Energetic Insight into the Formation of Solids from Aluminum Polyoxocations

Dana Reusser, William H. Casey, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

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. Downhill: Calorimetric measurements on the ε-AlAl127+ ion and its heterometal-substituted forms provide the first enthalpy of formation (ΔHf,el) values on these species in aqueous form independent of formation pathway. Data reveal the AlAl127+ ion lies closer in energy to solid aluminum (oxy)hydroxide phases than aqueous monomers, thus explaining its role as a precursor to amorphous aluminum hydroxides (flocs).

Original languageEnglish (US)
Pages (from-to)9253-9256
Number of pages4
JournalAngewandte Chemie - International Edition
Volume54
Issue number32
DOIs
StatePublished - Aug 1 2015
Externally publishedYes

Keywords

  • Al
  • Keggin ions
  • aluminum hydroxide
  • calorimetry
  • thermodynamics

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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