Tunable Fe3O4 nanoparticles assembled porous microspheres as catalysts for Fischer-Tropsch synthesis to lower olefins

Fe catalyzed Fischer-Tropsch synthesis to lower olefins (FTO) has been recognized as a structure-sensitive reaction. Nanostructure Fe-based catalysts can expose more active surface. But bulk nanostructure Fe catalysts without support interaction show easily sintered and agglomerated, which makes hard to investigate size effect. Therefore, the effect of particle size on bulk Fe catalysts have rarely been reported. Herein, nanoparticles assembled Fe3O4 bulk catalysts were synthesized via a PAA-mediated solvothermal method to solely investigate the size effect of Fe phase. By tuning preparation parameters, a series of porous-Fe3O4 microspheres assembled by different nanoparticle size (8.5–16.5 nm) was obtained, maintaining constant microspheres size. When Fe3O4 nanoparticles are smaller than 10 nm, catalysts show similar catalytic activity (FTY). Beyond 10 nm, the FTY obviously decreases with increasing of particle size. Particularly, Fe3O4 with nanoparticle size of 9.9 nm performs the highest activity as well as C2-C4= selectivity and O/P ratio. The smaller particles attribute to C5+ formation, while inhibit CH4 selectivity. By CO-TPD, TPH-MS and XRD analysis, we discuss size effect on CO adsorption, surface carbon species and carburization degree, building relationship between particle size and catalytic activity. It is found that the carburization degree of iron phase correlates positively with catalytic activity. These results deepen understanding of the structure-performance relationship for bulk iron catalysts in FTO.