Synthesis of Multiwalled Carbon Nanotubes on Fly Ash Derived

Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW, 2006, Australia. Environ. Sci. ...
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Environ. Sci. Technol. 2009, 43, 7889–7894

Synthesis of Multiwalled Carbon Nanotubes on Fly Ash Derived Catalysts OSCAR M. DUNENS,* KIERAN J. MACKENZIE, AND ANDREW T. HARRIS Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, The University of Sydney, NSW, 2006, Australia

Received June 18, 2009. Revised manuscript received July 23, 2009. Accepted August 25, 2009.

Carbon nanotubes (CNTs) are an allotrope of carbon with unique properties that make them potentially useful in a vast range of applications. However, CNTs are predominantly produced using expensive and/or nonrecyclable catalyst supports, e.g., mesoporous silica and alumina. In this work, coal combustion fly ash, a bulk waste product with limited uses, was impregnated with iron nitrate and successfully used as a substrate to produce industrial grade multiwalled carbon nanotubes (MWNTs) by fluidized bed chemical vapor deposition. CNTs were analyzed using thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The most successful catalyst trialed at 650 °C using ethylene as a carbon source was a 5 wt % Fe fly ash catalyst, which produced a CNT yield in respect to metal loading of approximately 82.5%. The MWNTs had outer diameters of between 12 and 20 nm with a reasonable degree of wall graphitization (IG/ID of 1.17). Advantages of utilizing fly ash as a catalyst support are its availability at low cost at the megaton scale, its high thermal stability, and suitability for use in industrial fluidized bed reactors. Potential applications for the fly ash produced CNTs include use in composite materials.

Introduction Fly ash, predominantly a byproduct from coal and biomass combustion, is being produced in increasing quantities. Consequently its disposal in an environmentally sound manner is becoming a matter of global concern (1, 2). Coal power station fly ash, the most abundant type of fly ash, is primarily comprised of small glassy aluminosilicate spheres, 1 µm to well over 100 µm in diameter (3). These spheres are formed by the rapid cooling of molten mineral matter in pulverized coal at high combustion temperatures. The chemical composition of fly ash varies with the chemical composition of the coal being fired and the combustion technology (2). Class F fly ash, as defined by ASTM C168 (4), is the principal type of fly ash in Australia and is produced from bituminous and anthracitic coals. The main chemical constituents in Australian fly ash are SiO2 (44.5-67.0 wt %), Al2O3 (22.2-30.7 wt %), Fe2O3 (1.1-14.4 wt %), TiO2 (0.9-1.9 wt %), CaO (0.4-4.2 wt %), MgO (0.3-1.6 wt %), and K2O (0.4-2.9 wt %) (5). In 2007 Australia produced an estimated 14.5 million tons of power station fly ash, with beneficial ash utilization below * Corresponding author e-mail: [email protected]. 10.1021/es901779c CCC: $40.75

