# Challenges and insights in CO2 adsorption using N-doped carbons under realistic conditions

R. Morales-Ospino, L. Reinert, M.T. Izquierdo, A. Celzard, L. Duclaux, V.

Fierro

► To cite this version:

R. Morales-Ospino, L. Reinert, M.T. Izquierdo, A. Celzard, L. Duclaux, et al., Challenges and insights in CO2 adsorption using N-doped carbons under realistic conditions. Environmental Research, 2025, 273, pp.121211. 10.1016/j.envres.2025.121211. hal-04976790

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# Challenges and insights in CO₂ adsorption using N-doped carbons under realistic conditions

R. Morales-Ospino<sup>a, *,</sup> , L. Reinert<sup>b, D</sup> , M.T. Izquierdo<sup>c</sup> , A. Celzard<sup>a, d D</sup> , L. Duclaux<sup>b</sup> , V. Fierro<sup>a, ** D</sup>

ARTICLE INFO

Keywords:

Activated carbons
Chitosan
CO₂ capture
Biogas-upgrading
Water vapor
Ammonia

A B S T R A C T

N-doped carbon was synthesized from an N-rich precursor, chitosan, and systematically compared with materials reported in the literature to evaluate its potential for CO₂ capture in post-combustion scenarios and biogas upgrading under realistic conditions. The synthesized material, with 5.2 at.% nitrogen and a high surface area of 1000 m² g⁻¹, exhibited moderate CO₂ uptake (2.6 mmol g⁻¹) and an ideal CO₂/N₂ selectivity (SCO₂/N₂) of 20.4 under post-combustion conditions (15% vol. CO₂, 1 bar, 298 K). For equimolar CO₂/CH₄ mixtures (50% vol. CO₂), the material showed an ideal CO₂/CH₄ selectivity (SCO₂/CH₄) of 3.3 under the same conditions. When exposed to humidity, the material adsorbed CO₂ efficiently at low relative humidity (RH) levels, but suffered a significant loss of capacity at 30% RH, attributed to water-induced site blocking. A comparison with other N-doped/enriched materials from the literature revealed difficulties in correlating CO₂ adsorption and selectivity with specific material properties, such as surface area and nitrogen content. This underscores the complexity of defining universal design principles for CO₂ capture.

1. Introduction

Reducing CO₂ emissions is a key environmental priority. Adsorption-based CO₂ capture offers energy savings over traditional amine-based methods, with activated carbons (ACs) showing promise for CO₂ capture from flue gases at 323–393 K (Raganati et al., 2021; García-Díez et al., 2019). Surface functionalization through heteroatom doping is being widely explored to improve the selective CO₂ adsorption performance of ACs. Among these, N doping is particularly appealing due to its basic nature, which promotes stronger interactions with acidic molecules such as CO₂. However, the effectiveness of N doping for CO₂ adsorption is still debated. Conflicting findings in the literature highlight the uncertainties regarding the influence of nitrogen content, specific N functionalities and their interplay with textural properties and other heteroatoms present in the carbon matrix.

Some studies indeed suggest that N-doping enhances both CO₂ adsorption and selectivity (Qi et al., 2019; Zhao et al., 2012), while

others argue that only selectivity improves under certain conditions (Luo et al., 2016; Chithra et al., 2018; Ying et al., 2022; Jin et al., 2018a; Zhang et al., 2016). For instance, Adeniran et al. (Adeniran and Mokaya, 2016) reported a slight decrease in CO₂ adsorption for N-doped carbons compared to undoped ones with similar properties. Sevilla et al. (2013) found that N functional groups in N-doped carbons do not significantly affect CO₂ capture, showing comparable capacities to undoped carbons. Luo et al. (2021), through GCMC simulations, showed that narrow micropores, with a pore diameter around 0.6 nm, increase CO₂ adsorption at higher pressures (0.16–3 bar), while N content has a greater impact at lower pressures (0–0.16 bar). Kumar et al. (2015) demonstrated through molecular simulations that while N-doping does not significantly enhance CO₂ uptake, it improves CO₂/N₂ selectivity (SCO₂/N₂). Xing et al. (2012) found that CO₂ adsorption capacity in N-doped ACs is strongly linked to N content, regardless of surface area or micropore volume, with higher N levels favoring CO₂ uptake. Morales-Ospino et al. (2023) studied the effects of N and O doping on CO₂ adsorption in ACs

<footnote><sup>a</sup> Université de Lorraine, CNRS, IJL, F-88000, Epinal, France</footnote>

<footnote><sup>b</sup> Université Savoie Mont Blanc, EDYTEM, F-73000, Chambéry, France</footnote>

<footnote><sup>c</sup> Instituto de Carboquímica (ICB-CSIC), 50018, Zaragoza, Spain</footnote>

<footnote><sup>d</sup> Institut Universitaire de France (IUF), 75231, Paris, France</footnote>

<footnote>This article is part of a special issue entitled: WWEM-GEET-GC-2024 published in Environmental Research.</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author.</footnote>

<footnote>E-mail addresses: rafael.morales-ospino@univ-lorraine.fr (R. Morales-Ospino), vanessa.fierro@univ-lorraine.fr (V. Fierro).</footnote>
across various temperatures and pressures. The findings showed that the AC textural properties primarily control CO₂ uptake up to 25 bar between 273 and 323 K. While N-O doping reduced textural properties and CO₂ adsorption, it improved S<sub>CO2/N2</sub> at pressures of up to 5 bar within the same temperature range. The results of the aforementioned studies highlight the inconclusive nature of the influence of N functionalities in N-doped carbons on CO₂ adsorption.

