DFT Study of N2O Adsorption onto the Surface of M-Decorated Graphene Oxide (M = Mg, Cu or Ag)

In order to reduce the harm of nitrous oxide (N2O) on the environment, it is very important to find an effective way to capture and decompose this nitrous oxide. Based on the density functional theory (DFT), the adsorption mechanism of N2O on the surfaces of M-decorated (M = Mg, Cu or Ag) graphene oxide (GO) was studied in this paper. The results show that the effects of N2O adsorbed onto the surfaces of Mg–GO by O-end and Cu–GO by N-end are favorable among all of the adsorption types studied, whose adsorption energies are −1.40 eV and −1.47 eV, respectively. Both adsorption manners belong to chemisorption. For Ag–GO, however, both the adsorption strength and electron transfer with the N2O molecule are relatively weak, indicating it may not be promising for N2O removal. Moreover, when Gibbs free energy analyses were applied for the two adsorption types on Mg–GO by O-end and Cu–GO by N-end, it was found that the lowest temperatures required to undergo a chemisorption process are 209 °C and 338 °C, respectively. After being adsorbed onto the surface of Mg–GO by O-end, the N2O molecule will decompose into an N2 molecule and an active oxygen atom. Because of containing active oxygen atom, the structure O–Mg–GO has strong oxidizability, and can be reduced to Mg–GO. Therefore, Mg–GO can be used as a catalyst for N2O adsorption and decomposition. Cu–GO can be used as a candidate material for its strong adsorption to N2O.


Introduction
As a kind of harmful gas, Nitrous oxide can cause severe environmental problems, such as greenhouse effect, ozone depletion, acid rain and photochemical pollution [1][2][3]. Generally, it comes from biomass combustion, industrial production, selective catalytic reduction of NO x as well as the nitrification and denitrification of microorganisms in soil and water [4,5]. Although the concentration of N 2 O in the atmosphere is only 322 ppb at present, its global warming potential is more than 300 times as much as that of CO 2 , because it has a long atmospheric life span of 120 years [6]. Therefore, reducing the anthropogenic emissions of N 2 O is conducive to the protection of the climate environment.
In recent years, many researches have been focused upon the adsorption and decomposition of N 2 O on various catalyst surfaces. Through the experimental studies, it is known that spinel Ni 0.75 Co 2.25 O 4 modified with cesium cations [7], Fe/SSZ-13 [8], Co-Mn-Al mixed oxides [9] and Cu-Zn/ZnAl 2 O 4 [10] have good catalytic effects on the N 2 O decomposition.
Carabineiro et al. [11] also found that the order of catalytic activity of the following metal oxides for N 2 O decomposition is: Fe 2 O 3 > CeO 2 > ZnO > TiO 2 > Al 2 O 3 , and doping Au into these metal oxides contributes to the reduction of an active oxygen atom on their surfaces. The decomposition

Model and Computational Methods
We performed the geometry optimization calculation by the density functional theory (DFT) of the first principles [35,36], using the DMol 3 module in the Materials Studio. The generalized gradient approximation (GGA) method with the Perdew-Burke-Emzerhof (PBE) function was selected for the spin unrestricted DFT-D2 computation [37][38][39][40][41]. In calculations, the double-numeric quality basis set with polarization functions (DNP) was selected, and the core treatment method DFT Semi-core Pseudopots (DSSP) was conducted. The calculation tasks were carried out with the accuracy of coarse-medium-fine, and in the fine accuracy, the convergence of energy and force were 1.0 × 10 −5 Ha and 2.0 × 10 −3 Ha/Å, respectively. The self-consistent field (SCF) tolerance was 1.0 × 10 −6 . In the electronic setting and density of state (DOS) calculation setting, the Brillouin zone is sampled with 8 × 8 × 1 k-points and 4 × 4 × 1 k-points under the Monkhorst-Pack scheme, respectively. The supercell of graphene was composed of 5 × 5 repeating units (containing 50 carbon atoms), where the length of crystal lattice parameters a × b × c is 12.30 Å × 12.30 Å × 25.00 Å. The length of the c direction is sufficient to eliminate the effect of the pseudopotential interaction between the adjacent cell systems. The computational formula of the adsorption energy for adsorbate (A) adsorbed on adsorbent (B) is E ads (A) = E (A + B) − E (A) − E (B), in the formula, E (A + B), E (A) and E (B) is the total energy of adsorption structure of A adsorbed on B, adsorbate (A) and adsorbent (B) at 0 K, respectively. The basis set superposition error (BSSE) was not considered, since previous studies have shown that the numerical basis sets implemented in DMol 3 can minimize or even eliminate BSSE [28,41,42].

