Methanol synthesis from H2 and CO2 on a Mo6S8 cluster: a density functional study.
Catalytic CO(2) hydrogenation to methanol has received considerable attention as an effective way to utilize CO(2). In this paper, density functional theory was employed to investigate the methanol synthesis from CO(2) and H(2) on a Mo(6)S(8) cluster. The Mo(6)S(8) cluster is the structural building block of the Chevrel phase of molybdenum sulfide, and has a cagelike structure with an octahedral Mo(6) metallic core. Our calculations indicate that the preferred catalytic pathway for methanol synthesis on the Mo(6)S(8) cluster is very different from that of bulklike MoS(2). MoS(2) promotes the C-O scission of H(x)CO intermediates, and therefore, only hydrocarbons are produced. The lower S/Mo ratio for the cluster compared to stoichiometric MoS(2) might be expected to lead to higher activity because more low-coordinated Mo sites are available for reaction. However, our results show that the Mo(6)S(8) cluster is not as reactive as bulk MoS(2) because it is unable to break the C-O bond of H(x)CO intermediates and therefore cannot produce hydrocarbons. Yet, the Mo(6)S(8) cluster is predicted to have moderate activity for converting CO(2) and H(2) to methanol. The overall reaction pathway involves the reverse water-gas shift reaction (CO(2) + H(2) --> CO + H(2)O), followed by CO hydrogenation via HCO (CO + 2H(2) --> CH(3)OH) to form methanol. The rate-limiting step is CO hydrogenation 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 + H(2)). Both the Mo and S sites participate in the reaction with CO(2), CO, and CH(x)O 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(6)S(8) cluster is the result of the interplay between shifts in the Mo d-band and S p-band and its unique cagelike geometry.
Liu
Ping
P
Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, USA. pingliu3@bnl.gov
Choi
YongMan
Y
Yang
Yixiong
Y
White
Michael G
MG