Regulating the Spin State of Nickel in Molecular Catalysts for Boosting Carbon Dioxide Reduction

Molecular catalysts have been extensively studied for fundamental understanding of the catalytic mechanism of specific active sites for carbon dioxide (CO2) reduction, and nitrogen-coordinated transition metal centers are considered active sites for high-efficiency catalysis. However, different spin states of the same metal atom have rarely been studied. In this study, a hydrazine-pretreatment strategy is presented toward high-spin-state nickel centers in nickel(II) phthalocyanine and porphyrin. Such high-spin-state nickel centers possess abundant unpaired 3d electrons and exhibit considerably enhanced activity for CO2 reduction. The high-spin-state nickel(II) phthalocyanine displays a higher CO faradaic efficiency than that of ground-state nickel(II) phthalocyanine (98.5% vs 93.2% at −0.7 V; the ratio of CO and H2 generated by high-spin-state nickel(II) phthalocyanine is approximately 5 times that generated by ground-state nickel(II) phthalocyanine). And the CO faradaic efficiency of nickel porphyrin increased from approximately 0% (ground state) to 25.0% (high spin state). Operando X-ray absorption fine structure analysis demonstrates that the high-spin-state nickel exhibits a stronger adsorption of CO2 and intermediates, which is beneficial to CO2 reduction. Theoretical calculations reveal that the high-spin-state nickel center shows a smaller highest–lowest occupied molecular orbital gap, which favors the production of the key intermediate *COOH, thus accelerating CO2 reduction.