CO2 capture using N-containing nanoporous ...

Journal of Environmental Chemical Engineering 6 (2018) 1995–2002

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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

CO2 capture using N-containing nanoporous activated carbon obtained from argan fruit shells

T

Ouassim Boujibara,b, Ahmed Souiknya, Fouad Ghamoussb, Ouafae Achaka, Mouad Dahbic, ⁎ Tarik Chafika, a

Laboratory LGCVR-UAE/L01FST, Faculty of Sciences and Techniques, University Abdelmalek Essaadi, Tangier, Morocco PCM2E, EA 6299 Université François Rabelais de Tours, Parc de Grandmont, 37200 Tours, France c Materials Science and Nano-Engineering Department, Mohammed VI Polytechnic University, 43150, Ben Guerir, Morocco b

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 capture Argan shells Activated carbon Nitrogen-containing Nanoporous

The present work investigates the CO2 capture by activated carbons prepared from Argan fruits shells. The protocol consist on carbonization followed by activation using wet impregnation or dry physical mixing with activating agents such as KOH or NaOH. The as-prepared samples have been subjected to textural investigations and comprehensive characterizations using scanning electron microscopy, energy dispersive X-ray diffraction, Raman and FTIR spectroscopy. Values of specific surface areas and pore volume up to 2251 m2/g and 1.04 cm3/ g, respectively, were extracted from adsorption isotherms that allow, also, determination of pores sizes and surface energy distributions. Of interest, the chemical composition given by EDX revealing significant N content up to 13.90 wt% and approved by FTIR spectroscopy. Moreover, the CO2 isotherms measured, under 1 bar and 25 °C, show uptake capacity reaching 5.63 mmol/g. This values is likely attributed to CO2 adsorption by the prepared activated carbon combining large surface area, narrow micropores and the N containing surface functionalities.

1. Introduction Global warming took on a much more worrying dimension due to the continuous increase in greenhouse gases (GHGs) emissions. Manmade CO2 is one of the major contributors to global warming as a result of the staggering amounts that are being released into the atmosphere on a daily basis [1]. In the past decades, considerable effort has been put into developing new technologies to reduce excessive CO2 emission and mitigate its long-term effects on climate change [2]. The main treatment technologies used to capture CO2 from flue gas streams are absorption, adsorption and membrane separation [3]. Generally, liquidphase absorption is the most commonly used method for CO2 capture because of its higher efficiency. However, this process presents several drawbacks, such as high energy consumption and serious concerns related to solvents useand waste management [4]. Hence, adsorption emerges as a promising contender technology, as it allows achieving high CO2 uptake capacities while maintaining high thermal stability, which is a prerequisite for operating under high-temperature conditions such as those involved in post-combustion power plants [5]. Another major advantage offered by the adsorption is the possibility to restore the initial performance of the used solid adsorbent through

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regeneration by electrical swing adsorption (ESA) [6] or pressure swing adsorption (PSA) and thermal swing adsorption (TSA) processes [7]. These adsorption facilities make use of appropriate porous materials such as activated carbons, zeolites and metal-organic frameworks regarding their large surface areaper unit of mass as well as suitable pore size distribution [8–10]. In general, activated carbons are preferentially used considering their relatively low cost and availability, especially when obtained from low-cost biomass/waste precursors [11,12]. Activated carbons also show good stability when operated under moist environments due to their hydrophobic character, and can be readily regenerated thermally or by evacuation with lower energy requirements [13]. Recently, biomass wastes have attracted considerable attention as efficient precursors for the preparation of activated carbons with a well-developed porous network [14,15]. Bio-wastes are largely abundant, inexpensive and naturally occurring materials usually rich lignocellulosic compounds and can be directly transformed into porous carbons through carbonization followed by activation [16,17]. Basically, the activation step can be conducted by employing a physical or a chemical process [18]. In physical activation, the precursor is first carbonized, then activated using steam or carbon dioxide [19,20]. Whereas for chemical activation, the precursor is mixed with a

Corresponding author. E-mail address: t.chafi[email protected] (T. Chafik).

https://doi.org/10.1016/j.jece.2018.03.005 Received 10 December 2017; Received in revised form 13 February 2018; Accepted 5 March 2018 Available online 06 March 2018 2213-3437/ © 2018 Published by Elsevier Ltd.


