Dissociation of O₂ and its reactivity on O/S doped graphene

Abstract

It is routinely believed that the oxidation of SO₂ to SO₃ dominates the removal rate of SO₂ on carbon-based catalysts. Recently, both experiment and theoretical calculations evidence that SO₂ is readily oxidized by epoxy groups on graphene oxides at room temperature. Based on this fact, we hypothesize in this study that the real rate-determining step for SO₂ catalytic oxidation under O₂ atmosphere could be the dissociation of molecular O₂, which further forms oxygen functional groups on the graphene surface. Density functional theory corrected with dispersion was employed to investigate the dissociation of O₂ on O or S doped graphene and then its reactivity for SO₂ oxidation. The results showed that O/S doping greatly promotes the dissociation, which leads to the formation of epoxy and/or carbonyl groups on the graphene surface. However, a high oxidation barrier for the oxidation of SO₂ by the carbonyl group was found, which implies that the carbonyl group is of low reactivity. Therefore, dopant screening or the design of doped structures should be carefully considered to avoid the formation of carbonyl during O₂ dissociation.

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1 Introduction

Sulfur dioxide (SO₂) is one of the notorious pollutants with toxicity, which is released by the burning of fossil fuels, such as in automobile engines and power plants, and contributes greatly to acid deposition. Great efforts have been made to develop techniques for its treatment. Among them, carbon materials, typically activated carbon (AC) and activated carbon fibres (ACF), have long been known to be general media for the catalytic removal of SO₂ at low temperature (20–150 °C).^1–4 However, their mechanism at the atomic level remains unclear, which hinders the development of more effective carbon-based catalysts for desulfurization.

It is conventionally thought that the rate-determining step for the catalytic removal of SO₂ by carbon is the oxidation of SO₂ to SO₃.⁵ However, it was found that the oxidation could be catalyzed by graphene oxides (GO) at room temperature.⁶ In addition, by density functional theory calculations, Zhang and coworkers^7,8 found that SO₂ can be oxidized by the epoxy groups of GO with a rather low energy barrier. Both experimental and theoretical results indicate that epoxy groups should be the active center for the oxidation of SO₂, which might not be the rate-determining step. Alternatively, we hypothesize that the real rate-determining step in the oxidation of SO₂ might be the formation of epoxy groups from the dissociation of sluggish O₂.

Graphene (GP) can be used as a model catalyst to provide insights for the catalytic oxidation of SO₂ on carbon materials.^6,9–11 Doping with foreign atoms has been confirmed to be an effective functionalization method.^12–16 For example, N-doped graphene was found to be active for the oxygen reduction reaction (ORR).^17–19 The ORR is a four-electron pathway, which is believed to depend on the spin density and atomic charge redistribution on the neighboring carbon atoms close to the doping sites.^18,20 Besides, Dai²¹ et al. proposed that the high activity of N-doped GP may be attributed to the larger electronegativity of N, which creates a positive charge on the adjacent C atoms. These results indicate that doping with other electronegative heteroatoms and the charge redistribution caused by them may favor the adsorption and reduction of O₂ as well. However, doping with some low electronegative elements, such as boron,^15,22 sulfur,^23,24 or their mixture,^25,26 have also significantly promoted the catalytic activity of GP. The charged sites created by breaking the electroneutrality of GP might be the key factor, regardless of the electronegativity of doping atoms.²²

In this study, S and O atoms were separately introduced into GP to investigate the probable dependence of O₂ molecule adsorption and dissociation on the electronegativity of the doping atoms, by using density functional theory (DFT) calculations. The electronegativity of the S atom is 2.58, which is close to that of the C atom, while the electronegativity of O is 3.44, which is much stronger than that of the C atom. Both experimental²⁷ and theoretical results^27,28 indicate that the substitution of internal atoms in graphene by heteroatoms is the key role for the promoted catalytic activity of graphene materials. Therefore, only the substitution of internal C atom(s) by O/S atoms will be considered. To determine the activity of oxygenic functional groups on doped-graphene, the adsorption and oxidation of SO₂ on the doped materials are calculated.

