Metal and Non-metal Atoms Modified Graphene as Efficient Catalysts

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Metal and Non-metal Atoms Modified Graphene as Efficient Catalysts for CO Oxidation Reactions Yanan Tang, Weiguang Chen, Huadou Chai, Gao Zhao, Yi Li, Dongwei Ma, and Xianqi Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00060 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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The Journal of Physical Chemistry

Metal and Non-metal Atoms Modified Graphene as Efficient Catalysts for CO Oxidation Reactions Yanan Tanga,1, Weiguang Chena,1, Huadou Chai a,b, Gao Zhaoa, Yi Lib, Dongwei Mac and Xianqi Daia,b* a

Quantum Materials Research Center, College of physics and Electronic Engineering, Zhengzhou Normal University, Zhengzhou 450044, China b College

of Physics and Materials Science, Henan Normal University, Xinxiang Henan, 453007, China

c

School of Physics, Anyang Normal University, Anyang 455000, China

Abstract This study is to explore the metal Co and nonmetal atoms co-doping graphene (CoNMx-graphene, x=1, 2, 3 and NM=N, Si, P) as substrates for CO oxidation reactions. By density functional theory (DFT) calculations, we show the formation mechanism of CoNM3-graphene configurations and their corresponding electronic structures and magnetic properties. On the CoNM3-graphene sheets, the adsorbed O2 is more stable than that of CO molecule and serves as the reactive species. Besides, the coadsorption of CO/O2 (or 2CO) has larger adsorption energies than that of the isolated O2 and CO molecule, which would be used as an initial state for the CO oxidation reactions. Furthermore, the possible reaction mechanisms for CO oxidation on CoNM3-graphene are investigated in detailed. It is found that the Eley-Rideal (ER) mechanism (CO + O2 → CO2 + Oads) on CoN3-graphene sheet has smaller energy barrier than that of another initial state (CO + O2 → CO3), which is an energetically more favorable than the Langmuir-Hinshelwood (LH) and new termolecular Eley-Rideal (TER) mechanism. Among the CoNM3-graphene sheets, the CO oxidation reactions through the completely ER reactions are more likely to proceed rapidly on CoN3-graphene (˂ 0.3 eV), which would be regarded as a potential graphene-based catalysts with high activity.

Corresponding

author. E-mail address: [email protected] (Y. Tang), [email protected] (X. Dai), Tel./Fax: +86 371 65501661. 1These authors contributed equally to this article and are considered co-ﬁrst authors.

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1. Introduction Graphene is a two-dimensional (2D) monolayer of sp2-hybridized carbon atoms, which has been considered as the most promising start in materials science due to its excellent physical and chemical properties

1-4.

The high electrical conductivity, chemical

stability and large surface area of graphene can be used as an ideal support for metal catalysts to realize new carbon-metal nanomaterials

5-7.

Unfortunately, there is weak

adsorption of metal adatoms or gas molecules on pristine graphene (pri-graphene), because there are inert nature of the -electrons conjugation formed by carbon atoms 8. Besides, the weak interaction allows the metal catalysts to move freely on pri-graphene and agglomerate to larger clusters, which limits the effectively use of metal atoms as robust. Therefore, the enormous efforts have been devoted to improve the catalytic performance of supported metal atoms by downing the metal nanoparticles 9-11. Currently, the single atom catalyst (SAC) as novel model systems exhibited long-term operation, high catalytic activity and excellent selectivity

12-14,

suggesting its great potential as

promising candidates for practical applications in the field of catalysis. Generally, the introduction of structural defects or impurity dopants have been proved to be an effective approachs to tune the surface reactivity and physical properties of graphene-based materials

15-21,

such as band structure, carrier concentration and

magnetism 18,22-24. In fact, structural defects do exist in graphene and can further enhance the functional performance of graphene. Meanwhile, defective structures can be deliberately introduced into graphene by ion irradiation or chemical treatments

25-28.

Recently, the theoretical studies of one or two metal dopants within graphene vacancy defects are observed

29,30.

The increased chemical reactivity through the created defects

sites offer the more possibilities for supported catalysts residing in the surface. On the other hand, much effort has been devoted to N, B, Si and P doping either during graphene synthesis or by post annealing in environment with foreign non-metal species 31-33, which increased catalytic properties due to the induced facile electron transfer on the graphene basal plane

34-36.

Furthermore, both experimentally and theoretical studies have been

reported that metal or non-metal co-doping (B-N, P-N, Co-Nx and Fe-Nx) could create a unique electronic structure of graphene (or carbon nanotube), the co-doping can be used as active sites to speed up the oxygen reduction reactions (ORR) 37-43. These observations

2


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illustrated that the co-doping can bring in special synergistic effects on the catalytic properties of graphene support. However, it is still very difficult to control the doping concentration, the exact doping and bonding formats, and need to understand the formation mechanism of co-doping elements on the surface of graphene, as well as their influence for the catalytic reactions. In contrast to the ORR behavior on the co-doping graphene, the less attention was devoted to the low-temperature carbon monoxide (CO) oxidation. In fact, the oxidation of CO is also a very importance reaction for solving the increasing environmental problems caused by automobiles exhaust and industrial processes

44-46.

Earlier theoretical studies

have demonstrated that doped heteroatom (Pd-47, Al-48, Pt-49, Fe-50 or Co-51) at the single vacancy of graphene (SV-graphene) is stable enough to be utilized for CO oxidation. Recently, the single-atom Si52, B53, N and P54 doped graphene could regulate the electronic structure, local curvature and surface reactivity of systems55. In addition, the metal-coordinated N-doped graphene (MNx) exhibits high catalytic activity for CO oxidation as compared with the normal metal catalysts 56-58. It is pointed that the nitrogen modified graphene could be used as the efficient catalyst or as the supported substrate to enhance the chemical activity of metal catalysts. Therefore, it is natural to explore whether the other non-metal atoms modified graphene configurations (NMx-graphene) can be more suitable for potential catalytic applications. Previous results found that the single-atom NM doped graphene can regulate the stability of metal catalyst

59

and exhibit excellent catalytic performance for CO

oxidation60. Because of the NM dopants within graphene are more inclined to provide free electrons and often as the anchoring sites to facilitate the interaction of catalyst and substrate. Recent studies found that the introduction of two or more heteroatoms into graphene is an available method to further improve its surface activity

43.

Herein, we

choose the N, Si and P atom due to their similar electronic configurations as the C atom and these non-metal impurities exist in carbon nanomaterials 61-64. Compared to the noble metal catalysts (Pt and Au), Co as a familiar metal element is low-cost, environmentally benign and readily available

65.

