Oxidation of activated aluminum with water as a method for hydrogen generation

Abstract

The activation of aluminum with low melting gallium based alloys makes it possible to obtain a material easily oxidizable by water at room temperature to release ~1.2 L of pure hydrogen per 1 g of metal. The kinetics and regularities of the interaction of activated alumi num with water were studied as a function of the amount and composition of metal activators, dispersity of the powder, and temperature. The phase composition of the solid reaction products was shown by X ray diffraction analysis to depend on temperature and duration of the process.Substantial increase in the cost of traditional energy carriers enforced noticeable intensification of works on search for alternative energy sources including those related to the use of hydrogen. However, the use of this energy carrier requires the development of new methods for preparation, storage, and transportation of hydrogen and methods and devices for the direct conversion of fuel to energy. Among the problems considered, the develop ment of safe and economically viable methods for prepa ration, storage, and transportation of hydrogen is most topical and at the same time most complicated.A possible route for the complex solution of these problems is related to the use of metals and their alloys and to the synthesis of hydrogen directly at the site of consumption. Economical and ecological considerations as well as physicochemical properties allows one to dis tinguish aluminum among all metals and alloys, because the oxidation of 1 g of aluminum with water gives 1.244 L of hydrogen.It is well known that under standard conditions alumi num is covered with very thin but dense oxide film pre venting the reaction of aluminum with water. Aluminum can be activated by various methods, for example, amal gamation, 1 oxidation in alkaline media, 2 and high tem * Dedicated to Academician I. I. Moiseev on the occasion of his 80th birthday. perature oxidation. 3 However, several drawbacks of these methods make them inappropriate for the creation of generators of hydrogen for domestic purposes.The addition of gallium based alloys with low melting temperatures (gallam) 4,5 to metallic aluminum leads to the destruction of the surface oxide layer 6-9 (Rebinder effect) and imparts high reactivity toward water, alcohols, and several functionalized hydrocarbons to the metal. 10,11 Depending on the oxidation reaction conditions, first of all on the temperature, different solid products (oxides, hydroxides) can be obtained at the same yield of hydrogen 12 2 Al + 3 H 2 O = Al 2 O 3 + 3 H 2 + Q (1) (15.1 MJ (kg Al) -1 ), 2 Al + 4 H 2 O = 2 AlOOH + 3 H 2 + Q (2) (15.5 MJ (kg Al) -1 ), 2 Al + 6 H 2 O = 2 Al(OH) 3 + 3 H 2 + Q (3) (16.3 MJ (kg Al) -1 ).Reaction (1) occurs if the metal is oxidized with water and/or water vapor at temperatures of the reaction mixture higher than 200-250 °С, whereas reactions (2) and (3) occur at lower temperatures.We have previously 13 developed the method for the preparation of the activated aluminum powder. The

Hydrogen generation from water by means of activated aluminum

“…After continuous mechanical treatment solid aluminum turns into the fine powder consisting of particles covered by gallam. Since the aluminum surface is not closed by oxide film, such aluminum powder reacts with water even at room temperatures [25,26]. The presence of liquid film (gallam) on the surface of aluminum particles is proved by DSC and X-ray diffraction data.…”

“…The aluminum activation was performed in two steps: 1) preparation of gallam (alloy on basis of gallium); 2) mechanical treatment of preliminarily crushed aluminum wire with gallam in a porcelain mortar [26]. For the activation of aluminum granules (99.9%, average diameter is 10 mm) the third stage was used, which consists of the mechanochemical treatment in high-energy ball mill of materials obtained in the second stage [27].…”

“…The activation was performed in two steps: manual treatment of aluminum granules with gallam (step 1) [25,26] and additional trituration in the high-energy ball mill (step 2) [27]. Such activated aluminum powders reacting with water give high rate of hydrogen production e 1000e2500 mL/ (g min).…”

Role of Ball Milling of Aluminum Powders in Promotion of Aluminum-Water Reaction to Generate Hydrogen

“…[6] Scientists have accomplished extensive research to promote the reaction of Al with water for hydrogen generation. Ball milling (BM) of monolithic Al powders [2,[7][8][9] or Al powders with low melting point (LMP) metals such as Ga, In, Bi, Hg, and Zn, [10][11][12][13][14][15][16][17] water soluble salts such as NaCl and KCl, [12,13,[18][19][20][21][22] and ceramic powders such as Al 2 O 3 (a), Al 2 O 3 (c), TiO 2 , C, SiO 2 , and Al(OH) 3 [22][23][24][25][26][27][28][29][30] have been considered to activate Al reaction with water.…”

“…Each type of the second phase particles can promote the reaction of Al with water in distinctive ways. The presence of LMP metals can promote the reaction by different mechanisms: destruction of Al intercrystalline contacts, [14,31] diffusion of Al nanoparticles through LMP metals to the reaction sites, [32] formation of microgalvanic cells, [11][12][13]15] destruction of the native alumina film, and diffusion of Al through a LMP metal layer to reach water, [14,15,[33][34][35] and also shifting the operating potential of Al to more negative values. [33] In the case of ceramic particles, Deng et al [24,25] explained that a dispersion of oxide particles in the Al matrix can modify the protective layer and make it less strong.…”