3.2.1. Modification of the ELD-42C Graphene Configuration by a Single Mn Atom
Jia et al. [
2] found that pristine graphene doped with heteroatoms or pristine graphene with structural defects is significantly better able to adsorb gas molecules. Xu et al. [
44] believed that the adsorption properties of graphene for gas molecules can be best improved by modifying graphene with alkali metals, alkaline earth metals and TM. Mn is an important TM element that is widely distributed throughout the Earth’s crust. Its valence electron configuration is 3d
54s
2, and chemical bonds can be easily formed between Mn and carbon atoms [
45,
46,
47]. Due to its weak adsorption properties for CH
4 molecules, the ELD-42C graphene configuration was modified with TM Mn atoms to construct iMn-ELD-42C graphene configurations (where
i indicates the number of Mn atoms,
i = 1, 2). Their adsorption properties for CH
4 were then studied.
When the ELD-42C graphene configuration was modified by a single Mn atom, there were six optional adsorption sites for the Mn atoms: the hole sites, H1, H2 and H3; the bridge sites, B1 and B2; and the top site, T1 (see
Figure 1). A single Mn atom, respectively placed at T1, B1 and B2 during the construction of the Mn-ELD-42C configurations, always moved to the top of the adjacent carbon ring under the action of the chemical bonds as the structure was optimized. This is in line with the optimal adsorption site of TM atoms determined by Zhao et al. [
6] and Liu et al. [
48]. The adsorption characteristics of the Mn atoms at H1, H2 and H3 were calculated, and the results are given in
Table 2.
In the table,
indicates the binding energy of a single Mn atom; BL1, BL2, BL3 and BL4, respectively, refer to the length of the bonds between the Mn atoms and C atoms; and
represents the charge transfer between the Mn atoms and the ELD-42C graphene configuration. It can be seen from
Table 2 that the binding energy of a single Mn atom was different at H1, H2 and H3. At H2 it was −3.453 eV. This was the largest absolute value out of the three hole sites. The binding energy of a single Mn atom at H1 was −2.922 eV, which was the smallest absolute value. These results indicated that Mn atoms adsorbed above H2 were the most stable, while Mn atoms adsorbed above H1 were the least stable. During modification, four Mn-C chemical bonds were formed between the Mn atoms and the four carbon atoms at H2, with lengths of 2.967 Å, 2.974 Å, 2.974 Å and 2.981 Å, respectively. This suggests that most of the Mn atoms were adsorbed on the central axis of H2. The charge transferred from the Mn atoms to the ELD-42C graphene configuration was 0.33 e at H3 and 0.29 e at H1. Therefore, the interaction between the ELD-42C graphene configuration and Mn atoms adsorbed at H1 and H3 was weaker than it was at H2. The most stable Mn-ELD-42C graphene system configuration (
Figure 5) was therefore obtained via structural optimization after it had been modified by a single Mn atom at H2.
The Mulliken layout of the Mn-ELD-42C graphene configuration before and after adsorbing a single CH
4 molecule was analyzed. It was found that the charge transferred from the Mn atoms to the ELD-42C graphene configuration was 0.69 e, indicating a strong electrostatic effect between the two. The partial density of states (PDOS) for the Mn-ELD-42C graphene configuration is shown in
Figure 6 (partial). There were resonance peaks between the d orbit of the Mn atoms and the
p orbit of the C atoms within the range of −1.958 to −1.381 eV, confirming that there was interaction between the two orbits. As a result, the valence band of the Mn-ELD-42C graphene configuration largely derives from the interaction between the d orbit of the Mn atoms and the
p orbit of the C atoms. This is similar to the results obtained by Wu et al. [
49] and Zhao et al. [
37], who modified graphene substrates by using TM atoms as a doping agent. As extra electrons were provided to the ELD-42C graphene configuration by the Mn atoms, the overall conduction band of the configuration moved to the Fermi level, where the conduction band intersected with the valence band and endowed the Mn-ELD-42C graphene configuration with typical metallic-phase characteristics.
