Periodic trends in hydrotreating catalysis: Thiophene hydrodesulfurizationover carbon-supported 4d transition metal sulfides

The kinetics of atmospheric gas-phase thiophene hydrodesulfurization (HDS) over five carbon-supported 4d transition metal sulfide catalysts (Mo, Ru, R h, Pd, and CoMo) were studied. Reaction orders (thiophene, H2S, and Hi), ap parent activation energies, and preexponential factors were determined. The activity trends for these catalysts follow the well-known volcano-shape cu rve. The mast active catalyst shows the lowest thiophene reaction order, wh ich is taken to imply that a strong interaction between transition metal su lfide (TMS) and thiophene results in a high HDS activity. The kinetic resul ts are interpreted in terms of trends in metal-sulfur bond energy. These tr ends are counter to commonly held correlations between metal-sulfur bond en ergy and periodic position of the transition metal. Both Sabatier's princip le and the "bond energy model" appear to be inadequate in explaining the ob served trends in kinetic parameters. Instead, an alternative proposal is ma de: the metal-sulfur bond strength at the TMS surface relevant to HDS catal ysis depends on the sulfur coordination number of the surface metal atoms, Transition metals (TM) at the left-hand side of the periodic table, i.e., M o, form stable sulfides, leading to a low sulfur addition energy under reac tion conditions, The sulfur addition energy is the energy gained upon addit ion of a sulfur atom (e.g., in the form of thiophene) to the TMS. Over to t he right-hand side of the periodic table, the stability of the TMS decrease s due to lower bulk metal-sulfur bond energies. This can result in more coo rdinative unsaturation of the TM surface atoms and possibly the formation o f incompletely sulfided phases with higher sulfur addition energies. At the right-hand side of the periodic table the activity decreases due to weak m etat-sulfur interactions, leading to poisoning of the metallic state. (C) 2 000 Academic Press.