Periodic Trends in Hydrotreating Catalysis: Thiophene Hydrodesulfurization over Carbon-Supported 4d Transition Metal Sulfides

Abstract

The kinetics of atmospheric gas-phase thiophene hydrodesulfurization (HDS) over five carbon-supported 4d transition metal sulfide catalysts (Mo, Ru, Rh, Pd, and CoMo) were studied. Reaction orders (thiophene, H2S, and H2), apparent activation energies, and pre-exponential factors were determined. The activity trends for these catalysts follow the well-known volcano-shape curve. The most active catalyst shows the lowest thiophene reaction order, which is taken to imply that a strong interaction between transition metal sulfide (TMS) and thiophene results in a high HDS activity. The kinetic results are interpreted in terms of trends in metal–sulfur bond energy. These trends are counter to commonly held correlations between metal–sulfur bond energy and periodic position of the transition metal. Both Sabatier's principle and the “bond energy model” appear to be inadequate in explaining the observed trends in kinetic parameters. Instead, an alternative proposal is made: the metal–sulfur bond strength at the TMS surface relevant to HDS catalysis 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., Mo, form stable sulfides, leading to a low sulfur addition energy under reaction conditions. The sulfur addition energy is the energy gained upon addition of a sulfur atom (e.g., in the form of thiophene) to the TMS. Over to the right-hand side of the periodic table, the stability of the TMS decreases due to lower bulk metal–sulfur bond energies. This can result in more coordinative unsaturation of the TM surface atoms and possibly the formation of incompletely sulfided phases with higher sulfur addition energies. At the right-hand side of the periodic table the activity decreases due to weak metal–sulfur interactions, leading to poisoning of the metallic state.