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Communication

The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S

by
Shiqian Wei
1,2,3,
Xinyu Tian
1,
Zhen Rao
1,
Chunxia Wang
1,
Rui Tang
1,
Ying He
1,
Yu Luo
1,
Qiang Fan
1,2,
Weifeng Fan
1,2 and
Yu Hu
1,2,*
1
School of New Energy Materials and Chemistry, Leshan Normal University, Leshan 614000, China
2
Leshan West Silicon Materials Photovoltaic and New Energy Industry Technology Research Institute, Leshan 614000, China
3
Material Corrosion and Protection Key Laboratory of Sichuan Province, Zigong 643000, China
*
Author to whom correspondence should be addressed.
Materials 2025, 18(12), 2907; https://doi.org/10.3390/ma18122907
Submission received: 9 May 2025 / Revised: 5 June 2025 / Accepted: 8 June 2025 / Published: 19 June 2025
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Abstract

Carbon dioxide (CO2) and hydrogen sulfide (H2S) as harmful gases are always associated with methane (CH4) in natural gas, biogas, and landfill gas. Given that chemisorption and physisorption are the key gas separation technologies in industry, selecting appropriate adsorbents is crucial to eliminate these harmful gases. The adsorption of CH4, CO2, and H2S has been studied based on the density functional theory (DFT) in this work to evaluate the feasibility of transition metal (M = Mn, Fe, Co, Ni, Cu, Mo) porphyrin-like moieties embedded in graphene sheets (MN4-GPs) as adsorbents. It was found that the interactions between gas molecules and MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) are different. The weaker interactions between CH4 and MN4-GPs (M = Co, Ni, Cu, Mo) than those between CO2 and MN4-GPs or between H2S and MN4-GPs are beneficial to the separation of CH4 from CO2 and H2S. The maximum difference in the interactions between gas molecules and MoN4-GPs means that MoN4-GPs have the greatest potential to become adsorbents. The different interfacial interactions are related to the amount of charge transfer, which could promote the formation of bonds between gas molecules and MN4-GPs to effectively enhance the interfacial interactions.

