Evaluation of Nitrogen-Doped Adsorbents Based on Reduced Graphene Oxide as Platforms for CO₂ Capture

Abstract

CO₂ emissions into the atmosphere have been rapidly rising due to human activities, resulting in the escalation of global warming. To mitigate climate change, it is imperative to develop materials for CO₂ capture with high CO₂ capacity and low production costs. Herein, we developed a facile method to obtain adsorbents based on reduced graphene oxide (rGO) sheets, NrGO(1 – X)700, where X represents the mass of diethylenetriamine (DETA) (X = 1, 2 and 4 g) used in the preparation. The materials NrGO(1 – 1)700, NrGO (1 – 2)700, and NrGO(1 – 4)700 were obtained from graphene oxide dispersions, followed by DETA impregnation and chemical activation with K₂CO₃. N₂ isotherms demonstrated that the materials simultaneously presented micro and mesopores with similar values of specific surface area (280.16 to 310.32 m² g^–1), pore volume (0.26 to 0.28 cm³ g^–1) and pore size (3.78 to 3.80 nm). CO₂ sorption experiments revealed that the material NrGO(1 – 4)700, containing the highest amount of pyridinic, graphitic, and amino nitrogen functionalities, showed the best CO₂ adsorption capacity. Diffuse reflectance Fourier transformed infrared spectroscopy experiments indicated stronger solid-gas interactions for NrGO(1 – 4)700 than for the other materials.

Keywords:
CO₂ capture; porous materials; reduced graphene oxide; nitrogen-doped adsorbent

Introduction

In the last decades, the atmospheric CO₂ concentration has rapidly increased due to anthropogenic activities, e.g., fossil fuel combustion, forest fires and agricultural activities, resulting in climate changes.¹,²,³ CO₂ capture and storage is considered a short to medium-term strategy to mitigate CO₂ release.⁴,⁵,⁶ In coal-fired power stations, CO₂ is removed from the flue gas stream by aqueous alkanolamines solutions, which is a costly technology.⁷,⁸,⁹ Thus, porous solids represent an attractive technology for CO₂ capture due to their easy preparation, eco-friendliness, and low operation costs. A wide variety of adsorbents (including mesoporous silica, activated carbon, graphene-based porous adsorbents, zeolites, metal organic framework (MOFs) and porous covalent organic network (COFs))¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰ has been extensively studied for remediation of toxic environmental pollutants, such as herbicide removal, extraction of heavy metal ions and CO₂ adsorption.²¹,²²,²³,²⁴ Among these solids, some exhibit low CO₂ adsorption, moisture intolerance, prolonged times to reach gas saturation due to low kinetic adsorption, complex syntheses, and weak mechanical properties. Therefore, there is an urgent need to develop new porous solids with advanced properties for application in CO₂ capture.

Graphene-based porous adsorbents are considered a next-generation solution due to the characteristics of graphene, which exhibits a large specific surface area (2630 m² g^–1), high mechanical strength and thermal stability.²⁵,²⁶,²⁷,²⁸ However, graphene sheets have a tendency to form irreversible agglomerates via π-π restacking, resulting in solids with inaccessible surface areas.²⁹,³⁰ Several synthesis methods, including aerogel preparation,³¹,³² self-assembly processes of sheets,¹⁵ pillared graphene layers,³³,³⁴ and chemical activation processes,³⁵ have been reported to improve the specific surface areas of the solids (170-1155 m² g^–1).³⁶ However, for CO₂ adsorption, not only is a large specific surface area necessary, but also other physical (pore volume and pore size distribution) and chemical properties (presence of amino groups and nitrogen atoms) can influence the performance of the materials.³⁷ For example, microporous materials present sufficient solid-gas forces to trap CO₂ molecules at 1 bar and room temperature.³⁸ On the other hand, larger pore sizes (meso and macropores) do not exhibit considerable CO₂ adsorption under the same conditions due to weak solid-gas interactions.³⁹ Yang et al.⁴⁰ studied the kinetic of CO₂ adsorption on SBA-15 (mesoporous material) and activated carbon (micro, meso, and macroporous materials), demonstrating that permeability into the meso/macropores is 3-6 orders of magnitude larger than into the micropores. Therefore, interconnected pore architectures containing bimodal distributions are desirable for CO₂ capture due to the higher CO₂ capacity of micropores and facile permeability of mesopores. Additionally, the introduction of nitrogen atoms into mesoporous graphene can improve physical solid-gas interactions.³⁵ Another strategy is to introduce amino groups because they can react with CO₂ molecules, yielding carbamate species.⁴¹

