Only 20% of the worldwide energy consumption is supplied by electrical power; our lifestyles mainly depend on the availability of chemical fuels . Photosynthetic processes represent in this regard the most important source of energy produced since the beginning of life on earth. A crucial role in this task has been played in living organisms by small transition-metal clusters embedded into a biochemical environment. For example, the Mn4CaO5 cluster contained into the transmembrane complex photosystem(II) promotes the water splitting to dioxygen, protons and “energized” electrons; the latter are used for CO2 reduction and chemical energy storage (e.g., sugar formation). Inspired by such natural processes, the goal of artificial photosynthesis is to develop simplified “artificial leaves” which can be exploited in a large-scale generation of fuels (e.g., H2) directly from sunlight . The step involving water oxidation is considered to be a main bottleneck, hampering progress in the development of applicable technologies. Several synthetic catalysts, containing different metal-oxo cores have been proposed, but earth-abundant Ni, Mn, Co amorphous (hydr)oxides likely represent the most promising materials for technologically relevant and low cost devices . A thorough understanding of water oxidation processes requires detailed insight into the atomistic texture of natural and artificial catalysts. The quest for exhaustive models is thwarted by the typically amorphous structure of water-oxidation catalysts, by the intrinsic complexity of natural systems and by the elusive experimental detection of protons, main players of the game. The lack of such insight represents the main hurdle on the way towards progress, and limits the development of mechanistic models to a too speculative level. We contributed to the understanding of photosynthesis through the close comparison between ab initio simulations and experimental results: combined DFT simulations and XAFS measurements shed light on a variety of atomistic motifs detected at the interface between water and Co/Mn-based catalysts ; a detailed analysis of the compatibility between XFEL measurements and ab initio results supports the occurrence of isomers in the dark state of photosystem II ; reliable structural models open the door to the investigation of reaction mechanisms promoted by such systems .
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