| Because of its role in determining the turnover rate of a catalytic reaction, the binding strength of reacting species to a solid surface is an important aspect to consider in the design of heterogeneous catalysts. With this idea in mind, the isosteric heat of adsorption of sulfur to molybdenum disulfide was investigated as a function of the atomic fraction of cobalt to total metal present in a series of alumina-supported hydrodesulfurization (HDS) catalysts. The heat comes from a van't Hoff plot by equilibrating a sulfided catalyst with mixtures of hydrogen sulfide in dihydrogen. The temperature dependence of the gas phase composition, defined by the ratio of pressures of the above gases, yields the isosteric heat and also the isosteric entropy. The entropy was estimated to have a reasonable value for the proposed reaction, and compared well to a theoretical value. For large coverages by sulfur, which probably are characteristic of the working catalyst, the heat depends little on the content of cobalt. Possible reasons for this result include the definition of sulfur coverage, the validity of comparing catalysts at the same value of coverage, and the possible contribution to the equilibria of sulfur other than that associated with the active phase.; Temperature-programmed reaction in flowing dihydrogen was used to identify the kinetic stability of sulfur in the non-catalytic phases present. Contributions of sulfur bound to the bulk phases of molybdenum disulfide, cobalt sulfide, and gamma-alumina were found to be negligible in terms of the thermodynamic experiments. A phase of fixed fraction of sulfur to molybdenum was identified in each of the catalysts. It seemed related to sulfur labile during HDS, but not necessarily to the active sites for HDS.; The difficulty of measuring binding energy is acknowledged and possible solutions are proposed. |