Published on Web 09/09/2009

 2009 American Chemical Society

20% (6). Fly ash is a prozzolan, which in the presence of moisture, reacts with calcium hydroxide to form insoluble compounds possessing cementitious properties. Currently the main uses for fly ash include bulk fill applications such as structural fill and mine site remediation, and as cement replacement in lightweight high performance concrete (10-20 wt % fly ash additive) (7). Fly ash applications that have been successfully investigated include use in agriculture and soil management (8), absorbents for heavy metals (9), and waste stabilization (10). Fly ash has also been shown to be a suitable catalyst support for hydrogen production (11) and waste gas cleaning (12), and as a catalyst for steam methane reforming (13) and gas phase oxidation of volatile organic compounds (VOCs) (14), however none of these applications are currently in commercial use (2). To date, there has been no published report on fly ash being used as a catalyst support for carbon nanotube (CNT) synthesis. Carbon nanotubes are unique cylindrical crystalline carbon structures that can be visualized as seamless rolled sheets of graphene (15). The carbon sp2 bonding and high aspect ratio (>104) of CNTs gives rise to a unique array of physical, chemical, mechanical, optical, and electrical properties. These extraordinary properties lend themselves to a vast array of potential applications, including in composite materials (16), energy storage (17), and electronics (18) among many. The CNT market is forecasted to reach multibillion dollar value within the next decade (19), and the fluidized bed chemical vapor deposition method (FBCVD) of CNT synthesis has been identified as possessing the most potential for large scale CNT synthesis, which has been defined previously as being of the order of 10 000 tons per plant per year (20). Chemical vapor deposition (CVD) is a CNT synthesis technique where a gaseous carbon source is catalytically decomposed at moderate temperatures (500-1100 °C) in the presence of a metal catalyst. The FBCVD method of CNT synthesis utilizes a supported catalyst, most commonly a transition metal on a porous substrate. A mixture of gases flow vertically through the bed of catalyst particles, resulting in increased bed mixing and “fluid-like” behavior of the particle bed, which significantly increases heat and mass transfer in the reaction zone. Further advantages of FBCVD include continuous production, low energy requirements, and a high level of control over reaction parameters. Substrates used for CNT synthesis via FBCVD typically include Al2O3, SiO2, and MgO, ranging in size from 25 µm (21) - 150 µm (22). The chief transitional metals used include Fe, Co, Ni, often doped with Mo to improve yield and selectivity. Total metal weightings are usually in the order of 1-15 wt % (23, 24). Although these commonly used catalysts and substrates provide a good fluidizing medium and support for CNT growth, they are produced using virgin materials and are rarely recycled due to destruction by CNT purification techniques using strong acid or base treatments. Therefore, the need exists for environmentally benign catalysts if large scale CNT production is to become less resource intensive. This move toward more environmentally friendly CNT production methods has resulted in a number of papers for CNT synthesis using naturally occurring materials, such as lava (25), natural minerals (26), and botanical hydrocarbons such as eucalyptus oil (27). The use of fly ash as a catalyst/ support for CNT synthesis presents a further route toward the development of less resource intensive CNT processes. Fly ash is similar in composition to many laboratorydeveloped catalysts for CNT synthesis, including Fe content, e.g., ref 23. Thus, fly ash has the potential to be used as a VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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catalyst for CNT growth without chemical treatment (especially if the fly ash has a high Fe content), or impregnated with Fe to increase metal loading. The favorable composition of fly ash offers the advantage of reducing the use of virgin resources and lowering costs associated with CNT catalyst manufacture.

