Thermal treatments of activated carbon fibres using a microwave furnace

doi:10.1016/S1387-1811(01)00384-5

Microporous and Mesoporous Materials

Volume 47, Issues 2–3, October 2001, Pages 243-252

https://doi.org/10.1016/S1387-1811(01)00384-5 Get rights and content

Abstract

Thermal treatment of activated carbon fibres (ACF) in a flow of N₂ gas has been carried out using a microwave device operating at 2450 MHz and with a power input of 1000 W, instead of a conventional furnace, and the samples were analysed by means of low temperature N₂ adsorption, elemental analysis and determination of points of zero charge. The results show that microwave treatment for periods between 5 and 30 min affects the porosity of the ACF, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACF. During microwave treatment surface groups are completely eliminated, whereas oxygen and nitrogen atoms bonded within the pseudo-graphitic layer planes are retained. On re-exposure to air the surface groups only reform to a very limited extent and as a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained.

Introduction

Carbon materials with low oxygen content, basic properties, highly hydrophobic character and resistance to aging can be obtained by means of appropriate thermal treatments in different gaseous environments [1], [2], [3], [4], [5], [6], [7], [8]. Generally, these thermal treatments have to be carried out at relatively high temperatures (>800°C) while flowing an inert or reducing gas (e.g. N₂, H₂) over a suitable carbon precursor, during at least 1 or 2 h. The high temperatures are invariably achieved by convective and/or conductive heating of the sample, which is placed in a conventional heating system, such as a tubular furnace. However, recent work has indicated that treatment using a microwave device instead of the conventional heating systems, can also be very effective [9], [10]. In view of the possible advantages associated with the use of microwave heating systems, it was therefore decided to study the effect of microwave heating in more detail using a carbon precursor (activated carbon fibre) with a better defined microstructure than that used previously.

In conventional heating the heat source is located outside the carbon bed which is heated by conduction and/or convection. With microwave heating, on the other hand, the microwaves supply energy to the carbon particles themselves. Some carbons have free electrons whose displacement is restricted by grain boundaries. When these carbons are subjected to an electromagnetic field, space charge polarisation takes place. Entire macroscopic regions of the material become either positive or negative synchronising their orientation with the field. This mechanism is often called the Maxwell–Wagner effect [11]. At low frequency the polarisation synchronises its orientation with the field, but as the frequency of the waves increases there is a phase lag between the polarisation and the applied field. This leads to an absorption of energy and Joule heating of the carbon particles [11].

Microwaves are now being used in various technological and scientific fields in order to heat dielectric materials [11], [12]. The main advantage of using microwave heating is that the treatment time can be considerably reduced, which in many cases represents a reduction in the energy consumption as well. Microwave energy is derived from electrical energy with a conversion efficiency of approximately 50% for 2450 MHz and 85% for 915 MHz [12]. In addition, the consumption of gases used in the treatment can also be reduced. Yet, in the particular case of carbon materials, there are relatively few publications that describe the use of microwaves for producing [13], [14] and regenerating [15], [16], [17] activated carbons. Surface chemistry modification of active carbons by means of microwave heating was also studied in previous works [9], [10]. The aim of the present work was to further explore the effect of microwave heating on the textural and surface chemistry properties of active carbon fibres.

Section snippets

Materials

The precursors used for the production of activated carbon fibres (ACF) were three acrylic textile fibres provided by Fisipe (Barreiro, Portugal). According to the manufacturer all of the fibres had been polymerised from acrylonitrile (∼90 wt.%) and vinyl acetate (∼10 wt.%) monomers. Fibres F1 and F1N were bright 3.3 dtex (g/10,000 m) filaments of varying length produced in 1995 (F1) and 1999 (F1N) with slightly different fabrication processes. Fibre F2 was also produced in 1995 but was in the

Microwave heating

Fig. 1 shows the evolution of the temperature of the carbon bed during the different microwave treatments of the ACF. In all cases heating of the samples was quite fast taking, in general, less than a minute to reach a temperature above 800°C. However, the temperature was not constant during the 15 min of the treatment. At the beginning there was a rapid increase in temperature until it reached a maximum around two min after commencement of the treatment. The maximum temperature was in most

Conclusions

The results presented show that microwave heating is a very effective means of modifying the porosity and, of special importance, the surface chemistry of ACF. Comparatively short heating periods are sufficient to completely remove the surface groups, but without removing oxygen and nitrogen atoms bonded within the pseudo-graphitic planes. As the surface groups are mainly acidic in nature, whereas ring oxygen and nitrogen atoms impart basic functionality, the resulting carbon materials are very

Acknowledgements

The authors are grateful to FISIPE – Fibras Sintéticas de Portugal for the provision of samples and to the Fundação para a Ciência e a Tecnologia (Portugal), the European Regional Development Fund (FEDER) and the program PRAXIS XXI for financial support (project PRAXIS/3/3.1/MMA/1781/95).

