Electrochemical CO₂ Reduction Facilitated by Manganese Bipyridine-Based Molecular Catalysts

TY - BOOK

T1 - Electrochemical CO2 Reduction Facilitated by Manganese Bipyridine-Based Molecular Catalysts

AU - Hong, Wanwan

PY - 2024/1

Y1 - 2024/1

N2 - The massive anthropogenic activities and excessive exploitation of petrochemical resources have contributed to the increased atmospheric CO2 concentration as a controlling component of greenhouse gases. Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy represents a sustainable strategy to convert CO2 into value-added fuels and chemicals while limiting its emissions into the atmosphere. However, challenges remain in the development of highly efficient and selective catalysts, along with the fundamental comprehension of the relevant catalytic mechanisms. The goal of this Ph.D. thesis was to develop novel molecular catalysts for the application of CO2RR with simultaneous mechanistic investigation in order to gain a deep understanding on how the molecular structure of catalysts in CO2RR influences the catalytic performance. This thesis mainly focuses on the modifications in the secondary coordination sphere of manganese tricarbonyl bipyridine complexes to comprehend the controlled selectivity in electrochemical CO2 reduction in competition with hydrogen evolution reaction (HER). The commonality of the three projects is to introduce amine-related functionalities into the vicinity of the Mn active site. In specific, Mn complexes with macrocyclic bpy-based ligand were designed in the first project. The rotation of the Mn(CO)3 core in solution brought protonated amine pendants near to the Mn center giving the crucial Mn hydride intermediate. Further protonation of the hydride species via the amine-assisted approach led to HER, while insertion of CO2 to the Mn-H bond was hindered by the protection of macrocyclic linker over the Mn center. Overall, H2 was predominantly generated over HCOO–. The importance of the external Brønsted acids on the selectivity of CO2RR was understored in the second study. For the new Mn complex bearing four benzylic diethylamine groups in the secondary coordination sphere, the selectivity of CO2RR was dominant by HCOO– in the presence of iPrOH, which was shifted to H2 when stronger acid PhOH was applied. In the third study, incorporation of the pyrrolidine groups to the bpy ligand of Mn complex induced a 92% high selectivity for CO2 reduction to HCOO–. The basic pyrrolidine groups functioned as proton relays and facilitated the Mn hydride formation with a lower energy barrier of 7.9 kcal mol–1, compared with its counterpart with diethylamine groups. Essentially, the work described in the thesis demonstrated that catalytic performance can be manipulated by the molecular structure of Mn bpy-based complexes. Hence, ligand design represents an effective approach for forthcoming electrocatalysts to achieve optimizing selectivity and activity. Caution, however, should be exercised in the future design as small modifications may lead to substantial variations in catalytic performance.

AB - The massive anthropogenic activities and excessive exploitation of petrochemical resources have contributed to the increased atmospheric CO2 concentration as a controlling component of greenhouse gases. Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy represents a sustainable strategy to convert CO2 into value-added fuels and chemicals while limiting its emissions into the atmosphere. However, challenges remain in the development of highly efficient and selective catalysts, along with the fundamental comprehension of the relevant catalytic mechanisms. The goal of this Ph.D. thesis was to develop novel molecular catalysts for the application of CO2RR with simultaneous mechanistic investigation in order to gain a deep understanding on how the molecular structure of catalysts in CO2RR influences the catalytic performance. This thesis mainly focuses on the modifications in the secondary coordination sphere of manganese tricarbonyl bipyridine complexes to comprehend the controlled selectivity in electrochemical CO2 reduction in competition with hydrogen evolution reaction (HER). The commonality of the three projects is to introduce amine-related functionalities into the vicinity of the Mn active site. In specific, Mn complexes with macrocyclic bpy-based ligand were designed in the first project. The rotation of the Mn(CO)3 core in solution brought protonated amine pendants near to the Mn center giving the crucial Mn hydride intermediate. Further protonation of the hydride species via the amine-assisted approach led to HER, while insertion of CO2 to the Mn-H bond was hindered by the protection of macrocyclic linker over the Mn center. Overall, H2 was predominantly generated over HCOO–. The importance of the external Brønsted acids on the selectivity of CO2RR was understored in the second study. For the new Mn complex bearing four benzylic diethylamine groups in the secondary coordination sphere, the selectivity of CO2RR was dominant by HCOO– in the presence of iPrOH, which was shifted to H2 when stronger acid PhOH was applied. In the third study, incorporation of the pyrrolidine groups to the bpy ligand of Mn complex induced a 92% high selectivity for CO2 reduction to HCOO–. The basic pyrrolidine groups functioned as proton relays and facilitated the Mn hydride formation with a lower energy barrier of 7.9 kcal mol–1, compared with its counterpart with diethylamine groups. Essentially, the work described in the thesis demonstrated that catalytic performance can be manipulated by the molecular structure of Mn bpy-based complexes. Hence, ligand design represents an effective approach for forthcoming electrocatalysts to achieve optimizing selectivity and activity. Caution, however, should be exercised in the future design as small modifications may lead to substantial variations in catalytic performance.

M3 - Ph.D. thesis

BT - Electrochemical CO2 Reduction Facilitated by Manganese Bipyridine-Based Molecular Catalysts

PB - Aarhus University

ER -