Effect of Fe3O4 content on the CO2 selectivity of iron-based catalyst for Fischer-Tropsch synthesis

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.