MS-1-P-1906 EDXS on MoS₂-base/Al₂O₃ HDS catalysts: A chemical distribution study of silicon

Electron microscopy (SEM and TEM) is a powerful tool for the characterization of a wide range of solid catalysts. Both microscope types give direct evidence of the morphology, chemical composition and crystalline structure in the different scales (micrometer to nanometer). The improvements of the instruments in the spatial resolution, energy resolution, efficiency of the detectors and data collection, has improved very much the quality of the obtained results and the silicon dispersion on gamma alumina particles used for the preparation of MoS₂-based (hydrodesulfurization) HDS catalysts shows these new capabilities.
MoS₂-base/Al₂O₃ HDS catalysts are widely used to remove sulphur from hard-to-desulfurize compounds such as 4,6-dimethyldibenzothiophene. Their catalytic performance is directly related to the dispersion of the MoS₂ structure and the current scientific research is focused on achieving higher dispersion of Co-Mo-S active catalytic sites. In this work, we add silicon atoms on the surface of the alumina particles to modify their acidic properties and increase the dispersion of the Co-Mo-S structure. The Si atoms were aggregated to tri-lobular extruded of alumina by an incipient wet process using a silicon solution. Subsequently, the extruded were calcined and characterized by SEM and TEM.
The concentration of O, Al and Si in the sample, obtained by EDXS, was 46.90, 49.74 and 3.36 wt% in average, respectively. This chemical quantification indicates that the Si was integrated as SiO₂ in the sample to a concentration around 7.2 %. The silicon permeation in the extruded was revealed by a composition study through the cross section of extruded. The result obtained is shown in Figure 1. A homogeneous concentration of silicon inside the extruded is observed. Therefore, this sample was the strongest candidate to impregnate the active phases (P, Co and Mo). Their dispersion was evidenced by concentration profiles (Figure 1) and chemical mapping, see Figure 2. Homogeneous dispersion of Co and Mo is appreciated in both results. However, the dispersion silicon was heterogeneous. This sample was subsequently sulfided to produce the Co-Mo-S structures. The result obtained is illustrated in Figure 3. MoS₂ structures fully dispersed on the Al₂O₃-7. 2%SiO₂ surface in HRTEM images is observed. Therefore, from these first results, the presence of SiO₂ on gamma-alumina contributes to the formation of the MoS₂ structures. However, still it is necessary improve the spreading of silicon in the extruded.