Influence of support-interaction on the sulfidation behavior and hydrodesulfurization activity of Al2O3-supported W, CoW, and NiW model catalysts

The interaction of Co (or Ni) and W with the Al2O3-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 degreesC, In bimetallic catalysts, the presence of W prevents the strong interaction of Co and Ni with the Al2O3 support and partially blocks the diffusion of Co and Ni into the support, thereby facilitating the sulfidation of Co and Ni. For CoW/Al2O3 catalysts bulk cobalt sulfide and WS2 are formed during suffidation and hence no promotion effect is observed. However, the HDS activity of NiW/Al2O3 is a factor 5-6 times higher than for W/Al2O3. This strong promotion effect is ascribed to the formation of the NiWS phase by redispersion of preformed bulk Ni sulfide to the edges of WS2 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/Al2O3 containing CyDTA shows the highest HDS activity. Due to the complexation of Ni with CyDTA, the sulfidation of Ni is retarded to temperatures where WS2 is already formed, thereby forming directly NiWS. However, these catalysts are highly unstable at high sulfidation temperature (T less than or equal to 550 degreesC) and segregation of the active phase is observed. As a result these catalysts loose their superior HDS activity.