N 2 O Adsorbed on the Surface of GO
The graphene oxide structure in this article contains an epoxy functional group, as showed in Figure 1a. The oxygen atom (O1) was located above the center of the bond of two neighboring carbon atoms (C1, C2), the lengths of O1-C1 bond and O1-C2 bond are both 1.46 Å, the bond angle of ∠O1-C1-C2 and ∠O1-C2-C1 are both 58.7 • (listed in Table 1). Compared with the primary graphene, the carbon atoms C1 and C2, which are under the oxygen atom O1 in the graphene oxide, are protruded from the plane of the primary graphene, while the distance between the two atoms and the plane is 0.42 Å. The length of C1-C2 bond is 1.51 Å, which is larger than the carbon-carbon bond length (1.42 Å) in the primary graphene. Figure 1b,c are the structural diagrams of an N 2 O molecule adsorbed on the surface of graphene oxide through O-end and N-end, respectively. In Figure 1b, the distance between the N 2 O molecule and oxygen atom (O1) is 2.97 Å, and the adsorption energy of this N 2 O molecule adsorbed on the surface of graphene oxide is −0.06 eV, while the distance is 3.35 Å and the adsorption energy is −0.03 eV in Figure 1c. It shows that the adsorption of the N 2 O molecule on the surface of graphene oxide by O-end is slightly stronger than by N-end, and both belong to physisorption. These adsorption energy values are lower than that of N 2 O molecules adsorbed on the surface of primary graphene, which is −0.07 eV [25]; this phenomenon indicates that the oxygen atom (O1) on graphene oxide weakens the interaction between the N 2 O molecule and graphene surface. The Hirshfeld charges of N 2 O, GO, N 2 O-GO (O-end) and N 2 O-GO (N-end) are listed in Table 2; the latter two are the structures of N 2 O molecules adsorbed onto the surface of graphene oxide through O-end and N-end. It can be found that the charge of each atom in N 2 O and GO does not change obviously before and after the adsorption of N 2 O on the surface of graphene oxide by O-end or N-end, which indicates that there is no charge transfer between N 2 O and GO, and means that the interaction between them is weak.

