SECM study of hydrogen photogeneration in a 1,2-dichloroethane | water biphasic system with decamethylruthenocene electron donor regeneration

This paper reports light driven hydrogen evolution reaction (HER) at 1,2-dichloroethane | water (DCE | W) interface using photoexcited decamethylruthenocene (DMRc) as electron donor. DMRc is in situ regenerated by electroreduction of its oxidized form (DMRc+) formed during HER as a by-product. This enables continuous HER using small amount of DMRc. Proton transfer from the acidic aqueous phase to the DCE phase is ensured by negative chemical polarization of the liquid | liquid interface. The reduction of protons in DCE occurs only after excitation of DMRc by light. Voltammetry performed with the organic droplet-modified glassy carbon electrode immersed in the aqueous electrolyte solution of various anions, indicated that oxidation of DMRc is followed by an anion insertion from water into the organic phase. We demonstrate that DMRc can be electrochemically regenerated at the microelectrode positioned close to the interface between two immiscible electrolyte solutions (ITIES) by the scanning electrochemical microscopy. Regeneration of the electron donor allows further development of biphasic system towards continuous hydrogen generation platform

Scanning electrochemical microscopy determination of hydrogen flux at liquid|liquid interface with potentiometric probe

Scanning electrochemical microscopy potentiometric determination of local hydrogen concentration and its flux next to the liquid|liquid interface was demonstrated. This method is based on the shift of open circuit potential of Pt-based reversible hydrogen electrode. The detection system was verified with a system generating hydrogen under galvanostatic conditions. Then, it was applied to aqueous|1,2-dichloroethane interface where hydrogen is produced with decamethylferrocene as electron donor

Electrochemical oxygen reduction at soft interfaces catalyzed by the transfer of hydrated lithium cations

The oxygen reduction reaction by decamethylferrocene (DMFc), triggered by hydrophilic metallic cations behaving as Lewis acids towards water molecules in a homogeneous organic phase reaction, was investigated using cyclic voltammetry at the water|1,2-dichloroethane (w|DCE) interface. Simulated CVs, prepared through a facile 1-dimensional geometry in COMSOL Multi-physics software and incorporating interfacial and homogeneous reactions, were compared to experimental ones in order to elucidate the kinetics, thermodynamics, and viability of the proposed mechanism. The predominant O2 reduction reactions were proposed to occur in bulk organic phase, or in the vicinity of the w|DCE interface; six organic phase reactions were put forward. The first step was hydrolysis made possible through polarization of the O−H bond of water molecules available in the cations hydration shell. The metal ion behaves as a Lewis acid coordinating to the oxygen and weakening the O−H bond, making the proton more acidic, thereby facilitating attack by decamethylferrocene (DMFc) to form DMFc-H+. DMFc-H+ then participates in dioxygen reduction, generating the O2H• radical species and DMFc+. Afterwards, the radical oxidizes another equivalent of DMFc to produce O2H−, that can then abstract a proton from the metal ions hydration sphere to generate hydrogen peroxide. The disproportionation of O2H− and the ion-pair formation of Li+ and OH− make up the other two reactions. The CV analysis was based on two curve features; the DMFc+ transfer wave and the positive limit of the polarizable potential window – the edge of scan potential profile – including the metal ion return peak. The goal of this article is to determine the kinetic/thermodynamic aspects of this mechanism from the experimental electrochemical data

H2O2 generation at carbon paste electrode with decamethylferrocene solution in 2-nitrophenyloctyl ether as a binder. The catalytic effect of MoS2 particles

Here, we report hydrogen peroxide generation at 2-nitrophenyloctyl ether (NPOE)-water interface with decamethylferrocene as an electron donor. The progress of this reaction was detected by the observation of color change of the organic and aqueous phases in series of shake-flask experiments. The shape change of cyclic voltammograms recorded at carbon paste electrode with decamethylferrocene solution in NPOE also indicates (electro)catalytic reaction. Hydrogen peroxide was electrochemically detected at Pt microelectrode tip positioned in front of carbon paste electrode. For this purpose, scanning electrochemical microscopy (SECM) approach curves were recorded. Analogous experiments demonstrated the possibility of electrochemical regeneration of the electron donor. The (electro)catalytic effect of MoS2 on hydrogen peroxide generation was found by both shake-flask and SECM experiments