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carbonized product is denoted as ARG-C.

chemical reagent and followed by a heating under inert atmosphere [21]. Compared to physical activation, chemical processing has been advantageous due to its ability to be performed in only one step at relatively low temperatures, leading to greater yields [22]. Besides, the nature of biomass used as a precursor, is considered as key parameter governing the production of high quality activated carbons [23]. In Morocco, the agricultural activity related to the production of Argan fruit (Argania Spinosa) for oil extraction is rapidly emerging because of worldwide growing interest regarding its uses for culinary and cosmetic purposes. So far, the increased popularity of Argan oil has prompted an annual production up to 4000 tons by Morocco, which leaves behind about 80.000 tons of hard shells [24]. The latter is currently considered as an agriculture by-product without any significant economic value and mainly used by the local population as domestic combustible [25]. Even more interesting, Argan shells are well known by rich lignocellulosic content [26], allowing high potential for use as raw material for the production of activated carbons. Indeed, we previously reported successful production of nanoporous activated carbon made out of Argan shells, using optimal preparation conditions following empirical approach [27]. The aim of the present work is to improve the surface area and the prosity of the activated carbons following the traditional protocole based on carbonization followed by activation. It is established that the carbonization step is carried out in order to ensure removal of moisture and the conversion of organic matter into elemental carbon. Note that appropriate carbonisation temperature is needed in order to avoid char yield decrease through further decomposition and release of gaseous and condensable products [28]. Subsequent activation step permits the development of prosity through the diffusion of activating agent into the charcoal. This step depend on several parameters such as the ratio char/reactant, the temperature and chemical nature of the activating agent. Recent works pointed out that chemical activation using impregnation or physical mixing with alkaline hydroxides e.g. NaOH and KOH, allows obtaining microporous activated carbon with larger surface area and narrow pore size, that can be appropriate candidate for CO2 capture at ambient temperature and pressure [29–31]. This approach has been adopted in the present work and yields to activated carbon with interesting presence of nitrogen without further doping with nitrogen-containing chemicals. The as-prepared activated carbons have been investigated with respect to the potential application as efficient CO2 adsorbent through the measurements of CO2 uptake under ambient conditions (25 °C and 1 bar). The obtained CO2 adsorption capacity values were compared to those reported in the literature, under similar conditions, for various biomass-derived activated carbons. The objective is to help creating economic value through valorization of available local agriculture by-product as efficient porous material for CO2 capture.

