MSc., Faculty of Science, University of the Witwatersrand, 2011 / Gold has for many years been regarded as being inert and catalytically inactive
compared to the PGMs (platinum group metals). However, in the past decade it has
attracted a lot of interest as both a heterogeneous and a homogenous catalyst and has
been shown to catalyse a wide range of reactions e.g. oxidation, hydrogenation and
reduction among others. Highly dispersed gold nanoparticles on metal oxides, like
titanium oxide (Degussa, P25) have predominantly been studied because they yield
some of the most active and stable catalysts. Modification of the catalysts and/or
supports has been shown to affect their catalytic properties.
Likewise, perovskites, which can be manipulated by partial substitution, are reported
to be active supports for CO oxidation, but only at high temperatures with no activity
shown for temperatures below 200°C. In this study, these perovskites were
investigated at low temperatures (below 100°C) with improved activity found upon
gold deposition. The presence of gold nanoparticles therefore significantly enhanced
the catalytic activity, while the support itself was suspected to be involved in the
reaction mechanism.
A series of perovskites of the type ABO3 (LaMnO3, LaFeO3, LaCoO3 and LaCuO3)
were prepared using the citrate method, while the gold was deposited on them using
the deposition-precipitation method. The supports were calcined at different
temperatures for optimisation. The catalysts were tested for carbon monoxide
oxidation and the active catalysts characterised by XRF, XPS, XRD, Raman
spectroscopy and BET surface area measurements.
With the support calcined at 800ºC, the best catalyst was then modified and compared
with the unmodified catalyst. The 1-wt%Au supported on LaFeO3 was found to give
the best catalytic performance. This support was then modified with various weight
loadings of calcium to determine the effect of calcium on the catalytic activity.
Calcium-doped materials showed decreased surface area, poorer crystallinity and a
drop in catalytic activity relative to the Au-LaFeO3 which indicated the best results
for CO oxidation. In addition, Au-LaFeO3 showed online stability over 21 hours.
Calcining the support improved the incorporation of gold nanoparticles into the
perovskite lattice, resulting in superior catalytic activity. Nevertheless, at higher
calcination temperatures, the catalytic activity of Au-CaTiO3 was depressed while
that of Au-LaFeO3 was enhanced. The activity of perovskites increased upon gold
deposition. XPS, revealed that in the active catalysts, both cationic and metallic gold
co-existed, whilst in the inactive catalysts the gold existed predominantly either as
cationic or metallic gold.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/10809 |
| Date | 18 November 2011 |
| Creators | Mokoena, Lebohang Vivacious |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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