Mg-, Cu-and Ag-Decorated Graphene Oxide
The top view and side view of the structures of the Mg, Cu and Ag atom-modified graphene oxide (Mg-GO, Cu-GO and Ag-GO) are shown in Figure 2. The detailed structural parameters of Mg-GO, Cu-GO and Ag-GO are listed in Table 1. The bond length of the C1-O1 bond in Mg-GO, Cu-GO and Ag-GO is 1.47 Å, 1.47 Å and 1.45 Å, respectively, which indicates that the differences between bond lengths are small. However, the bond angle of ∠O1-C1-C2 increases from 58.7° to 100°, and the distance between the oxygen atom O1 and the carbon atom C2 is greater than 2.26 Å, indicating that after Mg, Cu or Ag doping, the C2-O1 bond breaks on the surface of graphene oxide. Moreover, the oxygen atom O1 shifts from the top of the center of our C1-C2 bond to the top of the carbon atom C1. Thus, the C1-O1 bond becomes almost perpendicular from inclining to the graphene plane. The lengths of the Mg-O1 bond in Mg-GO, the Cu-O1 bond in Cu-GO, and the Ag-O1 bond in Ag-GO are 1.87 Å, 1.80 Å, and 2.10 Å, respectively. The first two are similar to the length of the Fe-O bond in Fe-GO (1.83 Å), but longer than the length of the Al-O bond in Al-GO (1.70 Å) and the Si-O bond in Si-GO (1.70 Å) [42]. It is clear that the interaction between Ag and O in Ag-GO is the weakest among these metal atoms-decorated graphene oxide, since the length of Ag-O1 bond is the longest. The bond angles of ∠C1-O1-Mg, ∠C1-O1-Cu and ∠C1-O1-Ag are 106.7°, 120.4° and 159.4°, respectively. From the top view in Figure 2, it can be seen that the angle between the Mg-O1 bond and C1-C2 bond is approximately 120°, while the angle between the Cu-O1 bond (or Ag-O1 bond) and C1-C2 bond is approximately 180°. In the side view of Figure 2, the vertical distances between the Mg, Cu and Ag atoms and the graphene plane are marked as 2.71 Å, 3.06 Å and 3.16 Å, respectively.   Table 1. The bond length of the C1-O1 bond in Mg-GO, Cu-GO and Ag-GO is 1.47 Å, 1.47 Å and 1.45 Å, respectively, which indicates that the differences between bond lengths are small. However, the bond angle of ∠O1-C1-C2 increases from 58.7 • to 100 • , and the distance between the oxygen atom O1 and the carbon atom C2 is greater than 2.26 Å, indicating that after Mg, Cu or Ag doping, the C2-O1 bond breaks on the surface of graphene oxide. Moreover, the oxygen atom O1 shifts from the top of the center of our C1-C2 bond to the top of the carbon atom C1. Thus, the C1-O1 bond becomes almost perpendicular from inclining to the graphene plane. The lengths of the Mg-O1 bond in Mg-GO, the Cu-O1 bond in Cu-GO, and the Ag-O1 bond in Ag-GO are 1.87 Å, 1.80 Å, and 2.10 Å, respectively. The first two are similar to the length of the Fe-O bond in Fe-GO (1.83 Å), but longer than the length of the Al-O bond in Al-GO (1.70 Å) and the Si-O bond in Si-GO (1.70 Å) [42]. It is clear that the interaction between Ag and O in Ag-GO is the weakest among these metal atoms-decorated graphene oxide, since the length of Ag-O1 bond is the longest. The bond angles of ∠C1-O1-Mg, ∠C1-O1-Cu and ∠C1-O1-Ag are 106.7 • , 120.4 • and 159.4 • , respectively. From the top view in Figure 2, it can be seen that the angle between the Mg-O1 bond and C1-C2 bond is approximately 120 • , while the angle between the Cu-O1 bond (or Ag-O1 bond) and C1-C2 bond is approximately 180 • . In the side view of Figure 2, the vertical distances between the Mg, Cu and Ag atoms and the graphene plane are marked as 2.71 Å, 3.06 Å and 3.16 Å, respectively.
The adsorption energies of Mg, Cu and Ag atoms adsorbed on the surface of GO are −1.76 eV, −1.84 eV and −1.13 eV, respectively, all belonging to chemisorption. The Hirshfeld charges of atoms or the partial structure in Mg-GO, Cu-GO and Ag-GO are listed in Table S1 in Supplementary Information. It can be found that the charges of the oxygen atom and graphene part in GO increase after doping the Mg, Cu and Ag atoms. Although the adsorption energy of Mg adsorbed on GO and the distance between the Mg atom and O1 atom are both slightly lower than those of Cu, the interaction between the Mg atom and the graphene surface is the strongest among the three metal atoms, because the distance between Mg and the graphene surface is the shortest, and the charge transfer after Mg doping is the strongest.   The adsorption energies of Mg, Cu and Ag atoms adsorbed on the surface of GO are −1.76 eV, −1.84 eV and −1.13 eV, respectively, all belonging to chemisorption. The Hirshfeld charges of atoms or the partial structure in Mg-GO, Cu-GO and Ag-GO are listed in Table S1 in Supplementary Information. It can be found that the charges of the oxygen atom and graphene part in GO increase after doping the Mg, Cu and Ag atoms. Although the adsorption energy of Mg adsorbed on GO and the distance between the Mg atom and O1 atom are both slightly lower than those of Cu, the interaction between the Mg atom and the graphene surface is the strongest among the three metal atoms, because the distance between Mg and the graphene surface is the shortest, and the charge transfer after Mg doping is the strongest. Figure 3a is the local density of state (LDOS) graph of the Mg-GO structure. It can be seen that there are two peaks at 0.13 eV higher than the Fermi energy for Mg-s (red) and −1.89 eV lower than Fermi energy for O-p (blue).