2.3. Chemical activation The ARG-C precursor was crushed and sieved in order to retain particles size fraction between 500 and 1000 μm. Chemical activation was carried out using sodium hydroxide (NaOH) or potassium hydroxide (KOH), following two different methods namely impregnation (Im) and physical mixing (PM). In the case of impregnation method, an amount of 4 g of ARG-C precursor was impregnated in a solution containing 16 g of KOH or NaOH dissolved in 50 ml of distilled water. The solution was kept under vigorous stirring for 2 h at 60 °C. The resulting slurry was dried overnight in an oven at 110 °C. The resulting samples denoted ARG-K-Im and ARG-N-Im, respectively, for the carbon activated using impregnation with KOH and NaOH. For the physical mixing activation, 16 g of NaOH or KOH beads were physically mixed with 4 g of the carbonized ARG-C sample at room temperature. It is to be noticed that this process was performed in the absence of water. The obtained samples using KOH or NaOH were denoted ARG-K-PM and ARG-N-PM, respectively. After impregnation and physical mixing processes, the resulting samples were heat-treated under 600 cm3/min of N2 flow in a temperature programmable oven, following the sequence below: i) Heating from room temperature to 850 °C at a rate of 5 °C.min−1; ii) Holding at 850 °C for one-hour; iii) Cooling to room temperature. After the heat treatment, all the samples were washed with HCl (5 M) solution, rinsed with distilled water until reaching neutral pH then dried at 110 °C overnight. 2.4. Characterization of the activated carbons The carbonization temperature of the Argan shells was decided based on the Thermo-gravimetric analysis using Perkin Elmer Pyris STA 6000 instrument under a nitrogen flow. The chemical composition was assesed with energy dispersive X-ray spectroscopy (EDX) using a Bruker Nano XFlash Detector 430-M. The surface functionalities were investigated with FT-IR spectroscopy, Jasco 410. The morphology was studied with scanning electron microscopy (SEM, Model SH–4000 M) at acceleration voltage of 15 kV. The crystalline structure was examined by X-ray diffraction (XRD) using Brukereco D8 Advance diffractometer. The graphitization degree was determined using LabRAM HR Raman spectrometer. The textural characteristics were studied by measing N2 adsorption isotherms at 77 K using Micromeritics ASAP 2020 apparatus. Furthermore, CO2 adsorption isotherms measured at 273 K were used to assess the ultramicropores size distribution (pores of width < 0.7 nm). Before adsorption experiments, all samples were outgassed for 4 h at 523 K under high vacuum (13.3 kPa) in order to achieve surface cleaning from moisture and others contaminants. BET surface area, SBET, was determined according to BET method [31], considering the molecular cross-sectional area of N2 at 77 K to be 0.162 nm2.The total pore volume, VT,was obtained from the amount of N2 adsorbed at a relative pressure of P/Po = 0.90. For reasons mainly related to N2 molecules diffusion inside the very narrow pores at 77 K, it is assumed that N2 isotherms correspond to the adsorption in mesopores and micropores larger than 0.7 nm. Whereas CO2 isotherms at low relative pressure (P/Po < 0.035) are assumed to be associated with adsorption in ultramicropores (0.4–0.8 nm). Thus, the Horvath–Kawazoe (H-K) equation [32] was applied to N2 and CO2 isotherms for the determination of the micropore and ultramicropore volumes denoted, respectively, VHK-N2 and VHK-CO2. Mesopores volume, Vmeso, was calculated from the difference between VT and VHK-N2 [33]. The pore size distribution (PSD) was determined by applying density functional theory (DFT) method to N2 and CO2 adsorption isotherms data, assuming a slit

2. Material and methods 2.1. Raw material Argan hard shells collected from the southern region of Morocco were first washed with distilled water and dried in an oven at 100 °C for 24 h, then crushed and sieved to retain the fraction between 1 and 3 mm that were subsequently, carbonized without any prior chemical treatment. 2.2. Carbonization of Argan shells The carbonization process was carried out with 50 g of the clean-dry shells contained in a crucible placed in a home-made stainless steel reactor, itself positioned inside a horizontal furnace. After 10 min of purge under N2 flow, the reactor was heated under a N2 flow of 100 cm3/min at a rate of 10 °C/min until reaching 700 °C, then maintained at this temperature for a period of 1 h. Finally, the reactor was allowed to cool down to room temperature under N2 flow. The resulting 1996


