| Summary: | A stable high temperature gold nano-catalyst:
synthesis, characterization and application
The ability of supported gold nanoparticles to catalyse many reactions even
at very low temperatures has spurred a great deal of research into the eld.
Reactions such as CO oxidation and NOx reduction have many industrial applications
as well as uses in the motor industry for catalytic converters. The
interest is both for scienti c as well as economic reasons as gold supplies far
exceed all PGM supplies. Scienti cally gold catalysts are able to catalyze reactions
from below 0°C, a feat that no PGM catalyst can achieve. The low
temperature activity of gold catalysts will reduce the emission of pollutants
during start up. Since the discovery and development of gold catalysts one of
the most researched topics has been nding ways to stabilise the gold nanoparticles
on the support surface. The importance of gold nanoparticle stability
is crucial as the catalysts are only highly active if the gold nanoparticles are
less than 5 nm in size. A number of companies have worked to develop gold
catalysts that are stable for long durations at temperatures over 450°C with
no signi cant progress made over the last two decades other than a catalyst
produced by Toyota.
In this thesis, literature reviews of current support materials as well as synthesis
methods are investigated in order to determine reasons for the instability
of current gold catalysts. Further, the Mintek Aurolite catalyst is tested and
its deactivation mechanisms probed using in-situ VT-PXRD, Rietveld re nement,
TEM, HR-TEM, as well as CO oxidation tests. Testing revealed aws in
the support structure of the catalyst which resulted in dramatic deactivation.
As titania is such a common support material for many reactions in industry
as well as being known to be one of the best supports for gold it was chosen
as a support material. However, as is revealed, in its current forms and
morphologies it is unable to provide the thermodynamically stable and high
surface areas that are required for a stable catalyst After the development of a
robust and reproducible synthesis method for the deposition of gold and other
PGM's a number of supports were tested. These include silica and zirconia as well as titania derivatives such as Degussa P25 and commercial anatase. Initially
these supports o er high usable surface areas but after a relatively small
amount of time complete deactivation occurs. Reasons for this deactivation
are determined and the information gained is used to develop supports that
can combat these deactivation processes. Phase pure nano anatase is synthesised
which produced a support with an incredibly large surface area compared
to the aforementioned supports. The catalyst was able to withstand temperatures
over 450°C for longer durations compared to other catalysts exposed
to the same conditions. However, the phase conversion of the anatase to its
thermodynamically stable form rutile once again deactivated the catalyst with
time. Finally a rutile nanosupport is developed with the desired morphology
and thermodynamic stability needed for high temperature applications. The
catalyst is able to withstand temperatures over 550°C for more than 200 hours
as well as still being active after exposure to 810°C. The industrial Aurolite
catalyst showed complete deactivation after just 12 hours at 500°C. The catalyst
produced in this thesis has been shown to be one of the most stable and
thermally resistant gold catalysts in the world.
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