Reduction of Oxygen on Dispersed Nanocrystalline CoS₂

The electrocatalytic properties of nanocrystalline CoS₂ have been investigated for the oxygen reduction reaction (ORR) in 0.1 M HClO₄. CoS₂ with pyrite structure was prepared by hydrothermal synthesis and attached to a glassy carbon electrode from solution with a mixture of carbon and Nafion. The prepared CoS₂ electrode layers showed high activity toward the ORR and very good stability under oxygen reducing conditions. Selectivity of the ORR toward H₂O₂ was determined by rotating (ring) disk electrode measurements, and relatively high selectivity was obtained with up to 80% H₂O₂ formation around 0.4 V (vs Ag/AgCl), but this dropped to zero for potentials below 0.0 V. The amount of H₂O₂ produced between 0.6 and 0.0 V was dependent on the quality of the CoS₂ dispersion within the electrode layer, and decreasing CoS₂ particle size resulted in significant improvement in the ORR electrocatalytic activity, both by increasing the turnover frequency and through decreasing the selectivity toward H₂O₂ production

Electrochemical Water-Splitting Based on Hypochlorite Oxidation

Effective catalytic water-splitting can be electro­chemically triggered in an alkaline solution of sodium hypo­chlorite. Hypo­chlorite oxidation on poly­crystalline platinum yields ClO<b>·</b> radicals, which initiate a radical-assisted water-splitting, yielding oxygen, hydrogen peroxide, and protons. The efficiency of the O₂ production corresponds to about two electrons per molecule of the produced O₂ and is controlled primarily by the hypo­chlorite concentration and pH

Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution

The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms of HIROFs display two reversible redox couples commonly assigned to the Ir­(III)/Ir­(IV) and Ir­(IV)/Ir­(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated with it. This effect is interpreted as partial oxidation of Ir­(IV) as limited by nearest neighbor repulsion of resulting Ir­(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the “super-nernstian” pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir­(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir­(V) oxide formation above the thermodynamic onset potential for the OER at low pH

Potential-Dependent Structural Memory Effects in Au–Pd Nanoalloys

Alloying of metals offers great opportunities for directing reactivity of catalytic reactions. For nanoalloys, this is critically dependent on near-surface composition, which is determined by the segregation energies of alloy components. Here Au–Pd surface composition and distribution of Pd within a Au_0.7Pd_0.3 nanoalloy were investigated by monitoring the electrocatalytic behavior for the oxygen reduction reaction used as a sensitive surface ensemble probe. A time-dependent selectivity toward the formation of H₂O₂ as the main oxygen reduction product has been observed, demonstrating that the applied potential history determines surface composition. DFT modeling suggests that these changes can result both from Pd surface diffusion and from exchange of Pd between the shell and the core. Importantly, it is shown that these reorganizations are controlled by surface adsorbate population, which results in a potential-dependent Au–Pd surface composition and in remarkable structural memory effects

Near Room Temperature Synthesis of Monodisperse TiO₂ Nanoparticles: Growth Mechanism

Hydrolysis of TiCl₄ was used to form monodisperse nanoparticles of TiO₂ with clean surfaces. The solid fraction and solution composition during synthesis were simulated using equilibrium data, and formation and growth was followed with two complementary techniques, an electrospray-scanning mobility particle sizer (ES-SMPS) and dynamic light scattering (DLS). In ES-SMPS the number density of particles is measured. Droplets formed in the spraying step mainly contain electrolyte, giving rise to residue particles that are detected together with the nanoparticles of interest. Discrimination between the two kinds of particles can be made by changing the flow conditions and applicability of the method for in situ measurements of particle size during growth is demonstrated. In DLS the hydrodynamic mobility is measured, and further insight into the initial growth mechanism was revealed by observation of slow, sustained oscillations in the scattered intensity, indicating a dissolution–precipitation mechanism at the lowest pH values. The size of the particles formed in the dissolution–precipitation step is most likely determined by the surface charge, and larger particles are formed by aggregation

α- and γ‑FeOOH: Stability, Reversibility, and Nature of the Active Phase under Hydrogen Evolution

α-FeOOH (goethite) and γ-FeOOH (lepidocrocite) were found to be the main corrosion products of the steel cathode in the sodium chlorate process; the identification of the phases formed under reducing potentials, along with the study of the electrodes during the reoxidation, is fundamental to understanding their role in this process. In this work, FeOOH-based electrodes were investigated through in situ and in operando X-ray absorption spectroscopy (XAS), combined to electrochemical measurements (e.g., voltammetry and chronoamperometry). At sufficiently negative potentials (below −0.4 V vs RHE ca.) and under hydrogen evolution conditions an unknown iron­(II)-containing phase is formed. A comprehensive analysis of the whole XAS spectrum allowed proposing a structure bearing a relation with that of green rust (space group <i>P</i>3̅1<i>m</i>). This phase occurs independently of the nature of the starting electrode (α- or γ-FeOOH). During electrochemical reoxidation, however, the original phase is restored, meaning that the reduced phase brings some memory of the structure of the starting material. Spontaneous reoxidation in air suppresses the memory effect, producing a mixture of α and γ phases

Understanding solid-gas reaction mechanisms by operando soft X-ray absorption spectroscopy at ambient pressure

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO2 nanoparticles toward reducing gases. H2 was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH4 and CH3CHCH2) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO2 – x layer at the surface of SnO2 is readily reoxidized to SnO2 by treating the sample with O2 at mild temperatures (>200 °C), revealing the nature of “electron sponge” of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO2, involving direct reduction of Sn sites at the surface via cleavage of C–H bonds and the formation of methoxy- and/or methyl-tin species at the surface