METAL-SUPPORT INTERACTIONS IN PRECIPITATED, MAGNESIUM-PROMOTED COBALT SILICA CATALYSTS

To define cobalt-silica interactions, precipitated, Mg-promoted catalysts were prepared by the cobalt nitrate-sodium carbonate reaction, without support and in the presence of silica supports. The major component of the unsupported Co catalyst was tentatively identified as Co2CO4 on the basis of elemental analyses and the IR carbonate absorptions at 1500 and ≈ 1400 cm-1. Upon heating in air at 360°C, the unsupported catalyst converted to Co3O4 according to IR and XRD analyses. TPR studies indicated easy reduction at 300-400°C, with one peak for the Co2CO4 and the Co(OH)2 components. A magnesium-promoted unsupported catalyst showed similar chemical properties and nearly identical IR spectrum; however, significantly different reducibility characteristics were observed by TPR. The composition of the silica-supported catalysts varied between two extremes. In the case of unreactive silica, IR and EM analyses showed a blend of the unsupported catalyst with the silica. In the other extreme, with reactive silicas, all the cobalt converted to cobalt silicates, as indicated by an SiO stretching vibration at 1034 cm-1 and by the absence of carbonate absorptions. TPR traces of the blend-type catalysts were similar to their unsupported analogues. The cobalt silicates required temperatures > 700°C for reduction according to TPR. The TPR of the intermediate-type catalysts that were, in part, blends of the unsupported catalyst with the silica and, in part, cobalt silicates showed cobalt silicate reductions starting above 400°C. This may be due to the catalytic effect of the cobalt that was reduced in the 300-400°C range. Electron microscopy, in combination with energy-dispersive X-ray analyses, further confirmed the cobalt-silica reaction. Cobalt silicates appeared as a "growth," often with a filamentous structure, leaving other areas of the silica completely intact. The formation of cobalt silicates by a solid-solute reaction is discussed. © 1992.