核壳结构NiCoPd@GO的可控构筑及其对镁基储氢材料的催化机理研究

Mg-based materials are a very promising candidate for hydrogen storage. However, its application is severely restricted by its high thermodynamic stability and sluggish hydrogen sorption-desorption kinetics. Compounding by adding carbon and transition metal elements has been adopted to improve the performance of Mg-based materials for hydrogen storage to resolve urgently these above problems. Graphene is widely used as catalyst support and has a spillover effect on the hydrogen. Transition metals are added to reduce MgH2 dissociation energy barrier, and increase the reactive sites during the process of absorption and desorption hydrogen. In the present work, core-shell structured NiCoPd@GO was designed, assembled and constructed by the assembly nanotechnology to achieve the catalytic versatility. Mg-based materials were prepared by introducing core-shell structured NiCoPd@GO catalyst to the process of hydriding combustion synthesis followed by mechanical milling. The synergistic catalytic effects of core-shell structured NiCoPd@GO catalyst on the hydrogen sorption-desorption thermodynamics and kinetics and cyclic stability of Mg-based materials are mainly studied. The phases structure of the hydrogenated and dehydrogenated samples are studied by X-ray diffraction, Fourier transform infrared spectroscopy, the morphologies of the samples are observed by a transmission electron microscopy by focusing on the microstructual characteristics of the interface between the Mg particles and the catalysts. The hydrogen storage performance and hydrogen absorption-desorption rate are studied by using the pressure-composition-temperature. With an in-situ synchrotron radiation X-ray diffraction the phase transformation mechanisms and catalytic effects during hydrogen absorption-desorption process are revealed. The relationship between the microstructures and cycle stability of hydrogen absorption-desorption is established. The aim of the project is to obtain Mg-based hydrogen storage materials with a high activity, high capacity and the lower hydrogen desorption temperature.

镁基储氢材料仍然是研究热点之一，但高的热力学稳定性和较差的吸放氢动力学限制了其实际应用。复合化特别是添加碳和过渡金属是解决问题的有效手段。石墨烯对氢气具有溢流效应，是催化剂的良载体；过渡金属能降低MgH2解离能垒，增加反应活性位点。本项目利用纳米组装方法在微观尺度上设计、组装并构筑核壳结构NiCoPd@GO催化剂，实现催化的多功能性。通过氢化燃烧及球磨将其与镁复合获得纳米镁基复合材料，重点研究NiCoPd@GO对材料吸放氢热力学和动力学性能的协同催化效应；采用X射线衍射、红外光谱仪分析物相结构；利用透射电镜观察微观形貌，研究核壳结构在镁颗粒界面的微观特征；采用压力－组分－温度测试储氢性能；利用同步辐射X射线衍射技术，研究核壳结构金属在吸放氢过程中相结构的动态转变过程，建立微观组织与吸放氢循环稳定性之间的模型，揭示氢化相形核长大和分解内在反应机理，获得高活性、高容量、低温放氢的镁基储氢材料。

镁基储氢材料仍然是研究热点之一，但高的热力学稳定性和较差的吸放氢动力学限制了其实际应用。复合化特别是添加碳和过渡金属是解决问题的有效手段。石墨烯对氢气具有溢流效应，是催化剂的良载体；过渡金属能降低MgH2解离能垒，增加反应活性位点。本项目首先采用两步法制备了PdNi/Graphene(GN)催化剂，并研究了HCS+MM制备Mg-PdNi/GN复合储氢材料的微结构和储氢性能。研究表明：球磨预处理样品可以促进晶粒细化，从而提高HCS过程中Mg的氢化程度。氢气氛下球磨除可减少样品氧化外，还可有效抑制MgH2的分解。Mg95-(Pd3Ni3/GN4)5的吸放氢性能最好。其次，通过第一性原理计算对Ni-Mg的加氢反应进行了预测和分析，并通过实验验证了预测的准确性；理论研究表明，Ni掺杂Mg的氢化反应是一个扩散控制过程，Ni掺杂使H2解离能载体显著降低，扩散成为限制步骤。实验研究证实了理论研究的结果。接着，通过化学还原法制备石墨烯负载纳米镍催化剂，并采用氢化燃烧合成与机械球磨（HCS+MM）复合工艺将其引入到Mg-10 at%Al体系中，探究石墨烯载镍催化剂对Mg-Al储氢合金储氢性能的影响。Mg90Al10-8(80wt%Ni@Gn)表现出最好的吸放氢性能，其在250 °C、400 s内吸收5.11 wt%H2，在300 °C，30 min内放出5.81 wt%H2。单质Ni和原位生成的Al均匀分布在MgH2颗粒表面，这对吸放氢的催化效果有重要作用。最后，采用溶胶凝胶法和高温煅烧合成了双金属氧化物Sc2O3/TiO2催化剂，并通过HCS+MM工艺将其引入到Mg中。MgH2@5wt%Sc2O3/TiO2样品放氢峰值温度最低，良好的吸放氢性能归功于双金属氧化物的协同催化作用，突出可控制备新型催化剂的重要性。