Juan M. Feliu

An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes

In this work we have studied the electrochemical behaviour of a bismuth surface compound formed spontaneously without applying external potential when platinum is put in contact with a solution of a Bi(III) salt. When studied in a sulphuric acid solution, the surface compound undergoes a redox reaction where oxidized and reduced species are both adsorbed at the platinum surface. The redox reaction depends on the crystalline surface structure of the platinum substrate. The behaviour of the adsorbate at the platinum (111), (100), (110) and (332) faces as well as at the surface of a spherical platinum single crystal has been investigated. The redox reaction has been found to be highly reversible at the Pt (111) electrode and has been studied as a function of pH and sweep rate at this orientation. This reaction appears as a good probe for detection of (111) surface domains on the platinum surface, as shown by the results obtained with Pt (332), otherwise noted in the terrace-step notation as 5 (111) × (110). The results show also that steps may be blocked selectively by the bismuth adsorbate while terrace sites remain free for hydrogen adsorption. The stability of the bismuth surface compound at the various surface orientations has been investigated. The voltammetric behaviour of the surface compound at Pt (100) has been compared to the voltammetric UPD of bismuth. The comparison allows the assignment of peaks in the voltammogram of the UPD of bismuth at this orientation to a change in the valency of the surface bismuth compound without change in the coverage because the adsorbate is already present at the surface. A similar conclusion is valid for other orientations. The spontaneous deposition has been assumed to be due to an incipient underpotential deposition of bismuth at potentials higher than that of incipient Pt-OH formation. Three surface compounds have been identified at the surface from the reversible behaviour of the adsorbate at Pt (111): Biad at low potential, (BiOH)ad as an intermediate in the redox process and (BiO)ad or (Bi(OH)2)ad, the stable species, at higher potential.

CO oxidation on stepped Pt[n(111) x (111)] electrodes

Abstract The oxidation of CO adlayers, formed by direct dosing from a CO-saturated solution, and bulk CO has been studied on Pt[ n (111)×(111)] single crystals in 0.5 M H 2 SO 4 . For the stepped Pt surfaces studied, CO is found to adsorb preferentially on the steps, blocking the electrochemical hydrogen adsorption there. A pronounced effect of electrode surface structure on CO oxidation has been observed. The overpotential for the oxidation of a saturated CO adlayer, as well as of submonolayer CO coverages, is found to increase in the sequence Pt(553)

Study of the charge displacement at constant potential during CO adsorption on Pt(110) and Pt(111) electrodes in contact with a perchloric acid solution

Abstract A new electrochemical approach has been made, employing the current—time transient responses when a CO adlayer is formed at a platinum electrode at various controlled potentials where CO oxidation does not take place. The case of Pt(110) is compared with those of Pt(111) and Pt(111) disordered after ten cycles of oxygen adsorption—desorption. In order to avoid interference with anion-specific adsorption, the study was carried out in a perchloric acid solution. There is good agreement between the charge measured by voltammetry in the absence of CO and the charges measured during the current—time transients. This is indicative that the latter charges are produced by the displacement of the species at the interface as a result of CO adlayer formations. The sign of the current transient has been found to depend on the potential at which CO adsorption is carried out. This dependence may be related to the nature of species which are present in the interfacial region, providing new complementary information that voltammetry cannot yield.

C-Type Cytochromes Wire Electricity-Producing Bacteria to Electrodes†

Fil: Busalmen, Juan Pablo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnologia de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingenieria. Instituto de Investigaciones en Ciencia y Tecnologia de Materiales; Argentina

Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)

The electrochemical oxidation of saturated and sub-saturated CO adlayers on Pt(111) in 0.5 M H2SO4 has been studied using chronoamperometry. For the saturated CO coverage the oxidation is initiated by an apparently zeroth-order process of removing 2–3% of the adlayer, followed by the main oxidation process, which is shown to be of the Langmuir–Hinshelwood type with a competitive adsorption of the two reactants, CO and OH. The Langmuir–Hinshelwood kinetics can be modeled using the mean-field approximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation and its potential dependence are determined by a fitting of the experimental data with the mean-field model. For sub-saturated CO coverages the overall picture is shown to be more complicated and remains to be understood.

Defining the transfer coefficient in electrochemistry: An assessment (IUPAC Technical Report)

Abstract The transfer coefficient α is a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3rd edition of the IUPAC Green Book, the cathodic transfer coefficient αc is defined as –(RT/nF)(dlnkc/dE), where kc is the electroreduction rate constant, E is the applied potential, and R, T, and F have their usual significance. This definition is equivalent to the other, -(RT/nF)(dln|jc|/dE), where jc is the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficient αa is defined similarly, by simply replacing jc with the anodic current density ja and the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake of n electrons from the electrode in the case of αc, or their release to the electrode in the case of αa. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions of αc and αa has led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity is dln|j|/dE, which can be made dimensionless upon multiplication by RT/F, yielding (RT/F)(dln|j|/dE). One common source of misinterpretation consists in setting this experimental quantity equal to αn, according to the above definition of the transfer coefficient, and in trying to estimate n from αn, upon ascribing an arbitrary value to α, often close to 0.5. The resulting n value is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantities αc = –(RT/F)(dln|jc|/dE) and αa = (RT/F)(dlnja/dE), which are independent of any mechanistic consideration.

