Heat capacity and thermodynamic functions of γ-Al2O3

Alumina (Al2O3) can be synthesized with a variety of crystalline structures, each with unique physical properties. Although the corundum phase (alpha-Al2O3) has been well studied, the other phases of alumina have had limited attention over the years. One of the low-density phases of alumina, gamma-Al2O3, is important in a variety of technical applications, largely because of its large surface area, pore volume, and high thermal stability when compared to mesoporous silicas. The mesoporous structure of gamma-Al2O3 causes it to be hygroscopic, and samples that are calcined at different temperatures can have widely varying amounts of water adsorbed to their surfaces as well as subtle changes in structure. We have measured the constant pressure heat capacities of four gamma-Al2O3 samples that were calcined at (300, 600, 900, and 1100) degrees C and have the chemical formulas Al2O3.1.540H(2)O, Al2O3.0.811H(2)O, Al2O3.0.537H(2)O, and Al2O3.0.204H(2)O, respectively. Molar heat capacities were measured from 1.8 to 300 K using a Quantum Design Physical Property Measurement System (PPMS), and the data was fit to a sum of theoretical functions below 15 K, orthogonal polynomials from 101(to 60 K, and a combination of Debye and Einstein functions above 50 K. These fits were then used to generate C-p,C-m degrees, Delta S-T(degrees)m degrees, Delta H-T(degrees)m degrees, and Phi(m)degrees values at smoothed temperatures from 0 K to 300 K for all samples. The differences in the thermodynamic functions for the samples is attributed to the differing amounts of water adsorbed to the surfaces and the corresponding change in the strength of water interactions with the surface. (C) 2017 Elsevier Ltd.