Structure and Nature of the Active Sites in CoMo Hydrotreating Catalysts Conversion of Thiophene

Abstract

The performance of sulfided (Co)Mo/alumina catalysts was studied in the conversion of thiophene between 423 and 773 K at atmospheric pressure. Reaction orders for thiophene, H 2 , and H 2 S were determined at several temperatures. The reaction order in thiophene increases with rising temperature, indicating a LangmuirHinshelwood mechanism. Beyond 550 K the conversion curve deviates from pseudo-first-order kinetics, demonstrating the influence of the surface coverage of thiophene on the reaction rate. Above 673 K the reaction rate rises exponentially with temperature which can be ascribed to the presence of a second type of active site that is only active at high temperatures. These catalytic data can be combined with recent structural studies using EXAFS spectroscopy to derive an activity-structure relation. It is proposed that at low temperature the active sites are terminal sulphur vacancies on the Co-promotor atom. In addition around 673 K a second type of sites becomes active, which is believed to consist of bridging sulphur vacancies in between Co and Mo atoms on the MoS 2 edges.

Surface-dependent sulfidation and orientation of MoS2 slabs on alumina-supported model hydrodesulfurization catalysts

“…The global shape of the XANES spectra of Mo/-Al 2 O 3 model catalysts exhibit similar features confirming sulfidation as shown above with XPS ( Figure 5) [32,42,43]. Partial erosion of the shoulder at 20 014 eV also confirms that a full sulfidation has not been reached for all model catalysts as expected from the sulfidation temperature (i.e 300 °C) [44].…”

Nature of cobalt active species in hydrodesulfurization catalysts: Combined support and preparation method effects

“…A synergy between the Co 9 S 8 and the CoO may occur. As reported for some supported CoMo systems, the sulfur vacancies of the Co 8 S 9 may serve as activation sites for the thiophene [35] and the cobalt oxide as activation sites for the H 2 [36]. From the XPS analyses of the catalyst after reaction, the surface atomic ratio Co/Si, as shown in Table 3, decreases noticeably.…”

Recent STM, DFT and HAADF-STEM studies of sulfide-based hydrotreating catalysts: Insight into mechanistic, structural and particle size effects

“…Their substitution of Mo at edge sites is believed to enhance vacancy formation and the creation of new and more active sites. Several studies have been carried out to correlate the structure of the active promoted phases to the activity [1,15,17], but the exact location and coordination of the promoter atoms have been debated extensively [1,14,[50][51][52][53][54]. Lack of structural insight has hampered the progress.…”