Mo catalysts were prepared by impregnation of titania synthesized by the pH swing method which provides a TiO
2 carrier with a high specific surface area (134 m
2·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% MoO
3 and then decreased for higher Mo loadings. The sulfur behavior on the sulfided Mo/TiO
2 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 H
2S release rate constant (
kRE) 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% MoO
3. The optimal Mo dispersion was 5.2 atom/nm
2, which is higher than the optimal Mo dispersion on 70 m
2·g
−1 TiO
2 (4.2 atom/nm
2). Comparison of
kRE and S
0 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 MoS
2 active phase by Ti atoms).
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