Particle-Size-Dependent Methane Selectivity Evolution in Cobalt-Based Fischer–Tropsch Synthesis

The effects of the cobalt particle size in the range of 6.5–26.3 nm on the catalyst structural transformation and methane selectivity evolution in Fischer–Tropsch synthesis (FTS) have been investigated. In situ X-ray diffraction experiments revealed that cobalt was maintained as CoO under a reaction environment when initial Co3O4 particles on SiO2 were below 8.4 nm. The large metallic Co particles can keep a stable size and phase state, indicating almost unchanged catalytic activity and methane selectivity. In contrast, small metallic Co or CoO particles show a gradual increase in methane selectivity with time on stream. Stable large metallic Co particles can be explained as the dominant surface high-coordinated Co atoms having a weak binding ability for H2O molecules, which are more difficult to be oxidized into CoO. It is believed that CoO rather than metallic Co atoms could interact with Si-OH groups to form inactive cobalt silicate (Co2SiO4), which leads to a higher methane selectivity and drives a decrease in the particle size during the FTS reaction. ZrO2 as a structural promoter highly dispersed on SiO2 was found to give a milder increase in methane selectivity due to fewer chances of Si-OH interacting with CoO species. Density functional theory (DFT) calculations combined with CO temperature-programmed desorption (CO-TPD), NH3-TPD, CO2-TPD, and Py-IR demonstrate that Co/ZrO2 interfacial sites play a crucial role in enhancing the catalytic activity as the electron accumulation in the interface region not only increases the CO adsorption amount but also promotes the CO dissociation on cobalt sites. At the same time, a higher surface C*/H concentration benefits a lower methane selectivity. ZrO2, which acts as a Lewis acid, can adsorb CO molecules abundantly but weakly by the interaction between Zr4+ cations and O atoms in CO. Especially, for the 15Co(C)/Q15 catalyst with initial CoO particles on SiO2, methane selectivity rapidly increases at an early reaction stage, which originates from easier accessible CoO reacting with Si-OH groups. In addition, both 15Co(C)/Q15 and 15Co(C)/10Zr(I)-Q15 catalysts show the dominant CoO phase, which is active in the FTS reaction. Indeed, the DFT calculations demonstrate that CO activation on the CoO(200) surface via the H-assisted CH2O dissociation pathway exhibits a lower activation energy of 1.21 eV compared to that on the face-centered cubic (fcc) Co(111) surface with an activation energy of 1.42 eV, despite that direct CO dissociation, CHO dissociation, and CHOH dissociation on CoO(200) show higher activation energies. This study provides a prime scientific understanding about regulating catalytic activity and methane selectivity.