Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Vladimir M. Shalaev
Yulia N. Pushkar
Vladimir M. Shalaev
Yulia N. Pushkar
Committee Member 1
Peter A. Bermel
Committee Member 2
Stephen M. Durbin
Utilization of sunlight requires solar capture, light-to-energy conversion and storage. One effective way to store energy is to convert it into chemical energy by fuel-forming reactions, such as water splitting into hydrogen and oxygen. Ruthenium complexes are among few molecular-defined catalysts capable of water splitting. Insight into the mechanism of their action will help to design future robust and economically feasible catalysts for light-to-energy conversion. Mechanistic insights about the design of such catalysts can be acquired through spectroscopic analysis of short-lived intermediates of catalytic water oxidation. Development of time-resolved approaches through stopped flow UV-Vis Spectroscopy to follow the catalysis of water oxidation is critical for understanding the dynamics of this reaction. In addition, use of techniques sensitive to the electronic states of molecules such as EPR and X-ray absorption spectroscopy (XAS) is implemented to determine the electronic requirements of catalytic water oxidation. Parallel Resonance Raman and oxygen evolution measurements are carried out to follow particular vibrations and confirm the presence of certain intermediates under conditions of active O2 evolution.
Spectroscopic characterization of the first ruthenium-based catalyst for water oxidation, known as the "blue dimer", + cis,cis[(bpy)2(H2O)-RuIIIORuIII(OH2)(bpy)2]4+ (bpy is 2,2-bipyridine)as well as single-site ruthenium catalysts, [RuII(L)(4-pic)2(OH2)]2,(L = 4-t-butyl-2,6-di(1',8'-naphthyrid-2'-yl)pyridine, pic = 4-picoline), [Ru(bpy)(tpy)Cl]+ (tpy is terpyridine), and [Ru(bda)(isoq)2] ] (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline) are carried out. These catalysts may be considered as artificial analogs of the oxygen-evolving complex (OEC) in the Photosystem II in green plants as they all undergo oxidative activation by proton coupled electron transfer (PCET) to reach higher oxidation states where water oxidation occurs. Intermediates of water oxidation are prepared chemically by oxidation of Ru-complexes with defined number of Ce (IV) equivalents and freeze-quenched at controlled times.
We demonstrate that correlated UV-Vis stopped flow, EPR, X-Ray Absorption Spectroscopy and O2 evolution measurements on the blue dimer water oxidation catalyst oxidized through single and multiple catalytic turnovers result in characterization of a new reactives intermediate denoted as BD[3,4]4+-prime and BD[4,5]4+. EXAFS analysis demonstrated a considerably modified ligand environment in those intermediates as compared to stable intermediates BD[3,3] and BD[3,4]. During O2 evolution at pH 1, most of the blue dimer catalyst exists of BD[3,4]-prime suggesting that it is a key oxygen evolving intermediate in the catalytic cycle where oxidation is a rate limiting reaction under the conditions of the experiments. Furthermore, Raman measurements gave strong support for the presence of a Ru-O stretch in the Ru-OOH peroxide fragment of BD[3,4]-prime. Extended x-ray absorption fine structure (EXAFS) analysis carried out on BD[4,5] as well as resonance Raman confirmed the assignment of its oxidation state and presence of a short Ru=O bond. Presence of a short Ru=O was also confirmed by EPR for 17O labeled BD[4,5] which provided further insights into its electronic structure, demonstrating high spin density on the RuV=O oxygen.
The H2O/D2O kinetic isotopic effects in water oxidation by the blue dimer are investigated and a kinetic isotope effect for blue dimer water oxidation reaction in D2O determined to be between 2.1-2.5 by combined UV-Vis stopped flow and EPR analysis. This study showed that the mechanism of O-O bond formation is atom proton transfer and revealed that the rate limiting steps in the overall catalytic cycle is not the O-O bond forming step consistent with the rate limiting oxidation of the oxygen evolving intermediate BD[3,4]-prime by Ce(IV). We also show that electron transfer processes in blue dimer water oxidation catalyst can be optimized by use of the photosensitizer [Ru(bpy)3]2+. This photosensitizer can work by redox shuttle mechanism and speed up several oxidation steps. These results are significant as they demonstrate that a redox mediator such as [Ru(bpy)3]3+ can shorten the lifetime of the BD[3,4]-prime oxygen evolving intermediate while enhancing the catalyst's oxygen evolution rate. Information obtained here about the physical, chemical, structural and electronic states of the reactive intermediates in the blue dimer catalytic cycle is critical and can contribute to the catalyst's optimization for better performance, as demonstrated by use of the electron-transfer mediator [Ru(bpy)3]3+.
Single-site catalysts have also been shown to be active in water oxidation and are attractive model compounds for both experimental and theoretical studies of water oxidation as they show improved catalytic activity compared to "blue dimer". Although 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 and in Ru-based catalysts, such intermediates were never isolated. Spectroscopic characterization of the electronic structure and molecular geometry of suspected peroxo intermediates and RuIV=O intermediates in single site water-oxidizing complexes complex [RuII(L)(4-pic)2(OH2)]2+ and [Ru(bda)(isoq)2] are reported for the first time. We also report that Ru-bound peroxide is formed by [RuIV(L)(4-pic)2=O]2+ rather than [RuV(L)(4-pic)2=O]3+ reaction with water which takes place on a 10-100 seconds time scale. Highly oxidized RuV=O intermediates have been implicated in the mechanism of water oxidation with Ru-based catalysts. Explanation of the origin of the [RuIV(L)(4pic)2=O]2+ reactivity towards O-O bond formation awaits further investigation; however we provide interesting structural and electronic insights about highly reactive and unstable intermediates in monomeric Ru complexes which were never isolated before.
Aside from investigating the peroxo intermediates in single-site Ru catalysts, simple ligand exchange from a Ru-Cl to a Ru-H2O bond on a mononuclear Ru(II)polypiridine catalyst was also observed. We demonstrate that the [Ru(bpy(tpy)Cl]+ catalyst cannot function as a water oxidation catalyst if freshly dissolved and immediately oxidized. Such type of XAS analysis at Ru L-edge and Ru K-edge show interestingly that the spontaneous reactivity and characterization of a Ru(H2O) aqua complex can only occur if a Ru(II)Cl polypiridine catalyst is incubated in water for a period of at least 4 hours.
Lastly, we demonstrate that chlorination of the photosensitizer [Ru(bpy)3]2+ result in a chlorinated product which show possible configuration of Ru(bpy)3(ClO3)3OOH peroxo product from EXAFS analysis. EXAFS fits indicate that addition of a Ru-peroxo distance improve the quality of the fit considerably. Such results proved to be extremely interesting as the chlorinated product had the same EPR signal as the peroxo species observed in monomeric Ru complexes [RuII(L)(4-pic)2(OH2)]2+ and [Ru(bda)(isoq)2]. Structure of this chlorinated product through XRD will undeniably shed more understanding on the catalytic cycle of monomeric Ru complexes which are by far efficient than the blue dimer.
In summary, this thesis outlines the structure and electronic configurations of the critical intermediates of water oxidation by catalytically active ruthenium complexes. Proposed spectroscopic approaches shown in this project have outstanding potential of uncovering the mechanism of the water splitting reaction and allow identification of the critical requirements for catalytic water oxidation paving the way for a light to fuel device.
Moonshiram, Dooshaye, "Spectroscopic Characterization of the Water-Oxidation Intermediates in the Ru-Based Catalysts for Artificial Photosynthesis" (2013). Open Access Dissertations. 101.