Evaluation of nanosized iron in slurry-phase fischer-tropsch synthesis

The Fischer - Tropsch (F-T) activity of three α-Fe₂O₃-based materials, two unsupported nanosized, NANOCAT (3 nm) and BASF (20-80 nm), and a supported micrometer-sized (32.5 μm) UCI, were measured with respect to total hydrocarbon production from synthesis gas (H₂/CO ∼ 2/1). All three oxides were initially reduced under CO at 553 K in ethylflopolyolefin-164 solvent, and the extent of their reduction was established by monitoring CO₂ evolution. The ease of reduction of α-Fe₂O₃ followed the order: UCI > NANOCAT > BASF. The Fischer-Tropsch (F-T) synthesis activities of the three reduced materials were measured at 513 K. Mössbauer and X-ray absorption fine structure (XAFS) measurements established that the initial oxides were in the α-Fe₂O₃ phase. Room-temperature XAFS together with low-temperature Mössbauer data of the quenched catalyst samples after 120 h F-T reaction suggest that all three catalysts were essentially a mixture of oxides and carbides with magnetite being the dominant phase. The observed high activity with NANOCAT and the presence of mainly (88%) magnetite phase indicate that either the oxide phase contributes as a catalyst or the minor (12%) carbide phase is extremely active for F-T synthesis. TEM images of the quenched samples remarkably showed that both unsupported nano materials, NANOCAT and BASF, avoided expected agglomeration and the micrometer-sized UCI converted into a nano material of <10 nm under F-T reaction conditions. The narrow particle size distribution of the quenched catalysts in the nano range (∼ 10-50 nm) explains the very similar activities observed with the three different-sized starting materials in the order: BASF > NANOCAT ∼ UCI. The data suggest a crucial role of nanosized Fe during F-T synthesis.