In this work, we describe the fabrication of graphene oxide (GO) containing interconnected micro and mesopores, simultaneously functionalized with amino groups and doped with different types of nitrogen atoms (pyridine, graphitic, and pyrrole nitrogen). The solids were named NrGO(1 – X)700, where X = 1, 2, 4 g represents the mass of diethylenetriamine (DETA) added to the graphene oxide (1 g) water dispersion. The adsorbents NrGO(1 – X)700 were fabricated from GO dispersions and DETA, followed by an activation process with K₂CO₃. The material NrGO(1 – 4)700, prepared with the highest GO/DETA ratio (1:4), demonstrated the best CO₂ adsorption capacity. This performance was correlated with its highest percentage of amino group and graphic nitrogen.

Experimental

Chemicals

The chemicals employed in this work were purchased from Sigma-Aldrich (St. Louis, USA) and Vetec (Rio de Janeiro, Brazil) and used as received: graphite flakes (≥ 99%), phosphorus pentoxide (P₄O₁₀, ≥ 99%), sodium nitrate (NaNO₃, ≥ 99%), potassium permanganate (KMnO₄, 97%), chloridric acid (HCl, 35-37%), sulfuric acid (H₂SO₄, 98%), hydrogen peroxide (H₂O₂, ≥ 30%), methanol (MeOH, 99.8%), potassium carbonate (K₂CO₃, ≥ 99%) and diethylenetriamine (DETA) (HN(CH₂CH₂NH₂)₂, ≥ 98.5%). CO₂ (99.9%) was acquired from White Martins (Rio de Janeiro, Brazil), He (99.999%) and N₂ (99.999) were acquired from Tecgases (Rio de Janeiro, Brazil).

Syntheses of NrGO(1 – X)700 materials

The graphene oxide used to fabricate the NrGO(1 – X)700 was obtained by a method previously reported by our group,⁴²,⁴³ and based on Hummer’s method.⁴⁴ A dispersion of GO (1.0 g) in 250 mL of distilled water was sonicated for 120 min, resulting in a brown dispersion with a concentration of 4 mg mL^–1. Then, 30, 60 or 120 mL of DETA solution (33.3 g L^–1) were added dropwise to the GO dispersions and transferred to an autoclave (Parr 4748) of 100 mL, which was maintained under magnetic stirring during 24 h at 120 °C. The dispersion was freeze-dried for 2 days to yield NrGO(1 – X). Then, each solid was chemically activated with K₂CO₃ using a NrGO(1 – X):K₂CO₃ ratio of 1:1, where 1.0 g of NrGO(1 – X) was added to a methanolic solution of K₂CO₃ (1.0 g in 70 mL of MeOH). The dispersion was heated up to 70 °C, under magnetic stirring. After the methanol evaporation, the solids were heated up to 200 °C under an N₂ flow (100 mL min^–1) for 1 h in a horizontal furnace. Then, the temperature was raised to 700 °C and maintained for 2 h. After cooling to room temperature, the solids were washed using Milli-Q water until the pH reached 7.0 and dried at room temperature, yielding NrGO(1 – 1)700 (0.0882 g), NrGO(1 – 2)700 (0.0586 g) and NrGO(1 – 4)700 (0.0849 g). Elemental analyses (CHN) for NrGO(1 – 1)700 were: C, 75.04; H, 0.82; N, 5.09%; NrGO(1 – 2)700: C, 76.36; H, 0.95; N, 4.78%; NrGO(1 – 4)700: C, 76.68; H, 1.22; N, 5.39%.

