Abstracts Category : Other

Electrochemical water oxidation with transition-metal based catalysts: structure-function relations by electrochemistry combined with X-ray absorption spectroscopy and UV-visible spectroscopy

by Diego Gonzlez-Flores

The importance of fundamental research in the field of heterogeneous water oxidation is reflected on its impact in solar fuel production. In biological systems, solar energy capture to produce fuels (sugars) involves water oxidation catalyzed by a pentanuclear Mn4CaO5 complex which can be seen as a source of inspiration for the development of synthetic catalysts. However, a better understanding of the structure-function relationships is still required in both synthetic and biological catalysts. More detailed mechanistic insights in any of the two type of systems are important to draw parallelisms in the other. In this thesis, I address this challenge by electrodepositing manganese-calcium oxides to study their properties by combining electrochemistry with UV-visible spectroscopy and X-ray absorption spectroscopy (XAS). The prepared oxides are amorphous with a birnessite type of structure. The developed synthesis protocol involves active site creation by annealing at moderated temperatures. Calcium can be introduced in the structure of the oxide by controlling the concentration in the solution during deposition. XAS studies suggest that reactive motives could involve Mn3Ca(-O)4 cubane structures. However, calcium results inessential for water oxidation activity, but tunes the electrocatalytic properties of the manganese oxide. By UV-visible measurements it is observed that the ability to undergo redox transitions and the presence of a minor fraction of MnIII is a prerequisite for catalysis. The redox tuning properties of the calcium might help to stabilize the corresponding active structures. Related to this study, I also investigated the effect of annealing of pre-synthesized manganese-calcium oxides attached to FTO electrodes by XAS. Similarly as in the electrodeposited oxides, it is observed that the annealing results in more defects and active site creation. Studies in structure-function relationships might be in many cases influenced by structural rearrangement during catalysis. This is especially important in crystalline materials where it is still unclear whether catalysis is always associated with some degree of amorphisation and structural rearrangement of the surface of the crystallites. I addressed those questions by studying the amorphisation of a crystalline cobalt phosphate Co3(PO4)2.8H2O under electrochemical operation combined with XAS measurements. By electrochemical analysis I observed a high catalytic activity at the surface of the cobalt phosphate catalyst; however, complete amorphisation (in the term of 5 to 8 h) results in bulk catalysis in all the volume of the amorphous oxide formed. In conclusion a good description of the kinetics of restructuring during operation is necessary to have a whole picture of catalysis in crystalline materials. Finally, the role of the pH is essential for understanding the catalytic mchanism and for practical applications. This question was addressed by a comparative study of cathodically electrodeposited Mn, Fe, Co, Ni and mixed NiFe catalyst at pH 7 and pH