Global warming represents a critical environmental challenge that requires immediate action. According to the Sustainable Development Goals (SDGs), particularly SDG 13, addressing climate change is a critical priority, especially the reduction of greenhouse gas emissions. Carbon dioxide (CO₂) is one of the greenhouse gases that is emitted through transportation, industrial processes, burning of fossil fuels, and agricultural burning. The World Meteorological Organization reports that CO₂ emissions are projected to reach 41.6 billion tons in 2024, up from 40.6 billion tons in 2023. Thus, reducing the release of CO₂ directly into the atmosphere is an urgent issue. Carbon capture, utilization and storage (CCUS) represents a promising technology for reducing CO₂ emissions. CCUS involves carbon capture, transportation, utilization and storage [1–3]. Adsorbent materials for carbon capture are an important component of CCUS. Porous materials such as silica, metal-organic frameworks, porous organic polymer, and zeolites, have been widely used as potential materials for carbon capture due to their high surface area and high porosity [4–7]. However, the drawbacks of these materials include the requirement for energy-intensive regeneration, limited selectivity in humid environments, and high costs.

Activated carbon (AC) is an effective material for CO₂ adsorption due to its high surface area, excellent thermal stability, chemical inertness, easily tunable structure, ability to be chemically modified on their surface, low cost, and low energy consumption for regeneration [8–12]. Many efforts have considered agricultural wastes as a resource for the production of AC due to their high carbon content, abundance, and low cost [13–16]. Previous reports have demonstrated that agricultural waste-derived AC is a potential adsorbent for CO₂ adsorption. Increasing the surface area and porosity by chemical and physical activations [17–20] and heteroatom doping [21–23] of agricultural waste-derived AC, has been widely studied to improve CO₂ adsorption performance and mitigate nitrogen oxide emissions [24]. Furthermore, the combination of chemical or physical activation processes with nitrogen doping has been shown to greatly increase CO₂ adsorption capacity [21,25–27]. For instance, as-prepared AC from tea seed shells using potassium hydroxide (KOH) activation, followed by nitrogen doping with melamine as the nitrogen source for enhanced CO₂ adsorption proposed [21]. As-prepared materials showed the best CO₂ capturing capacity due to their high surface area. The specific surface area played a significant role in CO₂ capture. Spessato et al. prepared nitrogen-doped activated carbon (N-AC) using the procedure of KOH-activation and nitrogen doping using melamine, hexamethylenetetramine, or tetramethylammonium hydroxide [27]. The results evidence that nitrogen doping, especially pyridinic-type doping, tends to contribute more to CO₂ adsorption than extremely high surface area values. However, the nitrogen-doping technique requires a solid agent of nitrogen source, chemical purification, or time-consuming procedures as reported in previous works [21,25–27]. Recently, Ruan et al. [28] demonstrated that ammonia (NH₃) activation is an effective method for preparing N-AC from pine sawdust. Heat treatment under NH₃ not only introduces nitrogen atoms onto the surface of AC but also increases the surface area through NH₃ etching. The CO₂ adsorption capacity of N-AC is more strongly related to the presence of nitrogen-containing groups than to surface area. NH₃ heat treatment has been proposed as a facile method for nitrogen doping.

Coconut is one of the top 10 most-produced fruits globally. In 2024, global coconut production was estimated at approximately 65.4 million metric tons. A large amount of coconut shells is discarded into the environment. The conversion of coconut shells into AC has attracted significant attention as a sustainable solution that aligns with SDG 12: Responsible Consumption and Production. Coconut shells widely abundant in Asian agricultural waste have emerged as a promising adsorbent for CO₂ capture. To the best of our knowledge, the combination of nitrogen doping using NH₃ and chemical activation of coconut shell-derived AC to improve CO₂ adsorption performance has not been fully explored. In this work, N-AC was successfully prepared for application in CO₂ adsorption. Coconut shell-derived AC was mixed with KOH and activated under an argon atmosphere, followed by heat treatment under an NH₃ atmosphere. KOH activation in the first step provided a porous structure that allowed NH₃ gas to enter the pores, resulting in a high nitrogen content. The KOH-activated and nitrogen-doped AC exhibits approximately 34% enhancement in CO₂ adsorption compared to AC without any modification. The improvement is attributed to the presence of nitrogen atoms, which serve as active sites for CO₂ adsorption, as well as the high surface area.

Fig. 3 shows SEM images and EDS spectra of AC (Fig. 3a,b), N-AC (Fig. 3c–h), heat-treated AC (Fig. 3i,j), KOH-AC (Fig. 3k,l), and KOH-N-AC (Fig. 3m,n). SEM analysis revealed that all samples have a smooth surface with irregular particle sizes, and no obvious pores were observed. However, the evidence of pores was analyzed by BET, which will be discussed later. The EDS spectra of all samples mainly consist of carbon and oxygen at ~0.3 keV (K_α) and ~0.5 keV (K_α), respectively. Sodium, magnesium, silicon, phosphorus, sulfur, potassium, and calcium were observed in low concentrations. The elemental compositions of all samples are summarized in Table S1. The EDS analysis results indicated that carbon content increased after heat treatment under NH₃ and Ar. Increasing the temperature from 700°C to 800°C under NH₃ heat treatment results in a decrease in oxygen content. Heat treatment under NH₃ and Ar atmosphere led to an increase volatile matter degradation. Raman spectroscopy was employed to analyze the crystallinity of all samples.

