Hydrogen production by reacting water with mechanically milled composite aluminum-metal oxide powders

Several composite aluminum-metal oxide powders were prepared by mechanical milling and considered for hydrogen production in the Al–water split reaction. The powders included compositions capable of independent, highly exothermic thermite reaction between components: Al·MoO3, Al·Bi2O3, and Al·CuO, as well as chemically inert compositions Al·MgO and Al·Al2O3. Experiments used a water displacement method to quantify hydrogen production. In most experiments, the flask containing water and composite powder was maintained at 80 °C; additional limited experiments were performed at varied temperatures. Condensed reaction products were collected and examined using electron microscopy and X-ray diffraction. For all compositions, the aluminum–water split reaction was nearly complete. Average reaction rates were comparable to those reported earlier for materials with similar particle sizes prepared by ball milling. Reaction rates were affected by the specific composition; the Al·Bi2O3 composite reacted substantially faster than other materials. It was observed that the Al–water split reaction initiated at 80 °C could be completely stopped by cooling the reacting flask to room temperature; the reaction did not restart at room temperature but could be resumed at its previous rate by heating the flask back to 80 °C. For Al·MoO3 composite, an interruption in the hydrogen production was observed at a constant temperature; it was associated with the formation of MoO2.4(OH)0.6, a hydrated MoO3 phase. Evidence of thermite reactions interfering with the Al–water split reaction and generating metallic Bi and Cu was obtained for experiments with Al·Bi2O3, and Al·CuO composites, respectively. A qualitative reaction mechanism is proposed assigning different rate controlling processes to different stages of the Al–water split reaction for the composites prepared by ball milling.