Hydrogen-Generation By The Reaction Of Pure Al Powder With Water And The Related Influencing Factors

Metal Al is a new hydrogen-generation material, which can continuously reactwith water and generate hydrogen under ambient condition. Al is the most abundantmetal in the earth’s crust, and has a relative low cost. Meanwhile, the reactionbyproduct of Al with water is bayerite or boehmite, which is environmentally benignand can be reduced to metal Al for recycle by a Hall-Heroult process or used to makerefractory and calcium aluminate cement.1kg Al reacts with water to generate asmuch as0.11kg H₂, making it a very promising in-situ hydrogen-generation materialfor portable and kW-grade fuel cells. The research on Al-water reaction to generatehydrogen has been an active area in the past few years, and various activationmethods for pure Al were developed. The hydrogen-generation performance ofAl-water reaction depends on many factors such as the reaction temperature, Alparticle size and morphology, activation method, water quality and type, stirring rate,Al to water weight ratio, etc. In this thesis, the hydrogen-generation performance bythe reaction of pure Al powder with water was investigated systematically, and therelated influencing factors were analyzed as well, which includes1. Hydrogen generation by pure Al powder with water under ambient conditionDifferent size Al powders were used to react with deionized water, and theirhydrogen generation performance was investigated systematically. The results showedthat both nano-and micro-sized Al powders could react with water and generatehydrogen under atmospheric pressure at mild temperature. A shrinking core modelwas used to analyze the reaction progress, indicating that the Al-water reaction iscontrolled by the surface chemical reaction at the initial stage, and then by the H₂Omolecule diffusion in the byproduct layer afterwards. Because the byproduct layerbecomes thick with the reaction progress, impeding the transport of water moleculestowards inner Al. The activation energy of Al-water reaction increased from64.2kJ/mol to88.7kJ/mol with increasing the Al particle size from98.38nm to24.94μm. X-ray analyses revealed that the reaction byproducts are bayerite, boehmite or amixture of them, depending on the reaction temperature.2. Dynamics difference between pure Al and modified Al powders with water-Al₂O₃modified Al powder was prepared by a ceramic processing procedure.The dynamics difference between the reaction of pure Al and γ-Al₂O₃modified Alpowders with water under the same condition was investigated. The results showedthat the reaction difference between two types of Al powders is just at the initial stage.The reaction induction time of modified Al powder with water is obviously shorterthan that of pure Al powder. This difference probably originates from a lower tensilestrength of the Al surface passive oxide films on modified Al particles, leading to alower critical breakage gas pressure in H₂bubbles at Al:Al₂O₃interface.3. Catalysis of oxides in hydrogen generation by the reaction of Al with waterHydrogen generation by the reaction of pure Al powder in Al（OH）₃,-Al₂O₃,α-Al₂O₃, or TiO₂suspensions was investigated. The results showed that the reactionof Al powder with water is promoted and the reaction induction time decreasesgreatly by the above hydroxide and oxides. X-ray diffraction analyses revealed thatthe 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. Amechanism analysis revealed that hydroxide and oxides could dissociate watermolecules and promote the hydration of the passive oxide film on Al particlesurfaces, leading to the decrease in the induction time.4. Effect of trace species in water on the reaction of Al with waterHydrogen generation by the reaction of pure Al powder with deionized water,distilled water, tap water and sea water was investigated systematically. The anions,cations and organic substance in the above water were analyzed in detail. The resultsshowed that the water type has a significant impact on Al-water reaction. The traceorganic acids 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 revealed that theorganic acids and F-ions form stable complexes with aluminol groups on Al particlesurfaces, impeding the hydration process of Al surface oxide film and retarding thereaction of Al with water.5. Effect of initial pressure on the reaction of Al with waterThe effect of initial gas pressure on Al-water reaction dynamics wasinvestigated systematically. It was found that there is a non-monotonic correlationbetween the initial pressure and the induction time for the beginning of Al-waterreaction. The induction time decreases with decreasing the initial pressure at the firststage, then reaches a minimum, and finally increases with further decreasing theinitial pressure. The mechanism analyses revealed that the special vacuum pressurecorresponding to the minimum induction time is close to the saturated vapor pressure,below which there are some vapor bubbles passing or adsorbing on Al particlesurfaces due to boiling, retarding the hydration process of Al surface oxide film andincreasing the reaction induction time.