Fig. 4 shows two strong peaks at around ~1340 and ~1590 cm⁻¹, which related to the defects in graphite (D-band) and crystalline graphite (G-band), respectively. The degree of order and defects of carbon materials are calculated by the intensity ratio of the D-band and G-band (I_D/I_G) [29–31]. The I_D/I_G values of AC, N-AC700, N-AC800, N-AC900, and AC800 were 1.12, 1.16, 1.22, 1.20, and 1.17, respectively. The I_D/I_G value increased with the increasing temperature of the heat treatment under NH₃ gas from 700°C to 800°C due to the incorporation of nitrogen atoms as defects into the carbon structure. However, the I_D/I_G value slightly decreased when the NH₃ treatment temperature increased to 900°C. It seems that the nitrogen doping in AC reaches saturation at a temperature of 800°C. This result was confirmed by XPS, which will be discussed later. The I_D/I_G values of KOH-AC800 and KOH-N-AC800 were 1.14 and 1.23, respectively. The increasing in the I_D/I_G of KOH-N-AC800 compared to KOH-AC800 implied that nitrogen atoms were successfully incorporated in the surface of KOH-activated AC [27,32,33].

Fig. 5a shows the XPS survey spectra of all samples. The two obvious peaks were observed at ~284 and ~532 eV, which are related to C1s and O1s, respectively. In addition, the presence of the N1s peak of N-AC and KOH-N-AC was observed at ~400 eV. This peak indicated that nitrogen atoms were successfully doped into AC. The C1s peak of all samples reveals four sub-peaks: C–C, C–O, C=O, and O–C=O as shown in Fig. S1 (Supplementary Materials). For the O1s peak, the four fitting peaks: C=O, C–O, O–C=O, and O–H as shown in Fig. S2 (Supplementary Materials) [34–36]. The N1s peak of N-AC and KOH-N-AC can be deconvoluted into three peaks: pyridinic-N, pyrrolic-N and graphitic-N as illustrated in Fig. 5b–e [37,38]. The composition (%) of C1s, O1s and N1s, and CO₂ uptake at 25°C and 1 bar of AC, N-AC, and KOH-N-AC are summarized in Table 1. The N/C ratios of N-AC700, N-AC800, N-AC900, and KOH-N-AC800 were 3.23, 4.84, 3.40, and 5.43 at%, respectively. The results indicated that the optimum temperature for nitrogen doping in AC is 800°C, which corresponds to the I_D/I_G value derived from Raman spectra. It was found that the content of pyridinic-N, pyrrolic-N and graphitic-N increases with the treatment under NH₃ gas from 700°C to 800°C, and then decreases after heat treatment under NH₃ gas at 900°C as shown in Fig. 5f. Furthermore, the nitrogen content of KOH-N-AC800 is higher than that of N-AC800.The increase in nitrogen content is attributed to the oxygen-containing functional groups in the AC, which are introduced by KOH activation and facilitate nitrogen enrichment [39,40]. The relationship between CO₂ adsorption performance and nitrogen content will be discussed later with the surface area analyzed by BET.