TABLE 1. Class F Fly Ash Composition As Determined by XRF

Experimental Section Catalysts were prepared via wet impregnation (28, 29). Fe(NO3)3 · 9H2O (A.R. grade, Sigma Aldrich) was dissolved in ethanol and sonicated for 15 min prior to the addition of a weighed amount of sieved fly ash substrate (-125 um, +150 um) in the appropriate proportions to result in catalysts with a total iron loading of either 2.5 or 5 wt % (inclusive of the 1.38 wt % Fe present in the as received fly ash). The mixture was stirred and air-dried at 40 °C for 15 h prior to calcination in air at 800 °C for 12 h. The synthesis apparatus consisted of a 52 mm internal diameter, 1000 mm long cylindrical fluidized bed reactor, constructed entirely of Inconel 601 and enclosed within a high temperature ceramic furnace. An expansion unit, 100 mm in diameter and 500 mm in height, was installed at the top of the reactor to minimize particle entrainment. An environmental mitigation system comprised of particle scrubbers was used to treat effluent gases prior to release. Gas flow to the reactor was controlled via a series of Alicat Scientific 16 Series mass flow controllers. A type K thermocouple situated 30 mm above the distributor plate measured the temperature of the particle bed in situ. In each experiment 80.0 g of catalyst was introduced into the reactor at room temperature under an N2 flow of 3 SLPM. The reactor was heated to 700 °C before being reduced in situ at 700 °C for 30 min under a flow of H2 and N2 (1:1) at 6 SLPM. The temperature of the furnace was reduced to 650 °C under a flow of N2 at 6 SLPM prior to reaction with the carbon source. Reaction conditions consisted of C2H4, H2, N2 (1:1:2) at a total flow rate of 6 SLPM for a duration of 30 min. Post reaction cooling was undertaken using an N2 flow of 3 SLPM. Purification of the as-synthesized reaction products was not undertaken to eliminate additional process variables. Twenty-five kg of fly ash with high cenoshere content (micro alumino-silicate hollow particles) and low bulk density (∼ 420 kg/m3) was obtained from Blue Circle Southern Cement (30). The chemical composition of the as-received material was measured using X-ray fluorescence (XRF; Philips PW2400). The loss on ignition (LOI), which provides an indication of the unburnt carbon present in the ash, was also measured. The particle distribution of 459 g of as-received fly ash was obtained using weight fractions from an ABI CHEM BS410 vibrating test sieve. As-synthesized products were analyzed using thermogravimetric analysis (TGA; TA Instruments SDT Q600), highresolution transmission electron microscopy (HRTEM; JEOL 300 kV Atomic Resolution), scanning electron microscopy (SEM; Zeiss Ultra Plus Field Emission), and Raman spectroscopy (inVia Renishaw Raman Spectrometer). As-received fly ash was also analyzed via TGA, SEM, and Raman. For TGA analysis, a sample weight of ∼4 mg was used and a temperature ramp rate of 5 °C/min to 1000 °C and an air: nitrogen flow (6:4) of 100 mL/min were employed. For SEM analysis, ∼3 mg samples were placed on carbon tape and were sputtered with a ∼5 nm layer of Au. For TEM analysis, ∼5 mg of as-synthesized product was added to 20 mL of ethanol and sonicated for 15 min. Three drops of this mixture were placed onto a 200 mesh copper grid (coated with lacey carbon film) and dried in air for 2 min; this process was repeated 10 times and air-dried overnight prior to analysis. Raman spectroscopy was conducted using argon excitation at 514 nm, 0.12 mW, for an exposure time of 10 s and 10 7890

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fly ash component

weight %

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 loss on ignition total

64.16 0.69 28.95 1.97 0.02 0.67 0.37 0.70 2.16 0.06 0.03 0.25 100.0

TABLE 2. A Summary of the Reaction Products from the Non-Impregnated, 2.5 wt % Fe Impregnated, and 5 wt % Fe Impregnated Fly Ash Catalysts

catalyst FA FA2.5 FA5

substrate

Fe carbon estimated CNT yield total yield CNT as % of wt% wt% yield % metal IG/ID

fly ash 1.38 Fe impregnated 2.5 fly ash Fe impregnated 5 fly ash

2.29 7.65

0 10

0 30.6%

1.20 0.78

8.25

50

82.5%

1.17

accumulations. An average of three different points per sample was used.

Results and Discussion Fly ash is comprised of small solid particles called precipitator, small hollow particles (Cenospheres) and thin-walled hollow spheres (plerospheres) containing both Cenospheres and precipitator (3). The as-received fly ash used in this study had been physically treated in order to increase the cenosphere and pleroshpere content; the lower bulk density and pozzolanic properties of these particles are desired for use in concrete applications. The low bulk density, high thermal stability, and spherical nature also make them readily fluidizable and suitable for use in large-scale fluidized bed reactors.

FIGURE 1. First order derivative weight loss profiles for the fly ash supported catalysts. The 5 wt % Fe catalyst produced carbon products that had a higher maximum oxidation temperature, and narrower peak width at half-maximum, indicating the presence of less amorphous products and narrower product distribution than the products obtained from the 2.5 wt % and nonimpregnated fly ash catalysts.

