Investigating CO₂ electro-reduction mechanisms: DFT insight into earth-abundant Mn diimine catalysts for CO₂ conversions over hydrogen evolution reaction, feasibility, and selectivity considerations

Abstract

This study investigates the detailed mechanism of CO₂ conversion to CO using the manganese(I) diimine electrocatalyst [Mn(pyrox)(CO)₃Br], synthesized by Christoph Steinlechner and coworkers. Employing density functional theory calculations, we thoroughly explore the electrocatalytic pathway of CO₂ reduction alongside the competing hydrogen evolution reaction. Our analysis reveals the significant role of diimine nitrogen coordination in enhancing the electron density of the Mn center, thereby favoring both CO₂ reduction and hydrogen evolution reaction thermodynamically. Furthermore, we observe that triethanolamine (TEOA) stabilizes transition states, aiding in CO₂ fixation and reduction. The critical steps influencing the reaction rate involve breaking the MnC(O)–OH bond during CO₂ reduction and cleaving the MnH–H–TEOA bond in the hydrogen evolution reaction. We explain the preference for CO₂ conversion to CO over H₂ evolution due to the higher energy barrier in forming the Mn-H₂ species during H₂ production. Our findings suggest the potential for tuning the electron density of the Mn center to enhance reactivity and selectivity in CO₂ reduction. Additionally, we analyze potential competing reactions, focusing on electrocatalytic processes for CO₂ reduction and evaluating “protonation-first” and “reduction-first” pathways through density functional theory calculations of redox potentials and Gibbs free energies. This analysis indicates the predominance of the “reduction-first” pathway in CO production, especially under high applied potential conditions. Moreover, our research highlights the selectivity of [Mn(pyrox)(CO)₃Br] toward CO production over HCOO^– and H₂ formation, proposing avenues for future research to expand upon these findings by using larger basis sets and exploring additional functionalized ligands.

Graphical abstract

Keywords

manganese carbonyl complex / CO₂ reduction reaction / hydrogen evolution reaction / selectivity / density functional theory studies

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

M. Panneerselvam and L. T. Costa acknowledge the financial support from FAPERJ (Grant No. 204.539/2021-SEI-260003/014963/2021), APQ-1, CAPES/PRINT 1038152P and 88881.465529/2019-01. MPS and L. T. Costa thanks for the Computing Resources from Feynman, DIRAC and CENAPAD Server. L. T. Costa also thanks to CNPq Fellowship (Grant No. 306032/2019-8) 88881.310460/2018-01). M. Panneerselvam extends sincere gratitude to Petrobras Research and Development/Applied Research Project (SAP No. 4600608425) and acknowledges financial support from Petrobras and COPPETEC (CNPJ 72.060.999/0001-75). Special thanks are also extended to the ATOMS Group for fostering an excellent work environment. Marcelo Albuquerque also acknowledges the support provided by CNPQ/INCT (Project No. 406447/2022-5).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-024-2502-5 and is accessible for authorized users.

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