Probing the Electric Field Effect on the Catalytic Performance of Mn

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Probing Electric Field Effect on the Catalytic Performance of Mn-Doped Graphene to CO Oxidation Xian-Yan Xu, Hui-Shi Guo, and Cunyuan Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08226 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry

Probing Electric Field Effect on the Catalytic Performance

of

Mn-doped

Graphene

to

CO

Oxidation Xian-Yan Xu,a* Huishi Guo,a Cunyuan Zhao,b

a

College of Chemistry and Environmental Engineering, Shaoguan University,

Shaoguan 512005, China. b

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of

Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China.

Corresponding Author Xian-Yan Xu*, E-mail: [email protected]

1

ACS Paragon Plus Environment

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ABSTRACT Electric field is an effective route to tune the performance of catalysts. Here, we have probed the reaction mechanism of CO oxidation on Mn-doped graphene (Mn-Gr), the effect as a result of the externally applied electric field, and provided a theoretical understanding of the rule by density functional theoy (DFT) calculation. On the basis of DFT calculations, we suggest that electric field has significant impact on the catalytic performance of CO oxidation on Mn-Gr. The reaction barriers for CO2 formation decrease with increasing CO/O2 adsorption ability on Mn-Gr as the electric field decreases from 0.5 to -0.75 V/Å, leading to a more activation of O-O bond and then accelerate the CO2 formation. However, strong binding between CO2 and Mn-Gr under a larger positive or negative electric field would result in CO2 desorption difficult and hinder the catalyst regeneration. Therefore, it is proposed that -0.50 F/Å are more appropriate for CO oxidation on Mn-Gr with a lower determined reaction barrier of 0.55 eV when considering the CO2 formation and desorption, in which the adsorption energy is neither too strong nor too weak. These findings highlight the possibility to manipulate the catalytic performance of the doped graphene to CO oxidation with the electric field controlled which would be helpful for future design and implementation of high performance catalysts.

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1.

Introduction

In the past decades, the catalytic oxidation of carbon monoxide (CO) has become one of the hottest topics in academic research which is not only because CO oxidation is a simple stoichiometric reaction and could be considered as a prototype catalytic reaction in surface chemistry to evaluating catalyst activity, selectivity and durability, but also CO oxidation plays an essential role in exhaust treatment of the post-combustion process of automobiles1 as well as in alleviating the poisoning effect on platinum catalyst in oxygen reduction reaction (ORR)2. As reported, the usually used catalysts for CO oxidation are traditional metals like gold,3-5 Pt,6-9 Rh,6, 10

Ir,6 Pd,6, 11 Ag,12 noble metals or alloy with metal oxides as support like Au/TiO2

and Au-Cu alloy systems13-14. However, these catalysts are scarce and relatively expensive, and moreover the durable operation is impeded for CO poisoning effect on platinum based catalysts. Therefore, it is meaningful to develop cheap and efficient catalysts to replace these expensive noble metal-based catalysts. Graphene, a form of carbon with a true two-dimensional structure consisting of sp2-hybridized carbon atoms, has attracted increasing attention in different scientific fields including catalysis due to its unique properties, such as the high specific surface area, high stability and durability, high thermal conductivity, electric conductivity. The experimental results have suggested P-doped, S-doped, N-doped graphene15 could serve as an efficient catalyst for ORR and C-H acetylene Hydrochlorination16, Direct Friedel–Crafts alkylation reactions17, aerobic oxidation of Alcohols18, and also CO oxidation which we would focus in here. As reported, palladium cluster supported on graphene has been experimentally demonstrated to show superior catalytic activity towards CO oxidation and the reaction was theoretically predicted to follow Langmuir-Hinshelwood (LH) mechanism.11 The experimental investigation of Zhang et al. has revealed the high activity of Pt-Ni cluster supported on graphene for CO oxidation.19 Moreover, several studies have proved the graphene doped with Au,20 Fe,21 Pt,22 Cu,23 Al,24 Mo,25 Sn,26 Ni,27 Si28 are 3



