Macroscopic analysis of polarization characteristics of gas-diffusion electrodes in contact with liquid electrolytes: Part I: First order reactions

In this paper principles of gas-liquid chemical reaction engineering are applied to analyse the current-potential characteristics of gas-diffusion electrodes (GDE) in contact with liquid electrolytes. A macroscopic electrode model is formulated which accounts for mass transfer in the external diffusion films, in the gas layer and in the flooded layer. The set of model equations accounts for material balances, mass transport kinetics and Butler-Volmer polarization kinetics. Several dimensionless parameter groups are introduced which allow a compact reformulation of the proposed model. For first order reactions its solution can be derived analytically. The introduced parameter groups allow a classification of the different operating modes of a GDE, that is, slow reaction, fast reaction and instantaneous reaction.     

Electrosynthesis of hydrogen peroxide by partial reduction of oxygen in alkaline media. Part II: Wall-jet ring disc electrode for electroreduction of dissolved oxygen on graphite and glassy carbon

A wall-jet ring disc electrode was constructed by adapting a wall-jet flow through electrochemical cell. Commercially available spectral graphite and glassy carbon were used as working disc electrodes and the ring electrode was made of stainless steel. The efficiency and rate constants, measured in a planar parallel flow hydrodynamic regime, indicated the partial electroreduction of dissolved oxygen as a quasi-reversible two-electron process for both electrode materials tested.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part II: Experiments in flow-by fixed-bed electrochemical cells with three-phase flow

The second part of the work concerned with mediated electrosynthesis of H₂O₂ in acidic solutions (pH 3) deals with investigations using divided flow-by fixed bed electrochemical cells operated with co-current three-phase flow (aqueous/organic emulsion and O₂ gas at 0.1 MPa). Graphite felt (GF) and reticulated vitreous carbon (RVC) were evaluated as cathodes at superficial current densities up to 3000 A m^–2. Typically, at current densities above 600 A m^–2 graphite felt yielded higher peroxide concentrations per pass and current efficiencies, most likely due to the almost an order of magnitude higher organic liquid to solid mass transfer capacity for 2-ethyl-9,10-anthraquinone (EtAQ) mediator, that is, 0.13 s^–1 in the case of GF vs 0.015 s^–1 for RVC with 39 ppc (pores per cm). Factorial experiments revealed a positive interaction effect between superficial current density and emulsion load with respect to the current efficiency for H₂O₂ electrosynthesis. Thus, at the highest investigated superficial current density of 3000 A m^–2, the current efficiency was 84% when the emulsion load was at the highest explored level of 11.7 kg m^–2 s^–1, whilest for the lowest level of emulsion load, 2.8 kg m^–2 s^–1, the current efficiency for H₂O₂ was 18%. Furthermore, the presence of 1 mM cationic surfactant, tricaprylmethylammonium chloride (CH₃(C₈H₁₇)₃N⁺Cl^–, A336), had a positive main effect of about 12% on H₂O₂ current efficiency and there was also a positive synergistic effect between surfactant and emulsion load, estimated at about 7%. The aqueous to organic phase volume ratio, in the range of 0.9/1 and 3/1, had a statistically insignificant effect on the current efficiency for H₂O₂ generation. A decrease of the aqueous to organic phase volume ratio from 3 to 0.9 increased the cell voltage from about 6.5 to 7.3 V.     

The Iron(II) Complex [Fe(CF3SO3)2(mcp)] as a Convenient,Readily Available Catalyst for the Selective Oxidation of Methylenic Sites in Alkanes

The efficient and selective oxidation of secondary C H sites of alkanes is achieved by using low catalyst loadings of a non‐expensive, readily available iron catalyst [Fe(II)(CF₃SO₃)₂(mcp)], { Fe‐mcp , [mcp=N,N′‐dimethyl‐N,N′‐bis(2‐pyridylmethyl)cyclohexane‐trans‐1,2‐diamine]}, and hydrogen peroxide (H₂O₂) as oxidant, via a simple reaction protocol. Natural products are selectively oxidized and isolated in synthetically amenable yields. The easy access to large quantities of the catalyst and the simplicity of the C H oxidation procedure make this system a particularly convenient tool to carry out alkane C H oxidation reactions on the preparative scale, and in short reaction times.