References (20)

C.A. León y León et al.
Carbon
(1992)
H.P. Boehm et al.
Carbon
(1970)
S.K. Verma et al.
Carbon
(1992)
R.C. Bansal et al.
Carbon
(1974)
J.A. Menéndez et al.
Carbon
(1999)
K.E. Haque
Int. J. Miner. Process
(1999)
H.F. Stoeckli et al.
Carbon
(1990)
J.A. Menéndez et al.
Langmuir
(1996)
J.A. Menéndez et al.
Langmuir
(1997)
J.A. Menéndez et al.
J. Phys. Chem.
(1996)

There are more references available in the full text version of this article.

Cited by (104)

Microwave post-treated activated carbons for capacitance boosted non-aqueous supercapacitors
2024, Journal of Alloys and Compounds
Herein, as a novel idea, a microwave (MW) post-treatment strategy is proposed to modify the structure and surface characteristics of already prepared activated carbons (ACs) for application in non-aqueous Electric Double-Layer Capacitors. Pistachio nutshell-derived carbon is first KOH-activated and then subjected to MW irradiation for 0, 2, 5, and 10 minutes. X-ray Diffraction and Raman analyses show that MW post-treatment leads to structural modifications, and FTIR and XPS analyses reveal relative elimination of surface functional groups which results in subsequent enhancement in water contact angle and renders more favorable surface wetting of carbon by non-aqueous organic electrolyte. The performance characteristics of symmetrical non-aqueous supercapacitors incorporated with the prepared ACs show a significant positive effect of MW irradiation in such a way that 0 and 10-minute-irradiated ACs demonstrate 152 and 392 F g⁻¹ capacities, respectively, at 1.75 A g⁻¹, with the corresponding specific energies and powers of 340 Wh kg⁻¹ and 11 kW kg⁻¹ for AC-10, respectively. This remarkable enhancement in the electrochemical performance is attributed to the effective role of the MW post-treatment in modifying the AC structure as well as providing AC surfaces that have better wettability with less polar non-aqueous electrolyte. Moreover, this strategy is additionally applicable to make hydrophobic activated carbons for other applications as the absorption of less polar contaminants from liquid or gaseous environments.
Efficient removal of triclosan from water through activated carbon adsorption and photodegradation processes
2024, Environmental Research
This study investigated the application of adsorption with activated carbons (ACs) and photodegradation to reduce the concentration of triclosan (TCS) in aqueous solutions. Concerning adsorption, ACs (Darco, Norit, and F400) were characterised and batch experiments were performed to elucidate the effect of pH on equilibrium. The results showed that at pH = 7, the maximum adsorption capacity of TCS onto the ACs was 18.5 mg g⁻¹ for Darco, 16.0 mg g⁻¹ for Norit, and 15.5 mg g⁻¹ for F400. The diffusional kinetic model allowed an adequate interpretation of the experimental data. The effective diffusivity varied and increased with the amount of TCS adsorbed, from 1.06 to 1.68 × 10⁻⁸ cm² s⁻¹. In the case of photodegradation, it was possible to ensure that the triclosan molecule was sensitive to UV light of 254 nm because the removal was over 80 % using UV light. The removal of TCS increased in the presence of sulfate radicals. It was possible to identify 2,4-dichlorophenol as one of the photolytic degradation products of triclosan, which does not represent an environmental hazard at low concentrations of triclosan in water. These results confirm that the use of AC Darco, Norit, and F400 and that photodegradation processes with UV light and persulfate radicals are effective in removing TCS from water, reaching concentration levels that do not constitute a risk to human health or environmental hazard. Both methods effectively eliminate pollutants with relatively easy techniques to implement.
The role of nitrogen plasma surface treatment and TiO<inf>2</inf> seeding onto carbon fibers on the microwave-assisted growth of radially aligned TiO<inf>2</inf> nanowires
2021, Materials Chemistry and Physics
Radially aligned titanium dioxide (TiO₂) nanowires were synthetized on nitrogen plasma-treated carbon fibers (CF) using a two-step microwave-assisted solvothermal method. The TiO₂ synthesis consisted in seeding TiO₂ nuclei on CF followed by TiO₂ nanowire growth. To assess the role of the nitrogen plasma surface treatment and TiO₂ seeding, CF and CF-TiO₂ hybrids were analyzed by Raman spectroscopy, X-ray diffraction, scanning electron and atomic force microscopies, after each step of the TiO₂ synthesis. In addition, X-ray photoelectron spectroscopy was used to analyze the CF chemical composition upon the plasma treatment and TiO₂ seeding. Single fiber tensile tests were conducted on CF and CF-TiO₂ hybrids to track the mechanical changes induced by the surface treatment and TiO₂ synthesis. It was found that both, the nitrogen plasma surface treatment and TiO₂ nuclei seeding were essential to homogeneously synthesize radially aligned TiO₂ nanowires onto CF. No significant changes in the CF surface roughness after plasma treatment and TiO₂ seeding were found; however, upon TiO₂ nanowire growth, the surface roughness increased by about twelve times with respect to the pristine ones. The plasma surface treatment generates oxygen- and nitrogen-containing functional groups that promote TiO₂ nucleation onto CF. In addition, the CF internal heating due to microwave irradiation triggers a preferential nucleation of TiO₂ on the fibers rather than in the reaction medium. The tensile strength of CF remained without significative changes after the surface treatment and TiO₂ nanowire growth. The CF-TiO₂ hybrid materials developed in this work could be exploited in the development of multifunctional structural composites.
