Oxygen- and hydrogen evolution triggered by surface-supported catalysts

During this research, we focus on significant efforts to develop solid state materials and molecular coordination complexes as catalyst for water oxidation. We will develop molecular catalysts and examine the activity of molecular catalysts in water oxidation following their attachment to conducting electrodes. For such molecular species to be useful in a solar water-splitting device it is preferable that they are securely and durably affixed to an electrode surface. We also consider the combination of the action of molecular catalysts with light absorption so that light driven water oxidation may be achieved. Most recently, our group has demonstrated the oxygen evolution properties of a water-soluble Mn(III) corroles and the non-dissociative adsorption of a similar complexes on weakly interacting gold surface support. In the underlying project, we will investigate the successful stabilization Mn(III/V), Fe(III/IV), Co(III,IV), and Cu(II/III) corrole complexes 1-4 on nanoparticulate Au, Ag, Graphite TiO2 and ITO surfaces. The metal corrole complexes are chemically modified at the tetrapyrrolic periphery to covalently attach the catalytic systems via well-defined linker moieties (L) on the surface substrate. Anchoring moiety linked via thiol- or ether- bridges, absorbs visible light and leads to photoinduced interfacial electron transfer into the TiO2 conduction band. We will characterize the electronic and structural properties of the (1-4)-L-TiO2 assemblies by using a combination of methods, including computational modeling and UV-visible, IR, and EPR spectroscopies. We will investigate the oxidation state of manganese, iron, cobalt and copper in the synthesized complexes and try to generate reversibly Mn, Fe, Co, Cu in higher oxidation states by visible-light photoexcitation of (1-4)-L-TiO2 nanoparticles (NPs) which might recombine back to the lower oxidation states in the dark, in the absence of electron scavengers. The reported results will be particularly relevant to the development of photo-catalytic devices for oxidation chemistry based on inexpensive materials (e.g., TiO 2, Graphite and Mn/Fe/Co/Cu complexes) that are robust under aqueous and oxidative conditions.

Oxygen reduction and water oxidation reaction are two key processes in fuel cell applications. The oxidation of water to dioxygen is a 4H+/4e process, while oxygen can be fully reduced to water by a 4e/4H+ process or partially reduced by fewer electrons to reactive oxygen species such as H2O2 and O2. During the project period we have established the synthesis of novel catalyst systems for the oxygen evolution-, the oxygen reduction-, the hydrogen evolution- and the CO2 reduction reaction. We have synthesized manganese corrole complexes, which exhibit bifunctional character in aqueous solution by oxidizing hydroxide ions through a fourelectron process in weak to moderate alkaline conditions to molecular oxygen and reducing O2 in a twoelectron process to hydrogen peroxide. The complexes can produce a steady OER current with a faradaic efficiency of >80% over 11h. The selectivity, fast kinetics, and stability of this complex suggest that manganese corroles should definitely be considered as potential candidates for bifunctional catalysts for the reversible conversion of oxygen into water. Our combined STM results-obtained at the solid-liquid and solid-vacuum interface-have revealed 1)the nondissociative and regular adsorption of manganese corrole molecules on electrode surfaces and 2)intact electronic properties of manganese corroles. Both results are crucial for heterogeneous catalysis at the solid-liquid as well as solid-gas interfaces. We have published this research work in the journal Angewandte Chemie (https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201508404). The second part of our research work has focused on the electrochemical conversion of CO2 to alcohols, which is one of the most challenging methods of conversion and storage of electrical energy in the form of high-energy fuels. The challenge lies in the catalyst design to enable its real-life implementation. Herein, we demonstrate the synthesis and characterization of a cobalt(III) triphenylphosphine corrole complex, which contains three polyethylene glycol residues attached at the meso-phenyl groups. Electrondonation and therefore reduction of the cobalt from cobalt(III) to cobalt(I) is accompanied by removal of the axial ligand, thus resulting in a square-planar cobalt(I) complex. The cobalt(I) as an electron-rich supernucleophilic d8-configurated metal centre, where two electrons occupy and fill up the antibonding dz2 orbital. This orbital possesses high affinity towards electrophiles, allowing for such electronically configurated metals reactions with carbon dioxide. Herein, we report the potential dependent heterogeneous electroreduction of CO2 to ethanol or methanol of an immobilized cobalt A3-corrole catalyst system. In moderately acidic aqueous medium (pH = 6.0), the cobalt corrole modified carbon paper electrode exhibits a Faradaic Efficiency (FE%) of 48 % towards ethanol production. We have published this research work in the journal Nature Communications (https://www.nature.com/articles/s41467-019-11868-5).