Characterization techniques

Elemental analyses were performed using a PerkinElmer (Shelton, USA) model 240C series ii at the Analytical Central IQ-USP. X-ray photoelectron spectroscopy (XPS) was conducted using a Scienta Omicron (Uppsala, Sweden) GmbH (10^–7 Pa) system. All spectra were measured using a monochromatic Al Kα radiation source (hv = 1486.6 eV). High-resolution spectra were obtained with a pass energy (E_pass) of 30 eV, and the peaks were fitted using CasaXPS software.⁴⁵ Attenuated total reflectance with Fourier transform infrared (ATR-FTIR) spectra recorded from 400 to 4000 cm^–1 on a Thermo Nicolet FTIR iS50 spectrometer (Waltham, USA). Raman spectra at 532 nm were acquired using a Witec Alpha 300R spectrometer (Ulm, Germany) equipped with a 50X objective lens. Powder X-ray diffraction (PXRD) patterns were obtained on a Miniflex-Rigaku (Tokyo, Japan) using a Cu Kα radiation of 1.5418 Å, generated at 30 kV and 15 mA. All data were acquired at room temperature over a 2θ range from 5 to 70°, in steps of 0.02°, and with a counting time of 1 s per step. Nitrogen isotherms, at –196 °C, were measured using ASAP 2020 Micromeritic (Norcross, USA) equipment, in the range of 0 to 1 bar. Specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) equation, and pore size distributions were determined by the Barrett-Joyner-Halenda (BJH) method. Before the analyses, 100 mg of the sample was degassed at 120 °C for 48 h. Scanning electron microscopy (SEM) images were acquired with a Magellan 450 (FEI Company, Hillsboro, USA) microscope using an FEG source at 5 KeV and a current of 50 pA. Approximately 1 mg of each sample was dispersed in 3 mL of isopropanol and sonicated for 1 h. Then, 10 μL of the dispersion was dropped onto a silicon substrate and the solvent was allowed to evaporate under ambient conditions.

CO₂ studies

CO₂ isotherms were measured using ASAP 2020 (Norcross, USA) e-serial 1200 equipment at 25 °C, in the range of 0 to 1 bar. The materials (60 mg) were degassed at 120 °C for 24 h before the measurements. Diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS) experiments were conducted on a Bruker Vertex 70 spectrophotometer (Billerica, USA), equipped with a Harrick reflectance accessory, containing a reaction chamber (HVC-DRP-4, Harrick) and ZnSe windows. The samples were initially dried in situ at 100 °C under He flow for 15 min and then cooled to 30 °C, at which point the background interferogram was collected under He flow. Subsequently, the samples were exposed to a CO₂ flow for 15 min. After this, the stream was switched to He and a spectrum was immediately collected. At the end of this measurement (approximately 47 s), two minutes were allowed to pass before acquiring a new spectrum. This procedure was repeated nine more times (2-20 min). The interferograms were obtained after 256 scans at a resolution of 16 cm^–1.

Results and Discussion

Syntheses of materials

The adsorbents NrGO(1 – X)700 were prepared from GO and DETA, serving as carbon and nitrogen sources, respectively. Initially, mixtures of GO dispersion and DETA solutions were heated up to 120 °C in a Parr autoclave (hydrothermal procedure) to produce reduced graphene oxide platforms containing nitrogen atoms. Subsequently, these materials were annealed at 700 °C with K₂CO₃ under an N₂ flow to produce NrGO(1 – X)700, where X represents 1, 2 and 4 g of DETA. At high temperature, K₂CO₃ reacts with carbon layers of GO, enlarging the voids and removing carbon atoms.⁴⁶,⁴⁷ These oxidation reactions produce CO and CO₂, creating holes in the carbon network (equations 1,2,3), and enhancing the specific surface area of the materials. Different amounts of DETA were used to evaluate the nitrogen contents and types incorporated into the adsorbents and to correlate them with the CO₂ adsorption capacities.

(1) ${\text{K}}_2{\text{CO}}_{3\left(\text{s}\right)}\to {\text{K}}_2{\text{O}}_{\left(\text{s}\right)}+{\text{CO}}_{2\left(\text{g}\right)}$

(2) ${\text{C}}_{\left(\text{s}\right)}+{\text{CO}}_{2\left(\text{g}\right)}\to 2{\text{CO}}_{\left(\text{g}\right)}$

(3) ${\text{K}}_2{\text{O}}_{\left(\text{s}\right)}+{\text{C}}_{\left(\text{s}\right)}\to 2{\text{K}}_{\left(\text{s}\right)}+{\text{CO}}_{\left(\text{g}\right)}$

