Reynaldo O. Lezna

A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium

In order to establish the kinetic laws which govern the oxidation process of ethanol on smooth platinum electrodes in acid medium, electrolyses of ethanol were carried out under different experimental conditions. The influence of the initial ethanol concentration and of the potential plateau of electrolysis were investigated and allowed us to improve our understanding of the reaction mechanism. In addition, some ir reflectance spectroscopy experiments were performed to identify the adsorbed intermediates and the reaction products.

In-situ FTIR study of the electrocatalytic oxidation of ethanol at iridium and rhodium electrodes

Abstract In-situ Fourier transform IR (FTIR) reflectance spectroscopy has been used to study the electroadsorption and oxidation of ethanol at polycrystalline Ir and Rh electrodes in HClO 4 solutions. The formation of surface and solution species has been followed optically during slow voltammetric scans. Ethanol electroadsorption leads to the formation of linearly bonded and bridge-bonded CO on Rh surfaces but only to linearly bonded CO on Ir. In the oxidation potential region, Ir electrodes show different catalytic properties from Rh electrodes, with Ir being a more selective catalyst than Rh. Oxidized states at early potentials play an activating role in ethanol electro-oxidation. Acetaldehyde is probably an intermediate product, but the major product on Ir electrodes is acetic acid whereas CO 2 is the main product at Rh electrodes.

Electromodulated infrared spectroscopy of methanol electrooxidation on electrodispersed platinum electrodes: Enhancement of reactive intermediates

Abstract EMIRS spectra of the surface species resulting from methanol electrosorption on electrodispersed Pt were obtained by working under a wide range of experimental conditions, i.e. electrode roughness factor, methanol concentration and mean modulation potential. The intensity of the absorption band related to linearly adsorbed CO-species, COL, at ca. 2030–2080 cm−1 decreases on increasing the electrode roughness factor, a fact which is interpreted as a decrease in the CO adsorbate poisoning effect on electrodispersed Pt surfaces. As a consequence, under well-defined conditions, several bands are observed which might be assigned to adsorbed intermediates and reactive species. EMIRS also reveals a competition between COB and COL at high surface coverages. Spectral data for methanol electrooxidation on electrodispersed electrodes correlate well with the expected structure of this type of electrode.

Modulated reflectance spectroscopy and voltammetry of the sulphide/gold system

Abstract The electroadsorption of sulphide species on gold and the growth of sulphur multilayers have been investigated by optical and electrochemical techniques in sodium tetraborate buffer as the supporting electrolyte (pH 7.0 and 9.2) at 25°C. Differential reflectance spectra indicate that the adsorption of sulphide begins inside the hydrogen evolution reaction (HER) region. The HER is catalysed by the presence of sulphide species on the electrode. During the growth of sulphur multilayers, soluble polysulphide species can be optically detected either as intermediates under diffusion-controlled kinetics or as products of a chemical reaction between sulphide ions and the deposited sulphur layer. The steep fall in the integral reflectance at 0.4 V (vs. SHE) is interpreted through the incorporation of light-absorbing polysulphides into the structure of the deposited sulphur layer.

Chemisorption of methanol on different platinum electrodes (smooth and rough polycrystalline, monocrystalline, and preferentially oriented), as studied by EMIRS

Abstract EMIR spectra of methanol electroadsorbates on smooth and rough polycrystalline, monocrystalline and preferentially oriented platinum electrodes are reported. The (100)-type preferred oriented platinum behaves approximately as Pt (100) single crystals. On electrodispersed platinum electrodes the poisoning phenomena due to CO adsorbates appear to be reduced as compared to smooth platinum electrodes. EMIRS results correlate well with the electrochemical behaviour of the different platinum electrodes for methanol electrooxidation.

In-situ spectroelectrochemistry (UV–visible and infrared) of anodic films on iron in neutral phosphate solutions

Abstract The composition of anodic films on iron formed at different potentials in neutral solutions has been investigated by in-situ UV–vis and IR spectroscopies in the presence of PO 4 3− , a non-contaminant inhibitor used in anticorrosive paints. Optical and electrochemical determinations were used to detect and characterize two well-defined current waves in the active region of Fe and a following passive zone at more positive potentials. The response has been associated with the presence on the surface, in the dissolution zone, of Fe 3 II [PO 4 ] 2 as the main species and probably some [Fe III PO 4 ]. [Fe III PO 4 ] has been detected in the passive region by in-situ IR spectroscopy. Only minor amounts of Fe 2+ and Fe 3+ species in solution have been detected optically. These phosphated overlayers are believed to provide extra protective properties to the passivating film.

