Thermodynamic Characterization of the Hydroxyl Group on the γ‑Alumina Surface by the Energy Distribution Function

Controlling water adsorption on γ-alumina, i.e., the quantity and nature of the surface hydroxyl groups, is essential for adjusting the acidic/basic properties of the surface. IR and DFT studies have shown that different OH groups, each of them characterized by its own chemical environment and adsorption properties, can be present on the surface of γ-alumina. However, quantifying this surface heterogeneity and predicting the influence of the synthesis and activation conditions is still a challenging problem. In this paper, a detailed experimental study is conducted on a γ-alumina sample obtained by thermal decomposition of a commercial boehmite. Using a thermogravimetric setup, both water-adsorption equilibrium and desorption kinetics were acquired in a large range of controlled experimental conditions (1 Pa < partial pressure of water <1400 Pa ; 100 °C < temperatures <600 °C). The energy distribution function (EDF) of water-adsorption enthalpy is evaluated on the basis of the theories developed for strongly heterogeneous surfaces but so far never applied to the water vapor interactions with γ-alumina. The OH adsorption enthalpy range of the EDF, and the experimental OH contents, are in good agreement with density functional theory simulations, making a bridge between macroscopic and atomistic features. It is also shown that the EDF of the γ-alumina surface is a very powerful tool to predict the hydroxyl coverage as a function of the pretreatment history of the sample.