Nanoscopic Insight into Water Adsorption and Desorption in Commercial Activated Alumina by Positron Annihilation Lifetime Spectroscopy

Activated alumina is widely used in industry as an adsorbent. Its strong affinity toward water allows for the profound dehydration of gas streams. To optimize such processes, a deeper insight into water interaction with activated alumina is required. This knowledge can be obtained using positron annihilation lifetime spectroscopy, a sensitive tool that unravels previously unknown aspects of adsorption processes. Activated alumina (Compalox® AN/V-813) was subjected to such a study supported by detailed characterization using scanning electron microscopy, X-ray diffraction, and N2 adsorption-desorption. A complex porous structure of the material, consisting mainly of boehmite and η-Al₂O₃ or γ-Al2O3, was found. It is responsible for significant differences in adsorption and desorption. The course of adsorption is close to the classical layer-by-layer description. However, there are indications of initial water capture at active sites and final water reorganization consisting of filling the smallest free volumes that remain empty. The narrow mesopore inlets that keep water in the pores even at a relative vapor pressure of 0.4 are primarily responsible for the course of the desorption process. During adsorption, water is mainly maintained in the form of small clusters up to the highest pressures, whereas during desorption, it is continuous until narrow pore openings.