FIGURE 2. SEM images of (a) 5 wt % Fe-Fly ash catalyst particle covered in MWNTs and carbon nanofibers (b) web of MWNTs and CNFs covering the surface of 5 wt % Fe loaded fly ash (c) CNFs on the surface of 2.5 wt % Fe loaded fly ash, (d) MWNTs intertwined within large CNFs on 2.5 wt % Fe loaded fly ash. The fly ash composition, determined by XRF analysis, is reported in Table 1. The fly ash contained 28.95 wt % Al2O3, 64.16 wt % SiO2, and a natural Fe loading of 1.37 wt % (1.97 wt % Fe2O3). The low loss on ignition of 0.25% indicated that few combustible impurities, e.g., carbon, were present. The fly ash contained small quantities of oxides other than Al2O3 and SiO2, e.g., Na2O and K2O. The effect of these compounds on CNT growth has not been reported to date, however sodium (from NaCl) and potassium (from KCl) have shown to inhibit carbon deposition and CNT growth quantitatively (31) . A relatively narrow size distribution of particles is desired for use in fluidized beds in order to achieve maximum bed mixing and predictable fluidization behavior. The particle size distribution of the fly ash according to weight fraction showed all particles under 300 µm, with the majority weight fraction between 125 and 150 µm (29.3 wt %). The bulk weight fraction was used in this study to achieve maximum fly ash utilization; the 125-150 µm size fraction is also comparable with other successful supported CNT catalysts (e.g., ref 32,). Three fly ash derived catalysts were trialed for CNT synthesis, (i) calcined as-received fly ash (FA), (ii) 2.5 wt % Fe loaded fly ash (FA2.5), and (iii) 5 wt % Fe loaded fly ash (FA5). The Fe content of the as-received fly ash was increased since most Fe catalysts for CNT synthesis have metal loadings between 2.5 and 10 wt % metal; lower metal loadings generally show lower catalytic activity and CNT yields, and higher metal loadings can cause excessive amorphous carbon deposition. A summary of the reaction products, evaluated using TGA, Raman, SEM, and HRTEM is shown in Table 2. Thermogravimetric analysis showed that the FA catalyst reaction products contained 2.29 wt % carbon, the FA2.5 catalyst products 7.65 wt % carbon, and the FA5 catalyst products 8.25 wt % carbon. This can be rationalized by the

increased metal loading increasing hydrocarbon decomposition and providing more active sites for CNT growth. The first order derivative weight loss profiles of the reaction products provide an indication of the type of carbon present (Figure 1). The thermal stability of the products can be deduced from the maximum oxidation temperature, and the distribution of products from the oxidation peak width at half-maximum; a narrow peak width indicates a narrow product distribution. There is no consensus in the literature concerning the oxidation temperature ranges of various carbon products produced during CNT synthesis, which may include amorphous carbon, carbon fibers, CNTs, damaged CNTs, and graphitic particles. This stems from the multitude of factors influencing CNT thermal stability, including the synthesis conditions (e.g., temperature and pressure), the catalyst metal type and loading, and purification conditions. For CNTs, the diameter and presence of defects also influences the oxidation temperature; a higher maximum oxidation temperature for CNTs does, however, indicate higher quality, i.e., a greater degree of wall graphitization (33). In addition, the TGA parameters of sample weight, sample compaction, temperature ramp rate, and oxidation gases are further variables in the programmed thermal oxidation process (33). Therefore, allocating the total carbon yield to the various carbon products present is subjective. Since no CNTs were observed in the small yield of products from the FA catalyst, and a large proportion of carbon products (>50%) being CNFs for the FA2.5 catalyst, we assumed that any weight loss below 550 °C was attributable to amorphous carbon and CNFs, and any weight loss above 550 °C was attributable to CNTs and graphitic carbon. Although this method of classification may contain some error, it is more accurate than estimating CNT yields from SEM and HRTEM alone. This classification is similar to VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Representative HRTEM images of carbon products produced using a 2.5 wt % Fe fly ash catalyst (a and b) and a 5 wt % Fe fly ash catalyst (c, d, and e) at 650 °C by fluidized bed chemical vapor deposition. (a) large CNF with an outer diameter of ∼80 nm surrounded by MWNTs of various diameters (∼10-40 nm), (b) MWNT with an outer diameter of ∼40 nm and length greater than 1 µm, (c) web of MWNTs with outer diameter of ∼15 nm, (d) CNT with ∼20 walls, an outer diameter of ∼12 nm and inner diameter of ∼4 nm, (e) MWNTs with outer diameter of ∼15 nm.

