Foreign Metal Adatoms Research Articles

Evidence of Water Reorientation on Model Electrocatalytic Surfaces from Nanosecond-Laser-Pulsed Experiments

The behavior of water at the interface formed between a quasi-perfect Pt(111) single-crystal electrode and an aqueous electrolyte solution is studied by means of the laser-induced temperature jump method. This method is based on the use of nanosecond laser pulses to suddenly increase the temperature at the interface. The measurement of the response of the interface toward the laser heating under coulostatic conditions provides evidence on the net orientation of water at the interface. Especially interesting is the study of the effect on the interfacial water caused by the selective deposition of foreign metal adatoms, because these bimetallic systems usually exhibit appealing electrocatalytic properties. The T-jump methodology shows that the surface composition strongly affects the interaction of water with the surface. The most representative parameter to characterize this interaction is the potential where water reorientation occurs; this potential shifts in different directions, depending on the relative values of the electronegativity of the adatom and the substrate. These results are discussed in the light of available information about the effect of adatom deposition on the work function and the surface potential of the modified surface. Finally, some implications on the enhancement of the electrocatalytic activity are briefly discussed.

Surface metal modifiers for methanol electrooxidation on platinum; silver and mercury

The catalytic activity of platinum surfaces towards methanol electrooxidation can be modified by the deposition of a second metal using different methodologies. There is little information about the catalytic performance of polycrystalline platinum modified by silver and mercury adatoms using spontaneous and electrochemical deposition methods. Cyclic voltammetrics have been performed to compare the current vs potential profiles of modified platinum surfaces in acid solution at room temperature. The inhibition of the hydrogen adatom voltammetric profile by foreign metal adatoms on platinum was used to calculate the degree of surface coverage by the metal. Poisoning effects were checked by anodic stripping experiments of methanol residues on the modified platinum surfaces at adsorption potentials in the hydrogen electrosorption region using a micro flux cell. Methanol solution oxidation was also evaluated at slow scan rates of up to 0.8 vs reversible hydrogen electrode (RHE) on the platinum-modified surfaces. The comparison between the amounts of carbon-monoxide-type residues and the solution oxidation of methanol was analysed to check for their utility as catalytic surfaces for direct methanol fuel cells.

Specific features of hydrogen sorption by palladium dispersed forms at α-phase potentials

A method using foreign metal adatoms is proposed for distinguishing adsorbed and dissolved hydrogen in palladium electrodes. It is shown that the solubility of hydrogen in the α phase of different dispersed palladium samples is higher than that in smooth Pd and increases as the dispersity increases. On the basis of the Bucur model, a quantitative estimate is made of the structural defectiveness of the palladium dispersed forms investigated.

On the determination of zero charge potentials of platinum group metals in the presence of foreign metal adatoms on the surface

Some theoretical and experimental aspects of the potential of zero total charge (p.z.t.c.) determination of platinum group metals in the presence of foreign metal adatoms on the surface are discussed. A thermodynamic method for the determination of p.z.t.c. for electrodes of the platinum group metals in the presence of foreign metal adatoms on their surface has been proposed. It is shown that the hardness method can be used for studied of the effect of adatoms on the p.z.t.c. The possibilities of the thermodynamic and the hardness methods are compared in relation to the p.z.t.c. determination. The estimation of the p.z.t.c. of electrodeposited Pt and Rh in the presence of differing amounts of adsorbed Ag was carried out by both methods. The values of p.z.t.c. determined both by the thermodynamic and by the hardness method are close. The p.z.t.c. of the studied electrodes shift in the negative direction in the presence of Ag adatoms. The possible causes of the observed shift are discussed.

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.

Peculiarities of the electrocatalytic properties of surface skeleton iridium electrodes

The peculiarities of electrocatalytic behaviour of surface skeleton (ss) Ir electrodes on the adsorption of hydrogen and oxygen, the formation of foreign metal adatoms and proceeding of the processes of adsorption and electrooxidation of simple organic substances (CH 3OH, H 2CO and HCOOH) have been studied. Special attention is paid to the formation of oxide layers and their effect on the electrocatalytic processes. It is shown that on ss Ir electrodes an oxide layer giving a “reversible” wave at E r = 1.0 V is formed but this layer is in a considerably smaller quantity than the one on smooth Ir. It has been established that the areas not covered by an oxide layer on the electrodes studied have the same specific activity with regard to the adsorption of foreign metal adatoms, organic particles and in the electrooxidation reactions of simple organic compound as the electrode surface had before the formation of the oxide layer.