The specific surface area (S_BET), total pore volume, micropore volume, pore diameter, N/C ratio, and CO₂ uptake at 25°C and 1 bar of all samples are summarized in Table 2. The S_BET and pore size distribution were calculated by N₂ adsorption/desorption isotherm using the BET and density functional theory method, respectively. The adsorption increases rapidly at the initial stage, and then gradually reaches saturation, indicating that the samples possess a high proportion of micropores. The adsorption isotherms of all samples exhibit both type I and type IV, corresponding to the IUPAC classification as shown in Fig. 6a,c. Isotherm types I and IV represent the microporous and mesoporous materials [41–43]. The results showed that the S_BET, total volume and micropore volume increased with an increasing NH₃ treatment temperature. NH₃ heat treatment not only introduces nitrogen atoms into AC but also improves surface area, micropore, and total pore volume. According to previous studies, NH₃ heat treatment at 700°C or higher enhances the surface area and porosity of activated carbon because the decomposed NH₂ and NH radicals from NH₃ attack the carbon surface and also form nitrogen-containing functional groups [44–46]. Fig. 6b shows the CO₂ adsorption profiles at 25°C of AC and N-AC. The results showed that the CO₂ adsorption of AC, N-AC700, N-AC800 and N-AC900 were 2.02, 2.35, 2.55, and 2.45 mmol/g, respectively. N-AC800 exhibits 26.24% higher CO₂ adsorption compared to AC without any modification. Although N-AC900 has the highest surface area, micropore volume and total pore volume, their CO₂ adsorption is lower than that of N-AC800. The results exhibited that nitrogen doping in AC plays a more significant role in CO₂ adsorption than surface area and porosity [27,28,47]. The enhancement of CO₂ adsorption capacity is attributed to the increase in pore structure and the presence of active sites introduced by nitrogen doping. For the pore structure, the surface area and micropore volume are important factors influencing CO₂ adsorption performance. A higher surface area and a large volume of micropores provide abundant adsorption sites for CO₂ molecules through van der Walls forces [48–50]. In the case of N-AC, a previous theoretical study based on density functional theory (DFT) demonstrated that the CO₂ adsorption capacity increases with higher nitrogen doping concentrations in carbon materials [51], which is consistent with our results. In addition, the enhancement of CO₂ adsorption on N-AC can be explained by nitrogen-derived functional groups like pyridinic-N, pyrrolic-N, and graphitic-N, which act as active sites for CO₂ adsorption [47,50]. According to a previous theoretical study based on DFT [52], pyridinic-N-doped carbon materials serve as chemisorption sites for CO₂ adsorption. The nitrogen atom functions as a mediator, attracting electrons from surrounding carbon atoms and donating them to facilitate bond formation with the CO₂ molecule. In contrast, pristine carbon materials and graphitic-N-doped carbon materials interact with CO₂ through physisorption. Pyridinic-N-doped carbon materials exhibited higher CO₂ adsorption energy compared to pristine carbon materials and graphitic-N-doped carbon materials, thereby enhancing the CO₂ adsorption capacity. Furthermore, experimental results indicate that the introduction of the Lewis base site onto the surface of AC enhances CO₂ adsorption [53–57]. The Lewis base sites in the carbon materials are generated by nitrogen doping. Pyridinic-N acts as an anchoring site for CO₂ through Lewis base interactions, thereby enhancing the specific adsorbent-adsorbate interactions for CO₂. Fig. 6d shows the CO₂ adsorption profiles at 25°C of AC800, N-AC800, KOH-AC800, and KOH-N-AC800. The CO₂ adsorption of AC800, N-AC800, KOH-AC800, and KOH-N-AC800 were 2.07, 2.55, 2.31, and 2.70 mmol/g, respectively. KOH-N-AC800 exhibits 33.66% higher CO₂ adsorption compared to AC. The improvement in CO₂ adsorption capacity of KOH-N-AC800 is ascribed to the synergistic effects of surface area enhancement and nitrogen doping, induced by sequential KOH activation and NH₃ heat treatment. Although the surface area of KOH-N-AC800 (903.74 m²/g) is lower than that of N-AC900 (926.32 m²/g), KOH-N-AC800 exhibits the highest CO₂ adsorption capacity, which is attributed to its higher N/C ratio that plays a crucial role in CO₂ adsorption [27,28,47].

Fig. 7a presents the CO₂ and N₂ adsorption isotherms of AC, N-AC800, and KOH-N-AC800 as a function of pressure at 25°C. At 1 bar, the CO₂ adsorption of AC, N-AC800, and KOH-N-AC800 were 2.02, 2.55, and 2.70 mmol/g, respectively, while the N₂ adsorption were 0.30, 0.30, and 0.32 mmol/g, respectively. The CO₂ adsorption capacity of KOH-N-AC800 is higher and more rapidly compared to N₂, indicating that KOH-N-AC800 can be used for selective capture of CO₂. Fig. 7b shows the CO₂/N₂ selectivity of AC, N-AC800, and KOH-N-AC800 as a function of pressure at 25°C. Selectivity of CO₂ over N₂ was determined using the ideal adsorption solution theory based on the adsorption isotherms of CO₂ and N₂ [58–60]. The results showed that KOH-N-AC800 and N-AC800 exhibit higher CO₂/N₂ selectivity values than AC, which is attributed to nitrogen doping in KOH-N-AC800 and N-AC800 that enhances preferential CO₂ adsorption [60].

The effectiveness of hybrid activation for enhancing CO₂ adsorption was successfully demonstrated using coconut shell-derived AC. The biomass-derived AC was initially activated with KOH, followed by heat treatment under an NH₃ atmosphere. KOH-N-AC800 exhibits a 34% improvement in CO₂ adsorption compared to untreated AC. The enhancement of CO₂ adsorption is achieved by the synergistic effect of surface area and the incorporation of nitrogen-containing functional groups, which serve as active sites for CO₂ adsorption. Our results exhibited that nitrogen doping is a more crucial factor for effective CO₂ adsorption than surface area and porosity. The enhancement of CO₂ adsorption performance is attributed to the presence of pyridinic-N, which introduces Lewis base sites on the AC surface for CO₂ adsorption. Furthermore, pyridinic-N exhibits strong interactions with CO₂ molecules through chemisorption. This work highlights the effective utilization of chemically activated and nitrogen-doped biomass-derived AC as a sustainable adsorbent for enhancing CO₂ adsorption, contributing to carbon capture technologies in alignment with SDG 12 (Responsible Consumption and Production) and SDG13 (Climate Action).