Seven-coordinate complexes for water oxidation catalysisfrom molecular characterization to solid state photocatalysis

Resumen

The main challenge in the water oxidation catalysis based on Ru complexes is to develop powerful anodes and photoanodes. The field of molecular water oxidation has exploited with the appearance of new mononuclear seven-coordinate water oxidation catalysts that their activity has exceeded that of the natural Mn4O5 cluster. This thesis aimed the development of photoanodes that integrate molecular Ru water oxidation catalysts. Approximately half of thesis develops powerful water oxidation catalysts in the homogeneous phase. The other half of thesis is focused in the generation of hybrid molecular anodes and photoanodes. At the start of the thesis, we envisioned potentially seven-coordinate complexes based on the [2,2'-bipyridine]-6,6'-dicarboxylate (bda2-), [2,2':6',2''-terpyridine]-6,6''-dicarboxylate (tda2-) and 2,5-bis(6-carboxylatopyridin-2-yl)pyrrol-1-ide (t5a3-) ligands as equatorial ligands. Two new family of complexes that bear the tda2- and the t5a3- in the equatorial plane that provide a pendant base close to the Ru center. These complexes turn to be the fastest in the literature with Turn Over Frequencies up to 7700 s-1 thanks to H intramolecular transfer between the dangling carboxylate and the Ru-OH group. Finally, we developed a novel electrocatalytic method to benchmark our new catalyst with the best in the literature and we have selected the best candidates to immobilize them on anodes and photoanodes. In our first attempt to prepare hybrid anodes we observe that immobilized Ru-bda catalysts evolve to RuO2 at high potentials. The generation of oxide layers is the consequence of the bimolecular nature of the water oxidation mechanism by Ru-bda complexes. Their impossibility to oxidize water in a bimolecular manner on the surface causes decomposition, such as the generation of RuO2. Interestingly, the resulting RuO2 film outperform when we benchmark it with other oxides in the literature. We then moved to the Ru-tda family of complexes due to their single-site mechanism and the high rate in catalysis. The immobilization of Ru-tda catalysts on glassy carbon electrodes coated with MWCNT leads in a-million-TON-anode that sustained water oxidation catalysis without sings of degradation. Finally, with collaboration with Prof. Nate S. Lewis group in Caltech we prepare n-Si electrodes decorated with Ru-tda catalysts to generate photoanodes that perform unassisted water oxidation.