Basic Research Needs for Solar Energy Utilization - Office of ...

While these essential features of catalyst design are widely recognized, the fundamental
knowledge base needed to control the steps following electron transfer is almost completely
lacking.
NEW SCIENTIFIC OPPORTUNITIES
Mechanisms of Complex, Coupled Reactions for the Solar Production of Fuels. It is
evident from the very limited number of active non-biological catalysts discovered that reactions
essential to solar production of fuels are exceedingly complex and require precise control of
molecular events. Structures that promote the coupling of productive reactions and suppress
those that are unproductive must be developed and refined. Mechanistic studies are therefore
essential to the rational design of advanced catalytic systems. This understanding can be
achieved by the isolation and structural/dynamical identification of reaction intermediates using
a combination of techniques ranging from classical spectroscopic, electrochemical and magnetic
analysis to transient spectroscopy and mass spectrometric/dynamical analyses using isotopic
labeling. One example of the power of combining these diverse techniques is the level of
understanding recently achieved in the catalytic conversion of H2O to O2 catalyzed by binuclear
ruthenium μ-oxo complexes.
Excited-state Bond Making and Breaking Processes. Photochemical bond breaking and
bond making reactions of many inorganic and organometallic compounds can directly lead to
end products in fuel production, including hydrogen from water and carbon dioxide reduction
products. Light-induced reactions are of interest because they can provide reaction paths that are
not accessible to ground states. The fundamental principles for developing new photosystems for
product formation exist for photo-driven inorganic and organometallic substances. Research is
needed to optimize photoreaction quantum yields. To accomplish this, a better understanding of
excited-state decay pathways in promising complexes is needed in order to channel excitation
energy into the fuel-producing reaction paths. These efforts can be enhanced by the development
of more accurate excited-state electronic structure calculations.
Theoretical and Experimental Studies of Rates and Mechanisms of Multielectron/atom
Transfer Reactions. There is a need for a systematic theory of atom and ion transfer, which is
analogous to the Marcus theory of electron transfer. This theory will emanate from and be tested
by systematic kinetics studies. In addition, mechanisms must be understood and developed for
redox leveling (as occurs in biological systems) as well as for coupling single and multiple
electron transfer reactions. A critical aspect of this development is the design of robust ligand
systems for sustained multiequivalent chemistry.
Proton-coupled Electron Transfer Reactions Including H Atom and Hydride Transfers.
Photochemical H2O and CO2 reduction to fuels poses scientific challenges including protoncoupled
multielectron transfer processes. For example, a number of photosynthetic systems show
promise in the photoreduction of CO2 to CO and/or formate, however, systems that demonstrate
the transformation to methanol (with 6 protons and 6 electrons) or methane (with 8 protons and
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