A thesis submitted to the Faculty of Science, University of the Witwatersrand;
Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy.
July 1991 / Precipitated-iron Fischer- Tropsch catalysts were sulfided in the range 500 - 20000 ppm
S/Fe with an aqueous sulfide source (Na2S, (NB4)zS, (NB4)zS5) during the precipitation
process. Sulfidation was performed at pH 10.75, 8.5 and 6.9. Sodium ions were removed
by centrifugation, and atomic absorption analysis confirmed low sodium levels (0-51
ppm). Based on solution speciation models, ferrous sulfide (FeS) which formed from
aqueous HS' species, was found to influence the iron-oxyhydroxide crystallite
morphology. It is proposed that, when sulfide was added at pH 10.75, FeS molecules
functioned as nuclei for crystallite growth, while a pH 6.9 they assisted 'with the
aggregation of particles. The processes of nucleation and aggregation appeared to be in
competition following sulfidation at pH 8.5, resulting in a composite morphology that
produced an inactive catalyst.
The bulk structure of the catalysts was elucidated using XRD, SEM and nitrogen
porosimetry, All sulfided catalysts exhibited enhanced BET surface areas and total pore
volumes with a maximum at 2000 ppm S (surface area = 166 m2/g,total pore volume =
0.254 cm3/g) compared to an unsulfided catalyst (surface area = 58 m2/g, total pore
volume = 0.184 cm3/g), Furthermore, for any series of catalysts at the same level of
sulfidation, the BET surface areas were observed to decrease as the pH of sulfide addition
decreased. Increasing levels of sulfidation (to 20000 ppm) brought about an increase in
crystallite size and therefore, improved crystallinity as determined by XRD measurements.
Materials with larger crystallites possess smaller surface areas, and thus the crystallinity
was found to increase as the pH of sulfidation decreased.
Surface characterisation by XPS after calcination at 400°C and reduction (400°C),
revealed sulfate species (169.4 eV) on catalysts sulfided with 500-2000 ppm, while sulfide
species (162.O eV) emerged at higher sulfide content. No sulfates were observed on
reduced catalysts following calcination at 200 C.
[Abbreviated Abstract. Open document to view full version]
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/20979 |
| Date | 31 August 2016 |
| Creators | Bromfield, Tracy Carolyn |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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