Resumen de A journey towards efficient molecular wocs: from mononuclear to polynuclear complexes
Over the last 40 years, the field of molecular water oxidation catalysis has witnessed a tremendous progress improving the catalytic performance up to 6 orders of magnitude higher than the first reported molecular catalyst. Despite the success, there are still several challenges that need to be addressed in order to transfer the high catalytic activity into functional devices. The present doctoral thesis focuses on some of these challenges which are exposed in Chapter 2 of the present thesis and are briefly summarized as follows. First of all, the catalyst should work very fast with minimum overpotential. Thus, one of the objectives of this thesis is to synthesize new ruthenium water oxidation catalysts with different number of coordinated carboxylate groups. These anionic ligands are expected to lower the working potential of the reaction, while geometrical and secondary effects of the ligands are be explored as a way to enhance catalytic performance. Secondly, in order to transfer the high catalytic activity of homogeneous catalysts into solid supports with the ultimate goal of building powerful water splitting devices, the design of molecular catalysts that are efficient, robust and easily transferable to heterogenous phase is highly desirable. In this direction, the next objective of this thesis is focused on the development of molecular water oxidation catalyst that can be supported on conductive substrates, starting from mononuclear complexes all the way to coordination polymers. The new family of materials have been used to understand the ruling factors and mechanistic details of the catalysis on surfaces. Finally, the last challenge addressed in this thesis is to explore water oxidation catalysts made of first raw transition metals. In particular, new mechanistic insights of the water oxidation catalysis by a dinuclear cobalt complex that is active for the water oxidation reaction are described.
To achieve these goals, advanced organic and inorganic synthetic methodologies have been employed together with a wide range of spectroscopic techniques (UV-Visible, resonance Raman, nuclear magnetic resonance, x-ray absorption spectroscopy, x-ray photoelectron spectroscopy etc.). Gas detection techniques such as manometry, gas cromathography with TCD detectors or Clark sensors and mass spectrometry have also been employed. In addition, a broad range of electrochemical techniques have been crucial to properly characterize the redox properties of the new complexes and materials as well as their catalytic activity.
In chapter 3 of the present thesis, a new family of Ruthenium complexes containing pyridine carboxylate ligands for the water oxidation reaction have been prepared and fully characterized. They contain one or two anionic carboxylate moieties connected to an aromatic pyridine ligands. The sigma-donating properties of these carboxylate ligands reduces the redox potential of the complexes and lowers the overpotential for the water oxidation catalysis process. It has been shown that each carboxylate moiety can reduce the overpotential in the range of 200-300 mV. In addition, the different orientation of the ligand arrangement around the metal center gives different isomeric complexes with different geometric constrains on the metal center. These differences have a strong influence on the redox as well as catalytic properties of the complexes.
In the fourth chapter, a seven coordinated Ruthenium complex containing the pyridine-2,6-dicarboxylate ligand for fast water oxidation catalysis has been prepared and fully characterized. This complex at high oxidation state Ru(IV) generates a seven coordination complex (CN7) with pentagonal bipyramidal geometry. It has been shown that this complex serves as a precursor of a highly active water oxidation catalyst which shows TOF values in the range of 2.4-3.4 × 103 s-1. The extremely fast kinetics of the catalyst is attributed to the involvement of intramolecular H-bonding of a dangling carboxylate group with the active site of the catalyst.
Chapters 5 and 6, focus on functional coordination polymers for the generation of powerful molecular electroanodes for water oxidation. These include polymers containing the [Ru(tda)(py’)2] core (where tda = 2,2':6’,2”-terpyridine-6,6”-dicarboxylate and py’ = functionalized pyridine) and 4,4´-bipyridine or 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine organic linkers. Different organic linkers allowed us to synthesize polymers with different dimensions (1D or 2D). The ratio of the core unit and the organic linker together with the solvent nature play a crucial role for the chain length of the polymers. These polymers exhibit very strong adsorption on multiwalled carbon nanotubes (MWCNT) with a unique CH-pi interaction that allows for the high mass loading of the polymer and stability on the surface of electrodes. These hybrid materials are extremely powerful catalyst precursors for the water oxidation catalysis giving current densities in the range of 0.1-0.4 A/cm2 and are comparable to those of commercial electrolizers. The current density provided by these materials is stable over long periods of time without any deactivation, thus proving the robustness of the hybrid materials and making them suitable for water splitting devices.
Chapter 7 explores different methodologies to anchor mononuclear ruthenium complexes on conductive supports such as graphitic materials or metal oxides. For instance, carboxylate groups have been used to attach a ruthenium complex onto metal oxides, diazonium salts for C-C bonding attachment to graphitic materials or pyrazine linkages to anodized graphitic materials. Preliminary tests of the resulting electroanodes have been performed with very distinct results depending on the anchoring group and the nature of conductive support. In the case of the carboxylate linkage, the stability is very poor, showing full deattachment from the surface after few minutes in aqueous conditions. On the other hand, the covalent linkages on graphitic materials are very strong but the hydrophobicity nature of the resulting hybrid anode hinders the activation of the catalyst precursor.
Finally, chapter 8 focuses on the characterization of peroxo and superoxo intermediate species involved in the water oxidation catalytic cycle. They have been characterized by resonance Raman and x-ray absorption spectroscopy. In addition, oxygen labeling experiments have been used to confirm the source of the oxygen atoms in molecular oxygen obtained from the oxidation of wat
