H2OSPLIT · WATER SPLITTING CATALYSTS FOR ARTIFICIAL PHOTOSYNTHESIS

WATER SPLITTING CATALYSTS FOR ARTIFICIAL PHOTOSYNTHESIS

Artificial photosynthesis is one of the most promising methods for the direct conversion of solar energy into renewable chemical fuels. The process involves splitting water by creating spatially separated electron-hole pairs, which then control the redox semi-reactions leading to evolution of molecular hydrogen and oxygen. This project aims at providing an electronic and structural characterization of novel highly-efficient catalysts for water oxidation, as well as at identifying the fundamental reaction mechanisms underlying their function and efficiency. To this end, we will use state-of-the-art first-principles numerical modeling based on density functional theory. In particular, we will focus on inorganic ruthenium-containing polyoxometalate homogeneous catalysts that have been recently synthesized and that displayed unprecedented reactivity and stability in solution. The project succeeded in providing an atomic-level characterization of the electronic and structural properties of the Ru-POM catalyst in the gas phase by means of density functional theory calculations, as well as describing the interaction of a water molecule with the RuO active cores of the system, providing initial screening and insight into the local structure of the first water coordination shell. Classical molecular dynamics simulations of the catalyst in solution revealed the solvent-solute interactions and the structural properties of the solvation shell. Advanced hybrid ab-initio simulations were used to study the effect of the solvent on the structural and electronic properties addressed previously in the gas phase as well as the reaction mechanism of water oxidation by means of meta-dynamics simulation techniques (WP5). The most important results were reported in the article "Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium–oxo molecular complex" by Simone Piccinin, Andrea Sartorel, Giuliana Aquilanti, Andrea Goldoni, Marcella Bonchio, and Stefano Fabris, that was published in the Proceedings of the National Academy of Sciences, PNAS vol. 110, no. 13, 4917–4922 (2013). The publication was the result of a very fruitful collaboration with experimental groups that was initiated thanks to the funding of the present project. In particular the main results can be summarized as follows: Solar-to-fuel energy conversion relies on the invention of efficient catalysts enabling water oxidation through low-energy pathways. Our aerobic life is based on this strategy, mastered by the natural Photosystem II enzyme, using a tetranuclear Mn–oxo complex as oxygen evolving center. Within artificial devices, water can be oxidized efficiently on tailored metal-oxide surfaces such as RuO2. The quest for catalyst optimization in vitro is plagued by the elusive description of the active sites on bulk oxides. Although molecular mimics of the natural catalyst have been proposed, they generally suffer from oxidative degradation under multiturnover regime. Here we investigate a nano-sized Ru4–polyoxometalate standing as an efficient artificial catalyst featuring a totally inor- ganic molecular structure with enhanced stability. Experimental and computational evidence reported herein indicates that this is a unique molecular species mimicking oxygenic RuO2 surfaces. Ru4–polyoxometalate bridges the gap between homogeneous and heterogeneous water oxidation catalysis, leading to a break- through system. Density functional theory calculations show that the catalytic efficiency stems from the optimal distribution of the free energy cost to form reaction intermediates, in analogy with metal-oxide catalysts, thus providing a unifying picture for the two realms of water oxidation catalysis. The potential impact of these results, namely the correlations among the mechanism of reaction, thermodynamic efficiency, and local structure of the active sites, is to provide the key guidelines for the rational design of superior molecular catalysts and composite materials designed with a bottom–up approach and atomic control.

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