CO2-activated Carbon Research Articles

From Green to Black Gold: Highly Microporous Carbons from Pistachio Shells by a Controlled Physical Activation Process.

Highly microporous carbons with BET surface areas of up to ca. 3300 m2 g-1 and pore volumes of up to 1.6 cm3 g-1 have been successfully synthesized from pistachio shells, a waste whose generation is growing on account of the nutritive value of pistachios and the resilience of this crop to climate change. Such a high pore development has been achieved by a simple and benign CO2 physical activation process assisted by a custom pre-treatment of the biomass. Herein, different approaches have been explored for the transformation into carbon materials with diverse microstructures, mineral matter content and particle size/morphology, tuning therebytheir reactivities towards CO2 and diffusion kinetics and, in this way,pore development. In particular, the most efficient route for the production of highly microporous carbonsinvolves a hydrothermal carbonization process which increases the degree of aromatization and effectively removes the mineral matter, enhancing thereby the efficiency of both carbon production and porosity generation. By increasing the activation temperature, substantial shortening of the operation time can be achieved without compromising pore development. This work provides new integral strategies towards the production of biomass-based, CO2-activated carbons with a focus on optimizing pore structure, minimizing energy consumption and maximizing product yield.

Hydrochars derived from real organic wastes as carbonaceous precursors of activated carbons for the removal of NO from contaminated gas streams

The improvement of air quality in densely-populated urban regions constitutes an environmental challenge of increasing concern. In this respect, the abatement of NO emissions, primarily emanating from combustion processes associated with motor-vehicles, along with industrial/domestic combustion systems, represents one of the main problems. Here, three hydrochars from diverse organic residues were used as activated carbon precursors for their evaluation in the NO removal in two potential application scenarios. Hydrochars were physically activated at 800 °C with pure-CO2 or diluted-O2. These materials were tested in a lab-scale biofilter at different conditions (NO concentration, temperature, relative humidity, NO-containing gas and carbon particle size) and in a larger-scale biofilter to evaluate the long-term NO removal capacity. Hydrochar-derived carbons present a relatively well-developed micro- and mesoporous structure, with BET areas of up to 421 m2/g, and a variety of oxygen surface functionalities (carboxylic, lactone, carbonyl and quinone groups), especially concerning CO2-activated carbons. These exhibited an excellent behaviour at low NO concentration (5 ppmv) between 25 and 75 °C with removal capacities of ≈97 % and > 82 %, respectively; and still good-performance (≈66 %) in a more concentrated gas (120 ppmv). Whilst, carbons obtained by diluted-O2 activation from the same hydrochars, evidenced a higher removal capacity loss at high NO concentration. The O2 presence in the gas stream was confirmed as a crucial factor in the NO elimination, since both co-adsorb on the carbon surface favouring NO oxidation to NO2. Besides, the humidity in the airstream diminished the NO removal capacity from 0.88 to 0.51 mgNO/gcarbon, but still remained at 0.54 mgNO/gcarbon, when the carbon (in pellet) was operated at larger-scale biofilter in 9-fold longer test under humid air. Therefore, this study highlights the potential of renewable carbons to serve as cost-effective component in urban biofilters, to mitigate NO emissions from exhaust gases in biomass boilers and urban semi-close areas.

Eco-friendly removal of methyl tert-butyl ether from contaminated water using steam and CO2-activated carbons

AbstractPetrol frequently contains the additive methyl tertiary butyl ether (MTBE). Because of its significant health risks, MTBE pollution of surface and ground water is a severe concern for the environment. Highly porous physically activated carbons, particularly CO2-activated carbon (CO2-AC) and steam-activated carbon (Steam-AC), were obtained from date stones as potential eco-friendly adsorbents for MTBE from contaminated water. The chemical composition, microstructure, textural, and structural characteristics of adsorbents were characterised by elemental analysis, SEM, N2 sorption, XRD, and FTIR. The adsorption process evaluation based on the initial MTBE concentration, liquid-to-solid ratio, and equilibrium contact time. CO2-AC and steam-AC adsorbents have high surface areas of 819.5 m2/g, and 567.7 m2/g, respectively. At 40 °C, CO2-AC has an adsorption capability of 181.36 mg/g. The adsorption result was best fitted by the Freundlich model. The two-step intraparticle diffusion process prevailed the adsorption process, and the pseudo-second-order model presented an optimal fit for the adsorption kinetics models. Spontaneous physical adsorption was endothermic when CO2-AC adsorbs at 40 °C because ∆G was − 6.34 kJ/mol. Finally, the water quality improved and the salt content, the alkalinity, and the hardness decreased with the use of CO2-AC as an environmentally friendly adsorbent for removing MTBE from the polluted water.

