Abstract:
A brief overview of this PhD thesis,
The emergence of nanotechnology has stimulated both fundamental and industrially relevant
studies of the catalytic activity of noble metal nanoparticles. Palladium, ruthenium and gold
are well known catalysts when used in nanoparticle- based systems. This body of work
endeavoured to investigate the catalytic activity of these noble metal nanoparticles through
three studies as a briefly overviewed below.
Study 1:
Palladium is a well-known catalyst, even in bulk phases, but its high cost had driven industry
towards its use in nanoparticle- based systems well before nanotechnology had attracted the
attention of the media. Palladium nanoparticles often show remarkable catalytic activity and
selectivity, particularly for the hydrogenation of some unsaturated hydrocarbons, such as
alkenes, alkynes and unsaturated carbonyl compounds. The nature of supports can affect the
catalytic activity and selectivity of metal-support interaction. Natural polymeric supports,
such as wool, can be suitable for new generation of composite materials incorporating
nanosized metal nanoparticles and have the added advantage of being “environmentally
friendly”.
Catalytic hydrogenation of cyclohexene to cyclohexane by palladium nanoparticles
immobilized on wool was demonstrated by using a Parr high pressure hydrogenation set-up.
The efficiency of the process was explored over loading rates from 1.6% to 2.6% of
palladium nanoparticles (by weight) with a variety of particle sizes. Optimization of the
reaction conditions including, stirring rate, amounts of reactants, gas pressure and target
temperature, led to series of catalytic activity tests carried out for 5 or 24 hours (each) at 400psi H2 and 40 oC using a stirring rate 750 rpm. Product mixtures were analysed using gas
chromatography (GC-FID) to determine conversions. Samples S1 and S2 proved to be the
most active catalysts because the average Pd particle size was around ~5 nm and the particles
were more accessible for the reactant (i.e., Pd particles were on the surface of wool).
However, under the catalytic testing conditions studied, wool (Pd/wool) did not show
advantages over commercially used palladium nanoparticles on activated carbon (Pd/C).
Study 2:
Ruthenium fabricated as noble metal nanoparticles can be catalytically active for
hydrogenation of organic compounds. However, a challenging issue for researchers is that Ru
nanocatalysts can be spontaneously deactivated due to effects, such as sintering or leaching of
active components, oxidation of noble metal nanoparticles, inactive metal or metal oxide
deposition and impurities in solvents and reagents. Calcination of noble metal nanoparticles
is one option for reactivation of Ru nanoparticles immobilized on SiO2 (Ru/SiO2) utilized as
nanocatalysts in chemical reactions. In fact, the catalytic activity of noble metal nanoparticles
is known to be proportional to the active part of the surface area. The effects of calcinations
on catalytic activity of “shape- specific” 0.1 wt% Ru/SiO2 for hydrogenation of cyclohexene
to cyclohexane were investigated. Optimization of calcinations by varying temperature and
time proved to be effective on the activity of nanocatalysts retaining the Ru nanocatalysts
shapes for the hydrogenation of cyclohexene. Product mixtures were analysed using gas
chromatography (GC-FID) to determine conversions. The Ru catalysts showed the highest
activity (100%) when they were activated by calcination following protocol No.1 in a furnace
under the mildest reductive conditions studied (temperature = 200 oC for 1 hour, which was
the shortest calcination time). HRTEM study showed only minor deformation of the Ru
nanoparticles and minimal aggregation for this type of activation.
Study 3:
Supported gold nanoparticles have excited much interest owing to their unusual and
somewhat unexpected catalytic activity particularly with the selective oxidation of organic
compounds. Gold nanoparticles immobilized on Norit activated carbon (Au101/C) via
colloidal deposition gave high selectivity of benzyl alcohol oxidation. The presence of a base
(K2CO3) increased the catalytic activity of gold nanocatalysts (which was negligible in the
absence of base) through dehydrogenation of the alcohol via deprotonation of a primary OH
groups, and helped overcome the rate-limitation step of the oxidation process. The interaction
between the gold species and the support was investigated by measuring change in catalytic
activity with different activation methods (i.e., washing with a solvent at elevated
temperature, and/or followed by calcinations). A mixture of benzyl alcohol as a reactant,
methanol as a solvent, K2CO3 as a base and oxygen gas was studied by the activated gold
nanocatalysts using a mini reactor set-up. The efficiency of the process was explored by
varying the amounts of benzyl alcohol and the base, target temperature, metal loading of the
gold catalysts rate and the solvent, between 3 and 24 hours at 73 psi O2 and a stirring rate
(750 rpm). The samples of the reaction mixture were centrifuged and analysed by highperformance
liquid chromatography (HPLC) to determine conversions.
The effect of size on the catalytic activity was studied for different types of gold particles
(Au101, Aunaked and Aucitrate) and clusters (Au8 and Au9) immobilized on powder Norit
activated carbon. The highest activity of benzyl alcohol oxidation was observed for activated
1.0 wt% Au101/C catalysts (washed with toluene and followed by calcination under vacuum at
100 oC for 3 h) for ~3.5 nm gold particles. Additionally, the support effect was studied for
gold particles immobilized on different types of carbons, such as Norit activated carbon
(powder, granular and powdered) and mesoporous carbons (CMK-3, CMK-8 and NCCR-41),
granular modified carbon (–SH and –SO3H groups) and Vulcan carbon. The highest activity was observed by activated 1.0 wt% Au101/C8 catalysts (washed with toluene and followed by
calcination under vacuum at 100 oC for 3 h). Activated 1% Au101/C41 (washed with toluene
followed by calcination under vacuum at 100 oC for 3 hours) with 2.6 ± 0.1 nm gold particle
size showed the highest selectivity towards methyl benzoate as a main product (S%: 88%)
after 3 hours reaction time. However, activated 1% Au101/C (calcination in O2 -H2 at 100 oC
for 3 hours) with 6.6 ± 0.3 nm gold particle size exhibited the highest selectivity towards
benzoic acid as a main product (S: 86%) after 24 hours reaction time.Therefore, particle size
and type of carbon support can be considered as playing crucial roles in defining the catalytic
activity of gold nanocatalysts which were used for benzyl alcohol oxidation.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/10042 |
| Date | January 2014 |
| Creators | Ghadamgahi, Sedigheh |
| Publisher | University of Canterbury. Chemistry |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Sedigheh Ghadamgahi, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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