(749e) On Mechanism of Splitting Water By Its Reaction With Aluminum and Magnesium

Reactions of metals with water are of interest for hydrogen generation for a wide range of applications, from in-situ operated fuel cells to propulsion of underwater vehicles. In our previous work, the mechanism of water split reaction with aluminum was studied by monitoring the heat generation with micro-calorimetry and weight loss in thermo-gravimetric measurements for liquid water and steam, respectively.  In both experiments, two spherical aluminum powders were used with different but overlapping particle size distributions.  The experimental results were interpreted taking into account actual particle size distributions of aluminum powders and assuming a location for the reaction interface at either the outer surface of the oxidizing particles or the inner metal-oxide interface. Matching the oxidation dynamics for particles of the same size and belonging to powders with different size distributions allows one to determine the location of the reaction interface.  For liquid water, results suggest that the reaction occurs at the surface of the shrinking aluminum core, but for reactions of aluminum with steam at elevated temperatures, the results indicate that the reaction is most likely occurring at the outer surface of the growing alumina shell. It is further observed that the alumina shell becomes fragile.  When the shell break once it grew thick enough, a new shell begins to grow around the fresh aluminum surface.  Here, these conclusions are used to develop a simplified, diffusion-limited reaction model considering different species diffusing for oxidation of aluminum in liquid water and in steam.  Reaction kinetics is quantified using previous thermo-analytical measurements and using isoconversion techniques.  In addition, the oxidation behavior of spherical magnesium powders in water is investigated using the same approach as was used previously for aluminum.  Two measurements for the reaction of spherical magnesium with liquid water and steam are performed with heat flow calorimetry and thermo-gravimetry, respectively.  The results are interpreted considering actual particle size distributions.  Once the mechanisms of metal-water reactions are elucidated, modified materials, including aluminum/magnesium alloys can be developed enabling one to optimize the hydrogen generation by the water split reaction.