Hydrogen from water is an optimal energy carrier, and considerable efforts are devoted to develop artificial photosynthetic systems for water splitting, including water oxidation reaction step 2H2O → O2 + 4e + 4H+ with complicated 4-electron cascade transformations. Fast and durable water splitting catalysts will be incorporated into future sunlight to chemical fuels conversion assemblies. Here we report a simple and synthetically accessible cis-[Ru(bpy)(5,5’-dcbpy)(H2O)2]2+ (bpy = 2,2’-bipyridine, 5,5’-dcbpy = 2,2’-bypyridine-5,5’-dicarboxylic acid) water oxidation catalyst. Its oxidation in solutions at pH = 1 by cerium (IV) ammonium nitrate results in activity ~ 40 / 230 times faster than that of parent cis-[Ru(bpy)2(H2O)2]2+ compound and widely studied [Ru(tpy)(bpy)(H2O)]2+ (tpy = terpyridine), correspondingly. In solution, the rate of O2 evolution by cis-[Ru(bpy)(5,5’-dcbpy)(H2O)2]2+ is first order on catalyst which indicates catalyst reactivity via water nucleophilic attack pathways. RuV intermediate, namely cis-[RuV=O,-OH (bpy)(5,5’-dcbpy)]2+, was detected in the reaction mixtures by X-ray absorption and resonance Raman spectroscopy. The RuV=O was found to be ~1.72 Å by EXAFS and to have ~810 cm-1 frequency in Resonance Raman (-40 cm-1 16O/18O isotope shift). DFT calculations of redox potentials, vibrations and dG of the reactions are in good agreement with the experiment. To avoid deactivation pathway through a dimer formation, the catalyst was post-synthetically incorporated into the UIO-67 molecular organic framework (MOF). cis-[Ru(bpy)(5,5’-dcbpy)(H2O)2]2+-UIO-67 catalyzes water oxidation without loss of catalytic activity. [RuV=O,-OH(bpy)(5,5’-dcbpy)]2+-UIO-67 intermediate in MOF was also detected and can form an O-O bond via water nucleophilic attack. Postsynthetic modification of UIO-67 thin film grown on FTO glass electrode surface with cis-[Ru(bpy)(5,5’-dcbpy)(H2O)2]2+ leads to an increase of the catalytic current by two orders of magnitude. Thus, [Ru(bpy)(5,5’-dcbpy)(H2O)2]2+-UIO-67 is a promising building block for integrated sunlight to chemical fuels devices.

Water oxidation catalyst cis-[Ru(bpy)(5,5’-dcbpy)(H2O)2]2+ and its stabilization in molecular organic framework

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.