Metal-support interactions in precipitated, magnesium-promoted cobalt [sbnd]silica catalysts

Abstract

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 Co 2CO 4 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 Co 3O 4 according to IR and XRD analyses. TPR studies indicated easy reduction at 300-400°C, with one peak for the Co 2CO 4 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 Si O 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.