Mechanism and kinetics of Al 2O 3 nanoparticles formation by reaction of bulk Al with H 2O and CO 2 at sub- and supercritical conditions

Abstract

Oxidation of bulk samples of 〈Al〉 by water and H 2O/CO 2 mixture at sub- and supercritical conditions for uniform temperature increase and at the injection of H 2O (665 K, 23.1 MPa) and H 2O/CO 2 (723 K, 38.0 MPa) fluids into the reactor has been studied. Transition of 〈Al〉 into AlOOH and Al 2O 3 nanoparticles has been found out. Aluminum samples oxidized by H 2O and H 2O/CO 2 fluids at the injection mostly consist of large particles (300–500 nm) of α-Al 2O 3. Those oxidized for uniform temperature increase contain smaller particles (20–70 nm) of γ-Al 2O 3 as well. Mechanism of this phenomenon is explained by orientation of oxygen in H 2O polar molecules to the metal in the electric field of contact voltage at Al/AlOOH and Al/Al 2O 3 boundary. Addition of CO 2 to water resulted in CO, CH 4, CH 3OH and condensed carbon, increase in size of Al 2O 3 nanoparticles and significant decrease in time delay. In pure CO 2 〈Al〉 oxidation resulted in oxide film. Using temperature and time dependences of gaseous reactant pressure and Redlich–Kwong state equation, kinetics of H 2 formation has been described and oxidation regularities determined. At aluminum oxidation by H 2O and H 2O/CO 2 fluids, self-heating of the samples followed by oxidation rate increase has been registered. The samples of oxidized aluminum have been studied with a transmission electronic microscope, a thermal analyzer and a device for specific surface measurement. The effect of oxidation conditions on the characteristics of synthesized nanoparticles has been found out.