Particle Size Effects of Cobalt Carbide for Fischer-Tropsch to Olefins

Particle size effects of the cobalt carbide (Co2C) catalyst on its catalytic performance for Fischer-Tropsch to olefins were investigated. When the Co2C nanoparticles were smaller than 7 nm, increasing the particle size led to enhanced intrinsic activity based on the turnover frequency (TOF), higher selectivity to lower olefins, higher ratio of olefin to paraffin, and lower methane selectivity. However, when the Co2C nanoparticles were larger than 7 nm, both intrinsic activity and product selectivity did not depend on the particle size. Further kinetic studies showed that both the apparent activation energy and the reaction order of H-2 decreased, while the reaction order of CO increased with decreasing Co2C particle size when the size was smaller than 7 nm. In contrast, these kinetic parameters were nearly constant when the Co2C particle size was larger than 7 nm. Theoretical analysis revealed a strong correlation between the exposed facets and Co2C particle sizes, leading to the observed dependence of catalytic performance on the catalyst particle size.