2 Models and methods

2.1 Computational models

A 3 × 6 graphene unit cell (with 72 carbon atoms, GP) was employed as the model substrate, as depicted in Fig. 1a. It was large enough for the reaction, SO₂ + ½O₂ → SO₃, according to our previous studies.^7,8 A vacuum region of 20 Å was added perpendicular to the graphene plane to minimize the interaction between different layers.²⁹ A single O atom took the place of one C atom (Fig. 1b), which is denoted as OG. A single S atom took the place of two neighboring C atoms (Fig. 1c), which is denoted as SG. The relaxed bond lengths of C–C in GP are 1.42 Å, which are consistent with the experimental data. The relaxed C–O bonds and C–S bonds are 1.48 and 1.86 Å, respectively. Although it has been confirmed that S can be doped in the plane of GP, the local structures remain unclear.²³ We tested two doping models: a single S atom replacing one and two internal C atom(s). It was found that the latter is more feasible (Fig. S1–S3†). Therefore, only the two C atoms replaced model was used, and is denoted as SG by default.

Fig. 1 Spin density (a–c) and total charge density (d–f) analysis of different surfaces. GP, OG and SG stand for pristine, O doped and S doped graphene, respectively. All the lengths are given in Å. The isosurface for (a)–(c) is 5 × 10⁻⁶ e Å⁻³.

2.2 Computational methods

All density functional theory (DFT) calculations were carried out with the code VASP5.2 (ref. 30 and 31) using the generalized gradient approximation with a Perdew–Burke–Ernzerhof (PBE) exchange and correlation functional.³² A plane-wave basis set with the cut-off energy of 400 eV was employed within the framework of the projector augmented-wave (PAW) method.³³ The Brillouin zone was sampled with 3 × 3 × 1 k-points sampled with Monkhorst–Pack method. Gaussian smearing was used, with a smearing width of 0.2 eV. The D2 method of Grimme³⁴ was used to describe the van der Waals (vdW) correction with default parameters. All the atoms, except those on the boundary, were relaxed and converged to 0.02 eV Å⁻¹. The carbon atoms on the boundary of the GP substrate were frozen in all directions. All the abovementioned parameters were sufficient to ensure that the total energy converged to within 1 meV per atom. The calculated bond lengths of C–C in graphene and O–O in an isolated O₂ molecule are 1.42 and 1.26 Å, respectively, which are consistent with published values.^35,36 The nudged elastic band (NEB) method^37,38 was used to search the minimum reaction pathway (MEP) of O₂ reduction by heteroatom-doped graphene from the initial state (IS) to its final state (FS) with 8–12 replicas interpolated. The transition state (TS) was localized with the climbing image method and verified with a single imaginary frequency.

The adsorption energy, ΔE_ads, is defined as follows:

ΔE_ads = E_tot − (E_mol + E_sheet)

where E_tot, E_mol, and E_sheet are the total energies of the adsorption complex, the isolated molecule and the GP/doped-GP sheet, respectively.

3 Results

3.1 S and O doped graphene

We first focus on the electronic structures of pristine and doped-graphene. It is believed that spin density and positive charge can be introduced by electronegative atoms doping, which are of importance for catalytic activity promotion. Spin density determines the positional selectivity of radical adsorption, while charge density determines the adsorption energy.²⁰ Fig. 1b shows that spin density was induced by O doping in an up and down pattern alternately among 1–4 C atoms from the dopant, while the spin was localized at the S atom for SG, as demonstrated in Fig. 1c. The charge density in Fig. 1e illustrates that electrons were transferred from C1–C3 atoms to the dopant O atom, which results in positively charged C atoms for species adsorption. The redistribution of charge densities of C atoms in SG is almost unperceivable. Consequently, the order for the catalytic activity of the three different samples might be GP < SG < OG, and the C atoms directly connected to dopants should be regarded as active sites. Density of states (DOS) analysis showed that the main electron structures of S- and O-doped GP remained as that of the pristine GP (Fig. S4†).

