Catalytic characteristics of active corner sites in Co-Mo-S nanostructure hydrodesulfurization - A mechanism study based on DFT calculations

A non-periodic computational Co-Mo-S 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 Co-Mo-S 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 Mo-S bonds; the former is more stable. The hydrogen dissociation on short Mo-S bond is endothermic and with high activation energy, whereas the hydrogen dissociation on long Mo-S bond is just the opposite. The Mo-X has only short Mo-S bonds. The Co atoms on the Mo-X and Corner exhibit a square planar coordinated structure with four sulfur atoms with a Co-Mo atom pair. The Co-Mo 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 C-S 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 Co-Mo and di-sulfur atom pairs. Hence, increasing Corner sites in Co-Mo-S is effective and beneficial to HDS catalysis. (C) 2016 Elsevier Inc. All rights reserved.