An Investigation On Hydrogen Generation By Hydrolysis Reaction Of Aluminum And Water

Hydrogen generation by hydrolysis reaction of aluminum and water isconsidered as the central issue in the portable hydrogen research field. However,aluminum would not react with water to yield hydrogen at ordinary conditions due tothe passive oxide film formed on its surface. Recent research is focused on how toeliminate the oxide film on the surface of aluminum and make aluminum keep itsreactivity during preparation and storage. In this thesis, the effects of amalgam andgraphite on hydrogen generation by hydrolysis of aluminum with water werereported. The effects of progress conditions on the hydrogen generation byhydrolysis of metal aluminum were discussed and the hydrolysis reactionmechanisms were proposed. The main results are as follows:（1） The hydrolysis reaction of aluminum was carried out by covering thereaction surface with mercury or zinc amalgam and then putting water into aself-built reactor. The self-built reactor was designed according to a Chinese patent.The results showed that the hydrolysis reactivity of aluminum was improved whenhydrolysis of aluminum-mercury or aluminum-zinc amalgam mixtures. Zincamalgam showed a more pronounced effect on aluminum activation and hydrogengeneration rate than mercury only. The maximum hydrogen generation rate of43.5cm³h^-1cm^-2was obtained at65oC for the case coated with zinc amalgam, theapparent activation energy was calculated to be43.4kJ mol^-1, while the maximumhydrogen generation rate of13.7cm³h^-1cm^-2was got for the case coated with Hg,the activation energy was74.8kJ mol^-1. The XRD patterns and microscopyobservation showed that the aluminum hydrolysis reaction was taken place at themercury/water interface and the aluminum particles could transport through themercury film rather than other particles. The aluminum hydrolysis model induced bymercury was proposed, that is （i） the dissolution of aluminum;（ii） the migration ofaluminum particles;（iii） the hydrolysis of aluminum particles.（2） The effect of graphite on the hydrogen generation by hydrolysis of metalaluminum was reported. The Al-C-NaCl powder was prepared by a ball milling process. Increasing the amount of carbon addition, prolonging milling time,increasing the reaction temperature could reduce the induction time of hydrogenproduction, improve the aluminum hydrolysis reactivity and obtain an increasedhydrogen yield. The Al particles were covered by graphite to form a core-shellstructure. The graphite layer acted as a “protective layer” on the surface of aluminumpower, which prohibited the formation of oxide film on aluminum surface. Such anAl/graphite composite material exhibited a pronounced hydrolysis reactivity with tapwater to generate hydrogen.（3） The effect of processing parameters on hydrogen generation by hydrolysisof Al-C-NaCl powder was discussed. It was found that increasing the amount ofNaCl additive could improve the hydrogen generation yields by hydrolysis of theAl-C composites. When the addition of NaCl into the Al-33wt.%C composite is10wt.%-50wt.%, the obtained hydrogen volume was1280cm³g^-1of Al and thehydrogen evolution yield reached to be94.2%; Using a fine aluminum powder couldimprove the aluminum hydrolysis reactivity and obtained an increased hydrogengeneration rate; Al powder in a pellet exhibited poor hydrolysis reactivity over in apowdered state; increasing the aging time significantly degraded the Al reactivity inthe present composite system. So the powder must be kept in an air-isolationatmosphere; the hydrogen evolution rate of the Al-C-NaCl composite materials isstrongly dependent on reaction temperature and the maximum hydrogen generationrate of52.4cm³min^-1g^-1of Al was obtained when the reaction temperature wasincreased to75oC.