CO₂ Capture on Prussian Blue Analogues (PBAs): Modeling Equilibrium, Kinetics, and Isosteric Heats of Adsorption

The demand to alleviate climate change produced by greenhouse gases, such as CO₂, is a fact at present. The use of porous adsorbents such as Prussian blue analogues (PBAs) possessing fast removal velocities and improved uptake capacity properties has shown promise as an alternative in the solution to this problem. In this work, the structural characterization and CO₂ adsorption data of a potassium cobalt hexacyanoferrate (CoHCFM11) are presented. FTIR demonstrates the presence of a mixture of two chains, Fe³⁺-CN-Co²⁺ and Fe²⁺-CN-Co³⁺, produced during the synthesis procedure. XRD reveals a face-centered cubic crystal, spatial group <i>Fm</i>3̅<i>m</i>, and corroborates the presence of cavities ideal to capture CO₂ molecules. XPS shows the existence of two oxidation states of Co and Fe in PBA. TEM measurements confirm the cubic shape nature of CoHCFM11 with an average size of 80-130 nm. A deep understanding of the CO₂ adsorption mechanism by CoHCFM11 is obtained by mathematical models. The validation of models in predicting experimental isotherm and kinetic adsorption data sets is presented. The goodness of fit is collated through the residual and normal probability plots to discriminate the appropriate model. Normal probability plots confirm the best stability of the 2-S Langmuir model in agreement between the measurements and model. Kinetic data are fitted using five models. Goodness-of-fit statistics confirm that Avrami's model better explains the data. Isosteric heats of adsorption are obtained through the Clausius-Clapeyron equation using the isosteres approach to which the heats obtained via the different isotherm model predictions are compared. The 2-S Langmuir model predicts closely the results obtained via the isosteres approach. The results of this study demonstrate the need to use goodness-of-fit statistics when selecting a model to recommend as the most appropriate in CO₂ adsorption data.