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microstructure. The chemical composition of the carbonized material given by EDX elemental analysis as displayed in Fig. 2b. It is shown, mainly, the presence of Carbon (84.05 wt%) and Oxygen (11.19 wt%) along with small amounts of Nitrogen (2.95 wt%),Potassium (1.01 wt %) and Calcium (0.80 wt%). The presence of nitrogen in the carbonized Argan shells was also reported by Laila Bouqbis and al., who pointed out a total nitrogen content about 150 ppm determined by Kjeldahl method as well as 100 ppm of NaNO3 contained in the char produced by Argan shells carbonization under nitrogen flow [37]. Further SEM micrographs are shown in Fig. 3a and b, corresponding to ARG-K-Im and ARG-K-PM samples respectively, obtained after activation with KOH using impregnation and mixing methods. The samples seems to present a honeycomb-like structure with large spherical cavities. Moreover, in contrast to the displayed smooth cavities of ARG-KPM, the surface of ARG-K-Im presents some rifts and crannies resulting from the chemical etching during liquid phase impregnation. In the case of activation with NaOH, the SEM images of ARG-N-PM and ARG-N-Im samples (Fig. 3c and d, respectively), show irregular and heterogeneous surface morphology displaying pores with different sizes and shapes. X-ray diffraction patterns of the activated carbons and carbonized precursor (ARG-C), are gathred in Fig. 4, for sake of comparison. The XRD profile of ARG-C sample does not reveal the presence of well-defined peaks that might be associated with any distinct mineral phase, which is completely consistent with the amorphous structure expected for this kind of carbonaceous material. However, two broad humps are noticeably present, one at 2θ = 25.56° and the other at approximately 2θ = 43.16°, that can be assigned, respectively, to (002) and (100) graphitic planes [38]. Although these features are present in all samples after activation, the peaks of (002) and (100) have been markedly broadened and their intensities decreased dramatically to become barely visible in the case of ARG-N-Im, ARG-K-PM and ARG-K-Im samples, and almost disappeared for ARG-N-PM. This phenomenon is due to the destruction of the graphite crystalline structure as result of the activation process leading to the predominance of thin-carbon sheets disorderly assembled. Noteworthy is the significantly high intensity at low-angle scatter (5–15°2θ) for the activated samples suggesting porosity development [39]. Fig. 5 shows the Raman spectrum of the prepared active carbons and the carbonized precursor. For all the as-prepared carbon samples, there are two peaks located at 1350 cm−1 and 1580 cm−1, corresponding, respectively, to the D and G bands of graphitic material. The D band is due to the vibration of disordered sp3 carbon, and G band is ascribed to the vibration of ordered sp2 carbon [40]. The intensity ratio of ID/IG indicates the graphitization degree of the samples. So that, graphitization degree has an inverse relationship to the intensity ratio of ID/IG. Analysis of the spectra revealed an increase in the intensity ratio of the activated carbons as compared with those of ARG-C. This is, apparently, associated with relative decrease in the graphitization degree of activated carbons due to the destruction of the graphitic crystalline structure within the ARG-C as a result of the activation treatment. Textural characterization of the activated carbons was investigated using nitrogen adsorption isotherms (measured at 77 K) shown in Fig. 6a. These isotherms are characteristic of type I, representative of microporous solids, according to the IUPAC classification [41]. All the isotherms present a plateau at relative pressure values higher than P/ Po = 0.5, corresponding to monolayer accommodation and saturation with adsorbed N2 within the accessible porosity. Details concerning micropores filling at very low relative pressures starting at P/Po below 10−5 are highlighted in the zoomed part shown in Fig. 6b. Interestingly, at P/Po = 5.10−4, the adsorbed amounts are already about 47%, 52%, 38% and 38% of total adsorption respectively, for, ARG-K-PM, ARG-KIm, ARG-N-Im and ARG-N-PM samples. Accordingly, the predominance of microporosity suggests the need for subsequent measurements with CO2 adsorption at 273 K, known to be rather appropriate than N2 isotherms for the assessment of ultramicroporosity (pores of width < 0.7

pore geometry [34]. The surface energy distribution was determined using the following Eq. (1) corresponding to the solid-fluid potential well depth function ξ (ϵsf/kB) [35]:

∈ k dS ξ ⎛ sf ⎞ = B k S0 d∈sf B ⎝ ⎠ ⎜

⎟

(1)