System
Bond  Figure 3a is the local density of state (LDOS) graph of the Mg-GO structure. It can be seen that there are two peaks at 0.13 eV higher than the Fermi energy for Mg-s (red) and −1.89 eV lower than Fermi energy for O-p (blue).
Both the two peaks contribute to the peak of the corresponding region of the total state density Mg-GO-sum (black), indicating the 3s bands for Mg and 2p bands for the oxygen atom O1 are well bound to yield a set of hybridized p bands, which leads to the formation of an Mg-O1 bond with a length of 1.87 Å. Figure 3b is the LDOS of the Cu-GO structure. There are two peaks at approximately −2.83 eV and −0.88 eV below the Fermi energy for O-p (blue) and a peak at −1.57 eV below the Fermi energy for Cu-d (red). All the three peaks contribute to the peak of the corresponding region of the total density of state density Cu-GO-sum (black), which shows that 3d bands for Cu and 2p bands for the oxygen atom O1 are well hybridized, which results in the formation of the Cu-O1 bond with a length of 1.80Å. Figure 3c is the LDOS of the Ag-GO structure. It can be observed that there is a peak at −3.60 eV below the Fermi energy for Ag-d (red), where a peak appears at the corresponding region for the total density of state density Ag-GO-sum (black). However, there is a peak at −1.20 eV below the Fermi energy for O-p (blue), where no clear peak appears for Ag-GO-sum (black). It indicates that the interaction between Ag and the oxygen atom O1 in GO is not strong.
Mg-GO-sum (black), indicating the 3s bands for Mg and 2p bands for the oxygen atom O1 are well bound to yield a set of hybridized p bands, which leads to the formation of an Mg-O1 bond with a length of 1.87 Å. Figure 3b is the LDOS of the Cu-GO structure. There are two peaks at approximately −2.83 eV and −0.88 eV below the Fermi energy for O-p (blue) and a peak at −1.57 eV below the Fermi energy for Cu-d (red). All the three peaks contribute to the peak of the corresponding region of the total density of state density Cu-GO-sum (black), which shows that 3d bands for Cu and 2p bands for the oxygen atom O1 are well hybridized, which results in the formation of the Cu-O1 bond with a length of 1.80Å. Figure 3c is the LDOS of the Ag-GO structure. It can be observed that there is a peak at −3.60 eV below the Fermi energy for Ag-d (red), where a peak appears at the corresponding region for the total density of state density Ag-GO-sum (black). However, there is a peak at −1.20 eV below the Fermi energy for O-p (blue), where no clear peak appears for Ag-GO-sum (black). It indicates that the interaction between Ag and the oxygen atom O1 in GO is not strong.    Figure 4. It can be seen from Figure 4a that the structure of the N 2 O molecule has obvious changes after being adsorbed on the surface of Mg-GO by O-end. The distance between the oxygen atom O2 and the nitrogen atom N1 in the N 2 O molecule increases from 1.19 Å to 2.99 Å, the N1-O2 bond breaks; the distance between the nitrogen atoms N1 and N2 shortens from 1.14 Å to 1.11 Å, which is close to the length of the triple bond     Figure 5a, there are two overlapped peaks near the Fermi level and in the region of 1.8-2.8 eV higher than Fermi level, for Mg-sum and O2-p. It indicates that the 3s bands for the Mg atom and 2p bands for the oxygen atom O2 in N 2 O are well hybridized, which leads to the formation of the Mg-O2 bond with a length of 1.84 Å. However, there is no obvious overlapping peak between the DOS curves for Mg-s and N 2 O-p in Figure 5b, indicating that no strong interaction appears between the Mg atom and the terminal nitrogen atom N2 in the N 2 O molecule. stronger than that by O-end, and also explains the reason why the distance between N2O and Ag atoms is closer and the adsorption energy is larger when the N2O molecule is adsorbed on the surface of Ag-GO by N-end.
