E. Lamy-Pitara

Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum

Abstract The hydrogenation reaction of maleic acid, catalyzed by platinized platinum, was investigated in the presence and in the absence of gaseous hydrogen, and with/without an external control of platinum potential, in different acid supporting electrolytes. The intrinsic catalytic activity of platinum, used as a catalyst and as an electrode, was measured at different potentials. A promotion of the catalytic activity of platinum, induced by an external control of its potential, in the presence of gaseous hydrogen, was found only in a perchloric acid supporting electrolyte. No promotion effect was obtained in the presence of strongly adsorbed anions (HSO 4 − , Cl − ). Moreover strongly adsorbed anions induce a decrease in the catalytic activity of platinum, whereas they do not affect the electrocatalytic activity of platinum catalyst-electrode. The observed anion effects on the hydrogenation of maleic acid were compared with those obtained in the electrocatalytic hydrogen oxidation (h.o.r.) and evolution (h.e.r.) reactions studied on a rotating disk platinum electrode. In electrocatalytic oxidation of hydrogen the current densities depend on the nature of the supporting electrolyte and decrease in the order, j (HClO 4 )> j (H 2 SO 4 )> j (HCl). However, the hydrogen evolution reaction, studied in underpotential and in low overpotential regions, does not depend on the nature of the supporting electrolyte. The ensemble of results is discussed in connection with the nature of active hydrogen species and/or active platinum sites involved in these reactions.

Catalytic and electrocatalytic oxidation of oxalic acid in aqueous solutions of different compositions

Oxalic acid oxidation in aqueous solutions of different compositions (different anions and pH), with a platinum catalyst–electrode, was studied. Different experimental methods were compared: the presence or absence of gaseous oxygen, external potential control or spontaneous establishment of the potential in the presence of the reactants. The intrinsic catalytic activity of platinum was determined by analytical techniques. The presence of strongly adsorbed anions in the solution induces a decrease of the catalytic activity of platinum. The weak ‘anion-like’ adsorption of oxalic acid, occurring in competition with the adsorption of different anions, can account for this result. Undissociated oxalic acid is more reactive than oxalate and dioxalate anions. The activity of platinum measured at a given potential, is higher in catalytic than in electrocatalytic oxidation of oxalic acid, suggesting that different adsorbed active oxygen species are involved in these two oxidation processes.

Adsorption of tin on platinum : an uncommon underpotential deposition

Abstract Tin deposited on platinum in underpotential conditions was studied by voltammetric and X-ray photoelectron spectroscopy (XPS) techniques. Tin induces a strengthening of bonds between platinum and adsorbed hydrogen or oxygen. The electrochemical and catalytic properties of the platinum substrate are irreversibly modified by tin deposition. Although it is possible to ”regenerate“ the first monolayers of platinum by voltammetric cycling (0–1.5 V/RHE), some of the tin deposit which diffuses into the platinum bulk (at a depth of 200 A) cannot be desorbed. This bulk tin is found in the oxidized state (Sn(II)), and probably forms a complex compound with platinum and oxygen. In this case, adsorption of SN(IV) on the regenerated platinum surface with partial or no reduction occurs. This complex behaviour particular to tin deposited underpotentially on platinum could explain conflicting reports of electrocatalytic effects in the literature.

The electrocatalytic reactions of oxidation and evolution of hydrogen on iridium electrodes modified by sulphur adsorption

Abstract Iridium electrodes were modified by adsorption of sulphur species produced by reaction with H 2 S, and tested for the electrocatalytic reactions of oxidation and cathodic evolution of hydrogen. At low sulphur coverages ( θ s θ s = 0.20. At higher sulphur coverages ( θ s > 0.50) a stronger decrease of current densities is seen. These effects of sulphur obtained in the case of iridium electrodes, are quite comparable with those obtained in the case of platinum electrodes. The Tafel slope increases with increasing sulphur coverage from 35 mV, for θ s = 0, to 60 mV, for θ s = 0.95, indicating a change of mechanism at high θ s , whereas in the case of platinum electrodes this parameter remains unchanged in the presence of adsorbed sulphur.

Electrochemical preparation of a Pt-Ru alloy

Abstract The electrochemical reduction of Pt(IV) and Ru(III), in separated and mixed solutions, has been studied on a platinum electrode in 1 N HCl solution. The chronoamperometric and chronocoulometric results showed that the simultaneous reduction of Ru(III) and Pt(IV) occurs, leading to alloy formation. The voltammogram of the obtained alloy is in agreement with such a conclusion.

Electrocatalytic hydrogenation of m-xylene on platinized-platinum electrodes

The electrocatalytic hydrogenation of m-xylene (1,3-dimethylbenzene) was investigated at room temperature in 0.5 M H2SO4 solutions on a platinized-platinum (Pt/Pt) electrode. It was found that the hydrogenation process of m-xylene took place only in the potential region corresponding to the weakly adsorbed hydrogen (0<EH<0.25 V) and that the electrocatalytic activity of the Pt/Pt electrode was dependent on the hydrogenation potential. The hydrogenation of m-xylene yielded steroisomeric product mixtures of the saturated 1,3-dimethylcyclohexane (1,3-DMCH), cis- and trans-1,3-DMCH. The cis/trans isomer ratio was found to be strongly dependent on the potential on which the hydrogenation of m-xylene has been carried out. These results were compared with those reported for the gas and liquid-phase catalytic hydrogenation of m-xylene and explained through a model of molecular reorientation dependent on the electrode potential.

Adsorption of iron on platinum

In the field of heterogeneous catalysis, platinum-iron bimetallic catalysts are known for their good selectivity in the hydrogenation of C=Q bonds conjugated with c--C bonds, leading to unsaturated alcohols [l-9]. Although these catalysts were discovered a long time ago (19251, in the work of Tutley and Adams [l] and later Rylander et al. [2], they have become the object of intensive study again during the last five years [3-91. However, the behaviour of these bimetallic catalysts is not yet well understood. One important question remains open: is the iron in the metallic state or not, under the experimental conditions of hydrogenation reactions in the aqueous phase? The purpose of this work was therefore to study the adsorption of iron on platinum under the conditions of underpotential deposition and then to characterize this Pt-Fe system by its activity for catalytic hydrogenation of maleic acid.

Depot en sous-tension de cuivre sur des electrodes de platine ruthenisee

The characterization of ruthenium deposited on platinum electrodes by hydrogen or oxygen adsorption is difficult, thus the use of UDP of copper is suggested. Different electrodes, with increasing factor of roughness (3 30, a multilayer growth of copper, before the completion of the first monolayer, is observed.