Energetic Insight into the Formation of Solids from Aluminum Polyoxocations |
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Authors: | Dr Dana Reusser Prof William H Casey Prof Alexandra Navrotsky |
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Institution: | 1. Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, One Shields Ave., Davis, CA 95616 (USA);2. Department of Chemistry and Department of Earth and Planetary Sciences, University of California Davis (USA) |
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Abstract: | The ε‐Keggin AlO4Al12(OH)24(H2O)12]7+ ion (AlAl127+) 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, GaAl127+ and GeAl128+, using solution calorimetry. Rather than measuring the enthalpies of the MAl127/8+ ions directly from solution hydrolysis, we measured the metathesis reaction of the crystallized forms with barium chloride creating an aqueous aluminum solution monospecific in MAl127/8+. Then, the contributions to the heat of formation from the crystallized forms were subtracted using referenced states. When comparing the aqueous AlAl127+ 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. |
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Keywords: | Al13 aluminum hydroxide calorimetry Keggin ions thermodynamics |
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