1. Introduction

Methane (CH4), a high-energy-intensity fuel and an important industrial chemical, can be obtained from natural gas, biogas, and landfill gas [1,2,3]. However, corresponding treatments have to be implemented to separate CH4 from harmful gases. For example, besides CH4 as the dominant component accounting for the 55~65% gas volume, biogas has 30~40% carbon dioxide (CO2) and trace water as well as hydrogen sulfide (H2S) and organic acid [3]. Among them, H2S as a toxic acidic gas is harmful to human health and has different adverse effects such as cognitive and motor impairment as well as olfactory fatigue or anosmia, which could occur at concentrations within 20 to 500 parts per million (ppm), while concentrations beyond 600 parts per million (ppm) carry the risk of causing death [4]. Moreover, the presence of H2S is responsible for the corrosion of industry facilities, especially pipelines, resulting in a great waste of materials and a significant safety hazard during production [5]. CO2 as an acid gas also needs to be eliminated given its corrosive property [6,7] and negative effect on calorific values [8,9]. Simultaneously, CO2 and H2S as problematic components also exist in natural gas and landfill gas [10]. Therefore, the effective separation of CH4 from CO2 and H2S is essential to avoid pipeline corrosion and to satisfy calorific value standards in practical applications [11].
Chemisorption and physisorption have become the key gas separation technologies in industry [12]. Zeolites, active alumina, activated carbon, silica gels, and molecular sieves as commercial adsorbents are commonly used for gas separation [13,14,15,16,17]. With higher demands and more complex application environments, a large number of new materials have emerged. Graphene, one of the most promising materials, exhibits striking physical and chemical properties due to its unique structure composed of sp2-hybridized carbon atoms [18,19,20,21,22,23,24]. The tunable structure and correspondingly changed properties confer graphene with a potential to effectively and selectively adsorb target gases. It has been demonstrated that the introduction of nonmetal and/or metal atoms is a feasible way to tune adsorbate–adsorbent interactions and realize gas separations [25,26,27,28]. Recently, transition metal porphyrin-like moieties embedded in graphene sheets (MN4-GPs) are frequently being studied as catalysts for oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and CO2 reduction reaction, among others [29]. In addition, MN4-GPs can be used as gas sensors with a high selectivity and sensitivity [30]. It is indicated that the interactions between gas molecules and MN4-GPs are tunable. Therefore, MN4-GPs as adsorbents have the potential to separate CH4, CO2, and H2S by their different interactions with the adsorbate.
Density functional theory (DFT) studies have been proved to be a reliable method to predict the properties of materials. Moreover, DFT studies can provide insights at the atomic and electronic levels. The adsorption behaviors of CH4, CO2, and H2S on pristine or modified graphene have been reported based on DFT calculations. Gao et al. [31] investigated the adsorption behaviors of CH4 and H2S and successfully improved the adsorption capacity of graphene by doping with Ni. Gui et al. [32] studied CH4, CO, and C2H2 adsorption on pristine graphene and Mn-doped graphene and found that C2H2 and CO adsorption can be enhanced after Mn doping, while CH4 adsorption is still weak. Besides transition metal doping, co-doping with nonmetal atoms, especially the formed transition metal porphyrin-like moieties, has also shown potential in effectively promoting tunable interactions between gas molecules and graphene. There are few works focusing on the adsorption of CH4, CO2, and H2S on MN4-GPs. Nosheen et al. [33] found that iron porphyrin-like moieties embedded in graphene sheets (FeN4-GPs) could adsorb CO, NO, and NO2 with stronger interactions than CO2, H2S, NH3, and SO2. The work of Chen et al. [34] revealed the different adsorption properties of MN4-GPs (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) for CH4. However, to the best of our knowledge, the adsorption behaviors of CH4, CO2, and H2S have not been simultaneously compared to those of MN4-GPs. This is not conducive to effectively designing a MN4-GP structure to separate CH4 from CO2 and H2S. Thus, a better understanding of adsorbate–adsorbent interactions is essential to evaluate the feasibility of MN4-GPs as adsorbents.
The adsorption mechanisms of CH4, CO2, and H2S on MN4-GPs were studied in this work based on DFT calculations. As for the MN4-GPs, common transition metals at low prices, such as Mn, Fe, Co, Ni, Cu, and Mo, were taken into account. It is expected that this work could gain an insight into the interactions between gas molecules (CH4, CO2, H2S) and MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) and furthermore provide ideas to effectively design selective adsorbents for the achievement of CH4, CO2, and H2S separation.

