Atomistic simulation of water adsorption and cation siting in polyoxoniobate materials

A new, transferable force field is developed for three α-Keggin polyoxoniobate materials with structural formulas of K12[Ti2O2][SiNb12O40] · 16H2O (K–Si–Ti), Na12[Ti2O2][SiNb12O40] · 4H2O (Na–Si–Ti), and Na10[Nb2O2][GeNb12O40] · 6H2O (Na–Ge–Nb). Water and cation siting is computed at various water loadings via replica exchange Monte Carlo (REMC) simulations. As water is introduced into the systems, the preferred adsorption sites are observed to change in response to cation redistribution. At low water loadings, cations prefer adsorption sites exhibiting high framework coordination. With an increase in water loading, secondary adsorption sites arise, where the cations are stabilized by water. Equilibrium water loadings are computed as a function of water partial pressure through grand canonical Monte Carlo (GCMC) simulations. The K–Si–Ti water loading at saturation is in excellent agreement with experimental single-crystal refinement data and thermogravimetry. The computed water uptake for Na–Si–Ti and Na–Ge–Nb is significantly higher than the amount of water found in powder X-ray diffraction refinements, but is in excellent agreement with thermogravimetric analysis. We conclude that powder diffraction refinement techniques have difficulty in determining water content, especially when water is highly mobile and weakly adsorbed.