The effect of crystal plane on Fe3O4 carbonization
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摘要: 在费托合成反应中,Fe基催化剂由于价格低廉、活性高、CH4选择性低等多种优势,被广泛应用于大规模煤炭间接液化工业中。催化性能与催化剂颗粒尺寸、表面结构、成分构成等性质密切相关。还原碳化是铁基催化剂活化的关键步骤,本工作通过改变晶体生长条件,制备了暴露{111}晶面的不同尺寸的Fe3O4-O,以及尺寸接近的Fe3O4-O和暴露{110}晶面的Fe3O4-RD,探究Fe3O4晶粒尺寸以及暴露晶面对碳化过程的影响。结果表明,尺寸达到微米级的Fe3O4-O晶体比50 nm的晶体更难被碳化。利用原位XRD表征尺寸均为150 nm的Fe3O4-O和Fe3O4-RD晶体在还原碳化过程中的物相组成变化,结果显示,两种晶体的碳化速率不同,且可碳化上限不同,因此,晶面取向会影响还原碳化过程。使用TEM表征暴露不同晶面的Fe3O4碳化后的晶体结构,发现两种晶体的形貌均发生改变,形成核壳结构。Abstract: In the Fischer-Tropsch synthesis reaction, Fe-based catalysts are widely used in large-scale indirect coal liquefaction industry due to their low price, high activity, and low CH4 selectivity. The catalytic performance is closely related to the catalyst particle size, surface structure and composition. Since reductive carbonization is a key step in the activation of iron-based catalysts, in this work, Fe3O4-O (expose the {111} crystal planes) with different particle size, and similar particle size but exposing different crystal planes, {111} and {110} (Fe3O4-RD), have been prepared to explore the effect of particle size and surface structure on the carbonization process. The results show that the 50 nm Fe3O4-O particles change more significantly than the one with large particle size (2–10 μm) after carbonization. In-situ XRD was used to monitor the phase change of Fe3O4 with exposing different surface planes during carbonization. The results show that 150 nm Fe3O4-O and Fe3O4-RD particles behave differently in carbonization rate and have different iron carbide concentration in the end, which indicates the carbonization process can be affected by exposed crystal planes. TEM analysis reveals that Fe3O4@FexC core-shell structure formed after carbonization.
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Key words:
- Fe3O4 /
- crystal plane effect /
- in-situ XRD /
- carbonization
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图 1 微米级Fe3O4-O1的SEM照片:(a)新鲜样品;(b)对(a)中虚线区域放大;(c) 300 ℃,5%CO气氛碳化12 h;(d)对(c)中虚线区域放大;(e) 300 ℃,100%CO气氛碳化12 h;(f)对(e)中虚线区域放大
Figure 1 SEM of micron-sized Fe3O4-O1: (a) as prepared sample; (b) zoomed-in image of marked area in (a); (c) carbonized for 12 h in 5%CO at 300 ℃; (d) zoomed-in image of marked area in (c); (e) carbonized for 12 h in 100%CO at 300 ℃; (f) zoomed-in image of marked area in (e)
图 4 50 nm Fe3O4-O3碳化前后TEM照片:(a)新鲜样品的TEM;(b)新鲜样品的HRTEM;(c) 对(b)中虚线区域放大;(d)碳化样品的TEM;(e)碳化样品的HRTEM;(f)对(e)中虚线区域放大
Figure 4 TEM images of 50 nm Fe3O4-O3: (a) TEM of fresh sample; (b) HRTEM of fresh sample; (c) zoomed-in image of marked area in (b); (d) TEM of carbonized sample; (e) HRTEM of carbonized sample; (f) zoomed-in image of marked area in (e)
图 6 原位XRD碳化实验的分析:(a1)、(a2)Fe3O4-O2碳化中特定时间点的XRD谱图和2D热图;(b1)、(b2) Fe3O4-RD碳化中特定时间点的XRD谱图和2D热图;(c)两种晶体的Fe3O4最强峰41.3°的相对衍射强度变化;(d)气相色谱分析尾气计算出的CO转化率
Figure 6 Analysis of in-situ XRD carbonization experiments: (a1), (a2) XRD patterns at specific time points and 2D heatmap of Fe3O4-O2 carbonization process; (b1), (b2) XRD patterns at specific time points and 2D heatmap of Fe3O4-RD carbonization process; (c) diffraction intensity change of the Fe3O4 strongest peak of two crystals at 41.3°; (d) CO conversion calculated by gas chromatographic analysis of exhaust gas
图 7 原位XRD碳化实验结束后样品的TEM照片: (a)Fe3O4-O2碳化后的TEM;(b)Fe3O4-O2碳化后HRTEM;(d)Fe3O4-RD碳化后的TEM;(e)Fe3O4-RD碳化后的HRTEM;(c)、(f)两个样品(b)和(e)中对应区域的EDX mapping
Figure 7 TEM images of the samples after the in-situ XRD carbonization experiments: (a) TEM of carbonized Fe3O4-O2; (b) HRTEM of carbonized Fe3O4-O2; (d)TEM of carbonized Fe3O4-RD; (e) HRTEM of carbonized Fe3O4-RD; (c), (f) EDX mapping of the corresponding regions of (b) and (e)
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