The effect of surface carbon on ethylene dimerization

Highlights: • The effect of surface carbon on ethylene dimerization is investigated on a Co₃C-Co/SiO₂ model catalyst. • The Co₃C-Co/SiO₂ catalyst shows a high selectivity of C₄ products compared with Co/SiO₂ catalyst. • Surface carbon blocks hydrogen adsorption suppressing hydrogenation of ethylene to ethane and promoting dimerization to C₄ products. • DFT calculations reveal that the effective barrier of CCH₃* dimerization on Co₃C is lower than that on Co, explaining the high selectivity to C₄ products on the Co₃C-Co/SiO₂ catalyst. The oligomerization of olefins is important to the production of the high hydrocarbons. In the oligomerization process, it is always accompanied by the reaction of hydrogenation. In the present article, we report that surface carbon affects the ethylene (C₂H₄) hydrogenation on the supported cobalt catalysts. Co/SiO₂ catalyst is carburized with CO and Co₃C structure as well as Co structure is revealed with Transmission Electron Microscopy (TEM). We investigate ethylene hydrogenation on the Co/SiO₂ catalyst and the carburized Co₃C-Co/SiO₂ catalyst. It exhibits a high selectivity of C₄ products on the carburized Co₃C-Co/SiO₂ catalyst. We further apply Density Functional Theory (DFT) calculation to study the effect of surface carbon on the dimerization of ethylene. Co(0001) and Co₃C(2 2 1) are chosen to represent the Co/SiO₂ and Co₃C-Co/SiO₂ catalysts, respectively. The dimerization of ethylene occurs by CCH₃* species coupling instead of CH₂CH₂* coupling. The effective barrier of CCH₃* coupling on Co(0001) is higher than that on Co₃C(2 2 1). This explains the high selectivity to C₄ products on the carburized Co₃C-Co/SiO₂ catalyst. Our study provides experimental and theoretical insight for the effect of surface carbon to the reaction of ethylene with hydrogen. It may guide us to reasonable design of the catalyst for olefin oligomerization.