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.

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