With the development of doping technology, the metal

elements (such as Co atom) have been embedded into the graphene lattice experimentally 26,27,

yet the formation progresses of the metal-vacancy complex are unclear and more

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detailed studies should be carried out. Among the defective structures, the 555-777 graphene as the common divacancy structure is more stable than that of the single vacancy

25,

but there is a lack of investigation on geometric, electronic and magnetic

properties of metal atom anchored 555-777 graphene sheet. Although co-doping using elements with different electronegativity could create a unique electronic structure and might bring in special synergistic effects, the formation mechanism about the exact doping process and doping concentration of non-metal and metal atoms on graphene sheet and their synthetic effect on catalytic performance are still much needed. Therefore, it is essential to investigate the structural and catalytic properties of metal Co and non-metal (NM=N, Si and P) atoms co-doping graphene (CoNMx-graphene) sheets. In this work, the formation configurations and catalytic properties of CoNM3-graphene are investigated by using the first-principle methods based on density functional theory (DFT). Our interest here is to study how the incorporation of non-metal (N, P and Si) atom and metal Co atom co-doping on the surface of graphene. Firstly, the formation patterns of co-doping graphene with one Co atom and NM dopants (CoNMx-graphene, x =1, 2 and 3) are comparably analyzed. Compared with the Si3- and P3- graphene sheet, the single Co adatom can be strongly trapped at the N3-graphene (CoN3-graphene) sheet. Secondly, the geometric stability, electronic structures and magnetic properties of reactants (CO, O2, CO/O2, 2CO) on CoNM3-graphene substrates are studied in detail. Furthermore, the possible reaction pathways for CO oxidation are considered comparatively through the Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and termolecular Eley–Rideal (TER) mechanisms. Which is the most favorable reaction mechanism for the oxidation of CO by O2 molecule? This provides new strategy for the development of low cost and high performance co-doping graphene for catalysis. 2. Computational model and methods All the spin-polarized DFT calculations are performed using the Vienna ab initio simulation package (VASP) 66,67. The projector augmented wave (PAW) tool is employed to describe the interaction between frozen cores and valence electrons

68.

Generalized

gradient approximation (GGA) with Perdew, Burke, and Ernzernhof (PBE) functional

69

is adopted in the calculations. The selection of exchange-correlation functional has evidential effect on the result of adsorption energies, while has much smaller effect on

4


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the reaction energy barriers48,70. The kinetic energy cutoff for the plane-wave basis set is chosen to be 450 eV. The C 2s22p2, N 2s22p3, Si 3s23p2, P 3s23p3, Co 3d74s2 and O 2s22p4 states are treated as valence electrons. As shown in Fig. 1(a), a divacancy structure (555-777) graphene sheet with a 6×6 supercell is adopted. A vacuum spacing of 20 Å was selected between the graphene layers and its neighboring images to avoid interactions along the z direction. In order to improve convergence of states near the Fermi level (EF), the Brillouin zone (BZ) integration is sampled using the Monkhorst-Pack (MP) grid with 3×3×1 k-points and 6×6×1 k-points is used for the density of states (DOS) calculations. Bader charge analysis

71

is used to evaluate the transferred charges of atoms. To

search for the most energetically preferable geometries, each type of adsorbates (metal atom or gas molecules) on the different sites of 555-777 graphene (or CoNM3-graphene) surfaces is tested. The minimum-energy path (MEP) for each reaction step is obtained by using the climbing image nudged elastic band (CI-NEB) methods

72-74.

The transition

states (TS) are verified by the vibrational frequencies, which has one imaginary frequency. The spring force between adjacent images was set to be 5.0 eV Å-1 and the forces acting on the atoms are less than 0.02 eV Å-1. Some images are constructed along the reaction pathways from the initial state (IS) to the final state (FS), and the energy barriers (Ebar) is calculated by the energy difference between IS and TS or intermediate states (MS) and TS. The adsorption energy (Eads) is calculated using a formula: Eads = EA + EB  EAB

(1)

where EA, EB and EAB are the total energies of the optimized adsorbates in the molecules or atoms (A: Co, O2, O, CO, CO/O2 and 2CO), the clean graphene substrates (B: 555-777 graphene and CoNM3-graphene) and the adsorbed systems, respectively. The formation energy (Eform) of a metal or non-metal dopant is given by the formula: Eform= (ECoNMx-gra + EC  ED)  ECoNM(x-1)-gra

(2)

where ECoNMx-gra and ECoNM(x-1)-gra is the total energy of the CoNMx-graphene and CoNM(x-1)-graphene sheets (x=1, 2 and 3); EC and ED is the energy of isolated C atom and Co, N, Si or P atom. 3. Results and discussion 3.1 Formation, electronic and magnetic properties of CoNM3-graphene

5



The calculated models of CoNM3-graphene sheets are constructed by extracting carbon atoms from 6×6 hexagonal supercell. After optimized structure, the formation of 555-777 graphene sheet by extracting two carbon atoms is shown in Fig. 1(a), which is more stable than that of single vacancy structure and composed of three pentagons and three heptagons

15.

Firstly, we consider the adsorption behaviors of single-atom Co at

different high symmetric sites of 555-777 graphene sheet, namely, the hollow sites of C-C rings in the heptagon (H1, H2), pentagon (H3) and hexagonal (H4), the bridge sites of the C-C bonds (B1, B2, B3 and B4) and the top site of carbon atom (T), as shown in Fig. 1(a). For the Co adatom, it is found that the B1, B2 and B3 are unstable sites and the Co atom at B4 site has a smaller Eads of 1.93 eV than that on the H1 (2.63 eV), H2 (2.63 eV), H3 (2.60 eV), H4 (2.30 eV) and T (2.66 eV) sites. Although the Co adatom has a relatively larger adsorption energies on 555-777 graphene than that on the pri-graphene (H site, 1.61 eV)51, yet the small energy difference between adsorption sites means that the metal adatom still easily mobile on the 555-777-graphene surface.

Fig.1. Optimized structures for (a)-(b) 555-777-graphene sheet without (or with) the Co dopant, (c)-(d) CoN1- and CoN2-graphene sheets, (e)-(f) CoN3- and CoSi3-graphene sheets. Black, green, gray and blue balls represent the C, Co, N and Si atom, respectively. Secondly, we examined the possible adsorption sites of Co atom at the surface of graphene layer (including B1, B2, B3, B4, T, H1, H2, H3, H4 and H5), as shown in Fig.

