Effect of cobalt metal loading on Fischer–Tropsch synthesis activities over Co/γ-Al₂O₃ catalysts: CO conversion, C₅₊ productivity, and α value

Abstract

This study investigates the effect of cobalt loading (5–20 wt% Co) on the physical properties of the alumina support and cobalt particle size of Co/γ-Al₂O₃ catalysts for Fischer–Tropsch synthesis (FTS) reaction (T = 210 °C (set), P = 20 bar, H₂/CO = 2). To characterize the catalysts and correlate these characteristics with their catalytic activities in FTS, N₂ adsorption, inductively coupled plasma, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) studies were conducted. N₂ adsorption and XRD analyses showed that as the cobalt loading was increased up to 15 wt%, the number of cobalt oxide particles increased by the direct interaction between cobalt oxide and the alumina support surface, but that when the cobalt loading was further increased to 20 wt%, the particle size of the cobalt oxide increased abruptly due to the additional cobalt loading on the previously loaded cobalt oxide. These physical properties of the supported cobalt catalysts were attributed to the pore structure of the alumina support. From the TEM micrographs, the size of cobalt particles was roughly estimated to increase from 20 to 50 nm at a cobalt loading up to 15 wt% Co/γ-Al₂O₃ to 70–100 nm at 20 wt% cobalt loading. For the 5–15 wt% Co/γ-Al₂O₃ catalysts, CO conversion and C₅₊ liquid oil productivity increased with increasing cobalt loading because they were strongly proportional to the number of cobalt particle active sites. However, the 20 wt% Co/γ-Al₂O₃ catalyst showed the highest value of α because the larger cobalt particles increased the opportunity for chain growth. The XPS data also supported the greater reducibility of the cobalt species in 20 wt% Co/γ-Al₂O₃ and hence its larger size compared to that at low cobalt loading.