Characterization valorized anthracite and its application in manganese (VII) adsorption from aqueous solution; batch and column studies
2021, Microporous and Mesoporous Materials
The current study spotlights the modification of Anthracite (An) by microwave beam to form modified anthracite (MAn). The precursor (An) and modified anthracite (MAn) were investigated and characterized by several techniques such as XRD-EDAX, TGA, DSC, SEM, Raman spectra and BET surface area (S_BET). The Mn (VII) isotherm notification was illustrated by a Langmuir model (R² = 0.999) and the maximum monolayer coverage (Q_O) 555.5 mg/g that approved the surface symmetry of the modified anthracite. Whereas, The Mn (VII) kinetic knowledge was designated by the pseudo-second-order equation as specified by the high determination coefficient (R² = 0.99342). The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) were determined and their values indicate that the sorption of Mn (VII) on (MAn) is endothermic and spontaneous processes as well as physisorption on the surfaces of MAn. Moreover, in the dynamic mode, the adsorption capacity Ts was 58.6 mg/g or 114 mg, finally, the column recycled at least five times using H₂SO₄ (1 M) for each regeneration without considerable loss in sorbent mass. Therefore, MAn is estimated as a better Alternative technique for the elimination of Mn (VII) owing to a low quotation and high efficacy in this implementation.
Hierarchical tannin-derived carbons as efficient tetracycline adsorbents
2020, Applied Surface Science
Mimosa tannin has been used to produce two different types of carbons materials: hydrothermal carbon (HTC) on the one hand, and ordered mesoporous carbon (OMC) on the other hand. Both were obtained after pyrolysis at 1173 K, resulting in materials with similar BET areas, close to 580 m²/g, but very different pore size distributions (PSDs). HTC was essentially microporous while OMC had a large fraction of mesopores. Tetracycline (TC) adsorption on these carbons was studied at equilibrium but kinetics experiments were also carried out at two different temperatures. The TC adsorption data were fitted by different mathematical, classical models, and new kinetic models were introduced to take into account their very different PSDs. While the two materials presented similar BET areas (A_BET), the OMC had a TC adsorption capacity six times higher than that of the HTC. Adsorption kinetics were also strongly affected by the PSD. The importance of having high accessible pore volumes and not only high surface areas in order to have good performances in TC adsorption has been evidenced not only with our materials but also in the light of the TC adsorption data reported in the open literature. We proved that A_BET, while easy to calculate and always reported, is not the best tool to assess the potential suitability of an adsorbent to the removal of aqueous pollutants.
Paracetamol removal by Kon-Tiki kiln-derived biochar and activated carbons
2020, Industrial Crops and Products
Biochar was obtained from Eucalyptus pruning residues with a non-conventional device named Kon-Tiki kiln. The average heat of combustion of the biochar, 27.3 MJ kg⁻¹, was higher than that of Eucalyptus wood, 17.8 MJ kg⁻¹. Activation with CO₂ was performed by varying the activation time from 0 to 60 minutes. The activated carbons (ACs) and the carbon precursor have been characterised and tested for paracetamol removal in the liquid phase, studied in both kinetic and equilibrium aspects. ACs presented an increase in BET area (up to 845 m²/g), total pore volume and microporosity with the activation time. The pseudo-second order model was the one that best fitted the experimental data. Elimination of paracetamol was much faster when using ACs, 5 h, than when using the biochar, 3 days. However, pollutant removal was greater than 95 % for all materials, which is a promising result for low-cost biochars in a difficult economic context. All the adsorption equilibrium experiments exhibited multilayer behaviour, showing values up to 98 mg g⁻¹ for the maximum monolayer-adsorption capacity.

View all citing articles on Scopus

View full text

Thermal treatments of activated carbon fibres using a microwave furnace

Abstract

Introduction

Section snippets

Materials

Microwave heating

Conclusions

Acknowledgements

Carbon

Carbon

Carbon

Carbon

Carbon

Int. J. Miner. Process

Carbon

Langmuir

Langmuir

J. Phys. Chem.