Characterizations

XPS survey spectra of NrGO(1 – X)700 are presented in Figures 1a-1c. The signals at 284.6, 531.6 and 398.6 eV were attributed to C 1s, O 1s and N 1s, respectively, confirming the presence of nitrogen atoms. The nitrogen content, derived from area integration of the N 1s peak was found to be 4.13, 3.43 and 5.58% for NrGO(1 – 1)700, NrGO(1 – 2)700 and NrGO(1 – 4)700, respectively. These findings are in agreement with the elemental analyses. High-resolution XPS spectra of N 1s, shown in Figures 1d-1f, were analyzed to identify the types of nitrogen moieties. The deconvolution of the signal at 398.6 eV (N 1s) revealed five new peaks centered at approximately 397.0, 398.1, 399.2, 400.3 and 402.5 eV. These correspond to pyridinic-N, amino, pyrrolic-N, graphitic-N and oxidized-N, respectively. These results confirmed that the nitrogen atoms were incorporated into reduced graphene oxide networks. The contributions of each nitrogen species are detailed in Table 1.

Figure 1
XPS survey spectra of (a) NrGO(1 – 1) 700, (b) NrGO(1 – 2) 700 and (c) NrGO(1 – 4) 700. High-resolution N 1s spectra of (d) NrGO(1 – 1) 700, (e) NrGO(1 – 2) 700 and (f) NrGO(1 – 4) 700.

Table 1
Energies and nitrogen percentages determined by high-resolution XPS of N 1s

Material	Energy (N / %) / eV
	Pyridinic-N	Amino	Pyrrolic-N	Graphitic-N	Oxidized-N
NrGO (1 – 1)700	397.0 (21.31)	398.3 (14.87)	399.3 (10.42)	400.5 (39.48)	402.9 (14.91)
NrGO (1 – 2)700	397.0 (33.27)	398.5 (18.88)	399.3 (6.17)	400.3 (26.40)	403.0 (15.28)
NrGO (1 – 4)700	397.0 (9.33)	398.1 (20.10)	399.2 (18.14)	400.3 (41.32)	402.5 (11.11)

In a recent report,³⁵ we demonstrated that incorporating nitrogen atoms into reduced graphene oxide sheets decreased the energies of the frontier orbitals, facilitating the polarization of CO₂ and resulting in improved solidgas interactions. Theoretical experiments indicated that pyridinic-N and graphitic-N are the most effective species for CO₂ adsorption due to their higher solid-gas interaction strength. Additionally, amino groups can react with CO₂ to form carbamate species.¹⁶,⁴⁸,⁴⁹ Thus, the high content of amino, pyridinic-N and graphitic-N species suggests that NrGO(1 – X)700 materials could be potential adsorbents for CO₂ capture.

ATR-FTIR spectra of graphite, GO and the intermediates NrGO(1 – 1), NrGO(1 – 2), NrGO(1 – 4) are presented in Figure S1 (Supplementary Information (SI) section). The graphite spectrum exhibited a band at 1546 cm^–1, attributed to C=C stretching. The GO spectrum showed bands at 3532 and 1621 cm^–1 ascribed to O–H bond related to hydroxyl groups and water molecules.⁵⁰ Bands at 1725, 1220 and 1057 cm^–1 were attributed to carboxylic, epoxide and alkoxy functional groups, confirming the oxidation of graphite flakes. In the spectra of NrGO(1 – 1), NrGO(1 – 2) and NrGO(1 – 4) the bands at 3346, 3283 and 1546 cm^–1 were attributed to ν_s(N–H), ν_as(N–H), and δ(N–H), respectively, confirming the presence of amino groups. For the activated materials, the absence of bands related to amino and oxygen-based functional groups in the spectra of NrGO(1 – X)700 suggested the decomposition of DETA and oxygen functional groups during the activation process (Figure 2a). The decomposition of DETA resulted in the incorporation of nitrogen atoms into graphene structures, as demonstrated by XPS spectroscopy.

Figure 2
(a) ATR-FTIR and (b) Raman spectra for NrGO(1 – X)700.

