(Digital Presentation) Investigate the Reaction Mechanism in the C3H7OH Electrochemical Reduction Reaction from CO2 Using Dinuclear Cuprous Molecular Catalyst
Authors: Sakamoto Naonari , Sekizawa Keita , Shirai Soichi , Nonaka Takamasa , Arai Takeo , Sato Shunsuke , Morikawa Takeshi 
Published: December 2024

The urgent global challenge of climate change and the depletion of energy resources have spotlighted the technology of converting carbon dioxide (CO2) into valuable carbon-based products. The CO2 reduction reaction (CO2RR) via metal complex molecules is expected to improve and enhance the electronic/chemical environment of the catalytic active sites through control with freely modulable organic ligands. The selectivity for the formation of formic acid and carbon monoxide from CO2 in organic solvents has been successfully controlled in some molecular catalysts by small structural differences in the ligands. For catalytic reactions in organic solvents, NMR and other common analytical methods can be used to follow structural changes and reaction intermediates during the reaction of unimolecular molecular catalysts. The study of detailed reaction processes has led to ligand design guidelines that contribute to higher reaction rates and improved selectivity.

However, analytical methods still need to be developed for heterogeneous molecular catalysts used in water on electrodes. Catalysts capable of producing C2 products, such as ethylene and ethanol, require methods to detect intermediates in multi-electron reduction processes involving C-C coupling reactions. Current C2 producing molecular catalysts are known to decompose during reactions, turning into metals and not fully utilizing their precisely controlled active sites. Therefore, developing structurally stable molecular catalysts during reactions is critical to understanding these mechanisms.

In this study, we have demonstrated a Br-bridged copper dinuclear molecular catalyst with high robustness for CO2/CO reduction reactions, producing C3+ products including C3H7OH beyond C2 products. Operando XAFS and various spectroscopic analyses show that the dinuclear molecular catalyst maintains a stable Cu(I) state during the reduction reaction, preventing decomposition. Operando spectroscopic analysis using localized surface plasmon resonance identifies key C2 and C3 coupling intermediates essential for C3 production. Reaction intermediates obtained by 13CO2-labeled operando analysis were also corroborated. DFT calculations propose a mechanism where the reaction proceeds at room temperature to form C3 coupling species. The transition state structures resulting from this mechanistic analysis suggest that the progression of C-C coupling in the Cu dinuclear molecular metal complex is facilitated by the formation of a CO2/CO-reduced intermediate species that bridges the two Cu active sites. The bridging intermediate can adjust the Cu-Cu distance during the reaction, attracting reducing species to one Cu side and accepting substrate to the other Cu side. C3H7OH is formed through C-C coupling at the bridging site in flexible Cu binuclear structures. The discovery of a robust molecular catalyst for C3+ production provides a molecular design guideline for developing next-generation catalysts for multicarbon CO2 reduction products.