2008 Volume 51 Issue 2 Pages 73-82
A 35S radioisotope pulse tracer method using 35S-labeled H2S was used to investigate the sulfidation state of Mo-based catalysts supported on TiO2. Despite the coverage of Mo species on the TiO2 support, sulfidation of the TiO2 support still occurred at 300°C. The sulfided catalysts could be reduced under H2 atmosphere. A 35S radioisotope tracer method using 35S-labeled dibenzothiophene ([35S]DBT) was used to determine the sulfur mobility on the working sulfided Mo/TiO2 catalysts. The amount of labile sulfur (So), which represents the amount of active sites, and the release rate constant of H2S, which represents the mobility of the active site (kRE), were estimated. Compared with Mo/Al2O3 catalyst, Mo/TiO2 catalyst had slightly higher So and about 2 times higher kRE, which indicate that new mobile active sites were formed on sulfided Mo/TiO2 catalyst. Comparison of the kRE and So of titania-based and alumina-based catalysts suggested that the active phase consisted of a ‘TiMoS’ phase exhibiting a promoting effect similar to the well-known ‘CoMoS’ phase (promotion of the MoS2 active phase by Ti atoms). Hydrodesulfurization (HDS) of DBT was evaluated over Mo catalysts supported on titania synthesized by the pH swing method, which provides a TiO2 carrier with a high specific surface area (134 m2·g-1) and excellent mechanical properties. The activity increased linearly with Mo content up to ca. 16 wt% MoO3 and then decreased for higher Mo loadings. The optimal Mo dispersion on this catalyst was 5.2 atom/nm2, which is higher than the optimal Mo dispersion on 70 m2·g-1 TiO2 (4.2 atom/nm2). [35S]DBT HDS catalyzed by CoMo/TiO2 showed that So increased in parallel with the catalytic activity up to a Co/Mo molar ratio of 0.4. Only a slight increase in kRE was observed with Co addition, unlike CoMo/Al2O3 catalysts.
