B. Beden

In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum–ruthenium bulk alloy electrodes

The electro-oxidation of carbon monoxide and methanol on platinum–ruthenium bulk alloys was investigated by cyclic voltammetry and in situ FTIR reflectance spectroscopy. Electrochemical measurements revealed that these two oxidation reactions are dependent on the ruthenium composition in the alloy. For CO oxidation, the alloy with an Ru concentration of 50 at.% allows this reaction to occur at lower potentials than on pure platinum or pure ruthenium. Conversely, methanol electro-oxidation gives higher current densities on alloys, containing 10–15 at.% Ru. With FTIRS measurements, it was found that, during methanol oxidation on Pt–Ru alloys, CO, but also small amounts of HCHO and HCOOH, are formed, the surface distribution of which depend on the alloy composition. The COads absorption bands are quite absent in spectra corresponding to alloys with the optimum composition (15–20 at.% Ru). On the other hand, during the oxidation of gaseous CO on alloys with the optimum composition (Pt0.5Ru0.5), the CO2 formed is not adsorbed on the electrode surface, the opposite of the other compositions. These results suggest strongly that the rate determining step for both reactions is the formation of adsorbed OH on the ruthenium surface.

In situ UV visible reflectance spectroscopic investigation of the nickel electrode-alkaline solution interface

Abstract An in situ uv visible reflectance spectroscopic technique is used to investigate the nickel electrode-alkaline solution interface. It is shown that by recording the change in relative reflectivity of the surface vs the potential applied to the electrode, at various fixed wavelengths, it is possible to get the spectra of superficial layers. Reflection-absorption spectra were thus obtained respectively for the hydroxide αNi(OH) 2 (two maxima at 360 and 500 nm) and the oxyhydroxide NiOOH (broad band with a maximum at 500–550 nm), providing new insights concerning the reversibility of some of the electrochemical processes which occur on the electrode surface when varying the potential.

Electrocatalytic oxidation of methanol on platinum-based binary electrodes

Abstract Four platinum-based binary catalysts (Pt+Sn, Pt+Pb, Pt+Re, Pt+Ru) used for the electrochemical oxidation of methanol were investigated by linear and cyclic voltammetry. Except for the Pt+Ru system, which always gives an enhanced electrocatalytic activity when compared to pure metals, thus leading to a synergistic effect, the other ad-atom+Pt systems show an increased activity only for some concentration ranges of the precursor salts, and for some ranges of electrode potentials. Outside of these experimental conditions, the three metal ad-atoms behave as relatively strong catalytic poisons. An explanation of these effects is suggested on the basis of a competitive adsorption between the second-metal ad-atoms, the adsorbed hydrogen and the strongly bound residue of methanol adsorption.

The electrooxidation of CO: a test reaction in electrocatalysis☆

Abstract This review paper aims to show how the electrochemical behaviour of CO plays a key role in the understanding of the reaction mechanism of many electrocatalytic oxidations of small organic molecules. For that purpose, the adsorption of CO on noble metal electrodes, eventually modified by foreign metal adatoms, is reviewed, taking into account both experimental (electrochemical and spectroscopic techniques) and theoretical (Extended Huckel Model) approaches. Data from the gas phase—solid metal interface are also considered.

Fourier transform infrared reflectance spectroscopic investigation of the electrocatalytic oxidation of d-glucose: Identification of reactive intermediates and reaction products

In situ reflectance infrared spectroscopic techniques (under the SPAIRS and SNIFTIRS variants) have been applied to the investigation of the electrooxidation process of d-glucose at platinum electrodes in alkaline medium. Various reactive or poisoning intermediates were identified and their potential dependence monitored. The following mechanism was postulated. At low potentials, the first step leads to an adsorbed intermediate (A) formed by dehydrogenation of the anomeric carbon of glucose. Oxidation of (A) at higher potentials follows two possible routes. At E < 0.6 V vs. rhe, species (A) is oxidised as weakly adsorbed gluconate, either linked by two oxygens (species I, at E < 0.3 V), or by only one oxygen at 0.3 < E < 0.6V (species II). The second route is observed at 0.6 < E < 1.6V. In this potential range, ie, all over the range where the surface is covered by oxygenated species, species (A) is oxidised as weakly adsorbed δ-gluconolactone (B). Desorption of (I), (II) and (B) gives the corresponding solution species, ie gluconate (C) and δ-gluconolactone (D). In solution, slow hydrolysis of (D) leads to (C), so that gluconate is the only final product accumulating in solution.