Greener carbon capture using microwave heating for the development of cellulose-based adsorbents

In this work, CO2-activated carbons were produced from microcrystalline cellulose using microwave heating during activation. Activations were thus completed at 400 °C to burn-offs of 10, 20 and 30 wt%. The activated carbons’ CO2 adsorption capacity was tested over 10 cycles of adsorption (25 °C) and desorption (100 °C). CO2 adsorption capacity was found to increase with increasing activation burn-off, whilst larger average dynamic adsorption capacities were achieved with activated carbons of 20 wt% (1.64 mmol/g) and 30 wt% (1.73 mmol/g) burn-off compared to commercial activated carbon Norit R2030CO2 (1.58 mmol/g). These microwave-prepared activated carbons were also compared with similar activated carbons produced using conventional heating in our previous work. The microwave-prepared activated carbons were found to possess 9.5–25.6 % larger CO2 adsorption capacities at equivalent burn-offs, despite being produced at 200 °C lower temperature, 83–94 % shorter activation times and 39–68 % lower heating energy consumption. These results represent the establishing of a more efficient means of producing microcrystalline cellulose-based activated carbons for a greener, sustainable carbon capture that contributes to the circular economy.

One-Pot Mechanochemical Synthesis of Carbons with High Microporosity and Ordered Mesopores for CO2 Uptake at Ambient Conditions.

Mechanochemical synthesis of ordered mesoporous carbons with tunable mesopores and well-developed irregular microporosity is investigated. This synthesis was carried out by the self-assembly of ecofriendly chemicals such as tannin and glyoxal used as carbon precursors, and triblock copolymer as a soft templating agent. The structural properties of the resulting carbons were tailored by using different block copolymers (Pluronic F127, and P123) as soft templates. The various weight ratios of tannin and block copolymer were employed to tune the textural properties of these carbons. The tannin: Pluronic F127 ratios (1:0.75, 1:1, 1:1.1) gave the ordered mesoporous carbons among a wide variety of the samples studied. The ordered mesoporosity was not observed in the case of Pluronic P123 templated mesoporous carbons. The CO2-activated carbon samples obtained for both Pluronic templates showed a high specific surface area (close to 900 m2/g), large pore volume (about 0.6-0.7 cm3g-1), narrow pore size distribution, and high CO2 uptake of about 3.0 mmol g-1 at 1 bar pressure and ambient temperature.

MgO-containing porous carbon spheres derived from magnesium lignosulfonate as sustainable basic catalysts

The presence of alkalis in lignosulfonate allows an easy preparation of sustainable MgO-containing carbon catalysts with surface basicity by carbonization of magnesium lignosulfonate and/or further partial gasification of the produced char with CO2. Carbon spheres with different chemical and physical properties were obtained from lignosulfonate treated at temperatures ranging from 500 to 900 ºC. Carbonization at 900 °C generates hollow porous carbon spheres (pore volume of 0.20 cm3/g and apparent surface area of 465 m2/g) with magnesium content of 12%. A kinetic study of CO2 gasification of the carbon spheres obtained at 900 °C at temperatures in the range of 700 – 800 °C revealed that the gasification rate can be accurately described by the random pore model up to conversion values of 0.5. Based on this study, in order to develop additional porosity on the carbon spheres obtained at 900 °C, a partial gasification with CO2 at 750 °C for 30 min was carried out, reaching surface areas higher than 700 m2/g and 15.3% of Mg loading, with an overall preparation yield of 30%. All the obtained carbon materials were tested as catalyst for 2-propanol decomposition, showing a high selectivity to acetone, evidencing the basic character of these carbon catalysts. The highest activity and selectivity were shown by the CO2-activated carbon spheres (conversion and acetone selectivity higher than 90% at 420 °C), indicating that magnesium lignosulfonate is an attractive raw material for the preparation of sustainable carbon catalysts for biorefinery applications.

