Abstract
With the achievements recorded in the development of fuel cell technology, arguably
hydrogen-driven systems would replace the combustion fossil fuel-based systems in
the nearest future for safer and pollution-free environment. The much desired
renewable and sustainable hydrogen infrastructure to replace or complement the
fast-depleting fossil-based hydrogen fuel for the full commercialisation of fuel cell
could be achieved through catalyst development and gasification of by-product
glycerol glut from biodiesel production activity seen as a waste at the moment.
In this study, the development of catalysts for the conversion of biomass vegetable oil
via transesterification reaction to biodiesel has been explored in parallel to the
subsequent catalytic gasification of pure and by-product glycerol from biodiesel
synthesis to generate hydrogen-rich gases for utilisation in fuel cells.
Reaction of tricalcium aluminate (C3A) with adsorbed water vapour under controlled
hydration process at elevated temperatures was found to modify its surface
morphology by formation of strongly basic hydroxide products. This was found to
increase its surface basic strength and ability to catalyse transesterification reaction to
biodiesel for the first time. Furthermore, basic alkaline earth metal oxides MgO, SrO
and transition metal oxide ZnO that are known to catalyses transesterification reaction
but suffered deactivation due to profuse leaching were doped and incorporated into
the non-hydrated tricalcium aluminate (C3A) lattice structure. The doped catalysts
were found to be not only active and selective to biodiesel formation but also
resistant to deactivation by leaching of the doped active metals for the first time.
The rapid deactivation of the nickel-based catalyst Ni/Al₂O₃ due to carbon deposition;
agglomeration and phase transformation especially during prolonged high
temperature operations using feedstock glycerol in steam reforming was minimised
through the use of promoters such as ceria (CeO₂) and LSCM (La₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O[sub](3-δ))
and alternative supports such as samarium-doped ceria (Ce₀.₈Sm₀.₂O[sub](2-δ)) and zirconia-doped
ceria
(Ce₀.₇₅Zr₀.₂₅O₂).
This
led
to
the
development
of
a
new
catalyst
system
NiLa₀.₇₅Sr₀.₂₅Cr₀.₅Mn₀.₅O[sub](3-δ)/
Ce₀.₇₅Zr₀.₂₅O₂ (Ni-LSCM/Ce-Zr) which was found to be very
active and offered much better suppression of carbon deposition and agglomeration
minimizing catalyst deactivation. However, the work revealed that, the ‘traditional’
wet impregnation method does not offer sufficient control over particle size, growth
and distribution. It takes time, is costly and results in weak interaction between the
active phase metal catalyst particles and support leading to agglomeration, instability
and deactivation at times even where a promoter was used; hence this offered poor
catalytic properties.
This study has demonstrated for the first time the use of a new phenomenon called
redox lattice reorganisation and already known redox exsolution as alternative
methods to wet impregnation in the preparation of oxide-supported nickel-based
metal catalysts in glycerol steam reforming (GSR). The work has revealed that unlike
what happens with the traditional wet impregnated catalysts where metal catalyst
superficially interact with the oxide support resulting in catalyst deactivation due to
agglomeration and carbon deposition or phase transformation. Redox lattice
reorganisation in spinel has shown that metal catalyst particles can be grown out from
the support itself and firmly anchored on the spinel oxide support leaving behind
elaborate macro porous channels. That provides good surface area, strong metal support-interaction
and
reduced
tendency
for
catalyst
deactivation
by
agglomeration
and
offered
effective
coking
suppression
and
good
catalytic
behaviour.
The
work
has
further
shown that particle size, population, metal-support interaction, size of the
channels in redox lattice reorganisation can all be tailored for better catalytic
behaviour by simple control of reduction temperature. The work revealed further that
redox exsolution in perovskite; particle size and distribution, metal-support interaction
and general morphology of the catalyst surface could be tailored for good catalytic
performance through control of B-site doping, careful choice of dopant metals for
both A-site and B-site cations and defect chemistry in glycerol steam reforming (GSR).
The metal exsoluted catalyst systems were found to be not only active and selective
toward the desired products but have also demonstrated great potentials to suppress
carbon deposition.
Type
Thesis, PhD Doctor of Philosophy
Rights
Embargo Date: 2017-08-29
Embargo Reason: Thesis restricted in accordance with University regulations. Electronic copy restricted until 29th August 2017.