6


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1(b), in order to find out the most energetically favorable configuration for the Co adatom around the SV site of 555-777 graphene sheet. It is found that the B1, B2, B3, B4 and H2 are unstable sites, e.g., the Co adatom at B1, B2 and H2 sites would be drawn back to the SV site, while the Co adatom at H3 site (3.32 eV) and T site (2.68 eV) are stable configurations. In comparison, the Co adatom at H1 site has the relatively larger energy (4.25 eV) than that on the H3 (3.32 eV), H4 (3.34 eV) and H5 (3.09 eV) sites. These results indicate that the adsorption of Co atom is more likely trapped at the SV site, namely, the formation of Co dopant within SV site is the most stable configuration as compared with the other sites (B, T and H). The SV defect within 555-777 graphene (SV 555-777 graphene) is shown in Fig. S1(a), the distance of C-C bonds is about 2.89 Å. The total magnetic moment is determined by the contribution of the localized dangling bond states of carbon atoms, resulting in the asymmetry of spin electron distribution at the SV site induce the local magnetic moment (2.0 uB), as shown in Fig. 2(a). The calculated adsorption energy is 10.91 eV for Co adatom, which is larger than those on the SV-graphene (8.51 eV) and 555-777 graphene sheets (2.66 eV). The distance of Co-C bond is 1.77 Å and the adsorption height of Co dopant is 0.64 Å, respectively. The Co dopants are displaced outside of the graphene surface due to its atomic radii is larger than that of the carbon atom. Based on the above results, the adsorption of Co atom has the larger Eads on different graphene substrates (SV graphene, 555-777 graphene and SV 555-777 graphene) than that on the pri-graphene sheet, belong to the chemisorption. Compared to the 555-777 graphene, the Co dopant within SV-graphene and SV 555-777 graphene (˃ 8.00 eV) exhibit the more stability, because there is strongly interaction between Co atom and dangling bonds of carbon atoms at the SV site due to the more transferred electrons. The spin charge density (SCD) of Co-graphene is shown in Fig. 2(b), where the corresponding contour lines in the plots are drawn at intervals of 0.0001 e/Å3. It is found that the doped Co atom induces the spin charge redistribution at SV site of 555-777 graphene, the spin electrons mainly localized between Co and neighboring carbon atoms. Compared with the SV-graphene system, the total magnetic moment of Co-graphene is reduced to 1.0 uB, since the transferred electrons (0.78 e) form Co dopant can decrease the number of unpaired spin electrons in system.

7



In order to justify the high stability of Co dopant at SV 555-777 graphene, we examine the diffusion barriers for Co adatom moving from the SV site to the neighboring stable sites (H1, H3 and H4) and the corresponding diffusion pathways are SV-H1, SV-H3 and SV-H4, respectively. As shown in Fig. S2, the calculated results show that the Co atom need overcome the large energy barriers, e.g., 6.83 eV, 7.89 eV and 7.92 eV for SV-H1, SV-H3 and SV-H4 eV, respectively. These results illustrate that the Co atom would be fixed by the SV site and the formation of Co-graphene configuration is quite stable at room temperature. In addition, the formation energy (Eform) is calculated using the formula Eform= (ECo-gra + EC  ECo)  Egra, where ECo-gra and Egra are the total energies of Co-doped SV 555-777 graphene or SV-graphene and 555-777- or pri-graphene sheets. The Co dopant within 555-777 graphene has relatively smaller Eform (5.53 eV) than that within the SV-graphene (7.50 eV), indicating that the metallic Co atom can be easily incorporated into the SV 555-777 graphene sheet. These results can explain the observations that the metal atoms remain immobile in the defective graphene materials27, which has profound effect for preparing functional metal-graphene materials.

Fig.2. Spin charge density plots for (a) single defect in 555-777-graphene, (b) Co doped 555-777-graphene, (c)-(d) CoN3- and CoSi3-graphene systems, and the corresponding contour lines in plots are drawn at 0.0001 e/Å3 intervals. 8


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Based on the stable Co-graphene configuration, the different CoNMx-graphene models are considered according to the increased doping concentration (CoNM1-, CoNM2- and CoNM3-graphene) and the doping non-metal elements include N, Si and P atoms. With the increased concentrations of N dopants, the corresponding formation energies and optimized configurations of different CoNx-graphene (x=1, 2, 3) are shown in Table 1 and Figs. 1(c)-(e). Compared with the single Co doping within SV 555-777 graphene (5.53 eV), it is clearly shown that the formation energies are significantly decreased following the increased N dopants (from 2.05 to 1.63 eV), illustrating that such complex defects are much easier to form in metal-embedded graphene sheet. Besides, the formation of Co and N3 co-doping is the most energetically favorable configurations than that of CoN1- and CoN2-graphene sheets. The Co-N bond distances within CoN1-, CoN2and CoN3-graphene sheets are 1.79, 1.81 and 1.81 Å, respectively, which are larger than that of C-C bonds in pristine graphene (1.42 Å). As shown in Fig. S1(b)-(f), the formation processes of CoSix- and CoPx-graphene configurations (x=1, 2 and 3) are investigated, the corresponding Eform and structural parameters are listed in Table.1. It is found that the formation of CoSi3-graphene and CoP3-graphene sheets have smaller Eform (2.53 and 3.41 eV) than those of CoNM1- and CoNM2-graphene systems (NM = Si and P). Compared with doped N atoms, the same number of Si and P dopants largely induce the surface wrinkles due to they have the larger atomic radii. The optimized configuration of CoSi3-graphene is shown in Fig. 1(f). Noting that the N dopants have smaller Eform than those of the P and Si dopants, illustrating that large electronegativity of N atoms tend to substitute those of C atoms around the Co dopant, thus they easily incorporate into the Co-graphene sheet and form the more stable CoN3-graphene configuration.