Raman spectra of NrGO(1 – X) displayed a D-band around 1360 cm^–1, attributed to the E_2g phonon of C sp² atoms, and a G-band at 1590 cm^–1 attributed to A_1g breathing mode, as shown in Figure 2b.⁵¹,⁵² The I_D/I_G ratio, which provides a measure of defects in the sp² domain, increased from graphite (0.131) to GO (0.947), as demonstrated in Figure S2 (SI section). This increase is due to the introduction of oxygen functional groups during the oxidation reaction. The I_D/I_G ratios for the intermediate materials (before activation with K₂CO₃ at 700 °C), NrGO(1 – 1), NrGO(1 – 2), and NrGO(1 – 4) were 1.105, 1.100 and 1.103, respectively, higher than that of GO due to the presence of DETA (Figure S2). The activated materials showed even higher I_D/I_G values, e.g., NrGO(1 – 1)700 (1.115), NrGO(1 – 2)700 (1.157), and NrGO(1 – 4)700 (1.139), indicating an enhancement in defects caused by the introduction of nitrogen atoms into the graphene network and the removal of carbon atoms during the activation process.

PXRD patterns are shown in Figure S3 (SI section). Graphite flakes exhibited a strong basal diffraction at 2θ = 26.3º attributed to the plane (002), corresponding to a d-spacing of 3.38 Å. A second peak at 2θ = 54.4º was related to the plane (004).¹⁵ In the case of GO, the (002) peak shifted to 2θ = 9.8º corresponding to a d-spacing of 9.01 Å. This increased d-spacing in GO, compared to graphite, was due to the presence of oxygen-containing functional groups and water molecules between carbon sheets.⁵³ PXRD patterns for NrGO(1 – X)700 exhibited a broad peak at 2θ = 23.0º (d-spacing = 3.83 Å), indicating a partial Π-π restacking due to the removal oxygen-containing functional groups during the activation process (Figure 3a).

Figure 3
(a) PXRD patterns obtained for NrGO(1 – X)700. Nitrogen isotherms for (b) NrGO(1 – 1)700, (c) NrGO(1 – 2)700 and (d) NrGO(1 – 4)700 (open cycles for adsorption branch and empty cycles for desorption branches). (e) Pore size distributions for NrGO(1 – X)700 obtained from BJH method. Scanning electron microscopy (SEM) images for (f) GO, (g) NrGO(1 – 1)700, (h) NrGO(1 – 2)700 and (i) NrGO(1 – 4)700.

The nitrogen (N₂) adsorption and desorption isotherms obtained for NrGO(1 – 1)700, NrGO(1 – 2)700, and NrGO(1 – 4)700, collected at –196 °C, are displayed in Figures 3b-3d. The materials exhibited a combination of type I and type IV profiles, according to the International Union of Pure and Applied Chemistry (IUPAC) classification.⁵⁴ The initial part of the isotherms (P/P₀ < 0.2 bar) presented a concave curve characteristic of microporous materials and type I classification. In the range of 0.5 < P/P₀ < 1 bar, the isotherms displayed an H2-type hysteresis loop typical of bottle-like pore network. This hysteresis is attributed to the difference in the N₂ condensation and evaporation phenomena. The specific surface areas S_BET were calculated using Brunauer-Emmet-Teller (BET) model, pore volumes and pore size distributions (Figure 3e) were obtained by Barret-Joyner-Halenda (BJH) method. The S_BET values were 283.47, 310.32, and 280.16 m² g^–1 for NrGO(1 – 1)700, NrGO(1 – 2)700, and (1 – 4)700, respectively (Table 2). During the activation process, K₂CO₃ attacks both DETA and graphene structures. A slight increase in S_BET and pore volumes was observed from NrGO(1 – 1)700 to NrGO(1 – 2)700. However, this trend was not observed for NrGO(1 – 4)700, most likely because the amount of K₂CO₃ used mainly reacted with DETA, resulting in lower S_BET and pore volume for this material.

Table 2
Textural properties of the NrGO(1 – X)700, nitrogen contents and CO₂ capacities in comparison with other adsorbents in the literature

