ZnAl-LDHs热致拓扑转变的理论及实验研究  

Theoretical and experimental study on the topotactic transformation mechanism of Zn-Al layered double hydroxides

作　　者：孟庆婷[1] 鄢红[1] Qingting Meng Hong Yan(State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China)

机构地区：[1]化工资源有效利用国家重点实验室、北京化工大学,北京100029

出　　处：《中国科学：化学》2017年第4期493-502,共10页SCIENTIA SINICA Chimica

基　　金：国家重点基础研究发展计划(编号:2014CB932101);国家自然科学基金(编号:21627813)资助项目

摘　　要：层状复合金属氢氧化物(LDHs)材料具有纳米尺度的二维层状结构.其晶体结构在煅烧的条件下可发生热致拓扑转变,利用此性质可以LDHs为前驱体制备高分散催化剂.本文采用基于密度泛函的第一性原理分子动力学模拟方法并结合热重分析(TG-DTA)研究了ZnAl-LDHs在升温过程的拓扑转变机理.通过TG-DTA数据明确了发生分解反应的两个关键性温度273和800℃,并在这两个温度进行模拟.在273℃,模拟了LDHs层间阴离子分解及层板脱羟基过程中金属离子的迁移机理,结果表明,层间阴离子CO_3^(2-)通过与主体层板作用形成单齿配体进行分解,产物水分子先于CO_2释放到层间区域内.此时LDHs层板结构已经坍塌,金属离子在层板方向及垂直层板方向都发生了显著的迁移,拓扑不变量(层板方向迁移度)变化明显.因此,ZnAl-LDHs不存在记忆效应.在800℃,模拟发现LDHs结构已经完全坍塌,生成了多孔性的混合金属氧化物,与文献实验结果相符合.本文从原子水平上理解了LDHs整个结构从分解的起始阶段到完全脱水的演变过程,并解释了ZnAl-LDHs没有记忆效应的原因,为认识LDHs热致拓扑转变机理,设计高分散催化剂提供了有益的理论信息和指导.Layered double hydroxides （LDHs） are a class of minerals with nanoscale two-dimensional layered structure. The topotactic transformation of LDHs under calcination and the reconstruction behavior （memory effect） of the calcined LDHs in aqueous solution play a key role in the fabrication of high dispersed metal nanocatalysts. The thermal topotactic transformation mechanism of ZnAl-layered double hydroxides （LDHs） and memory effect of LDHs were investigated by density functional theory based molecular dynamics （MD） simulation, combined with thermogravimetric/differential thermal analysis （TG-DTA）. TG-DTA results reveal that the LDH phase undergoes two key endothermic events at 273 and 800 ℃. The results show that the anion in LDHs decomposes to CO2 and H20 via a monodentate intermediate at 273 ℃, and H20 releases prior to CO2. After the decomposition of interlayer CO32- at 273℃, the distribution of metal cations in LDH matrix changed heavily along both LDH（001） facet and the c-axis direction perpendicular to the （001） facet. The dehydrated products cannot reconstruct back to the hydroxide phase, showing no memory effect. At 800 ~C, a complete collapse of layered structure occurs, resulting in a totally disordered cation distribution and plenty of holes in the final product. This work will be helpful for the design and preparation of highly dispersed nanocatalysts derived from LDHs precursors.

关 键 词：ZnAl-LDHs 热分解 拓扑转变 分子动力学模拟 密度泛函理论 

分 类 号：O611.64[理学—无机化学] TB383.1[理学—化学]