P. Gajardo

Structure of Oxide Como - Gamma-al2o3 Hydrodesulfurization Catalysts - An Xps and Drs Study

A series of CoMoγ-Al2O3 oxide catalysts containing 15.0 ± 0.6 wt% of cobalt and molybdenum oxides (considered as Co3O4 and MoO3) and r = Co(Co + Mo) atomic composition ranging from 0.00 to 1.00 were studied by XPS and diffuse reflectance spectroscopy. These techniques and previously reported results on electron microscopy show that molybdenum occurs on γ-Al2O3 as tetrahedral oxidic Mo(VI) according to the Mo(VI) monolayer model, and also as Mo(VI) in multilayers having a bulk like MoO3 or paramolybdate surrounding. Cobalt is poorly dispersed when alone (15%) on a γ-Al2O3 surface; in that case, the formation of Co3O4 clusters is extensive, and very little CoAl2O4 is formed. In the r = 0.25 – 0.75 composition range, a strong interaction takes place between molybdenum of the monolayer and cobalt, which brings about a high dispersion of cobalt and seems, in turn, to increase the dispersion of molybdenum. As a consequence of the dispersion of cobalt, much CoAl2O4 is formed. The results, as well as the properties of the catalysts in the middle range of composition r, are interpreted in terms of a model of the catalyst surface in which a bilayer of Co-Mo is formed on the CoAl2O4.

Structure and catalytic activity of CoMo/γ-Al2O3 and CoMo/Sio2 hydrodesulphurization catalysts: an xps and esr characterization of sulfided used catalysts

A series of sul-fided Co, Mo and CoMo supported on γ-Al2O3 and SiO2 catalysts has been investigated, after they had been used as catalysts for 8 h at 305°C and 30 bar pressure in contact with a mixture of thiophene, cyclohexene, cyclohexane and hydrogen. The catalytic activity in the hydrodesulphurization (HDS) of thiophene and hydrogenation of cyclohexene was measured. The sulphided and used catalysts were investigated by X-ray diffraction, diffuse reflectance spectroscopy, XPS and ESR. Samples were transferred from the catalytic reactor to the XPS and ESR equipments under controlled argon atmosphere. Several cobalt and molybdenum surface species were identified and their functions in HDS and hydrogenation reactions were evaluated. Cobalt is in its full sulphided state only if molybdenum is present in the catalyst. The high HDS and hydrogenation performance of CoMo supported on Al2O3 is basically due to a strong CoMo-support interaction; this leads to a high dispersion of the non-sulphided species (precursor) and, in consequence, of the active sulphided species.

XPS study of the supported phase — SiO2 interaction in Mo/SiO2 and CoMo/SiO2 hydrodesulphurization catalysts in their oxidic precursor form

Abstract XPS results on two series of catalysts, namely Mo/SiO2 and CoMo/SiO2 in the oxidic form, are reported. In the first series the molybdenum oxide content deposited on SiO2 ranges from 2.8 to 20.6 weight %. The second series was obtained by cobalt impregnation of the first series; in this case the atomic ratio Co/(Co + Mo) was fixed at 0.36. XPS binding energy and intensities measurements enable estimation of the interaction and dispersion between the oxide phase and the support to be made. In the case of molybdenum deposited alone on the support, the binding energies of the Mo and O levels both increase with MoO3 content. When cobalt is also present on the support, the binding energies of the Mo and O levels remain constant. The relative intensities IMo/ISi and ICo/ISi remain constant up to 12% active phase and then drastically increase for higher amounts of oxide. These results confirm the existence of a relatively strong Mo/SiO2 interaction. This interaction is substantially weaker when Co is supported on the MoSiO2 solid, leading to the formation of CoMoO4 at the expense of the Mo/SiO2 interaction. Furthermore, it is shown that at high content of oxide the active phase is deposited on the outer surface of the particle and not inside the pores.

Comparative study of unsupported and supported Co-Mo hydrodesulphurisation catalysts

Catalytic activities of unsupported and alumina- or silica-supported cobalt-molybdenum catalysts in the oxidic and sulphided form are compared. Silica-supported catalysts behave essentially as unsupported catalysts. The behaviour of alumina-supported catalysts is quite different. The synergistic effect is much stronger, especially in the sulphided form, and the domain in which the synergism is exhibited is much broader. In addition, alumina brings about a strong hydrogenating activity. ESCA measurements have helped to investigate further the interaction between the cobalt-molybdenum system and alumina.

Gravimetric Study of Reduction By H-2 of Hydrodesulfurization Como-gamma-al2o3 Catalysts in Their Oxidic Precursor Forms

A CoMo/γ-Al2O3 oxidic catalyst series, where the r= Co/(Co + Mo) atomic composition changes from 0.00 to 1.00 and the total cobalt and molybdenum oxide content remains constant (≈ 15% wt. since Co and Mo are present as Co3O4 and MoO3), were prepared. Their hydrogen reducing capabilities were studied gravimetrically and compared with those of well defined crystalline bulk oxide phases. This technique allows a better understanding of the structure of these amorphous solids and confirms other physico-chemical measurements.The simultaneous presence of a molybdenum monolayer and bulk like MoO3 explains well the reduction behaviour of molybdenum-rich catalysts. Reduction of catalysts containing only Mo is very heterogeneous, but no independent molybdenum oxide species were found.Much of the cobalt in cobalt-rich catalysts behaves as Co3O4 during their reduction. Several independent cobalt oxide species are present in catalyst containing only cobalt.Inhibition of the reducing capability is observed in catalysts whose composition lies in the r= 0.25–0.75 range. It is proposed that a strong interaction between the oxides causes this phenomenon.

Genesis, nature and dispersion of active phase in sulphided CoMo/γ-Al2O3 hydrodesulphurization catalysts. Investigation by analytical electron microscopy and X-ray photoelectron spectroscopy

A series of CoMo/γ-Al2O3 catalysts with a varying Co/(Co + Mo) atomic ratio has been investigated by analytical electron microscopy (a.e.m.) and X-ray photoelectron spectroscopy (X.p.s.) after sulphidation and catalytic testing by hydrodesulphurization and hydrogenation reactions. The crystalline nature of the active species has been characterized by dark field and high resolution imaging. Their dispersion has been measured in more detail using electron probe microanalysis and X.p.s. Comparison with the results of previous work on oxide precursors allows us to describe the transformations brought about by sulphidation. The complementary aspects of a.e.m. and X.p.s. for the study of supported catalysts are discussed.