Advanced Time-Resolved Studies of the O-O Bond Formation Mechanisms: Interplay of the Metal and Ligand Redox Reactivity

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In this project supported by the Chemical Structure, Dynamic & Mechanism,B Program of the Chemistry Division, Professor Yulia Pushkar of the Department of Physics and Astronomy at Purdue University studies the time-resolved mechanism of dioxygen (O2) formation in artificial photosynthesis. In artificial photosynthesis, solar energy is converted into chemical energy through generation of the clean fuels hydrogen and oxygen, a process which requires rearrangement of chemical bonds. Fundamental understanding of this process is required for the development of new catalysts and devices which are able to mimic natural photosynthesis. The development of artificial photosynthesis and its large-scale implementation can address energy needs of modern society. This research lies at the interface of physics, chemistry and materials science, with results expected to impact diverse fields and contribute to fundamental science, education and national energy security. Planned research and educational activities are designed to increase participation of under-represented students from economically disadvantaged backgrounds, improve experiences of female students in STEM (Science, Technology, Engineering and Mathematics), enhance training of students via integration of research results into curricula and to deliver teaching modules to schools. Research in this project focuses on the complex multi-electron chemical process of artificial photosynthesis. A major project goal is to determine the structure, electronic configurations and dynamics of the critical intermediates involved in water oxidation. In this multi-scale approach, time-resolved techniques monitor the evolution of structure and electronic states in newly designed ruthenium catalysts, with a focus on the key mechanism of oxygen-oxygen bond formation and its dependence on ligand structure. The relationship between molecular structure and catalytic activity is tested by a combination of experiments and quantum-mechanical computational models. Experimental techniques in this study of in situ catalytic water oxidation are synchrotron-based X-ray spectroscopy, including X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), electron paramagnetic resonance (EPR) and multi-wavelength kinetic resonance Raman spectroscopy. These experimental techniques deliver information on the structure of the intermediates and their electronic configuration as they evolve during the catalytic process.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.