Biomimetic Electrocatalysis Of Oxygen Reduction Reaction At Liquid/Liquid Interfaces

Oxygen reduction reaction (ORR) is one of the most important reactions of metabolism in organisms. ORR occurs at biomembranes to build up a transmembrane proton gradient, which can power the synthesis of ATP to sustain life activities. However, reduction of molecular oxygen in its ground state is spin-forbidden and kinetically inert, thus requires the catalytic activation of appropriate catalysts. In nature, efficient ORR is catalyzed by terminal oxidases, such as cytochrome c oxidases (CcO). The catalytic unit of CcO comprises a heme site and a trisimidazole-ligated Cu center, so-called Fe/Cu unit, to which O2 binds and is reduced to H2O rapidly, without releasing partially reduction yet cytotoxic oxygen species (such as peroxide and superoxide). However, there is substantial debate in the literature regarding the catalytic role of the Cu site.Liquid/liquid interfaces, or the so-called the interface between two immiscible electrolyte solutions (ITIES), which provide a physical separation of the reactants and products, can be considered as a simple model of biomembrane. In comparison with the solid/solution interface, electrochemistry at ITIES allows for monitoring both ion and electron transfer, which is particular interesting to study ion transfer coupled electron transfer reaction, e.g., ORR. So far, the catalytic activities of a variety of catalysts, such as nanoparticles, metalated porphyrin, free-base porphyrins and metallic phthalocyanines, have been studied at the liquid/liquid interfaces. These catalysts have been demonstrated to catalyze either four-electron reduction or two-electron reduction to produce H2O or H2O2. The next step will be the comparative study of more bio-inspired oxygen reduction catalysts, such as bio-enzymes and synthetic heterodinuclear heme/Cu analogues, for the deeper understanding of biological ORR catalysis mechanism. This is also the primary motivation of the present thesis work. This thesis has been devided into five parts.In the first chapter, the development of electrochemistry at the ITIES was introduced, as well as the theory of electrochemistry at the ITIES and its application in electrocatalysis. The latter was divided into four parts according to the electrocatalysis type. In addition, the catalysts for electrocatalytic ORR at ITIES and their catalytic mechanism were reviewed.In chapter 2, the catalytic activity of iron phthalocyanine (FePc) towards ORR by 1,1'-dimethylferrocene (DFc) and tetrathiafulvalene (TTF) at water/1,2-dichloroethane interface was studied. Cyclic voltammetry and biphasic reaction results provided evidence of catalyic ability of FePc on ORR via a four-electron reduction mechanism. And the reaction rate can be enhanced by approximately three orders.Chapter 3 presented a work on the reduction of oxygen by electron dornors (TTF and DFc) catalyzed by microperoxidase-11 (MP-11) at a polarized water/1,2-dichloroethane interface. Cyclic voltammetry and differential capacitance measurments suggested the adsorption of MP-11 at the interface. The negative value of ?G obtained from the Langmuir type adsorption isotherm indicated that the adsorption was thermodynamically favorable and was rationalized to the spontaneous nature of the adsorption process. The ORR catalyzed by MP-11 proceed as a proton coupled electron transfer reaction to hydrogen peroxide (the selectivity was more than 30%). Moreover, it unambiguously shows that MP-11 cannot selectively catalyze the four-electron reduction of oxygen to H2O. Hence it can be inferred that Cu center plays a crucial role in the respiratory ORR.In the fourth chapter, the adsorption behavior of water-soluble natural porphyrin vitamin B12 (VB12) at the liquid/liquid interface was investigated firstly. It was found that the adsorption was thermodynamically feasible and spontaneous. Both cyclic voltammetry measurements and biphasic reaction experiment proved VB12 could catalyze ORR by DFc and TTF, mainly via four-electron reduction mechanism to produce H2O.Last chapter summarized the work presented in this thesis. An attempt was also made to propose the furture trend of ORR at liquid/liquid interface.