2. Calculation Methods

Vienna ab initio simulation package (VASP v5.3) is frequently chosen to implement DFT calculations, and the reliability of its calculation results is widely recognized [35,36,37,38]. Thus, all DFT calculations within this work were performed using VASP. Core–electron interactions were described with the projector augmented wave (PAW) method [39]. Exchange–correlation energies were calculated by the Perdew, Burke, and Ernzerhof (PBE) method based on generalized gradient approximation (GGA) [40]. Spin polarization and dipole correction were employed to eliminate spurious interactions in periodic boundary calculations. Owing to the long-range van der Waals (vdW) interactions between gas molecules and MN4-GPs, the dispersion force-corrected DFT (DFT-D) method introduced by Grimme was employed [41]. Cut-off energies of plane-wave bases were set as 450 eV. The k-point meshes of 4 × 4 × 1 were used for all the calculations in this work. The convergence thresholds of energies and forces were 1 × 10−4 eV and 0.02 eV/Å, respectively. In order to ensure the reliability of the calculation results, convergence tests of cut-off energies, k-point mesh, force, and energy thresholds were implemented, as shown in the supporting information. The energy values remained stable at approximately −464.1 eV when the cut-off energy was set above 450 eV (Figure S1). This implied that a cut-off energy of 450 eV is sufficient for the calculations, and this setting, which has been used in other works [42], also demonstrated good performance. The energy values were consistently stable with a k-point mesh of 4 × 4 × 1, an energy convergence threshold of 1 × 10−4 eV, and a force convergence threshold of 0.02 eV/Å (Figures S2–S4). Therefore, the calculation parameter settings were validated as appropriate.
The structures of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) were constructed by loading transition metal single atoms coordinated with four nitrogen atoms on the (5 × 5) graphene supercells (a = b = 12.30 Å, c = 15.00 Å), as displayed in Figure 1. To evaluate the stability of MN4-GPs, the binding energies (Eb) and the cohesive energies (Ec) of transition metal single atoms (Mn, Fe, Co, Ni, Cu, Mo) loaded on nitrogen-doped graphene should be calculated.
Eb [43] can be obtained using the following equation:
Eb = EMN4-GPs − EM − EN4-GPs
where EMN4-GPS is the total energy of different MN4-GPs, EM is the energy of an isolated transition metal single atom, and EN4-GPS is the energy of MN4-GPs without loading transition metal single atoms.
Ec [44] can be calculated using the following equation:
Ec = (Ebulk − n × EM)/n
where Ebulk is the energy of the transition metal bulk phase, n is the number of atoms contained in the bulk phase, and EM is the energy of an isolated transition metal single atom.
Different adsorption sites and orientations were considered to construct adsorption structures of CH4, CO2, and H2S on MN4-GPs. A total of 48 configurations were designed to study the adsorption behaviors of these molecules on MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo). As shown in Figures S5–S10, CH4 molecules were adsorbed in three orientations: three hydrogen atoms were downward (labeled as (CH4)3H-down), three hydrogen atoms were upward (labeled as (CH4)3H-up), and two hydrogen atoms were downward (labeled as (CH4)2H-down). For CO2 molecules parallel to MN4-GPs, two adsorption structures were studied: an oxygen atom was adsorbed on the transition metal center (labeled as (CO2)O-parallel), and a carbon atom was adsorbed on the metal center (labeled as (CO2)C-parallel) (Figures S11–S16). In addition, the vertical adsorption structures of CO2 (labeled as (CO2)O-vertical) were also studied. For H2S molecules, both parallel and vertical adsorption structures were constructed on MN4-GPs, as shown in Figures S17–S22 (labeled as (H2S)parallel and (H2S)vertical, respectively).
The thermodynamically stability of the adsorption structures can be evaluated by adsorption energies Eads [45]:
Eads = Etotal − Esubstrate − Egas
where Etotal is the total energy of the MN4-GPs system with adsorbed gas molecules, Esubstrate is the energy of the pristine MN4-GPs substrate, and Egas is the energy of isolated gas molecules in the vacuum.

3. Results and Discussion

3.1. The Thermodynamic Stability of MN4-GPs

First of all, the thermodynamic stability of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) should be taken into account. The binding energies (Eb) and the cohesive energies (Ec) are key data to evaluate the thermodynamic stability of MN4-GPs. The negative value of Eb means that the transition metal single atoms could be stably embedded in the graphene sheets as porphyrin-like moieties. According to Equation 1, the Eb values of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) are −6.93, −7.72, −8.41, −8.22, −5.46, and −6.55 eV, respectively. Compared to the binding energy, a more negative cohesive energy indicates a greater tendency for transition metal atoms to agglomerate. Namely, for stable MN4-GPs, the binding energy must be more negative than the cohesive energy. According to Equation 2, the Ec values of Mn, Fe, Co, Ni, Cu, and Mo atoms are −4.28, −5.46, −5.93, −5.55, −3.88, and −6.58 eV, respectively. To facilitate the comparison of Eb and Ec values, the results have been compiled in Figure 2. It is evident that the binding energies of Mn, Fe, Co, Ni, and Cu single atoms are more negative than their cohesive energies, while the cohesive energy and binding energy of Mo single atom are comparable. Thus, transition metal single atoms prefer to coordinate with nitrogen atoms to form porphyrin-like moieties embedded in graphene sheets. In previous experimental studies [46,47,48,49], these transition metal single atoms have been successfully anchored on nitrogen-doped graphene. It is indicated that Mn, Fe, Co, Ni, Cu, and Mo atoms can be stably embedded in the graphene sheets. Therefore, the thermodynamically stable MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) have the potential to be used as adsorbents.

