Victor Climent

Potential of zero total charge of palladium modified Pt(111) electrodes in perchloric acid solutions

The electrochemical behaviour of palladium-modified Pt(111) electrodes in perchloric acid solutions has been studied by means of cyclic voltammetry and CO charge displacement. From the results of the charge displacement experiments, values of the potentials of zero total charge (pztc) have been determined and their dependence on the palladium coverage has been studied in solutions without specifically adsorbing anions. These pztc values are compared to those obtained in sulfuric acid solutions. Whilst in the presence of bisulfate anions n the pztc decreases significantly as a consequence of the deposited palladium, in the case of perchloric acid solutions the pztc remains constant within experimental error. This constancy has been explained by means of a compensation between the displacement of the potential of zero free charge toward less positive potentials and the increase of hydrogen coverage on the palladium-covered electrode surfaces. The influence of the concentration of specifically adsorbed (bi)sulfate anions at constant pH has been pointed out. The oxidation of the CO adlayer resulting from the charge displacement has also been studied. Despite the apparently easier OH adsorption, the oxidation of adsorbed CO takes place at higher potentials on the modified surfaces. CO coverage values, calculated from knowledge of the pztc value and the CO stripping charge, diminish as the amount of palladium increases.

Quantitative SNIFTIRS studies of (bi)sulfate adsorption at the Pt(111) electrode surface

Subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) was applied to study (bi)sulfate adsorption on a Pt(111) surface in solutions of variable pH while maintaining a constant total bisulfate/sulfate ((bi)sulfate) concentration without the addition of an inert supporting electrolyte. The spectra were recorded for both the p- and s-polarizations of the IR radiation in order to differentiate between the IR bands of the (bi)sulfate species adsorbed on the electrode surface from those species located in the thin layer of electrolyte. The spectra recorded with p-polarized light consist of the IR bands from both the species adsorbed at the electrode surface and those present in the thin layer of electrolyte between the electrode surface and ZnSe window whereas the s-polarized spectra contain only the IR bands from the species located in the thin layer of electrolyte. A new procedure was developed to calculate the angle of incidence and thickness of the electrolyte between the Pt(111) electrode surface and the ZnSe IR transparent window. By combining these values with the knowledge of the optical constants for Pt, H(2)O and ZnSe, the mean square electric field strength (MSEFS) at the Pt(111) electrode surface and for thin layer of solution were accurately calculated. The spectra recorded using s-polarization were multiplied by the ratio of the average MSEFS for p- and s-polarizations and subtracted from the spectra recorded using p-polarization in order to remove the IR bands that arise from the species present within the thin layer cavity. In this manner, the resulting IR spectra contain only the IR bands for the anions adsorbed on the Pt(111) electrode surface. The spectra of adsorbed anions show little change with respect to the pH ranging from 1 to 5.6. This behavior indicates that the same species is predominantly adsorbed on the metal surface for this broad range of pH values and the results suggest that sulfate is the most likely candidate for this adsorbate.

The electrochemistry of nitrogen-containing compounds at platinum single crystal electrodes : Part 2. Semicarbazide on Pt(100) electrodes

Abstract The electrochemical behaviour of Pt(100) electrodes of immersed in semicarbazide containing acidic solutions has been studied by means of cyclic voltammetry and in situ FTIR spectroscopy. Various adsorbed and solution intermediates have been detected spectroscopically in the potential range between 0.10 and 1.20 V RHE. Adsorbed CO, both linearly- and bridge-bonded, is formed upon the dissociative adsorption of semicarbazide for potentials above 0.25 V RHE. This seems to be the main path for the oxidation of semicarbazide to CO2 and molecular nitrogen. Other reaction paths involve the formation of urea and nitric oxide. These species have been detected in the adsorbed state for potentials between 0.50 and 0.90 V. The in situ infrared spectra also show bands in the Cue5fbN stretching region which have been tentatively assigned to the formation of cyanamide and adsorbed cyanate anions.

The hanging meniscus contact: geometry induced diffusional overpotential. The reduction of oxygen in dimethylsulphoxide at Au(111)

Abstract Mass transport in the hanging meniscus configuration has been studied. The effects of this geometry on an electrochemical reaction of a gas phase species with a high solubility in the electrolyte phase have been investigated, both numerically and experimentally. The steady-state process is modelled in cylindrical polar coordinates using a fully implicit finite-difference numerical method and solved with a preconditioned Krylov subspace method. The numerical method incorporates an expanding mesh technique to optimise electrode flux error-cancelling. The nature of the meniscus boundary such as to permit the ingress of electroactive material mentioned, but not the egress of electrode reaction products is shown to result in a diffusional overpotential in the steady-state voltammogram. This observation has been examined experimentally by using the reduction of O 2 in dimethylsulphoxide solution on a Au(111) single crystal electrode.

The electrochemistry of nitrogen-containing compounds at platinum single crystal electrodes: Part 3. Carbohydrazide on Pt(hkl) electrodes

Abstract The electrochemical behaviour of platinum single crystal electrodes in carbohydrazide containing solutions has been investigated by means of cyclic voltammetry and in situ FTIR spectroscopy. The spectroscopic and voltammetric results have shown that the main reaction path of carbohydrazide oxidation goes through formation of CO ads at potentials between 0.2 and 0.4 V RHE. At potentials higher than 0.4 V, a direct oxidation path to CO 2 can be proposed. On Pt(100) and Pt(111) a second reaction path involves the cleavage of N–N bonds leading to the formation of semicarbazide and urea at potentials below 0.25 V. Formation of adsorbed urea and NO has also been detected at potentials higher than 0.5 V on these surfaces. On Pt(110), the fast formation of adsorbed CO impedes other possible reaction pathways.