	Sample	SBET / (m2 g–1)	VTotal / (cm3 g–1)	Pore size / nm	N / wt.%	S / wt.%	CO2 uptakea / (mmol g–1)	Reference
Porous solids based on graphene	NrGO(1 – 1)700	283.47	0.305	3.78	3.39b/4.13c	–	1.21	this work
	NrGO(1 – 2)700	310.32		3.78	3.18b/3.43c	–	2.00	this work
	NrGO(1 – 4)700	280.16		3.78	3.59b/5.58c	–	4.60	this work
	a-RGO-950	1315.98	1.07	–	–	–	3.06	Chowdhury et al.18
	a-GDC-2	3240	2.23	2.75	–	–	2.0	Ganesan et al.55
	MEGO-I	310	0.94	2.4	–	–	0.85	dos Santos et al.15
Carbon microporous materials activated carbon	MNSM-700-2	1604	0.66	< 1	2.62	–	4.35	Bai et al.56
	CSC-750-0.5	1177	0.52	< 1	0.26	–	4.15	Bai et al.57
	CSPT-700-1	1188	0.47	< 1	0.27	7.07	3.59	Bai et al.58
	PPSC-650–2	1094	0.46	< 1	0.22	4.68	3.64	Bai et al.59
Silica functionalized with amino group	MCM-41-NH2	17	0.04	–	3.47	–	1.01	Mello et al.17
	MCM-41-N3	266	0.24	< 2.0	3.50	–	1.01	dos Santos et al.16
	W-AG-150A	73.1	0.31	12.5	5.12	–	1.97	Anyanwu et al.60

• ^a CO₂ uptake: obtained at 25° C and 1 bar; ^b data obtained from elemental analyses; ^c data obtained from Survey XPS experiments. S_BET: specific surface area calculated by the BET method; V_Total: total pore volume, pore size distributions were determined by the Barrett-Joyner-Halenda (BJH).

SEM image of GO revealed a bidimensional structure containing stacked sheets with a wrinkled morphology (Figure 3f). Comparing GO and NrGO(1 – X)700, SEM images of all NrGO(1 – X)700 exhibited three-dimensional porous structures, composed of highly crumpled and interconnected GO sheets (Figures 3g-3i). The holes observed in the NrGO(1 – X)700 structures were introduced by the activation process with K₂CO₃ during syntheses.

CO₂ adsorption studies

Isotherms were conducted at 25 °C to evaluate the CO₂ adsorption capacities of the NrGO(1 – X)700 materials (Figure 4). At 1 bar, the CO₂ capacities were 1.21, 2.00 and 4.60 mmol g^–1 for NrGO(1 – 1)700, NrGO(1 – 2)700 and NrGO(1 – 4)700, respectively. Given that the specific surface areas, pore volumes and pore size distributions of NrGO(1 – X)700 were very similar, the higher capacity exhibited by NrGO(1 – 4)700 might be attributed to the stronger physical interactions between nitrogen functional groups present in this material and CO₂ molecules (Table 2). Moreover, NrGO(1 – 4)700 demonstrated higher CO₂ adsorption than other porous solids based on graphene,¹⁵,¹⁸,⁵⁵ carbon microporous materials,⁵⁷,⁵⁸,⁵⁹,⁶¹,⁶²,⁶³ and silica functionalized with amino group¹⁶,¹⁷,⁶⁰ reported in the literature, as indicated in Table 2.

Figure 4
CO₂ isotherms obtained for NrGO(1 – X)700 obtained at 25 °C.