N-enriched porous carbons are typically prepared using three methods (1) pyrolysis or activation of N-rich precursors (e.g., melamine-based polymers, polyacrylonitrile, vinylpyridine resin, chitosan): under an inert atmosphere (Jin et al., 2018b; Chagas et al., 2020; Kamran and Park, 2020); (2) co-carbonization of N-containing organic compounds with carbonaceous materials (Morales-Ospino et al., 2023; Rehman and Park, 2020); and (3) heat treatment or activation of chars using N-containing gases like ammonia (Horikawa et al., 2012; Stöhr et al., 1991; Feng et al., 2021; Mochizuki et al., 2022a; Han et al., 2016a).

Derived from crustacean waste, chitosan is a linear copolymer of β-(1–4)-linked D-glucosamine and N-acetyl-D-glucosamine, containing approximately 7.4 wt% N. Chitosan is both renewable and eco-friendly, and serves as a dual source of carbon and nitrogen, simplifying the synthesis process and enabling the production of N-rich carbon materials with well-developed porosity. Chitosan-based N-enriched ACs can be produced through pyrolysis (Lourenço et al., 2023; Nisticò et al., 2020; Kaczmarek and Zawadzki, 2010a) or hydrothermal treatment (Chagas et al., 2020). Pyrolysis can transform chitosan into a carbonaceous materials with N content between 6 and 10 wt% for samples obtained between 673 and 1073 K (Nisticò et al., 2020). Numerous studies have demonstrated the preparation of N-enriched ACs from chitosan through thermochemical activation (Kamran and Park, 2020; Rehman and Park, 2020; Li et al., 2022; Yang et al., 2022; Shao et al., 2023; Xie et al., 2023). Li et al. (2022) produced ACs with surface N-content between 2.9 and 8.4 at.% and BET areas ranging from 1054 to 2039 m² g⁻¹ by activating chitosan with KOH, K₂CO₃, and KHCO₃, achieving a CO₂ capacity of 3.9 mmol g⁻¹ at 298 K and 1 bar, and a S<sub>CO2/N2</sub> of 15–17. Rheman and Park (Rehman and Park, 2020) synthesized N-enriched ACs by pyrolyzing chitosan at 873 K and activating it with KOH and urea at 1073 K, resulting in ultramicroporous carbon with BET areas between 386 and 2150 m² g⁻¹ with N-content decreasing from 11.6 to 1.2 at.% of N with A<sub>BET</sub>. The materials showed S<sub>CO2/N2</sub> between 12 and 25. Xie et al. (2023) prepared N-enriched carbon spheres by combining chitosan with zeolite imidazole framework 8 (ZIF8), followed by pyrolysis at 1173 K. These spheres had a N content of 6.98 wt% and a BET area of 887 m² g⁻¹, with CO₂ uptake of 1.3 mmol g⁻¹ at 303 K and a S<sub>CO2/N2</sub> of 40. Shao et al. (2023) obtained microporous ACs by carbonizing a sodium lignosulfonate-chitosan mixture at 873 K, yielding a material with 89.2% microporosity and high heteroatom content (31 at.% of N + O), which achieved 4.8 mmol g⁻¹ CO₂ uptake at 273 K and S<sub>CO2/N2</sub> over 60. Kamram et al. (Kamran and Park, 2020) activated chars from hydrothermally treated chitosan using KOH and NaOH, producing materials with surface N contents ranging from 2.3 to 6.5 at.%. These materials exhibited CO₂ adsorption capacities between 7 and 8 mmol g⁻¹ at 273 K and S<sub>CO2/N2</sub> ranging from 17 to 69. Yang et al. (2022) activated chitosan through pyrolysis with NaNH₂ at 1173 K, resulting in N-enriched ACs with BET areas ranging from 1460 to 2400 m² g⁻¹, N contents decreasing from 7.2 to 3.6 at.% and CO₂ capacities between 1.7 and 6.3 mmol g⁻¹ at 273 K. However, S<sub>CO2/N2</sub> was not reported.

This study employs chitosan as a precursor to synthesize an N-enriched AC through pyrolysis and ammonia activation. It focuses on applications such as carbon capture from flue gases in post-combustion scenarios and biogas purification, with the aim of improving the material's performance by incorporating N functionalities into its surface chemistry. The material's CO₂ adsorption performance is evaluated under both dry and humid conditions to simulate realistic applications. The study compares CO₂ adsorption and selectivity in two capture scenarios with literature data to explore the influence of nitrogen content and surface properties. The review reveals inconsistent findings,

highlighting the complex interplay of factors affecting CO₂ capture. This paper therefore underscores the important need for systematic studies under realistic conditions to clarify these relationships and guide the optimization of N-doped activated carbons.

## 2. Experimental and methods

### 2.1. Synthesis of the N-rich activated carbon

Chitosan (supplied by AOXIN®, molecular weight 193 kDa, deacetylation degree >90%) was used as the precursor. A weighted amount of chitosan (5 g) was placed in an alumina crucible and introduced into a tubular furnace (Nabertherm RSH 50/500/13) equipped with a quartz tube. Nitrogen was used as the carrier gas at a flow rate of 200 mL min⁻¹ and the activation of chitosan was carried out in two steps, as shown in Fig. 1.

(1) **Carbonization:** Chitosan was pyrolyzed at a heating rate of 10 K min⁻¹ under a nitrogen flow of 200 mL min⁻¹, reaching a temperature of 1173 K. Once the target temperature was achieved, the nitrogen flow was redirected to a flask containing a 15 wt% aqueous ammonia solution maintained at 303 K.

