A study on the synthesis of a model CoMnO catalyst for FTO reactions
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In this work, the synthesis of a model CoMnO catalyst is described. MnO was used in large amounts (up to 80 wt%) to enhance the lower olefin selectivity of cobalt. The lower olefin selectivity promotion by MnO was probed by the use of a bulk CoMnO compound. This bulk system was characterised using SEM, XRD and TPR and was used to determine FTO reaction optima (temperature, pressure). TPR indicated that MnO may have a positive effect on the reducibility of cobalt oxide. In a range of 5-20 bar and 220-280°C, optima were found at 10 bar and 240°C. Subsequently, using colloidal synthesis, model CoMnO catalysts were made. Three classes of colloidal particles were synthesised: tripod, tetrahedral and hexagonal colloids of 60, 20 and 16 nm, respectively. These particles were characterised using TEM, STEM-EDX, XRD and XPS. The colloidal particles were made up of a uniform distribution (STEM-EDX) of cobalt and manganese which are present as separate CoO and MnO species (XRD, XPS), in contrast to a mixed CoMnO spinel structure for the bulk system (XRD). At low pressures (1 bar), the tetrahedral catalyst behaves as a FTS catalyst with C5+ as its main fraction (45%) and a lower olefin fraction of 35%. Through the calcination of the tetrahedral particles, the crystal structure was changed to a mixed spinel phase. This crystal structure change resulted in an increased activity, a decreased lower olefin selectivity (24%) and an increased C5+ selectivity (60%). It appeared that the crystal structure of the CoMnO was of large influence on the catalytic properties of the catalysts.