Abstract
The CO2 adsorption on various Prussian blue analogue hexacyanoferrates was evaluated by thermogravimetric analysis. Compositions of prepared phases were verified by energy-dispersive X-ray spectroscopy, infra-red spectroscopy and powder X-ray diffraction. The influence of different alkali cations in the cubic Fm3m structures was investigated for nominal compositions A2/3Cu[Fe(CN)6]2/3 with A = vacant, Li, Na, K, Rb, Cs. The Rb and Cs compounds show the highest CO2 adsorption per unit cell, ~3.3 molecules of CO2 at 20 °C and 1 bar, while in terms of mmol/g the Na compound exhibits the highest adsorption capability, ~3.8 mmol/g at 20 °C and 1 bar. The fastest adsorption/desorption is exhibited by the A-cation free compound and the Li compound. The influence of the amount of Fe(CN)6 vacancies were assessed by determining the CO2 adsorption capabilities of Cu[Fe(CN)6]1/2 (Fm3m symmetry, nominally 50% vacancies), KCu[Fe(CN)6]3/4 (Fm3m symmetry, nominally 25% vacancies), and CsCu[Fe(CN)6] (I-4m2 symmetry, nominally 0% vacancies). Higher adsorption was, as expected, shown on compounds with higher vacancy concentrations.
Highlights
Prussian blue analogues (PBAs) have recently received attention as potential candidates for CO2 gas adsorption
In this work we have investigated the CO2 adsorption capabilities on hexacyanoferrates
The results show that the present K, Rb, and Cs samples contain essentially only Fe2+, the Na sample a small amount of Fe3+ ; 5(2)%, but that the Li sample contains a significant fraction of Fe3+, 17(2)%, tentatively attributable to a too short time for reduction of all Fe to Fe2+
Summary
Prussian blue analogues (PBAs) have recently received attention as potential candidates for CO2 gas adsorption. The studies have mainly been on compounds M3 [Co(CN)6 ]2 and M3 [Fe(CN)6 ]2 with. M, a transition metal [1,2]. They have shown that PBAs can adsorb up to ~3.0 mmol/g of CO2. For CO2 capture and separation, adsorbers are sought that show rapid uptake, high capacity, high selectivity and stable cyclic performance [3]. We found in a recent study that PBAs K2x/3 Cu2+ [Fe3+ 1-x Fe2+ x (CN)6 ]2/3, with nominally K-free x = 0.0 and K-rich x = 1.0 satisfy these criteria [4]. The maximum CO2 uptake is for both compounds ~4.5 mmol/g at 1 bar and 0 ◦ C.
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