superficial structure of the metallic deposit.

Preparación electroquímica de electrodos bimetálicos PtRu soportados sobre sustratos carbonosos y su caracterización para la oxidación de metanol en medio ácido

In this work the influence of the electrochemical oxidation of different carbon substrates, such as glassy carbon, graphite cloth, graphite felt, and carbon fiber paper, on the electrodeposition of bimetallic PtRu catalysts was studied. The oxidation of the different substrates prior to the catalyst deposition, leads to remarkable improvement of catalyst dispersion, reduction of particle size and higher specific surface area of the catalyst, when was compared than that electrodes with the unnoxidized substrates. All the electrodes were used to study the methanol oxidation in acid media.