2线水铁矿的晶胞与团簇模型

Unit cell and cluster model of 2-line ferrihydrite

费托合成是合成清洁能源的重要化学反应。2线水铁矿是目前公认的费托合成铁基催化剂的前躯体。由于2线水铁矿热力学稳定性差,因此结构模型的推导主要基于X射线衍射结果和透射电子显微镜。但目前还鲜有将实验与模拟相结合,研究2线水铁矿的纳米团簇的报道。本研究采用实验与模拟相结合的方法,其中,分子模拟采用Material Studio(简称MS)中的Dmol3模块和Forcite模块分别对2线水铁矿的晶胞与团簇模型进行结构优化;实验研究利用沉淀法合成铁基催化剂前驱体2线水铁矿样品,洗涤方法选择离心,干燥方式选择冷冻干燥;表征方法包括、X射线衍射分析和红外光谱分析。从实验制备的2线水铁矿的X射线衍射结果出发,并结合米歇尔等给出的晶胞模型参数,搭建了2线水铁矿的晶胞模型、直径为2.66 nm的2线水铁矿球形纳米团簇及边长为3.22 nm的2线水铁矿立方体纳米团簇模型。在对模型进行了结构最优化后,通过模拟的X射线衍射对比图验证了晶胞与团簇模型的合理性,即实验与模拟的衍射峰差别控制在1.0°以内。结果表明,通过不同泛函结构优化后得到的模型的X射线衍射图出峰位置分别为35.9°/62.7°(GGA/PBE)和35.8°/61.9°(m-GGA/M06-L)。比较实验值(35.1°/62.0°),2种计算结果的差值均在1.0°以内,且m-GGA/M06-L泛函计算结果更加接近实验值。对于两种纳米团簇模型,Fine精度计算得到的结果最准确,且两种不同形状的团簇模型均为最优模型。本研究强调了实验与模拟的紧密性,从实验结果出发,结合MS软件的X射线衍射模拟功能,成功搭建了合理的晶胞与团簇模型,对2线水铁矿结构的研究提供了依据。

Fischer-Tropsch synthesis is an important chemical reaction for the synthesis of clean energy.2-line ferrihydrite is currently recognized as the precursor of Fe-based synthesis of Fe-based catalysts.Due to the poor thermodynamic stability of 2-line ferrihydrite,the derivation of the structural model is mainly based on X-ray diffraction results and transmission electron microscopy.However,there have been no reports on the combination of experiments and simulations to study the nanoclusters of 2-line ferrihydrite.This research used a combination of experiments and simulations.Among them,the molecular simulation used the Dmol3 module and the Forceite module in Material Studio(MS for short)to optimize the structure of the unit cell and cluster model of 2-line ferrihydrite,respectively.Precipitation method was used to synthesize iron-based catalyst precursor 2-line ferrihydrite sample.The washing method was centrifugation and the drying method was freeze-drying.Characterization methods included X-ray diffraction analysis and infrared spectrum analysis.Based on the X-ray diffraction results of the experimentally prepared 2-line ferrihydrite,and in combination with the cell model parameters given by Michel etc,the unit cell model of 2-line ferrihydrite,spherical 2-line ferrihydrite nanocluster model with a diameter of 2.66 nm and cubic 2-line ferrihydrite nanocluster model with a side length of 3.22 nm were constructed.After the structure optimization of the model,the rationality of the unit cell and cluster model was verified by the simulated X-ray diffraction comparison chart,that is,the difference between the experimental and simulated diffraction peaks was controlled within 1.0°.The results show that the peak positions of the X-ray diffraction patterns of the model obtained by the optimization of different functional structures are 35.9°/62.7°(GGA/PBE)and 35.8°/61.9°(m-GGA/M06-L).Comparing the experimental values(35.1°/62.0°),the difference between the two calculation results is within 1.0°,and the m-GGA/M06-L functional calculation results are closer to the experimental values.For the two nano-cluster models,the results obtained by fine precision calculation are the most accurate,and the two cluster models with different shapes are the optimal models.This study emphasizes the closeness of experiments and simulations.Starting from the experimental results and combining the X-ray diffraction simulation function of MS software,a reasonable unit cell and cluster model has been successfully established,which provides a basis for the study of ferrihydrite structure.