The water oxidation reaction to molecular oxygen is attracting an increasing importance in the field of chemistry research due to its potential involvement in new sustainable energy schemes based on artificial photosynthesis. This Doctoral Thesis focuses on the study of molecular catalysts for water oxidation based on copper and nickel since their high abundance and inexpensive character make them potential candidates for their use in catalytic systems. Despite these advantages, there is a current lack of information regarding their reaction mechanisms and the factors that determine their activity.
Therefore, we first develop new molecular catalysts based on copper as metal center. Their mechanistic study reveals the essential role that the ligand oxidation has in the catalytic cycle as a tool to control the reaction overpotential. In collaboration with Prof. Maseras group (ICIQ) a new mechanism for the oxygen-oxygen bond formation step is found to operate in different copper catalysts based on single electron transfers. This unprecedented mechanism, so-called Single Electron Transfer-Water Nucleophilic Attack (SET-WNA) allows us to redefine the mechanistic scenario in water oxidation.
Later on, the design of new ligands with different redox properties is addressed. That allows to study the factors influencing the activity and reaction mechanism and provide valuable information for the design of more active, stable and efficient new catalysts based on copper complexes.
The knowledge developed with copper complexes is applied in nickel catalysis. This allows to study the labile character of nickel complexes in basic media that determine the presence of two different operating mechanism based on molecular species and nickel oxide respectively.
Finally, the immobilization of molecular catalysts on the surface of graphene-based electrodes and its implications on the stability and catalytic activity are also studied due to its importance for the design of practical devices. We discover the essential role of the π-delocalization in increasing the reaction kinetic for water oxidation by more than two orders of magnitude.