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Table 1 The 555-777 graphene and the NMx-graphene (x=1, 2, 3, NM=N, Si, P) systems with the adsorbed Co atom, including the formation energy (Eform, in eV), the number of transferred electrons between Co atom and substrates (△q1, in e) (the ‘‘+”or‘‘’’ denotes losing or gaining electrons), the bond length between Co and neighboring C (or NM) atoms (d1, in Å) and the adsorption height of Co adatom (d2, in Å). substrates

Eform

△q1

d1

d2

555-777-gra

5.53

+0.78

1.77

0.64

N1-gra

2.05

+0.84

1.79

0.57

N2-gra

1.87

+0.93

1.81

0.57

N3-gra

1.63

+1.02

1.81

0.42

Si1-gra

2.98

0.89

2.02

0.76

Si2-gra

2.82

1.39

2.16

1.58

Si3-gra

2.53

2.22

2.10-2.16

1.34

P1-gra

3.81

+0.59

1.99

0.95

P2-gra

3.70

+0.46

2.10

1.50

P3-gra

3.41

+0.29

2.09

1.53

Generally, the dopants in graphene sheet distort the  conjugation network of carbon surface and induce the local surface curvature of graphene surface 60, which may play an important role in enhancing the interaction of support-catalyst. As shown in Fig. 1(f) and S1(f), the adsorption energy (5.72 eV) of single-atom Co on Si3-graphene surface is larger than that on P3-graphene sheet (5.28 eV), the corresponding Co-Si bonds and adsorption height is 2.10-2.16 Å and 1.34 Å, respectively. In comparison, the adsorbed Co atom on N3-graphene is the most stable configuration with the largest Eads (7.61 eV). The calculated results show that the adsorption energies (˃ 5.0 eV) of Co adatom on NM3-graphene are larger than cohesive energy value of Co (4.43 eV) bulk in experiment75, illustrating that the formation of CoNM3-graphene configurations are enough stable at room temperature. In addition, we analyze the degree of interactions between Co atom and neighboring non-metal atoms by investigation the DOS plots. It is found that the broadened partial DOS (PDOS) of Co 3d states strongly hybridize with the PDOS of NM atoms 2p or 3p states around the Fermi level (EF), as shown in Fig. S3. Compared with the

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CoP3-graphene system, the more electronic states overlap between the PDOS of Co 3d orbitals and N3-2p or Si3-3p orbitals at the EF, illustrating that there are more transferred electrons between the p orbitals of NM atoms and the d orbitals of Co atom, resulting in enhancing the Co-NM interactions. Hence, the formations of CoN3-graphene and CoSi3-graphene configurations are quite stable. Furthermore, the electronegativity difference between Co and NM atoms make the Co adatom tends to gain the more electrons from the Si3-graphene (2.22 e), whereas the less electrons are transferred from the Co atom to the N3-graphene (1.02 e) and P3-graphene (0.29 e) sheets due to the large electronegativity of N and P atoms. Therefore, the different NM3-graphene substrates can effectively regulate the stability of single-atom Co through the transferred electrons and the positively (or negatively) charged of Co atom as active site helps to regulate the adsorption property of gas molecules. To understand the change in electronic property of NM3-graphene, the DOS plots for Co adatom on NM3-graphene sheets are analyzed, as shown in Fig. 3. For the 555-777 graphene, the symmetry of spin-up and spin-down channels illustrate that this system exhibits a non-magnetic property, as shown in Fig. 3(a). The extracting one C atom from 555-777 graphene breaks the symmetry of the sublattices, so the spin channels of system is asymmetric, representing the magnetic properties. Compared with the N3-graphene, the DOS peaks of system are increased around the Fermi level (EF) when the Co atom is adsorbed. The large hybridization between PDOS of Co 3d states and total DOS (TDOS) of system nearby EF indicate that the formation of a covalent bond between the adsorbed Co atom and N3-graphene substrate. The asymmetry of spin channels confirms that CoN3-graphene exhibits the magnetic character. As shown in Fig. 3(b)-(c), the TDOS of Si3- and P3-graphene systems have been obviously altered after the Co atom is adsorbed. For the CoSi3-graphene, there is a hybridized peak at the EF in the spin-up states and a small band gap occurs at the EF in the spin-down states, illustrating that the CoSi3-graphene system exhibits semimetal property. In the DOS plots of CoP3-graphene, the vanishing of the DOS peaks at the EF and there is a small non-zero band gap about 0.20 eV, indicating that the CoP3-graphene system exhibits the semiconducting property. In addition, the SCD of CoN3- and CoSi3-graphene systems are shown in Fig. 2(c)-(d). The more spin electrons are

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accumulated at Co-N3 and Co-Si3 interfaces and the less spin electrons are located at neighboring carbon atoms. The transferred electrons between adsorbates and substrates can increase (or decrease) the number of unpaired spin electrons in systems, so the gaining more electrons of Co atom on Si3-graphene exhibits the larger magnetic moment (0.96 uB) than that on the N3-graphene system (0.56 uB), while the CoP3-graphene system exhibits non-magnetic property (0.0 uB). Therefore, the adsorbed Co atom can regulate the change in electronic structures and magnetic properties of NM3-graphene sheets, namely, CoN3-graphene is the metallic property, CoSi3-graphene and CoP3-graphene exhibit the semimetal the semiconducting characters.

Fig.3. Spin-resolved DOS plots for (a)-(c) CoN3-, CoSi3- and CoP3-graphene systems. The black, red, green, blue and pink curves represent the TDOS of 555-777 graphene, SV-graphene, NM3-graphene, CoNM3-graphene and the PDOS of Co-3d states. The Fermi level is set to zero. 3.2 Adsorption of reaction species on CoNM3-graphene In order to explore the catalytic activity of CoNM3-graphene sheets for the CO oxidation reaction, we firstly investigate the adsorption properties of reaction species (O2, CO, 2CO, CO/O2 and O) on CoNM3-graphene. Several adsorption geometries are taken into consideration and the most stable configurations are shown in Fig. 4, and the corresponding adsorption energies and structural parameters are listed in Table. 2.

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Table 2 The adsorption energy (Eads, in eV), adsorption distance (d3, in Å), bond length (d4, in Å) and the number of transferred electrons (△q2, the “” denotes gaining electrons) for adsorbed gases (O2, CO and 2CO) on graphene substrates.

system CoN3-gra CoSi3-gra CoP3-gra

Eads 1.95 1.40 2.46 1.76 1.68 1.34

O2 (parallel/tilted) △q2 d3 0.61 1.84 0.51 1.74 0.65 1.84 0.55 1.71 0.68 1.87 0.53 1.67

d4 1.36 1.30 1.37 1.29 1.38 1.28

Eads 0.65 1.07 1.56 1.71 0.95 1.29

CO/2CO △q2 d3 0.19 1.85 0.14 1.76 0.31 1.77 0.26 1.77 0.23 1.86 0.22 1.75

d4 1.15 1.16 1.16 1.16 1.15 1.16

Fig.4. Optimized geometries for CoN3-graphene with (a)-(c) the adsorbed O2 and CO, (d)-(f) the single O atom, the coadsorption of O2/CO and 2CO molecules. Black, green, gray and red balls represent the C, Co, N and O atom, respectively. As shown in Fig. 4(a)-(b), the individual O2 molecule is adsorbed at the top site of Co atom. The O2 prefers to form two bonds with the Co atom with an Eads of 1.95 eV and the O-O bond is parallel to the graphene surface (1.84 Å), which is slightly larger than that of tilted O2 configuration (1.40 eV). According to the Bader analysis, there are about 0.61 electrons transferred from CoN3-graphene to O2, which occupy the 2* orbital of O2 and make the adsorbed O2 exhibits negatively charged. As shown in Fig. 5(a), the DOS

13



plots of CoN3-graphenen system has been obviously altered when the O2 is adsorbed. The broadened PDOS of Co-3d states strongly overlapped with the O2 5σ, 1 orbitals from -5.0 eV to the EF and the TDOS of system. Besides, the largely decreased Co-3d states in the O2 adsorbed on CoN3-graphenen verifies the transferred charges (0.61 e) from the occupied Co 3d states to the O2 molecule, indicating that intermolecular orbitals interaction can enhance the stability of gas molecule. In the DOS plots, the asymmetry of spin channels are also evident the magnetic property of adsorbed O2 on CoN3-graphene.