3.2 Adsorption and dissociation of O₂ molecule

3.2.1 O₂ adsorption. The adsorption of O₂ on GP, OG and SG surfaces were calculated both with non-spin and spin polarization, as shown in Fig. 2a–c and d–f, respectively. Spin polarization appears to relieve the adsorption of O₂ by pristine or doped planes, because the distances from the adsorbed O₂ to the substrates are much longer (by 0.1–0.3 Å) than those without spin polarization. In both the conditions, O₂ is pulled closer to the surface in the case of S/O doping. For the adsorption of O₂ on GP, the adsorbed O₂ is parallel above the GP surface with a distance of 3.13 Å (Fig. 2d), which is in good agreement with the published value of 3.09 Å.³⁹ The adsorption distances increase in the order OG < SG < GP. However, all the bond lengths of adsorbed O₂ are ca. 1.26 Å (Table 1), which is quite close to the value of isolated gaseous molecules, thus denoting that only physisorption exists.

Fig. 2 Relaxed adsorption configurations of O₂ on GP, OG and SG surfaces. Spin polarization was ignored in (a)–(c), while it was included for (d)–(f). All the distances are given in Å.

Table 1 Summary of adsorption energy for the adsorption of O₂ on GP, OG and SG surfaces

Conf.	O2/GP		O2/SG		O2/OG
	NSP^a	SP^b	NSP	SP	NSP	SP
a Non-spin polarization.b Spin polarization.
ΔEads, eV	0.34	0.13	0.36	0.16	0.57	0.26
O–O bond length, Å	1.26	1.25	1.27	1.25	1.26	1.26

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As tabulated in Table 1, the adsorption energies obtained with spin polarization are lower than those with non-spin polarization correspondingly, and the adsorption energies increase in the order GP < SG < OG, both of which are consistent with the geometric trends mentioned above. The adsorption energy of O₂ on GP in Fig. 2d is 0.14 eV and this is also in good agreement with the previous calculated value of 0.11 eV,³⁹ while it is slightly larger than the experimentally tested value of 0.1 eV.⁴⁰

Based on the geometric and energetic results, spin polarization possesses few effects and can be ignored for qualitative conclusions.

3.2.2 O₂ dissociation. Fig. 3 shows the MEP for the oxygen dissociation reaction on pristine GP. Totally, the oxygen dissociation on GP is endothermic by 0.72 eV, which indicates that the process is thermodynamically unfavorable. During the dissociation process, a middle state (MS) emerged, which divided the reaction into two sections. In the MS configuration, two oxygen atoms and the two carbon atoms exactly below them formed a square, with a side length of 1.50 Å. From the IS to MS, the energy barrier is 1.78 eV, and a net energy consumption is 1.23 eV. During the process, O₂ is pulled closer to the carbon basal plane. The bond length of the O–O bond in the initial state (IS) is 1.26 Å, which is equal to the calculated bond length of an isolated O₂ molecule in the gaseous phase. The TS1 is more similar to the middle state (MS) than the reactant (IS) in the geometric structure. The controlling factor appears to pull the O₂ molecule close to the substrate and elongate the O–O bond to form the peroxide ion, O₂²⁻. From the MS to FS, the energy barrier is 0.62 eV, with a net energy release of 0.51 eV. In the final state (FS), the bond lengths of the O–C bonds are 1.45 or 1.48 Å, which are equivalent to the bond length of epoxy. Finally, the O₂ molecule was dissociated and transferred to two epoxy groups endothermically with a rather high barrier, implying that the reaction on pristine graphene is unfeasible under mild conditions.

Fig. 3 Minimum energy pathway (MEP) for the dissociation of oxygen on pristine graphene (GP). All the lengths are given in Å.