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efficient catalysts for CO oxidation. The reported investigations suggest that Al-doped graphene exhibits a similar catalytic performance in CO oxidation to Au-doped graphene, of which the energy barrier of the determined step is 0.32 eV for Al-doped graphene and 0.31 eV for Au-doped graphene.20, 24 The overall energy barrier for CO oxidation on Sn-doped graphene is 0.41 eV26. Pt,22 Fe,21 Cu,23 Mo, 25 and Ni27 doped graphene are suggested a modest reactivity to CO oxidation with the energy barrier of the determined step to be 0.58, 0.57, 0.54, 0.60, 0.63 eV, respectively. Zhao et al. also found Si-doped graphene could be an efficient and metal-free catalyst for CO oxidation by N2O or O2.28 Besides graphene, other two-dimensional monolayer materials, such as 2D polymeric phthalocyanine sheets,29 germanene, silicene,30 molybdenum disulfide monolayers,31 graphdiyne,32 graphitic carbon nitride (C2N33), and hexagonal boronnitride(h-BN)

34-36

have been

investigated as catalysts for CO oxidation. Among the reported two-dimensional monolayer materials, two well-established reaction mechanisms have been conducted for CO oxidation, which are Eley-Rideal (ER) mechanism and Langmuir-Hinshelwood (LH) mechanism. ER mechanism features with the direct interaction between the free-standing CO and a surface activated O2 or remaining atomic O after the first O removal. While LH mechanism is characterized by the coadsorption of CO and O2 that interact with each other to form a peroxide-type like intermediate. Sabatier principle37 states that an efficient catalyst would show a right interaction with the reactants, that is, neither be too strong nor too weak. As for CO oxidation on the two-dimensional monolayer materials, oxygen molecule are considered as oxidant and its reaction mechanism shows a close relation with O2. It is proposed the catalytic activity of materials might be related with the activation of the O-O bond. Aided by the doped atoms, the O-O bond is more activated, which might cause an easier formation of CO2. Therefore, it is expected an effective route to activate O-O bond could promote CO oxidation. However, it is noted that the activation of O-O bond derives the interaction between oxygen and the catalyst, which implies that the high activation of O-O bond is accompanied with the strong binding O atom and the active site which would hinder 4


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CO2 diffusion to regenerate the catalyst when CO2 is formed. Therefore, an effective route to get a higher activated O-O bond with a easier desorption of CO2 might develop an efficient catalyst to CO oxidation. Electric field, which is advanced in an clean, easily acquirable and adjustable in both direction, have been revealed to extensively affect the physical properties of molecules including the interface properties. For instance, hydrogen storage have been theoretically identified to realize according to applying an electric field to control the adsorption and releasing or desorption of H2 on BN sheet, Li-doped graphene and N-doped graphene.38-39 N2O on Al, Ga, or Mn-doped graphene40 in an applied electric field could be effectively captured and decomposed. Besides, the electric field has been revealed to effectively tune the adsorption of O2 molecule on Au-doped graphene.41 Therefore, on the basis of above analysis, it is natural to expect that an electric field could be effective control the activity of CO oxidation on the doped graphene. Herein, we would perform an investigation on the reaction mechanism of CO oxidation on Mn-doped graphene (Mn-Gr) by DFT calculations, and then further probe the applied external electric field effect on the catalytic performance of Mn-Gr to CO oxidation.

2.

Computational details

All calculation were performed on DMol3 package42-43 using spin-unrestricted density functional theory (DFT) with Perdew-Burke-Ernzerhof (PBE)44 of generalized-gradient approximation (GGA) as exchange-correlation functions. Long-range dispersion correction via Grimme scheme45 was adopted to better express the van der Waals (vdW) interactions between the molecules and the substrate. A double numerical basis set including polarization function (DNP)42 and the DFT semi-core pseudopotentials (DSPPs) were employed to describe the ion-electron interactions. The convergence criteria were set to be 1.0×10-5 Hartree in energy, 2.0×10-3 Hartree Å-1 in force, and 5.0×10-3 Å in displacement. The complete linear synchronous transit/quadratic synchronous 5



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transit (LST/QST) method46 have been well validated to search transition state geometries, which were further confirmed by minimum-energy pathway (MEP) calculation using the nudged elastic band (NEB) method47 A hexagonal graphene supercell (4 × 4 graphene unit cell) containing 32 atoms was chosen to model a system with one carbon atom substituted by a manganese atom(see Figure 1a), similar to our previous work of CO oxidation on Ni-doped graphene.27 The minimum distance between the neighbouring graphene sheets is greater than 20 Å to avoid the layer-to-layer interactions. The Brillouin zone integration were respectively set as 5 × 5 × 1 and 15 × 15 × 1 special k-points sampling using the Monkhorst-Pack scheme48 during the geometry optimization and the density of states, while orbital analysis was computed at Γ point. The real-space global orbital cutoff was set in fine quality. Hirshfeld charge analysis method49 was adopted for the population analysis.

