Effect of Fe3O4 content on the CO2 selectivity of iron-based catalyst for Fischer-Tropsch synthesis
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摘要: 本研究以共沉淀法制备的α-Fe2O3催化剂为前驱体，通过调变碳化温度和碳化时间制备了不同物相组成的系列催化剂，采用XRD、Mössbauer谱、XPS和Raman光谱等技术考察了催化剂体相和表面物相组成，在此基础上研究了不同条件下（不同CO转化率和H2O分压）催化剂的物相组成与催化剂性能之间的关系，重点探究了费托合成条件下CO2生成的活性相。结果表明，升高碳化温度和延长碳化时间有利于Fe3O4向碳化铁转变。在典型的费托合成条件下，催化剂的活性受到碳化铁含量和积炭程度的共同影响。当H2O分压较低时，动力学因素限制了水煤气变换（WGS）反应的进行，CO2选择性仅受CO转化率的影响，Fe3O4含量变化对CO2选择性无明显影响；而在较高的H2O分压下，随着催化剂中Fe3O4含量增加， CO2选择性也随之增加。本文初步阐明了Fe3O4是铁基费托合成催化剂中WGS反应的主要活性相，为认识Fe基费托合成催化剂CO2生成的活性相提供了新的信息，为新型低CO2选择性费托合成工业催化剂的设计奠定了基础。Abstract: In this study, a series of catalysts with different Fe3O4 to iron carbide ratios were obtained by carburizing the α-Fe2O3 precursor prepared by co-precipitation method, under various carburization conditions. XRD, Mössbauer spectroscopy, XPS, and Raman spectroscopy were used to characterize the bulk and surface phase compositions of the Fe-based catalysts. The results show that increasing the carburization temperature and prolonging the carburization time lead to higher iron carbide concentration. To explore the active phase of CO2 formation, the catalysts were tested under different reaction conditions by tuning either CO conversion or H2O partial pressure. It turns out that the catalytic performance of the Fe-based catalyst in the FTS and water-gas shift (WGS) reactions is influenced by both the content of iron carbide and the degree of carbon deposition. Under typical Fischer-Tropsch reaction condition, the CO2 selectivity is determined by the CO conversion rather than the Fe3O4 content in the catalyst, meaning that the WGS reaction is here limited by the kinetic factors. On the contrary, adding H2O to the reaction gas results in the trend that higher CO2 selectivity is promoted by higher content of Fe3O4 in the Fe-based catalyst. It seems that Fe3O4 is the main active phase for the WGS reaction in the iron-based catalyst for FTS. These results provide a new insight into the active phase of CO2 generation on the Fe-based catalysts, which could be the theoretical basis for the design of new industrial FTS catalysts with low CO2 selectivity.
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Key words:
- Fischer-Tropsch synthesis /
- iron-based catalyst /
- Fe3O4 /
- CO2 selectivity /
- WGS reaction
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图 1 不同条件碳化后Fe催化剂的XRD谱图
Figure 1 XRD pattern of the Fe-based catalysts pretreated under different carburization conditions
图 2 不同条件碳化后Fe催化剂的Mössbauer谱图
Figure 2 Mössbauer spectra of the Fe-based catalysts pretreated under different carburization conditions
图 3 不同条件碳化后催化剂的Fe 2p XPS谱图
Figure 3 Fe 2p XPS spectra of Fe-based catalysts pretreated under different carburization conditions
图 4 不同条件碳化后Fe催化剂Raman谱图
Figure 4 Raman spectra of various carburized Fe-based catalysts
图 5 Fe催化剂在费托合成评价中的CO转化率（a）与CO2选择性（b）
Figure 5 CO conversion (a) and CO2 selectivity (b) of FTS over various Fe-based catalysts
图 6 反应后催化剂的XRD谱图
Figure 6 XRD patterns of various spent catalysts after the FTS reaction test
图 7 反应后催化剂的Mössbauer谱图
Figure 7 Mössbauer spectra of various spent catalysts after the FTS reaction test
图 8 反应后催化剂的Fe 2p XPS谱图
Figure 8 Fe 2p XPS spectra of various spent catalysts after the FTS reaction test
图 9 不同Fe3O4含量催化剂的CO2选择性-CO转化率坐标分布位置（TOS = 800 min）
Figure 9 Relationship between the CO2 selectivity and CO conversion for FTS over various Fe-based catalysts with different Fe3O4 contents (TOS = 800 min)
图 10 CO转化率水平下的CO2选择性
Figure 10 CO2 selectivity at same CO conversion for FTS over various Fe-based catalysts
图 11 添加3%H2O条件下Fe催化剂在费托合成评价中的CO转化率（a）与CO2选择性（b）
Figure 11 CO conversion (a) and CO2 selectivity (b) of various Fe-based catalysts during the FTS reaction with the addition of 3% H2O in the feed
图 12 添加3%H2O条件下（调整空速后）Fe催化剂在费托合成评价中的CO转化率（a）与CO2选择性（b）
Figure 12 CO conversion (a) and CO2 selectivity (b) of various Fe-based catalysts during the FTS reaction with the addition of 3% H2O in the feed; the CO conversion over different catalysts was controlled at a similar level (15%–20%) by adjusting the space velocity
图 13 不同Fe3O4含量Fe催化剂的H2O-TPD谱图
Figure 13 H2O-TPD profiles of Fe-based catalysts with different Fe3O4 content
表 1 Fe催化剂的碳化条件
Table 1 Conditions for the carburization of the Fe-based catalysts
Catalyst Carburization temp. t/℃ Time t/h Fe-270a-1b 270 1 Fe-270-4 270 4 Fe-290-4 290 4 Fe-310-4 310 4 a: carburization temperature (℃); b: carburization time (h) 
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表 2 不同条件碳化后Fe催化剂的物相组成及其含量
Table 2 Iron phase composition and content of Fe-based catalysts pretreated under different carburization conditions
Catalyst Phase composition /% Fe3O4 FexC χ-Fe5C2 ε-Fe2C Fe-270-1 87.79 9.55 2.66 Fe-270-4 68.51 6.75 24.73 Fe-290-4 39.76 25.27 34.97 Fe-310-4 6.28 37.03 56.69
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表 3 反应后催化剂的物相组成及其含量
Table 3 Iron phase composition of various spent catalysts after the FTS reaction test
Catalyst Phase composition /% Fe3O4 FexC χ-Fe5C2 ε-Fe2C Fe-270-1 75.84 9.50 14.67 Fe-270-4 65.10 25.80 9.08 Fe-290-4 28.19 32.03 39.78 Fe-310-4 11.95 43.67 44.38
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表 4 添加3%H2O反应后催化剂的物相组成及其含量
Table 4 Iron phase composition of various spent catalysts after FTS upon adding 3% H2O in the feed
Catalyst Phase composition /% Fe3O4 FexC χ-Fe5C2 ε-Fe2C Fe-270-1 90.61 1.00 8.40 Fe-270-4 88.62 6.32 5.06 Fe-290-4 52.90 35.67 11.43 Fe-310-4 7.55 70.30 22.14
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