Catalysis Letters, Vol.144, No.1, 123-132, 2014
Fischer-Tropsch Synthesis: Impact of H-2 or CO Activation on Methane Selectivity
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, H-2/CO = 2 and GHSV = 0.4-6 Nl g (-1)(cat) h(-1). The cobalt catalysts were activated by H-2 and CO, respectively, in two separate runs. For the H-2 activated catalyst, the methane selectivity decreased (15-6 %) and C5+ selectivity increased (80-91 %) linearly with increasing CO conversion from 10 to 60 %. The C-2-C-4 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 H-2 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 H-2 activated catalyst, with a decrease in methane and an increase in C5+ selectivity with CO conversion.