3D printing CO2-activated carbon nanotubes host to promote sulfur loading for high areal capacity lithium-sulfur batteries

3D printing CO2-activated carbon nanotubes host to promote sulfur loading for high areal capacity lithium-sulfur batteries

Cu(II) and Au(III) recovery with electrospun lignosulfonate CO2-activated carbon fiber

The objectives of this study were twofold: developing lignosulfonate activated carbon fibers (LACFs) and determining the corresponding metal recovery mechanisms with batch experiments and non-linear modeling. LACFs were developed through electrospinning, followed by CO2-based physical activation. Physical and chemical characterizations revealed that the LACF sample that was activated for 60 min exhibited a higher specific surface area (376.54 m2/g), larger total pore volume (0.30 cm3/g), higher micropore ratio (32%), and more acidic and sulfur functional groups than did the other samples. Cu(II) and Au(III) adsorption behaviors on the LACF could be described with the Freundlich and Langmuir model, respectively. Both systems consist of physisorption and chemisorption, and the mechanisms include electrostatic forces, Van der Walls forces, cation exchange, surface complexation. In particular, Au(III) adsorption was faster, and LACF-Au bonds were stronger due to the additional microprecipitation. Furthermore, the LACF sample could regenerate after three adsorption-desorption cycles. Overall, this study provides the foundation for developing physically activated lignosulfonate carbon and its application in recovering valuable metal ions.

Structural and Electrochemical Properties of Physically and Chemically Activated Carbon Nanoparticles for Supercapacitors.

The demand for supercapacitors has been high during the integration of renewable energy devices into the electrical grid. Although activated carbon materials have been widely utilized as supercapacitor electrodes, the need for economic and sustainable processes to extract and activate carbon nanomaterials is still crucial. In this work, the biomass waste of date palm fronds is converted to a hierarchical porous nanostructure of activated carbon using simple ball-milling and sonication methods. Chemical and physical activation agents of NaOH and CO2, receptively, were applied on two samples separately. Compared with the specific surface area of 603.5 m2/g for the CO2-activated carbon, the NaOH-activated carbon shows a higher specific surface area of 1011 m2/g with a finer nanostructure. Their structural and electrochemical properties are functionalized to enhance electrode–electrolyte contact, ion diffusion, charge accumulation, and redox reactions. Consequently, when used as electrodes in an H2SO4 electrolyte for supercapacitors, the NaOH-activated carbon exhibits an almost two-fold higher specific capacitance (125.9 vs. 56.8 F/g) than that of the CO2-activated carbon at the same current density of 1 A/g. Moreover, using carbon cloth as a current collector provides mechanical flexibility to our electrodes. Our practical approach produces cost-effective, eco-friendly, and flexible activated carbon electrodes with a hierarchical porous nanostructure for supercapacitor applications.

Isotherm and thermodynamic modelling of malachite green on CO2-activated carbon fibers

We designed porous carbon fibers with the aid of CO2 during activation for effective malachite green (MG) adsorption. The physiochemical mechanisms have been characterized both experimentally and numerically to decipher the underlying relevance of CO2-activation on the MG adsorption capacity. The obtained samples are dominated by micropores, resulting in a high specific surface up to 1012 m2 g−1 via 60 min of CO2-activation. The adsorption isotherm was fitted to Langmuir with the maximum adsorption capacity of 555.56 mg L-1. The regeneration was secured over 80% during 3 trials. Thermodynamics revealed endothermic in nature and the spontaneous adsorption process.

