【作者】 崔洁；

【导师】 朱敏； 姚向东；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2014， 博士

【摘要】 尽管Mg基储氢材料是最有应用前景的储氢材料之一，然而其脱氢温度过高及脱氢速率较慢的问题仍未解决。本文首先综述了近年来在改善Mg基储氢合金的储氢性能所取得的研究进展，并指出：从动力学调控方面看，通过纳米化或添加催化剂等是改善其储氢性能的有效方法，但对于纳米结构而言，结构的稳定性问题必须解决，而就添加催化剂而言，传统添加催化剂的方法使得催化效率仍旧较低；从热力学调控方面看，虽然脱氢焓的变化得到了重视，但脱氢熵的的影响不可忽略，需寻找抑制熵降低或使熵增加的方法，并进行熵变的理论计算。本文以Mg基储氢材料为研究对象，分别对其结构稳定性、动力学性能以及热力学性能的调控方法开展了探索，研究了纳米限域结构及催化剂包覆结构的Mg基储氢材料循环性能、动力学及热力学性能；制备过程的结构调控和吸放氢过程中的结构变化及其对储氢性能的影响；探讨了纳米尺寸及催化剂对脱氢熵的热力学理论计算和实验调控。首先，采用气相传输沉积的方法获得了纳米AAO模板限域的Mg基复合材料。在Mg蒸气经氩气吹扫通过气相传输实现纳米多孔氧化铝限域Mg的过程中，与AAO孔壁表面接触，发生副反应生成MgO和Mg17Al12相。而生成的这一层MgO和Mg17Al12相将阻碍Mg进一步与孔壁接触，使后续进入孔道内的Mg蒸气则沿着孔壁形核生长，最终获得Mg-AAO复合材料，其有效负载率可达35wt%，主要相组成为Mg、Mg17Al12及MgO。该材料在300oC下25min内能完成吸氢，30min内能基本完成脱氢。吸氢后生成MgH2及Mg3Al2相，再脱氢后，MgH2转变成Mg，Mg3Al2相几乎不变。其脱氢焓和熵分别为73.2kJ mol-1及130.1J mol-1K-1，相比于相关纳米多孔材料限域Mg的报道，其负载率得到较大提高，动力学性能也得到更为明显的改善。通过球磨预制微米Mg粉并与TiCl3在THF溶液中反应，我们制备了多相、多价态Ti基催化剂（TiH2、TiCl3及TiO2）包覆微米级Mg颗粒表面的、具有核壳结构的Mg基储氢材料。与传统的通过球磨添加催化剂的材料相比，其性能改善十分显著。脱氢活化能降低至30.8kJ mol-1，起始脱氢温度为175oC，比球磨Mg-TiCl3材料降低了65oC。在250oC时15min内脱氢量达到5wt%，且在200oC时15min内能脱氢1.5wt%。这主要是因为多相、多价态Ti基催化剂作为吸/放氢中电子转移的媒介，成为催化活性中心，且与Mg/MgH2接触面积大。且该材料循环性能和结构稳定性良好，在275oC下，经10次吸/放氢循环后动力学性能未发生衰减，基本保持多相、多价态Ti基催化剂包覆的核壳结构形貌。此外，该材料脱氢焓和熵分别为75.1kJ mol-1与136.3J mol-1K-1，其熵增大了6.3J mol-1K-1，一定程度上降低了MgH2的稳定性。在此基础上，我们研究不同过渡族金属催化剂（TM=Ti、Nb、V、Co、Ni、Mo）构成的核壳结构，发现其脱氢动力学由快至慢的顺序依次为Mg-Ti、Mg-Nb、Mg-Ni、Mg-V、Mg-Co以及Mg-Mo。除Mg-Ni材料外，Mg-TM材料脱氢速率随着TM电负性χ的增大而减小，脱氢活化能随TM χ的增大而增大。Mg-Ni材料出现奇点的原因是因为Ni与Mg形成Mg2Ni，当将Ni基催化剂以Mg2Ni/Mg2NiH4为基相时，Mg-TM材料的脱氢活化能随TM基氢化物形成焓的增大而减小，仍与实验规律吻合。因此，TM基催化剂的催化效果本质上是和TM与H作用强度有关，TM与H作用越强，越能够扰动TM基催化剂与MgH2界面上的Mg-H，进而起到更强的弱化作用，使得Mg-TM材料中的Mg-H键更容易断裂，改善了脱氢反应动力学性能。将不同尺度的Mg颗粒与TiCl3在THF溶液中反应，制备了纳米晶Ti基催化剂包覆在不同尺寸的Mg材料。Mg平均颗粒尺寸约为40nm及500nm的材料分别记为N-Mg-Ti-with wash及N-Mg-L-Ti材料，相比于未包覆催化剂的材料，其吸/放氢动力学性能得到显著改善，其中N-Mg-Ti-with wash在275°C仅需0.3h即基本完成脱氢。我们发现脱氢动力学快慢的次序与材料中催化剂Ti含量由多至少的次序相符，杂质元素的含量及组成对材料动力学的影响作用也很明显。但当颗粒尺寸小于2μm时，其动力学与颗粒尺寸关系不明显，即催化剂及杂质元素的含量及组成直接影响Mg/MgH2颗粒的表面状态，因而成为影响吸/放氢动力学的关键因素。最后，通过建立理论模型，计算研究颗粒/晶粒尺寸、以及催化剂等因素对熵的作用。由于纳米化导致过剩体积增加，使Mg及MgH2的平动熵及振动熵均增加。并且MgH2的平动熵及振动熵均增加大于Mg的，导致其脱氢熵随颗粒/晶粒尺寸的减小而减小。当颗粒/晶粒尺寸小于3nm时，其脱氢熵将小于120.1J mol-1K-1。在类核壳结构的过渡族金属催化剂包覆Mg储氢材料表面，H2分子在过渡族金属催化剂表的解离活化能远低于在纯Mg表面，因此须考虑体系中解离态H的含量，这导致MgH2的脱氢熵随着解离活化能的增加而降低。我们的理论模型计算得出的脱氢熵的结果与文献报道结果及本文的实验结果相符。更多还原