Where kB is Boltzmann’s constant, ϵsf is the depth of the potential well of the solid-fluid interaction, S is the area of the pore wall surface having a solid-fluid potential well depth less than ϵsf, and S0 is the total pore wall surface. 2.5. CO2 capture measurements The CO2 adsorption capacity of the prepared carbon samples was measured using the same volumetric gas adsorption apparatus (Micromeritics ASAP 2020). Prior to the experiment, the samples (0.10 g) were first purged at 0.67 kPa and 30 °C for 20 min, then heated up to 350 °C at 10 °C/min under a vacuum of 13.3 kPa and kept under these condition for 4 h [14]. CO2 adsorption measurements were then carried out by multiple dosing of 0.2230 mmol/g of high-purity CO2 (Oxynord, Morocco, 99.99%) at low absolute pressure (5% tolerance), until reaching equilibrium. The temperature was maintained at 25 °C using ethylene glycolin and ultra-thermostatic bath (Julado F25), to ensure further control and stabilization of adsorption temperature. Accordingly, CO2 adsorption isotherms were obtained in the pressure range between 0.01–1.0 bar. 3. Results and discussion 3.1. Thermal textural and structural investigations As displayed in Fig. 1, the profile of thermal gravimetric analysis (TGA) obtained with Argan shells clearly shows several weight loss occurring as function of temperature increase. This profile is also of interest regarding the carbonization temperature range required for the char production. In agreement with the literature [36], The first weight loss of 5.78%, is attributed to the released of moisture content and volatile matter at temperature range between 60 °C and 190 °C. The second decomposition stage of the profile shows a weight loss of 53.72% at a temperature range of 230 °C to 370 °C, is due to the decomposition of hemicellulose and cellulose. The final stage of the profile exhibited weight loss of 33.66% attributed to the decomposition of lignin at a temperature above 370 °C. Stabilization of the material was observed above 700 °C and justify the consideration of this temperature for carbonization. Fig. 2a shows the SEM image of the carbonized precursor ARG-C, indicating a morphology characterized by bumpy and rough

Fig. 1. TGA curve of the Argan shells under nitrogen atmosphere.

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Fig. 2. (a) SEM micrograph of ARG-C and (b) EDX analysis of the encircled region.

Fig. 3. SEM micrographs of the prepared activated carbons: (a) ARG-K-PM, (b) ARG-K-Im, (c) ARG-N-PM and (d) ARG-N-Im.

Fig. 4. X-ray diffraction patterns of the prepared active carbons and the carbonized precursor. Fig. 5. Raman spectra of the prepared active carbons and the carbonized precursor.

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Fig. 6. (a) Nitrogen adsorption isotherms measured at 77 K for the prepared activated carbons (b) zoomed up inset of the low-pressure region.

nm), mainly, due to diffusional difficulties of N2 molecules inside the very narrow pores [42]. Furthermore, applying Horvath–Kawazoeequation to N2 and CO2 isotherms allows calculation of different pore volume values; VHK-N2 and VHK-CO2, as well as the VHK-N2/VT ratio, for the estimation of micropores contribution to the total pore volume. Meanwhile, BET equation was applied to N2 isotherms to obtain BET surface area; SBET [43]. The obtained values corresponding to all the aforementioned parameters are gathered in Table 1. Among the prepared activated carbons, the largest SBET and VT values were obtained for ARG-K-PM activated by physical mixing with KOH beads (2251.04 m2/g and 1.04 cm3/g), while impregnation with an aqueous KOH solution has yielded to a slightly reduced SBET and VT (1889.63 m2/g and 0.87 cm3/ g). In addition, it can be straight forwardly concluded that these surface areas are mainly derived from micropores as it is revealed by the fairly high values of Smic for both samples. Note, the largest contribution of Smic as observed for ARG-K-Im, eventhough its smaller SBET when compared to ARG-K-PM. On the other hand, using NaOH as an activating agent, produced smaller surface areas and lower pore volumes. On top of that, an opposite trend of the above mentioned was observed, i. e. impregnation in aqueous NaOH solution created more developed texture as indicated by SBET and VT values as compared to those obtained with physical mixing method. Furthermore, although the prepared activated carbons are essentially microporous, there is a minor contribution of mesopores as revealed by the calculated value of Vmeso reported in Table 1. The greatest contribution of mesopores was noticed for ARG-N-Im (23.95%), while the lowest for ARG-K-Im (8.04%). The above observations are perfectly in agreement with the pores size distribution (PSD) derived from DFT calculations applied to N2 and CO2 adsorption isotherms data (Fig. 7a and b). The porosity of the prepared active carbons consists mostly of micropores with minor mesopores as depicted by the shoulder between 20 and 40 Å (Fig. 7a).