3.

Results and discussion

3.1. CO oxidation on Mn-Gr Firstly, we calculated the binding energy for manganese(Mn) atom on the single vacancy in graphene and the migration barrier for Mn at graphene to assess the stability of Mn-Gr. The binding energy Eb is defined by Eb= EMn-Gr – EGr-sv – EMn. Here, EMn-Gr, EGr-sv, EMn respectively present the total energy of Mn-Gr, graphene with single vacancy (Gr-SV), Mn atom. And it is calculated to be -6.95 eV, consistent with the result of Krasheninnikov50. It is a strong binding causing by the strong hybridization of the carbon 2p orbitals and Mn 3d orbital around the Fermi level (see Figure 1b), especially between the C 2p orbitals and Mn 3dz2 orbitals as shown HOMO and HOMO-1 in Figure 1c. Such strong binding indicates that Mn-Gr is stable and the metal atom cannot be removed thermally or by irradiation with sub-MeV51, similar to Ti-doped graphene(-8.00

eV),

Al

doped-graphene(-5.60

eV)24,

and

Ni-doped 6


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graphene(-7.57 eV)27 with a binding energy around 6 eV. The diffusion energy barrier of Mn in graphene is considered by rotating one of the Mn-C bonds, which is calculated to be 3.45 eV(Figure 1d), higher than Au doped graphene (2.1 eV), Pt doped graphene (3.1 eV)50, and Cu-doped graphene (2.34 eV)23, and comparable with Ni-doped graphene(3.41 eV)27. The imaginary frequency of the transition state is -419.25 cm-1. The high activation barrier implies Mn would not move at room temperature and thus hinders metal atoms to form a cluster. Hence, we believe that Mn-Gr is relatively stable and feasible to be a substrate for CO oxidation when compared to Cu-doped graphene23 and Ni-doped graphene27.

Figure 1. (a) The structural configuration and (b) the projected density of states (DOS) of Mn-Gr. (c) Orbital charge density isosurfaces in Mn-Gr, isovalue=0.3. (d) The diffusion potential energy surface for Mn atom on Mn-Gr. Gray and purple balls represent the C and Mn atom, respectively.

We then turn to study CO and O2 adsorption on Mn-Gr. The adsorption energy Ead is defined by Ead= EMn-Gr + molecule –EMn-Gr - Emolecule, where EMn-Gr, EMn-Gr

+ molecule,

Emolecule respectively present the total energy of Mn-Gr,

molecule adsorbed onto Mn-Gr, and molecules. By this definition, positive 7



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(negative) value of Eads indicates endothermic (exothermic) process. The calculated results show that the more stable CO adsorption configuration is that the CO molecule binds to Mn-Gr via the interaction between C in CO molecule and Mn, and the adsorption energy is -1.62 eV (see Figure 2a). The new formed Mn-C bond is 1.91 Å. The C-O bond in CO elongates from 1.14 Å to 1.15 Å after the adsorption. While O2 forms two almost equivalent Mn-O bonds with Mn-Gr in the most stable O2 adsorption configuration (see Figure 2b). The two Mn-O bonds are respectively 1.83 Å and 1.84 Å. The O-O are in-plane with one Mn-C bond and the O-O bond is stretched from 1.23 Å of O2 molecule to 1.39 Å. We can see Ead(O2) is larger than Ead(CO), and thus Mn-Gr will be dominantly covered by O2 and be preserved from the poisoning effect if the CO/O2 mixture is injected with a constant gas flow rate.

Figure 2. Optimized adsorption structure (top and side view) and calculated adsorption energies of (a) CO, (b) O2 and (c) coadsorption CO/O2 on Mn-Gr. Gray, purple and red balls represent the C, Mn and O atom, respectively.

