Combined effect of noble metals (Pd, Au) and support properties on HDS activity of Co/SiO2 catalysts Original Research Article

Cobalt-based catalysts supported on different types of SiO2 are studied in the hydrodesulfurization of thiophene. Amorphous silica and siliceous MCM-41 and HMS, characterized by different texture and surface acidity are used as carriers. The effects due to the modification of the support by impregnation with palladium precursor and to the co-impregnation of cobalt and gold are considered. The catalysts are characterized by N2 physisorption (BET), XRD, TPR and XPS techniques. The use as supports of the ordered mesoporous silica with higher surface area with respect to amorphous silica, produces a better dispersion of the cobalt oxides particles. Moreover, the addition of palladium to the supports inhibits the interaction between cobalt and silica and favors the reducibility of the cobalt oxide. Co-impregnation of gold and cobalt decreases the Co3O4 crystallite size. Due to the combination of these effects an increase in the cobalt activity is observed. The extent of the improvement is highly dependent on the support characteristics. As compared to the monometallic Co catalysts, the monometallic Pd catalysts supported on amorphous SiO2 and on MCM-41 are less active. However quite unexpectedly the Pd/HMS exhibits the best performance among all the studied samples. Catalytic tests of HDS of tetrahydrothiophene (THT) support the direct route of the C–S bond cleavage as the rate-determining step of the reaction.