Reaction and free-energy pathways of hydrogen activation on partially promoted metal edge of CoMoS and NiMoS: A DFT and thermodynamics study

Understanding of molecular hydrogen (H-2) activation mechanism on MoS2-based catalysts is crucial for enhancement of catalytic hydrotreating processes. In this work, H-2 activation reaction pathways including adsorption, dissociation, and diffusion phases on metal edge of partially Co-promoted MoS2 (CoMoS) and partially Ni-promoted MoS2 (NiMoS) catalysts under hydrotreating conditions have been investigated using density functional theory and thermodynamic calculations. Here, investigation of H-2 adsorption on CoMoS and NiMoS catalysts shows that H-2 molecule prefers to adsorb on the promoter site rather than the sulfur site, while the H-2 molecule adsorbs firmly on CoMoS but substantially weaker on NiMoS. H-2 dissociation is the rate-determining step for both CoMoS and NiMoS catalysts and the activation energy (E-a) of rate-determining step for both catalysts is identical (E-a = 0.79 eV). However, thermodynamic result indicates that CoMoS is more reactive toward H-2 activation than NiMoS (free energy of activation (Delta G(double dagger)) at 575 K = 0.65 and 1.14 eV for CoMoS and NiMoS, respectively). In terms of diffusion, hydrogen atom migrates relatively easy (E-a < 0.55 eV) on both CoMoS and NiMoS surfaces. Partial charge analysis reveals that both heterolytic and homolytic H-2 dissociation characteristics are observed on CoMoS and NiMoS depending on the reaction site. In addition, dissociated hydrogen atoms are more stable in terms of thiol group (S-H) on CoMoS while metal-hydrogen pairs (Mo-H and Ni-H) are more stable on NiMoS. (C) 2017 Elsevier B.V. All rights reserved.