FIGURE 4. Raman spectra for the three fly ash catalysts (a) non impregnated fly ash, (b) 2.5 wt % Fe fly ash, and (c) 5 wt % fly ash catalyst. The ratio of the intensity of the G peak (∼1590 cm-1) and D peak (∼1340 cm-1) provides an indication of the degree of graphitization of the carbon products. The nonimpregnated and 5 wt % Fe catalysts show similar G/D ratios, whereas the 2.5 wt % Fe catalyst has a lower ratio, implying that more disordered sp3 carbon is present in the reaction products. previous studies using similar reaction conditions and experimental apparatus (29, 34). From Figure 1 it can be seen that the FA5 catalyst products had a higher maximum oxidation temperature and narrower 7892

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peak width at half-maximum compared to the FA and FA2.5 catalyst products. This result implies the presence of less amorphous carbon and a narrower product distribution, as confirmed by SEM and HRTEM analysis. The estimated CNT fraction of total carbon present for the three catalysts was FA 0, FA2.5 10, and FA5 50%. The SEM images of the FA2.5 and FA5 reaction products are shown in Figure 2 (the small quantity of FA catalyst reaction products consisted predominantly of amorphous carbon and is not shown). The FA5 catalyst products (Figure 2a and b) consisted mainly of webs of CNTs covering the fly ash support surface, however large CNFs and amorphous impurities are also present. Figure 2a shows a broken plerosphere encompassing a cenosphere. CNTs and CNFs have grown on both the outer and inner surfaces of the fly ash particle, most probably due to the iron nitrate solution filling the cracked particle during catalyst impregnation; this phenomenon was not observed for nonimpregnated fly ash. Figure 2c and d show the reaction products on the surface of the FA2.5 catalyst. Products mainly consisted of CNFs (∼200 nm in diameter), however small amounts of CNTs can be seen intertwined in the CNFs (Figure 2d). From HRTEM analysis (Figure 3) it can be seen that the CNTs produced by the FA2.5 and FA5 have diameters ranging from 10 to 40 nm, and contain little amorphous overcoat (Figure 3d and e). Even dispersion of CNTs over the fly ash support surface has not been obtained (Figure 2a). This relatively poor dispersion could be due to a weak interaction between the metal catalyst and the fly ash support. A weak catalyst/support interaction can result in an increase in metal sintering during

