The texture evolution of g-C3N4 nanosheets supported Fe catalyst during Fischer-Tropsch synthesis

Abstract

Graphitic carbon nitride (GCN) nanosheets decorated with iron nanoparticles (Fe/g-C3N4) were synthesized and used as catalysts in FTS. The crystal structure and morphology of Fe/g-C3N4 were characterized with XRD, FT-IR, HRTEM, EXAFS, BET, and its texture evolution during Fischer-Tropsch synthesis was monitored. It was shown that there were two types of iron species in Fe/GCN, i.e., “dissolved iron”, in which irons dissolved into g-C3N4 matrix as single atoms, and formed mainly at low Fe loadings. In contrast, when the loading exceeded the dissolved saturation, the irons would coordinate with (-N-CN-)2− to form the second type of iron species, Fe(NCN) like species. The “dissolved iron” was more difficult to thermally decompose and convert into iron carbide than Fe(NCN) like species. Such a slow transformation gave birth to unique induction period for low loading Fe catalysts. Moreover, the N-containing bases or defects of g-C3N4 might play a critical role in enhancing O/P ratio. As a result, g-C3N4 supported 40wt% Fe sample exhibited highest activity and C2=-C4=selectivity. Besides, the g-C3N4 possessed a “structure memory effect” analogous to hydrotalcite.