Since the surface coadsorbed and activated CO and O2 molecules is the precursor to form a peroxo-type like intermediate via a concerted reaction pathway in LH mechanism while the free-standing CO directly reacts with the pre-adsorbed O2 or atomic O in ER mechanism, so we have also investigated the probability for CO and O2 coadsorbed onto Mn-Gr and the dissociation for O2 on Mn-Gr. After full optimization, an coadsorption mode is obtained with 1.33 Å of O-O bond length for the adsorbed O2 and 1.15 Å of C-O bond length for the adsorbed CO, see Figure 2c. It releases an adsorption heat of 2.78 eV, 8


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larger than Ead(O2) and Ead(CO). Therefore, it is proposed that for this system the CO oxidation can be initiated via LH mechanism, which is CO + O2 OC*OO* CO2 + O. In addition, the O2 dissociation on Mn-Gr show that the dissociation barrier is 1.00 eV and thus O2 is stable at adsorption mode without dissociation. As a result, it is probable for CO oxidation on Mn-Gr initiated via ER mechanism CO+ O2 CO2 + O.

Figure 3. Schematic energy profile along (a) LH and ER process for the initiation steps, (b) ER process for the following steps to remove the left atomic O, and (c) the corresponding local configuration involved in surface-catalyzed CO oxidation.

Figure 3 shows the energy profiles of MEPs via LH and ER mechanisms as well as the corresponding local configurations along MEPs. We find that CO2 is formed via the ER mechanism when CO directly attacks the activated O2, see Figure 3a. The initial reactant (ISa) first climbs over an transition state (TSa) by overcoming an energy barrier of 0.86 eV, and then reaches final state (FS1) releasing a large energy of 3.22 eV. The O-O bond along is gradually elongated and finally broken in the process CO+ O2 CO2 + O. In LH mechanism, it is a free energy process for CO attaching the central Mn atom in Mn-Gr with preadsorbed O2 to give the coadsorption configuration ISb. Upon the coadsorption of CO and O2, the CO and O2 molecules approach each other 9



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to form the first transition state (TSb-1), with an energy barrier of 0.72 eV along the reaction pathway. Passing over TSb-1, an peroxide-type intermediate OC*OO* (MSb) is formed, which is a slight endothermic process with the reaction energy of 0.10 eV for CO + O2 OC*OO*. Mediated by the central Mn atom, the O-O bond length of the peroxo-type intermediate continuously elongates and passes over the second transition state (TSb-2) with a substantially low energy barrier of only 0.13 eV to come to the final state(FS1), releasing a large energy of 3.06 eV in the overall LH process (ISb FS1). We can find LH process show a lower determined energy barrier and is more thermodynamically favourable when comparing to the ER process for CO+ O2

CO2 + O. Therefore, the LH process is more feasible for the initial CO oxidation rather than the ER process. After reaching FS1, the resulting CO2 molecule is easy to desorb from the substrate due to the weaker interaction between CO2 and the substrate, thus leaving an atomic O strongly interacting with the Mn atom. Such an activated surface O atom can readily bind another CO molecule to form an another intermediate state, surmounting an energy barrier of 0.41 eV and releasing a energy of 1.36 eV (see ISc FS2 in Figure 3b). The intermediate state will soon turn to the final state by desorbing another CO2 with an energy barrier of 0.50 eV to regenerate Mn-Gr. Overall the whole CO oxidation, the highest energy barrier of this catalytic cycle is therefore only 0.72 eV, higher than that of Ni-Gr27 (0.63 eV) at the same calculation level of theory. Moreover, the same determined step CO + O2 OC*OO* with O2 activation is observed on Mn-Gr and Ni-Gr27. But such the disparate energy barriers exist between Mn-Gr and Ni-Gr. It might corroborate the fact that O2 on Mn-Gr is not so activated as Ni-Gr. For the case of Mn-Gr, the O-O bond is only 1.33 Å for the CO/O2 coadsorption configuration, while for that of Ni-Gr the O-O bond is 1.36 Å. O2 is more active on Ni-Gr and thus easier to reach the peroxide-type like intermediate OC*OO*. Since Zhang et. al. have revealed that the adsorption of O2 molecule can be tuned on Au-doped graphene by an external 10


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electric field and it has pointed out the findings can provide a new route for tuning the O2 adsorption process.41 Hence, we propose an appropriate electric field applied in CO oxidation on Mn-Gr would help O2 activation and promote CO oxidation. Therefore, a detail investigation of CO oxidation on Mn-Gr under various electric field are explored in the following part.

3.2. CO oxidation on Mn-Gr under electric field

Figure 4. (a) Sideview of Mn-Gr with the direction of external electric field, Variation of (b)Ead and (c) Ea under different electric field.

