Realization of artificial photosynthesis carries the promise of cheap and abundant energy. The H2O is an ideal source of electrons and protons for fuel forming reactions, but the chemical complexity of water splitting makes practical realization challenging. To advance the catalyst’s rational design, detailed information on the structure of the catalyst under reaction conditions and mechanisms of O-O bond formation are required. Here, we used a combination of EPR, freeze quench, X-ray absorption spectroscopy, Resonance Raman and DFT to follow in situ catalyst dynamic under conditions of water oxidation. In my presentation I will compare and contrast principles of action of Ru based WOC with WOCs containing 3d transition metals such as Fe, Co and Mn. Currently, Ru and Fe based molecular WOCs are known to function at acidic pH (~1). For Ru based water oxidation catalysts (WOC) formation of RuV=O (S=1/2) intermediate appears to be essential for the activation of the H2O molecule towards O-O bond formation. RuV=O is forming via proton coupled electron transfer from 6 or 7-coordinate RuIV-OH or RuIV(OH)2 species. Similar intermediate [FeV=O,(OH)(Pytacn)]2+ (Pytacn=1-(2’-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane) with FeV=O ~1.60 Å was detected by us in the Fe based WOC. However, spin state of this Fe species was assigned as S=3/2. Cobalt based [Co4O4Py4Ac4]0, Py = pyridine and Ac = CH3COO-) cubane WOC functions at the basic pH (~12). Spectroscopically, under in situ conditions, we were able to characterize CoIV=O fragment at ~1.67 Å. This species is able to activate water via water nucleophilic attack (WNA). In Fe and Co catalysts there are groups present which are bases (-OH or CH3COO-) available for the abstraction of the proton from the water during the O-O bond formation via WNA. Ru based catalysts were integrated with metal organic frameworks such as UIO-67 and MIL-126. Assemblies of the Ru based catalyst with MIL-126 and its dcbpy analog MIL88 are photoactive in water splitting and can be used as photoanodes at pH=1. Current mechanistic understanding allows to further expand groups of Ru and Fe based WOCs with systematic increase of their WOC activities.

Mechanistic analysis of water oxidation catalysis: from Ru- to 3d-transition metal (Fe, Co, Mn) based catalysts and their functional assemblies for artificial photosynthesis

ACS Spring 2022

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