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Energetic Insight into the Formation of Solids from Aluminum Polyoxocations
Author(s) -
Reusser Dana,
Casey William H.,
Navrotsky Alexandra
Publication year - 2015
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201503544
Subject(s) - aqueous solution , hydroxide , chemistry , standard enthalpy of formation , inorganic chemistry , aluminium , ion , metastability , hydrolysis , chloride , crystallization , calorimetry , amorphous solid , ion exchange , crystallography , organic chemistry , thermodynamics , physics
The ε‐Keggin [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ ion (AlAl 12 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, GaAl 12 7+ and GeAl 12 8+ , using solution calorimetry. Rather than measuring the enthalpies of the MAl 12 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 MAl 12 7/8+ . Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl 12 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.