Investigation of the deactivation behavior of Co catalysts in Fischer-Tropsch synthesis by using encapsulated Co nanoparticles with controlled SiO2 shell layer thickness
The stability of cobalt catalysts in the Fischer-Tropsch synthesis (FTS) is one of the most challenging issues, which requires to be solved for its industrial application. Herein, a serial of the model silica-encapsulated core-shell Co@SiO2 catalysts have been synthesized with different thickness of the SiO2 shell layer. Our results show that the thickness of porous silica layer and the reaction temperature play crucial roles in helping us to search for the origin of catalyst deactivation. At a high temperature (240 °C), the Co@SiO2 catalyst exhibits a high catalytic activity, producing a high water vapor concentration inside catalyst particles along with higher content of the light hydrocarbons. Therefore, the oxidation of metallic Co is the dominant deactivation behavior as the thinner shell layer facilitates the water vapor diffusion out of the catalyst surface and the pore channels to alleviate catalyst deactivation. At a low temperature (220 °C), the Co@SiO2 catalyst initially reaches a relatively high catalytic activity and then rapidly deactivates for all investigated catalysts, which is mainly attributed to the blocking of pore channels of silica layer and the covering of catalyst active sites by heavier hydrocarbons leading to the syngas inefficient access onto the active sites. Note that the thickness of SiO2 shell layer indicates much less impact on the catalyst deactivation from the wax accumulation. As a result, the 40Co@SiO2 catalyst at 240 °C indicates an excellent stability with almost ~100% CO conversion due to the efficient avoiding of metallic Co oxidation and pore channels blocking with thinner shell layer. The proposed new strategy can provide viable information to elucidate the catalyst deactivation behavior and thereof direct us to develop the stable cobalt catalysts in FTS reaction.