费托合成铁基催化剂中Fe_(3)O_(4)含量对CO_(2)选择性的影响

Effect of Fe_(3)O_(4)content on the CO_(2)selectivity of iron-based catalyst for Fischer-Tropsch synthesis

本研究以共沉淀法制备的α-Fe_(2)O_(3)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H_(2)O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CO_(2)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Fe_(3)O_(4)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H_(2)O分压较低时,动力学因素限制了水煤气变换(WGS)反应的进行,CO_(2)选择性仅受CO转化率的影响,Fe_(3)O_(4)含量变化对CO_(2)选择性无明显影响;而在较高的H_(2)O分压下,随着催化剂中Fe_(3)O_(4)含量增加,CO_(2)选择性也随之增加。本文初步阐明了Fe_(3)O_(4)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CO_(2)生成的活性相提供了新的信息,为新型低CO_(2)选择性费托合成工业催化剂的设计奠定了基础。

In this study,a series of catalysts with different Fe_(3)O_(4)to iron carbide ratios were obtained by carburizing theα-Fe_(2)O_(3)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 CO_(2)formation,the catalysts were tested under different reaction conditions by tuning either CO conversion or H_(2)O 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 CO_(2)selectivity is determined by the CO conversion rather than the Fe_(3)O_(4)content in the catalyst,meaning that the WGS reaction is here limited by the kinetic factors.On the contrary,adding H_(2)O to the reaction gas results in the trend that higher CO_(2)selectivity is promoted by higher content of Fe_(3)O_(4)in the Fe-based catalyst.It seems that Fe_(3)O_(4)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 CO_(2)generation on the Fe-based catalysts,which could be the theoretical basis for the design of new industrial FTS catalysts with low CO_(2)selectivity.