3.2.2. Adsorption of CH4 by the Mn-ELD-42C Graphene Configuration
DFT was used to study the CH
4 adsorption capacity of the Mn-ELD-42C graphene configuration by adding CH
4 molecules to one side. A stable CH
4Mn-ELD-42C adsorption configuration (
Figure 7a) was obtained after the Mn-ELD-42C graphene configuration with the first adsorbed CH
4 molecule had been optimized. The first CH
4 molecule was located above the Mn atom, proving that this was where the adsorption energy was the largest. The adsorption energy of this configuration was −1.717 eV, which is larger than that of the ELD-42C graphene configuration for CH
4 molecules (−0.847 eV), of Li-modified carbon nanotubes for CH
4 (−0.464 eV) [
50] and of Pt-modified graphene for CH
4 (−0.488 eV) [
51]. Apparently, modifying the ELD-42C graphene configuration with a single Mn atom improved its adsorption properties for CH
4 molecules. A 2CH
4Mn-ELD-42C adsorption configuration was obtained after a second CH
4 molecule had been added (
Figure 7b). Here, both the first and the second CH
4 molecules were located above the Mn atoms and close to the Mn-ELD-42C graphene configuration. The combined action of the mutual repulsion of the CH
4 molecules and their adsorption by the Mn-ELD-42C graphene configuration enabled a third CH
4 molecule to be adsorbed above the hexagonal carbon ring that was close to the Mn atoms (
Figure 7c). A fourth CH
4 molecule was adsorbed at T1 above the C atoms (
Figure 7d), and a fifth above the octagonal carbon ring (
Figure 7e). Limited by the adsorption space, the repulsive force between the molecules gradually increased as more CH
4 molecules were adsorbed. The adsorption configuration began to arc when an eighth molecule was adsorbed (
Figure 7f), and there was a stratification phenomenon when the ninth molecule was adsorbed (
Figure 7g). Due to their layered adsorption, the distances between the 9th–16th CH
4 molecules and the Mn atom became larger, and the adsorption energy was reduced. The 16th CH
4 molecule was nowhere near the Mn atom and was the most distant from the Mn-ELD-42C graphene configuration. It also had the lowest adsorption energy (−0.755 eV). When a 17th CH
4 molecule was placed on one side of the configuration, the calculated adsorption energy became positive, indicating that the gas molecule had not been adsorbed. This proved that the Mn-ELD-42C graphene configuration could only stably adsorb up to 16 CH
4 molecules on each side. The geometrical configuration is shown in
Figure 7h.
Table 3 shows the average adsorption energy,
, and the adsorption energy,
, of the jCH
4Mn-ELD-42C
jCH
4 adsorption configuration for CH
4 adsorption on one side and both sides; the distance,
, between the CH
4 molecules and the plane of the Mn-ELD-42C graphene configuration; the distance,
, between the CH
4 molecules and the Mn atoms; and the adsorption capacity (PBW) of the Mn-ELD-42C graphene configuration for CH
4 in the jCH
4Mn-ELD-42C
jCH
4 adsorption configuration. Analysis of these data reveals that the absolute value of the average adsorption energy,
, of the CH
4 gradually decreased as j, the number of CH
4 molecules adsorbed, increased. The adsorption energy,
, of 16 CH
4 molecules adsorbed on one side was compared. When the gas molecules were not stratified (the first–eighth molecules), the first, third, and seventh CH
4 molecules presented a higher adsorption energy than the other CH
4 molecules because their adsorption sites were close to Mn atoms and their distance from the plane of the graphene configuration was relatively small. When the gas molecules were stratified (the ninth–sixteenth molecules), the interaction between the CH
4 and Mn steadily decreased as the distance between them increased, leading to reduced adsorption properties.
The above results indicate that the adsorption properties of a Mn-ELD-42C graphene configuration are affected by Mn atom modification and that this can play an important role in CH4 adsorption. The adsorption energy was also affected by the distance between the CH4 molecules and the plane of the graphene configuration. The adsorption distance, , between the 16th CH4 molecule and the Mn-ELD-42C graphene configuration was 11.550 Å, which was the largest out of the 16 CH4 molecules. At this point, both the average adsorption energy, , and the adsorption energy, , of the configuration were at their lowest (−0.897 eV/CH4 and −0.755 eV, respectively).
Up to 16 CH
4 molecules could be stably adsorbed on one side of the Mn-ELD-42C graphene configuration, with an average adsorption energy of −0.897 eV/CH
4. On this basis, it can be calculated that the Mn-ELD-42C graphene configuration is able to stably adsorb up to 14 CH
4 molecules on the other side, making a total of 30 CH
4 molecules overall (
Figure 8), with an average adsorption energy of −0.867 eV/CH
4 and an adsorption capacity of 46.25 wt%. This is much closer to the proposed DOE standard (50 wt%) [
9]. The adsorption capacity of the Mn-ELD-42C graphene configuration was 1.02 times that of the basic ELD-42C graphene configuration (45.26 wt%). This makes it clear that the adsorption capacity for CH
4 molecules can be effectively improved by the modification of Mn atoms.
