Synthesis, characterisation and activity of ruthenium/N-doped multi-walled carbon nanotubes catalysts

Mabena, Letlhogonolo Fortunate
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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.
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.