(2) **Activation:** The carbonized chitosan (N-rich char) was activated at 1173 K for 1 h using ammonia vapor. The NH₃ vapor was generated by bubbling nitrogen through the aqueous ammonia solution, maintaining a constant flow rate (~25 mg min⁻¹, as determined by HCl titration). The saturated ammonia vapor (90 wt% NH₃, determined from the Enthalpy-Concentration diagram for ammonia-water solutions (R. and A.-C.E. American Society of Heating, 2001)) was introduced into the furnace at a nitrogen flow rate of 200 mL min⁻¹. This volumetric nitrogen flow rate was chosen because it is sufficient to obtain the desired ammonia concentration.

It is worth clarifying that steps 1 and 2 occur consecutively within the same furnace, with raw chitosan undergoing treatment during step 1, followed by the carbonized chitosan (the product of step 1) being activated during step 2. Following activation, the material was cooled to room temperature under a continuous nitrogen flow. The final product was labelled as AC-NH3. For comparison, a separate sample of carbonized chitosan obtained after Step 1 was retained for physicochemical characterization.

### 2.2. Material characterization

The AC-NH₃ sample was characterized through N₂ and H₂ adsorption-desorption at 77 K using a 3Flex Micromeritics manometric system. N₂ adsorption data were utilized to determine the BET area (A<sub>BET</sub>), while the combination of N₂ and H₂ isotherms, analyzed using the two-dimension non-local density functional theory (2D-NLDFT) method, provided information on pore size distribution (PSD), specific surface area (S<sub>2D-NLDFT</sub>), micropore volume (V<sub>micro, 2D-NLDFT</sub>), and total pore volume (V<sub>total, 2D-NLDFT</sub>). Water adsorption was also examined using a Belsorp Max II manometric device (Microtrac) at 323 K. After doping, the N content in the bulk phase of the AC-NH₃ sample and the carbonized chitosan was determined through elemental analysis (EA) using a Vario El Cube analyzer (Elementar). Surface N content was also measured on the AC-NH₃ sample and the carbonized chitosan by X-ray photoelectron spectroscopy (XPS) using an ESCAPlus OMICRON system with a hemispherical electron energy analyzer. The analysis area was 1.75 mm × 2.75 mm, and measurements were conducted under vacuum with a 15 kV voltage, 20 mA current, and a non-monochromatized Mg X-ray source (1253.6 eV).
<figure><img src="image_4.png" /><figcaption>Fig. 1. Experimental steps to obtain N-rich AC (AC-NH3) from chitosan, including (1) pyrolysis of chitosan at 1173 K and (2) subsequent activation with NH3 solution of carbonized chitosan.</figcaption></figure>

## 2.3. High-pressure adsorption isotherms

CO₂, N₂, and CH₄ adsorption isotherms were measured using a HPVA II high-pressure manometric system (Micromeritics) at 298 K, 323 K, and 348 K, with pressures up to 35 bar. Approximately 1 g of the AC-NH3 was loaded into a 10 cm³ sample cell and was degassed under vacuum at 383 K for 12 h before starting gas adsorption. Equilibrium points (pressure stabilization with a maximum difference of 0.3 mbar) were recorded at 13 pressure intervals. Background corrections were applied using an empty sample cell under the same conditions.

Single-component adsorption data were fitted using the Sips adsorption model (Eq. (1)), (Eq. (1)), while binary equilibria for CO₂/N₂ and CO₂/CH₄ gas mixtures were predicted using the multicomponent Sips model (Eq. (2)). Ideal selectivity was calculated using Eq. (3), and the isosteric enthalpy of adsorption (-ΔHads) (J mol⁻¹ K⁻¹) was determined using the Clausius-Clapeyron equation (Eq. (4)).

$$q_{eq,i} = \frac{q_{max,i}(b_i P)^n}{[1 + (b_i P)^n]^n} \qquad (\text{Eq. 1})$$

$$q_{eq,i}^{\text{mix}} = \frac{q_{\text{max},i}(b_i P)^{n_i}}{\left[ 1 + \sum_{j=1}^{\text{NoC}} (b_j P)^{n_j} \right]} \qquad (\text{Eq. 2})$$

$$\text{Selectivity}_{i/j} = \frac{q_{eq,i}^{\text{mix}}}{q_{eq,j}^{\text{mix}}} \times \frac{y_j}{y_i} \qquad (\text{Eq. 3})$$

$$\frac{(-\Delta H_{ads})}{R_g} = \frac{\partial \ln(P)}{\partial \left(\frac{1}{T}\right)} \qquad (\text{Eq. 4})$$

Here, qeq,i (mmol g⁻¹) represents the equilibrium adsorbed amount of component i, while P (bar) is the total equilibrium pressure, and Pi or Pj denotes the partial pressure of components i or j in the gas mixture. The parameter qmax,i (mmol g⁻¹) is the maximum adsorption capacity for component i at saturation, with bi (bar⁻¹) representing the affinity constant of component i and bj the affinity constant for the mixture components (j=1,2, ..., NoC, the number of components). The term ni or nj (dimensionless) is the heterogeneity factor for each respective component i or j. The adsorbed amount of each component in the mixture, qeq,i mix or qeq,j mix, corresponds to the amount of i (CO₂) or j (N₂ or CH₄) adsorbed at a given temperature T (K) and pressure. yi and yj are the gas phase volumetric fractions of components i and j in the mixture. Finally, Rg (8.314 J mol⁻¹ K⁻¹) represents the universal gas constant.

