Fischer–Tropsch Synthesis: Impact of H2 or CO Activation on Methane Selectivity

Abstract

The effect of CO conversion on methane selectivity of a 10 % Co/TiO2 catalyst was investigated using a 1L continuously stirred tank reactor at 493–503 K, 2.4 MPa, H2/CO = 2 and GHSV = 0.4–6 Nl g cat −1 h−1. The cobalt catalysts were activated by H2 and CO, respectively, in two separate runs. For the H2 activated catalyst, the methane selectivity decreased (15–6 %) and C5+ selectivity increased (80–91 %) linearly with increasing CO conversion from 10 to 60 %. The C2–C4 hydrocarbon selectivity decreased slightly with CO conversion. The enhancement in C5+ selectivity was mainly due to the decrease in methane selectivity. The CO activated cobalt catalyst was initially in a carburized form after activation, and then converted to a form with greater metallic character when exposed to syngas at Fischer–Tropsch conditions. The CO activated catalyst at pseudo steady-state had higher methane selectivity in general as compared to the H2 activated one, possibly due to the presence of the cobalt carbide phase. Two kinetic regions appear to exist for the CO activated catalyst. When CO conversion is lower than 20 %, the methane selectivity increased exponentially with CO conversion, which is attributed to enhanced hydrogenation. When CO conversion is higher than 20 %, the CO activated catalyst behaves more like a H2 activated catalyst, with a decrease in methane and an increase in C5+ selectivity with CO conversion.