Fig.5. Spin-resolved DOS plots for CoN3-graphene sheet without (or with) (a) O2 and (b) CO molecule. The black, red, green, blue and pink curves represent the TDOS of CoN3-graphene and the PDOS of Co 3d states without O2 and CO adsorption, the TDOS of CoN3-graphene and the PDOS of Co 3d states with O2 and CO adsorption, and the LDOS of O2 and CO. The Fermi level is set to zero. As shown in Fig. 4(c), the adsorbed CO is nearly vertical on the graphene surface with the Co-CO distance of 1.85 Å and it has relatively smaller adsorption energy (0.65 eV) as compared with the adsorbed O2 (1.95 eV). As shown in Fig. 5(b), the DOS plots for CO on the CoN3-graphene reveal that the overlap between Co-3d orbitals and CO 5σ, 1 orbitals. The orbital interactions cause the narrow and sharp peaks of CO 5σ and 2* orbitals to be broadened and delocalized nearby the EF. The transferred charges from the Co atom to the adsorbed CO (0.19 e) and then lead to the elongation of C-O bond (1.15 Å). In all, the distributions of Co-3d states become narrow and decreased at the EF after the O2 (or CO) adsorption on CoN3-graphene. The more electronic states overlap between the local DOS (LDOS) of O2 and PDOS of Co atom, illustrating that the adsorption of O2 is more stable than that of CO molecule. For the adsorbed CO on CoN3-graphene, the

14


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spin-up and spin-down channels are asymmetric and thus this system exhibits magnetic properties. As shown in Fig. 4(d), the single O atom prefers to anchor at the top site of Co catalyst and the distance between Co-O is 1.60 Å. Compared with the adsorbed O2 and CO, the atomic O has the largest Eads (5.05 eV) and exhibits high stability. The large electronegativity of O atom gains more electrons (0.64 e) from the CoN3-graphene and exhibits the negatively charged (O). The O species may exhibits more activity than O2 for CO oxidation. In addition, if CO and O2 mixture is injected as the reaction gases, the adsorbed O2 prefers to occupy the active site of Co catalyst due to the O2 has large Eads. Furthermore, the coadsorption of CO and O2 on the Co catalyst has larger adsorption energy (2.41 eV) than the isolated O2 or CO molecule, as shown in Fig. 4(e), the corresponding O-O and C-O bonds are 1.35 and 1.15 Å, respectively. Furthermore, the average Eads (1.07 eV) of two CO molecules on the CoN3-graphene is slightly more stable than that of isolated CO molecule (0.65 eV), as shown in Fig. 4(f). The corresponding distance of CO-Co is 1.76 Å and the elongation of C-O bond is 1.16 Å. It is worth to note that the single CO and O2 molecule on CoN3-graphene sheet have larger adsorption energies (0.65 and 1.95 eV) than those on the Co doped 555-777-graphene (0.61 and 0.97 eV) and the Co doped SV-graphene (0.62 and 1.75 eV)51. This result illustrates that the CoN3-graphene substrate can enhance the stability of adsorbed gases, which may facilitate the interaction between reactants. As shown in Fig. 6, the most stable configurations of adsorbed species on CoN3-graphene are investigated by using the valence charge distribution. It is found that the more electrons are accumulated at the O-Co and N-Co interfaces and there are fewer electrons are distributed at the N-C bonds. Because of the more electrons are transferred from the CoN3-graphene to the adsorbed gases. Compared with the single CO (0.19 e), the quite stability of O2 gains more electrons (0.61 e) and the O-O bond is elongated at 1.35 Å. Thus, the elongated (or shorted) bond and the strong (or weak) adsorption of reactants are correlated with the number of transferred electrons between adsorbates and substrates. Bader charge analysis shows that the supported Co atom provides the varied transferred electrons from 1.10 e to 1.35 e for 2CO and O2, respectively. It is clearly shown that the Co adatom serves as donor to supply electrons, which are partly

15



transferred to the adsorbed gases and the rest ones to the supported substrate, resulting in enhancing the interaction of adsorbate-substrate.

Fig.6. Charge distribution plots for (a) O2, (b) CO, (c) O, (d) O2/CO and (e) 2CO adsorbed on the CoN3-graphene sheet, and the corresponding contour lines in plots are drawn at 0.01 e/Å3 intervals. Black, green, gray and red balls represent the C, Co, N and O atom, respectively. As shown in Fig. 7, we further analyze the spin electron distribution between adsorbed gases and reactive substrates, where the corresponding yellow and blue areas represent the spin up and spin down electrons, respectively. Compared with the bare CoN3-graphene system, the redistribution of spin electrons of Co atom is largely changed after the O2 is adsorbed. The spin-up electrons are accumulated at the O2 molecule and the spin-down electrons are placed at the Co and N atoms, as shown in Fig. 7(a). For the adsorbed CO, the less spin electrons are located at the CO and CoN3-graphene, thus this system has the smaller magnetic moment (0.90 uB) than that of the adsorbed O2 (2.80 uB), as shown in Fig. 7(b). There are more spin-down electrons in the O adsorbed at the CoN3-graphene system, since the O atom gains more transferred electrons from substrate and then increase the number of unpaired electrons (1.10 uB), as shown in Fig. 7(c). As shown in Fig. 7(d), the redistribution of spin electrons on the coadsorbed CO and O2 is different from the isolated CO or O2 molecule. It is found that the more spin up electrons

16


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are accumulated at the O2 molecule and Co-N3 atoms, and the less ones are placed at the CO molecule, so this system exhibits the small magnetic moment (0.60 uB). For the coadsorption of 2CO molecules, it is shown that the spin down electrons are accumulated at the CO molecules and the spin up electrons are placed at CoN3-graphene sheet, which has relatively larger magnetic moment (0.80 uB) than that of CO/O2, as shown in Fig. 7(e). There results illustrate that the different gas adsorption or coadsorption can effectively regulate the size of magnetic moment for CoN3-graphene system, which may be used to distinguish the kinds of adsorbed species in sensor devices.

