Asymmetric Porphyrinoids with Proton and Electron Relays Immobilized on Carbon Materials as Efficient Catalysts for Electrochemical CO 2 Reduction

A critical and ongoing challenge is the development of catalysts that can convert carbon dioxide (CO2) into useful reduced products. The rational structural design of molecular catalysts has attracted increasing attention owing to their well-defined active sites. Herein, a ligand-tuned strategy is developed to enhance the catalytic performance and unveil the ligand effect of a series of carbon-immobilized Co(5-(4-hydroxyphenyl)-10,15,20-tris(3,4-dimethoxyphenyl)) porphyrin, Co(OMe)3THPP(1a) and its analogues on the carbon dioxide reduction (CO2RR). Experimental results indicate that Co(OMe)3THPP bearing electron and proton-donating substituents (hydroxyl and OMe) induces electronic localization at the Co site, which greatly enhances CO2 adsorption and activation. This work offers a strategy to regulate the electronic structure of active sites by designing ligands and discloses the directed ligand catalysis of tailored Co(OMe)3THPP composites and their analogues for a highly efficient CO2RR. The immobilization of Co(OMe)3THPP onto multiwalled carbon nanotubes results in enhanced electrocatalytic activity, with CO2 being selectively reduced to CO (>98%) at a low overpotential in an aqueous medium. This effect is ascribed to the particular environment created by the aqueous medium at the catalytic site of the immobilized catalyst that facilitates the adsorption and further reaction of CO2. As electrocatalysts at −1.0 V vs the reversible hydrogen electrode (RHE), asymmetric methoxy-substituted Co(II) porphyrin Co(OMe)3THPP (1a) shows a higher CO2 to CO Faradaic efficiency (FECO) of ∼85% and turnover frequency (TOF) of 7.7 s–1(2.8 × 104 h–1) due to the electron- and proton-donating ability of different substituents. Immobilization of Co(OMe)3THPP(1a) onto carbon nanotubes results in a remarkable enhancement of the electrocatalytic abilities, with CO2 being selectively reduced to CO(>98%) at an overpotential of −1.0 V with a turnover frequency of 14.4 s–1(5.2 × 104 h–1) in an aqueous medium. This work highlights the significance of understanding the role of different ratios of proton and electron donors in CO2 reduction in aqueous media.