Chemistry Department Seminar
"In situ analysis of Ru-based catalysts under water oxidation conditions"
Presented by Yulia Pushkar, Purdue University
Tuesday, February 27, 2018, 10:00 am — Room 300, 3rd Floor, Chemistry Bldg. 555
Realization of artificial photosynthesis carries the promise of cheap and abundant energy. The water molecule 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 and stopped flow spectroscopy with ms-s time resolution, X-ray absorption spectroscopy (XAS), Resonance Raman (RR) and DFT to follow in situ catalyst dynamic under conditions of water oxidation.1-3 Two representative Ru –based catalysts were analyzed: [RuII(NPM)(4-pic)2(H2O)]2+ and [RuII(pic)2(dpp)]2+. First system has water coordinated to Ru center and forms [RuIV(NPM)(4-pic)2=O]2+ upon oxidation. This intermediate undergoes fast dynamics (on few sec time scale) of oxygen atom transfer from the RuIV=O oxo species to uncoordinated nitrogen of the NPM ligand. NPM ligand modification occurs on the time scale of catalyst activation and results in [RuIII(NPM-NO)(4-pic)2(H2O)]3+ and [RuIII(NPM-NO,NO)(4-pic)2]3+ complexes with unique EPR signals.
[RuII(pic)2(dpp)]2+ complex was proposed to activate via formation of the 7-coordinate [RuV=O(pic)2(dpp)]3+ species. We report the first detection of the ligand protected 7-coordinate species in catalytic mixtures by combination of the spectroscopic techniques. Over a few minutes this intermediate transfers oxygen from the RuV=O group to a pyridyl nitrogen of the dpp ligand. This reaction proceeds twice resulting in the dpp-di-N-oxide ligand. This ligand modification results in the catalyst activation.
[Ru(bda)(pic)2] complex is also proposed to activate via formation of 7-coordinate [RuV=O(bda)(pic)2]+ intermediate which is highly reactive in solution via radical coupling pathway. Site isolation of the catalyst on the electrode
Hosted by: Dmitry Polyansky
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