Al-water Reaction Research Articles

Alumina Hydrate Polymorphism Control in Al–Water Reaction Crystallization by Seeding to Change the Metastable Zone Width

The polymorph of alumina hydrate from Al–H2O reaction crystallization is hard to control due to its complexity. In this study, seeding is adopted to control the polymorphism by changing the metastable zone width (MSZW) which provides the easiest way to quantify the system stability. First, the influence of seed amount on the MSZW and polymorph was investigated. The MSZW declines with the increase of seed loading, and the critical seed loading for obtaining single gibbsite phase products is determined to be 15 g/L. Then the effect of seeding time on the polymorph indicates that the seed should be added before nucleation occurrence in order to control polymorphism effectively. The MSZW and polymorph of both seed-free and seeded Al–H2O reaction crystallization is controlled by changing the temperature and alkali concentration. It is found that if the critical concentration is lower than the saturation solubility curve of bayerite in the diagram, single gibbsite phase crystals can be obtained. Besides, the me...

Kinetics study of the Al–water reaction promoted by an ultrasonically prepared Al(OH)3 suspension

The hydrogen-generation rate of Al in a Al(OH)3 suspension is comparable to the behavior of Al in a NaOH solution.

Clarification of activation mechanism in oxide-modified aluminum

Previous works showed that oxide-modified Al powder could react with water and generate hydrogen under ambient condition. The activation mechanism was believed to be the weakening of passive oxide film on Al particle surface due to all its constituents transforming into loose γ-Al2O3 phase during the heat-treatment. In this work, a model experiment was performed, in which pure Al powder and the mixture of Al + γ-Al2O3 powders were heat-treated in vacuum from room temperature to 600 °C. It was found that the induction time for the reaction of heat-treated pure Al powder with water decreased obviously when the heat-treating temperature was above 200 °C. However, the reaction induction time of the mixture of Al + γ-Al2O3 powders was always considerably shorter than that of pure Al powder even if no heat-treatment was done. This implies that the catalytic effect of external oxides on the hydration of passive oxide film on Al particle surface plays a dominant role in promoting the Al-water reaction and hydrogen-generation. This clarification is also helpful in understanding the corrosion behavior of metal Al in different environment.

Hydrogen generation by the reaction of Al with water promoted by an ultrasonically prepared Al(OH)3 suspension

A high-activity Al(OH)3 suspension is prepared by the reaction of Al with water using an ultrasonic procedure. The above Al(OH)3 suspension could considerably promote the Al-water reaction and hydrogen-generation under ambient condition. This Al(OH)3 suspension has a good stability in air and its activity increases with decreasing the particle sizes of Al powder to prepare it. The mechanism analyses reveal that the Al(OH)3 particles in ultrasonically prepared Al(OH)3 suspensions are very fine, which could effectively dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces, speeding up the Al-water reaction. As the present Al(OH)3 suspension is chemically neutral and there is no special treatment for Al powder, the present method provides a viable way to generate hydrogen for portable application.

Effect of initial gas pressure on the reaction of Al with water

The effect of initial gas pressure on Al–water reaction was investigated systematically. It was found that there is a non-monotonic relationship between the initial pressure and the induction time for the beginning of Al–water reaction. The induction time decreases with decreasing the initial pressure at the first stage from atmospheric pressure, then reaches a minimum, and finally increases with further decreasing the initial pressure. The mechanism analyses revealed that the special vacuum pressure corresponding to the minimum induction time is close to the saturated vapor pressure, below which probably there are some vapor bubbles passing or adsorbing on Al particle surfaces due to boiling, retarding the hydration process of Al surface oxide film and increasing the reaction induction time. The present results imply that a suitable initial gas pressure should be chosen for Al–water reaction to generate hydrogen.

Effect of Ti on the microstructure and Al–water reactivity of Al-rich alloy

Ti-bearing Al alloys (0.1–1 wt.%) were prepared using arc melting techniques. Their microstructures were investigated using XRD and SEM/EDX, and found to depend strongly on Ti contents. Al grains are columnar as Ti contents are lower, but they are refined and turn into equiaxed ones when Ti contents are higher. The particle sizes of Ga–In–Sn phase decrease with Al grain refinement. Al–water reactivities were also investigated under different water temperatures. Kinetic measurements found that Ti prohibits Al–water reaction and reduces hydrogen yields when alloys contain little Ti. However, Al reacts with water fast and hydrogen yields rise with the increase of Ti contents of alloys. Reasons concerning the variations of microstructures and Al–water reactivities with Ti additions are discussed.