Poison formation reaction from formic acid and methanol on Pt(111) electrodes modified by irreversibly adsorbed Bi and As

Abstract Dissociative adsorption of formic acid and methanol on adatom-modified Pt(111) electrodes has been carried out as a way of studying poison formation reactions on these surfaces. The electrodes were prepared using irreversible adsorption of Bi and As. A modification of the dissociative adsorption technique used for poison formation studies has been employed. For clean Pt(111) surfaces the behaviour of the poison formation reaction of the two organic molecules is almost the same, but on adatom-modified Pt(111) electrodes different results are found. Bi and As show an important long-range electronic effect which inhibits poison formation from the dissociative adsorption of formic acid at very low adatom coverage. However, for methanol, the inhibition due to the presence of Bi adatoms on the surface can be explained by a simple third-body effect. A computer simulation of the dissociative adsorption of formic acid on Bi-modified Pt(111) electrodes has been carried out in order to calculate the width of the domains affected by the presence of Bi adatoms.

Validity of double-layer charge-corrected voltammetry for assaying carbon monoxide coverages on ordered transition metals : comparisons with adlayer structures in electrochemical and ultrahigh vacuum environments

Abstract A coulometric procedure enabling the reliable and accurate evaluation of saturated CO coverages, θ sat CO , on Pt-group transition-metal electrodes is outlined, and applied to CO adlayers on ordered low-index platinum, rhodium, and iridium surfaces in acidic aqueous media. Along with voltammetric data, the method utilizes previously described measurements of the charge displaced upon CO adsorption. The reverse of this charge, Q dis , together with the “background” charge Q b flowing between a suitable pair of electrode potentials in the absence of CO, constitutes the overall “double-layer” correction Q dl to the total voltammetric charge Q tot measured for the electrooxidation of adsorbed CO between the same potentials. Significantly, the Q dis as well as the Q b component of Q dl typically constitutes moderate or even large corrections to Q tot , so that the deduced θ sat CO values are noticeably (20–30%) smaller than some voltammetric-based estimates reported earlier. However, the revised coulometric θ sat CO values are in consistently good agreement with the corresponding coverages obtained by means of an infrared spectrophotometric procedure. These θ sat CO values are compared with adlayer structural information obtained recently from in situ scanning tunneling microscopy along with infrared spectroscopy, and also with structural data for corresponding adlayers in ultrahigh vacuum (UHV). In most cases, the electrochemical and UHV-based θ sat CO values are not greatly different (within 5–10%), even though the CO binding site arrangements are often dissimilar in these two environments. The role of the electrode potential in affecting θ sat CO under some conditions via alterations in binding-site energetics, however, is noted for the Pt(111)/CO system.

Surface structure effects on the electrochemical oxidation of ethanol on platinum single crystal electrodes

Ethanol oxidation has been studied on Pt(111), Pt(100) and Pt(110) electrodes in order to investigate the effect of the surface structure and adsorbing anions using electrochemical and FTIR techniques. The results indicate that the surface structure and anion adsorption affect significantly the reactivity of the electrode. Thus, the main product of the oxidation of ethanol on the Pt(111) electrode is acetic acid, and acetaldehyde is formed as secondary product. Moreover, the amount of CO formed is very small, and probably associated with the defects present on the electrode surface. For that reason, the amount of CO2 is also small. This electrode has the highest catalytic activity for the formation of acetic acid in perchloric acid. However, the formation of acetic acid is inhibited by the presence of specifically adsorbed anions, such as (bi)sulfate or acetate, which is the result of the formation of acetic acid. On the other hand, CO is readily formed at low potentials on the Pt(100) electrode, blocking completely the surface. Between 0.65 and 0.80 V, the CO layer is oxidized and the production of acetaldehyde and acetic acid is detected. The Pt(110) electrode displays the highest catalytic activity for the splitting of the C-C bond. Reactions giving rise to CO formation, from either ethanol or acetaldehyde, occur at high rate at any potential. On the other hand, the oxidation of acetaldehyde to acetic acid has probably the lower reaction rate of the three basal planes.

Formic acid oxidation on Pdad + Pt(100) and Pdad + Pt(111) electrodes

Irreversible adsorption of palladium on Pt(100) and Pt(111) is used to form electrocatalysts combining two catalytic elements. Their electrocatalytic activity towards formic acid oxidation is examined. A dramatic effect of the presence of adsorbed palladium on Pt(100) on this reaction is observed with a considerable lowering of the oxidation potential and the absence of self-poisoning on open circuit. In contrast, the activity of the Pt(111) substrate is not greatly changed by adsorbed Pd. A deactivation of the Pdad + Pt(100) electrocatalyst is observed when the oxidation of formic acid takes place, suggesting a slow step in the formation of an adsorbed species which blocks the adsorption of formic acid as a first step of the overall reaction. At all palladium coverages of Pt(100), even with a multilayer of palladium, the activity of the modified electrode towards the electro-oxidation of formic acid was found to be higher than that of bulk palladium.