【Abstract】 Mg-base hydrogen storage material is one of the most promising candidates of hydrogenstorage materials. However, the dehydrogenation temperature is still too high and thedehydrogenation rate remains slow. In this thesis, the improvements in recent years onMg-base hydrogen storage material were firtly reviewed. It can be pointed out that addingcatalysts or nano-structuring could improve the kinetics of Mg/MgH2. For nano-structuring,the problem of the structure stability must be considered. For adding catalyst, it indeed needsto improve the catalytic efficiency which is relatively low by traditional method for addingcatalyst. In terms of thermodynamic tuning, although the enthalpy change is given attention,the effect of dehydrogenation entropy has rarely reported. It is indeed important to find waysto avoid the negative effects of the decrease of entropy (absolute value–all following isdefined as it), and theoretically calcutate for the entropy change.Hence, this thesis investigates the tunning methods for enhance the structure stability,dynamic performance and thermodynamic properties of Mg-based hydrogen storage materials.The kinetics, cycylic and thermodynamics properties of AAO nano-confined Mg andTM-based catalyst coating on Mg particle surfaces were studied. The structure tunning bypreparation process and its impact on kinetics, cyclic and thermodynamics properties werealso studied. The influence of nano-size and the catalysts coating on the dehydrogenationentropy of MgH2based on the thermodynamic models and the experiments are investigated.A new approach has been developed to successfully load Mg into the nanometre-sizedpores of an Anodic Aluminium Oxide (AAO) template for realizing the nano-confinement ofMg. Mg nano-particles were nucleated along the AAO pore walls. Mg-AAO can absorbhydrogen within25mins and desorb hydrogen within30mins under300°C. Afterhydrogenation, Mg and Mg17Al12phases transformed into MgH2and Mg3Al2. Although smallamount of MgO and Mg17Al12formed as by-products, the effective filling was about35wt%which is higher than that reported by other groups in Mg nano-comfinement. The confinedMg/MgH2shows favourable kinetics with high stability. Furthermore, the slight reduction inhydrogen desorption enthalpy and entropy of MgH2is found from74.4kJ mol-1to73.2kJmol-1and131.0J mol-1K-1to130.1J mol-1K-1, respectively, in the presentnano-confinement.The core-shell structured Mg with Ti-based catalyst (denoted as Mg-Ti) is prepared bythe chemical reaction between Mg powders and TiClxin THF solution, which is of~10nm in thickness and contains multiple phases and valences. Compared with Mg-TiCl3by traditionalball-milling method, the Mg-Ti system has superior dehydrogenation properties, which canstart to release H2at about175oC and release5wt%H2within15mins under250oC. And thecyclic kinetics is relatively stable as the kinetics performance from the3rdcycle to the10thcycle maintains well. The deh drogenation reaction entrop (ΔS) of the system is changedfrom130.5J K-1mol-1H2to136.1JK-1mol-1H2, which reduces the Tplateauto279oC from300oC at equilibrium pressure of1bar. A new mechanism has been proposed that the mulitiplevalence Ti sites act as the intermediate for electron transfers between Mg2+and H-, whichenables recombination of H2on Ti easier.Mg is coated by different transition metals (TM: Ti, Nb, V, Co, Mo, or Ni) with a crystalsize of nano-scale (less than10nm) to form a core (Mg)-shell (TM) nano-structure by areaction of Mg powders in THF solution with TMClx. It is experimentally confirmed that thesignificance of catalytic effect on the dehydrogenation is in a sequence of Mg-Ti, Mg-Nb,Mg-Ni, Mg-V, Mg-Co and Mg-Mo. This may be contributed to the decrease ofelectro-negativity (χ) from Ti to Mo. However, Ni shows a special case with high catalyticeffect in spite of the electro-negativity. It is supposed that the formation of Mg2Ni compoundmay play an important role to enhance the hydrogen de/hydrogenation of Mg-Ni system. Itcan also be found that the larger formation enthalpy, the worse dehydrogenation kinetics.On the basis of the above, Mg with different sizes (40nm,500nm,1μm) coated withTi-based catalyst is compared both in structure and properties. Although the particle size ofmicro-sized Mg-Ti is larger than nano-sized Mg-Ti, the micro-sized still show betterdehydrogenation kinetics than nano-sized Mg-Ti. It is suggested that the key factor effect onthe kinetics could be the surface conditions (the contents of positive catalyst and negativeimpurity element) of Mg/MgH2while the particle-size is under2μm.The particle/grain size and catalysts effect on dehydrogenation entropy of MgH2arecalculated by a theoretical model. The excess volume of Mg and MgH2is increased alongwith the decrease of particle/grain size. Thus, the translational and vibrational entropy of Mgand MgH2are both increased, while the increase in MgH2is larger than Mg. Hence, thedehydrogenation entropy decreases with the decrease of the particle/grain size. Besides, theconcentration of dissociative H on TM surface is considered as the catalyst can dissociate H2more easily. The dehydrogenation entropy increases with the concentration of dissociative H.The the experimental results support our theoretical calculations.更多还原