## 2.4. CO₂ isotherms with pre-loaded humidity

To evaluate the impact of humidity on CO₂ uptake, adsorption isotherms were measured under pre-loaded humidity conditions using a gravimetric gas/vapor sorption analyzer (Surface Measurements Systems, UK). Approximately 20 mg of the adsorbent was outgassed at 363 K under vacuum before introducing controlled humidity. Once equilibrium was reached, CO₂ adsorption was measured up to 1 bar. Equilibrium is determined based on the rate of mass change (dm/dt), expressed as a percentage change in mass per minute (% min⁻¹). An equilibrium criterion of dm/dt = 0.004% min⁻¹ was applied for typical sample sizes of 10–20 mg used in the equipment, in accordance with the manufacturer’s recommendations.

The CO₂ isotherm after pre-loaded humidity was then compared to a dry CO₂ isotherm measured under identical conditions up to 1 bar, as illustrated in Fig. 2. As a consequence of water pre-adsorption, CO₂ adsorption is reduced. This method assumes that water vapor adsorbed is not displaced during subsequent CO₂ adsorption.

# 3. Results and discussion

The yield of activated carbon (AC) from raw chitosan was 21 wt%. When considering only the carbonized chitosan as the starting material, the yield increased significantly to 72 wt%. This indicates that 72% of the mass of the intermediate carbonized product was successfully converted into activated carbon during the activation step.

## 3.1. Textural properties and chemical composition

Fig. 3a and b shows the N₂ and H₂ adsorption-desorption isotherms on AC-NH3 at 77 K. The N₂ isotherm in Fig. 1a is classified as type Ia by IUPAC, which is characteristic of highly microporous materials having mainly narrow micropores (of width < ~ 1 nm). Fig. 3b displays the H₂ isotherm, which is useful for characterizing ultramicropores that N₂ adsorption may struggle to access due to steric hindrance and diffusion limitations (Jagiello et al., 2020). Fig. 3c shows the pore size distribution (PSD) derived from the 2D-NLDFT model. AC-NH3 sample has negligible mesoporosity, with microporosity comprising nearly 47% ultra- and 53% super-micropores. Fig. 3d, shows the cumulative pore volume; the total pore volume is 0.41 cm³ g⁻¹, with 0.21 cm³ g⁻¹ attributed to ultramicropores and 0.19 cm³ g⁻¹ to super-micropores.

The textural properties, including BET area (ABET) and specific surface area obtained by the 2D-NLDFT method (S2D-NLDFT), are presented in Table 1. Notably, the ABET of carbonized chitosan is minimal, confirming the need for activation to produce a porous material. After activation vapor from NH₃ solution, the sample achieves an ABET of nearly 1000 m² g⁻¹. Although ammonia is less commonly employed as
<figure><img src="image_5.png" /><figcaption>Fig. 2. Illustrative example of a CO₂ isotherm with pre-loaded humidity (blue) compared with a pure CO₂ isotherm (green) up to 1 bar. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Adsorption-desorption isotherm of (a) N₂ and (b) H₂ at 77 K, (c) pore size distribution by 2D-NLDFT, and (d) cumulative pore volume of AC-NH3 sample. Full symbols in Figures (a) and (b) represent adsorption while empty symbols represent desorption.</figcaption></figure>

<table><caption>**Table 1**

Textural properties and elemental analysis (EA) of carbonized chitosan and AC-NH3.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="4">Textural properties</th><th colspan="4">Elemental analysis (wt. %)</th></tr><tr><th>A<sub>BET</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>S<sub>2D-NLDFT</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>V<sub>total, 2D-NLDFT</sub><br>(cm<sup>3</sup> g<sup>-1</sup>)</th><th>V<sub>micro, 2D-NLDFT</sub><br>(cm<sup>3</sup> g<sup>-1</sup>)</th><th>C</th><th>H</th><th>N</th><th>O</th></tr></thead><tbody><tr><td>Carbonized Chitosan</td><td>&lt;10</td><td>-</td><td>-</td><td>-</td><td>84.7</td><td>0.8</td><td>7.4</td><td>7.1</td></tr><tr><td>AC-NH3</td><td>1000</td><td>1200</td><td>0.41</td><td>0.40</td><td>78.7</td><td>1.2</td><td>6.3</td><td>13.2</td></tr></tbody></table>

an activating agent, NH₃ activation is classified as a physical activation process, similar to the use of oxidizing gases such as steam, carbon dioxide or air. Previous studies have demonstrated the effectiveness of ammonia as an activating agent at elevated temperatures ranging from 773 to 1273 K (Zuo et al., 2020; Liu et al., 2022; Świetlik et al., 2014; Yang et al., 2014; Luo et al., 2014; Han et al., 2016b; Indayaningsih et al., 2017).

The A<sub>BET</sub> is approximately 20% lower than that obtained by the 2D-NLDFT method.

This discrepancy is due to the presence of a significant amount of ultramicropores, which leads to an underestimation of the A<sub>BET</sub>, due to the limitations of the BET method (Ramirez-Vidal et al., 2021) but also because it only uses the N₂ adsorption isotherm. In contrast, S<sub>2D-NLDFT</sub> incorporates two probe molecules, with the smallest pores better characterized using H₂ at 77 K (Jagiello et al., 2020). Due to its smaller kinetic diameter (0.29 nm compared to 0.364 nm) and supercritical state at isotherm conditions, H₂ can more effectively diffuse
into the ultra-micropores, resulting in a more accurate assessment of the surface area.

Table 1 also shows the elemental composition of carbonized chitosan and AC-NH3. The carbonized chitosan consists of 84.7 wt % C and already possess a high N content (7.4 wt%) since chitosan is an N-rich precursor, 7.7 wt % (Kaczmarek and Zawadzki, 2010b). After ammonia activation, the AC-NH3 sample shows a reduced C content of 78.7 wt%, an increase in H to 1.2%, a slight decrease in N to 6.3 wt %, and a significant rise in O to 13.2 wt %.

