Inverse model systems to investigate metal-support interactions in Fischer-Tropsch catalysis

• Main Author: Macheli, Lebohang
• Other Authors: Van Steen, Eric
• Format: Doctoral Thesis
• Language: English
• Published: Faculty of Engineering and the Built Environment 2020
• Subjects: Chemical Engineering
• Online Access: http://hdl.handle.net/11427/30921

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.