Mechanism of oxygen reduction by metallocenes near liquid|liquid interfaces

The mechanism of the oxygen reduction reaction (ORR) at a liquid|liquid interface, employing ferrocene (Fc) derivatives – such as decamethylferrocene (DMFc) – as a lipophilic electron donor along with sulfuric acid as an aqueous proton source, was elucidated through comparison of experimentally obtained cyclic voltammograms (CVs) to simulated CVs generated through COMSOL Multiphysics software which employs the finite element method (FEM). The simulations incorporated a potential dependent proton transfer (i.e . ion transfer, IT) step from the water (w) to organic (o) phases along with two homogeneous reactions (C1C2) occurring in the organic phase – an IT-C1C2 mechanism. The reaction of DMFc with H+(o) to form DMFc-hydride (DMFc-H+) was considered the first step (reaction 1), while reaction of DMFc-H+ with oxygen to form a peroxyl radical species, View the MathML sourceHO2, and DMFc+ was deemed the second step (reaction 2). Subsequent reactions, between View the MathML sourceHO2 and either DMFc or H+, were considered to be fast and irreversible so that 2 was a ‘proton-sink’, such that further reactions were not included; in this way, the simulation was greatly simplified. The rate of 1, kcf, and 2, kchem, were determined to be 5 × 102 and 1 × 104 L mol−1 s−1, respectively, for DMFc as the electron donor. Similarly, the rates of biphasic ORR for 1,1′-dimethylferrocene (DFc) and Fc were considered equivalent in terms of this reaction mechanism; therefore, their rates were determined to be 1 × 102 and 5 × 102 L mol−1 s−1 for 1 and 2, respectively. The reactive and diffusive layer thicknesses are also discussed

Hydrogen Peroxide Generation at Liquid|Liquid Interface under Conditions Unfavorable for Proton Transfer from Aqueous to Organic Phase

The charge transfer processes across the interface between two immiscible electrolyte solutions (ITIES) can be employed for energy storage and conversion, solvent extraction, or sensing or in life sciences. Among them are catalytic reactions, which have only been recently studied. Here H2O2 generation is studied with decamethylferrocene (DMFc) as electron donor at the interface between tetrahexylammonium perchlorate solution in 1,2- dichloroethane (1,2-DCE) and aqueous HClO4. These conditions are unfavorable for proton transfer across ITIES because of positive Galvani potential difference. Voltammetry with 1,2-DCE droplet modified electrode shows that DMFc oxidation is accompanied by ClO4− insertion into the organic phase. The reaction progress was followed by UV−vis spectroscopy, voltammetry, and scanning electrochemical microscopy (SECM). In the first and last method, horseradish peroxidase was used as catalyst. It is concluded that O2 is reduced to H2O2 at the liquid|liquid interface not only under conditions when proton transfer to organic phase is strongly favored, namely, when Galvani potential difference is negative (Angew. Chem., Int. Ed. 2008, 47, 4675−4678)

Hydrogen and Hydrogen Peroxide Formation in Trifluorotoluene-Water Biphasic Systems

Hydrogen or hydrogen peroxide can be generated in liquid-liquid biphasic systems, where the organic phase contains sufficiently strong electron donor (whose redox potential is lower than the potential of reversible hydrogen electrode). H2O2 generation with acidified aqueous phase occurs prior to H2 evolution when oxygen is present. No other organic solvent than highly toxic 1,2-dichloroethane (DCE) has been reported in biphasic system for H2 or H2O2 generation. In this work, we have used trifluorotoluene (TFT) instead of carcinogenic DCE, and studied these reactions in TFT-water biphasic system. To evaluate H2 flux, scanning electrochemical microscopy potentiometric approach curves to the TFT-water interface were recorded. H2O2 was detected voltametrically at a microelectrode located in the vicinity of the interface. H2 and H2O2 are formed and both reactions occur also in the absence of a hydrophobic salt in the organic phase. Their thermodynamics was discussed on the basis of Gibbs energies determined electrochemically with droplet-modified electrodes. The results show that DCE can be replaced by a noncarcinogenic solvent and the biphasic system for H2 and H2O2 generation can be simplified by elimination of the uncommon hydrophobic salt from the organic phase

Catalysis of water oxidation in acetonitrile by iridium oxide nanoparticles

Water oxidation catalysed by iridium oxide nanoparticles (IrO2 NPs) in water–acetonitrile mixtures using [RuIII(bpy)3]3+ as oxidant was studied as a function of the water content, the acidity of the reaction media and the catalyst concentration. It was observed that under acidic conditions (HClO4) and at high water contents (80% (v/v)) the reaction is slow, but its rate increases as the water content decreases, reaching a maximum at approximately equimolar proportions (≈25% H2O (v/v)). The results can be rationalized based on the structure of water in water–acetonitrile mixtures. At high water fractions, water is present in highly hydrogen-bonded arrangements and is less reactive. As the water content decreases, water clustering gives rise to the formation of water-rich micro-domains, and the number of bonded water molecules decreases monotonically. The results presented herein indicate that non-bonded water present in the water micro-domains is considerably more reactive towards oxygen production. Finally, long term electrolysis of water–acetonitrile mixtures containing [RuII(bpy)3]2+ and IrO2 NPs in solution show that the amount of oxygen produced is constant with time demonstrating that the redox mediator is stable under these experimental conditions