Heat capacities and thermodynamics of formation of ε-Keggin MAl12 Selenates (M = Al(III), Ga(III), or Ge(IV))

Abstract

a b s t r a c tWe report enthalpies of formation from the elements and oxides, D f H = À (22,673.44 ± 30.19) kJ Á mol À1 and D f;ox H = À(869.68 ± 28.75) kJ Á mol À1 , for [GeO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (GeAl 12 ) measured using high temperature oxide-melt solution calorimetry. This material is the selenate salt of the germanium-substituted polynuclear Al 7þ 13 ion in the e-Keggin structure. We also report heat capacities from temperatures of (2 to 300) K on this GeAl 12 selenate and its other heterometal substituted forms, Na[AlO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (AlAl 12 ) and Na[GaO 4 Al 12 (OH) 24 (H 2 O) 12 ](SeO 4 ) 4 Á12H 2 O (cr) (GaAl 12 ), measured using a Quantum Design Physical Property Measurement System (PPMS). These measurements were used to calculate entropies and subsequently free energies of formation for these three materials. All three MAl 12 selenates, where M = Al(III), Ga(III), or Ge(IV), have similar heat capacities from temperatures (2 to 300) K and similar characteristic Debye temperatures, H D , suggesting similar lattice vibrational densities of states.

Heat capacities and thermodynamics of formation of flat-Al13 nitrate – [Al13(OH)24(H2O)24](NO3)15·11H2O

“…Here we continue our work on the thermodynamic properties of aluminum hydroxide clusters [18,19] by studying this different class of aluminum polyoxocation. We report the enthalpy of formation and low crystallized with nitrate counterions in the triclinic space group, P 1 (figure 1).…”

“…The enthalpy of solution, D sol H, of the [Al 13 (l 3 -OH) 6 (l 2 -OH) 18 (H 2 O) 24 ](NO 3 ) 15 Á11H 2 O (cr) (flat-Al 13 ) sample and reference materials were measured using acid solution calorimetry according to previously reported methods [19,20]. Briefly, solid samples were formed into pellets and dropped from room temperature into hydrochloric acid (5.00 g, 16.9%) held isothermally at T = 301 K. The calorimeter was calibrated using the measured heat of solution of KCl (NIST SRM 1655) in deionized water, and a consistent final molality of each run was ensured.…”

“…The second main class contains clusters consisting solely of distorted octahedrally coordinated aluminum ions and hydroxyl linkages arranged analogously to those in a hexagonally close-packed layered hydroxide structure such as brucite [4]. These species increase in nuclearity and charge in a ring-like manner from [Al 8 (l 3 -OH) 3 (l 2 -OH) 12 (H 2 O) 18 ] 10+ (aq) [5] to [Al 13 (l 3 -OH) 6 (l-OH) 18 (H 2 O) 24 ] 15+ (aq) [6][7][8][9][10] (figure 1) and are termed ''flat'' structures, reflecting their more sheet-like topology compared to the Keggin Al 13 ions. These clusters have proven to be useful experimental models for understanding aqueous exchange reactions at the atomic level, as they expose similar (aluminum + oxygen) coordination chemistries found in soil minerals [11].…”

“…These clusters have proven to be useful experimental models for understanding aqueous exchange reactions at the atomic level, as they expose similar (aluminum + oxygen) coordination chemistries found in soil minerals [11]. Until recently the [Al 13 (l 3 -OH) 6 (l-OH) 18 (H 2 O) 24 ] 15+ (aq) (flat-Al 13 ) polynuclear aluminum hydroxide ions could not be synthesized in high yield or with fast reaction times [7,8,10]; thus they have often been omitted from speciation studies and relatively few reports are currently available on them, e.g. [4,[12][13][14].…”

“…In a companion study [18], we reported complete thermodynamic characterization of the heterometal-substituted e-Keggin MAl 12 selenates (M = Al III , Ga III or Ge IV ), including standard enthalpies, entropies, and free energies of formation. Here we continue our work on the thermodynamic properties of aluminum hydroxide clusters [18,19] by studying this different class of aluminum polyoxocation.…”