Fig. 4 presents X-ray Photoelectron Spectroscopy (XPS) analysis of the atomic composition and N functionalities in carbonized chitosan and the AC-NH3 sample, along with the curve fitting of the N1s spectra. In Fig. 4a, pie charts illustrate the atomic composition by XPS for both samples. For the AC-NH3 sample, the atomic composition is shown as 79.1% C, 15.7% O, and 5.2% N. The increased O and N content in AC-NH3 suggests significant incorporation of these elements following activation with ammonia, when compared to the composition of carbonized chitosan, which consists of 88.6% C, 6.7% O, and 4.7% N. The increase in surface N content measured by XPS, alongside a decrease in bulk N content determined by EA after ammonia activation, may appear contradictory. However, this discrepancy reflects the different regions analyzed by the two techniques: XPS assesses surface N, whereas EA examines bulk N. Ammonia activation introduces a certain amount of N, while high-temperature activation (1173 K) removes some of the N from the carbonized chitosan precursor. Notably, ammonia activation is unlikely to act as a major N donor at such high temperatures. Instead, its primary role is to retain as much N as possible from the precursor while enriching the material surface with nitrogen.

Since saturated vapor from NH₃ solution also contains a certain amount of steam (10 wt%), the activation process also involves water, which explains the increased O content in the sample compared to carbonized chitosan. This observation is consistent with previous reports on N-doping of AC using dry NH₃, without water, where the N content increased while the O content decreased (Mochizuki et al., 2022b). Studies on bamboo charcoals activated with NH₃, steam, and a combination of NH₃/steam further confirm this pattern: activation with NH₃ alone results in higher N content and lower O content, steam alone increases O content and decreases N, and the combination of NH₃ and steam activation leads to both higher N and O contents (Liu et al., 2022). Fig. 4b shows the XPS spectra for the N1s, C1s, and O1s peaks in both

samples. In the N1s spectrum, a clear difference is observed, with AC-NH3 displaying a much stronger N1s peak than carbonized chitosan. The C1s spectrum shows subtle differences in peak shape between the two samples, suggesting changes in C bonding in AC-NH3. Finally, the O1s spectrum reveals a more pronounced peak in AC-NH3 compared to carbonized chitosan, reinforcing the observation of increased O content after activation.

Fig. 4c provides the curve fitting of the N1s spectra, highlighting various N functionalities in the AC-NH3 and carbonized chitosan samples. For AC-NH3, the curve fitting reveals several distinct N species: N6 (35.3%), identified as pyridinic N is a dominant component, indicating a significant presence of these functionalities after NH₃ activation. The other N species include N5 (32.5%), representing pyridonic/pyrrolic N, NQ (18.5%), representing quaternary N, and NX (13.6%), indicating oxidized N. In the carbonized chitosan sample (bottom plot), a similar distribution of N functionalities is observed but with different proportions: N6 (38.0%) is the most prominent, followed by NQ (24.4%), N5 (23.2%), and NX (14.4%). The increase amount of N5 in AC-NH3 underscores the impact of NH₃ activation on altering the N functionalities.

In general, at temperatures above 873 K, N5 species gradually transform into N6 (Pels et al., 1995) due to the loss of oxygen from pyridone groups (Quílez-Bermejo et al., 2022). At even higher temperatures, these N6 species can further transform into edge-type NQ. The increase in the percentage of N5, in contrast to the decrease expected from heat treatment alone, suggests that ammonia activation primarily enhances the N5 content. This increase likely offsets the potential conversion of some of the original N5 to N6. The observed decrease in N6 can be attributed to its transformation into NQ at high temperatures.

## 3.2. High pressure isotherms of CO₂, N₂ and CH₄ and selectivity

Fig. 5 presents the high-pressure adsorption isotherms and selectivity analysis for CO₂, N₂ and CH₄ on the AC-NH3 sample at different temperatures. Fig. 5a–c shows the adsorption isotherms for CO₂, N₂ and CH₄, respectively, at temperatures of 298 K, 323 K and 348 K. CO₂ uptake is significantly higher than that of N₂ and CH₄ across all temperatures, highlighting its greater affinity for the adsorbent, especially at lower temperatures. Since all three molecules are non-polar, the order of adsorbed amounts is primarily determined by their polarizability

<figure><img src="image_7.png" /><figcaption>Fig. 4. (a) Surface quantification (at. %) of C, N and O by XPS, (b) C1s, O1s and N1s XPS spectra and (c) curve fitting of N1s XPS spectra with different N-functional groups for carbonized chitosan and AC-NH3 sample.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** High-pressure isotherms of (a) CO₂, (b) N₂ and (c) CH₄ at 298, 323 and 348 K up to 35 bars. Ideal selectivity of (d) CO₂ over N₂ (S<sub>CO2/N2</sub>) and (e) CO₂ over CH₄ (S<sub>CO2/CH4</sub>) of AC-NH₃ sample at 298 and 323 K. (f) Isosteric enthalpy of adsorption of CO₂, N₂ and CH₄ on AC-NH₃ sample.</figcaption></figure>

volume, with CO₂ having the highest, followed by CH₄, and finally N₂ with the lowest (Ribeiro et al., 2008).

