DFT computations on surface physical adsorption of hydrocarbons produced in the Fischer-Tropsch synthesis on a CNT/Co nanocatalyst

Abstract

The physical sorption of several paraffin and olefin hydrocarbons produced in the Fischer-Tropsch (FT) synthesis on the surface of cobalt nanocatalyst supported on (4,4)-armchair carbon nanotube (CNT) was studied using density functional theory (DFT) computations accomplished at B3LYP level of theory. The most negative binding energy (ΔEbinding) was obtained for (4,4)-CNT-Co/C5H10 (compound 10) in which pentene was adsorbed on the CNT/Co surface reflecting this compound had the most retention time to participate in secondary reactions on the surface of the (4,4)-CNT-Co nanocatalyst. The adsorption of all olefin species on the surface of (4,4)-CNT-Co nanocatalyst was exergonic (ΔGadsorption < 0). The negative ΔHadsorption values for compounds 4, 6, 10 and 11 indicated their exothermic adsorption nature. The QTAIM data supported the covalent character of the CC bonds in the (4,4)-CNT and the electrostatic nature of CCo bonds in the (4,4)-CNT-Co nanocatalyst. The nuclear quadrupole coupling constants for the quadrupole 2H, 59Co and 60Co were measured about 180–220 kHz, 23–140 MHz and 26–155 MHz, respectively. According to greater ΔEbinding values for the olefin containing structures, it was inferred that adsorption of the olefins were preferred on the catalyst surface compared with their related paraffin hydrocarbons. Hence, it was established that the (4,4)-CNT-Co nanocatalyst had a superior performance to produce heavy hydrocarbons instead of light types which is very beneficial for the FT process.