Spectroscopic Characterization of Ru-based Catalysts for Artificial Photosynthesis
Photosynthetic water oxidation is a fundamental process in the biosphere that results in the sunlight-driven formation of O2 from water. Biological photosynthesis encompasses a series of complicated processes involving several transition states and intermediates that scientists continue to investigate. Mimicking this reaction in a man-made device will allow for sunlight-to-chemical energy conversion, with water providing electrons and protons for the formation of oxygen and reduced chemicals(1-2). Such processes are best suited for sustainable and clean generation of H2. The first synthetic catalyst designed to mimic the portion of biological photosynthesis involved in water oxidation, i.e. the catalyzed evolution of O2 from H2O, was the ruthenium-based compound commonly referred to as blue dimer.
Although the water-oxidizing capabilities of blue dimer were first reported around three decades ago, several aspects of this catalytic process remained hidden. A variety of spectroscopic techniques namely stopped-flow UV-Vis Spectroscopy, Electron Paramagnetic Resonance, X-Ray Absorption Spectroscopy and Resonance Raman are used to probe the catalytic process of blue dimer as well as single site monomeric ruthenium complexes with higher turnover rate. EPR, Raman and XAS characterization of the electronic structure and molecular geometry of peroxo intermediates in blue dimer as well as single-site water-oxidizing complexes are reported.
Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts(3). However, such intermediates were never isolated and their structural and electronic characterization has not been reported. The intermediates described here are direct products of the O-O bond formation step in the studied catalysts. The combination of all these techniques enabled identification of the critical requirements for catalytic water oxidation for the design of new economical and efficient catalysts.
1. Esper B, Badura A, & Rogner M (2006) Photosynthesis as a power supply for (bio) hydrogen production. Trends in Plant Science 11.
2. Moonshiram D, et al. (2012) Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis. J.Am.Chem.Soc. 134(10):4625-4636.
3. Concepcion JJ, Jurss JW, Templeton JL, & Meyer TJ (2008) One Site is Enough. Catalytic Water Oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+. J Am Chem Soc 130(49):16462-16463.