Entropy, enthalpy, and gibbs free energy variations of 133Cs via CO2-activated carbon filter and ferric ferrocyanide hybrid composites

The addition of ferric ferrocyanide (Prussian blue; PB) to adsorbents could enhance the adsorption performance of 133Cs. Toward this goal, we present a heterogeneously integrated carbonaceous material platform consisting of PB in direct contact with CO2-activated carbon filters (PB-CACF). The resulted sample retains 24.39% more PB than vice versa probed by the ultraviolet–visible spectrometer. We leverage this effect to capture 133Cs in the aqueous environment via the increase in ionic strength and micropores. We note that the amount of PB was likely to be the key factor for 133Cs adsorption compared with specific surface characteristics. The revealed adsorption capacity of PB-CACF was 21.69% higher than the bare support. The adsorption characteristics were feasible and spontaneous. Positive values of ΔHo and ΔSo show the endothermic nature and increased randomness. Based on the concept of capturing hazardous materials via hazardous materials, our work will be of interest within the relevant academia for collecting radionuclides in a sufficient manner.

Performance of activated carbons prepared from spent tyres in the adsorption of rhodamine B in aqueous solutions.

Activated carbons were produced from spent tyre pyrolysis char by steam or CO2 activation and evaluated for their performance in rhodamine B (RhB) adsorption in aqueous solutions. The effect of RhB starting concentration (80-150 mg L-1), contact time (0-80 min), temperature (298-318 K) and initial pH on the adsorption process was examined. Pseudo-first-order and pseudo-second-order models were carried out to fit the experimental data to derive RhB adsorption kinetics. Langmuir, Freundlich and Temkin isotherm models were applied to depict RhB adsorption behaviour of the prepared activated carbons. Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were calculated. It has been found that the activated carbons can effectively adsorb RhB due to high mesoporosity and RhB equilibrium adsorption capacity (qe) increased almost linearly with increasing total mesopore volumes, regardless of the activation agents. When BET surface areas are similar, CO2-activated carbon obtained higher qe than steam due to higher mesoporosity of CO2-activated carbon. The results show that pseudo-second-order well fitted the experimental data. RhB starting concentration increased from 80 to 150 mg L-1 causing qe increased from 158 to 251 mg g-1 but RhB removal decreased from 99.7 to 84.5%. The RhB adsorption process follows the Langmuir model and thermodynamic calculation, indicating RhB adsorption is an endothermic, spontaneous process, dominated by both chemisorption and physisorption.

Effects of geometric and heat transfer parameters on adsorption–desorption characteristics of CO2-activated carbon pair

A comprehensive 2-D transient heat and mass transfer analysis is carried out to identify the best reactor configuration in terms of better charge and discharge characteristics for a CO2-activated carbon (Maxsorb III)-based sorption systems. Reactors with different aspect ratios (AR) ranging from 0.35 to 7.8 are analysed for a wide range of convective heat transfer coefficient (h), constant pressure charging, and discharging cases. Effects of external cooling/heating fluid temperature, convective heat transfer coefficient (h), operating pressures are studied for both the charging (1–100 bar) and discharging (65–110 bar) cases. The adsorption cell with AR= 7.8 showed the best performance for CO2 adsorption/desorption in a fixed charge/discharge time of 300 s. For charging at 100 bar pressure, the reactor with AR= 7.8 resulted in an increment of 23.34% in CO2 uptake and reduction in maximum bed temperature by 27 K compared to that of the reactor with AR = 0.35. For h = 700 and 500 W/m2 K, the reactor with AR = 7.8 adsorbs 1300 g and desorbs 832 g of CO2/kg of adsorbent at 100 bar and 65 bar for external cooling and heating fluid temperature of 293 K and 800 K, respectively. The study concludes that better discharge performance can be attained by proper selection of AR even at a lower heating fluid temperature as the reactor with AR = 7.8 at 600 K can desorb 46 to 131 g of extra CO2 w.r.t. all ARs at 800 K. The proposed reactor configurations are supposed to play a vital role in designing of adsorption-based green refrigeration and carbon capture systems.