3.2. The Adsorption of CH4, CO2, and H2S on MN4-GPs

To identify the most thermodynamically stable adsorption structures of CH4, CO2, and H2S, various adsorption sites and orientations were explored (details are provided in Section 2). The negative values of adsorption energies (as shown in Table S1) represent the feasibilities of gas molecules’ adsorption on MN4-GPs. The more negative adsorption energies are, the more thermodynamically stable adsorption structures become. For example, the adsorption energies of (CH4)3H-down, (CH4)2H-down, and (CH4)1H-down on MnN4-GPs are −0.19, −0.11, and −0.18 eV, respectively. It is indicated that (CH4)3H-down on MnN4-GPs is the most thermodynamically stable among these three structures. Namely, CH4 molecules preferentially adsorbed on MnN4-GPs with three hydrogen atoms oriented downward (as shown in Figure 3a). In addition, (CO2)O-parallel and (H2S)S-parallel are the most thermodynamically stable configurations for CO2 and H2S on MnN4-GPs because the corresponding adsorption energies are the most negative, −0.18 and −0.34 eV, respectively. This indicates that CO2 and H2S adsorbed preferentially at Mn sites with their oxygen and sulfur atoms parallel to the MnN4-GPs substrate, respectively (as shown in Figure 3b,c). Similarly, the most thermodynamically stable structures of CH4, CO2, and H2S on FeN4-GPs, CoN4-GPs, NiN4-GPs, CuN4-GPs, and MoN4-GPs are screened out, as shown in Figure 3. CH4 molecules adsorb more readily on MN4-GPs, with three hydrogen atoms oriented down. The most thermodynamically stable orientations of CO2 are parallel to MN4-GPs, although the adsorption sites are different. H2S molecules adsorb horizontally on MN4-GPs, except NiN4-GPs. In summary, gas molecules tend to utilize as many atoms as possible to bond with MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo), thereby achieving stable adsorption. Previous studies have also demonstrated the importance of interactions between gas molecules and material surfaces for thermodynamic stability [50,51,52]. In addition, the adsorption energies of CH4, CO2, and H2S on MN4-GPs obtained from other studies have been collected and are shown in Table S1. The adsorption energies of CO2 and H2S on FeN4-GPs calculated by Nosheen et al. [33] were −0.154 and −0.371 eV. Close values (−0.184 and −0.380 eV, respectively) are obtained in this work. Our calculation results are also consistent with the adsorption energies of CO2 and CH4 on FeN4-GPs, CoN4-GPs, NiN4-GPs, and CuN4-GPs in previous work [34]. This confirms the reliability of our results and provides theoretical support to gain an insight into CH4, CO2, and H2S adsorption behaviors on MN4-GPs.

