SUPPORT COMPOSITION EFFECT ON SUPERFICIAL STRUCTURES OF NICKEL AND MOLYBDENUM OXIDES SUPPORTED ON TiO2-Al2O3 MIXED OXIDES
In order to synthesize new hydrodesulfurization catalysts with high dispersion of active sites, in this work it is presented a study of the effect of the support on superficial Ni-Mo-support interactions during impregnation process. TiO2-Al2O3 mixed oxides with 5, 10 and 90 % mol of Al2O3 were synthesized by the sol-gel method. Characterization of the support was performed by X-ray diffraction (XRD), ζ-potential and N2 physisorption. Ni and Mo superficial species were analyzed by UV-vis diffuse reflectance (UV-vis-DRS) and Raman spectroscopies, respectively. DRX results show that TiO2-Al2O3 mixed oxides present a gamma-alumina and amorphous-titania structures. The ζ-potential results show that the supports have surfaces with similar charges and net surface pH between 7-8. Raman spectroscopy shows that the net surface pH of support controls the symmetry of Mo oxide, which is octahedral, whereas MoO4 2-, Mo7O24 6- and Mo8O36 4- clusters distribution is controlled by Al2O3 concentration. The catalysts with high Al2O3 content show vibration modes due to Mo-O-Mo stretching, whereas the low Al2O3 content catalysts these modes are of the terminal Mo=Ot bond. UV-vis-DRS results show the coexistence of Ni2+ ion with tetrahedral [Ni2+4O2-] and octahedral [Ni2+6O2-] coordination. Catalysts with high contents of Al2O3 favor the tetrahedral Ni2+ ion that forms the Ni2+/γ-Al2O3 spinel. Catalysts with low contents of Al2O3 favor the octahedral Ni2+ ion, indicating a mayor interaction with octahedral Mo oxide species.
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