Table 4 gives the Mulliken layout of the Mn-ELD-42C graphene configuration before and after adsorbing one CH
4 molecule, where H
1, H
2, H
3 and H
4 stand for the H atoms and C represents the C atom of the CH
4 molecule. For the CH
4 molecule adsorbed above the Mn atom, H
1, H
2 and H
3 faced the plane of the Mn-ELD-42C graphene configuration, while H4 faced away from it (
Figure 6a). The charge for H
4 was 0.27 e before the CH
4 molecule was adsorbed and 0.36 e after the CH
4 molecule was adsorbed, with 0.09 e of charge having been lost. For free CH
4 molecules, the C atom is negatively charged, and the four peripheral H atoms are positively charged. This results in a strong repulsive force between CH
4 molecules, making it difficult for multiple CH
4 molecules to gather at the same adsorption site. For the Mn-ELD-42C graphene configuration, the ELD-42C graphene substrate was negatively charged, allowing the positively charged CH
4 molecules on the outer surface to be adsorbed more easily via electrostatic interaction. In the CH
4 molecules adsorbed on the Mn-ELD-42C graphene configuration, both H
1 and H
2 lost their partial positive charge because they received equal numbers of electrons. This reduced the surface area of the positively charged CH
4 molecule, weakening the repulsive force between the CH
4 molecules. In addition, before and after a single CH
4 molecule had been adsorbed by the Mn-ELD-42C graphene configuration, a relatively large charge transfer occurred with the Mn atoms, with 0.29 e of charge being lost. When CH
4 molecules were adsorbed, the electrons of the Mn atoms were transferred to the CH
4 molecules; therefore, a strong Coulomb force was produced between the two, creating favorable conditions for CH
4 adsorption.
Figure 9 illustrates the charge density difference for the CH
4Mn-ELD-42C adsorption configuration. This directly reveals the charge transfer between the Mn atoms and CH
4 molecules. The blue elements are the electron gain zone, where the CH
4 molecules obtained electrons, and the yellow elements are the electron loss zone, where the Mn atoms lost electrons. As the large charge transfer between the Mn atoms and CH
4 molecules produced a Coulomb force between them, the Mn atoms had a significant effect on CH
4 adsorption. This is consistent with the analysis of the Mulliken layout in
Table 4.
The interaction between the Mn-ELD-42C graphene configuration and CH
4 molecules was also analyzed in terms of the PDOS of the CH
4 molecules.
Figure 10a shows the PDOS of the CH
4Mn-ELD-42C adsorption configuration after adsorbing a single CH
4 molecule. The density of states (DOS) peak of the Mn atoms increased from 4.031 eV (before adsorption, as shown in
Figure 6) to 4.763 eV (after adsorption), and the energy range enlarged from (−4.107, 1.547 eV) before adsorption to (−4.341, 1.637 eV) after adsorption. As a consequence, the CH
4 adsorption enhanced the interaction between the Mn atoms and the Mn-ELD-42C graphene configuration, which is in accord with the analysis of the Mulliken layout in
Table 4. After a single CH
4 molecule had been adsorbed, the DOS valence band peak of the CH
4Mn-ELD-42C adsorption configuration improved because of the hybridization between the 3d orbit of the Mn atoms and the 1s orbit of the H atoms. The DOS of the C atom in the Mn-ELD-42C graphene configuration also changed slightly.
Figure 10b was used to analyze the interaction between the d orbit of the Mn atoms, the s orbit of the H atoms and the
p orbit of the C atom on the eight unstratified CH
4 molecules adsorbed on one side of the Mn-ELD-42C graphene configuration. It can be seen that the s orbit of the H atoms of the first CH
4 molecule overlapped with the 3d orbit of the Mn atoms near −16.0 eV and −8.0 eV. This suggests that there is an interaction between the first CH
4 molecule and the Mn atoms. Compared with the first CH
4 molecule, the 1s orbit of the second CH
4 molecule had shifted to the right, indicating that the interaction between the second CH
4 molecule and the Mn atoms had weakened, making the adsorption energy of the second CH
4 molecule smaller than that of the first CH
4 molecule. The displacement of the PDOS peak for the CH
4 molecules correlated with changes in the adsorption energy, with the PDOS peak moving to the left when the adsorption energy increased and to the right when the adsorption energy reduced. As the number of CH
4 molecules adsorbed increased, the PDOS peak reduced and moved to the right, showing that the interaction between the CH
4 molecules and Mn atoms was gradually weakening. In the interval [−6.989, −4.893 eV], the PDOS peak of the eighth CH
4 molecule was significantly lower than that of the other seven CH
4 molecules, indicating that the interaction between the 1s orbit of the H atoms of the eight CH
4 molecule and the 3d orbit of the Mn atoms was the weakest. This is consistent with the gradual decrease in the average adsorption energy when the CH
4 molecules were adsorbed by the Mn-ELD-42C graphene configuration.