Kinetics and dynamic adsorption of methylene blue by CO2-activated resorcinol formaldehyde carbon gels

The present work is aimed at evaluating the kinetics and dynamic adsorption of methylene blue by CO2-activated carbon gels. The carbon gels were characterized by textural properties, thermal degradation and surface chemistry. The result shows that the carbon gels are highly microporous with surface area of 514 m2/g and 745 m2/g for resorcinol-to-catalyst ratios of 1000 (AC1) and 2000 (AC2), respectively. The kinetics data could be described by pseudo-first-order model, with a longer duration to attain equilibrium due to restricted pore diffusion as concentration increases. Also, AC1 exhibits insignificant kinetics with fluctuating adsorption with time at concentrations of 20 and 25 mg/L. However, AC1 reveals a better performance than AC2 in dynamic adsorption due to concentration gradient for molecules diffusion to active sites. The applicability of Yoon–Nelson and Thomas models indicates that the dynamic adsorption is controlled by external and internal diffusion.

Adsorptive and capacitive properties of the activated carbons derived from pig manure residues

Converting pig manure residues (PMR) into value-added activated carbon is a promising method to cope with the challenge of PMR disposal. In the present study, PMR were used as the precursors to prepare activated carbon (AC) materials. The performances of the AC materials prepared by chemical activation routes (with KOH and H3PO4), and by the physical activation route (with CO2 gas) were accordingly compared. Elevated heavy metal Ni was detected in both the PMR and the prepared AC materials, but the Ni contents in all the samples still met the threshold values for agricultural use. The investigation of pore structure revealed that the KOH-activated carbon, which was prepared by the activation of PMR-derived biochar, had the maximum surface area (2335.9 m2 g−1) and the largest micropore volume. H3PO4-activated carbon exhibited the most developed mesoporosity and the lowest surface area of 500.7 m2 g−1. Adsorption studies were conducted to evaluate the adsorption properties of the carbons toward methylene blue (MB). The obtained biochar did not show significant adsorptive performance for MB, whereas the activated carbons showed different performances. The adsorption of MB onto the activated carbons was well described by the pseudo-second order kinetic model and Langmuir isotherm. KOH-activated carbon exhibited the highest maximum monolayer adsorption capacity of 717.0 mg g−1, followed by H3PO4 and CO2-activated carbons. The adsorption property of KOH-activated carbon was apparently superior to that of three commercial activated carbons. In terms of the capacitive properties, KOH and CO2-activated carbons showed excellent double-layer capacitance.

Stevia residue as new precursor of CO2-activated carbon: Optimization of preparation condition and adsorption study of triclosan

The present work reports the preparation of CO2-activated carbon (AC) using Stevia rebaudiana (Bertoni) residue as a new carbon precursor. The experimental parameters were optimized via chemometrics tools to obtain an AC with high BET surface area (SBET). The found optimum condition was: activation temperature of 900 °C, CO2 flow of 165 cm3 g−1 and activation time of 60 min, providing an ACop with SBET of 874 m2 g−1. The ACop was characterized from several analytical techniques, which showed that it has heterogeneous morphology features and different surface chemical groups, predominating the acidic character. The adsorption performance of ACop for triclosan (TCS) removal from solution was investigated by kinetic, equilibrium and thermodynamic studies. The results showed that TCS adsorption process onto ACop is spontaneous and endothermic, wherein the mechanism occurs by different steps, which equally play important roles. Additionally, the monolayer adsorption capacity (Qm) was found to be 117.00 mg g−1.

The effect of activation time on water sorption behavior of nitrogen-doped, physically activated, monolithic carbon for adsorption cooling

Abstract Monolithic, nitrogen-doped carbon sorbents were prepared from resorcinol-urea-formaldehyde resins and physically activated with CO2 for different activation times. The effect of the activation time on the water sorption behavior and the physicochemical properties were investigated. Longer activation times lead to a steeper slope of the water sorption isotherm and, due to a higher specific surface area and micropore volume, an increased water sorption capacity. It was found that, after physical activation for 3 h at 800 °C, the physically activated, nitrogen-doped carbon has a high surface area (>1000 m2/g) and a high water sorption capacity (50 wt%). In a miniaturized adsorption heat pump test stand, the best candidate material was assessed alongside commercial silica gel for reference. At a temperature swing from 90 °C → 50 °C, the CO2-activated carbon exhibits a maximal specific cooling power which is a factor of 1.7 higher in comparison with the reference silica gel (429 W/kg versus 255 W/kg). At a more applicable temperature swing, 60 °C → 30 °C, the CO2-activated carbon yields a specific cooling power 3.8 times higher than that of the silica gel reference (932 W/kg versus 240 W/kg).