To probe the electric field effect, we applied an external electric field perpendicular to the graphene sheet, of which the upward “+” (downward “-”) direction was defined as a positive (negative) one, see in Figure 4a. The electric field imposed on our studied system varies from -1.00 to 0.50 V/Å with a step of 0.25 V/Å. Figure 4b depicts the adsorption energy Ead variations under different electric field for O2, CO, CO2, and the coadsorption of CO/O2 on Mn-Gr. It shows Ead decreases with increasing electric field for O2, CO, and CO/O2, suggesting a weaker interaction under a positive electric field while stronger interaction under a negative electric field. Except that, O2 adsorption on Mn-Gr remains stronger than CO adsorption, suggesting Mn-Gr can be applied to catalytic oxidation of CO with negligible CO poisoning effect under different electric field when the CO/O2 mixture is injected with a 11



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constant gas flow rate. Moreover, CO/O2 on Mn-Gr also shows a larger adsorption than the isolated O2 or CO molecule adsorption. Therefore, both LH and ER mechanism involved in the initiation step of CO oxidation are considered to explore the electric field effect. We have also detect the rule of O2 dissociation under different electric field and the calculated results show that the dissociation barrier Ea for O2 decreases with decreasing electric field, see Figure 4c. It increases to 1.14 eV under 0.50 V/Å and reduces to 0.69 eV under -1.00 V/Å. However, it is not the case for CO2. The data in Figure 4b shows that the weakest adsorption is about -0.40 eV under the electric field of -0.50 and -0.25 V/Å. Increase the external electric field from -0.25 to 0.5 V/Å or decrease the external electric field from -0.50 to -1.00 V/Å, the CO2 adsorption ability on Mn-Gr increases. Such case would increase a difficulty for CO2 desorption and hinder the catalyst recycling. Therefore, considering CO2 desorption to regenerate the catalyst and then obtain the optimal electric field applied in CO oxidation, we have only investigated the reaction mechanism of CO oxidation under the external electric field of -0.75, -0.50, 0, and 0.5 V/Å, of which the potential energy surfaces are diagrammed in Figure 5a-c.

12


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Figure 5. Schematic energy profile along (a) ER, (b) LH process for the initiation steps, (c) ER process for the following steps to remove the left atomic O together with CO2 desorption. (d) The activation energy Ea of the determined step in CO oxidation, as a function of Ead of CO/O2 coadsorption on Mn-Gr. Red line represent the linear fit to the data with an equation of Ea=0.519Ead+0.657, R2=0.922. (e) Diagram for comparison of Ea between CO oxidation and CO2 desorption under various external electric field.

Table 1. The structural parameters, Hirshfeld charge, HOMO and LUMO, and Fermi level of the CO/O2 coadsorption configuration under different electric field. Electric field/(FÅ-1)

dC-O/Å

0.50

1.14

0 -0.50 -0.75

1.15 1.15 1.16

Ef/eV

Q(CO) /e

Q(O2)/e

1.32

0.14

-0.06

0.16

-0.079

-5.61

1.33 1.35 1.36

0.08 0.02 -0.02

-0.15 -0.24 -0.28

0.14 0.13 0.12

0.068 0.22 0.30

-6.00 -6.42 -6.63

dO-O/Å

Q(Mn)/e

Q(Gr-Mn)/e

HOMO/eV

LUMO/eV upspin

downspin

-4.68

-4.90

-4.86

-5.15 -5.65 -5.91

-5.28 -5.70 -5.94

-5.24 -5.64 -5.85

Figure 5a shows a decreasing energy barrier with the decreasing electric field in the ER initiation process and it reduces to 0.60 eV under -0.75 F/Å. Similar to the ER process, the LH initiation process (Figure 5b) and the formation of the second CO2 via CO attacking the left atomic oxygen (Figure 5c) are presented the same trends. When the applied electric field is -0.75 F/Å, Ea for CO + O2 OC*OO*(ISbMSb) decreases to 0.35 eV, Ea for OC*OO* CO2 + O* (MSbFS1) decreases to 0.07 eV, Ea for CO + O* CO2 (IScFS2) is 0.17 eV. Overall the whole CO oxidation process, LH mechanism in the initiation step is always more preferred than ER mechanism under various electric field and the formation of the peroxide OC*OO* intermediate in CO + O2 OC*OO*(ISbMSb) always determines the whole reaction process with its elementary activation energy being consideration as the overall energy barrier. As proposed the correlation between O2 activation and the catalytic reactivity, Ea can be roughly interpreted as a linear function of Ead(CO/O2) shown in Figure 5d. The linear relationship implies that the 13



