Investigation of Sulfur Behavior on Mo-based Hydrodesulfurization Catalysts Supported on High Surface Area TiO₂ by ³⁵S Radioisotope Tracer Method

Abstract

Mo catalysts were prepared by impregnation of 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. Dibenzothiophene (DBT) hydrodesulfurization (HDS) activity was estimated over the obtained catalysts under typical HDS reaction conditions for various Mo contents. The activity increased linearly with Mo content up to ca. 16 wt% MoO₃ and then decreased for higher Mo loadings. The sulfur behavior on the sulfided Mo/TiO₂ catalysts was elucidated under the reaction working conditions using a ³⁵S radioisotope tracer method, or the HDS of ³⁵S-labeled DBT. The results indicated that at a given temperature the H₂S release rate constant (k_RE) was almost constant irrespective of the Mo content, and the amount of labile sulfur (S₀) increased linearly with the Mo content in parallel with the activity up to ca. 16 wt% MoO₃. The optimal Mo dispersion was 5.2 atom/nm², which is higher than the optimal Mo dispersion on 70 m²·g⁻¹ TiO₂ (4.2 atom/nm²). Comparison of k_RE and S₀ of the titania-based catalysts and the alumina-based catalysts suggested that the active phase consists of a 'TiMoS' phase exhibiting a promoting effect similar to the well-known 'CoMoS' phase (promotion of the MoS₂ active phase by Ti atoms).