Mechanisms and Energies of Water Gas Shift Reaction on Fe-, Co-, and Ni-Promoted MoS₂ Catalysts

Abstract

The reaction mechanisms and promoting effect in the water gas shift reaction on (Fe, Co, Ni)-MoS 2 have been computed at the level of density functional theory. The Mo-edge with 25% M substitution and 25% sulfur coverage is identified as the active site. On the redox reaction path (CO + H 2 O → CO + OH + H; OH → O + H; CO + O + 2H → CO 2 + 2H), surface OH dissociation into O + H is the rate-determining step. On the carboxyl reaction path (CO + OH → COOH; COOH → CO 2 + H), surface COOH dissociation into CO 2 + H is the rate-determining step. On the basis of the computed effective barriers, the redox mechanism is kinetically more preferable than the carboxyl mechanism for Mo, Fe, and Co, whereas both mechanisms have very close effective barriers for Ni. Compared with pure MoS 2, Ni has the largest promotion effect in reducing the effective barrier (0.72 eV), followed by Co (0.45 eV), whereas Fe has the smallest effect (0.04 eV). This promotion effect is tightly associated with the electronegativities and number of the valence electrons of the metals. As the best catalyst, Ni-MoS 2 has the lowest adsorption energies of the surface intermediates and also the lowest effective barrier.