Analysis of Anhydrous and Hydrated Surface Energies of gamma-Al₂O₃ by Water Adsorption Microcalorimetry

Abstract

Surface energies have postulated importance to catalysis, crystal growth, sintering, polymorphism, and many other fields. This importance is even more critical when dealing with nanostructured materials, where the surface-to-volume ratio is considerably higher and the surface term accounts for a much larger fraction of the total free energy. Here we present a novel approach to experimentally assess the average anhydrous and hydrated surface energies of oxides, and used the method to determine the surface energy of gamma-alumina. The method uses a water adsorption setup combined with a microcalorimeter where the heat of adsorption can be monitored as a function of the adsorbed amount. Maintaining a closed system, the approach enables the correlation of the molecular configuration of absorbed water with the thermodynamic data, and hence the definition of the point where a liquid water configuration exists (at high relative humidity). This information allows the calculation of the surface energy at room temperature for any coverage state by using the adsorption calorimetric data. For gamma-alumina, a close relationship between the water adsorption behavior and the surface energy was observed, evidencing that higher surface energies are associated with highly energetic dissociative behaviors of water and a continuous surface energy decrease upon water adsorption. Three adsorption stages were clearly observed from the combination of adsorption isotherm and microcalorimetric data, consistently with presented models.

Surface and grain boundary energies of tin dioxide at low and high temperatures and effects on densification behavior

“…At this point, the second derivative of the adsorption isotherm curve is zero. 17,29 The corresponding coverage is 10.13 H 2 O nm À2 for P 1 and 5.68 H 2 O nm À2 for P S ; the lower coverage of P S is in good agreement with the observation on isotherm curves that the transition region begins a small coverage on P S . By using Eq.…”

“…Fully oxidized samples were compared with those quenched from sintering conditions, and interface energies (both surface and grain boundary) measured by both hightemperature oxide melt drop solution calorimetry [10][11][12][13][14][15][16] and water adsorption microcalorimetry. 17 In addition, experimental assessment on the heat of adsorption of water on the surface of the SnO 2 is reported and correlated with recent theoretical calculations. A discussion on the thermodynamic versus kinetic effects on the sintering of SnO 2 is finally presented.…”

“…17 In this method, the heat of adsorption of water molecules on SnO 2 nanoparticles (DH ads , kJ mol À1 ) is monitored as a function of the water coverage (adsorbed water amount on the surface). While one may expect a strong interaction between the surface and the water molecules at low coverage (close to anhydrous state), with a high enthalpy of adsorption, as coverage increases the energy of adsorption decreases.…”

“…The instruments and methodology are described in more detail by Ushakov et al 22 and Castro and Quach. 17 All samples were degassed with equivalent heat treatment to the BET method, and evacuated for 3 h at 25°C before the water adsorption experiment.…”

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

“…The low SE of the layered structure CaMnO materials (significantly smaller than for Mn 3 O 4 , Mn 2 O 3 , or MnO 2 ) suggests that water is not strongly bound to the surface sites, because, for many oxides, the enthalpy of H 2 O chemisorption becomes more exothermic with increasing SE (28,(34)(35)(36). In comparing TiO 2 and SnO 2 (rutile structure), Ma et al (35) have argued that the better gas-sensing ability of the latter stems from a lower surface energy, lower water coverage on the surface, and weaker binding of surface water, enabling energetically easier access of other gas molecules to the SnO 2 surface.…”