calcination, reduction, and reaction. Large metal agglomerates are thought to predominantly result in the formation of unwanted CNFs and amorphous carbon (35). HRTEM analysis in Figure 3 also provides evidence of weak metal/ support interactions, with CNT tip growth visible; the tipgrowth CNT growth mechanism is suggested to occur when a low strength interaction exists between the metal and the support (36). The use of less crude catalyst preparation methods, such as more efficient drying techniques (e.g., rotary evaporation), may result in a more even dispersion of metal across the support surface and should be considered in future studies. The Raman spectra of the reaction products are given in Figure 4. The ratio of the intensity of the G peak (graphitic carbon) at ∼1590 cm-1 and D peak (defective carbon) at ∼1340 cm-1 provides an indication of the degree of graphitization of carbon in the reaction products (IG/ID). A high IG/ID ratio is indicative of a greater degree of wall graphitization and thus superior CNT quality. The IG/ID ratio for the FA products (Figure 4a), which consisted mainly of amorphous and graphitic carbon, was ∼1.2. The IG/ID ratio of FA5 was slightly lower at ∼1.17 (Figure 4c), and FA2.5 lower again at ∼0.8 (Figure 4b), indicating that the 2.5 wt % Fe catalyst contained more defective carbon than that produced by the 5 wt % Fe catalyst. The IG/ID ratios reported here are comparable to IG/ID ratios for MWNTs produced in several previous studies using Fe-supported catalysts (29, 37). Improvement of the CNT wall quality are possible via postsynthesis high temperature annealing, although this incurs a significant energy burden as temperatures >2000 K are generally required (38). A small peak at ∼280 cm-1 can be observed for the FA5 catalyst products (Figure 4c), but not for as-received fly ash or 2.5 wt % Fe fly ash (Figure 4a and b). The peak could be due to small amounts of SWNTs present in the sample, as SWNTs produce unique peaks between 100 and 350 cm-1 depending on tube diameter due to tube isotropic radial expansion (33). Even so, the yield of SWNTs in the sample is considered negligible, as none were observed by HRTEM analysis. While the FA2.5 and FA5 catalysts were both found to be active for CNT synthesis, the FA5 catalyst produced less carbon fibers and a higher CNT yield. A possible rationalization of this behavior is that FA2.5 catalyst potentially has a higher carbon supply at the CNT nucleation site. It has been shown that if the carbon supply rate exceeds the rate of CNT growth, the excess carbon may form amorphous carbon and carbon fibers in preference to CNTs (35). The natural iron content in fly ash has been shown to be inactive for CNT growth under the selected reaction conditions, most likely due to the inherent heterogeneity of the iron location and composition in the as received fly ash. Diamond et al. (3) have demonstrated, using a succession of acid etching treatments, that the location of the iron and its morphology varies considerably between fly ash particles from the same sample due to the nonhomogenous nature of coal. Furthermore, iron has been shown to be present in a multitude of forms including Fe3O4 (magnetite), MgFe2O4 (magnesioferrite), γ-Fe2O3 (maghemite), Fe2TiO4 (ulvospinel) and intermediate compositions between them (3), however the required catalyst for CNT growth is iron in reduced form (39). H2 may not be able to reduce the iron due to its location in the fly ash particle. We therefore suggest that the location of iron in as received fly ash is predominantly not located on the surface and/or is not in a form that is catalytically active for CNT growth (it is futile undertaking iron mapping of fly ash particles because of the large variability within the same sample and hence the time required to obtain a representative analysis).

Due to their low bulk density, high strength, and intrinsic stiffness there is potential for the use of CNTs in composite materials including polymers and concretes (40, 41). Since fly ash is already used as a value added cement product (Portland cement substitute) the use of fly ash produced CNTs in concrete applications appears well matched. CNTs have the potential to be purified from the fly ash substrate (e.g., using microwave acid digestion (42)), or added to concrete as a fly ash/CNT composite, after thermal treatment in air at moderate temperature (e.g., 500 °C) to remove unwanted amorphous carbon. Potential advantages of using fly ash/CNTs in concrete applications include the conservation of virgin materials and fuels that would have been used to produce products such as cement. Furthermore, concrete reinforcement with low-density materials reduces weight and thus transportation costs. However, several barriers need to be overcome before CNTs can be widely used in cement applications. In depth testing and optimization of CNT/ concrete composites has yet to be completed and poor binding at the macroscopic scale needs to be overcome, due to low surface friction of CNTs (40). Furthermore, building codes need to be altered to allow application of carbon nanotube and carbon fiber containing concretes (40); this is not a trivial process and will involve lengthy development and testing periods.

Acknowledgments O.M.D. gratefully acknowledges the financial support of the University of Sydney for providing the Chemical Engineering Postgraduate Research Scholarship. K.J.M. gratefully acknowledges the financial support of the University of Sydney for providing the Chemical Engineering Postgraduate Research Scholarship and St Paul’s College for the Arnott Postgraduate Scholarship. We acknowledge Drs. I. Kaplain and S. Bullock from the Electron Microscopy Unit, University of Sydney, for their assistance with SEM and HRTEM analysis, Dr. L. Carter from the Department of Chemistry, University of Sydney, for her assistance with Raman spectroscopy analysis, and I. Wainwright from the University of New South Wales Analytic Centre, for her assistance with XRF analysis.

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