【作者】 李永涛；

【导师】 孙大林；

【作者基本信息】 复旦大学 ， 物理电子学， 2011， 博士

【副题名】从纳米约束、催化到化学改性

【摘要】 实现氢的高效、经济、安全储／运是氢能利用的关键环节。化学储氢因其在存储密度、能效及安全性等方面颇具技术优势而备受关注。具有较高重量储氢密度的配位氢化物是当前化学储氢材料研究中的热点之一,但是动力学缓慢和可逆性差制约了其实际应用。本文在综述配位氢化物储氢材料研究进展及其存在问题的基础上,选取NaAlH4和NH3BH3 (AB)作为研究对象,并做了如下工作：首先通过构建纳米约束体系来改善NaAlH4脱/加氢特性；其次,研究了纳米碳材料催化NaAlH4体系的脱氢性能；最后,采用化学改性来改善AB的热解脱氢行为。具体研究内容及结论如下：(1)以孔径为4 nm左右的有序介孔碳MC为纳米骨架,运用熔融浸渍法制备了MC部分约束NaAlH4体系(NaAlH4/MC)。即分布在MC外表面的晶态NaAlH4大颗粒与受MC纳米孔道约束的纳米或非晶态NaAlH4共存。热分析结果表明与纯NaAlH4相比,NaAlH4/MC的热稳定性显著降低。初始脱氢温度从220℃降低至150℃左右,完全脱氢温度从320℃降低至210℃左右。脱/加氢性能测试表明NaAlH4/MC具有良好的可逆加氢性能。在无金属催化剂的条件下,在100～150℃/3.0～7.0 MPa H2的较温和条件下可实现脱氢产物的再加氢。上述脱/加氢性能改善主要归因于MC的纳米约束和化学催化的协同作用,其中,纳米约束起主要作用。(2)通过熔融浸渍、脱/加氢相结合的方法进一步构建了MC完全约束NaAlH4体系(Space-confined NaAlH4/MC)。即一部分NaAlH4及其脱氢产物NaH和Al被完全约束在MC纳米孔中,以无定形态或细小晶态的形式存在,在150℃,7 MPa较温和的条件下实现可逆再加氢；而另一部分NaAlH4分布在MC外表面通过脱/加氢处理来去除,即其脱氢产物NaHH和Al在上述脱/加氢条件下不参加可逆循环。脱/加氢性能测试表明MC完全约束NaAlH4的循环性能显著增强。经15次循环后的容量保持率大于80%,远高于纯NaAlH4经5次循环后的50%。进一步能谱分析表明,纯NaAlH4的脱氢产物Al经5次循环后长大成为2-3μm甚至8μm大颗粒,而Space-confined NaAlH4/MC即使经过15次循环后,Al元素分布仍比较均匀。分析认为,MC的纳米孔道作为“纳米反应器”,将NaAlH4及其脱氢产物NaHH和Al约束在纳米空间内,这不仅阻止颗粒团聚和相分离发生,而且还缩短了扩散距离,降低反应能垒,从而使NaAlH4在较温和的条件下实现可逆循环。(3)MC完全约束NaAlH4具有优异的脱氢动力学性能。等温脱氢测试表明Space-confined NaAlH4/MC的脱氢过程无明显孕育期,180℃时90 min内的脱氢量由纯NaAlH4的0.5wt.%增至5.0wt.%,而反应激活能Ea由116 kJ/mol降至46kJ/mol。动力学模型计算表明NaAlH4纳米约束体系的脱氢过程分为两个阶段：第一阶段由一维形核长大控制,而第二阶段则由三维相界迁移和扩散共同控制。(4)采用溶解—再结晶的方法制备了纳米碳材料负载NaAlH4体系。扫描电镜显示石墨烯(Graphene)负载样品中NaAlH4呈现层状连续体,富勒烯(C60)负载样品中NaAlH4为5-10μm的花瓣状颗粒,而MC负载样品中NaAlH4为1-3μm的球状颗粒。热分析表明纯NaAlH4在220℃开始脱氢,而Graphene、C60和MC负载NaAlH4的初始脱氢温度分别降低至190、185和160℃。进一步通过Kissinger方程计算可知,与纯NaAlH4脱氢生成Na3AlH6、Na3AlH6脱氢生成NaH以及NaH分解的三步反应激活能相比,Graphene、C60和MC负载NaAlH4的第一步反应激活能分别降低了13、19和40 kJ/mol,第二步反应激活能分别降低了77、125和148 kJ/mol,第三步反应激活能分别降低了59、122和131 kJ/mol。27Al NMR谱分析表明Al原子的局域结构发生改变,证实了纳米碳材料与NaAlH4之间存在相互作用,这可能是引起NaAlH4的热稳定性降低和动力学性能改善的原因。综上分析可得,纳米碳材料对NaAlH4脱氢过程的作用规律为MC>C60>Graphene。(5)通过机械球磨法添加碱土金属氯化物可显著改善AB的价键特性和热解脱氢行为。与纯AB相比,MgCl2/AB样品中B-H和N-H键的拉伸振动频率发生明显偏移,初始分解温度降低了60℃左右,且无NH3、B2H6和N383H6释放。进一步研究发现碱土金属氯化物对AB的热解行为具有相似作用,但MgCl2对抑制NH3释放的作用强于CaCl2。分析认为,碱土金属氯化物对AB具有双重化学改性作用,即Cl替代H激活B-H键和碱土金属与AB分子相互作用激活N-H键,是降低脱氢温度和抑制杂质气体释放的内在原因。上述结果对氨硼烷基化合物和硼氢化物氨络合物的性能改善具有借鉴意义。更多还原

