Ab initio DFT study of hydrogen dissociation on MoS2, NiMoS, and CoMoS: mechanism, kinetics, and vibrational frequencies

The present study provides detailed discussions about the structures, relative stabilities, and vibrational frequencies of hydrogen species on MoS2, NiMoS, and CoMoS catalyst edge surfaces. The transition states and activation energies for molecular hydrogen dissociation and surface migration of atomic hydrogen on catalyst edge surfaces have been calculated by complete linear synchronous transit (LST) and quadratic synchronous transit (QST) search methods. It has been found that the heterolytic dissociation of molecular hydrogen at a pair of sulfur-metal sites to form an SH group and a metal hydride is energetically preferred. The dissociation of molecular hydrogen on the Ni-promoted (101¯0) metal edge of NiMoS requires slightly lower activation energy than that on the unpromoted (101¯0) Mo-edge of MoS2 (0.87 and 0.91 eV, respectively). The dissociation of molecular hydrogen on the unpromoted (1¯010) S-edge requires a large activation energy (about 1.0 eV), and the addition of cobalt to the (1¯010) S-edge significantly lowers the dissociation energy to approximately 0.6 eV. The atomic hydrogen species on the (1¯010) S-edge and the Co-promoted (1¯010) S-edge are less mobile than on the (101¯0) Mo-edge of MoS2 or the Ni-promoted (101¯0) metal edge of NiMoS. The calculated vibrational frequencies of different surface hydrogen species agree well with reported experimental observations and have provided references for further spectroscopic experiments.