Nanotubular titanates were synthesized by a simple methodology using a commercial
TiO2 (Degussa P25 containing anatase and rutile phases) and a base (KOH) solution.
Prior to the removal of KOH, the samples of TiO2 were aged for three different time
intervals (0, 2 days, 61 days). The freshly prepared synthetic samples were
characterized for their structural and morphological properties by BET, XRD,
Raman, TEM, HRTEM, EDX and SEM. Both TEM and SEM analysis revealed that
ageing time influenced the tubular structure and morphology of the new materials.
Raman and surface analysis data also showed that ageing time affected both the
structural and surface properties of TiO2. The XRD results showed that the
crystallinity of the TiO2 decreased with increasing ageing time. Energy dispersive Xray
spectroscopy (EDX) showed that the tubes derived from TiO2 are comprised of
potassium, titanium and oxygen. Catalysts A, B and C were prepared by the addition
of 1 wt% Pd (wet impregnation) to the titanate formed after ageing of the TiO2 in
KOH for 0, 2 and 61 days, respectively. The catalysts were tested for the vapour
phase hydrogenation of phenol in a fixed-bed micro reactor within the temperature
range of 165 to 300oC under atmospheric pressure. Of the three catalysts, catalyst B
showed the best activity (conversion 97%) and total selectivity to cyclohexanone
(99%). In contrast, catalyst C, which showed a moderate activity favoured selectivity
to cyclohexanol. These results are attributed to differences in surface morphologies
between the two catalysts B and C, associated with the surface area and a change in
the surface acid-base properties. Catalyst B also showed a higher resistance towards
deactivation and maintained a higher total selectivity to cyclohexanone than did
catalyst C.
A hydrothermal treatment of NaOH and TiO2 was employed to prepare two
materials, TiO2-B and TiO2-C with relatively small crystallite size and large specific
surface area. The hydrogenation of phenol was used to evaluate the activity of the
catalysts Pd/TiO2-B and Pd/TiO2-C. The reaction proceeds in a single step and
involves the formation of a partially hydrogenated product, namely cyclohexanone.
The larger surface area catalyst (Pd/TiO2-C, 89 m2/g) showed better activity and
selectivity to cyclohexanone than its counterpart (Pd/TiO2-B, 45 m2/g). The catalyst
activity showed significant dependency on the surface area whereas the selectivity
was greatly influenced by surface basicity.
Titania derived nanotubes synthesized by treating P25 Degussa TiO2 with a
concentrated KOH solution and aged for 2 days was used as a catalyst support for the
hydrogenation of o-chloronitrobenzene (O-CNB) with Pd as the active phase. The
vapour-phase hydrogenation of O-CNB was carried out in ethanol at 250 oC and
atmospheric pressure over a Pd/TiO2 derived nanotube catalyst (Pd/TiO2-M).
Pd/TiO2-M gave complete conversion (100%) of O-CNB with a selectivity to orthochloroaniline
(O-CAN) of 86 %. The stability of the Pd/TiO2 catalyst was tested over
5 hours during which time the conversion slowly dropped to 80 % (selectivity 93 %)
due to poisoning. TPR analysis revealed the existence of a strong palladium-support
interaction and this was found to be crucial to the overall activity of the catalyst.
It has been found that gold supported on potassium titanate, KTiO2(OH) can, under
some circumstances, exhibit a superior performance for the oxidation of carbon
monoxide, relative to that obtained with titania as a support. It appears that the
dispersions of gold on the two types of support are sufficiently similar that other
factors are responsible for the improved activity noted. It may be that the higher
basic character and detailed structural features of the titanate surface play a role.
The effect of the addition of alkali metal ions on the anatase to rutile transformation
of titanium dioxide (P25 Degussa) was investigated using X-ray diffraction, Raman
spectroscopy, and surface area measurements. Both Li and Cs ions accelerated the
anatase to rutile transformation whereas Na and K ions did not show any effect.
Furthermore, the effect was more pronounced after addition of the Li ions so that the
transformation temperature dramatically decreased from ~800 oC for commercial
TiO2 to ~600 oC. The surface area of the TiO2 material decreased with sintering due
to the increase in crystalline size. Moreover, the acceleration of the transformation
occurred at lower temperatures and at higher Li content.
Mesoporous nanocrystalline TiO2 (HSA TiO2) was prepared by hydrothermal
treatment of TiO2 with NaOH. The material was very amorphous and underwent the
phase transformation from amorphous to anatase phase and subsequently from
anatase to rutile phase with sintering. The anatase to rutile transformation was
delayed after doping and grain growth was inhibited. After sintering at 800 oC the
material (HSA TiO2) still contained a significant amount of the anatase phase. The
complete transformation only occurred at ~1000 oC.
The esterification of benzoic acid and butyric acid with propanol over alkali metal
ions supported on TiO2 was investigated. K/TiO2-D showed the highest conversion
for both benzoic acid and butyric acid. The selectivity to propylbenzoate and
propylbutyrate was influenced by the basic nature of the catalysts. Butyric acid was
found to be more reactive than benzoic acid. The difference in reactivity was
explained in terms of steric and inductive effects. The differences in boiling points
and pH values were also considered.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5953 |
| Date | 20 January 2009 |
| Creators | Sikhwivhilu, Lucky Mashudu |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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