3.3. The Feasibility of MN4-GPs for Efficient Separation of CH4 from CO2 and H2S

The negative adsorption energies of CH4, CO2, and H2S indicate that gas molecules can be stably adsorbed on MN4-GPs, while their values are different, as shown in Table S1. Adsorption energies are important references to evaluate the strengths of interactions. More negative adsorption energies imply stronger interactions. Thus, the varying interactions between gas molecules and MN4-GPs can be utilized to separate CH4 from CO2 and H2S. To conveniently observe these differences, the adsorption energies of the optimal adsorption configurations of gas molecules have been complied and are shown in Figure 4.
As shown in Figure 4a, the adsorption energies of (CH4)3H-down, (CO2)O-parallel, and (H2S)S-parallel on MnN4-GPs are −0.19, −0.18, and −0.34 eV, respectively. This indicates stronger interactions between H2S and MnN4-GPs, while interactions involving CH4 and CO2 are comparable. Consequently, H2S can be separated from CH4 and CO2 due to the stronger adsorption on MnN4-GPs, while separating CH4 and CO2 is challenging because of the negligible difference in interactions. Similarly, the separation of CH4 and CO2 on FeN4-GPs is difficult given their close adsorption energies (−0.19 vs. −0.18 eV, as shown in Figure 4b). As atomic numbers increase, the interactions between CH4 and MN4-GPs (M = Co, Ni, Cu, Mo) (Eads = −0.16~−0.10 eV) become weaker than the interactions between CO2 and MN4-GPs (M = Co, Ni, Cu, Mo) (Eads = −1.59~−0.17 eV). Simultaneously, the interactions between H2S and MN4-GPs (M = Co, Ni, Cu, Mo) keep strong (Eads = −0.90~−0.19 eV). Thus, adsorption on MN4-GPs (M = Co, Ni, Cu, Mo) enables the separation of CH4 from CO2 and H2S.
In addition, the adsorption capacity of MN4-GPs (M = Co, Ni, Cu, Mo) can be determined by the adsorption energy differences in CH4, CO2, and H2S. As shown in Figure 4, the adsorption energies of CH4 on CoN4-GPs, NiN4-GPs, CuN4-GPs, and MoN4-GPs are −0.12 eV, −0.14 eV, −0.16 eV, and −0.10 eV, respectively. This reveals that MoN4-GPs have the weakest adsorption capacity for CH4. The adsorption energies of CO2 on CoN4-GPs, NiN4-GPs, CuN4-GPs, and MoN4-GPs are −0.19 eV, −0.17 eV, −0.17 eV, and −1.59 eV, respectively. This implies that MoN4-GPs have the strongest adsorption capacity for CO2. The adsorption energies of H2S on CoN4-GPs, NiN4-GPs, CuN4-GPs, and MoN4-GPs are −0.32 eV, −0.19 eV, −0.19 eV, and −0.90 eV, respectively. MoN4-GPs exhibit the strongest adsorption capacity for H2S. In summary, MoN4-GPs show the weakest adsorption capacity for CH4 molecules but the strongest adsorption capacity for CO2 and H2S molecules. Therefore, among MN4-GPs (M = Co, Ni, Cu, Mo), MoN4-GPs are the optimal adsorbent for separating CH4 from CO2 and H2S.
Previous calculational studies have investigated adsorbent feasibility for CH4/CO2/H2S separation. Braga et al. [53] used DFT calculations to screen transition metal-exchanged Y zeolites for selective H2S/COS/CO2 removal from natural gas. Sokhanvaran et al. [12] employed grand canonical Monte Carlo simulations, demonstrating metal–organic frameworks (MOFs) as effective H2S-selective adsorbents. Zhan et al. [54] combined grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations to investigate the adsorption behavior of CO2/CH4 and H2S/CH4 mixtures within quartz nanopores. Aksu et al. [17] implemented the multi-level computational screening of covalent organic frameworks (COFs) for the capture of H2S+CO2 from natural gas. No prior work has examined MN4-GPs as adsorbents for the efficient separation of CH4 from CO2 and H2S. Our calculations reveal the adsorption behaviors of CH4, CO2, and H2S on different MN4-GPs, demonstrating that MN4-GPs (M = Co, Ni, Cu, Mo), particularly MoN4-GPs, are promising selective adsorbents for removing H2S and CO2 from natural gas, biogas, and landfill gas.

