Modeling Ru-based Molecular Catalysts for Water Oxidation

The discovery of solar-driven water splitting catalysts with high activity and stability has been instrumental for storing solar energy to chemical fuels that can satisfy the increasing demand of sustainable energy. Ru-based catalysts continue to serve as the springboard for the development of solar energy conversion into chemical fuels. It is clear that the functionality of these catalysts is controlled by Ru metal centers, but in several cases their working mechanisms and the origin of their different efficiencies are still under debate. In particular we are interested in exploring the active role of the solvent in the water oxidation reaction, the catalysis and importance of multi-connectivity between adjacent active sites. To this end we focus on two recently discovered homogenous and heterogeneous Ru-based catalysts: 1) an organic mononuclear Ru(II)-pincer complex and 2) a fully inorganic multi-center Ru-based polyoxometalate (POM) assembled at functionalized multiwalled carbon nanotubes (MWCNT-dend). The single metal center Ru(II)-pincer recently synthesized[1] has been shown to promote both the thermal driven formation of H2 and the UV-vis driven evolution of O2. Here we investigate, through DFT calculations, the formation of hydrogen. We adopt an explicit description of the solvent and employ metadynamics coupled with the CarParrinello method to study the reaction mechanism and determine the activation free energies. We propose an alternative catalytic cycle, with considerably lower activation energies compared to earlier proposals. Our simulations demonstrate the importance of an explicit description of the solvent for modeling chemical reactions that involve the active participation of several solvent molecules. [2] Recent studies of the inorganic Ru-based POM catalyst with high turnover rate and low overpotential have been reported both experimentally and theoretically[3, 4]. Assembling of this catalyst on MWCNT-dend allows for obtaining a stable and efficient electrode for water oxidation that can be interfaced with photovoltaic units[5]. The complexity of the system together with the presence of explicit solvent requires combining state-of-the-art ab initio and classical atomistic simulations. We build a realistic model of this promising nano scale material, to provide insights into the formation properties, and the electronic structures of the system, in particular of the Ru-based POM / functionalized groups interface. We investigate the energetics of the