The formation of Al2O3 nanoparticles in the oxidation of aluminum by water under sub- and supercritical conditions

Massive aluminum samples were oxidized by sub- and supercritical water with the formation of (AlOOH)n and (Al2O3)n nanoparticles. The release of H2 began at 523 K when the reagents were heated uniformly to 700 K. The time lag of the beginning of oxidation was 140 s when supercritical water was injected into a reactor with aluminum samples at 665 K and 23.1 MPa. Oxidized aluminum powders were analyzed using a transmission electron microscope. Predominantly large (300–400 nm) α-Al2O3 particles were formed when supercritical water was injected into a reactor with aluminum. Smaller (20–50 nm) γ-Al2O3 particles were also observed in samples oxidized by water vapor under temperature increase conditions. Kinetic equations describing the rate of H2 formation in the reaction of H2O with aluminum were obtained. Possible nanostructuring mechanisms are discussed.