3.4. The Adsorption Mechanisms of CH4, CO2, and H2S on MN4-GPs

The electronic properties of CH4, CO2, and H2S on MN4-GPs (M = Co, Ni, Cu, Mo) have been studied to gain an insight into their adsorption behaviors, which could explain the potential of MN4-GPs for efficiently separating CH4 from CO2 and H2S. According to the difference charge density maps, as shown in Figure 5, significant electron transfer occurs at the interfaces, where the yellow and blue regions represent electron accumulation and depletion, respectively. Thus, interactions between gas molecules and MN4-GPs (M = Co, Ni, Cu, Mo) are confirmed by interfacial charge redistribution. However, interaction strength varies due to differences in the spatial extent of charge redistribution. For example, the interfacial charge redistribution regions between CH4 and CoN4-GPs (Figure 5a) are smaller than those for CO2 (Figure 5b) or H2S (Figure 5c). Moreover, Bader charge analyses have been performed to quantify the amount of charge transfer Δq (as labeled in Figure 5). There are 0.008 e electrons flowing from CoN4-GPs to CH4 (Figure 5a), 0.050 e electrons flowing from CoN4-GPs to CO2 (Figure 5b), and 0.103 e electrons flowing from H2S to CoN4-GPs (Figure 5c). Namely, compared with CH4 on CoN4-GPs, more electrons transfer at the interfaces of CO2 or H2S on CoN4-GPs. This trend is similar to that for other MN4-GPs (M = Ni, Cu, Mo). Therefore, the interactions between CO2 and MN4-GPs or between H2S and MN4-GPs are stronger than those between CH4 and MN4-GPs. As a result, the adsorption energies of CH4 are more positive than those of CO2 and H2S, as described in Section 3.3.
Crystal Orbital Hamilton Populations (COHPs) [55] are usually employed to analyze chemical bonds, while Integrated Crystal Orbital Hamilton Populations (ICOHPs), which are integral values of COHPs below Fermi levels, can quantify bonding electrons and evaluate bonding strength. More negative ICOHP values indicate stronger interactions. Thus, to further study the interactions between gas molecules and MN4-GPs (M = Co, Ni, Cu, Mo), COHP curves and ICOHP values are presented in Figure 6. For CH4 on MN4-GPs (M = Co, Ni, Cu), there are only COHP curves of C-H bonds (Figure 6a–c). Although COHP curves of Mo-H bonds are observed for CH4 on MoN4-GPs, those of C-H bonds show no significant changes, as shown in Figure 6d. In addition, the ICOHP values of C-H bonds in CH4 on MN4-GPs (M = Co, Ni, Cu, Mo) are −26.39, −26.44, −26.59, and −26.93 eV, respectively. Similar COHP curves and comparable ICOHP values indicate that minimal charge transfer (|Δq| = 0.002~0.020) at the interface has a negligible impact on C-H bond strength, merely facilitating‌ weak adsorption interactions between CH4 and MN4-GPs (M = Co, Ni, Cu, Mo). The interactions between CO2 and MN4-GPs (M = Co, Ni, Cu) are comparable according to the similar COHP curves (Figure 6e–g) and close ICOHP values (−38.04, −38.30, and −38.34 eV). In contrast, the interaction between CO2 and MoN4-GPs strengthens significantly due to the formation of Mo-C (ICOHP = −3.92 eV), Mo-O (ICOHP = −3.94 eV), and N-O (ICOHP = −0.09 eV) bonds. This weakens C-O bonds in CO2, which can be verified by the appearance of an anti-bonding orbital peak below the Fermi energy level (Figure 6h) and a more positive ICOHP value (−28.13 eV). S–H bond strength is similarly modulated upon H2S adsorption on MN4-GPs (M = Co, Ni, Cu, Mo) (Figure 6i–l). For MoN4-GPs, the COHP curve of S-H bonds exhibits distinct changes, including a prominent ‌anti-bonding orbital peak below the Fermi energy level (Figure 6l). This is attributed to Mo-S bond formation (ICOHP = −3.16 eV). In conclusion, interfacial charge transfer critically drives bond formation between gas molecules and MN4-GPs, which plays an important role in enhancing their interactions. Therefore, among MN4-GPs (M = Co, Ni, Cu, Mo), MoN4-GPs exhibit the strongest adsorption capacity for CO2 and H2S molecules, which is consistent with the findings outlined in Section 3.3.
Physical adsorption is an alternative technology for the removal of H2S and CO2 from natural gas, biogas, and landfill gas, and the development of new/modified materials is an attractive research area in this regard [56]. Previous studies confirm that adsorbent selectivity can be tuned by introducing different transition metals [53,57]. Through DFT calculations, this work further reveals the essential reasons for atomic and electronic levels. Additionally‌, factors including gas molecular ratios, pressure, and temperature significantly influence adsorbent selectivity [58]. Future studies must quantify these parameters to establish a complete theoretical framework for designing MN4-GPs as adsorbents.