Preparation, photoelectrochemical characterization, and photoelectrochromic behavior of metal hexacyanoferrate–titanium dioxide composite films☆

Abstract Composite films comprising of particulate semiconductors and molecular redox systems present interesting frameworks for exploring interfacial photoinduced electron transfer. Metal hexacyanoferrates as candidates for the molecular redox component have the virtue that they are both redox active and electrochromic. On the other hand TiO2 is an inorganic semiconductor that is extremely stable toward photocorrosion. Therefore we describe in this paper an approach to mate the two components. Specifically we describe the preparation and characterization of metal hexacyanoferrate (MHCF)–TiO2 composite films. The variant routes to the electrosynthesis of these films and the corresponding photoelectrochemical/photoelectrochromic behavior are also described for CuHCF–TiO2 and NiHCF–TiO2 composites. Finally, preliminary data on InHCF films are presented.

Potentiodynamic characteristics of ruthenized platinum electrodes in sulphuric acid solutions

Abstract The potentiodynamic response of ruthenized platinum electrodes in 0.5 M and 12 M H2SO4 at 25°C is investigated. The E/I profiles depend on the electrode history and involve sintering-type effects. In 12 M H2SO4 they can be analysed in more detail because the separation of the H-adatom and O-electroadsorption potential ranges is greater than in 0.5 M H2SO4. Thus, four different stages related to the oxide layer are distinguished. The interpretation of the corresponding overall processes is given in terms of the equilibrium potentials of the Ru/H2O system and the reversibility characteristics of the different stages derived from the various potential/time perturbation programs are discussed through reaction patterns previously considered for these reactions on other noble metals.

Driving Multi-electron Reactions with Photons: Dinuclear Ruthenium Complexes Capable of Stepwise and Concerted Multi-electron Reduction

Using biological precedents, it is expected that concerted, multi-electron reduction processes will play a significant role in the development of efficient artificial photosynthetic systems. We have found that the dinuclear ruthenium complexes [(phen)2Ru(tatpp)Ru(phen)2]4+ (P) and [(phen)2Ru(tatpq) Ru(phen)2]4+ (Q) undergo photodriven 2- and 4-electron reductions, respectively, in the presence of a sacrificial reductant. Importantly, these processes are completely reversible upon exposure to air, and consequently, these complexes have the potential to be used catalytically in multi-electron transfer reactions. A localized molecular orbital description of the ligands and complexes is used to explain both the function and spectroscopy of these complexes. In both complexes, the reducing equivalents are stored in the π* orbitals of the bridging ligands and depending on the solution pH, various protonation states of the reduced species of P and Q are obtained. Under basic conditions, the photochemical pathway favors sequential single-electron reductions, while neutral or slightly acidic conditions give rise to proton-coupled multi-electron transfer. In fact, at sufficiently acidic pH, only a coupled two-electron, 2-proton process is seen. Few molecular photocatalysts are capable of proton-coupled multi-electron transfer, which is believed to be a fundamental component of light-activated energy storage in nature.

Spectroelectrochemistry of palladium hexacyanoferrate films on platinum substrates

Abstract Among the Prussian blue analogs, palladium hexacyanoferrate (PdHCF) is of interest from electrochromic and catalytic perspectives, but has not been extensively studied to date. In this paper we report on the combined use of cyclic voltammetry and in situ infrared and UV–vis spectroelectrochemistry of PdHCF films potentiodynamically grown on platinum electrodes. Infrared spectroscopy (in the cyanide stretching frequency region) showed two pairs of interrelated bands at 2075 cm −1 /2180 cm −1 and 2110 cm −1 /2130 cm −1 . UV–vis reflectance spectroscopy also showed two distinct sets of features that are entirely in agreement with the voltammetric peaks and infrared signatures at corresponding potentials. These redox processes have excellent stability and correspond to two PdHCF compound stoichiometries bearing Pd/Fe elemental ratios of 1:1 and 3:2, respectively. A comparison of redox processes and compound stoichiometries in PdHCF with counterparts of Group VIII B metals (namely Ni and Pt) is finally presented.