Curved nanostructures of unsupported and Al2O3-supported MoS2 catalysts: Synthesis and HDS catalytic properties
Résumé
The active phase of hydrodesulfurization (HDS) catalysts is usually composed of MoS2 slabs promoted by cobalt and/or nickel. Because of the anisotropic structure of molybdenum disulfide, the active sites are believed to be present only on the edges of the slabs, exhibiting dangling bonds. However, slabs of MoS2-based industrial catalysts often exhibit disordered bent morphology which might generate strain effects creating new active sites. In order to evaluate a possible effect of bending MoS2 slabs on HDS catalytic properties, synthesis of well-defined curved morphologies was carried out in the present study. Two different strategies were evaluated herein: acidification of ammonium tetrathiomolybdate, (NH4)(2)MoS4 (ATM) in the presence of polyethyleneglycol or decomposition of ammonium thiodimolybdate, (NH4)(2) Mo2S12 (ATOM). The resulting amorphous sulfides, deposited or not on gamma-Al2O3, were then post-treated at 400 degrees C or 750 degrees C under N-2 or H-2 in the presence or not of H2S. Cobalt promotion was also attempted on the final post-treated solids. The results show that the nature of the sulfur-containing precursor strongly influences the final morphology. Using ATM, the as-obtained nanospheres are formed of bundles of slabs while using ATDM, fullerenic-like MoS2 particles are obtained after post-treatment at 750 degrees C. These fullerenic-like particles exhibit non constant radius of curvature with flat regions separated by strongly localized distorted zones suggesting the formation of defect sites on basal planes. The possibility of promoting these systems by Co sustains the presence of such surface sites. XPS and TPR results also show a marked weakening of the metal-sulfur bond strength when compared to flat crystals. Finally, catalytic activities determined for the HDS of thiophene and the simultaneous hydrogenation (HYD) of toluene and isomerization of cyclohexane are rather noticeable despite the absence of conventional edge sites. Bending MoS2 slabs therefore seems to lead to the creation of new active sites on basal planes with original catalytic properties.(C) 2012 Elsevier B.V. All rights reserved.