4. Conclusions

In this work, the adsorption of CH4, CO2, and H2S on MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) has been studied by constructing 48 adsorption configurations based on DFT calculations. The results reveal distinct adsorption orientations: CH4 molecules prefer a three-hydrogen-down geometry, while CO2 and H2S molecules adopt horizontal orientations. The adsorption energies of CH4 (−0.16~−0.10 eV) on MN4-GPs (M = Co, Ni, Cu, Mo) are more positive than those of CO2 (−1.59~−0.17 eV) and H2S (−0.90~−0.19 eV), indicating weaker interactions between CH4 and MN4-GPs. This difference facilitates separating CH4 from CO2 and H2S via MN4-GPs (M = Co, Ni, Cu, Mo). Gas–adsorbent interaction strength ‌scales linearly with charge transfer magnitude. Compared with CH4 (0.0023~0.019 e), more electrons are transferred during CO2 (0.020~0.82 e) and H2S (0.010~0.10 e) adsorption on MN4-GPs (M = Co, Ni, Cu, Mo). Moreover, on MoN4-GPs, the adsorption energy of CH4 (−0.10 eV) is the most positive, while the adsorption energies of CO2 (−1.59 eV) and H2S (−0.90 eV) are the most negative. Namely, MoN4-GPs exhibit the weakest adsorption for CH4 but the strongest for CO2 and H2S. The strong interactions of H2S and CO2 with MoN4-GPs are attributed to chemical bonding, specifically Mo-C and Mo-O bonds for CO2 and Mo-S bonds for H2S. Therefore, MoN4-GPs are the most promising adsorbent for the effective separation of CH4 from CO2 and H2S. It is expected that this work will provide a deep insight into the fundamental reasons for the different selectivity of MN4-GPs and offer a theoretical basis for designing effective adsorbents for the removal of H2S and CO2 from natural gas, biogas, and landfill gas.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma18122907/s1, Figure S1: The convergence test of cut-off energy; Figure S2: The convergence test of K-point mesh; Figure S3: The convergence test of force thresholds; Figure S4: The convergence test of energy thresholds; Figure S5: The structures of CH4 on MnN4-GPs; Figure S6: The structures of CH4 on FeN4-GPs; Figure S7: The structures of CH4 on CoN4-GPs; Figure S8: The structures of CH4 on NiN4-GPs; Figure S9: The structures of CH4 on CuN4-GPs; Figure S10: The structures of CH4 on MoN4-GPs; Figure S11: The structures of CO2 on MnN4-GPs; Figure S12: The structures of CO2 on FeN4-GPs; Figure S13: The structures of CO2 on CoN4-GPs; Figure S14: The structures of CO2 on NiN4-GPs; Figure S15: The structures of CO2 on CuN4-GPs; Figure S16: The structures of CO2 on MoN4-GPs; Figure S17: The structures of H2S on MnN4-GPs; Figure S18: The structures of H2S on FeN4-GPs; Figure S19: The structures of H2S on CoN4-GPs; Figure S20: The structures of H2S on NiN4-GPs; Figure S21: The structures of H2S on CuN4-GPs; Figure S22: The structures of H2S on MoN4-GPs; Table S1: The adsorption energies (eV) of CH4, CO2, and H2S on MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo).

Author Contributions

Conceptualization, S.W. and Y.H. (Yu Hu); methodology, R.T. and Y.L.; software, Q.F.; validation, S.W., X.T. and Z.R.; formal analysis, S.W., X.T. and Z.R.; investigation, S.W.; resources, Y.H. (Yu Hu); data curation, Y.H. (Ying He) and C.W.; writing—original draft preparation, S.W.; writing—review and editing, S.W. and W.F.; visualization, S.W., Y.H. (Ying He) and R.T.; supervision, S.W. and Y.H. (Yu Hu); project administration, S.W. and W.F.; funding acquisition, S.W. and W.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Leshan Normal University Doctoral Talent Launch Project (RC202007), Open Project of Crystal Silicon Photovoltaic New Energy Research Institute (2022CHXK005), Leshan Normal University Scientific Research Cultivation Project (No.KYPY2024-0001), Opening Project of Material Corrosion and Protection Key Laboratory of Sichuan province (2023CL10), Scientific Research Initiation Project for the Introduction of High-level Talents of Leshan Normal University (RC2024029), and Innovation and Entrepreneurship Training Program for College Students (S202210649162).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