Hydrogen generation by the reaction of Al with water using oxides as catalysts

Hydrogen generation by the reaction of pure Al powder in water with the addition of Al(OH)3, γ-Al2O3, α-Al2O3, or TiO2 at mild temperatures was investigated. It was found that the reaction of Al with water is promoted and the reaction induction time decreases greatly by the above hydroxide and oxides. X-ray diffraction analyses revealed that the hydroxide and oxide phases have no any change during the Al–water reaction, indicating that they are just as catalysts to assist the reaction of Al with water. A possible mechanism was proposed, which shows that hydroxide and oxides could dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces. Copyright © 2013 John Wiley & Sons, Ltd.

Effect of trace species in water on the reaction of Al with water

The Al–water reaction has attracted considerable attention in the past few years, because it is an economically viable way to supply hydrogen for portable and kW-grade fuel cells. The water type is found to have a significant impact on Al–water reaction. In this work, the effect of trace species in water on Al–water reaction is investigated systematically. It is found that the trace organic acids originated from the decay of botanies and animals in nature and trace F− ions, rather than other cations and anions, play a key role in the reaction dynamics of Al with water. The mechanism analyses reveal that the organic acids and F− ions form complexes with aluminol groups on Al particle surfaces to impede the hydration process of Al surface oxide film and retard the reaction of Al with water. The present results imply that a suitable water should be chosen for the Al–water reaction to generate hydrogen. At the same time, the organic acids and trace F− ions could be used as the agents to prevent undesirable Al–water reaction.

Modifying aluminum–water reaction to generate nano-sized aluminum hydroxide particles beside hydrogen

Al–water reaction is used to produce nanoparticles of aluminum hydroxide. Ball-milling of aluminum powder in water was examined for promoting this reaction. Scanning electron microscopy and X-ray diffraction assisted by Rietveld refinement were used to study the by-products of the reaction. Results showed that the activated Al powder completely reacted with water at room temperature. Hydrogen, gibbsite and bayerite were formed at the end of the reaction. A mechanism is proposed to explain how wet milling of Al powder can promote the reaction. The morphology of the by-products changed with the milling time. Notably the samples milled for 7 and 12 h had the shapes of nanospheres, nanospindles and nanorods. The calcination process made in conventional furnace and microwave oven changed the morphology of the produced alumina phases.

Effect of structural evolution of aluminum powder during ball milling on hydrogen generation in aluminum–water reaction

Effect of milling time on structure of pure Al powder and consequently on efficiency of Al-water reaction and hydrogen production is studied. Progress in milling process is studied considering changes in morphology, size, lattice imperfections and grains orientation of Al particles. Morphology of particles before and after the reaction is studied to establish a role of size and shape of particles on kinetics of the reaction. It is demonstrated that the formation of interlayer spaces within the investigated Al particles effectively increases hydrogen yield. Moreover, prolonged ball milling eliminates the interlayer spaces and decreases the rate of reaction of water with the powders.

Reaction of Al powder with water for hydrogen generation under ambient condition

Hydrogen-generation by the reaction of pure Al powder with deionized water under ambient condition was investigated systematically. The results showed that both nano- and micro-sized Al powders could react with water and generate hydrogen under ambient pressure at mild temperature. The nanometer Al powder almost completely reacted with water at 20 °C, most of micrometer Al powders could react with water at a temperature of >40 °C, implying that small-sized Al powder could directly be able to generate hydrogen without any activation or modification. A shrinking core model was used to analyze the reaction progress, indicating that the Al-water reaction was controlled by the surface chemical mechanism at the initial stage, and then by the H2O molecule diffusion in the byproduct layer afterwards. The activation energy was calculated, which increased from 64.2 kJ/mol to 88.7 kJ/mol with increasing the Al particle sizes from 98.38 nm to 24.94 μm. X-ray analyses revealed that the reaction byproducts are bayerite, boehmite or a mixture of them, depending on the reaction temperature. The reaction dynamics mechanisms are discussed.

Role of Modification Agent Coverage in Hydrogen Generation by the Reaction of Al with Water

Previous works indicated that Al particle surfaces could be modified by fine γ‐Al2O3 grains, which can be used as a hydrogen‐generation material, where the fine γ‐Al2O3 grains were produced by the decomposition of the Al(OH)3 phase in the mixture. In this work, commercially available γ‐Al2O3 powder was directly used as the modification agent and mixed with Al powder and heat treated at an elevated temperature. It was found that the modified Al powder produced by directly using γ‐Al2O3 has an obvious shorter time for complete hydrogen generation than that using Al(OH)3‐produced γ‐Al2O3. Microstructure analyses revealed that directly using γ‐Al2O3 powder has a better coverage of fine γ‐Al2O3 grains on Al particle surfaces than the Al(OH)3‐produced γ‐Al2O3. This implies that a uniform distribution of modification agents on Al particle surfaces is an important factor for the Al–water reaction dynamics.