Fig. 7. Pore size distribution determined from N2 (a) and CO2 (b) adsorption isotherms using DFT calculations.

Micropores distribution comprises three groups of ultramicropores; a narrow and high-intensity peak centered at pore diameter width of 8 Å and two small peaks at 12 and 16 Å. The PSD derived from CO2 adsorption data (Fig. 7b) shows a quite similar distribution in the nanopore region with a maximum centered at 8.5 Å, depending on the kind of activated carbon. Moreover, a very heterogeneous distribution is illustrated by ultramicropores wide distribution ranging from 4.5 to 7 Å. The observed higher extent of nanopores obtained in the case of KOH activated sample as compared to those activated with NaOH. This is partly attributed to partial gasification and expansion of the interlayer spacing between graphitic planes through consecutive intercalation and deintercalation of K with larger radii than Na [44]. Thus, K is expected to cause much more widening of the interlayer space and produce more nanopores, which is in a good agreement with the observed developed porosity. The multi-stage feature of the adsorption isotherm implies that there are various adsorption sites with different surface energy, e.g. adsorption sites, located on surface of different predisposed porosity; ultramicropore, micropore, and mesopore [45]. Therefore, subsequent relevant information about the energetic heterogeneity of adsorbent’s

Table 1 Textural parameters, CO2 adsorption capacity and chemical composition of the obtained activated carbons. Sample

ARG-K-PM ARG-K-Im ARG-N-PM ARG-N-Im a b

Surface area (m2/g) SBET

Samic

2251.04 1889.63 1462.71 1826.96

1834.04 1581.87 1153.80 1428.30

Smic/SBET (%)

81.47 83.71 78.88 78.17

Pore volume (cm3/g)

VHK-N2/VT (%)

Vb T

VHK-N2

VHK-CO2

Vmeso

1.04 0.87 0.74 0.96

0.93 0.80 0.58 0.73

0.13 0.15 0.09 0.10

0.11 0.07 0.16 0.23

89.42 91.95 78.37 76.04

Calculated from the DFT equation applied to the N2 adsorption isotherms. Determined at P/PO = 0.90 in the N2 adsorption isotherms.

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CO2 uptake at 25 °C and 1 bar (mmol/g)

5.51 5.63 3.64 3.73

Elemental composition (wt%) C

N

O

85.08 82.68 67.74 82.14

9.49 13.90 9.07 12.61

5.43 3.42 23.19 5.25


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Table 2 Comparative survey of the CO2 capture capacity of the activated carbon in this work with other carbon materials from different resources.

Fig. 8. Surface energy distribution determined for the prepared active carbons.

Resource

Surface area (m2/g)

Volume of micropores (cm3/g)

CO2 uptake at 25 °C and 1 bar (mmol/g)

Reference

Argan shell Cigarette butts Palm shell Yellow mombin fruit Pineconeshell Gelatin Sucrose (N-enriched) Chitosan (N-doped) Coconut shell (N-doped)

1889 2016 1250 1384 3135 1636 1745 907 1535

0.87 0.72 0.55 0.49 – < 0.51 0.53 0.39 0.73

5.63 3.66 4.40 7.30 4.73 3.80 4.30 4.26 4.80

This work [60] [61] [62] [63] [64] [65] [66] [67]