Fig. 5d displays the ideal selectivity of CO₂ over N₂ (S<sub>CO2/N2</sub>) at 298 K and 323 K, representative of typical CO₂ concentration in post-combustion scenarios (15% vol.). The data indicate that S<sub>CO2/N2</sub> decreases with increasing fugacity but remains relatively high at lower fugacities, particularly at 298 K. Fig. 5e illustrates the ideal selectivity of CO₂ over CH₄ (S<sub>CO2/CH4</sub>) at 298 K and 323 K for varying CH₄ concentrations (40%, 50% and 60% vol.). It demonstrates that S<sub>CO2/CH4</sub> is higher at 298 K and reduces as fugacity increases, particularly at higher CH₄ concentrations. It is worth noting that S<sub>CO2/CH4</sub> is considerably lower than that of S<sub>CO2/N2</sub>, indicating that this type of N-doped AC might be more appropriate for CO₂/N₂ separations at post-combustion scenario. Separation of CO₂ from N₂ or CH₄ during adsorption by ACs is primarily governed by equilibrium, with no significant influence from kinetic effects or molecular sieving. Selectivity improves as the adsorption of the undesired component in the binary mixture decreases. Since ACs adsorb more CH₄ than N₂ under the same conditions, the lower selectivity observed for CO₂/CH₄ compared with CO₂/N₂ is to be expected.

Finally, Fig. 5f shows the isosteric enthalpy of adsorption for CO₂, N₂, and CH₄, where CO₂ exhibits the highest enthalpy of adsorption (between 37 and 25 kJ mol⁻¹), indicating its stronger interaction with the AC-NH₃ sample. CH₄ adsorption on AC-NH₃ shows as a moderate isosteric enthalpy (between 29 and 20 kJ mol⁻¹), while N₂ shows the lowest values (between 20 and 13 kJ mol⁻¹), consistent with its lower adsorption on AC-NH₃. These isosteric enthalpy trends suggest that the AC-NH₃ sample preferentially adsorbs CO₂, making it promising for selective CO₂ capture. All samples showed a decreasing isosteric enthalpy trend when increasing the surface coverage, indicating stronger energetic interactions at lower adsorbed amounts, particularly at the onset of the adsorption process.

Fig. 6 provides a comparative analysis of CO₂ uptake and S<sub>CO2/N2</sub> in N-doped activated carbons (ACs) and other porous materials at 1 bar and

298 K. In Fig. 6a, the CO₂ uptake is plotted against the S<sub>CO2/N2</sub> for N-doped ACs at 15% CO₂ volume. This graph demonstrates that the AC-NH₃ sample (marked as "This work"), despite having a moderately high area (1000 m² g⁻¹) and N content (5.2 at.%), exhibits a lower CO₂ uptake and S<sub>CO2/N2</sub> compared to some previously reported N-doped ACs. The graph also represents the N content using the size of the symbols, where larger symbols correspond to higher N content. To provide context for the N content, the maximum (max) and minimum (min) values are indicated in the figure. Depending on the data availability, the N content can be reported either in atomic percentage (from XPS analysis) or in weight percentage (from Elemental Analysis). The heterogeneity of the results highlights the significant difficulty in correlating the N content of the samples as the sole parameter influencing higher S<sub>CO2/N2</sub>. Even when comparisons are made using the same reference, a higher N content does not always result in greater S<sub>CO2/N2</sub>.

The same observation applies if we plot an identical graph but use A<sub>BET</sub> as the parameter determining the size of the symbols (Fig. 6b). It remains difficult to draw a conclusion regarding the apparent surface area of the samples, as the highest areas do not always correspond to the highest CO₂ uptake. This makes it challenging to identify which characteristics most significantly influence either ideal selectivity or uptake. However, one seemingly conclusive observation is that samples with high N content generally exhibit lower A<sub>BET</sub> values, likely due to the pore-blocking effect of N-doping.

The N-doped carbons have also been compared to other types of porous materials, as shown in Fig. 6c. The AC-NH₃ sample again shows moderate performance compared to higher-performing materials such as zeolites, which exhibit high S<sub>CO2/N2</sub> under dry conditions due to the strong electrostatic interactions provided by their cations.

Fig. 6d compares the CO₂ uptake as a function of S<sub>CO2/CH4</sub> at 50% vol. of CH₄ (1 bar and 298 K) for N-doped carbons. It is worth noting that reports on the use of N-doped carbons for biogas upgrading through CO₂ removal are scarce, resulting in limited data for comparison. In terms of S<sub>CO2/CH4</sub>, the highest values observed are significantly lower than those
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Comparison of CO₂ uptake vs. CO₂/N₂ selectivity (S<sub>CO₂/N₂</sub>) for N-doped ACs (Morales-Ospino et al., 2023; Shi et al., 2023; Nazir et al., 2022; Liu et al., 2020; Pang et al., 2021; Ma et al., 2022), with symbol size indicating (a) N content or (b) A<sub>BET</sub>, and (c) CO₂ uptake vs. S<sub>CO₂/N₂</sub> for different porous materials reported in the literature (Morales-Ospino et al., 2023; Shi et al., 2023; Nazir et al., 2022; Liu et al., 2020; Pang et al., 2021; Ma et al., 2022; Bae et al., 2013; Dabbawala et al., 2020; Hudson et al., 2012; Lin et al., 2021; ZHANG et al., 2024; Dautzenberg et al., 2023) at 1 bar, 298 K and 15% vol. CO₂. Similarly, comparison of CO₂ uptake vs. CO₂/CH₄ selectivity (S<sub>CO₂/CH₄</sub>) for N-doped ACs (Li et al., 2019, 2020a; Abdelmoaty et al., 2017; Min et al., 2023; Arami-Niya et al., 2016) with symbol size indicating (d) N content or (e) A<sub>BET</sub>, and (f) CO₂ uptake vs. S<sub>CO₂/CH₄</sub> different porous materials reported in the literature at 1 bar, 298 K and 50% vol. CH₄ (Li et al., 2019, 2020b, 2023; Kostoglou et al., 2017; Wang et al., 2014, 2015a, 2015b; Yuan et al., 2013; Lv et al., 2018; Xian et al., 2015; Duan et al., 2018; Zhang et al., 2018; Ben et al., 2011; Rabbani and El-Kaderi, 2012; Yang et al., 2013; Belmabkhout and Sayari, 2009). The size of the symbols (circles) in Fig. 6(a), (b), (d), and (e) reflects the N content or A<sub>BET</sub> of the samples, with larger circles indicating higher values. Maximum (max) and minimum (min) values of N content and A<sub>BET</sub> are also indicated.</figcaption></figure>