The MEP for the oxygen dissociation on doped-GP is demonstrated as Fig. 4 for OG and Fig. 5 for SG. On the approach of O₂ to OG (from the IS to MS), the bond length of O₂ was elongated from 1.26 to 1.38 Å, where O₂ was converted to O₂⁻ (superoxide ion). The barrier of this process was about 0.01 eV according to our testing calculations, with a net energy release of 2.04 eV. The C atom connected to the adsorbed O₂ was drawn out of the OG surface, which was one of the nearest C atoms originally connected to the doped O atom.

Fig. 4 Minimum energy pathway (MEP) for the dissociation of oxygen on O-doped graphene (OG). All lengths are given in Å.

Fig. 5 Minimum energy pathway (MEP) for the dissociation of oxygen on S-doped graphene (SG). All the lengths are given in Å.

From the MS to FS in Fig. 4, the reaction energy barrier is as low as 0.05 eV, with a net energy release of 1.07 eV. During this process, O₂⁻ was pulled closer to the substrate. The bond length of the O–O bond was further elongated from 1.38 Å to 1.41 Å in the TS, and then to 2.68 Å in the FS, which means that the O–O bond in oxygen was thoroughly broken. The determining step appears to be the breaking of the O–O bond of the adsorbed superoxide ion. After the dissociation, one O atom of the O₂ molecule was connected with the two C atoms on the OG surface to form an epoxy group, and the other O atom with one C atom to form a carbonyl group. The two bonds of the epoxy are about 1.48 and 1.42 Å, which are in accordance with the calculation results in ref. 8.

The MEP of O₂ dissociation on SG is shown in Fig. 5. Moreover, there are two energy barriers for the dissociation of O₂, and obviously the first energy barrier is the main one, with a value of 0.39 eV. Similar to the situation on GP, the determining step was the activation of the adsorbed O₂ molecule to form the superoxide ion, with the O–O bond elongated from 1.26 to 1.34 Å. Then, the O–O bond was further elongated, with a barrier of 0.19 eV to form two carbonyl groups (FS), with the O–C bond length of 1.23 Å.

3.3 SO₂ oxidation by dissociated O₂

It is known that heteroatom doping can promote the dissociation of oxygen on graphene. However, the reactivity of the formed oxygen groups is unknown. Thus, we tested the oxidation of SO₂ on two doped-GPs. For conciseness, we denoted the dissociated adsorption configuration of O₂ on OG (SG) as 2O_OG (2O_SG). When one oxygen group was consumed through the oxidation reaction, the left species was denoted as 1O_OG (1O_SG). Selected adsorption and reaction energies for the SO₂ oxidation are listed in Table 2. Combined with energetic data analysis for the dissociation of O₂, it is interesting to find that the two catalytic loops are thermodynamically favorable. However, the rate-determining step for the catalytic oxidation of SO₂ by OG is SO₂/1O_OG → SO₃/OG, with a barrier of 2.46 eV, and that by SG for SO₂/2O_SG → SO₃/1O_SG is 2.57 eV. Both pathways are not kinetically promising as expected because their rate-determining barriers are unsatisfactory.

Table 2 Summary of adsorption energies (ΔE_ads), energy barriers (E_b) and reaction energies (E_r) for the adsorption of SO₂/SO₃ on different surfaces

Adsorption	ΔEads, eV	Reaction	Eb, eV	Er, eV
SO2/2O_OG	−0.35	SO2/2O_OG → SO3/1O_OG	0.79	−1.79
SO2/1O_OG	−0.46	SO2/1O_OG → SO3/OG	2.46	1.48
SO2/2O_SG	−0.27	SO2/2O_SG → SO3/1O_SG	2.57	0.62
SO2/1O_SG	−0.37	SO2/1O_SG → SO3/SG	1.41	−0.03

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According to previous studies,^6,8 the epoxy group should be responsible for the high activity of graphene oxides for the oxidation of SO₂. When O₂ was dissociated on OG, an epoxy group and a carbonyl group were formed in the configuration 2O_OG, while for 2O_SG, there were two carbonyl groups formed. The carbonyl group is supposed to be sluggish for oxidation. The MEP for the oxidation of SO₂ by 2O_OG is shown in Fig. 6 to provide some evidence for further understanding the oxidation mechanism.

