Understanding the Mechanism of Mn-Promoted H2O Oxidation

University Of Washington, Seattle WA

Investigators

Abstract

Clean energy alternatives inspired by Nature's photosynthetic oxidation of water require an understanding of the important reaction steps. Photosynthetic water (H2O) oxidation produces oxygen (O2) as one of the reaction products. A key step in this process is the formation of the oxygen-oxygen bond present in O2. For example, fuel cells based on artificial photosynthesis are limited by their slow rates of H2O oxidation. This provides a compelling motivation to understand how Nature's catalyst works. Professor Kovacs' group is synthesizing molecules that contain both manganese and calcium, elements know to be important components in the natural photosynthetic formation of oxygen from water. This research is (a) testing proposed mechanisms for photosynthetic O-O bond formation, and (b) providing evidence as to why calcium is essential. Professor Kovacs is a member of a mentoring group for female faculty organized by the University of Washington NSF-funded ADVANCE program, and serves as a mentor to female faculty, post docs, and graduate students at Washington and other academic institutions. Her graduate students are judging science fairs (Bainbridge Elementary Science Fair, Washington State Science and Engineering Fair), presenting demonstrations at the Shoreline STEM Festival, and volunteering at the Pacific Science Center. These activities are engaging children (ages 3-12) in science-related activities. Fundamental scientific research is needed in order to develop methods for efficiently capturing sunlight, and converting its energy into storable fuels. Nature accomplishes this via photosynthetic H2O oxidation, which converts solar energy into chemical bond energy in the form of electrons and protons, and simultaneously produces the O2 necessary to sustain life on our planet. Sluggish H2O oxidation catalysts limit the performance of existing fuel cells. Nature had billions of years to optimize its catalyst, which incorporates both Mn and Ca, and this provides compelling motivation to understand how Nature's catalyst works. The mechanism of the key O-O bond-forming step to form a peroxo intermediate is not well-understood. Professor Kovacs' research group is synthesizing site-differentiated heterobimetallic MnCa complexes designed to promote O-O bond formation. These complexes incorporate a Ca-OH moiety in close proximity to Mn, in order to rapidly trap MnV oxo intermediates, generated via sequential oxidative charging. Calculations show that the Ca-Mn separations in the synthetic targets require minimal rearrangement for O-O bond formation to occur. In order to verify that the designed molecules are capable of supporting the peroxo intermediate expected to form following O-O bond formation, independent routes to peroxos are being explored, using a reagent (H2O2) containing an intact, preformed, O-O bond. Reactions are being monitored in situ at low T, using electronic absorption spectroscopy and EPR, and oxidants are added in a step-wise fashion, thereby providing more control over the O-O bond-forming step. Professor Kovacs is a member of a Mentoring group for female faculty organized by the UW NSF-funded ADVANCE program, and serves as a mentor to female faculty, post docs, and graduate students at the UW, and at other academic institutions. Her graduate students judge local Science Fairs (Bainbridge Elementary Science Fair, Washington State Science and Engineering Fair in Bremerton, WA), present demonstrations at the Shoreline STEM Festival, and volunteer at the Pacific Science Center, engaging children (ages 3-12) in science-related activities.