CO2-activated porous carbon derived from cattail biomass for removal of malachite green dye and application as supercapacitors

In this study, we developed a feasible and cost-effective method for obtaining porous carbon from cattail biomass as a promising material to be applied in various applications. The obtained carbon was further treated via CO2 activation without any other chemical reagents. The results showed that the as-prepared activated carbon had a high specific surface area of 441.12m2/g after CO2 activation. Besides, the activated carbon showed excellent capacity to adsorb malachite green dye, and the effects of initial dye concentration, adsorption time, initial pH, and adsorption temperature were studied. Moreover, the activated carbon exhibited exceptional electrochemical performance as supercapacitors, with specific capacitance reaching 126.5F/g at a current density of 0.5A/g within a potential range of −1.0 to 0V in a 6M KOH solution. The excellent adsorption capacities and electrochemical performance suggested that the obtained activated carbon could be a promising candidate as an adsorbent and supercapacitor.

Hydrogen storage properties of nanoconfined LiBH4–Ca(BH4)2

The hydrogen storage properties of the eutectic melting metal borohydrides, 0.7LiBH4–0.3Ca(BH4)2, nanoconfined in two carbon aerogel scaffolds with different surface areas and pore volumes (pristine and CO2-activated) are presented and compared to the bulk properties. The temperature of hydrogen release investigated by temperature programmed desorption mass spectroscopy is reduced by 83°C for nanoconfined LiBH4–Ca(BH4)2 in the pristine scaffold and by 95°C in the CO2-activated scaffold, compared to that of the bulk. This corresponds to apparent activation energies, EA, of 204, 156 and 130kJ/mol. Several cycles of reversible, continuous release and uptake of hydrogen is investigated by the Sieverts׳ method. Nanoconfined LiBH4–Ca(BH4)2 in the CO2-activated scaffolds demonstrate high degree of stability, releasing 80% and 73% of the original hydrogen content in the second and third hydrogen release cycle, respectively. However most importantly, this study shows that CO2-activated carbon aerogel, CA-6, is more stabile against reaction with the metal hydride and a lower amount of borates and oxides are formed during melt infiltration and hydrogen release and uptake cycling. We conclude that the CO2-activated scaffold is more inert, provides faster kinetics and higher stability over several cycles of hydrogen release and uptake and has the potential to provide useful hydrogen storage densities in the range ~12wt% H2.

Preparation of porous diatomite-templated carbons with large adsorption capacity and mesoporous zeolite K-H as a byproduct

In this study, KOH activation was performed to enhance the porosity of the diatomite-templated carbon and to increase its adsorption capacity of methylene blue (MB). In addition to serving as the activation agent, KOH was also used as the etchant to remove the diatomite templates. Zeolite K-H was synthesized as a byproduct via utilization of the resultant silicon- and potassium-containing solutions created from the KOH etching of the diatomite templates. The obtained diatomite-based carbons were composed of macroporous carbon pillars and tubes, which were derived from the replication of the diatomite templates and were well preserved after KOH activation. The abundant micropores in the walls of the carbon pillars and tubes were derived from the break and reconfiguration of carbon films during both the removal of the diatomite templates and KOH activation. Compared with the original diatomite-templated carbons and CO2-activated carbons, the KOH-activated carbons had much higher specific surface areas (988m2/g) and pore volumes (0.675cm3/g). Moreover, the KOH-activated carbons possessed larger MB adsorption capacity (the maximum Langmuir adsorption capacity: 645.2mg/g) than those of the original carbons and CO2-activated carbons. These results showed that KOH activation was a high effective activation method. The zeolite K-H byproduct was obtained by utilizing the silicon- and potassium-containing solution as the silicon and potassium sources. The zeolite exhibited a stick-like morphology and possessed nanosized particles with a mesopore-predominant porous structure which was observed by TEM for the first time.