Mechanism of coke deposition on the surface of molybdenite

The formation of coke is studied by SEM/TEM on the basal plane of a thin (ca. 500 Å) single-crystal film of MoS₂. From exposure to benzene at 550 and 700 °C, which is exothermic in decomposition, carbon islands are formed on the face opposite to the exposure face; whereas for the endothermic decomposition of methane (at 700 °C), carbon islands are formed on the exposure face. This result provides a direct evidence for the carbon diffusion mechanism for coke deposition in which a temperature gradient is the driving force for carbon diffusion. It is demonstrated that the formation and growth of thin (perhaps monolayer) carbon islands can be studied by gold decoration/TEM. A gold-decoration/TEM study of coke formation of benzene on MoS₂ show that screw dislocations are not preferred sites for carbon deposition, and that steps on the basal plane are likely the active sites for decomposition of benzene.