Oxidation of methanol on a platinum electrode in alkaline medium: Effect of metal ad-atoms on the electrocatalytic activity

Abstract A mechanistic investigation of the electrochemical oxidation of methanol was worked out in alkaline medium in order to interpret the effect of some foreign metal ad-atoms (Pb. Bi, Cd, Tl) on the electrocatalytic activity of platinum. The rate-determining step being a surface reaction between adsorbed hydroxyls and a methanol chemisorption intermediate, the positive effect met with Pb and Bi may be explained through the bifunctional theory of electrocatalysis, whereas the negative effect met with Cd and Tl may come from the inhibiting behaviour of these latter ad-atoms.

Infrared spectroscopic study of the methanol adsorbates at a platinum electrode: Part I. Influence of the bulk concentration of methanol upon the nature of the adsorbates

The dependence of the adsorbed layer formed at a platinum electrode on the bulk concentration of methanol has been investigated using Electrochemically Modulated Infrared Reflectance Spectroscopy (EMIRS). The major conclusions are: (1) CO species are formed at all concentrations, the linearly bonded CO being predominant for concentrations above 5×10−3M in CH3OH. However, at lower concentrations, almost equal quantities of linearly and bridge-bonded CO species are detected. (2) When the methanol concentration is < 5×10−3M, a new single sided band appears at ca. 1690 cm−1. This band corresponds well to the carbonyl stretching vibration of an aldehyde and might therefore be consistent with ·CHO-like species.

Synergistic effect in the electrocatalytic oxidation of methanol on platinum+palladium alloy electrodes

Abstract The electrocatalytic oxidation of methanol has been investigated on platinium+palladium alloy electrodes of different compositions in acid, neutral and alkaline aqueous solutions. The surface characteristics (composition and roughness factor) of the alloys and the stability of the electrodes in contact with different electrolytic solutions have been studied using cyclic voltammetry. In particular, a surface enrichment in platinum due to a preferential dissolution of palladium and an increase of the roughness factor with an increase of the palladium content has been shown. The electrocatalytic activity of different alloys for methanol oxidation has been characterized by exchange current densities obtained from extrapolation of Tafel lines of calculated equilibrium potential. The plot of these current densities vs. the surface composition leads to a synergistic effect, particularly important in alkaline medium. A reasonable explanation of this enhanced electroactivity at about 15 at.% in Pd is given on the basis of a decrease of electrode poisoning.

Electrocatalytic oxidation of ethylene-glycol: Part I. Behaviour of platinum ad-atom electrodes in acid medium

Abstract The influence of six different foreign metal ad-atoms (Cd, Re, Pb, Cu, Bi, Tl) on the electrocatalytic activity of a platinum-based electrode was investigated in the case of the oxidation of ethylene-glycol in acid medium. Only Cd and Re ad-atoms lead to an electrocatalytic effect, mainly due to a cathodic shift of the whole polarization curve. An explanation of the different behaviour of these six ad-atoms is given on the basis of their potential range of redissolution, compared to the potential range of the first anodic peak of ethylene-glycol electrooxidation. As far as metal ad-atoms remain present on the platinum surface in the potential range of the hydrogen region, they prevent the formation of adsorbed hydrogen and thus the occurrence of a strongly bound intermediate.

“In situ” analysis by infrared reflectance spectroscopy of the adsorbed species resulting from the electrosorption of ethanol on platinum in acid medium

Abstract The adsorption of ethanol at a platinum electrode from a perchloric acid electrolytic solution has been investigated thoroughly, using infrared reflectance spectroscopy. Depending on the electrode potential, numerous infrared bands were detected, which could be attributed to various potential-dependent co-existing species. Among them, adsorbed CO, mainly linearly bonded to the surface, was detected unambiguously, giving rise to a strong absorption band at 2070 cm −1 . when the potential range was chosen so that the CO species did not cover the whole surface, new bands appeared, due to various CH stretches together with a strong CO stretch at 1055 cm −1 . Different models for the adsorbed species are discussed, including molecular adsorption of ethanol (evidence of weak infrared bands), adsorption of the ethoxy group [CH 3 CH 2 O] − , and the adsorbed ethylate species [CH 3 CO] − .