Desulfurization Reactions on Surfaces of Metal Carbides: Photoemission and Density–Functional Studies

High-resolution photoemission and density functional (DF) calculations were used to study the interaction of atomic sulfur and S-containing molecules with metal carbides in which the carbon/metal ratio varies from 0.5 to 1 (M₂C and MC, M = Ti, V or Mo). In these compounds, the C sites cannot be considered as simple spectators. They moderate the reactivity of the metal centers and provide bonding sites for adsorbates. For example, the adsorption of S on TiC(001) induces a large positive shift (1.0–1.3 eV) in the C 1s core level. DF calculations give a CTiTi hollow as the most stable site for the S adatoms. There is a correlation between the adsorption energy of S or thiophene and shifts in the centroid of the metal d band induced by metal–carbon bonding in the metal carbides. The M₂C and MC carbides have difficulty obeying Sabatier’s principle for being good HDS catalysts because some of them interact too strongly with the products (M₂C stoichiometry) and the others have problems dissociating the reactants (MC stoichiometry). The addition of small Au nanoparticles is an efficient way for enhancing the HDS activity of MC catalysts. In spite of the very poor desulfurization performance of TiC and MoC, the Au/TiC and Au/MoC systems display an HDS activity comparable or higher than that of conventional Ni/MoS _x catalysts. The Au nanoparticles probably increase the HDS activity of the metal carbides by enhancing the adsorption energy of thiophene and by helping in the dissociation of H₂ to produce the hydrogen necessary for the hydrogenolysis of C–S bonds and the removal of sulfur.