reported for S<sub>CO₂/N₂</sub>. In this scenario, as with the previous one, it was challenging to establish a clear correlation with N content. This difficulty was compounded by the fact that only bulk-phase N content data were available for the referenced work, as surface N content measurements were not reported. Fig. 6e, which displays CO₂ uptake versus S<sub>CO₂/CH₄</sub> with symbol size corresponding to A<sub>BET</sub>, also reveals conflicting

results regarding the factors driving higher CO₂ uptake. Nevertheless, as seen in Fig. 6d, samples with lower N content tend to exhibit higher A<sub>BET</sub> values in Fig. 6e.

Additionally, S<sub>CO₂/CH₄</sub> was compared with various porous materials, as shown in Fig. 6f. These materials include ACs, metal-organic frameworks (MOFs), porous aromatic frameworks (PAFs), and polymers. AC-
NH3 sample shows competitive $S_{CO_2/CH_4}$ but do not achieve the highest $CO_2$ uptake observed in some MOFs and other N-doped ACs. Finally, Fig. 6d plots $CO_2$ uptake against $S_{CO_2/CH_4}$. In this graph, AC-NH3 sample achieves a balance between $CO_2$ uptake and $S_{CO_2/CH_4}$, although it is outperformed by other ACs.

The overall lack of correlation between N content, BET area, $CO_2$ uptake and selectivity observed in Fig. 6 indicates that other factors likely play a critical role in adsorption performance. This inconsistency may be due to variations in experimental methods, non-standardized data treatment (for $CO_2$ selectivity) and the availability and reliability of data reported in the literature. Several hypotheses may help to explain these discrepancies. Firstly, $CO_2$ adsorption may depend more on the distribution of narrow micropores (<1 nm) than on surface area or N content, as these pores are particularly effective for size-selective adsorption. Secondly, types of N functionalities, such as pyridinic nitrogen (N6), may be more influential than overall N content, with some species, such as pyrrolic nitrogen (N5), contributing less to adsorption. Thirdly, O-containing functional groups on the carbon surface may interact synergistically with N species, influencing $CO_2$ binding in ways that N content alone cannot explain. Finally, adsorption site accessibility

may be a limiting factor, as pore blockage or steric hindrance can render otherwise active sites ineffective. Further standardized experiments focusing on these variables could provide a better understanding of the underlying mechanisms.

### 3.3. Water vapor isotherms and water vapor/$CO_2$ co-adsorption

Most of the results reported in the literature are based on experiments conducted at 273 K under dry conditions, which do not accurately reflect realistic scenarios. Fig. 7a presents the water adsorption-desorption isotherm for AC-NH3. Data on other N-enriched material, WKN, from previous work (Morales-Ospino et al., 2025) was also included for the sake of comparison. WKN is an N-doped carbon material derived from walnut shells, which were activated with KOH and then doped with N using urea in a post-synthesis treatment. Although both AC-NH3 and WKN have similar N content on their surface (5.2 at. % for AC-NH3 and 4.5 at. % for WKN), they differ in synthesis methods. AC-NH3 originates from an N-rich precursor, while WKN was produced by introducing N after KOH activation. In the case of WKN, N was incorporated by mixing the AC with urea, followed by heat treatment at

<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) H₂O vapor isotherms at 323 K for samples AC-NH3 and WKN, comparison of the pure $CO_2$ isotherm with $CO_2$ isotherms after a pre-loaded amount of relative humidity (RH) for samples (b) AC-NH3 and (c) WKN at 323 K. (d) Comparison of the relative $CO_2$ adsorbed amount at different 10 and 30 % RH with other samples from the literature. The size of the symbols (circles) in Fig. 6a and c reflects the $A_{BET}$ (m² g⁻¹) of the sample, with larger circles indicating $A_{BET}$ with maximum (max) and minimum (min) values indicated.</figcaption></figure>
523 K under a stream of air.

The inset in Fig. 7a reveals that at relative pressures (P/P₀) below 0.6, AC-NH3 adsorbs more H₂O than WKN, indicating greater hydrophilicity of the AC-NH3 material. However, as P/P₀ increases, the H₂O uptake of WKN surpasses that of AC-NH3, ultimately achieving a much higher maximum H₂O adsorption near P/P₀ = 1. The latter is due to the much higher BET area of WKN, 1900 m² g⁻¹, compared to AC-NH3, 1000 m² g⁻¹.

Fig. 7b shows CO₂ adsorption isotherms for AC-NH3 at 323 K, measured after pre-loading the sample with varying RH levels, ranging from dry conditions to 30% RH. The data indicate that the CO₂ adsorption capacity of AC-NH3 decreases progressively as RH increases. At 30% RH, AC-NH3 shows no CO₂ adsorption, suggesting that H₂O strongly competes for active sites, preventing CO₂ from adsorbing. This behavior highlights the higher sensitivity of AC-NH3 to moisture, where H₂O molecules effectively block CO₂ adsorption sites at 30% RH.

