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

Elementary reaction steps for the catalytic cycle of thiophene desulfurization on Ni₃S_y and Ni₄S_y, clusters are investigated using density functional quantum chemical calculations. The Ni₃S₂ cluster is active while the Ni₄S_y, 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 molecularly and dissociates 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 η⁴-coordinated, while in the presence of coadsorbed hydrogen thiophene prefers the η¹ site. 2,5-Dihydrothiophene (DHT) adsorbs somewhat stronger than thiophene on the Ni₃S₂ cluster. In the preferred η³ configuration, the ethylene moiety of the DHT adsorbs at one nickel atom site while its sulfur adsorbs at the neighboring nickel atom site. For the HDS cycles initiated by η¹ or η⁴ thiophene adsorption, the energy change associated 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.