E.P.M. Leiva

Collision as a way of forming bimetallic nanoclusters of various structures and chemical compositions

In the present work, a new way to obtain bimetallic nanoclusters of different structures and chemical compositions is proposed, which is based on computer simulations. Collision processes between two metal clusters of different natures are simulated through molecular-dynamics simulations using many-body potentials. Diverse diffusion mechanisms and structures can be observed, depending on the metals combined and the initial kinetic energies. The nanostructures we have found are core-shell (Pt-Au), alloyed (Pd-Au), and three-shell onionlike (Cu-Ag).

The oxygen and chlorine evolution reactions at titanium oxide electrodes modified with platinum

Abstract The oxygen (OER) and chlorine (CLER) evolution reactions were used as probes to test the electronic properties of modified electrodes (Ti/TiO 2 -Pt). Both reactions were studied in acidic solutions by means of current-potential (j-E) and impedance measurements under steady state conditions. The effect of dopant concentration, TiO 2 thickness, and the concentration of the electroactive species in the solution was analyzed. A diminution of the electron transfer current with titanium oxide thickness was obtained whereas an increase of the current was found on augmenting Pt amount. At low titanium oxide thickness both inner-sphere reactions behave in the same way as in the case of pure Pt, indicating that platinum sites are the active ones. However, at higher titanium oxide thickness, the platinum sites become less active and the valve metal oxide (TiO 2 ) begins to block OER and CLER reactions. The electrochemical impedance spectroscopy results are compared with those of the current-potential measurements and a good correlation is found between them.

Quantitative Study of Non‐Covalent Interactions at the Electrode–Electrolyte Interface Using Cyanide‐Modified Pt(111) Electrodes

Cations at the outer Helmholtz plane (OHP) can interact through non-covalent interactions with species at the inner Helmholtz plane (IHP), which are covalently bonded to the electrode surface, thereby affecting the structure and the properties of the electrochemical double layer. These non-covalent interactions can be studied quantitatively using cyanide-modified Pt(111) electrodes.

Generation of palladium clusters on Au(111) electrodes: Experiments and simulations

The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium–gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer.

An embedded atom approach to underpotential deposition phenomena

Abstract We have performed embedded atom calculations for a number of systems of electrochemical interest involving a metallic single-crystal substrate and a metallic adsorbate. Different thermodynamic contributions to the so-called underpotential shift are calculated and analyzed comparatively, drawing some general trends. The metal pairs considered involve silver, gold, platinum, palladium and copper. We consider the possibility of underpotential deposition through the excess of binding energy, arriving at two novel conclusions. First, for some systems consisting of metal M 1 and M 2 , underpotential deposition should be energetically possible in both cases, that is M 1 on M 2 and M 2 on M 1 . Second, anions may play a decisive role in changing the energetics of some systems. In particular cases like copper on Au(111) they may be responsible to a large extent for the existence of an underpotential deposition. Entropic contributions were neglected in the present analysis.

Multiple current components for methanol electrosorption and electro-oxidation at platinum in acidic solutions

Abstract Methanol electrosorption and residue electro-oxidation was studied on activated platinum electrodes by means of potentiodynamic profiles. Results obtained point to CO adsorbed on two sites as the main stable intermediate after methanol adsorption. Current peaks for both electrosorption and oxidation processes show the splitting on two components that strongly depend on electrode pretreatment and ionic composition of the base electrolyte. An explanation of the different behaviour observed is attempted in terms of two different surface states for the same intermediate.

Monte Carlo simulation for the formation and growth of low dimensionality phases during underpotential deposition of Ag on Au(100)

Simulation studies are undertaken for the system Ag/Au(100) by means of grand canonical Monte Carlo applied to a large lattice system. The interactions are calculated using the embedded atom model. The formation of adsorbed Ag phases of low dimensionality on Ag(100) is investigated and the influence of surface defects on the shape of the adsorption isotherms is studied. The results of the simulations are discussed in the light of experimental information available from electrochemical measurements.

A model for the adsorption of a monolayer of a metal on a foreign metal substrate

Abstract We present a model for the electrochemical adsorption of a monolayer of a metal on a foreign metal substrate in which the substrate and the adsorbate are represented as electronic plasmas of different densities embedded in a positive background charge. Explicit model calculations give a correlation between the adsorption potential and the work function difference between the two metals which is similar to the Kolb-Przasnyski-Gerischer relation found for adsorption on polycrystalline substrates.

Cu underpotential deposition on Au(111) and Au(100). Can this be explained in terms of the energetics of the Cu/Au system?

In this work we undertake a first principles study of the deposition of Copper on Au(111) and Au(100) surfaces from a density functional point of view. The analysis of the different energetic contributions to the so-called underpotential shift indicates that the deposition of Cu on any of these surfaces should occur at overpotentials. We conclude that some other factors must be taken into account to explain the experimental results, such as anion coadsorption. We also analyze the work function and the electronic density in the system.

Complex kinetic behaviour of “reduced” CO2 electro-oxidation at Pt electrodes

The electro-oxidation of “reduced” CO2 intermediate adsorbed at different adsorption times and potentials was studied by means of potentiodynamic oxidation profiles and current transients at constant potential. At potentials >0.75 V a Langmuir isotherm is obeyed, while a ·er isotherm is found at potentials <0.75 V. The kinetic behaviour of (CO2)r electro-oxidation can be explained through the existence of two intermediate species of similar chemical composition but different energy content. These species are related through an exchange equilibrium, assisted by the catalyst surface.