Co-based molecular catalysts for efficient CO2 reduction via regulating spin states

• Published in: Applied catalysis. B, Environmental Environmental, 2021-08, Vol.290, p.120067, Article 120067
• Main Authors: Kong, Xiangdong, Ke, Jingwen, Wang, Zhiqiang, Liu, Yan, Wang, Yibo, Zhou, Weiran, Yang, Zhengwu, Yan, Wensheng, Geng, Zhigang, Zeng, Jie
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Catalysts; Co-based molecular catalysts; CO2 activation; CO2 electroreduction; Cobalt; Density functional theory; Electron spin; Electron states; Electron transfer; Electrons; Spin state; Transition metals
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2021.120067

[Display omitted] •We synthesized Co-salophen with halogen as peripheral substitution.•The introduction of halogen atom into Co-salophen regulated the spin state of Co.•Co-salophen-Br with the highest spin state exhibited the highest activity for the formation of CO.•Mechanistic studies revealed that Co sites with high-spin state facilitated CO2 activation. A typical mode of CO2 activation is that d electrons at the d orbital of transition metals transfer to the unoccupied π* orbital of CO2. Thus the exploration of the relationship between d-electron behaviors and CO2 activation is of great importance. Herein, we demonstrate that high-spin state of 3d electrons in Co2+ facilitated the activation of CO2 over Co-salophen-X (X represents to Cl, Br, or I). Among these catalysts, Co-salophen-Br exhibited the highest Faradaic efficiency for CO. Notably, the Faradaic efficiency for CO over Co-salophen-Br reached 98.5 % at −0.70 V versus reversible hydrogen electrode, which was 1.5 and 1.2 times as high as those over Co-salophen-Cl (64.8 %) and Co-salophen-I (81.8 %), respectively. Density functional theory calculations revealed that high-spin state of Co sites decreased the reaction energy barrier for the formation of CO. Based on the analysis of electronic state, the ratio of high-spin state was 65.6 % for Co-salophen-Br, which was the highest among the three Co-based molecules. The Co sites with high-spin state promoted the electron transfer from high-energy 3d orbital (3dz2 and 3dx2-y2) of Co to the unoccupied π* orbital of CO2, improving catalytic performance.