Fig.7. Spin charge density plots for (a) O2, (b) CO, (c) O, (d) O2/CO and (e) 2CO adsorbed on the CoN3-graphene sheet, and the corresponding contour lines in plots are drawn at 0.0001 e/Å3 intervals. As shown in Table 2 and 3, the adsorbed gas molecules (CO, O2, 2CO and CO/O2) on the CoNM3-graphene sheets exhibit different stability. Compared with the CoN3-graphene sheet, the single O2 has larger adsorption energy on CoSi3-graphene sheet (2.46 eV), yet it exhibits the less stability on CoP3-graphene sheet (1.68 eV). In addition, the CoNM3-graphene prefers to be covered by adsorbed O2 due to it has a larger adsorption energy, resulting in the physisorbed CO molecule above preadsorbed O2 on Co catalyst is selected as the initial step (IS) through the ER mechanism. Furthermore, the coadsorption of O2 and CO molecules on the CoN3- (2.41 eV), CoSi3- (3.36 eV) and CoP3-graphene (2.32 eV) are larger than that of the isolated O2 (or CO) molecule, which

17



Page 18 of 35

may enhance the reaction possibility through the LH mechanism. It is interesting note that the coadsorption of 2CO molecules is more stable than that of the isolated one. This means that a new reaction pathway for CO oxidation named as TER mechanism will be investigated

47,58,76,

which includes that one O2 molecule be directly activated by two

preadsorbed CO molecules. For the CO oxidation reactions, the CO2 molecule is a final product by the interactions between CO and O2 molecules and its stability on CoNM3-graphene sheets is considered. It is found that the CO2 products have smaller adsorption energies on CoN3-graphene (0.19 eV) and CoSi3-graphene (0.04 eV) sheets than that of the O2 and CO (CO2 is not adsorbed on CoP3-graphene), belong to the physisorption, suggesting that the generated CO2 molecule is more easily desorbed from the active site of catalyst. Therefore, the possible reaction mechanisms for CO oxidation on Co-NM3-graphene are comparably analyzed. Table 3 The coadsorption energy of O2 and CO (Eads, in eV) and their bond length (d5, in Å), as well as the energy barrier (Ebar, in eV) for LH, ER and TER of CO oxidation. CO, O2 system

Eads

CoN3-gra 2.41 CoSi3-gra 3.36 CoP3-gra

2.32

d5 1.15 1.35 1.16 1.32 1.15 1.34

reaction mechanism LH(CO+O2) ER(O+CO) TER(2CO+O2) Ebar1, Ebar2

Ebar3

Ebar1, Ebar2

0.81, 0.04

0.20

0.77, 0.15

1.11, 0.00

0.07

1.45, 0.00

0.97, 0.11

0.15

0.66, 0.00

ER(CO+O2) Ebar1, Ebar2 Ebar3, Ebar4 0.63, 0.87, 0.34, 0.24 0.76, 0.83, 0.81, 0.55 0.43, 1.36, 1.15, 0.43

3.3 Reaction mechanisms for CO oxidation on CoNM3-graphene 3.3.1 LH mechanism Generally, there is a close relationship between adsorption stability and reaction pathways for the reactive gases on supported substrates 77. Firstly, two traditional reaction mechanisms (including LH and ER) for CO oxidation on CoN3-graphene sheet are investigated in this study. As shown in Fig. 8(a), the coadsorption configuration of O2

18


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and CO molecules are anchored at the Co catalyst as initial state (IS), where the CO and O2 are tilted and parallel to the graphene surface. The distance between O2-CO is 2.53 Å, and the corresponding distance of Co-O2 and Co-CO are 1.90 and 1.78 Å, respectively, as shown in Table 4(a). To proceed, the adsorbed O2 molecule turns around one of the oxygen atoms (O2) approaching the carbon atom of CO and then from the OOCO intermediate state (MS), where the O-O bond is elongated from 1.35 to 1.50 Å and the distance between CO-O2 is gradually decreased from 2.53 to 1.36 Å. Passing over the TS1, the reaction process needs overcome the Ebar of 0.81 eV. Finally, the peroxo-type OOCO complex is converted to CO2 molecule through the broken O1-O2 bond and leaving an O1 atom (Oads) adsorbs at the Co catalyst (FS). From the MS to FS, the TS2 has a much smaller energy barrier (0.04 eV) than that of TS1.

Fig.8. Different states and energy barriers for CO oxidation reactions on CoN3-graphene sheet through (a) LH and (b) ER mechanism. Black, green, gray and red balls represent the C, Co, N and O atom, respectively.

19



Page 20 of 35

Table 4 Structural parameters for the CO oxidation reactions on CoN3-graphene through the LH mechanism (CO + O2 → OOCO → CO2 + Oads) and ER mechanism (CO + Oads→ CO2), the corresponding IS, TS, MS and FS are display in Fig. 8(a)-(b). (a) distance (Å)

IS

TS1

MS

TS2

FS

dCo-C

1.78

1.88

1.95

1.96

4.58

dC-O2

2.53

1.43

1.36

1.35

1.17

dO1-O2

1.35

1.49

1.50

1.51

3.46

dCo-O1

1.90

1.87

1.82

1.81

1.60

dCo-O2

1.92

2.26

2.51

2.51

4.95

(b)

IS

TS

FS

dCo-O1

1.60

1.60

2.20

dC-O1

3.04

2.56

1.18

dC-O

1.14

1.15

1.17

As shown in Fig. 8(b), the remaining O1 atom can readily react with another CO molecule to form the second CO2 molecule. Once CO is physisorbed above the CoN3-graphene surface (IS2), it begins to approach the preadsorbed O atom until the transition state (TS3) is obtained. As shown in Table. 4(b), the distance between O1-C2 is deceased from 3.04 to 2.56 Å, resulting in the formation of second CO2 molecule. Passing over TS3, a new C-O bond is formed (1.18 Å) through the ER reaction, which has a relatively smaller energy barrier (0.20 eV) than that of the TS1 (0.83 eV). Hence, the adsorbed O species on Co catalyst exhibits more activity than O2 for CO oxidation reaction. In addition, the similar reactions of CO oxidation on the CoSi3- and CoP3-graphene sheets are also investigated through the LH and ER mechanisms, and the corresponding energy barriers are shown in Table 3. It is found that the formation of OOCO complex on CoSi3- and CoP3-graphene have larger energy barriers (1.11 and 0.97 eV) than those of the followed reaction (0.00 and 0.11 eV), indicating that it is much difficult to form the OOCO complex, thus this reaction can be viewed as rate controlling

