Mechanistic Studies of the Photoactivation of Metal- Hydride, Halide and Oxo Bonds

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With this award, the Chemical Catalysis Program of the Chemistry Division is funding Professor Daniel Nocera of Harvard University to study catalytic processes that can generate hydrogen from light and simple compounds. Fuels are compounds that react with an oxidant, usually oxygen, to release energy. At the molecular level, when a fuel is burned, the bonds in high energy compounds rearrange to form new bonds in lower energy compounds, and the energy released in this bond rearrangement is used to power our world. Most fuels in our society are derived from coal, oil and natural gas; namely fossil fuels, the bonds of which are rearranged with the bonds of oxygen to generate carbon dioxide and water, which enter the atmosphere. One way to avoid the formation of carbon dioxide is to cut the connection between fuels and carbon and to use hydrogen as a fuel. There are two sources of hydrogen that can be generated using solar irradiation: water or hydrohalic acid (for example, a hydrogen atom bonded to a chlorine atom); however, hydrogen cannot be generated by shining light directly on water or hydrohalic acid. A catalyst must be used that will capture the light and then act as an intermediary in the bond rearrangement reaction. Professor Nocera is developing new catalysts that can absorb light and then convert hydrohalic acids into hydrogen and elemental halogen. The new compounds, reactions and techniques resulting from this research are likely to impact realted areas of science and could result in new energy generation technologies. Professor Nocera continues to be involved in raising public awareness of the global energy problem and the role that science, and especially chemistry, can play in addressing the energy challenge. Research discoveries in this area are being conveyed to the public through a variety of forums including news media, radio programs, movies and other broadcast venues. The Nocera group is developing new catalysts for photochemical HX splitting and, in parallel, is studying the mechanisms of such reactions. The storage of renewable energy in the form of fuels requires the rearrangement of bonds with low energy content to ones of high energy content, and bonds of important energy consequence are H-H, X-X and O=O. New metal compounds based on Rh, Ni, Fe and Mn are being synthesized that can provide insight into these energy conversion processes. Critical intermediates that promote the requisite bond activation are being studied using advanced spectroscopic techniques, including the application of time-resolved X-ray crystallography and nanoparticle-based time-resolved resonance Raman spectroscopy for the identification of the critical M-H and M-X intermediates, and magnetic resonance techniques (intermediate-field ENDOR, ELDOR-detected NMR and single-crystal polarized neutron diffraction) for the identification of M-O intermediates. With knowledge of the stereoelectronic properties of the critical intermediates, photocycles are being constructed to drive energetically uphill reactions, which are at the basis for renewable energy conversion.