On the activation energy of the formic acid oxidation reaction on platinum electrodes

A temperature dependent study on the formic acid oxidation reaction has been carried out in order to determine the activation energy of this reaction on different platinum single crystal electrodes, namely Pt(1 0 0), Pt(1 1 1), Pt(5 5 4) and Pt(5 4 4) surfaces. The chronoamperometric transients obtained with pulsed voltammetry have been analyzed to determine the current densities through the active intermediate and the CO formation rate. From the temperature dependency of those parameters, the activation energy for the direct reaction and the CO formation step have been calculated. For the active intermediate path, the activation energy are in the range of 50–60 kJ/mol. On the other hand, a large dependence on the electrode potential is found for the activation energy of the CO formation reaction on the Pt(1 0 0) electrode, and the activation energy values for this process range between 20 and 100 kJ/mol. These results have been explained using a reaction mechanism in which the oxidation of formic acid requires the presence of a pre-adsorbed anion on the electrode surface

Effects of the anion adsorption and pH on the formic acid oxidation reaction on Pt(111) electrodes

The effects of solution pH and anion adsorption for the formic acid oxidation reaction on the Pt(111) electrode have been examined using electrochemical techniques. Regarding the pH effects, it has been found that oxidation currents for this reaction increase with pH, which indicates that solution formate is involved in the reaction mechanism. Unexpectedly, the adsorption of sulfate on the Pt(111) electrode has a positive effect on the oxidation of formic acid, which also suggests that adsorbed anions are also involved in the mechanism. The activation energy calculated from temperature dependent measurements diminishes with the solution pH and also in the presence of adsorbed sulfate. These measurements corroborate the involvement of solution formate and anions in the oxidation mechanism. Using these results, a rate equation for the oxidation of formic acid is proposed. The current values calculated from this equation are in very good agreement with the experimental currents in perchloric acid solutions.This work has been financially supported by the MICINN (Spain)(project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER)

Role of oxygen-containing species at Pt(111) on the oxygen reduction reaction in acid media

The oxygen reduction reaction (ORR) is one of the fundamental reactions in electrochemistry and has been widely studied, but the mechanistic details of ORR still remain elusive. In this work, the role of electrochemically oxygenated species, such as adsorbed hydroxide, OHads, adsorbed oxygen, Oads, and Pt(111) oxide, PtO, in the ORR dynamics is studied by employing electrochemical techniques, i.e., combining rotating disk mass-transport control with potential sweep rate perturbation. In this framework, a reduction peak at 0.85 V, E ORR, is detected. This peak shows a different electrochemical dynamics than that of Pt(111) oxides. The data analysis suggests that neither OHads nor Oads are the main bottleneck in the mechanism. Instead, results support the reduction of a soluble intermediate species as the rate determining step in the mechanism. On the other hand, PtO species, which are generated at relatively high potentials and are responsible of surface disordering, strongly inhibit the ORR as long as they are adsorbed in the electrode surface.This study has been carried out in the framework of the European Commission FP7 Initial Training Network “ELCAT,” Grant Agreement No. 214936–2. Supports from the Spanish MINECO though project CTQ2013–44083-P and GV through PROMETEOII/2014/013 (FEDER) are greatly acknowledged

Thermodynamic properties of hydrogen–water adsorption at terraces and steps of Pt(111) vicinal surface electrodes

In this work, the effect of temperature on the adsorption states of Pt(111) vicinal surface electrodes in perchloric acid is studied through a thermodynamic analysis. The method allows calculating thermodynamic properties of the interface. In this framework, the concept of the generalized isothermand the statistical thermodynamics description are applied to calculate formal entropies, enthalpies and Gibbs energies,ΔGi0, of the adsorption processes at two-dimensional terraces and one-dimensional steps. These values are compared with data from literature. Additionally, the effect of the step density onΔGi0 and on the lateral interactions between adsorbed species,ωij, at terraces and steps is also determined. Calculated ΔGi0 , entropies and enthalpies are almost temperature-independent, especially at steps, but they depend on the step orientation. In contrast,ΔGi0 and ωij at terraces depend on the step density, following a linear tendency for terrace lengths larger than 5 atoms. However, while ΔGi0 increases with the step density, ωij decreases. Results were explained by considering the modification in the energetic surface balance by hydrogen, Hads, andwater,H2Oads, co-adsorption on the electrode, which in turn determines the whole adsorption processes on terraces and steps.This study has been carried out in the framework of the European Commission FP7 Initial Training Network “ELCAT”, Grant Agreement No. 214936-2. Support from the Spanish MINECO though project CTQ2013-44083-P and GV through PROMETEOII/2014/013 (FEDER) are greatly acknowledged