【Abstract】 Storing hydrogen in an efficient, compact and safe manner is a technical challenge of utilizing hydrogen as an alternative energy carrier. One novel branch of hydrogen storage that attracts the attention of chemists and researches is chemical hydrogen storage owning to its prominent advantages in storing density, efficiency and safety. Recently, complex hydrides are becoming attractive materials for chemical hydrogen storage due to their intrinsic high hydrogen content. However, their practical applications are blocked by sluggish kinetics and poor reversibility. Based on the review of the research development and existing problems of complex hydrides, NaAlH4 and NH3BH3 (AB) were selected as the subject of the present work, which focused on the following aspects:Firstly, an improved de-/re-hydrogenation properties was achieved by using space-confining NaAlH4 in nanoporous materials; and then the dehydrogenation of NaAlH4 cayalyzed by nanocarbon materials were studied; finally a "chemical modification" method was applied to promote the hydrogen release from AB. All the research contents and results are as follows:(1) Through thermal melting impregnation NaAlH4 was loaded partially into ordered mesoporous carbon (MC) host with a pore size of～4 nm (as denoted as NaAlH4/MC) where the larger NaAlH4 particles reside on the external surface and amorphous and/ or nanosize ones on the MC internal surface. The thermal analysis results indicate that with respect to the pristine NaAlH4, the onset temperature for dehydrogenation of NaAlH4/MC decreases from about 220℃to around 150℃and the temperature for the complete dehydrogenation is reduced from about 320℃to about 210℃. Moreover, an enhanced reversibility of NaAlH4/MC is also observed. Without a metal catalyst, the re-hydrogenation in a dehydrogenated NaAlH4/MC system can be achieved even under the temperatures of 100～150℃and the hydrogen pressures of 3.0～7.0 MPa. These improvements are attributed to the synergistic effects of both nanoconfinement and chemical catalysis caused by the MC where the nanoconfinement plays a dominant role.(2) A nanoconfinement system with NaAlH4 exclusively embedded in MC was synthesized firstly by a three-step procedure; viz., thermal melting impregnation plus de-/re-hydrogenation. Through the consecutive steps, NaAlH4 is successfully loaded by the impregnation step, and its dehydrogenated products of NaH and Al are also confined in the MC pores; while some of the NaAlH4 resided on the MC external surfaces is eliminated by the de-/re-hydrogenation steps, and its resulting products are essentially in a "dormant" state and are no longer functional in the following de-/re-hydrogenation experiments, as denoted as Space-confined NaAlH4/MC. The capacity retention for Space-confined NaAlH4/MC is>80% after fifteen cycles and higher than 50% for pristine NaAlH4 over five cycles. Furthermore, as suggested by EDX analysis, after five cycles, the resulting Al of pristine NaAlH4 grows to around 2-3μm and even up to 8μm. In contrast, for Space-confined NaAlH4/MC, after fifteen cycles, the distribution of Al elements is still uniform. It is believed that the MC pore serves as a "nano-reactor" where both NaAlH4 particles and their resulting NaH and Al products are physically limited at a nanoscale level, which facilitates the mass transfer of the solid phases by shortening the diffusion distance and thus leads to an enhanced cycling stability.