Fig. 6 Minimum energy pathway (MEP) for the oxidation of SO₂ by 2O_OG. All the lengths are given in Å.

During the first section from the IS to MS, the epoxy group was transferred to the carbonyl group as well, with a small barrier of 0.23 eV. The oxidation barrier from the MS to FS was 0.79 eV, with a net energy release of 1.79 eV. Such a high barrier confirms that the carbonyl group is inertial for oxidation. This is quite different for the oxidation of SO₂ by the epoxy group located on pristine GP,⁸ where the barrier is as low as ca. 0.2 eV. The main difference should be the extension of the O–C bond during oxidation. For the epoxy group, the O–C bond was extended from 1.48 to 1.81 Å, with an extension of 0.33 Å, while for the carbonyl group, the O–C bond was extended from 1.27 to 1.62 Å, with an extension of 0.35 Å. The shorter initial O–C bond length and the longer extension result in a much higher oxidation barrier. Accordingly, it is rationally expected that the barriers for the oxidation processes SO₂/1O_OG → SO₃/OG, SO₂/2O_SG → SO₃/1O_SG and SO₂/1O_SG → SO₃/SG are also much higher, as carbonyl groups were involved.

4 Discussion

It has been found that the dissociation of O₂ on graphene can be efficiently promoted by heteroatom doping; however, the oxidation activities of oxygen species derived from O₂ dissociation are quite different and still unsatisfactory. Herein, attention is paid to the electronic structures of different oxygen species to obtain a fundamental understanding.

As shown in Fig. 7, compared with the situation of the epoxy group on GP (Fig. 7a), the valence band of the epoxy group on OG (Fig. 7b) is shifted to a lower level. Besides the shift of the primary highest occupied molecular orbital (HOMO, marked as A in the plot), a new peak (marked as B) appears, as presented in Fig. 7b. This implies that the epoxy group of OG is more inertial than that of GP. Fig. 7c shows a low level peak (marked as C), which is present below −20 eV. This confirms that the carbonyl group is considerably sluggish for oxidation, because the cost of energy is prohibitive to activate it.

Fig. 7 Density of states (DOS) analysis for oxygen functional groups on different surfaces. (a) Epoxy group on GP surfaces, (b) epoxy group on OG surfaces, and (c) carbonyl group on SG surface.

Based on electronic structure analysis, it was found that the formation of the highly active epoxy group from O₂ dissociation should be the key point for the design and synthesis of graphene-based redox catalysts. In the present study, both O and S doping resulted in the formation of a carbonyl group, which is inertial in the catalytic oxidation process. Other dopants or doping patterns might be more promising.

5 Conclusion

Density functional theory corrected with dispersion was used to investigate the potential promotion effects of O and S doping in graphene on oxygen dissociation and catalytic oxidation of SO₂. It was found that O/S doping tremendously promotes the dissociation of sluggish molecular oxygen, which leads to the formation of epoxy and/or carbonyl groups on the doped graphene surfaces. Due to the formation of carbonyl group, the catalytic loop for the oxidation of SO₂ is terminated because of its high oxidation barrier. These results indicate that more attention should be paid on dopant screening or the design of doped structures to avoid the formation of carbonyl in O₂ dissociation. Further exploration is needed to screen other dopants or doping patterns.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (51508356, 51378325) and Young Talent Program for Science and Technology Innovation of Sichuan Province in China (2015MZGC001). We also acknowledge the National Supercomputer Center in Shenzhen of China for computational service support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16789b

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