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stronger adsorption of CO/O2, the lower the reaction barrier and CO formation are promoted. Table 1 has summarized the structural parameters, Hirshfeld charge, HOMO and LUMO, and Fermi level of the CO/O2 coadsorption configuration under various electric field. It is found that the bond length of CO and O2 both increase as the electric field decreases. The Hirshfeld charge suggests that when electric field decreases from 0.50 to -0.75 F/Å, the positive charged CO becomes negative charged, O2 becomes more negative charged, and Mn atom decreases its positive charge. It can be conducted that when electric field decreases from 0.50 to -0.75 F/Å, more electron is accumulated on O2 due to the electron donation by Gr-Mn mediated by Mn atom. Thus O2 are more activated under more negative electric field which might be the reason for a higher catalytic activity of Mn-Gr in the case. However, when the electric field is negative than -0.50 F/Å, increasing interaction between substrate and oxygen makes stronger adsorption of CO2 on Mn-Gr, leading a difficulty for CO2 desorption. Thus the catalyst Mn-Gr regeneration is hampered, consisting with the Sabatier principle37. As shown in Figure 5e, for a cycle of CO oxidation with catalyst regeneration, the determined energy barrier is 0.81 eV for CO2 desorption under -0.75 F/Å, and 0.55, 0.72, 0.88 eV for CO2 formation under -0.50, 0, 0.5 F/Å, respectively. Therefore, it is recommended that -0.50 F/Å is a better choice to promote CO oxidation on Mn-Gr with a lower determined reaction energy barrier of 0.55 eV when compared with others. When compared with the commonly used noble metal catalysts Pt (>1.0 eV)52 and Au3 (0.93 eV), the CO oxidation reactions are more likely to proceed rapidly on the Mn-Gr under -0.50 F/Å, because CO oxidation through LH mechanisms has the lower determined energy barrier (0.55 eV).

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Figure 6. (a) Orbital charge density isosurfaces in the CO/O2 coadsorption configuration (isovalue = 0.03). Projected DOS for CO/O2 coadsorption configuration on Mn-Gr under an applied external electric field of (b) -0.75, (c) -0.5, (d) 0, and (e) 0.5 F/Å.

To unveil the microscopic scenario behind the peculiar catalytic activity of Mn-Gr, we calculated the frontier orbitals and the projected density of state (DOS) of CO/O2 coadsorption mode under various electric field. Figure 6a shows that HOMO and LUMO are mainly contributed by the significant hybridization between Mn-3dπ, CO-2π*, and O2-2π* orbitals without electric field. And further calculations indicate that it is the same case for those under an external electric field. But HOMO, LUMO, and Fermi level are blue shifted with decreasing electric field (Table 1) due to the increasing interaction between molecules and Mn-Gr. DOS shows the superposition of the CO, O2 and Mn states in Figure 6b-e. We can find that the level of Mn-d below Fermi level is partially depressed under -0.75, -0.5 F/Å when compared with that without electric field or under 0.5 F/Å, consist with the increasing positive charged Mn atom as the electric field decreases (Table 1). The occupied 15



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O2-2π* are slightly raising its density and the energy levels of O2-5σ and O2-1π become more close to the Fermi level when the electric decreases from 0.5 to -0.75 F/Å upon the increasing adsorption and more electron transfers from Mn-Gr to O2. But the case for CO is complicated due to the donation of CO-5σ to Mn-3dz2 orbitals and the back-donation of Mn-3dπ to CO-2π* orbitals. Nevertheless, strong hybridization exhibits between the O2-1π, O2-2π* orbitals, CO-5σ, CO-2π* orbitals, and Mn-3d orbitals due to the increasing adsorption under negative electric field, leading to the activation of O-O bond and thus accelerating the catalytic performance of CO2 formation.

