Elizabeth Santos

Volcano plots in hydrogen electrocatalysis – uses and abuses

Summary Sabatier’s principle suggests, that for hydrogen evolution a plot of the rate constant versus the hydrogen adsorption energy should result in a volcano, and several such plots have been presented in the literature. A thorough examination of the data shows, that there is no volcano once the oxide-covered metals are left out. We examine the factors that govern the reaction rate in the light of our own theory and conclude, that Sabatier’s principle is only one of several factors that determine the rate. With the exception of nickel and cobalt, the reaction rate does not decrease for highly exothermic hydrogen adsorption as predicted, because the reaction passes through more suitable intermediate states. The case of nickel is given special attention; since it is a 3d metal, its orbitals are compact and the overlap with hydrogen is too low to make it a good catalyst.

Identification of the adsorbate during methanol oxidation

Abstract The formation and oxidation of the methanol adsorbate at platinum in acid solutions is performed under potential control using a flow cell technique. The charges in both processes are equal; this fact confirms the results of Podlovchenko et al. obtained from charging curves after open circuit adsorption. It is shown, using the on-line MS technique and labelled methanol, CD 3 OH, that the adsorbed intermediate is probably COH.

Why is Gold such a Good Catalyst for Oxygen Reduction in Alkaline Media

The two faces of gold: the reduction of oxygen on gold electrodes in alkaline solutions has been investigated theoretically. The most favorable reaction leads directly to adsorbed O(2)(-), but the activation energy for a two-step pathway, in which the first step is an outer-sphere electron transfer to give solvated O(2)(-), is only slightly higher. d-band catalysis, which dominates oxygen reduction in acid media, plays no role. The reason why the reaction is slow in acid media is also explained.

CO adsorbate on Pt(111) single crystal surfaces

Abstract The electrooxidation of CO adsorbate on well ordered Pt(111) surfaces is studied through a flow-cell technique. Potentiodynamic sweeps and potential steps are used to characterize the kinetic behaviour of the adsorbate. The effect of surface disturbance when the electrode is cycled into the oxide formation region is analysed. The oxidation of CO is faster in the case of disturbed surfaces. The results are interpreted in terms of a simple kinetic model.

Catalysis in Electrochemistry: From Fundamentals to Strategies for Fuel Cell Development

Preface ix Preface to the Wiley Series on Electrocatalysis and Electrochemistry xi Contributors xiii 1. Volcano Curves in Electrochemistry 1 Roger Parsons 2. Electrocatalysis: A Survey of Fundamental Concepts 17 Alejandro Jorge Arvia, Agustin Eduardo Bolzan, and Miguel Angel Pasquale 3. Dynamics and Stability of Surface Structures 67 Margret Giesen and Guillermo Beltramo 4. Electrocatalytic Properties of Stepped Surfaces 127 Juan M. Feliu, Enrique Herrero, and Victor Climent 5. Computational Chemistry Applied to Reactions in Electrocatalysis 165 Axel Gross and Sebastian Schnur 6. Catalysis of Electron Transfer at Metal Electrodes 197 Elizabeth Santos and Wolfgang Schmickler 7. Combining Vibrational Spectroscopy and Density Functional Theory for Probing Electrosorption and Electrocatalytic Reactions 223 Marc T. M. Koper 8. Electrochemical Catalysts: From Electrocatalysis to Bioelectrocatalysis 249 Alejandro Jorge Arvia, Agustin Eduardo Bolzan, and Miguel Angel Pasquale 9. Electrocatalysis at Bimetallic Surfaces Obtained by Surface Decoration 297 Helmut Baltruschat, Siegfried Ernst, and Nicky Bogolowski 10. CO Adsorption on Platinum Electrodes 339 Angel Cuesta and Claudio Gutierrez 11. Exploring Metal Oxides: A Theoretical Approach 375 Monica Calatayud and Frederik Tielens 12. Electrocatalysis at Liquid Liquid Interfaces 427 Bin Su, Hubert H. Girault, and Zdenek Samec 13. Platinum-Based Supported Nanocatalysts for Oxidation of Methanol and Ethanol 453 Ernesto R. Gonzalez, Edson A. Ticianelli, and Ermete Antolini 14. Impact of Electrochemical Science on Energy Problems 487 Elizabeth Santos and Wolfgang Schmickler Index 499

Hydrogen evolution on silver single crystal electrodes : first results

Abstract The discharge of the first proton in the hydrogen evolution reaction on Ag(111) and Ag(110) has been investigated by potentiostatic pulses. For Ag(111), Tafel plots yield a straight line over the investigated range; however, the temperature dependence shows substantial deviations from the behavior predicted by the Butler–Volmer law. On Ag(110) the current density is substantially lower than on Ag(111), and the Tafel plots exhibit a change of slope.

Hydrogen Electrocatalysis on Single Crystals and on Nanostructured Electrodes

We investigate hydrogen evolution on plain and nanostructured electrodes with a theory developed by us. On electrodes involving transition metals the most strongly adsorbed hydrogen is often only a spectator, while the reaction proceeds via a weakly adsorbed species. For Pt(111) the isotherms for both species are calculated. We explain why a nanostructure consisting of a monolayer of Pd on Au(111) is a good catalysts, and predict that Rh/Au(111) should be even better. Our calculations for a fair number of metals are in good agreement with experiment.

Comparative study of CO absorbates for different structures of platinum surfaces

The behaviour of CO adsorbates is studied in 0.05 M HClO4 at Pt surfaces in a flux cell by means of potentiodynamic and potentiostatic transients. Simultaneously, the production of CO2 is followed by on-line mass spectroscopy. Four different signals can be distinguished. Results for polycrystalline platinum are compared with those of (111) and (100) single crystal electrodes. A correlation is made between the experimental observations and the nature of the platinum surface.

The effect of adsorbed carbon monoxide on hydrogen adsorption and hydrogen evolution on platinum

Abstract The hydrogen evolution reaction (HER) on platinum was studied in the presence of adsorbed carbon monoxide. Preferentially oriented Pt surfaces, obtained by means of high-frequency triangular pulses, show specific behaviour for the HER. The results for different degrees of coverage with carbon monoxide are discussed in terms of a modification of the Pt surface, resulting in an induced heterogeneity for hydrogen adsorption. The effect of the adsorption potential of CO on the hydrogen adsorption and evolution are interpreted in terms of different geometrical distributions of the free platinum sites.

Electrooxidation of adsorbed CO on polycrystalline platinum in alkaline solutions

Abstract The electrooxidation of adsorbed CO on polycristalline platinum in 1 M KOH was investigated under potentiodynamic conditions. A micro flux cell was used to avoid CO readsorption during the oxidation process. Three oxidation current peaks are observed at low sweep rates for the electrooxidation of (CO)ad obtained from CO-saturated electrolyte. The corresponding peak potentials shift towards more positive values, but the three peaks gradually overlap to give a single broad asymmetric current peak as v is increased. The multiplicity in the oxidation current peaks for (CO)ad under adsorption saturation is interpreted in terms of reactions involving different OH—substrate and CO—substrate interactions, instead of the existence of the “linear” or “bridge” adsorbed structures for (CO)ad. For (CO)ad below saturation values, a complex model involving different (CO)ad structures interacting with energetically different (OH)ad should be considered. From the dependence of peak potential on log v, a chemical reaction between adsorbed CO and platinum oxides can in principle be postulated as the rds. The splitting of the initial stages of oxide electroformation in the presence of adsorbed CO is evident from the triangular modulated linear potential sweep (TMLPS).