Porous Structural Properties of K or Na-Co Hexacyanoferrates as Efficient Materials for CO₂ Capture

The stoichiometry of the components of hexacyanoferrate materials affecting their final porosity properties and applications in CO₂ capture is an issue that is rarely studied. In this work, the effect that stoichiometry of all element components and oxidation states of transition metals has on the structures of mesoporous K or Na-cobalt hexacyanoferrates (CoHCFs) and CO₂ removal is reported. A series of CoHCFs model systems are synthesized using the co-precipitation method with varying amounts of Co ions. CoHCFs are characterized by N₂ adsorption, TGA, FTIR-ATR, XRD, and XPS. N₂ adsorption results reveal a more developed external surface area (72.69-172.18 m²/g) generated in samples containing mixtures of K⁺/Fe²⁺/Fe³⁺ ions (system III) compared to samples with Na⁺/Fe²⁺ ions (systems I, II). TGA results show that the porous structure of CoHCFs is affected by Fe and Co ions oxidation states, the number of water molecules, and alkali ions. The formation of two crystalline cells (FCC and triclinic) is confirmed by XRD results. Fe and Co oxidation states are authenticated by XPS and allow for the confirmation of charges involved in the stabilization of CoCHFs. CO₂ removal capacities (3.04 mmol/g) are comparable with other materials reported. CO₂ adsorption kinetics is fast (3-6 s), making CoHCFs attractive for continuous operations. Q_st (24.3 kJ/mol) reveals a physical adsorption process. Regeneration effectiveness for adsorption/desorption cycles indicates ~1.6% loss and selectivity (~47) for gas mixtures (CO₂:N₂ = 15:85). The results of this study demonstrate that the CoHCFs have practical implications in the potential use of CO₂ capture and flue gas separations.