To investigate the interactions between NrGO(1 – X)700 and CO₂, we recorded their DRIFTS spectra after CO₂ adsorption. Before the experiments, the samples were heated to 100 °C under He flow for 1 h to remove adsorbed species. The CO₂ adsorption stage was performed at room temperature by introducing a CO₂ flow for 15 min. After stopping the CO₂ flow, the spectra were recorded (Figure 5a). The band at 2350 cm^–1 was ascribed to the asymmetric stretching mode of CO₂ in the gas phase and physically adsorbed gas. The bands at 3630 and 3730 cm^–1 were related to the combinations of (2ν₂ + ν₃) and (ν₁ + ν₃), respectively.⁶⁴ To understand the differences among the materials regarding the strength of solid-gas interaction, a CO₂ desorption experiment was conducted, at room temperature. Under a He flow (15 mL min^–1), DRIFTS spectra were recorded every 2 min, and the area of the band at 2350 cm^–1 was used to estimate the parameter λ (Figure S4-S6, SI section). The λ parameter was calculated by the ratio between the area of the band at 2350 cm^–1 obtained at 0 min, and the area of the same band obtained at different times under He flow (desorption step). The values of λ parameter are shown in Table S1 (SI section). The plot of λ as function of desorption time is shown in Figure 5b. Comparing the values of λ parameter, we observed that NrGO(1 – 2)700 exhibited the highest tendency in the increase of the λ value, while NrGO(1 – 4)700 presented the lowest tendency. The λ parameters indicate that the strongest solid-gas interaction occurs for NrGO(1 – 4)700, and the weakest is for NrGO(1 – 2)700. In a recent report, we demonstrated from theoretical and experimental results that pyridinic and graphitic nitrogen atoms were more efficient for CO₂ capture than pyrrolic nitrogen.³⁵ The interaction involving pyridinic-N and CO₂ results from the nitrogen lone pair that attacks CO₂ molecule. Pyrrolic-N presents weak interactions with CO₂ because the lone pair of the nitrogen atom is committed in the conjugated π-system of the pyrrolic ring. Graphitic-N is also efficient for CO₂ adsorption because graphitic meta-N-doped can efficiently polarize CO₂ molecules, favoring solid-gas interactions. Additionally, amino groups also present lone pairs to attack CO₂ molecules.⁶⁵ Table 1 shows the percentage of different types of nitrogen atoms in the NrGO(1 – X)700. Among the materials, NrGO(1 – 4)700 showed the highest percentage of amino groups and graphitic-N, suggesting that the strongest solid-gas interactions are expected. Most likely, the presence of the highest percentage of the amino and graphitic nitrogen within the micro and mesopores in NrGO(1 – 4)700 allows for the highest CO₂ adsorption for this solid. Therefore, in the nitrogen-doped graphene materials with similar textural properties, the results indicate a tendency for the increase of CO₂ adsorption with the increase of the percentages of amino groups and graphitic nitrogen in their structures.

Figure 5
(a) DRIFTS spectra obtained after CO₂ adsorption on NrGO(1 – X)700. (b) Plot of parameter λ versus CO₂ desorption time.

Conclusions

This study successfully synthesized three porous adsorbent solids based on reduced graphene oxide using GO and DETA as precursors and K₂CO₃ as an activating agent. XPS analysis confirmed the incorporation of various nitrogen functionalities (pyridinic-N, amino, pyrrolic-N, graphitic-N and oxidized-N) into the adsorbent networks. N₂ isotherms showed that the NrGO(1 – X)700 series consisted of a mixture of micro and mesopores. CO₂ isotherms highlighted that NrGO(1 – 4)700 exhibited the highest CO₂ adsorption capacity, likely due to its higher percentage of amino groups and graphitic nitrogen. DRIFTS experiments indicated that NrGO(1 – 4)700 possesses the strongest solid-gas interactions. This is attributed to its high content of amino groups and graphitic nitrogen, which induce strong interactions with CO₂, resulting in superior CO₂ adsorption. Thus, the CO₂ adsorption capacity of nitrogen-doped adsorbents based on reduced graphene oxide, with similar values of textural properties (specific surface area, pore volume and pore size) increases with the amount of amino groups and graphitic nitrogen. This investigation not only highlights the synthesis of cost-effective and environmentally CO₂ adsorbent materials from graphite but also positions NrGO(1 – 4)700 as a viable material for efficient CO₂ capture in low-pressure applications, marking a significant stride towards addressing global carbon capture challenges.

Acknowledgments

This work was supported by the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Programa Institucional de Bolsas de Iniciação Científica (PIBIC) and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ). We are grateful to Laboratórios Multiusuário de Caracterização de Materiais e Espectroscopia Molecular from Universidade Federal Fluminense.

Supplementary Information

Supplementary information (ATR-FTIR, Raman spectra and PXRD patterns of GO, graphite, NrGO(1 – 1), NrGO(1 – 2) and NrGO(1 – 4); in situ DRIFTS spectra of CO₂ desorption on the NrGO(1 – X)700 sample as a function of time during helium purge; evaluation of CO₂ desorption with time in helium purge in DRIFTS analysis through parameter λ) is available free of charge at http://jbcs.sbq.org.br as PDF file.

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Edited by

Editor handled this article: Aldo José Gorgatti Zarbin (Guest)

Dedicated to the memory of Prof Oswaldo Luiz Alves for his great contribution to the field of solid-state chemistry.

Publication Dates

• Publication in this collection
  28 June 2024
• Date of issue
  2024

History

• Received
  25 Feb 2024
• Published
  29 May 2024