Physicochemical Mechanism for the Continuous Reaction of γ‐Al₂O₃‐Modified Aluminum Powder with Water

Abstract

Previous experiments showed that γ‐Al2O3‐modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH− ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH− ions react with Al and release H2. Because of the limited H‐soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H2 molecules accumulate and form small H2 gas bubbles at the Al:Al2O3 interface. When the reaction equilibrium pressure in H2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as‐received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H2 bubbles decrease as well.

Effect of Water Vapor on the Spallation of Thermal Barrier Coating Systems During Laboratory Cyclic Oxidation Testing

“…In another study dealing with aluminum powder, Deng et al [14][15][16] showed that water is in fact more reactive with the gamma alumina/aluminum system than with pure aluminum. It was shown that aluminum powder covered with cracked gamma alumina reacts with water to produce bohemite (AlOOH) and hydrogen gas, following the neutral reaction:…”

Effects of solvation shells and cluster size on the reaction of aluminum clusters with water

“…[1][2][3][4][5][6][7] Although the reaction, 2Al + 6H 2 O → 2Al(OH) 3 + 3H 2 , is exothermic, formation of an aluminum-oxide layer on the aluminum surface prevents continuous reaction. 8 In order to overcome this bottleneck, continual removal of the oxide layer has been attempted using various promoters such as hydroxides, 5 oxides, 9 and salts. 10 Unfortunately, none of these techniques has achieved a sufficiently fast rate of H 2 production for commercialization.…”

Flower-shaped ALOOH nanostructures synthesized by the reaction of an AlN/Al composite nanopowder in water

“…Typically, alumina nanoparticles are produced by a sol-gel method [9], hydrothermal method or mechanosynthesis. New methods for synthesizing nano-alumina have been developed, such as precipitation from ammonium carbonate aqueous solution and aluminum nitrate ethanol solution [10], flame spray pyrolysis [11], and high-gravity reactive precipitation [12]. Nano-sized aluminum oxyhydroxide is generated by the reaction of aluminum or aluminum nitride powders with water [13][14][15].…”