According to above discussion, it can be concluded that the N2O molecule decomposes into N2 and an active oxygen atom O, after being adsorbed on the surface of Mg-GO by O-end. It is the interaction between the N2O molecule and the Mg atom of the GO system that activates the N2O molecule and catalyzes the decomposition of N2O. The desorption energy of N2 from the structure N2-O-Mg-GO is −0.32 eV, and the deformed structure O-Mg-GO has strong oxidizability for the active oxygen atom O, which can be reduced to Mg-GO by reductants, such as CO. The adsorption energy is the highest and the distance between N2O and GO is the nearest. Thus, Mg-GO can be used as a kind of cyclic catalyst for N2O adsorption and decomposition.  Figure 6. It can be seen that there is only an overlapping peak for the DOS curves of Cu-d and N 2 O-sum, which is at the region of 1.8-2.7 eV higher than Fermi level, in Figure 6a. While in Figure 6b, it can be observed that there are four overlapping peaks for the DOS curves of Cu-d and N 2 O-sum, two of which are in the region of −12.0~−10.0 eV lower than the Fermi level, while the others are near −5.0 eV lower than and 1.6 eV higher than the Fermi level, respectively. It can be seen that the 3d bands for Cu and the p bands for N 2 O hybrid well, contributing to the formation of a Cu-N2 bond with a length of 1.79 Å. It indicates that the adsorption of N 2 O on the surface of Cu-GO by N-end is stronger than that by O-end, and the reason that the adsorption value of the former is larger than that of latter is also explained. The graphs in Figure 7 are the LDOS of the structures N 2 O-Ag-GO (O-end) and N 2 O-Ag-GO (N-end). Observing the DOS curves of Ag-d and N 2 O-sum, it can be found that there are two overlapping peaks near −5.5 eV lower than the Fermi level and in the region of 2.2~2.8 eV, respectively, which are higher than the Fermi level in Figure 7a. Whereas, no overlapping peak is found in Figure 7b. It shows that that the adsorption of N 2 O on the surface of Ag-GO by N-end is stronger than that by O-end, and also explains the reason why the distance between N 2 O and Ag atoms is closer and the adsorption energy is larger when the N 2 O molecule is adsorbed on the surface of Ag-GO by N-end.
According to above discussion, it can be concluded that the N 2 O molecule decomposes into N 2 and an active oxygen atom O, after being adsorbed on the surface of Mg-GO by O-end. It is the interaction between the N 2 O molecule and the Mg atom of the GO system that activates the N 2 O molecule and catalyzes the decomposition of N 2 O. The desorption energy of N 2 from the structure N 2 -O-Mg-GO is −0.32 eV, and the deformed structure O-Mg-GO has strong oxidizability for the active oxygen atom O, which can be reduced to Mg-GO by reductants, such as CO. The adsorption energy is the highest and the distance between N 2 O and GO is the nearest. Thus, Mg-GO can be used as a kind of cyclic catalyst for N 2 O adsorption and decomposition.

