Metal-support interactions on Fe-based Fischer-Tropsch catalysts

‘Metal support interactions’ is a term used to describe a phenomenon whereby the interaction between the active metal and the support material is strong enough to affect the catalytic activity and selectivity of the active phase. Primarily, there are two theories described in literature to explain the manner in which the development of these interactions alters catalytic behavior in supported catalysts. The first theory is ‘the contact affect’, which is generally associated with partially reducible supports such as TiO2 [Tauster et al., 1978]. It is believed that the intimate contact between the partially reduced surface of the support and the surface of the active phase results in the creation of special contact sites at the interface. These sites are thought to be responsible for the improved activity observed in TiO2 supported catalysts [Burch and Flambard, 1982; Vannice and Sudhakar, 1984; Tauster, 1987]. The second theory is ‘the ligand effect’. With this hypothesis, it is proposed that the development of chemical bonds at the interface between the active metal and the support material is responsible for the altered catalytic behavior in supported catalysts [Qing et al., 2011; Sou et al., 2012]. The presence of these bonds is believed to alter the strength of CO and H2 absorption on the surface of the active phase, resulting in different activity and selectivity. These chemical bonds might be viewed as ligand attachments [Haller and Resasco, 1989], holding the active metal in place. The ligand effect is commonly associated with irreducible support material such as silica [Hou et al., 2008; Sou et al., 2012] and alumina [Taniguchi, et al., 1988; Wan et al., 2007]. The aim of this study was to investigate metal support interactions as a ligand effect. The objective was to prepare model catalysts and modify the surface of the iron oxide using alkoxide compounds, viz. tetra ethoxy-silane (TEOS) and titanium butoxide (TBO), to generate the Fe-O-Si and Fe-O-Ti interactions respectively in a controlled and varying manner in order to investigate how these interactions affect the behaviour of the catalysts. The presence of both the surface silicate and surface titanate groups in the calcined catalyst precursor was confirmed using DRIFTS. Characterization of the calcined samples, containing Fe2O3, showed an overall decrease in the average crystallite size with increasing alkoxide loading (for both TEOS and TBO). However, this effect was more severe for the TEOS modified samples.

Grantor dc:publisher.institution

Department of Chemical Engineering

Year dc:date.issued

2012

Author dc:creator

• Mogorosi, Ramoshibidu Patrick

Advisor dc:contributor.advisor

• Van Steen, Eric

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/5438

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/5438