超声协同下电化还原偏硼酸铵合成氨硼烷的机理研究

Ammonia borane, or simply “AB” for short, is an excellent hydride-based hydrogen storage material with the highest hydrogen capacity of 19.59 wt% ever found by now. AB takes solid state at room temperature with high stabilty. The cyclic regeneration of ammonium metaborate which is the “spent fuel” after hydrogen release by AB hydrolysis is the very important way for reducing production cost of AB and saving boron resource. The electrochemical reduction approach will be adopted in this work for chemical conversion of ammonium metaborate to ammonia borane. The pore formation, filming and pressing, and surface modification technique will be used to fabricate a series of multidimensional cathodes with special features, whose catalytic activity and selectivity should be scientifically evaluated. Through substituting alcohol for water, adding conductive agent and antioxidant, and adjusting the concentration relation of ammonia to alkyl ammonium hydroxide, the electrolytic solution with reasonable component configuration could be obtained. In the homemade electrochemical reactor, ultrasonic wave is imported to fully agitate the reaction solution for speeding up mass and energy transfer. By introducing periodic reverse pulse current and duly regulating the pulse period, the repulsive interaction among like charges could be reduced so as to accelerate the reaction process. The feasible route for accurate analysis, separation, decomposition and transformation of the intermediate products from electrochemical reduction should be fully investigated. By way of measuring potentiodynamic polarization curve for the different electrodes and solutions, the variation characteristics of the redox reactions are analyzed so as to reveal electrochemical behavior. Quantitative description for the electrochemical reaction mechanisms according to the principle of kinetics should be proposed on base of the research results of electrochemical reduction under ultrasonic synergy. The scientific thoughts for efficient electrochemical reduction of ammonium metaborate should therefore be provided.

氨硼烷是迄今发现的氢含量最高(19.59%)、常温下呈稳定固态的化合物型储氢材料，其水解释氢后的“乏燃料”—偏硼酸铵的循环再生，是节约硼源、降低氨硼烷成本的重要途径。本项目拟用电化还原方法使偏硼酸铵再生为氨硼烷。利用致孔、膜压和表面修饰技术，研制若干种具有特色的多维阴极材料，科学评价其电化还原能力；通过以醇代水、加入导电和抗氧化助剂、调适氨水和烷基铵碱的比例，获得组分配置合理的电化溶液体系；自制特殊反应器，引入超声波外场，强化反应液的无死角扰动，加快传质传能；导入周期换向脉冲电流，适时调控脉冲周期，减小同性电荷间的排斥，加速反应进程；寻求电化还原中间产物的精确分析、分离和分解转化的科学途径；测定不同电极材料和反应液的极化曲线，分析氧化还原反应的变化特征，揭示反应液的电化学行为；探索超声协同下电化还原的反应规律，运用动力学原理定量描述电化反应的微观机制，为偏硼酸铵的高效电化还原提供科学思路。

本项目着重开展储氢材料氨硼烷水解释氢副产物偏硼酸铵的电化学还原、循环再生为氨硼烷的基础理论研究。借助电化学理论和方法，将偏硼酸铵还原再生为氨硼烷，实现硼源的循环利用，以降低氨硼烷的制备和应用成本。针对硼酸盐晶体和溶液体系中硼氧配阴离子的复杂多变性，对其进行了XRD、TG-DTA、FTIR和Raman综合表征，分析了其晶相结构、组成和热稳定性。以铜氨络合物、硼氢化钠分别作为氧化剂和还原剂，于非水混合溶剂中在室温下回流反应，再用乙醚重结晶纯化，获得了氨硼烷产物，同时考察了不同金属氨合物、不同溶剂和反应温度等条件对氨硼烷产率的影响；通过化学分析和仪器分析手段对产物进行了分析与表征。根据氨硼烷的化学组成和水解产物特性，分别采用甘露醇滴定、甲醛滴定和水解释氢法测定了B、N、H的含量，以此实现对氨硼烷含量的精确分析，同时运用TG-DTA、XRD、FT-IR、Laser-Raman和固/液核磁共振(1H NMR、11B NMR)等现代测试技术，科学地解析了氨硼烷的基本谱学性质。通过水热、阳极氧化等不同途径在Ti片上生长TiO2纳米线/纳米管阵列，再经氨气氮化得到TiN@Ti-NTAs纳米管阵列；在TiN@Ti-NTAs表面均匀沉积金属Ni，得到多孔Ni/TiN@Ti-NTAs电极；分别以Ti、TiO2@Ti、TiN@Ti、Ni/TiN@Ti、Ni/TiN@Ti-NTAs作为基板，通过等离子溅射方式沉积可控厚度的贵金属Au薄层，形成Au/Ti、Au/TiO2@Ti、Au/TiN@Ti、Ni/TiN@Ti和Au(Ni)/TiN@Ti-NTAs纳米管阵列复合结构电极体系。依次将TiO2@Ti、TiN@Ti、Au/Ti、Au/TiO2@Ti、Au/TiN@Ti、Ni/TiN@Ti和Au(Ni) /TiN@Ti-NTAs复合电极用作阴极，与石墨(Pt)对电极、饱和甘汞参比电极构成三电极体系，分别研究了BO2-碱性溶液的电化学行为，同时加装阳离子交换膜，适当调变脉冲电源、电解时间、电压、脉冲比例等条件，进一步探索了电极反应过程，对BO2-的电化学还原机理进行了分析和推测。