| Summary: | A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg,
in fulfilment of the requirements for the degree of Masters of Science.
Johannesburg, 2016 === Gold nanoparticles are ideally suited as catalysts for selected low temperature reactions such as
CO oxidation for catalytic convertors in the motor industry due to their high activity. But they are
prone to sintering at high temperatures. Platinum-group-metal based catalysts are efficient at
elevated temperatures and generally inactive at lower temperatures. This study explored the CO
oxidation efficiency of gold nanoparticles and of a combination of gold and rhodium
nanoparticles. Variables such as pH, loading concentration and type of support were varied to
control the final properties of the Au based catalysts. Possible bimetallic systems of gold and
rhodium were explored for wider temperature range activity than gold alone. All catalysts were
characterised using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray
Spectroscopy (EDS) and X-Ray Diffraction (XRD). Activity was measured using a temperature
controlled, custom-built reactor linked to a gas chromatograph.
The conditions yielding the smallest gold nanoparticles were established by adding 5, 8 or 10
wt.% loadings of chloroauric acid to aqueous suspensions of either TiO2 or SiO2 at pH 5, 7 or 9
and at 70-75 °C over 60 minutes. Each preparation was sealed in parafilm, aged in the dark at
room temperature for 3 days, vacuum-filtered and subsequently calcined at 300 °C. Gold
nanoparticles were smallest when deposited onto TiO2 instead of SiO2, at pH 7 and at a loading of
5 wt. %. A combination of gold and rhodium catalysts were subsequently prepared using these
conditions, with the simultaneous addition of rhodium at 1, 3, 5 or 10 wt. % loading.
Hydrolysis of gold is highly dependent upon pH, resulting in the synthesis of smaller particles
under alkaline conditions. Catalytic activity of samples analysed at 70 and 150 °C was highest for
gold nanoparticles below 5 nm, in agreement with previous studies. In the proposed bimetallic
catalysts, it was difficult to distinguish gold and rhodium nanoparticles in TEM images, although
EDS confirmed their combined presence on the TiO2 support. Particle sizes remained below 5 nm,
appearing monodispersed on the TiO2 support except at 10 % rhodium loading where some
nanoparticle aggregation was observed. CO oxidation activity showed an apparent temperaturedependent
shift in the optimal rhodium loading. Au-TiO2 catalysts with a 5% loading showed the
highest activity up to 350 °C for a period of 10 hours and the catalyst deactivated due to sintering.
At 150 and 200 °C the Au/Rh-TiO2 catalyst remained active for more than 12 hours. It was
concluded that the inclusion of rhodium is a potentially-favourable method for stabilising the
activity of gold catalysts.
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