Electrocatalysis at liquid/liquid interfaces

This thesis is devoted to the study of oxygen reduction reaction catalysed by porphyrins at the interface between two immiscible electrolyte solutions (ITIES). Electrochemical and spectrophotometric techniques are introduced to these interfaces in order to gather more information about the transfer mechanism. Furthermore, the reduction of oxygen and the oxidation of decamethylferrocene (DMFc) in 1,2-DCE and production of hydrogen peroxide (H2O2) in the aqueous phase, on the basis of the two-phase reaction controlled by a common ion are investigated. Mass spectrometric measurements were carried out for the 1,2-DCE phase before and after two-phase reaction with an aqueous phase containing acid to indicate the stability of DMFc and the DMFc+ over the course of the two-phase reaction. Density function theory (DFT) computations have been performed based on developed a reaction pathway. Catalytic effect of 5,10,15,20-tetraphenylporphyrinatocobalt(II) [Co(tpp)], 2,3,7,8,12,13,17,18-Octaethyl-porphyrin cobalt(II) (CoOEP) and two free-base porphyrins 5,10,15,20-meso-tetraphenylporphyrin (H2TPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin (H2OEP) have been investigated as a catalyst for a two electron reduction of O2 in presence of an electron donor at various pH values at the polarized water|1,2-DCE interface. Using voltammetry, it is possible to drive this catalytic reduction at the interface as a function of the applied potential difference, where aqueous protons and organic electron donors combine to reduce O2. The signal observed corresponds to a proton-coupled electron transfer (PCET) reaction, as no current and no reaction can be observed in the absence of either catalyst, acid or O2. [Co(tpp)] and CoOEP catalysis work like conventional cobalt porphyrins, activating O2 via coordination by the formation of a superoxide structure. The advantages of the present system is that, by controlling the interfacial potential difference, the proton transfer from water to 1,2-DCE can be accurately controlled. Accordingly, the driving force for proton-coupled electron transfer reactions is also effectively harnessed. Assisted proton transfer (APT) reactions were studied across the water|1,2-DCE interface facilitated by two free-base porphyrins such as H2TPP and H2OEP. At a water|1,2-DCE interface, the interfacial formation of di-acid H4TPP2+ and H4OEP2+ are observed by ion-transfer voltammetry and UV-Visible spectroscopy, due to the double protonation of H2TPP and H2OEP at the tertiary nitrogens in the ring. Additionally, "Ionic Partition Diagram" of neutral and ionisable H2TPP compounds is plotted to illustrate the various contributions of H2TPP

Ionic partition diagram of tetraphenylporphyrin at the water|1,2-dichloroethane interface

diagram of 5,10,15,20-tetraphenyl-21H,23H-porphine (H2TPP) at the water|1,2-dichloroethane interface using a simple Born solvation model. This zone diagram shows under which form this porphyrin is present, i.e. neutral, monoprotonated or diprotonated, and in which phase i.e. either in the aqueous or the organic phase as a function of the aqueous pH and the interface polarisation that can be controlled externally or by the distribution of supporting electrolytes. This diagram explains why the monoprotonated form has been difficult to observe when doing biphasic pH titration

Evidence of tetraphenylporphyrin monoacids by ion-transfer voltammetry at polarized liquid|liquid interfaces

We present a simple methodology to illustrate the existence of tetraphenylporphyrin monoacid based on ion-transfer voltammetry at a polarized water|1,2-dichloroethane interface and organic pK values are also estimated

H2O2 generation by decamethylferrocene at a liquid | liquid interface

Hydrogen peroxide generation at a liquid|liquid interface occurs with a yield of 20 % with respect to the concentration of reducing agent (decamethylferrocene). The liquid|liquid interface supplies electrons from the reducing agent and protons from the aqueous phase to drive the reduction of O2 into H2O2, which is extracted into the aqueous phase during the course of reaction (see picture; DCE=1,2-dichloroethane)

Hydrogen Evolution at Liquid–Liquid Interfaces

Blowing bubbles: Hydrogen evolution by proton reduction with [(C5Me5)2Fe] occurs at a soft interface between water and 1,2-dichloroethane (DCE). The reaction proceeds by proton transfer assisted by [(C5Me5)2Fe] across the water–DCE interface with subsequent proton reduction in DCE. The interface essentially acts as a proton pump, allowing hydrogen evolution by directly using the aqueous proton

Oxygen reduction by decamethylferrocene at liquid/liquid interfaces catalyzed by dodecylaniline

Molecular oxygen (O2) reduction by decamethylferrocene (DMFc) was investigated at a polarized water/ 1,2-dichloroethane (DCE) interface. Electrochemical results point to a mechanism similar to the EC type reaction at the conventional electrode/solution interface, in which an assisted proton transfer (APT) by DMFc across the water/DCE interface via the formation of DMFcH+ corresponds to the electrochemical step and O2 reduction to hydrogen peroxide (H2O2) represents the chemical step. The proton transfer step can also be driven using lipophilic bases such as 4-dodecylaniline. Finally, voltammetric data shows that lipophilic DMFc can also be extracted to the aqueous acidic phase to react homogeneously with oxygen

Dioxygen Reduction by Cobalt(II) Octaethylporphyrin at Liquid / Liquid Interfaces

Oxygen reduction catalyzed by cobalt(II) (2,3,7,8,12,13,17,18-octaethylporphyrin) [Co(OEP)] at soft interfaces is studied by voltammetry and biphasic reactions. When Co(OEP) is present in a solution of 1,2-dichloroethane in contact with an aqueous acidic solution, oxygen is reduced if the interface is positively polarized (water phase versus organic phase). This reduction reaction is facilitated when an additional electron donor, here ferrocene, is present in excess in the organic phase

Evidence of tetraphenylporphyrin monoacids by ion-transfer voltammetry at polarized liquid/liquid interfaces

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