The role of crystallite structure on mechanisms of coke and sulphur poisoning in catalytic reforming

The activity of typical platinum-based catalysts is dependent on the surface crystallite configuration. Although the ensemble requirement determines the reaction pathway the coordination of metal atoms in the crystallite i.e. face or corner atoms, has some influence on reaction selectivity; the electron deficient character of corner atoms makes these atoms more active for certain reactions. Sulphur is shown to preferentially bond irreversibly to the face atoms of a platinum crystallite, which is in a sulphided state under industrial conditions. Thus the mechanism of further sulphur poisoning is reversible attachment to crystallite corner atoms. The corner atoms are also found to be coke free due to high coke cleaning rates there, as well as steric influences. The coke deposition during long-term reformer operation (20 wt%) is found to be on the alumina; however the observed deactivation in octane yields is due to the change in nature of coke (gradual grafitization) on the metal sites of the catalyst. Thus, two types of coke are distinguished on the metal sites; one being easily removed by hydrogen, the other less readily; they are labeled reversible and irreversible (graphitic) coke. Hydrogen limits the mutual site deactivation rate by removal of the reversible and irreversible coke types by catalyzed hydrogeneration and hydrogasification, respectively. In addition to coke removal by hydrogen, hydrogenolysis of coke precursors serves to limit catalyst deactivation. The enhanced activity maintenance of platinum-iridium is due to high rates of hydrogenolysis and hydrogasification. In the case of platinum-rhenium catalysts the graphitization rate of metallic coke is sterically hindered by the preferential sulphidation of rhenium atoms