Observation of O<sub>2</sub> Molecules Inserting into Fe–H Bonds in a Ferrous Metal–Organic Framework
Haolin Zhu, Jia‐Run Huang, Feifei Zhang, Pei‐Qin Liao, Xiao‐Ming Chen
Abstract
Exploring the interactions between oxygen molecules and metal sites has been a significant topic. Most previous studies concentrated on enzyme-mimicking metal sites interacting with O 2 to form M–OO species, leaving the development of new types of O 2 -activating metal sites and novel adsorption mechanisms largely overlooked. In this study, we reported an Fe(II)-doped metal–organic framework (MOF) [Fe 3 Zn 2 H 4 (bibtz) 3 ] ( MAF-203, H 2 bibtz = 1 H,1’ H -5,5′-bibenzo[ d ][1,2,3]triazole), featuring an unprecedented tetrahedral Fe(II)HN 3 site. This MOF exhibits selective adsorption behavior for O 2 from air, achieving an O 2 /N 2 separation selectivity of up to 67.1. Breakthrough experiments confirmed that MAF-203 can effectively capture O 2 from the air even under a high relative humidity of 60%. X-ray absorption spectroscopy, in situ diffuse reflectance infrared Fourier transform spectra, and ab initio molecular dynamics simulations were utilized to monitor the O 2 loading process on the Fe(II)HN 3 site. Interestingly, O 2 molecules could insert into the Fe–H bonds of the tetrahedral Fe II HN 3 sites, forming Fe III –OOH species (instead of the commonly observed Fe–OO species) with an ultrahigh adsorption enthalpy of −99.2 kJ mol –1 . Consequently, the O 2 capture behavior of MAF-203 enables efficient electrochemical 2e – oxygen reduction for the production of H 2 O 2 with air as the feedstock. Specifically, in a solid-state electrolyte electrolyzer without any liquid electrolyte, MAF-203 achieved selective O 2 capture and continuous production of medical-grade H 2 O 2 (3.2 wt %) solution without salts for 70 h, with performance comparable to that under pure O 2 conditions. The O 2 adsorption and activation mechanisms inaugurate a fresh chapter in grasping the interaction between O 2 molecules and metal sites.