Catalytic characteristics of active corner sites in Co[formula omitted]Mo[formula omitted]S nanostructure hydrodesulfurization – A mechanism study based on DFT calculations

Abstract

A non-periodic computational CoMoS model consisting of un-promoted Mo-edge (Mo-E), Co-mixed Mo-edge (Mo-X), Co-edge (Co-E) and corner (Corner) sites was used to examine the structural effect of CoMoS nanocluster on its hydrodesulfurization (HDS) reaction activity. A density functional theory (DFT) with numerical atomic functions was used to predict the fine structures of Mo-E, Mo-X, Co-E and Corner under common HDS condition. The elementary thiophene HDS reactions on Mo-X, Co-E and Corner sites were compared. The results show that the Mo-E has short and long MoS bonds; the former is more stable. The hydrogen dissociation on short MoS bond is endothermic and with high activation energy, whereas the hydrogen dissociation on long MoS bond is just the opposite. The Mo-X has only short MoS bonds. The Co atoms on the Mo-X and Corner exhibit a square planar coordinated structure with four sulfur atoms with a CoMo atom pair. The CoMo atom pair can easily form coordinatively unsaturated active site (CUS) by H2S desorption. The CUSs on the Mo-X and Corner sites preferably bond with radical containing sulfides to spin-restricted sulfides, leading to high hydrogenolysis activity of the CS bonds. The di-sulfur atom pairs on the Co-E and Corner sites are effective to dissociate the hydrogen molecule and reduce steric hindrance from adsorbing sulfur intermediates during HDS, leading to high hydrogenation activity at the Co-E and Corner sites. Thiophene HDS reactions can process completely on the Corner site which contains CoMo and di-sulfur atom pairs. Hence, increasing Corner sites in CoMoS is effective and beneficial to HDS catalysis.