Shane H. Durbach

Direct synthesis of carbon nanofibers from South African coal fly ash

Carbon nanofibers (CNFs), cylindrical nanostructures containing graphene, were synthesized directly from South African fly ash (a waste product formed during the combustion of coal). The CNFs (as well as other carbonaceous materials like carbon nanotubes (CNTs)) were produced by the catalytic chemical vapour deposition method (CCVD) in the presence of acetylene gas at temperatures ranging from 400°C to 700°C. The fly ash and its carbonaceous products were characterized by transmission electron microscopy (TEM), thermogravimetric analysis (TGA), laser Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurements. It was observed that as-received fly ash was capable of producing CNFs in high yield by CCVD, starting at a relatively low temperature of 400°C. Laser Raman spectra and TGA thermograms showed that the carbonaceous products which formed were mostly disordered. Small bundles of CNTs and CNFs observed by TEM and energy-dispersive spectroscopy (EDS) showed that the catalyst most likely responsible for CNF formation was iron in the form of cementite; X-ray diffraction (XRD) and Mössbauer spectroscopy confirmed these findings.

The effect of CO2 on the CVD synthesis of carbon nanomaterials using fly ash as a catalyst

The efficient use of fly ash is a worldwide issue due to its high production and harmful effects on the environment. In this work the synthesis of carbon nanomaterials (CNMs) via the chemical vapour deposition (CVD) method, using fly ash as a catalyst and CO2 as an alternate carbon source, was investigated. Here CO2 was employed in three different ways: (1) as a sole carbon source, (2) as an additive to C2H2 and (3) as a carbon source prior to the reaction of C2H2 with fly ash. SEM, TEM and laser Raman spectroscopy confirmed that CNMs were formed in all three cases. In the first case, when CO2 was used as a sole carbon source, CNMs were formed in low yields with a considerable amount of amorphous carbon. However, in the second case when CO2 was used as an additive to C2H2, a drastic increase in CNM formation was observed. In both cases optimal yields were observed at 600 °C. However in the third case, when CO2 was used as a carbon source prior to the reaction with C2H2, uniform-sized nanofibers of the highest yields of all three cases were formed. Likewise these CNMs were found to be the most thermally stable. Hence this study has shown that the use of waste materials such as fly ash as a catalyst and CO2 as a carbon source prior to the reaction with C2H2, results in a very simple and cost efficient process to make uniformally shaped, thermally stable CNMs.

Morphological and crystallinity differences in nitrogen-doped carbon nanotubes grown by chemical vapour deposition decomposition of melamine over coal fly ash

Millions of tons of coal fly ash (CFA) are produced each year in thermoelectric coal powered stations as a waste-product. Until recently few researchers have endeavored to use CFA as a catalyst in the formation of carbon nanomaterials (CNMs). In this study a two-stage tube furnace was used to synthesize N-doped carbon nanotubes (NCNTs) by chemical vapor deposition using melamine as the source of nitrogen and carbon, with CFA as a catalyst, at temperatures ranging from 800 to 900 °C. The masses of the NCNTs formed were found to have increased with increased synthesis temperature. The morphology and crystallinity of the NCNTs along with the amount of nitrogen incorporated into these were found to vary with the synthesis temperature. NCNTs synthesized at 800 °C were found to be typical multiwalled carbon nanotubes by transmission electron microscopy, whereas those at 850 and 900 °C were found to be chain-like and bamboo-like compartmentalised nanotubes respectively. The NCNTs synthesized at 800 °C were found to contain the least incorporation of nitrogen by elemental analysis and were the most crystalline (as determined by using the IG/ID ratio and the G-band position from laser Raman spectroscopy), whereas those at 850 °C were the least crystalline but had the highest incorporation of nitrogen. NCNTs synthesised at 800 °C were the most thermally stable, whereas those synthesized at 850 °C were the least stable. NCNTs synthesized at 900 °C had a crystallinity, thermal stability and nitrogen content which lay between the other two.

Ion implantation of Raney copper catalysts

Abstract Raney copper catalysts promoted with zinc are known to catalyse both methanol synthesis and the water-gas shift reaction. In this work, samples were implanted with Zn+ ions using a specially designed free-powder target holder, and then tested for water-gas shift activity in a chemical microreactor. Beneficial effects in the form of longer sustanined activity at higher conversion were observed, extending over several hundred hours, especially for the higher zinc doses. Similar results were obtained with a chromium implant. Control implants with inert gas ions produced no beneficial effects. This study represents the first example of the use of ion implanted promoters in catalysis, on a bench top reactor scale.

Novel synthesis of Ag decorated TiO2 anchored on zeolites derived from coal fly ash for the photodegradation of bisphenol-A

