A. Guevara, A. Alvarez, M. Vrinat, Effect of TiO2-Al2O3 sol-gel supports on the superficial Ni and Mo species in oxidized and sulfided state and their performance on dibenzothiophene hydrodesulfurization. Catalysis Letter 126(2008) pp. 268-274. ISSN:1011-372X (Print) 1572-879X (Online).
The present work presents a comparative study of NiMo catalysts supported on solgel TiO2Al2O3 mixed oxides with 5 and 95 mol% content of Al2O3. The DRX and N2 physisorption results showed that the solgel method allows preparation of TiO2Al2O3 mixed oxides possessing high superficial area and an amorphous TiO2 structure. Results of f-potential showed that the net surface pH of the supports depends on their structure and composition. According to UVVis and Raman spectra obtained from the solids after impregnation, catalysts with high content of Al2O3 showed Mo7O24 2- and Mo8O26 4- species displaying MoOMo 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=Ot 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.