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

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 Mo7O242− and Mo8O264− species displaying Mo–O–Mo stretching vibration modes. On the other hand, catalysts with high content of TiO2 showed Mo7O242− and Mo8O264− species with vibration modes corresponding to terminal Mo=Ot bonds. Therefore, it appears that impregnation of catalysts with a pH 9 solution allows a polymerization process of MoO42− and [Ni2+4O2−] solution species to Mo8O264− and Mo7O242− 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 Mo7O242− and Mo8O264− in combination with [Ni2+6O2−] species produce a better Ni/(Ni + Mo) ratio and NiMoS phase.