Hydrogen spillover effects in the Fischer–Tropsch reaction over carbon nanotube supported cobalt catalysts

Abstract

Fischer–Tropsch synthesis (FTS) plays a crucial role in the conversion of syngas derived from various carbon-based resources into value-added fuels and chemicals. As a complicated reaction network-involved hydrogenation process, the hydrogen spillover onto the support surface influencing the catalytic performance has not been thoroughly investigated. Herein, we synthesized pretreated carbon nanotube (CNT) supported cobalt catalysts along with tuning the cubic cobalt particle size, which exhibited a significant difference in atomic H species spillover due to the changed surface defects (or possibly related to the oxidized carbon surface functionalities) on CNTs. Interestingly, it was found that weaker atomic H species spillover resulted in much higher TOF values and lower/stable methane selectivity over larger metallic Co particles, which was ascribed to the lower support surface atomic H* species storage capacity. In contrast, smaller Co particles on CNTs led to lower TOF values and continuously increased methane selectivity till the late stage of the 20 h catalytic testing, which might be due to unstable surface H* species distribution triggered by increasing surface defects (and/or oxidized surface functionalities) to accept more spillover H species and reverse H species spillover. Additionally, it was also demonstrated that the SiO₂-supported cobalt catalysts exhibited a notably lower and stable selectivity to methane, which was attributed to the fact that the absence of surface defects did not bring about the modified and higher H species concentration due to the negligible hydrogen spillover effect. This work conceptually constructed model cobalt catalysts to illustrate the effects of surface active reactants and intermediate dynamic distribution on the catalytic activity and product selectivity.