20


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step. Although the adsorption of O atom has the largest Eads, the corresponding reaction processes (CO + Oads→ CO2) have smaller energy barriers (˂ 0.2 eV), indicating that the adsorbed O can be easily converted into CO2 molecule and do not generate the poisoning effect of the active sites by O atom. From the energy points of view, the high catalytic activity for CO oxidation on CoN3-graphene sheet have lower energy barriers than those on other substrates, illustrating that the different NM3-graphene sheets can regulate the stability and catalytic activity of Co adatom and the LH reactions as starting step is a more favorable process on CoN3-graphene substrate. 3.3.2 ER mechanism The ER mechanism as an important reference has been considered, the optimized structures of each state for CO oxidation on CoN3-graphene are shown in Fig. 9(a), and the corresponding structure parameters are listed in Table 5(a). For the IS, the physisorbed CO molecule is suspended above the O2 and the CO-O2 distance is 4.07 Å, as well as the C-O and O-O bonds are 1.14 and 1.36 Å, respectively. The CO molecule approachs the O2 and then inserts into the O-O bond to from a carbonate-like CO3 complex (MS). Passing over the TS1, the C-O bond of CO is elongated to 1.22 Å and the O-O bond is 2.15 Å. The ER mechanism as an IS has a relatively smaller energy barrier (0.63 eV) than that of LH mechanism (0.81 eV). The second reaction is that the formed CO3 complex dissociates into CO2 molecule and a atomic Oads, where the C-O2 bond is elongated from 1.33 to 3.32 Å. The corresponding energy barrier (TS2) is 0.87 eV, which is larger than the first reaction (CO + O2 → CO3, 0.63 eV). Noting that the reversible reaction on the CoN3-graphene (CO2 + Oads → CO3, 0.38 eV) has a small energy barrier, indicating that the active O atom can be used to capture the CO2 molecule in chemical environment and the formation of CO3 complex is more stable than the dissociated products, thus the CoN3-graphene sheet would be covered by the carbonate-like CO3.

21



Fig.9. Different states and energy barriers for CO oxidation reactions on CoN3-graphene sheet through (a)-(c) ER mechanisms. Black, green, gray and red balls represent the C, Co, N and O atom, respectively.

22


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Table 5 Structural parameters for the CO oxidation on the CoN3-graphene through the ER mechanisms (a) CO + O2 → CO3 → CO2 + Oads, (b) CO + CO3 → 2CO2 and (c) CO + O2 → CO2 + Oads, the corresponding IS, TS, MS and FS are display in Fig. 9(a)-(c). (a) distance (Å)

IS

TS1

MS

TS2

FS

dC-O

1.14

1.14

1.22

1.17

1.17

dC-O1

4.07

2.48

1.35

1.18

1.18

dC-O2

4.16

2.21

1.33

2.29

3.32

dO1-O2

1.36

1.40

2.15

2.61

3.22

dCo-O1

1.85

1.79

1.86

2.66

3.95

dCo-O2

1.84

1.83

1.93

1.64

1.61

(b)

IS2

TS3

FS2

dC2-O3

1.14

1.14

1.17

dC2-O1

3.58

2.76

1.18

dC-O1

1.35

1.38

3.11

dO1-O2

2.15

2.16

3.44

dCo-O1

1.85

1.86

3.60

dCo-O2

1.94

1.92

3.94

(c)

IS3

TS4

FS3

dC-O

1.14

1.14

1.17

dC-O1

3.53

2.68

1.17

dO1-O2

1.35

1.30

2.76

dCo-O1

1.78

2.47

3.85

dCo-O2

1.95

1.75

1.60

Furthermore, the quite stable configuration of CO3 complex reacts with another CO (IS2) and then to produce two CO2 molecules (FS2), as shown in Fig. 9(b), as well as the corresponding structure parameters are listed in Table 5(b). For the IS2, the second CO molecule is suspended above the CO3 complex and their distance is 3.58 Å. Passing over 23



Page 24 of 35

the TS3, the C2-O3 bond is elongated to 1.17 Å and the new O1-C2 bond is about 1.18 Å. In this reaction, the calculated energy barrier is about 0.34 eV, which is much smaller than the dissociated reaction of CO3 complex (0.87 eV). This result indicates that the presence of CO molecule will promote the dissociation of CO3 complex on CoN3-graphene surface. On the other hand, we consider another ER mechanism for the interaction between CO and O2 to generate CO2 and Oads, without the intermediate state (CO3), as shown in Fig. 9(c), and the corresponding structure parameters are listed in Table 5(c). For the IS3, the physisorbed CO is placed above O2 on the Co atom and the distance of CO and O2 is 3.53 Å. Next, the carbon atom of CO starts to approach the O atoms of O2 molecule to reach the TS4, where the Co-O1 bond length is elongated (2.47 Å) and while the distance between CO and O2 is decreased (2.68 Å). An energy barrier of 0.24 eV is needed to reach TS4 on the CoN3-graphene, which is quite smaller than those of other surfaces like Fe-doped graphene (0.56 eV)

50

and BN3-graphene (0.39 eV)

78.

Passing over the TS4,

the O-O bond is dissociated and the new C-O1 bond is formed, resulting in a CO2 molecule is generated and leaving an Oads anchors at the Co atom (FS3). After the CO2 is desorbed, the CO molecule reacts with the atomic Oads to produce CO2 and the corresponding energy barrier is 0.20 eV, as shown in Fig. 8(b). For the different ER mechanism, the CO oxidation reactions on CoN3-graphene sheet through the intermediate state (CO + O2 → CO3) have larger energy barriers (˃ 0.6 eV) than that without MS (CO + O2 → CO2 + Oads) (˂ 0.3 eV). For other CoNM3-graphene sheets, the CO directly reacts with preadsorbed O2 molecule to generate CO2 with smaller energy barriers (0.55 eV for CoSi3-graphene, 0.43 eV for CoP3-graphene) than the formation reaction of CO3 complex. Compared with the LH mechanism (˃ 0.8 eV), it is found that the ER mechanism as initial step (CO + O2 → CO2 + Oads) has smaller energy barriers (˂ 0.6 eV) and is energetically more favorable for CO oxidation. Therefore, the ER reaction without CO3 complex is more prone to table place with low energy barriers on CoNM3-graphene sheets. 3.3.3 TER mechanism In order to explore the preferred mechanism for CO oxidation and the new TER mechanism on CoNM3-graphene sheets are systemically investigated. The gaseous O2

