Investigation of Room-Temperature Oxygen Adsorption by Bimetallic Iron–Cobalt Metal–Organic Framework-74

Metal–organic frameworks (MOFs) have emerged as promising candidates for selective gas adsorption due to their tunable structures, high porosity, and numerous open metal sites (OMSs). This research synthesizes and characterizes bimetallic Fe x Co 1– x -MOF-74 frameworks containing both iron (Fe) and cobalt (Co) metal centers to investigate and improve their oxygen (O 2 ) adsorption performances. Monometallic Fe-MOF-74 and Co-MOF-74, as well as bimetallic versions with Fe/Co ratios of 0.1:0.9, 0.5:0.5, and 0.9:0.1, were synthesized by a solvothermal method. A thorough characterization analysis was performed to verify their structural integrity, porosity, thermal stability, and metal composition. O 2 adsorption isotherms were assessed at 204, 273, and 298 K to analyze adsorption capacity and temperature-dependent characteristics. Cyclability experiments at 298 K examined the reusability of the materials during multiple adsorption–desorption cycles. Advanced characterization techniques, such as vibrating sample magnetometry (VSM), 57 Fe–Mössbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS), unveiled the possible adsorption mechanism. The findings indicate that monometallic Fe-MOF-74 has a significant initial O 2 adsorption capacity of 2.58 mmol/g at 298 K due to strong chemisorption at Fe sites. However, its cyclability is poor due to the irreversible oxidation of Fe 2+ to Fe 3+ at 298 K, resulting in a large drop after the first cycle. In contrast, Co-MOF-74 has a lower initial capacity of 0.36 mmol/g at 298 K, but demonstrates superior reversibility, maintaining consistent performance throughout several cycles owing to weaker, reversible physisorption. Among the bimetallic variants, Fe 0.5 Co 0.5 -MOF-74 achieves an optimum balance, exhibiting a moderate initial capacity of 0.82 mmol/g at 298 K and maintaining steady performance of around 0.65 mmol/g over seven cycles. The addition of Co stabilizes the Fe sites against oxidation and improves the material’s reusability. Mechanistic investigations demonstrate that Co mitigates Fe oxidation by facilitating reversible interactions and preventing the formation of irreversible Fe–O 2 bonds. The results underscore the promise of bimetallic MOFs as adaptable platforms for effective and sustainable O 2 capture, where the Fe/Co ratio can be adjusted to optimize adsorption capacity and cyclability.