The reactivities and the stabilities of the paramagnetic species formed on the surface of Al 2O 3MoO 3CoO and SiO 2MoO 3CoO: The carrier effects of Al 2O 3 and SiO 2

Abstract

The rates of formation and decay, the concentration and the stability of the paramagnetic species on the surface of the catalysts (carrier-MoO 3, carrier-MoO 3-CoO) prepared from two oxide carriers, γ-alumina and silica gel, having identical surface areas and pore radius distributions, were studied by electron spin resonance (e.s.r.) using a mixture in which an aromatic hydrocarbon was added to the oxide with or without a solvent (benzene). The measurements were made either at 20 °C or at 20 · 300 °C. The cobalt ions did not interfere with the Mo 6+ ions on γ-alumina, but reduced the activity of the Mo 6+ ions on silica gel. R +, a hydrocarbon cation, was formed only when Mo 6+ was present, while R +O★ 2− a complex ion which is different from R +, can be formed in the absence of Mo 6+, though its amount was increased by Mo 6+. R + appeared only in the presence of solvent molecules while R +O★ 2− appeared without a solvent. The stabilities of R +O★ 2− and Mo 5+ at 300 °C were far larger on γ-alumina than on silica gel. The effects of the carriers on the activities of the hydrodesulphurisation catalysts are in accord with the catalytic reaction scheme already reported.