STUDIES OF MOLYBDENA ALUMINA CATALYSTS .17. SULFIDED CATALYSTS EXPOSED TO AIR

Catalysts containing various levels of Mo supported on gamma-alumina were activated by: (1) sulfiding at 400°C; or (2) sulfiding in steps to 400°C; or (3) sulfiding in steps to 400°C, followed by cooling to room temperature, exposing to air, and finally resulfiding at 400°C. The latter catalysts exhibited significantly lower NO chemisorption, but only slightly lower hydrodesulfurization activity as compared to the former catalysts. We propose that oxygen adsorbed during air exposure is not completely removed during resulfiding, leading to more active reaction centers but less of them. We further propose that the more active center consists of an oxygen anion adjacent to a vacancy site, instead of the regular sulfide center of a sulfide anion and a vacancy site. Step-sulfided catalysts not exposed to air also displayed higher intrinsic activities, which was attributed to incomplete sulfiding. © 1990.