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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 complex metal hydroxides (LDHs) have a two-dimensional layered structure with nanoscale structure, and its crystal structure can undergo thermal induced topological transformation under the conditions of calcination, and LDHs can be used as precursors to prepare highly dispersed catalysts. The topological transformation mechanism of ZnAl-LDHs during the heating process was investigated by first-principles density functional-based molecular dynamics simulation combined with thermogravimetric analysis (TG-DTA). The TG-DTA data showed that the decomposition reaction of two The critical temperatures of 273 and 800 ℃ were simulated at these two temperatures.The migration mechanism of metal ions during the anion decomposition of LDHs and dehydroxylation of the layers was simulated at 273 ℃ .The results showed that the interlayer anions CO 3 ~ (2-) Decomposition of monodentate ligands by interaction with the host lamellae leads to the release of water molecules from the product to the interlayer region prior to CO 2. At this time, the LDHs lamella structure has collapsed, and the metal ions in the lamella direction and vertical laminates There is no obvious memory effect in ZnAl-LDHs. At 800 ℃, the structure of LDHs has been completely collapsed and porous structure has been formed Metalloxides, which are consistent with the experimental results in literature.From the atomic level, we understand the evolution of whole structure of LDHs from the initial stage of decomposition to complete dehydration and explain the reason why ZnAl-LDHs have no memory effect, Thermal induced topology transformation mechanism, the design of highly dispersed catalyst provides useful theoretical information and guidance.