The disposal of millions of tons of coal fly ash (CFA) threatens the environment, hence means to reuse CFA are highly sought after. In this study, CFA was reused to make materials which were tested for water purification. Zeolitic material (CFA_Zeo) was derived from CFA by a 2-step alkali-fusion hydrothermal method and then composited with TiO2 nanoparticles using a novel resin-gel technique. CFA_Zeo loadings were 15 and 30 wt% in the resulting TiO2/CFA_Zeo composites. These composites were then loaded with 1 wt% Ag nanoparticles by a deposition–precipitation technique using NaOH and urea. CFA_Zeo rods (morphology confirmed by TEM) were confirmed by PXRD to be sodium aluminum silicate hydrate. TEM analyses of the CFA_Zeo rods in the composites revealed them to be completely coated with TiO2 nanoparticles that had Ag nanoparticles on their surfaces. The photoluminescence emission peak of TiO2 was found to be significantly higher than that of TiO2/CFA_Zeo composites, with the TiO2/CFA_Zeo composites that were loaded with Ag having even lower emission intensities. UV-vis DRS spectra showed that CFA_Zeo had no effect on the band gap of TiO2, while composites that contained Ag had a wide absorption band in the visible region. The photocatalytic efficiency of these materials was then determined using bisphenol-A (BPA) as a model compound under both UV and visible light. Except for the 30 wt% TiO2/CFA_Zeo composites without Ag, all of the composites had superior photoactivity to uncomposited TiO2 under both UV and visible light. On the other hand, composites with Ag nanoparticles showed the best photoactivities. The superior photoactivities of these composites under UV-light were mainly attributed to the separation of charge carriers, whereas under visible light it was attributed to the ability of silver to harvest visible light through surface plasmon resonance (SPR).

The Effect of Hydrogen, Helium and Their Mixtures on the Synthesis of Carbon Nanotubes in a DC Arc‐Discharger

Abstract Multiwalled carbon nanotubes (MWCNTs) have been produced with a DC arc‐discharger in the presence of hydrogen, helium or hydrogen‐helium mixtures using high gas pressures (60 kPa–140 kPa) and varying gas pressure ratios (10%–100% hydrogen in helium at 100 kPa). Variation of He pressure hardly affected the cathode deposit formation while increased H2 pressure resulted in an increase in chamber soot and cathode material formation but a relative decrease in the % MWCNT production. Varying H2/He gas ratios at a constant total pressure of 100 kPa revealed that increasing H2 content gave increased cathode deposit formation because of a faster anode consumption rate as well as more total gaseous product formation. Further, a maximum MWCNT deposit was obtained from a 50/50 gas mixture. GC‐MS analysis of the gaseous products trapped from these reactions showed evidence for the presence of linear alkynes (ethyne, butyne, pentyne, hexyne etc.), benzene, naphthalene, anthracene, and their methylated analogues. The relative in‐situ temperature of the reaction with the pure gases was monitored and revealed that the highest temperature was recorded for the reaction with 60 kPa H2, which also produced the maximum amount of MWCNTs (80%). Impure samples from the cathode deposit were analysed by TEM and showed that in the majority of cases rectangular closed‐tipped nanotubes were synthesised, together with varying amounts of onion‐like sheets and folded graphene sheets.

TiO2 composited with carbon nanofibers or nitrogen-doped carbon nanotubes synthesized using coal fly ash as a catalyst: bisphenol-A photodegradation efficiency evaluation

Coal fly ash (CFA) was used as a catalyst for the synthesis of nitrogen-doped carbon nanotubes (NCNTs) and carbon nanofibers (CNFs) by chemical vapor deposition. Carbon nanomaterials (CNMs) were successfully purified by sequential treatment in 5% HF and then in a dil. HNO3/H2SO4 mixture, as was shown by SEM, TGA, and XRD. The purified NCNTs and CNFs were composited with TiO2 nanoparticles at varying loadings (i.e. 1, 5 and 20% CNF/NCNT loadings) by a surfactant wrapping sol–gel/hydrothermal method and used for the photodegradation of bisphenol-A (BPA) in water with light being sourced from a solar simulator. It was shown by TEM that the CNMs were completely coated with TiO2 nanoparticles and interactions between the CNMs and TiO2 were demonstrated using PXRD and laser Raman spectroscopy. Photoluminescence measurements showed that compositing TiO2 with CNMs, especially NCNTs significantly reduced its emission intensity suggesting a reduced electron/hole recombination rate. Unbound TiO2 was used to optimize the photodegradation experimental conditions, i.e. solution pH, the mass of the photocatalyst, the initial concentration of BPA and solution temperature. The photocatalytic efficiency of the various TIO2 and CNFs/NCNTs was assessed using the optimized conditions where it was observed that the composites containing 1 and 5% loadings of CNMs outperformed TiO2. The photocatalytic efficiency of the NCNT based composites was higher than that of the CNF based composites. This work shows that CFA, a toxic material, could be used to synthesize materials useful for cleaning water.

Synthesis of Multi-Walled Carbon Nanotubes from Plastic Waste Using a Stainless-Steel CVD Reactor as Catalyst

The disposal of non-biodegradable plastic waste without further upgrading/downgrading is not environmentally acceptable and many methods to overcome the problem have been proposed. Herein we indicate a simple method to make high-value nanomaterials from plastic waste as a partial solution to the environmental problem. Laboratory-based waste centrifuge tubes made of polypropylene were chosen as a carbon source to show the process principle. In the process, multi-walled carbon nanotubes (MWCNTs) were synthesized from plastic waste in a two-stage stainless steel 316 (SS 316) metal tube that acted as both reactor vessel and catalyst. The steel reactor contains Fe (and Ni, and various alloys), which act as the catalyst for the carbon conversion process. The reaction and products were studied using electron probe microanalysis, thermogravimetric analysis, Raman spectroscopy and transmission electron microscopy and scanning electron microscopy. Optimization studies to determine the effect of different parameters on the process showed that the highest yield and most graphitized MWCNTs were formed at 900 °C under the reaction conditions used (yield 42%; Raman ID/IG ratio = 0.48). The high quality and high yield of the MWCNTs that were produced in a flow reactor from plastic waste using a two stage SS 316 chemical vapor deposition (CVD) furnace did not require the use of an added catalyst.