A kinetic study of the oxidation of glyoxal on platinum-based electrodes by chromatographic techniques

The electrocatalytic oxidation of glyoxal in perchloric acid on smooth and modified platinum electrodes was investigated by quantitative analysis of the electrolysis products using gas and liquid chromatography. A mechanistic study confirmed that glyoxal adsorption is a dissociative process leading mainly to CO 2 (60%) and formic acid (40%). Modification of the electrode surface by lead adatoms allowed glyoxal oxidation to be carried out at different controlled potentials. On the other hand, glyoxal consumption was shown to be a first-order reaction. It was concluded that the most important parameters controlling the selectivity of glyoxal oxidation are the electrode potential and the nature of the foreign metal adatoms.

Oxidation of formic acid at a high surface area supported platinum modified by foreign metal adatoms

The anodic oxidation of formic acid was studied on platinum distributed in the form of small particles in a porous substrate modified by metal adatoms deposited at underpotentials. The rate of the reaction was greatly enhanced on the commercial catalyst Powercat 3000 (PC 3000). The hydrogen adsorption and the underpotential deposition of several metals at PC 3000 were shown to be dissimilar to that at smooth polycrystalline Pt. Steady-state and long-term experiments were performed to determine the stability of this electrode.

Structural effects in electrocatalysis: Oxidation of D-glucose on pt (100), (110) and (111) single crystal electrodes and the effect of upd adlayers of Pb, Tl and Bi

The electrocatalytic oxidation of D-glucose on Pt (100), (110) and (111) single crystal electrodes has been studied in acid solutions. Pronounced structural effects have been detected. The single crystal electrodes show much higher activity for D-glucose electrooxidation than polycrystalline Pt, particularly the Pt(111) face which appears to be less sensitive to poison formation. Foreign metal adatoms, such as Pb, Tl and Bi, deposited at underpotentials, cause workable catalytic effects as regards the oxidation current density, particularly on the Pt (100) and (110) planes. At complete adatom coverages the oxidation of D-glucose proceeds at higher overpotentials than on clean Pt surfaces, but higher oxidation current densities are obtained due to minor self-poisoning effects. In this work, the underpotential deposition of Tl and Bi on the three low index planes of Pt single crystals has also been studied.

Electrocatalytic oxidation of glyoxalate in alkaline medium on platinum adatom electrodes

The influence of the foreign metal adatoms deposited at underpotentials on the oxidation of glyoxalate on platinum was studied in alkaline medium. Pronounced catalytic effects caused by underpotential submonolayers of Pb, Tl, Cd, Cu and Cr were observed. The enhancement of the oxidation processes by underpotential submonolayers has been interpreted according to the bifunctional theory of the electrocatalysis.

Some specific features of the catalytic role of UPD foreign metal adatoms and water on the anodic oxidation of absolute methanol on Pt

In this work the influence of underpotential-deposited foreign metal adatoms on the anodic oxidation of methanol on Pt in absolute methanol solutions of H 2SO 4 and LiClO 4 is studied. Unliked aqueous solutions, the underpotential deposition (upd) of heavy metal ions causes striking catalytic effects for methanol oxidation in absolute methanol solutions. The maximum catalytic activity occurs at particularly high positive potentials where the upd monolayers are already anodically dissolved. An alternative interpretation based on double layer changes as a result of the redissolution of the upd monolayer has been postulated. The effect of small additions of water on the processes both on Pt and Pt/M(upd) surfaces has also been examined.

Structural effects in electrocatalysis: Oxidation of ethylene glycol on single crystal gold electrodes in alkaline solutions

Ethylene glycol is a potential fuel of practical interest whose electrooxidation has not been studied in detail. This is a complex reaction whose mechanism is far from being understood. It is quite clear, however, that some kind of strongly bound intermediate is formed in the oxidation of ethylene glycol on platinum. This is indicated also by the effects of foreign metal adatoms covering a part of the Pt surface. Thus, pronounced catalytic effects of Pb, Tl and Bi have been observed in alkaline solutions and these are partly based on a suppression of formation and/or adsorption of strongly bound intermediates. In this work the oxidation of ethylene glycol on gold single crystals with the (100), (110) and (111) orientations has been studied with the aim of investigating whether there is structural sensitivity in this reaction. Structural sensitivity, as well as the role of AuOH species formed on surfaces, could be of interest for both electrocatalysis and catalysis. 13 references.

Electrocatalytic oxidation of benzyl alcohol on a platinum electrode modified by underpotential-deposited heavy metal adatoms

The oxidation of benzyl alcohol on platinum surfaces modified by foreign metal adatoms (Pb, Tl, Bi) deposited at an underpotential region was studied in both acid and alkaline media; it was found that Pb, Tl, and Bi submonolayers intensively catalyse the benzyl alcohol oxidation. The enhancement of the electrocatalytic activity of Pt for the benzyl alcohol oxidation in acid solutions has been explained through the mechanism of ăthird body" effect in electrocatalysis. On the other hand, the enhanced catalytic activity in alkaline media, which in the case of Bi adatoms yields current densities close to the diffusion-limited values, has been interpreted in terms of the bifunctional theory in electrocatalysis.

