In situ XAFS study on the formation process of cobalt carbide by Fischer–Tropsch reaction

Abstract

Fischer–Tropsch (F–T) synthesis is an effective approach to convert the syngas of H₂ and CO into lower olefin and other valuable products for the chemical industry. Cobalt carbide (Co₂C), which was regarded as the sign of activity loss in the past, has recently been recognized as a highly-active phase for F–T synthesis. However, systematic study on the formation process of Co₂C by F–T reaction is still lacking. Herein, for the first time, in situ XAFS (X-ray Absorption Fine Structure) experiments were conducted to elaborate the Co₂C formation under operando conditions. F–T reaction processes starting from Co and CoO were analysed with the conclusion that Co₂C could be formed under both conditions. For the CoO process, Co₂C was transformed directly from CoO as a wavelet transform and EXAFS fitting results revealed that there was no sign of Co metal in the whole process. Thermodynamic analysis indicated that the ΔG value of the CoO process is much smaller than that of the Co process, which means that CoO is thermodynamically easier to transform to Co₂C. Combining with the shorter reduction time from Co₃O₄ to CoO, it can be concluded that CoO is more favourable as the precursor to synthesize Co₂C, which might be applied to the F–T industry. Besides, catalytic evaluation shows that the CO₂ selectivity, CO conversion and the ratio of olefin/paraffin for the CoO process are different from those of the Co process. In addition, the reaction temperatures were also investigated wherein Co₂C would be partially transformed to metallic Co when the temperature was increased up to 270 °C. This work provides fundamental and applicable guidance towards the synthesis of Co₂C by F–T reaction.