Use of Plasma-Synthesized Nano-Catalysts for CO Hydrogenation in Low-Temperature Fischer⁻Tropsch Synthesis: Effect of...

A study was done on the effect of temperature and catalyst pre-treatment on CO hydrogenation over plasma-synthesized catalysts during the Fischer⁻Tropsch synthesis (FTS). Nanometric Co/C, Fe/C, and 50%Co-50%Fe/C catalysts with BET specific surface area of ~80 m² g^⁻1 were tested at a 2 MPa pressure and a gas hourly space velocity (GHSV) of 2000 cm³ h^-1 g^-1 of a catalyst (at STP) in hydrogen-rich FTS feed gas (H₂:CO = 2.2). After pre-treatment in both H₂ and CO, transmission electron microscopy (TEM) showed that the used catalysts shifted from a mono-modal particle-size distribution (mean ~11 nm) to a multi-modal distribution with a substantial increase in the smaller nanoparticles (~5 nm), which was statistically significant. Further characterization was conducted by scanning electron microscopy (SEM with EDX elemental mapping), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The average CO conversion at 500 K was 18% (Co/C), 17% (Fe/C), and 16% (Co-Fe/C); 46%, 37%, and 57% at 520 K; and 85%, 86% and 71% at 540 K respectively. The selectivity of Co/C for C₅₊ was ~98% with 8% gasoline, 61%, diesel and 28% wax (fractions) at 500 K; 22% gasoline, 50% diesel, and 19% wax at 520 K; and 24% gasoline, 34% diesel, and 11% wax at 540 K, besides CO₂ and CH₄ as by-products. Fe-containing catalysts manifested similar trends, with a poor conformity to the Anderson⁻Schulz⁻Flory (ASF) product distribution.