Computational Mechanistic Studies of Water Oxidation by Ru Based Catalysts
EMSL Project ID
39896
Abstract
The development of catalysts which can produce H2 or other fuels and are made from earth-abundant materials is dependent upon the fundamental nature of the catalytic reactions and is closely linked to the EMSL scientific theme of interfacial phenomena. In the last 30 years, much progress has been made in understanding the micro-oxo ruthienium blue dimer [(bpy)2Ru(OH2)]2O4+, first reported in 1985. Our past contributions to study of these catalysts have included pioneering adaptations of an array of experimental methods based upon spectroscopic and electrochemical characterization. Collectively, these studies have provided a self-consistent picture of how these reactions might occur. Based upon the experimental studies of J. K. H., we have proposed explicit mechanisms for the catalytic mechanisms of blue-dimer type catalysts. These involve reactions initiated by H-atom abstraction of solvent by a ruthenyl oxo atom of the 4e-oxidized complex ion (i.e. [cis,cis-(bpy)2Ru(O)]2O4+). A different proposed pathway differs in the fate of the developing OH radical, which adds in concerted fashion to either the second ruthenyl O to give a bound peroxo group or to a bipyridine ring to give an intermediary bpy-OH radical species. We speculate that the ligand radical can then react further to incorporate a second solvent water, ultimately producing ligand based peroxo compounds that decompose to release O2. Non-innocent participation of bpy ligands has been previously suggested by several researchers to account of unusual reactivites of transition metal polyimmine complexes. These mechanisms has never been established however, and remain controversial. To explicitly probe our proposed schemes we have initiated our own DFT calculations that have tested the thermodynamic stability of various products created from water addition to the bpy ligand. Within the proposed work we will continue these studies, including explicit solvent effects and via the calculation of the complete intrinsic reaction coordinate for the proposed mechanisms with identification of the relevant transition states. Collectively, our independent experimental work and the theoretical studies performed at EMSL will constitute the most stringent test yet applied to the concept of bipyridine 'noninnocence' and should allow confirmation or denial of the plausibility of this type of pathway in water oxidation. As such, these studies are also relevant to a broader set of immine-based water oxidation catalysts, including the recently described mononuclear 6-coordinate ions that contain no aqua bound ligands.
Project Details
Project type
Exploratory Research
Start Date
2010-06-28
End Date
2011-07-03
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members