and function of surface energy (K). Accordinly, Fig. 8 indicates a wide surface energy distribution, with four peaks being detected, suggesting the existence of different type of adsorption sites. In agreement with the work reported by Ustinov and Do, the peak with the high energy around 65 K could be ascribed to adsorption sites located on the pore edges, while lower energy peaks are apparently attributed to different surface sites located on different types of accessible porosity (i.e. ultramicropore, micropore and mesopore) [46]. On the other hand, the acidic character of the CO2 molecule, justify the recent development of adsorbent doped with nitrogen-bearing groups to endow the surface with basic sites in order to improve CO2 capture efficiency [47–50]. The data concerning chemical composition, textural caracteritics and CO2 adsorption capacity obtained with the prepared activated carbons are summarised in Table 1 Textural parameters, CO2 adsorption capacity and chemical composition of the obtained activated carbons. Of interest, the increased nirtogen content obtained with ARG-K-Im up to 13.90 wt%. This finding is approved by FTIR spectrum recorded with ARG-K-Im sample mixed with appropriate amount of KBr in order to help identification of the targeted IR bands (Fig. 9). In agreement with the literature [51], the presence of IR bands at 1190 cm−1, 1560 cm−1 and 3370 cm−1 are assigned, respectively, to CeN stretching [52], NeH in-plane deformation [53] and NeH and/or eOH stretching [54]. In this sense, the activation under nitrogen flow using chemical agent such as KOH was reported to yield to significant amount of cyanide as result of nitrogen integration within the carbone of graphite lattice [55]. This may explain the increased nitrogen contain in the samples of prepared activated in the present work, seen in Table 1 Textural parameters, CO2 adsorption capacity and chemical composition of the obtained activated carbons., in agreement with data reported by E. Fuente and al. [55].

Fig. 9. FTIR spectra of ARG-K-Im sample.

3.2. CO2 capture investigation The aforementioned procedure for the measurement of CO2 adsorption at 25 °C, was adopted and permitted to obtain the curves shown in Fig. 10, representing the variation of the adsorbed CO2 amount (mmol/g) as a function of CO2 pressure (bar). These curves were used for the estimation of CO2 uptake capacity values gathered in Table 1 Textural parameters, CO2 adsorption capacity and chemical composition of the obtained activated carbons., for different prepared active carbons samples along with their elemental composition determined by EDX analysis. It is clearly noticed that KOH-activated samples exhibited higher CO2 uptake capacities as compared with those of samples activated with NaOH. The highest capacity was obtained with ARG-K-Im followed by ARG-K-PM with values corresponding, respectively, to 5.63 and 5.51 mmol/g. It is worth while noting that, although ARG-K-Im shows comparable SBET to ARG-N-Im, the resulting CO2 capture capacity is 1.5-fold higher. Even more interesting is that despite the larger SBET and micropores volumes obtained with ARG-K-

Fig. 10. CO2 uptake as a function of absolute pressure measured for the prepared activated carbons at 25 °C and 1 bar.

surface sites can be extracted through DFT calculations using the aefromentioned isotherms data. The use of Eq (1) allows determination of surface energy distribution for different activated carbons samples prepared in this work, expressed as differential surface area (m2/g K) 2000


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Table 3 Kinetic parameters associated with different models fitted to CO2 adsorption data. Sample

ARG-K-PM ARG-K-Im ARG-N-PM ARG-N-Im

Langmuir

Freundlich

Temkin

qm (mmol/g)

KL (kPa−1)

R2

KF (mmol/g)

N

R2

KT (kPa−1)

Qm (kJ/mol)

R2

11.0249 9.2565 6.5542 6.8228

0.009243 0.013697 0.010896 0.010730

0.971 0.976 0.954 0.967

0.1543 0.2129 0.1256 0.1211

1.2757 1.3900 1.3711 1.3381

0.999 0.998 0.999 0.999

0.2451 0.2350 0.2444 0.2432

1.5553 1.4541 2.3593 2.2863

0.928 0.953 0.930 0.934

qm: maximum monolayer capacity. KL: Langmuir parameter related to the affinity of the binding sites. KF: constant indicative of the adsorption capacity of the adsorbent. n: empirical constant related to the magnitude of the adsorption driving force. KT: Temkin constant related to equilibrium binding constant. Qm: Temkin constant related to the heat of adsorption. R: correlation coefficient.

