Fischer-Tropsch synthesis: Effect of pretreatment conditions of cobalt on activity and selectivity for hydrogenation of carbon dioxide

The effect of pretreatment conditions of cobalt on activity and product selectivity for the hydrogenation of carbon dioxide was studied over a 1%Na/20%Co-SiO₂ catalyst using a fixed-bed catalytic reactor operated at 220 °C and 1.89 MPa. The metallic form of cobalt was obtained from the reduction of cobalt oxide (Co₃O₄) by H₂ at 350 °C and produced primarily methane and lower hydrocarbons (C₂-C₄) from CO₂. Pretreatments with pure H₂ or syngas (H₂:CO) at 250 °C yield a fraction of partially reduced cobalt (CoO), which tends to suppress the hydrogenation activity of cobalt somewhat, reducing methane selectivity to a limited degree. After CO activation, partially reduced cobalt oxide (CoO) and cobalt carbide phases formed and produced significantly less methane (i.e., selectivity of 15.3%), and surprisingly, the selectivity for alcohols increased to 73.2%. In the absence of sodium, direct methanation is still a preferred reaction for CO₂ for CO pretreated cobalt; the XRD results of used catalysts revealed that the cobalt carbide phase converted to metallic cobalt. The results indicate that Na doping and carbide are important for low methane and high oxygenate selectivities. However, further work is needed to determine whether the role of Na is merely to stabilize the cobalt carbide phase, or whether Na is involved in promoting the catalytic cycle.