Active sites and support effects in hydrodesulfurization catalysts

Abstract

In the present article we will both review some of the current research on hydrodesulfurization (HDS) catalysts and present some new results. Five topics will be given special attention: (i) the nature of active sites in unpromoted and promoted HDS catalysts; (ii) the origin of promotional behaviors; (iii) studies of thermal ageing processes using high temperature sulfiding procedures; (iv) the nature of the phases present in catalysts after use under industrial conditions; and (v) support effects in carbon- and alumina-supported Co and Co-Mo catalysts. Regarding the first two topics, EXAFS results on Mo/Al 2O 3 catalysts show that the HDS activity is related to the concentration of MoS 2 edge sites. For most promoted catalysts the activity behavior can be related to the concentration of promoted edge sites (related to Co-Mo-S) as revealed by Mössbauer emission spectroscopy (MES) or NO adsorption using infrared spectroscopy. High temperature sulfiding studies have been found useful to elucidate the structural changes occurring during industrial operation. For alumina-supported Co-Mo catalysts these high temperature sulfiding studies reveal the existence of a “low-temperature” (Type I) and a “high-temperature” (Type II) Co Mo S structure. The differences in the activity of Type I and Type II Co Mo S and different carbon- and alumina-supported catalysts are explained in terms of the nature of the interactions with the support. The interactions may lead to changes in the electronic properties of the active phase. In general, the highest activity is observed for systems exhibiting the highest “metallic” character (e.g., least oxygen in the structure).