Theory of Carbon-Sulfur Bond Activation by Small Metal Sulfide Particles

Elementary reaction steps for the catalytic cycle of thiophene desulfurization on Ni3Sy and Ni4Sy clusters are investigated using d. functional quantum chem. calcns. The Ni3S2 cluster is active while the Ni4Sy cluster is relatively inactive for HDS catalysis. Adsorption and overall reaction energies are computed on complete geometry-optimized cluster-adsorbate systems. The nickel-sulfide cluster is found to significantly reorganize upon interaction with adsorbates. Sulfur readily rearranges between 3-fold and 2-fold binding sites. Hydrogen adsorbs mol. and dissocs. heterolytically over Ni3S2 to form both adsorbed sulfhydryl (SH) and hydryl (MH) species. The presence of coadsorbed hydrogen affects both the heat of adsorption and the coordination of thiophene. On the bare Ni3S2 cluster thiophene binds with h4-coordination, while in the presence of coadsorbed hydrogen thiophene prefers the h1 site. 2,5-Dihydrothiophene (DHT) adsorbs somewhat more strongly than thiophene on the Ni3S2 cluster. In the preferred h3 configuration, the ethylene moiety of DHT adsorbs at one nickel atom site while its sulfur adsorbs at the neighboring nickel atom site. For the HDS cycles initiated by h1 or h4 thiophene adsorption,the energy change assocd. with the carbon-sulfur bond scission step of adsorbed dihydrothiophene and that for the removal of sulfur via H2S are the most endothermic steps and are speculated to be rate-limiting. Their comparable values indicate that the two steps compete. The cycle which is initiated by the removal of sulfur from Ni3S2 is energetically unfavorable. [on SciFinder (R)]