Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study.

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO4 +0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.

Long-Life and pH-Stable SnO2-Coated Au Nanoparticles for SHINERS.

Shell-isolated nanoparticles (SHINs) with a 37 nm gold core and an 11 nm tin dioxide (SnO2) coating exhibited long-life Raman enhancement for 3 months and a wide pH stability of pH 2-13 in comparison with conventional SiO2-coated SHINs. Herein, Au-SnO2 is demonstrated as a more durable SHIN for use in the technique Shell-Isolated Nanoparticles for Enhanced Raman Spectroscopy (SHINERS).

Role of Transition Metals on TM/Mo2C Composites: Hydrogen Evolution Activity in Mildly Acidic and Alkaline Media.

Modification of electronic and chemical properties of a material by the introduction of another element into its lattice is one of the most common methods for designing new catalysts for different applications. In this work the effect of modifying molybdenum carbide with transition metals (Fe, Co, Ni, Cu), TM-Mo2C composites, upon the catalytic activity toward hydrogen evolution reaction (HER) in mild acidic and alkaline media has been studied. Catalysts were prepared by carbothermal reduction of molybdenum and TM oxides precursors and were characterized by different physicochemical techniques. Results evidenced a strong pH effect on the catalytic performance of TM-Mo2C, while, at pH = 5, inclusion of TM into the Mo2C lattice has a deleterious effect on the HER activity and, at pH = 9, a promoting effect was observed, highlighting the importance of considering specific operation conditions during the catalyst design process. Analysis of in situ near-edge X-ray adsorption data reveals a decrease on the oxidation state and average bond ionicity of dopant metal upon a pH increase, shedding light of the different effects of TMs on the resulting HER activity in acidic and alkaline media. Finally, stability tests demonstrated no deterioration on catalysts' performance after 8 h of continuous cycling within the HER working range, confirming the suitability of Mo2C materials as promising HER catalysts.

Theoretical voltammetric response of electrodes coated by solid polymer electrolyte membranes.

A model for the differential capacitance of metal electrodes coated by solid polymer electrolyte membranes, with acid/base groups attached to the membrane backbone, and in contact with an electrolyte solution is developed. With proper model parameters, the model is able to predict a limit response, given by Mott-Schottky or Gouy-Chapman-Stern theories depending on the dissociation degree and the density of ionizable acid/base groups. The model is also valid for other ionic membranes with proton donor/acceptor molecules as membrane counterions. Results are discussed in light of the electron transfer rate at membrane-coated electrodes for electrochemical reactions that strongly depend on the double layer structure. In this sense, the model provides a tool towards the understanding of the electro-catalytic activity on modified electrodes. It is shown that local maxima and minima in the differential capacitance as a function of the electrode potential may occur as consequence of the dissociation of acid/base molecular species, in absence of specific adsorption of immobile polymer anions on the electrode surface. Although the model extends the conceptual framework for the interpretation of cyclic voltammograms for these systems and the general theory about electrified interfaces, structural features of real systems are more complex and so, presented results only are qualitatively compared with experiments.

New insights into the oxygen reduction reaction mechanism on Pt(111): a detailed electrochemical study.

The oxygen reduction reaction (ORR) is undoubtedly the most important fuel-cell cathodic reaction. In this work, a detailed electrochemical analysis of the ORR on Pt(111) in nonadsorbing electrolytes was performed, which included the high-potential region Eup =1.15 V while ensuring the electrode surface structure stability. Our results suggest that the reduction of a soluble intermediate species formed during the ORR is the rate-determining step in the whole reaction mechanism. This species does not undergo any other electrochemical reaction at E>0.9 V and may accumulate close to the electrode surface. Together with dissolved O2, this intermediate may modify the oxide-growth dynamics on Pt(111). Hence, both species interact with the electrode surface through complex catalytic networks. Under certain experimental conditions, oxygenated species from the oxidation of Pt(111) may enhance the overall ORR current. These results propose an alternative to explain the current state of the art for this fundamental process.

Some reflections on the understanding of the oxygen reduction reaction at Pt(111).

The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments.