Influence of Support-Interaction on the Sulfidation Behavior and Hydrodesulfurization Activity of Al_2O_3-Supported W, CoW, and NiW Model Catalysts

摘要

The interaction of Co (or Ni) and W with the Al_2O_3-support influences the sulfidation behavior and thiophene hydrodesulfurization (HDS) activity of CoW and NiW model catalysts. High calcination temperatures retard the sulfidation of Co, Ni, and W to high temperature and lead to incomplete sulfidation at 400 ℃. In bimetallic catalysts, the presence of W prevents the strong interaction of Co and Ni with the Al_2O_3 support and partially blocks the diffusion of Co and Ni into the support, thereby facilitating the sulfidation of Co and Ni. For CoW/Al_2O_3 catalysts bulk cobalt sulfide and WS_2 are formed during sulfidation and hence no promotion effect is observed. However, the HDS activity of NiW/Al_2O_3 in a factor 5-6 times higher than for W/Al_2O_3. This strong promotion effect is ascribed to the formation of the NiWS phase by redispersion of preformed bulk Ni sulfide to the edges of WS_2 slabs as observed with X-ray photoelectron spectroscopy (XPS). Both the calcination and sulfidation temperature have a strong influence on the HDS activity. Incomplete sulfidation due to either high calcination temperature or low sulfidation temperature decreases the HDS activity. NiW/Al_2O_3 containing CyDTA shows the highest HDS activity. Due to the complexation of Ni with CyDTA, the sulfidation of Ni is retarded to temperatures where WS_2 is already formed, thereby forming directly NiWS. However, these catalysts are highly unstable at high sulfidation temperature (T ≤ 550 ℃) and segregation of the active phase is observed. As a result these catalysts loose their superior HDS activity.