Influence of the Presence of Different Alkali Cations and the Amount of Fe(CN)(6) Vacancies on CO(2) Adsorption on Copper Hexacyanoferrates

The CO(2) 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 A(2/3)Cu[Fe(CN)(6)](2/3) with A = vacant, Li, Na, K, Rb, Cs. The Rb and Cs compounds show the highest CO(2) adsorption per unit cell, ~3.3 molecules of CO(2) 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 CO(2) 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.