Research On Reaction Behavior Of Hydrided Mg-Mn Compsites With Water

Efficient hydrogen source and real-time hydrogen production and supply are the core technologies for the hydrogen application in the future. MgH₂ reacts with water to obtain H₂,Mg(OH)₂ and release heat. It is expected to be a new portable hydrogen source technology with highly efficient. However, the bottleneck problem that retards its application is the characteristic that the product -- Mg(OH)₂ is easily attached to the surface of unreacted MgH₂ to stop the reaction. So the focus of this paper is the formulation of materials and the study on the principles of reaction with water.In this paper, Mg-Mn composite powders and pure Mg powders were prepared by ball milling in hydrogen, and their hydrogenated powders were characterized by X-ray diffraction (XRD) and the scanning electron microscope (SEM). The basic law of the hydrolysis kinetics of these hydrogenated composites measured by home-made device was analyzed. The morphology of reaction products -- Mg(OH)₂ was also characterized and the basic law of morphology progression was qualitatively analyzed.The results showed that hydrogenated composite powders could be prepared by ball milling in hydrogen. Both the hydrogenation process and the final grain sizes of the composite powders were influenced by the ratio of ball to powder and the equal-fine particles approximated to the equiaxed was the final morphology. Due to the increased activity of magnesium and the mixing of a small amount of oxygen into hydrogen, hydrogenated Mg-MgO composite powders were prepared ultimately. The particle sizes of hydrogenated Mg-Mn composites and hydrogenated Mg-MgO composites varied in rang of 1⁷μm and 1⁴μm respectively.The hydrolysis kinetics of hydrogenated Mg-Mn composites is marked influenced by the reaction temperature and water quality, and the kinetics is optimized and hydrolysis rate and reaction rate are improved as the temperature rises in the temperature range of this experiment is the basic rule. When react with secondary deionized water and sodium chloride solution, there is a critical temperature Tr. The kinetics of the reaction with sodium chloride solution is superior to that with secondary deionized water when the temperature is lower than Tr, while it shows opposite reaction kinetics when the temperature is higher than Tr. It is almost due to the specific adsorption of the chloride ions in sodium chloride solution on the surface of Mg(OH)₂ when the temperature is lower than Tr. The formation of MgCl42- complex compound in a certain concentration of soluble could make the Mg(OH)₂ film loose and porous enough for the rise of hydrolysis ratio and rate. Over the Tr, temperature, the high temperature and reaction heat may increase the nucleation rate of Mg(OH)₂ and cause the reaction modification that dramatically keep the reaction process and speed. The lower impurities in the water would bring benefit to improve the hydrolysis kinetics. The optimal hydrolysis kinetics is that the hydrogen up to 94% of desorption capacity was released within 7 min.The theoretical analysis of the reaction is that the Avrami index n in the reaction process between hydrogenated Mg-Mn composites and secondary deionized water and NaCl solution changes with temperature. The transform from diffusion-controlled to three-dimensional-reaction-controlled is the possible changing law.The flaky morphology of products obtained from the reaction between the hydrogenated Mg-Mn composites and secondary deionized water are clearly visible with increasing temperature, and show a stable dispersion when the reaction occurred in a high temperature becacuce of the more exposed nonpolar (001) crystal face. The products obtained from the reaction between hydrogenated Mg-Mn composites and NaCl solution grow up cross-flakily along the normal direction of the particles. The presence of Cl- ions makes more obvious agglomeration and poor dispersion. The concept of "reaction modification" is more suitable for the reaction system between the hydrogenated Mg-Mn composites and secondary deionized water.Exploratory experiment about different reaction weight of hydride suggested that the more the reaction weight of hydride and higher temperature, the better dispersion and the more obvious effect of modification becacuce of the sharp increased exposed nonpolar (001) crystal face. It promises to be a new technology of preparation and modification of Mg(OH)₂ in a short process.