Research (Prof. Ohki)
Development of efficient processes for energy conversion and material synthesis is one of the key issues to be solved for sustainable societies. An innovative approach is to seek the frontier of materials with full of potentials, which may be discovered in between typical small molecular materials and nano materials. Our study starts from the synthesis of sophisticated “clusters”, which stand for molecules with multiple transition metal atoms. Their sizes are around 1 nm, the range of which has not been explored in molecular chemistry of transition and main-group elements, particularly those with iron-group metals. These cluster molecules are then applied as catalysts or functional materials, for instance in the catalytic reduction of nitrogen molecule or carbon dioxide, or they contribute to uncover the secrets of complicated metal centers in biology.
(1) metal-sulfur clusters
Metal-sulfur clusters, particularly those with Fe atoms, are ubiquitous in proteins mediating electron transfer and catalytic conversion of small molecules, such as the reduction of N2, reversible conversion between H2 and protons+electrons, and inter-conversion between carbon monoxide and carbon dioxide. These unique functions are useful as fundamental information for future development of highly efficient energy conversion processes, however, the complicated structures of metal-sulfur clusters often make us difficult to imagine or understand the relationships between the structures and the functions. Thus, chemical synthesis of such complicated clusters as molecular entities provides us with opportunities to uncover the secrets of enzymes. We have previously designed and prepared metal-sulfur clusters analogous to the largest and most complex metallo-clusters, e.g. the P-cluster with an [Fe8S7] core and the FeMo-cofactor with a [MoFe7S9C] core, which are indispensable for biological reduction of N2. We have been also developing metal-sulfur clusters with catalytic N2 reducing functions. Furthermore, a new method for carbon fixation to convert CO2 directly into hydrocarbons has been developed. We hope to improve this new reaction to realize a fuel regeneration process, where gasoline (C4-C10 hydrocarbons) can be directly prepared from CO2.
(2) metal clusters
Transition-metal particles with a diameter of a few nanometers or smaller are termed as nano-clusters. They have been of interest for their physical properties and potential applications as catalyst precursors, as their properties and reactivity vary by subtle changes in the size and structures. The past a few decades have witnessed a dramatic growth in this area, although the target elements have been mostly limited to coinage metals (Au, Ag, Cu) owing to their stability in the cluster forms. In other words, coinage metal clusters are so stable that their applications in catalysis have limitations. With particular interest in catalytic applications, we have been interested in the precise synthesis of metal-clusters and metal-hydride clusters with iron-group elements.
Our previous achievements include the synthesis of Fe-hydride clusters, which are relevant to possible intermediates in both biological and industrial N2 reducing processes. The reaction conditions of the enzyme and the industrial process are quite different, and thereby the their reaction pathways are also distinct. Even so, and interestingly, we can find two common keywords, multiple Fe atoms and Fe-bridging hydrides. Our Fe4- and Fe6-hydride clusters were found to serve as catalyst precursors for the conversion of N2 into N(SiMe3)3. This study demonstrates the importance to seek possible interfaces of two disciplines, in this case enzyme and solid-state catalysis.