Silica modified Co3O4 nanocubes as a model system for metal-support interaction in Co/SiO2 catalyst for Fischer-Tropsch synthesis
The aim of this study was to study the interaction between the silica and cobalt crystallites using a model of Co3O4-nanocubes whose surface was modified with tetraethyl orthosilicate. The model systems were prepared in two steps, viz. synthesis of the cobalt(III,II) oxide nanocubes via the sodium dodecylsulphate assisted oxidative precipitation and a surface modification of the nanocubes with tetraethyl orthosilicate. The obtained morphology and in particular the average crystallite size of Co3O4 nanocubes is affected by the temperature at which the reagents are mixed, the rate at which they are mixed as well as the time of reaction. The modification of the surface of the Co3O4-nanocubes with tetraethyl orthosilicate resulted in the formation of Co-O-Si ligands on the surface of Co3O4. This was confirmed in addition to (amorphous) SiO2 using Fourier transformer infrared (FTIR) spectrometry. The surface treatment did not yield the formation of crystalline silica or a crystalline cobalt silicate phase as indicated by the absence of characteristic diffraction bands using XRD. Furthermore, there was no hard-to reduce material (e.g. cobalt silicate) as the TPR profiles showed only two peaks corresponding to the reduction of Co3O4 to metallic cobalt. The reduction behaviour of Co3O4 nano-cubes was, however, affected by the surface treatment with tetraethyl orthosilicate. A shift in the reduction profile towards higher temperatures was observed with increasing SiO2 loading. This occurs as a result of increased activation energy for the reduction of the model catalysts upon modifying with tetraethyl orthosilicate, which might be ascribed to a strong interaction between cobalt and silica. Surface modification of Co3O4 with SiO2 prohibits sintering and the silica species may act as spacers between the cobalt particles. The Co-O-Si species are still present after the reduction of these model materials and during the Fischer-Tropsch synthesis. The Fischer-Tropsch activity is improved with a maximum activity improvement of about 15 times. The activity enhancement is larger than what is expected based on the increased metal surface area indicating that Co-O-Si surface species may also act as a promoter for the Fischer-Tropsch synthesis.
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