Finalist EPFL doctorate Award 2005 - Marselli Béatrice
31.12.05 - Béatrice Marselli's scientific work is (according to the opinion of the jury) ranking among the best 4 % (9 out of 250) PhD theses awarded by EPFL in 2004. Thesis EPFL, n° 3057 (2004). Dir.: Prof. Comninellis Christos.
Synthetic boron-doped diamond thin film is a new promising anode material. Because of its properties (high anodic stability under drastic conditions and wide potential window), it is widely investigated for numerous possible electrochemical applications such as electrosynthesis, preparation of powerful oxidants and electroincineration. In the first part of this work, simple charge transfer was investigated at boron-doped diamond electrode through the study of an outer sphere system in the potential region of water stability. In a second part of this work, the electrochemical oxygen transfer reaction (EOTR) was studied in more details. Hydroxyl radicals are one of the most important intermediates produced during EOTR. Their formation depends on the electrode material as well as the potential and implies different mechanisms and reactivities. At low potential, hydroxyl radicals are produced by the dissociative adsorption of water followed by the hydrogen discharge. This reaction is assumed to take place at electrocatalytic material like platinum. When the potential is higher than 1.23 V vs SHE (thermodynamic potential of water decomposition in acidic medium), the water discharge occurs, leading to the formation of hydroxyl radicals. From this, two classes of materials can be distinguished: active and non active electrodes. It is well established that at active electrodes, a strong interaction with hydroxyl radicals exists and the EOTR occurs via the formation of an higher oxide. In contrast, at non active electrodes, the substrate does not participate in the process and the oxidation is assisted by hydroxyl radicals that are weakly adsorbed at the electrode surface. Assuming that hydroxyl radicals are the main intermediates of the reaction, a model was developed to predict the organic compounds oxidation (COD-ICE model). Another part of this work deals with the validation of the theoretical models. In addition to the COD-ICE model, another model describing the oxidation reaction in terms of flux of both hydroxyl radicals and organics (γ-ν model) was developed. Both models permitted on the one hand to predict and describe the evolution of the oxidation reaction, and on the other hand to confirm the role of hydroxyl radicals. Moreover, it was possible to perform, depending on the conditions of applied current, either a partial oxidation (into intermediates) or a total incineration (into CO2) of the organic compound. The models, developed for a one-compartment electrochemical flow cell, were also validated in both a two-compartments cell and a new electrochemical cell, called turbine cell. In addition, the development of this cell allowed us to work with well established hydrodynamic conditions. The wide potential window that exists at boron-doped diamond electrode (BDD) theoretically allows the formation of free hydroxyl radicals, whose redox potential is estimated at about 2.6 V (vs SHE). The principal aim of this work was to highlight the presence of hydroxyl radicals at BDD electrode and to study their reactivity. First, we have investigated the production of hydrogen peroxide and the competitive reaction of carboxylic acids, both of which indicated the presence of hydroxyl radicals. Then, spin trapping was performed to detect hydroxyl radicals. This method consists in trapping the radical with an appropriate scavenger to produce a stable adduct, which can be analyzed by different techniques such as electron spin resonance (ESR), UV-visible and liquid chromatography (HPLC) measurements. The spin trapping at BDD electrode was performed through three experiments, viz., the electrolysis of a solution of 5,5-dimethyl- 1-pyrroline-N-oxide (DMPO) or 4-nitroso-N,N-dimethylaniline (p-nitrosoaniline or RNO) and the hydroxylation of salicylic acid using ESR, UV and HPLC analysis, respectively. These results have confirmed the presence and the key role of hydroxyl radicals during oxidative processes at BDD electrode. The hydroxylation of salicylic acid, whose oxidation mechanism is well established and yields to two dihydroxylated isomers (2,3- and 2,5-DHBA), was investigated in more details to study the reactivity of hydroxyl radicals. The results were compared to the reactivity of hydroxyl radicals chemically produced by Fenton reaction and UV-photolysis. The comparison was based on the investigation of the isomer distribution. On the basis of our results and by analogy with chemical and biological results, a mechanism for salicylic acid hydroxylation was proposed.