Acid-assisted strategy for efficient carbon dioxide reduction on molecular transition-metal catalysts

This project is mainly concerned with the fundamental studies of transition metal-catalyzed carbon dioxide (CO2) reduction as a means of energy conversion. This reaction may convert CO2 into useful compounds such as carbon monoxide (CO), formate, formaldehyde and fuels such as methanol and methane. The stability of CO2, however, imposes a high energy barrier for its reduction and a catalyst is required for it to occur under mild conditions. Various homogeneous/heterogeneous organic and metal catalysts have been reported. However, most of these catalysts suffer large overpotentials and low efficiencies. In some cases, nature of the catalytic species and the reaction pathway were not well studied and documented. We will investigate various structural/electronic factors, e.g. metals, ligand electron density and on-site intramolecular proton sources, governing the catalytic properties of a series of earth-abundant transition metal (Co, Cu, Fe and Ni) CO2 reduction catalysts. In particular, the use of acids to activate the reduction of CO2 by these catalysts will be studied in details. Multidentate π-acid ligands (imines and pyridines) will be used to stabilize the low-valent intermediate involved. In principle, the presence of Brønsted or Lewis acid should facilitate the reduction of CO2 by promoting concerted atom and electron transfer: O=C=O + E (acid) = O=C=O→E. To date, however, studies on such synergistic effects on CO2 reduction are limited to iron porphyrin complexes. CO2 reduction of our catalysts will be investigated and compared using an electrochemical approach. Variation of intrinsic catalytic properties such as apparent catalytic rate constant (kcat), turnover frequency (TOF), and overpotential (η) in the presence of acids will be studied. The catalytic reactions will also be evaluated in terms of catalyst stability, nature and selectivity of product (CO, HCOOH, CH3OH, and so on), current efficiency, and product turnover number (TON). Reaction mechanism and active intermediates in Brønsted/Lewis acid-assisted CO2 reduction will be investigated as well. The above investigations will be complemented with theoretical computation. The light-driven CO2 reduction of selected catalysts will also be explored by using various photosensitizers and sacrificial reductants.

Funding Source: RGC - Early Career Scheme (ECS)