Methanol synthesis from H₂ and CO₂ on a Mo ₆S₈ cluster: A density functional study

Catalytic CO₂ hydrogénation to methanol has received considerable attention as an effective way to utilize CO₂. In this paper, density functional theory was employed to investigate the methanol synthesis from CO₂ and H₂ on a Mo₆S₈ cluster. The Mo₆S₈ cluster is the structural building block of the Chevrel phase of molybdenum sulfide, and has a cagelike structure with an octahedral Mo₆ metallic core. Our calculations indicate that the preferred catalytic pathway for methanol synthesis on the Mo ₆S₈ cluster is very different from that of bulklike MoS₂. MoS₂ promotes the C-O scission of H_xCO intermediates, and therefore, only hydrocarbons are produced. The lower S/Mo ratio for the cluster compared to stoichiometric MoS₂ might be expected to lead to higher activity because more low-coordinated Mo sites are available for reaction. However, our results show that the Mo₆S ₈ cluster is not as reactive as bulk MoS₂ because it is unable to break the C-O bond of H_xCO intermediates and therefore cannot produce hydrocarbons. Yet, the Mo₆S₈ cluster is predicted to have moderate activity for converting CO₂ and H ₂ to methanol. The overall reaction pathway involves the reverse water-gas shift reaction (CO₂ + H₂ → CO + H ₂O), followed by CO hydrogenation via HCO (CO + 2H₂ → CH₃OH) to form methanol. The rate-limiting step is CO hydrogénation to the HCO with a calculated barrier of +1 eV. This barrier is much lower than that calculated for a comparably sized Cu nanoparticle, which is the prototypical metal catalyst used for methanol synthesis from syngas (CO + H₂). Both the Mo and S sites participate in the reaction with CO₂, CO, and CH_xO preferentially binding to the Mo sites, whereas S atoms facilitate H-H bond cleavage by forming relatively strong S-H bonds. Our study reveals that the unexpected activity of the Mo ₆S₈ cluster is the result of the interplay between shifts in the Mo d-band and S p-band and its unique cagelike geometry.