Nanostructured Mg for hydrogen production by hydrolysis obtained by MgH2 milling and dehydriding

Abstract

The synthesis, characterization and hydrolysis properties of nanostructured MgH2 and Mg in a 0.6 M MgCl2 aqueous solution are reported. The microstructural properties of commercial MgH2 are modified by mechanical milling without additives under H2 atmosphere for 5 h. The nanostructured Mg is obtained by dehydriding the as-milled MgH2 under vacuum at 355 °C for 2.5 h. The as-milled MgH2 shows average hydrolysis properties with good hydrogen production capacity (1390 mL/g H2) and kinetics (50% of the total yield in 2.3 min). The nanostructured Mg exhibits excellent hydrolysis properties with a high yield (910 mL/g H2), fast kinetics (90% of the total yield in 2.6 min) and practically no effect of a MgO/Mg(OH)2 passivating layer. The hydrolysis properties of both materials are attributed to their morphology and microstructure. The activation energies of the reaction are estimated and the mechanisms are discussed.

Recent Progress on Materials for Hydrogen Generation via Hydrolysis

“…Demirci [41] 综述了 NH [42] , 态密度计算结果 与测得的反应活化能转变温度高度契合, 验证了上 述机理。 陈勇课题组 [43] 通过共沉淀法和固相反应制备 的 Ni-Co-P/GO 三元催化剂, 是一种高活性的非贵 金属催化剂, 标准状态下 TOF(Time of frequency)值 高达 513.9 mol H2 •mol -1 cat [42] Fig. 3 Arrhenius plot for NH 3 •BH 3 hydrolysis catalyzed by Rh/VO 2 (A), calculated PDOS of several catalysts/supports (B) [42] 墨烯 ( [44] 、减小颗粒尺寸 [45] 、合金化 [46] 以及加入强酸弱 碱盐 [47][48][49] 。加酸能很好地去除钝化层, 但酸作为反 应物改变了水解反应路径, 使体系理论制氢量大大 减少。引入酸对设备耐腐蚀性的要求提升, 增加了 设备成本。减小 Mg 的颗粒尺寸能提升反应转化率, 但是过小的颗粒尺寸增加了原料加注的困难, 并且 发生爆炸的风险急剧上升。 加入强酸弱碱盐促进 Mg 水解是一种切实可行 且非常有效的方式。本课题组 [49] 前期研究过不同阳 离子的氯盐对 Mg 水解的促进作用。如图 4 所示 [49] , 随着阳离子对 OH -亲和力的增大, 相应盐类对 Mg 不同于 Mg、Al 在反应之前其表面氧化形成的 钝化层就能够阻止它与水反应, 而在反应过程中的 钝化问题则与 Mg 类似。但 Al 作为两性金属, 反应 生成的氧化物及氢氧化物能够在酸性以及碱性溶液 中分别溶解, 形成 Al 3+ 或者 AlO 2 -。 黄岳祥课题组 [50] 总结了 Al 钝化层的几种主要消去方式, 包括碱促 进、合金化、盐促进和氧化物促进等。 修饰 Al 表面的氧化层是一种可取的方式, Deng 图 4 不同阳离子对于 OH -的亲和力与其促进 Mg 水解转化 率的关系 [49] Fig. 4 Relationship between promotion of Mg hydrolysis and affinity to OHfor different cations [49] 等 [51] 采用球磨法在 Al 表面构造了一层 γ- [53][54][55][56] 把镁铝合金与 NaBH 4 进行混合, 并加入少量过渡金属盐。合金与 NaBH 4 各自的水解 产物能相互促进水解。 黄岳祥课题组 [57] 利用 Al 和 Hg 合金化, 构造了 一种特殊的 Al 水解制氢机制, 如图 5 所示 [57] , 他们 在 Al 棒料的上底面覆盖一层 Hg(或 Zn 汞齐), 然后 用聚丙烯管将 Al 和 Hg 包裹住, 并在 Hg 上方加入 [58] , 在 5×10 -4 mL/mL 的 O 2 中就会被氧化, 遇水容易发生燃烧。Al 往往与其 它不同的金属形成混合型的金属氢化物, 如 LiAlH 4 、 Mg(AlH 4 ) 2 等, 单独的 AlH 3 的制备和保存 [59] 也极其 困难。 MgH 2 的保存相对容易, 虽然现阶段制备成本 较高, 但是其理论制氢量高, 也被认为是有潜力的 水解制氢材料。本节主要介绍 MgH 2 水解制氢的研 图 5 Hg 促进 Al 水解的构造和机理示意图 [57] Fig.…”

Light-activated hydrolysis properties of Mg-based materials

“…Various strategies, such as the acidification of the reaction environment, 5,6 the use of salt solutions, 5,7 the modification of the morphology to obtain nanostructured hydrolysable materials, 8,9 and the incorporation of additives with different functions, 10,11 have been adopted and verified effective for activating the reaction of MgH 2 with water. However, these strategies still face many challenges, such as complex operation, high-cost or the pollution of additives.…”

Comparative investigation on feasible hydrolysis H 2 production behavior of commercial Mg‐M (M = Ni, Ce, and La) binary alloys modified by high‐energy ball milling—Feasible modification strategy for Mg‐based hydrogen producing alloys

“…Nowadays, the wide usages of H 2 energy are still seriously prevented by the H 2 production and H 2 storage issues 9,14,15 . Appropriate methods for H 2 generation and storage should be preferentially sought 16,17 . Although the emerging solid‐state H 2 storage method has been paid increasing attention, direct storage H 2 in high pressure tank or low temperature liquid state which have a higher energy density, high energy consumption, and latent unsafety are currently the main methods 18‐25 …”

“…9,14,15 Appropriate methods for H 2 generation and storage should be preferentially sought. 16,17 Although the emerging solidstate H 2 storage method has been paid increasing attention, direct storage H 2 in high pressure tank or low temperature liquid state which have a higher energy density, high energy consumption, and latent unsafety are currently the main methods. [18][19][20][21][22][23][24][25] Nowadays, the demands of hydrogen energy by emerging application fields become bigger and bigger.…”