24


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molecule is activated by preadsorbed 2CO molecules on the CoN3-graphene, as shown in Fig. 10, and the corresponding structure parameters are shown in Table 6. For the IS, two CO molecules are coadsorbed at the Co catalyst and from the CO–Co–CO configuration, where the C-O bond and the distance of CO-Co are 1.16 and 1.76 Å, respectively. Passing over TS1, the suspended O2 molecule gradually approaches two C atoms of coadsorbed CO molecules with the energy barrier of 0.77 eV (TS1) and the O-O bond is elongated from 1.24 to 1.46 Å, resulting in the formation of a pentagonal ring OCO–Co–OCO complex (MS). The calculated results show that the initial state of TER on CoN3-graphene has a relatively smaller (or larger) energy barrier than that of the LH (or ER) mechanism. Upon the dissociated O-O bond, the OCO–Co–OCO intermediate can generate two CO2 molecules through the TS2 of 0.15 eV. For the TER reactions, it is found that the CO oxidation reactions on CoN3-graphene has moderate energies, which is smaller (˂ 0.8 eV) than that of the common noble metal catalysts (Pt and Au)

44,79,80.

Compared with CoP3-graphene (0.66 eV), the CO oxidation reactions have larger energy barriers on CoSi3-graphene (1.45 eV) and CoN3-graphene (0.77 eV) sheet. Hence, the TER mechanism on CoP3-graphene sheet is more favorable reaction, which is useful to accelerate the CO oxidation reaction.

Fig.10. Different states and energy barriers for CO oxidation reactions on CoN3-graphene sheet through TER mechanism. Black, green, gray and red balls represent the C, Co, N and O atom, respectively.

25



Page 26 of 35

Table 6 Structural parameters for the CO oxidation on the CoN3-graphene through the TER mechanism (2CO + O2 → 2CO2), the corresponding IS, TS, MS and FS are display in Fig. 10. distance (Å)

IS

TS1

MS

TS2

FS

dCo-C1

1.76

1.81

1.98

1.95

4.28

dCo-C2

1.76

1.82

1.96

1.95

4.00

dO3-O4

1.24

1.26

1.46

1.49

2.97

dC1-O3

3.39

2.27

1.37

1.39

1.17

dC2-O4

3.59

2.24

1.39

1.43

1.17

Based on the above discussions, the change in catalytic activity of CoNM3-graphene for the CO oxidation reactions have been compared by modifying the different kinds of co-doping metal/non-metal atoms. The supported Co catalyst on NM3-graphene sheets exhibit positively (or negatively) charged, because of the large electronegativity difference between N, Si, P and Co atoms. Table 3 shows the catalytic reactions of CO oxidation on different CoNM3-graphene sheets through the similar reaction processes (including the LH, ER and TER mechanisms). For the LH mechanism, the reaction process (CO + O2 → OOCO2, 0.81 eV) on CoN3-graphene has smaller energy barriers than those on the CoSi3- and CoP3-graphene surfaces. For the TER mechanism, the CO oxidation reactions on CoP3-graphene sheet have the smaller energy barriers (2CO + O2 → 2CO2, 0.66 eV) than those on the CoSi3- and CoN3-graphene sheets. Compared with the LH and TER mechanisms, it is concluded that the sequential ER reactions (CO + O2 → CO2 + Oads, 0.24 eV) and (CO + Oads → CO2, 0.20 eV) on CoN3-graphene exhibits excellent catalytic activity for CO oxidation (˂ 0.3 eV). In general, the small energy barrier (˂ 0.5 eV) for CO oxidation reactions is expected to occur at room temperature 81. Therefore, the CO oxidation reactions through ER mechanism without CO3 complex on the CoNM3-graphene are likely to proceed rapidly due to their low activation barriers involved, which is comparable to that of the other single-atom catalysts 48,82,83. 4. Conclusion We perform periodic DFT calculations to study the formation mechanism, electronic structure and catalytic activity of CoNM3-graphene sheets. According to the calculated

26


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results, the stable configurations of CoNM3-graphene (NM=N, Si and P) have less formation

energy

than

that

of

CoNM1-

and

CoNM2-graphene

sheets.

The

electronegativity difference of NM atoms make the Co adatom positively (or negatively) charged, which helps to regulate the stability of gas reactants. In addition, it is found that the adsorbed O2 on CoNM3-graphene is more stable than that of CO molecule. The coadsorption of CO and O2 (or 2CO) has the larger adsorption energy than that of isolated O2 (or CO) molecule, thus the LH and TER mechanisms as initial state for CO oxidation reactions are systematically considered. Compared with CoSi3- and CoP3-graphene sheets, the ER mechanisms (including CO + O2 → CO2 + Oads, CO + Oads → CO2) on CoN3-graphene sheet are more prone to take place than that of the LH and TER mechanisms due to the smaller energy barriers (˂ 0.3 eV). Therefore, these results demonstrate that the CoN3-graphene sheet can be easily performed for CO oxidation reaction and exhibit a high catalytic activity. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. U1404109, 11504334, 11881240254, 11704005 and 61674053), the Natural Science Foundation of Henan Province (Grant No. 162300410325), the key Young Teachers of Henan Province (Grant No. 2017GGJS179) and Program for Science & Technology Innovation Talents in Universities of Henan Province (Grant No. 18HASTIT030). Aid program for Science and Technology Innovative Research Team of Zhengzhou Normal University.

27



Supplementary information

Fig.S1. The optimized structures for (a) SV 555-777 graphene, (b)-(c) CoSi1- and CoSi2-graphene sheets, (d)-(f) CoP1-, CoP2- and CoP3-graphene sheets. Black, green, yellow and blue balls represent the C, Co, P and Si atom, respectively.

28


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Fig.S2. The diffusion pathways and corresponding energy barriers for Co adatom from SV to H1, from SV to H3 and from SV to H4 sites. Black and green balls represent the C and Co atom, respectively.

29



Fig.S3. Spin-resolved PDOS of Co and N3, Si3 or P3 atoms. The pink and red curves represent the PDOS of Co 3d states and N3 of 2p states, Si3 3p states or P3 3p states. The Fermi level is set to zero.

30


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Table of Contents graphic

The possible reaction pathways for CO oxidation on CoNM3-graphene sheets (NM= N, Si, P) are investigated comparatively, which provides new strategy for the development of low cost and high performance graphene-based catalysis.

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Metal and Non-metal Atoms Modified Graphene as Efficient Catalysts

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