Synthesis, characterisation and activity of ruthenium/N-doped multi-walled carbon nanotubes catalysts
Date
2013-07-29
Authors
Mabena, Letlhogonolo Fortunate
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Abstract
Nitrogen doped carbon nanotubes (N-CNTs) were synthesised using
thermal-Chemical Vapour Deposition (CVD). The obtained material was
purified, characterised and used as a support for ruthenium nanoparticles.
The catalytic performance of the Ru/N-CNTs was investigated in different
chemical reactions. Thus, this thesis is divided into two sections. The
synthesis of the nanomaterials, the catalyst performance of nanomaterials
in the oxygen reaction reduction (ORR) and activity in the oxidation of
styrene and benzyl alcohol.
In the first section N-CNTs were synthesised using a thermal-CVD method
in a horizontal split-tube furnace. The reactions were carried out in a
tubular quartz reactor. Cyclohexanol was used as carbon source, aniline
as a nitrogen source and ferrocene as catalyst. A mixture of
cyclohexanol-aniline-ferrocene was placed in a quartz boat that was
directly introduced in the centre of the first furnace and vaporised at 280
°C. The resultant vapours were transferred to the second furnace where
the N-CNTs were grown at a temperature of 900°C under the carrier gas
flow (nitrogen or 5% H2 balanced in argon gas). The N-CNTs formed had a
fairly crystalline structure, constituted by a periodical bamboo like structure
with tubes diameters of 35 - 100 nm and nitrogen content up to
1.3 at. %.The N-CNTs with 0.8 at.% were selected to be used becaused of
the quality and the amount of CNTs produced.
N-CNTs were then used to support ruthenium (Ru) nanoparticles using a
microwave assisted reduction technique. The synthesised nanostructured
materials were characterised by TEM, SEM, TGA, and XRD.
The TEM images of the Ru catalysts supported on N-CNTs revealed
homogenous dispersion of Ru nanoparticles with a narrow sizes
distribution and small particle size with an average diameter of 2.5 nm
when 500 W power was used.
In the second section, part A; four catalysts with different Ru wt. %
supported on N-CNTs were prepared: the amount of Ru deposited on the
N-CNTs was varied between 0 –10 wt. %. The activity of the prepared
nanocatalysts towards the oxygen reduction reaction (ORR) was
characterised using the rotating disk electrode and voltammetry
techniques. The ORR activity was higher at lower concentrations of Ru on
N-CNTs. The 4e- pathway of ORR was more favourable on 2 and 5 % Ru
loaded N-CNTs than as 10 % Ru loaded N-CNTs.
In Part B; prepared Ru/CNT and Ru/N-CNT catalysts were calcined and
used for the liquid-phase oxidation reaction of styrene and benzyl alcohol.
The influence of various reaction parameters such as reaction time,
catalyst mass, solvent nature and reaction temperature were evaluated. It
is interesting to note that the RuO2 on carbon material catalyst was more
active for styrene oxidation than for benzyl alcohol oxidation reaction. The
conversion of styrene was 41 % and the selectivity to benzaldehyde was
85 % when 5 % RuO2/CNTs catalyst was used with 1,4-dioxane as a
solvent at 80 °C in 4 h. The highest conversion of benzyl alcohol was 11 %
also with 85 % benzaldehyde selectivity. The benzyl alcohol oxidation
was performed at 110 °C for 5 h.
Ru/N-CNTs were shown to exhibit better activity for a styrene oxidation
reaction. Therefore further investigations on the activity of nitrogen doped
carbon nanotubes (N-CNTs*) prepared by reaction of acetylene (C2H2)
and acetonitrile (CH3CN) at 700 °C over a 10 % Fe-Co supported on
calcium carbonate (CaCO3) catalyst was investigated for styrene
oxidation. In this case the nitrogen doped carbon nanotubes (N-CNTs*)
with 2.2 at. % nitrogen content was used. A 5 % Ru/N-CNT* catalyst was
highly selective as compared to the previous N-CNT supports used in the
styrene oxidation reaction. Comparing the support it was deduced that the
nitrogen present in the support is playing a major role. With the increase in
the nitrogen content in the matrix of the CNTs the conversion of styrene
decreased but with an increase in the selectivity. The selectivity towards
benzaldehyde was 96 % after 4 h when N-CNTs* were used as support for
the styrene conversion reaction. In comparison for the RuO2 on CNTs and
N-CNTs the styrene conversions were 85 and 87 % respectively.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree, Doctor of philosophy Degree (PhD) in chemistry Johannesburg, 2013.