Oxygen reduction reaction on stepped platinum surfaces in alkaline media

The oxygen reduction reaction (ORR) in 0.1 M NaOH on platinum single crystal electrodes has been studied using hanging meniscus rotating disk electrode configuration. Basal planes and stepped surfaces with (111) and (100) terraces have been employed. The results indicate that the Pt(111) electrode has the highest electrocatalytic activity among all the studied surfaces. The addition of steps on this electrode surface significantly diminishes the reactivity of the surface towards the ORR. In fact, the reactivity of the steps on the surfaces with wide terraces can be considered negligible with respect to that measured for the terrace. On the other hand, Pt(100) and Pt(110) electrodes have much lower activity than the Pt(111) electrode. These results have been compared with those obtained in acid media to understand the effect of the pH and the adsorbed OH on the mechanism. It is proposed that the surface covered by adsorbed OH is active for the reduction of the oxygen molecules.This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271-FEDER) and Generalitat Valenciana (project PROMETEO/2009/045, -FEDER)

Cu UPD at Pt(100) and stepped faces Pt(610), Pt(410) of platinum single crystal electrodes

Published in Russian in Elektrokhimiya, 2016, Vol. 52, No. 9, pp. 999–1010The processes of adsorption/desorption of copper adatoms on the basal Pt(100) face and stepped Pt(610), Pt(410) surfaces have been studied in perchloric acid solution by cyclic voltammetry. It has been shown that the positions of the Cu stripping peaks are determined by perfection of the adlayer. The “island” model is suggested to describe electrochemical behavior of the Pt(hkl)+Cuad system. Obtained results are important for target modification of shape-controlled nanoparticles that are used in electrocatalysis.Financial support from the Russian Foundation for Basic Research (project no. 14-03-00530), MINECO (CTQ2013-44083-P) and Generalitat Valenciana (PrometeoII/2014/013) (Feder, Spain) is gratefully acknowledged

Oxygen reduction at platinum electrodes: The interplay between surface and surroundings properties

In this work, recent progress in the understanding of the mechanism of the oxygen reduction reaction at Pt surfaces is shortly reviewed. Specifically, the presence of a soluble and short-lived intermediate different to H2O2 in the ORR reaction path and the interrelated effect between the surface arrangement, adsorption of oxygen-containing species and water structure in the ORR reactivity in acid environments are discussed. Besides, the influence of the proton concentration on the ORR product distribution, the existence of a chemical step and the possible role of the soluble intermediate as a bifurcation point in the mechanism are also analyzed.The authors would like to thank Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP - Procs. 2013/16930-7 and 2014/23486-9), Brazil (A.M.G-M) and the Spanish MINECO though project CTQ2016-76221-P (FEDER) and GV through PROMETEOII/2014/013 ( FEDER) (J.M.F.), for financial supports

New insights into the hydrogen peroxide reduction reaction and its comparison with the oxygen reduction reaction in alkaline media on well-defined platinum surfaces

The hydrogen peroxide reduction reaction (HPRR) is investigated at pH = 13 on the Pt basal planes and stepped surfaces with (1 1 1) terraces separated by either monatomic (1 0 0) or (1 1 0) steps. A quantitative analysis of the surface structure effect revealed that Pt(1 1 1) is the most active surface and its activity progressively decreases when steps are introduced. Additionally, inhibition of the HPRR is observed at low potentials, which onset potential is governed by the OHads and the point of maximum entropy (pme) of the interphase. Experiments with different rotation rates suggest the formation of an HPRR intermediate linked to the inhibition process, which is more strongly adsorbed on (1 1 0) than (1 0 0) steps. Finally, a comparison of the HPRR and ORR (oxygen reduction reaction) illustrated the important differences for both reactions, which are dependent on the step density. These divergences have been discussed based on adsorbed intermediates and O2 interactions with the Pt surface.Financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653 GB-100) and Generalitat Valenciana (Project PROMETEO/2020/063) is acknowledged

Thermodynamic studies of anion adsorption at the Pt(111) electrode surface from glycolic acid solutions

Kinetic glycolic acid (GA) oxidation and thermodynamic glycolate adsorption have been studied on Pt single crystal electrodes. The voltammetric profiles of Pt(111), Pt(100), and Pt(110) in 0.1 M GA are shown, and the effect of the inclusion of steps on the Pt(111) surface has been studied by cyclic voltammetry. For Pt(111) electrode, different concentrations and sweep rates have been applied, revealing that both adsorption and oxidation processes take place. By establishing the appropriate conditions, a complete thermodynamic analysis has been performed by using the electrode potential and the charge as independent variables. Total charge density curves, surface pressure at total charge density and at constant electrode potential were determined to calculate the Gibbs excess and the charge number at constant electrode potential for glycolate adsorption on Pt(111). Maximum glycolate coverage on the surface reaches a value of ∼6.0 × 1014 ions/cm2. Spectroscopic results show the formation of CO2 during the oxidation of glycolic acid, indicating that the cleavage of the C–C bond occurs during the oxidation process.The work was carried out under the financial support by the MINECO (Spain) (project CTQ2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013)

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.This work has been financially supported by the MICINN (Spain) (project 2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013)