carbonization of Argan shells followed by chemical activation through wet impregnation or physical mixing using either KOH or NaOH. Textural characteristics of the activated carbons; BET specific surface area (SBET), pore volume (VT), pores sizes distribution (PSD) and surface energy distribution were determined using N2 (at 77 K) and CO2 (at 273 K) isotherms. It was shown that both chemical agents employed in the activation process, enhance the surface area and porosity development. The sample prepared via dry physical mixing with KOH beads yields to higher SBET reaching the value of 2250 m2/g. The PSD profile revealed different types of microporosities for all the carbon materials, irrespective of their preparation route. Moreover, the existence of heterogeneous adsorption sites was also illustrated by surface energy distribution using DFT calculations. CO2 capture measurements (at 25 °C and 1 bar), indicate interesting uptake capacities by the prepared active carbons. The interesting CO2 capture capacity value being 5.63 mmol/ g, which is among the highest reported, so far, for biomass-derived carbons. was attributed to a combination of large surface area, appropriate porosity and the presence of surface functionalities containing N and O acting as basic sites. Hence, activated carbon obtained from this abundant, and renewable agriculture-by product, may offer promissing contribution to local economic value chain involving Argan shells, paving the way to potential application as precursor for CO2 capture.

PM as compared to ARG-K-Im, the latter enables higher CO2 capture per unit of mass. Therefore, textural properties of the prepared active carbons do not appear to be the only parameters governing CO2 capture. This phenomenon can be attributed to a combination of appropriate porosity and surface functions, in agreement with a recent study, pointing out the crucial role of basic sites towards CO2 capture [56]. Therfore, not only physical adsorption is involved but also chemical bonding, particularly, through interaction of CO2 molecules with surface functionalities containing N and O belongingto activated carbons [57,58]. Accordingly, and based on FTIR spectrum (Fig. 9)as well as EDX results given in Table 1, the higher CO2 uptake obtained with ARGK-Im sample seems to be partly associated with the presence of significant content of non-carbon elements, such as N and O within the carbon matrix [59]. The highest CO2 capture capacity value obtained with the active carbon prepared in our work (i.e. ARG-K-Im) is presented in Table 2, for shake of comparaison with the most relevant values reported in the literature for several activated carbons prepared from different raw mterials, including some synthesized nitrogen-doped activated carbons. Accordignly, the sample ARG-K-Im revealed promising potential, as indicated by the up take value of 5.63 mmol/g, slightly lower than 7.3 mmol/g reported for activated carbon made from yellow mombin fruit stones. On the other hand, the CO2 adsorption isotherms obtained with the prepared activated carbons were fitted with Langmuir, Freundlich and Temkin models [68]. Related adsorption parameters are summarised in Table 3, and the correlation coefficients (R2) has been used to check the best fit accommodation. It was found that Freundlich model was consistently the most appropriate for the description of CO2 adsorption process, showing the best fit with R2 > 0.998. This is in agreement with the surface energy heterogenity shown in Fig. 8, as well as some relevant reported results concerning CO2 capture by biomass-derived carbon adsorbents [69,70]. While it is commonly accepted that the Freundlich parameters “KF” and “n” (presented in Table 3) may help illustrating adsorption capacity and adsorption intensity/favorability, respectively. The calculated values of “n” (n > 1) suggest an easy and favorable adsorption of CO2 on the surface of the studied activated carbons samples [59]. Nevertheless, the value of correlation coefficient Langmuir model might be, also, valid. This suggests interactions between the adsorbate and the predisposed sites belonging to the microporous surface so that no multilayer adsorption occurs. Moreover, the obtained value of Temkin model’s correlation coefficient may suppose, also, additional adsorbate–adsorbate interactions [71].

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4. Conclusions In summary, ultra microporous active carbons derived from agriculture by-product; Argan shells, were successfully synthesized and tested with respect to CO2 capture. The samples were prepared via 2001


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