Energetic Insight into the Formation of Solids from Aluminum Polyoxocations

Abstract

The ε-Keggin [AlO4Al12(OH)24(H2O)12](7+) ion (AlAl12(7+)) is a metastable precursor in the formation of aluminum oxyhydroxide solids. It also serves as a useful model for the chemistry of aluminous mineral surfaces. Herein we calculate the enthalpies of formation for this aqueous ion and its heterometal-substituted forms, GaAl12(7+) and GeAl12(8+), using solution calorimetry. Rather than measuring the enthalpies of the MAl12(7/8+) ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl12(7/8+). Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl12(7+) ion to solid aluminum (oxy)-hydroxide phases, we found that this ion lies closer in energy to solid phases than to aqueous aluminum monomers, thus explaining its role as a precursor to amorphous aluminum hydroxide phases.

Chemical trigger toward phase separation in the aqueous Al(III) system revealed

“…It should be noted that with respect to the PNC pathway, Keggin-Al 13 ions should be considered phase-separated nanosolids occurring as later intermediates in the nucleation mechanism that undergo further aggregation processes due to their high structural stability and slow dynamics (20). Energetic calculations indicating that Keggin-Al 13 species lie close in energy to the solid Al-(oxy)hydroxide phase corroborate this interpretation (21).…”

Systematic Study of Aluminum Nanoclusters and Anion Adsorbates

“…Another way to extend the work presented here is to compute the energetics and thermodynamics associated with the chemical reactions of Al nanoclusters, such as formation, dissociation, and cation exchange, which have been detailed in recent experimental studies. ,,, One approach to calculating these experimentally verifiable quantities is a first-principles thermodynamics analysis, which combines DFT-computed total energies and chemical potentials to map out the stability of different bulk and surface phases. , A more recent Hess’ law approach, developed by Rong and Kolpak, combines DFT total energies and tabulated Δ G SHE 0 values to form a Δ G expression that includes experimentally tunable parameters such as the concentration, pH, and applied potential.…”

“…Thus, efforts to obtain a molecular-level understanding of environmental nanoclusters often employ representative structural models. The Keggin isomers, and related materials, ,,− are a well-known example set of geochemical models that share many structural features with naturally occurring nanomaterials.…”

Aggregation and Dissociation of Aqueous Al13 Induced by Fluoride Substitution

“…) polymer has been demonstrated thermodynamically to be a crucial intermediate species in the transformation of aqueous Al to Al hydroxide precipitates. 7,8 With multiple coordinated hydroxyl groups and high charges, the ε-K Al 13 7+ structure includes 12 edge-shared AlO 6 octahedra in four sets of triple linked trimers around a central AlO 4 tetrahedron. 9,10 The ε-K Al 13 7+ polymer has been detected in watersheds polluted by mine drainage and acid rain 11,12 and has a toxicity that is rather general and more virulent to plants and fishes than Al monomers.…”