Distinct ionic adsorption sites in defective Prussian blue: a 3D-RISM study

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Chemistry

Background:

• Ferric hexacyanoferrate (FeHCF), also known as Prussian blue (PB), is a material with selective alkali ion adsorption properties.
• Its unique structure offers potential for diverse applications, particularly in ion separation and storage.
• Understanding ion and water interactions within PB is crucial for optimizing its performance.

Purpose of the Study:

• To investigate the configurations and distributions of alkali ions (Li+, Na+, K+, Cs+) and water molecules within defective Prussian blue (d-PB).
• To elucidate the differences in ion-water interactions based on ion size.
• To quantify the dehydration effects associated with alkali ion adsorption in d-PB.

Main Methods:

• Utilized the statistical mechanics of molecular liquids to model ion and water behavior.
• Solved the three-dimensional reference interaction site model (3D-RISM) equation for d-PB systems in alkali chloride solutions.
• Analyzed the resulting three-dimensional distribution functions to determine ion and water configurations.

Main Results:

• Observed distinct ion-water configurations and distributions for small (Li+, Na+) versus large (K+, Cs+) alkali ions.
• Determined that Li+ and Na+ adsorb at off-center, diagonal positions within the d-PB lattice.
• Found that K+ and Cs+ adsorb at the center of the d-PB unit cell.
• Quantified dehydration: Li+ and Na+ adsorption caused no water exchange, while K+ and Cs+ adsorption removed two and three water molecules, respectively.

Conclusions:

• The adsorption site and dehydration behavior of alkali ions in defective Prussian blue are strongly dependent on ion size.
• Small alkali ions (Li+, Na+) exhibit minimal disruption to the water structure upon adsorption.
• Larger alkali ions (K+, Cs+) induce significant dehydration, suggesting a more substantial interaction with the PB framework.