Gibbs Free Energy Analysis
The above analysis showed that the probability of having an N2O molecule adsorbed on the surface of Mg-GO via O-end is larger than that via N-end, while on the surface of Cu-GO, an  Figure 8. When│∆G│> 0.4 eV, the adsorption of N2O on the surface is strong, which could be classified as chemisorption; when│∆G│< 0.4 eV, the adsorption of N2O on the surface is weak, which could be seen as physisorption. From Figure 8, it can be found that when the temperature T1 > 482 K = 209 °C, and the Gibbs free energy│∆G1│> 0.4 eV, the adsorption of N2O on the surface of Mg-GO by the O-end is a chemisorption reaction. While for the adsorption of N2O on the surface of Cu-GO by N-end, the temperature T2 > 615 K = 338 °C, and the Gibbs free energy│∆G2│> 0.4 eV. It shows that the temperature range, in which the chemisorption of N2O on the surface of Mg-GO can successfully occur, is larger than that of Cu-GO.  Figure 8. When |∆G| > 0.4 eV, the adsorption of N 2 O on the surface is strong, which could be classified as chemisorption; when |∆G| < 0.4 eV, the adsorption of N 2 O on the surface is weak, which could be seen as physisorption. From Figure 8, it can be found that when the temperature T1 > 482 K = 209 • C, and the Gibbs free energy |∆G 1 | > 0.4 eV, the adsorption of N 2 O on the surface of Mg-GO by the O-end is a chemisorption reaction. While for the adsorption of N 2 O on the surface of Cu-GO by N-end, the temperature T2 > 615 K = 338 • C, and the Gibbs free energy |∆G 2 | > 0.4 eV. It shows that the temperature range, in which the chemisorption of N 2 O on the surface of Mg-GO can successfully occur, is larger than that of Cu-GO.

Conclusions
In this article, the adsorption of an N2O molecule on the surfaces of Mg-, Cu-or Ag-modified graphene oxide by O-end or N-end had been studied with the density functional theory. The results show that Mg, Cu and Ag modification have different effects for N2O catalytic decomposition. Compared to N-end, the adsorption of the N2O molecule on the surface of Mg-GO by O-end is more likely to occur, whose adsorption energy is −1.47 eV. After adsorption, an N2O molecule will decompose into an N2 molecule and an active oxygen atom, the latter is in the structure O-Mg-GO, which can be reduced by reductants to Mg-GO. While on the surface of Cu-GO, the N2O molecule is more likely to be adsorbed through the N-end, the adsorption energy is −1.40 eV, and there is no change for the configuration of this N2O molecule during the adsorption. According the Gibbs free energy analysis, the minimum temperature for the chemisorption of N2O on the surface of Mg-GO by O-end is 209 °C, and that of Cu-GO by the N-end is 338 °C. Therefore, Mg-GO can be used as a catalyst for N2O adsorption and decomposition. Cu-GO can be used as a candidate material for its strong adsorption to N2O.

Conclusions
In this article, the adsorption of an N 2 O molecule on the surfaces of Mg-, Cu-or Ag-modified graphene oxide by O-end or N-end had been studied with the density functional theory. The results show that Mg, Cu and Ag modification have different effects for N 2 O catalytic decomposition. Compared to N-end, the adsorption of the N 2 O molecule on the surface of Mg-GO by O-end is more likely to occur, whose adsorption energy is −1.47 eV. After adsorption, an N 2 O molecule will decompose into an N 2 molecule and an active oxygen atom, the latter is in the structure O-Mg-GO, which can be reduced by reductants to Mg-GO. While on the surface of Cu-GO, the N 2 O molecule is more likely to be adsorbed through the N-end, the adsorption energy is −1.40 eV, and there is no change for the configuration of this N 2 O molecule during the adsorption. According the Gibbs free energy analysis, the minimum temperature for the chemisorption of N 2 O on the surface of Mg-GO by O-end is 209 • C, and that of Cu-GO by the N-end is 338 • C. Therefore, Mg-GO can be used as a catalyst for N 2 O adsorption and decomposition. Cu-GO can be used as a candidate material for its strong adsorption to N 2 O.  Table S1: Hirshfeld charges of atoms or partial structure in those systems below (M = Mg, Cu or Ag).