Figure 7. Orbital charge density isosurfaces in the CO2 adsorbed on Mn-Gr (isovalue = 0.03). 16


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While for CO2 on Mn-Gr, the frontier orbitals in Figure 7 show that HOMO, HOMO-1, HOMO-2, HOMO-3 are mainly consisted of Mn-dπ and Mn-dz2 components under 0.5 F/Å or without electric field, but HOMO under -0.50 and -0.75 are mainly contributed by Mn-dπ and CO2-2πu. The Hirshfeld charge analysis indicates charge on CO2 when adsorbed on Mn-Gr is respectively 0.26, 0.20, -0.26, -0.39 e under 0.5, 0, -0.5, -0.75 F/Å, implying that CO2 loses its electron to Mn-Gr mediated by Mn atom under 0.5 F/Å and without electric field, while CO2 accumulates electron from Mn-Gr under -0.5, -0.75 F/Å. As known, HOMO is the doubly degenerate nonbonding πg orbitals and LUMO is the doubly degenerate 2πu orbitals in isolated CO2 molecule, therefore CO2 on Mn-Gr under 0.5 F/Å or without electric field would donate its electron from its πg orbitals to Mn-dπ orbitals, and thus CO2-πg orbitals can be observed at the bottom of the conduction band while Mn-dπ orbitals at the top of the valence band near Fermi level, see Figure 8c-d. Moreover, the orbital contribution is more for 0.5 F/Å which implies a stronger binding between CO2 and Mn-Gr under 0.5 F/Å when comparing the projected DOS without electric field. When -0.5, -0.75 F/Å applied, electron transfers from Mn-dπ to CO2-2πu orbitals and a depressed Mn-dπ orbitals is shown in the valence band when compared to that of 0.5 F/Å and without electric field, see Figure 8a-b. Moreover, the strong hybridization exists between Mn-dπ and the occupied CO2-2πu orbitals at the top of the valence band and the bottom of the conduction band is mainly contributed by Mn-dπ orbitals. Except that, a stronger hybridization is observed under -0.75 F/Å when comparing to -0.5 F/Å. So that explains why a larger electric field like 0.5 F/Å and -0.75 F/Å applied would induce CO2 desorption difficult and then hinder the whole reaction cycle. Therefore, it is meaningful to seek an appropriate electric field to accelerate the catalytic elimination of the hazardous CO molecules with the catalysts sustainable. 17



Page 18 of 23

Figure 8. Projected DOS for CO2 adsorbed on Mn-Gr under an applied external electric field of (a) -0.75, (b) -0.5, (c) 0, and (d) 0.5 F/Å.

4.

Conclusions

By utilizing DFT calculations, we explored the stability of Mn-Gr, the adsorption ability of CO, O2 and CO/O2 onto Mn-Gr, the reaction mechanism of CO oxidation on Mn-Gr within or without electric field. The results indicate that Mn-Gr can be stabilized without metal clustering problem and preserved from CO poisoning when the CO/O2 mixture is injected with a constant gas flow rate due to a stronger adsorption for O2 than CO on Mn-Gr. The investigation of the reaction mechanisms for CO oxidation on Mn-Gr suggest that the reaction remains to proceed first via the Langmuir-Hinshelwood (LH) mechanism CO + O2 OC*OO* CO2 + O and then via the Eley-Rideal (ER) mechanism CO + O* CO2 under various electric field, of which the determined step is always CO + O2 OC*OO*. Moreover, the reaction energy barriers for CO2 formation along CO oxidation decrease as the electric field decreases from 0.5 to -0.75 V/Å, which is analyzed to be correlated with the activation of O2 on Mn-Gr. As the decreasing electric field strengthen the adsorption ability for CO/O2 on Mn-Gr, it leads to a more activation of O-O bond and then accelerate the CO2 formation. However, a stronger adsorption is observed for CO2 on Mn-Gr under a more negative electric field like -0.75 F/Å, which could induce a difficulty for CO2 desorption and the catalyst regeneration. Therefore, 18


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considering the CO2 formation and desorption, -0.50 F/Å are found to be more appropriate for CO oxidation on Mn-Gr with a determined reaction barrier of 0.55 eV, in which the adsorption energy is neither too strong nor too weak and the reaction barrier is lower than other studied systems. The electric field effect on the catalytic performance of Mn-Gr to CO oxidation could be interpreted by electronic structure analysis, which implies the important role of electric field applied in electron transfer between Mn-Gr and the adsorbates.

Notes The authors declare no competing financial interest.

Acknowledgements We gratefully acknowledge the support of Shaoguan University (Nos. S201501027, Nos. SZ2016KJ03) and the Guangdong Provincial Natural Science Foundation (Nos. 2015A030313185).

Supporting Information The imaginary frequency of the transition states in O2 dissociation and CO oxidation under various electric field are collected.

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TOC Graphic

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Probing the Electric Field Effect on the Catalytic Performance of Mn

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