(3) The Space-confined NaAlH4/MC was found to exhibit faster kinetics for dehydrogenation. For pristine NaAlH4, about 0.5 wt.% hydrogen is released at 180℃in 90 min. In contrast, the value for Space-confined NaAlH4/MC increases to about 5.0 wt.%, without an apparent induction period. Moreover, the activation energy Ea for hydrogen release is 46 kJ/mol for Space-confined NaAlH4/MC, and much lower than 116 kJ/mol for the pristine one. Furthermore, kinetic modeling studies suggest that the hydrogen release from the confined NaAlH4 system is governed by two processes; the initial process can be expressed by one-dimensional nucleation and growth, and the second process is jointly controlled by three-dimensional phase boundary migration and diffusion.(4) A "dissolution-recrystallization" method was applied to prepare nanocarbon materials supported NaAlH4. The NaAlH4 particles in Graphene supported NaAlH4 exhibit a heterogeneous matrix with layered structure, the ones in C60 supported NaAlH4 sample have a flower-like shape with size of 5～10μm, and the ones for MC supported NaAlH4 are shaped like sphere with 1～3μm in size. The thermal analysis indicates that the pristine NaAlH4 starts to release hydrogen at about 220℃, but the onset temperature for dehydrogenation of Graphene, C6o and MC supported NaAlH4 is remarkably reduced to 190,185 and 160℃, respectively. Moreover, as compared to the energies for three-step dehydrogenation of pristine NaAlH4 determined by using the Kissinger analysis, the activation energy for first-step dehydrogenation of Graphene supported NaAlH4, C6o supported NaAlH4 and MC supported NaAlH4 is reduced by 13,19 and 40 kJ/mol, the value for second-step is reduced by 77,125 and 148 kJ/mol, and the value for third-step is reduced by 59,122 and 131 kJ/mol, respectively. Furthermore,27Al NMR spectra show that the local structure of Al atom in nanocarbon materials supported NaAlH4 is different from that of prisitine one and suggests the existence of interaction between nanocarbon materials and NaAlH4, which may be the reason for these property improvements. By comparing with the above results, the destabilizing effect from high to low is in the following sequence: MC>C60>Graphene.(5) The reaction for hydrogen generation from AB via a solid-state can be promoted by alkaline earth metal chlorides. It is found that tuning the reactivity of both B-H and N-H bonds in AB by alkaline earth metal chlorides not only results in a significantly decrease in the onset dehydrogenation temperature by 60℃but also suppresses undesirable volatile by-products, such as NH3, B2H6 and N3B3H6. Moreover, alkaline earth metal chlorides such as MgCl2 and CaCl2 are found to exhibit a similar effect on improving the decomposition behaviors of AB, but the MgCl2 is more efficient than CaCl2 in suppressing the release of NH3. These improvements are attributed to the dual modification involving changing the reactivity of both B-H and N-H bonds by reaction with the additives which provides further insights into the promotion of hydrogen release from amidoboranes and related borohydride ammine complexes.更多还原