Justine P. Roth , Johns Hopkins University
Since its inception, this PRF award has allowed my research group to carve out a new area of research in which our efforts are devoted to the synthesis and study of a range of water-splitting catalysts for the production of dihydrogen as a clean-burning and sustainable fuel. This research area, focuses primarily upon photochemical reactivity, as it is well accepted that this approach is the vital solution to satisfying the increasing energy needs, while minimizing CO2 emissions and perturbation of the environment. We aim to develop mechanistic understanding applicable to the development of water oxidation catalysts that utilize visible light and operate at low over-potentials. In the process, we have been developing natural abundance isotopic methods that probe mechanisms of O–O bond formation.
We proposed these first of a kind competitive isotope measurements for the codification of water oxidation mechanisms according to the identity of the O–O bond-forming step. Earlier studies frequently used measurements of solvent 18O-labeling, together with water exchange rates, to infer mechanisms which can now be compared to those being formulated in our laboratories on the basis of competitive kinetic isotope effects (18O KIEs) and equilibrium isotope effects (18O EIEs) which give rise to different ratios of 16,16O2 and 18,16O2 from natural abundance water.
We have initially worked with a range of complexes, under a variety of conditions, to determine how best to probe the mechanism of the O–O bond-formation step. These complexes include stoichiometric reagents such as potassium ferrate and the catalysts below including: T. J. Meyer’s classic blue dimer, a related diruthenyl dimer prepared and provided by A. Llobet, a single site ruthenium catalyst originally reported by K.Sakai, the dimanganyl dimer of G. Brudvig and R. Crabtree, various organometallic iridium complexes reported by Crabtree and colleagues and a recently prepared Dawson-type polyoxometallate, [Co4(H2O)2(α-PW9O34)2]10-, reported by C. Hill. With this assortment of catalysts, we are systematically investigating the 18O KIEs upon catalytic water oxidation to O2. In addition to the isotopic measurements under the commonly used acidic conditions with CeIV reagents, we have made analogous measurements with photo-generated [RuIII(bpy)3][(ClO4)3] in phosphate buffer as well as the pre-synthesized complex. The latter assists in our kinetic measurements accompanying all isotopic studies.
Typically we use a Clark-type O2 electrode to study the kinetic order with respect to (pre)catalyst, oxidant, and base. Inverse dependency upon CeIII or RuII and H+ are also tested as is the accumulation of unbound H2O2, which may occur in strongly acidic media. In addition, we have begun to study how O2 yields may vary under sub-stoichiometric conditions depending on whether the kinetic mechanism involves additional oxidizing equivalents not accounted for by the simple equation: 2H2O → O2 + 4H+ + 4e−. Reports have begun to appear retracting the catalytic mechanisms proposed largely on the basis of Density Functional Theory (DFT) Calculations and assumed reaction stoichiometries.
We have been developing calibrated density functional methods for predicting 18O KIEs on water oxidation reactions. In addition to the manuscript published last year (2010) and a book chapter due out this year (Wiley Series on Reactive Intermediates in Chemistry and Biology (Steven E. Rokita, Editor) Volume on Copper-Oxygen Chemistry (Editors: Kenneth D. Karlin & Shinobu Itoh), we are in the process of submitting three separate manuscripts which will cite the funds provided by the present grant (50046-ND3). These manuscripts cover experimental and computational aspects of the chemistry as summarized in the preliminary TOC entries and “nuggets” which follow.
In brief, we have collaborations in place to obtain calibrated calculations at the density functional level of theory using the Gaussian suite of programs. Our approach generally involves the analysis of vibrational frequencies following identification of the precursor complex, the transition-state for O–O bond formation and the product complex. Calculations of this type are now being extended to investigate all steps along the reaction coordinates. The rate expressions, together with the competitive 18O KIEs, are being used to put the DFT methodology to the test. Our objective is often to reproduce (or predict) the limiting 18O KIE. While exploring agreement with experiment on a case by case basis, we are and attempting to correlate the O–O bond forming mechanisms to the magnitudes of the 18O KIEs. Thus, we are testing the proposal of whether we can use the combined experimental and computational approach to illuminate at unprecedented detail, the bonding changes that occur within transition states. Our promising result from this series of studies has shown that more often than not, O–O bond formation does represent the barrier to the turnover-limiting step in water oxidation catalysis.