In contrast, Fig. 7c shows the CO₂ adsorption capacity of WKN under similar RH conditions. Under dry conditions, AC-NH3 exhibits higher CO₂ uptake at 323 K and 1 bar, despite the WKN sample having a higher BET area. This is attributed to the specific types of N functional groups present in these samples, which have a greater impact at low pressures. AC-NH3 contains N5 and N6 N-types, which appear to interact more strongly with CO₂ (Ma et al., 2018) compared to the N functionalities in the WKN sample, such as amines or imines, which exhibit weaker interactions. Although AC-NH3 also exhibit N-Q functionality, previous studies have showed that N5 and N6 N-types have higher binding energies with CO₂ than N-Q, with N5 and N6 N-types interacting electrostatically, while N-Q interacted through dispersion forces (Sun et al., 2016). At higher pressures (25 bar), N-functional groups play a less significant role, and surface area becomes the dominant factor. Indeed, the WKN sample adsorbs more CO₂ than AC-NH3 at 323 K (12 mmol g⁻¹ vs. 7 mmol g⁻¹), contrary to the CO₂ uptakes at 1 bar where AC-NH3 showed a greater adsorbed amount (1.7 mmol g⁻¹ vs. 1.3 mmol g⁻¹). WKN retains some CO₂ adsorption capacity up to 30% RH, only losing it entirely at 40% RH. This implies that although WKN adsorbs more H₂O at high RH, it is less affected by moderate RH in terms of CO₂ capture compared to AC-NH3. WKN texture, composed of a higher volume of supermicropores and small mesopores, may allow it to accommodate better both H₂O and CO₂ under conditions of moderate RH. To address the issue of hydrophilicity due to N-doping, hydrophobic coatings, such as functionalization with silanes (Nambafu et al., 2022; Lu et al., 2020; Delgado-Sánchez et al., 2016), hydroxylation and a subsequent grafting of siloxane (Cosnier et al., 2005) or fluorinated compounds (Liu et al., 2019; Parmentier et al., 2012), may help reduce moisture interaction. However, additional synthesis steps could introduce complexity and scalability challenges. An alternative approach is partial drying of the flue gas stream before adsorption, balancing performance with humidity control.

Fig. 7d expands the analysis by comparing AC-NH3 and WKN with other carbon materials from the literature, showing how well each material retains CO₂ adsorption capacity under two values of RH (10 and 30 %). These levels of RH, while not fully representative of saturation (which is not practical in laboratory experiments), provide a meaningful intermediate condition for evaluating the impact of humidity on CO₂ adsorption performance. This graph represents the ratio of CO₂ adsorbed in humid versus dry conditions across the total heteroatom (N + O) content. AC-NH3 (marked as “This work”) retains its CO₂ adsorption relatively well at 10 % RH but loses it more drastically than some other materials at 30% RH. In contrast, WKN shows a higher retention ratio at moderate RH (30%), suggesting it may be better suited for CO₂ capture in moderately humid environments. However, WKN sample, the N-free counterpart of WKN maintained its CO₂ uptake despite the increase of RH, confirming that N-content develops hydrophilicity in the carbon surface. Other materials, including the N-CMK series (N-rich ordered mesoporous carbons) (Maruccia et al., 2023) and biochar-sorbents (BC-sorbents) (Zhang et al., 2022) samples, display

varying degrees of moisture tolerance, with BC sorbents performing very poorly under humid conditions even at 10 % RH. However, not much difference was observed between 10% and 30% RH, as most of the CO₂ uptake had already been lost at 10% RH. Excluding the N-CMK samples, it appears that materials with higher heteroatom content tend to perform less effectively in coping with moisture during CO₂ adsorption. Nevertheless, the available literature on heteroatom-doped carbons for CO₂ adsorption under humid conditions is very limited, making it challenging to draw definitive conclusions. Overall, this study shows the need for systematic studies where N-functional groups are precisely determined by XPS studies and CO₂ adsorption/separation is carried out under realistic conditions of humidity and temperature.

## 4. Conclusion

High surface area and nitrogen (N) content alone are not sufficient to achieve superior CO₂/N₂ and CO₂/CH₄ adsorption selectivities, S<sub>CO₂/N₂</sub> and S<sub>CO₂/CH₄</sub>, respectively. The N-rich activated carbon (AC-NH₃), with a BET area of 1000 m² g⁻¹, demonstrated a moderate CO₂ uptake of 2.6 mmol g⁻¹ and S<sub>CO₂/N₂</sub> of 20.4 at 1 bar and 298 K under post-combustion conditions (15% CO₂). For biogas upgrading, it showed a S<sub>CO₂/CH₄</sub> of 3.3 in an equimolar CO₂/CH₄ mixture. Although AC-NH₃ showed competitive CO₂ adsorption at low relative humidity (RH), compared to others N-doped activated carbons, its CO₂ capacity decreased significantly at 30% RH. This reduction can be attributed to surface N-functionalities present, such as N6 and N5, which promote water adsorption. A thorough review of the literature emphasizes the need for systematic studies to investigate the effects of N functionalities on S<sub>CO₂/N₂</sub> and S<sub>CO₂/CH₄</sub> under realistic conditions of RH and temperature. These findings highlight the challenges posed by CO₂ capture under practical conditions, particularly in the presence of humidity. Future research should focus on targeted modifications to improve selectivity and stability in humid environments.

### CRediT authorship contribution statement

R. Morales-Ospino: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. L. Reinert: Writing – review & editing, Methodology, Investigation. M.T. Izquierdo: Writing – review & editing, Methodology, Investigation. A. Celzard: Writing – review & editing, Supervision. L. Duclaux: Writing – review & editing, Methodology, Investigation. V. Fierro: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This work was supported by the TALiSMAN and TALiSMAN2 projects funded by ERDF and the CATALPA project (ANR-22-PESP-0007) funded by the ANR France 2030 program.

### Data availability

Data will be made available on request.

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