We gratefully acknowledge financial support from the funders. All molecular structures were visualized using the Vesta program.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Materials 18 02907 g001
Figure 1. The structure of MN4-GPs: (a) MnN4-GPs; (b) FeN4-GPs; (c) CoN4-GPs; (d) NiN4-GPs; (e) CuN4-GPs; (f) MoN4-GPs.
Figure 1. The structure of MN4-GPs: (a) MnN4-GPs; (b) FeN4-GPs; (c) CoN4-GPs; (d) NiN4-GPs; (e) CuN4-GPs; (f) MoN4-GPs.
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Figure 2. The binding energies Eb and cohesive energies Ec of transition metal single atoms in MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo).
Figure 2. The binding energies Eb and cohesive energies Ec of transition metal single atoms in MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo).
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Materials 18 02907 g003
Figure 3. The most thermodynamically stable adsorption structures of CH4, CO2, and H2S on (a–c) MnN4-GPs, (d–f) FeN4-GPs, (g–i) CoN4-GPs, (j–l) NiN4-GPs, (m–o) CuN4-GPs, and (p–r) MoN4-GPs, respectively.
Figure 3. The most thermodynamically stable adsorption structures of CH4, CO2, and H2S on (a–c) MnN4-GPs, (d–f) FeN4-GPs, (g–i) CoN4-GPs, (j–l) NiN4-GPs, (m–o) CuN4-GPs, and (p–r) MoN4-GPs, respectively.
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Figure 4. The adsorption energies of the optimal adsorption configurations of gas molecules on MN4-GPs: (a) MnN4-GPs, (b) FeN4-GPs, (c) CoN4-GPs, (d) NiN4-GPs, (e) CuN4-GPs, and (f) MoN4-GPs.
Figure 4. The adsorption energies of the optimal adsorption configurations of gas molecules on MN4-GPs: (a) MnN4-GPs, (b) FeN4-GPs, (c) CoN4-GPs, (d) NiN4-GPs, (e) CuN4-GPs, and (f) MoN4-GPs.
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Materials 18 02907 g005
Figure 5. The difference charge density maps of CH4, CO2, and H2S on (a–c) CoN4-GPs, (d–f) NiN4-GPs, (g–i) CuN4-GPs, and (j–l) MoN4-GPs with the same isosurface value of 5 × 10−4 electrons/Å3, respectively.
Figure 5. The difference charge density maps of CH4, CO2, and H2S on (a–c) CoN4-GPs, (d–f) NiN4-GPs, (g–i) CuN4-GPs, and (j–l) MoN4-GPs with the same isosurface value of 5 × 10−4 electrons/Å3, respectively.
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Materials 18 02907 g006
Figure 6. The COHP curves and ICOHP values of (a–d) CH4 on MN4-GPs (M = Co, Ni, Cu, Mo), (e–h) CO2 on MN4-GPs (M = Co, Ni, Cu, Mo), and (i–l) H2S on MN4-GPs (M = Co, Ni, Cu, Mo), respectively.
Figure 6. The COHP curves and ICOHP values of (a–d) CH4 on MN4-GPs (M = Co, Ni, Cu, Mo), (e–h) CO2 on MN4-GPs (M = Co, Ni, Cu, Mo), and (i–l) H2S on MN4-GPs (M = Co, Ni, Cu, Mo), respectively.
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MDPI and ACS Style

Wei, S.; Tian, X.; Rao, Z.; Wang, C.; Tang, R.; He, Y.; Luo, Y.; Fan, Q.; Fan, W.; Hu, Y. The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S. Materials 2025, 18, 2907. https://doi.org/10.3390/ma18122907

AMA Style

Wei S, Tian X, Rao Z, Wang C, Tang R, He Y, Luo Y, Fan Q, Fan W, Hu Y. The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S. Materials. 2025; 18(12):2907. https://doi.org/10.3390/ma18122907

Chicago/Turabian Style

Wei, Shiqian, Xinyu Tian, Zhen Rao, Chunxia Wang, Rui Tang, Ying He, Yu Luo, Qiang Fan, Weifeng Fan, and Yu Hu. 2025. "The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S" Materials 18, no. 12: 2907. https://doi.org/10.3390/ma18122907

APA Style

Wei, S., Tian, X., Rao, Z., Wang, C., Tang, R., He, Y., Luo, Y., Fan, Q., Fan, W., & Hu, Y. (2025). The Potential of MN4-GPs (M = Mn, Fe, Co, Ni, Cu, Mo) as Adsorbents for the Efficient Separation of CH4 from CO2 and H2S. Materials, 18(12), 2907. https://doi.org/10.3390/ma18122907

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