Comparative study of the electrocatalytic influence of underpotential heavy metal adatoms on the anodic oxidation of monosaccharides on Pt in acid solutions

The oxidation of monosaccharides on platinum is markedly catalyzed by foreign metal adatoms deposited in the underpotential range. The catalytic effects were correlated with the metal ad-atom coverage and with the steric and structural characteristics of the carbohydrates. The catalytic action caused by underpotential metal submonolayers was interpreted in terms of the decrease of the electrode poisoning resulting from a stable lactone-type adsorbate, according to the third-body effect in electro-catalysis. A qualitative analysis of the third-body mechanism is presented, which is consistent with a general reaction pattern for the oxidation of aldoses.

Catalysis of oxidation of alcohols on Pt by underpotential submonolayers

The oxidation of aliphatic alcohols on Pt is markedly catalysed by foreign metal adatoms deposited in the underpotential range. In the case of methanol, the catalytic effect of underpotential submonolayers is more pronounced in basic media. In acidic media the catalytic activity depends on the length of the carbon chain and the number of hydroxyl groups. On bare platinum the formation and strong adsorption of organic intermediates results in blocking the active sites of the electrocatalyst surface. The enhancement of the oxidation processes by underpotential submonolayers has been interpreted in terms of preventing electrode poisoning and the specific forces of the reaction intermediates from interacting with the modified electrode surface.

Catalysis of oxidation of aliphatic alcohols on Pt by underpotential submonolayers

The oxidation of aliphatic alcohols on Pt is markedly catalysed by foreign metal adatoms deposited in the underpotential range. In the case of methanol, the catalytic effect of underpotential submonolayers is more pronounced in basic media. In acidic media the catalytic activity depends on the length of the carbon chain and the number of hydroxyl groups. On bare platinum the formation and strong adsorption of organic intermediates results in blocking the active sites of the electrocatalyst surface. The enhancement of the oxidation processes by underpotential submonolayers has been interpreted in terms of preventing electrode poisoning and the specific forces of the reaction intermediates from interacting with the modified electrode surface.

Structural effects in electrocatalysis Oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms

Structural effects in electrocatalysis Oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms

Structural effects in electrocatalysis: Oxidation of formic acid and oxygen reduction on single-crystal electrodes and the effects of foreign metal adatoms

Structural effects in electrocatalytic reactions have been demonstrated on examples of the oxidation of HCOOH on Pt and the O 2 reduction on Au. Catalytic effects of foreign metal adatoms on these reactions have been studied on single-crystal surfaces. Hypothetical superlattice structures of foreign adatoms have been considered in explaining the coverage effect on the kinetics of the HCOOH oxidation. The OH chemisorption on Au in alkaline solution is strongly dependent on the crystallographic orientation of the electrode surface which is the origin of the structural dependence of the O 2 reduction. Thallium adsorbates at small overpotentials inhibit the O 2 reduction on the Au(100) plane, but catalyse it at the Au(111) and the Au(110) faces. At higher overpotentials a four-electron reduction is observed with all three planes covered with thallium adatoms.

Electrocatalytic oxidation of ethylene-glycol Part II. Behaviour of platinum-ad-atom electrodes in alkaline medium

Seven different foreign metal ad-atoms (Bi, Cd, Cu, Pb, Re, Ru, Tl) were investigated as activity modifiers of a platinum electrode for the electrocatalytic oxidation of ethylene-glycol in alkaline medium. None of these metal ad-atoms has an inhibiting behaviour at the optimum concentration of the precursor salt in solution. Among them, Pb, Bi and Tl greatly enhance the electrocatalytic activity of platinum, leading to current densities as high as 100 mA cm −2 for 0.1 M ethylene-glycol at room temperature; Cd shifts the whole polarization curve towards negative potentials; and Cu, Ru and Re leave the electrocatalytic activity of platinum practically unaffected. A qualitative explanation is suggested, based on the modification (due to the presence of the metal ad-atom) of the coverage of the electrode surface both by the organic adsorption residue and adsorbed hydroxyl.

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

Seven different foreign metal ad-atoms (Bi, Cd, Cu, Pb, Re, Ru, Tl) were investigated as activity modifiers of a platinum electrode for the electrocatalytic oxidation of ethylene-glycol in alkaline medium. None of these metal ad-atoms has an inhibiting behaviour at the optimum concentration of the precursor salt in solution. Among them, Pb, Bi and Tl greatly enhance the electrocatalytic activity of platinum, leading to current densities as high as 100 mA cm −2 for 0.1 M ethylene-glycol at room temperature; Cd shifts the whole polarization curve towards negative potentials; and Cu, Ru and Re leave the electrocatalytic activity of platinum practically unaffected. A qualitative explanation is suggested, based on the modification (due to the presence of the metal ad-atom) of the coverage of the electrode surface both by the organic adsorption residue and adsorbed hydroxyl.