| Abstract
| - X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study theinteraction of thiophene, H2S, and S2 with Ni2P(001), α-Mo2C(001), and polycrystalline MoC. In general, thereactivity of the surfaces increases following the sequence MoC < Ni2P(001) < α-Mo2C(001). At 300 K,thiophene does not adsorb on MoC. In contrast, Ni2P(001) and α-Mo2C(001) can dissociate the moleculeeasily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition productsof thiophene (CxHy fragments and S) from these surfaces. Our experimental and theoretical studies indicatethat the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbedsulfur into gaseous H2S. Ni2P is a better catalyst for HDS than Mo2C or MoC. The P sites in the phosphideplay a complex and important role. First, the formation of Ni−P bonds produces a weak “ligand effect”(minor stabilization of the Ni 3d levels and a small Ni → P charge transfer) that allows a high activity for thedissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surfacedecreases due to an “ensemble effect” of P, which prevents the system from deactivation induced by highcoverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding tothe products of the decomposition of thiophene and the H adatoms necessary for hydrogenation.
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