Browsing by Author "Macheli, Lebohang"

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    Inverse model systems to investigate metal-support interactions in Fischer-Tropsch catalysis
    (2019) Macheli, Lebohang; Van Steen, Eric
    This thesis aims to gain insight into the effects of metal-support interaction in Co/SiO2 catalysts using an inverse model system by modifying Co3O4 nanocrystallites with silanes (i.e. tetraethoxysilane, trimethyl chlorosilane, triphenyl ethoxysilane). It is postulated that the formation of Co-O-Si will alter the catalytic properties of the cobalt site to which silane is bonded and the adjacent ligand-free sites. The desire is to understand the effects of metal support interactions obtained after reduction of the model system on its performance in FischerTropsch synthesis, which was used as a test reaction, taking into account the change in the reducibility and metal surface area. Cobalt oxide was synthesize using sodium dodecylsulphate assisted oxidative precipitation or cobalt carbonyl decomposition and was contacted with the respective silanes in different solvents. The presence of silanes on the surface of cobalt oxide both prior and after reduction was confirmed using infrared spectroscopy. Modification of cobalt oxide with the silanes does not affect the morphology of cobalt oxide nano-crystallite and may have formed silicate islands on the surface of cobalt oxide. The presence of the silane retards the reduction process by changing the activation energy required for the reduction process. The presence of this material on the surface of cobalt decrease the surface area. However, the catalytic activity increased drastically. The interaction of Co3O4 with tetraethoxysilane was manipulated by using different solvents (mixtures) (i.e. anhydrous environment vs acid/base hydrolysing environment). For the modification in anhydrous environment, the formation of silica on the surface of Co3O4 is facilitated by heat treatment. However, in hydrous environment, the formation of silica on the surface of Co3O4 is initiated by the hydrolysis of tetraethoxysilane. The effects of modification using different solvent was tested in the Fischer-Tropsch synthesis. The modification under acid catalysed hydrolysis facilitate the formation of Co-O-Si contact points followed by increase catalytic activity compared to catalysts modified in other solvents. An insight about the effects of calcination on the interaction of silica and cobalt was obtained by calcining a modified sample at different temperatures. Low temperature (573 K) calcination facilitate stronger interactions. The interaction becomes weaker in increasing temperature up to 873 K. Calcination at temperature from 973 K-1173 K result in very strong interaction that leads to the formation of Co2SiO4. To further obtain an enlightenment regarding the electronic effects caused by the modification of cobalt oxide with silane, silanes of different electronic behaviour were used. In addition to electron withdrawing tetraethoxysilane, electron donor silane (triphenyl ethoxysilane and trimethyl chlorosilane) were used to modify the surface of cobalt oxide. While the activity was increased, the product selectivity was altered differently by the different silanes used.
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    Silica modified Co3O4 nanocubes as a model system for metal-support interaction in Co/SiO2 catalyst for Fischer-Tropsch synthesis
    (2014) Macheli, Lebohang; Van Steen, Eric
    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.