Changes in metal-sulfur bond energy in promoted and unpromoted molybdenum catalysts

Insight into the changes in the metal-sulfur bond energy in unpromoted and promoted (Co, Ni, or Fe) MoS2 catalysts is obtained by temperature-programm ed reduction (TPR) studies of in situ sulfided catalysts. The experiments r eveal a significant decrease in the metal-sulfur bond energy for the more a ctive Co-Mo-S and Ni-Mo-S structures as compared to MoS2. A similar decreas e is not observed for the less active Fe-Mo-S structure. The sulfur coordin ation is directly measured by in situ EXAFS and the results support that on e of the roles of the promoter atoms is to facilitate vacancy formation. Th e findings are in accord with recent density functional theory calculations .