Effect of TiO₂–Al₂O₃ Sol–Gel Supports on the Superficial Ni and Mo Species in Oxidized and Sulfided NiMo/TiO₂–Al₂O₃ Catalysts: Influence on Dibenzothiophene Hydrodesulfurization

Abstract

The present work presents a comparative study of NiMo catalysts supported on sol–gel TiO₂–Al₂O₃ mixed oxides with 5 and 95 mol% content of Al₂O₃. The DRX and N₂ physisorption results showed that the sol–gel method allows preparation of TiO₂–Al₂O₃ mixed oxides possessing high superficial area and an amorphous TiO₂ structure. Results of ζ-potential showed that the net surface pH of the supports depends on their structure and composition. According to UV–Vis and Raman spectra obtained from the solids after impregnation, catalysts with high content of Al₂O₃ showed Mo₇O₂₄ ²⁻ and Mo₈O₂₆ ⁴⁻ species displaying Mo–O–Mo stretching vibration modes. On the other hand, catalysts with high content of TiO₂ showed Mo₇O₂₄ ²⁻ and Mo₈O₂₆ ⁴⁻ species with vibration modes corresponding to terminal Mo=O_t bonds. Therefore, it appears that impregnation of catalysts with a pH 9 solution allows a polymerization process of MoO₄ ²⁻ and [Ni²⁺4O²⁻] solution species to Mo₈O₂₆ ⁴⁻ and Mo₇O₂₄ ²⁻ species with a close interaction with [Ni²⁺6O²⁻] species. However, these species have low interaction with the support. Thus, composition of the support appears to be more important than net surface pH in order to obtain a better distribution of superficial Mo species. XPS results suggest a higher proportion of “NiMoS” phase on the TiO₂ rich support. The most active catalyst in the dibenzothiophene hydrodesulfurization was NiMo/TiO₂–Al₂O₃ with 5 mol% Al₂O₃. This suggests that Mo₇O₂₄ ²⁻ and Mo₈O₂₆ ⁴⁻ in combination with [Ni²⁺6O²⁻] species produce a better Ni/(Ni + Mo) ratio and NiMoS phase.