Journal ofMaterials Chemistry A

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A novel single-at

aCollege of Mechanics and Materials, Ho

E-mail: jfzhang@hhu.edu.cnbGuangzhou Key Laboratory Environmen

Guangdong Key Laboratory of Environmen

Institute of Environmental Health and Pol

Science and Engineering, Guangdong Unive

China. E-mail: zhimin.ao@gdut.edu.cn

† Electronic supplementary informationand gures. See DOI: 10.1039/c9ta08525d

Cite this: J. Mater. Chem. A, 2020, 8,287

Received 5th August 2019Accepted 31st October 2019

DOI: 10.1039/c9ta08525d

rsc.li/materials-a

This journal is © The Royal Society of C

om catalyst for CO oxidation inhumid environmental conditions: Ni-embeddeddivacancy graphene†

Quanguo Jiang, a Jianfeng Zhang, *a Huajie Huang, a Yuping Wua

and Zhimin Ao *b

The degradation of catalysts for CO oxidation in humid air is a common issue owing to the blocking of the

active site by the adsorption and dissociation of water molecules. In order to evaluate the effect of humidity

on the CO oxidation, the adsorption and dissociation of the common gas molecules in air, namely CO, O2,

H2O, and N2, on single Ni-embedded divacancy graphene (Ni-DG) have been investigated using first-

principles calculations. It was found that all the molecules keep their molecular state and the adsorption

energy of CO is much larger than those of the other molecules. In addition, the CO molecules can

sufficiently substitute the pre-adsorbed O2, H2O, and N2 molecules with small energy barriers, indicating

that the active site of the Ni-DG will not be blocked by water molecules in humid environments. At most

two CO molecules can be chemically adsorbed on the Ni-DG, indicating that a new termolecular Eley–

Rideal (TER) mechanism is preferred, and the energy barrier for the rate-limiting step (2CO + O2 /

OCOOCO) is only 0.34 eV. Hirshfeld charge analysis shows that the charge transfer from the O2-2p*

orbital to the CO-2p* orbital plays an important role in the CO oxidation via the TER mechanism.

Overall, our results show that the low-cost Ni-DG is an efficient catalyst for CO oxidation, even in humid

air at low temperature.

1. Introduction

With environmental pollution getting worse, the trans-formation and elimination of waste gases has become particu-larly important.1 Among these waste gases, CO is very harmfulto human health, even in small quantities, thus CO conversionplays an important role in solving the environmental issues,where the most common method is the oxidation of CO intoCO2.1–3 Some noble metals, such as Au,4 Pt,5,6 Pd7 and Rh,8 havebeen studied as catalysts for CO oxidation. However, thesenoble metal catalysts are costly and usually require high reac-tion temperatures for efficient operation owing to the highenergy barriers, which is energy-consuming and may triggerexplosions due to the high temperatures. Therefore, reducing orreplacing the use of noble metals in the catalysts is needed.Non-noble metal oxide catalysts, such as Co3O4, show highcatalytic activity at low temperature.9–11 However, the Co3O4

hai University, Nanjing 210098, China.

tal Catalysis and Pollution Control,

tal Catalysis and Health Risk Control,

lution Control, School of Environmental

rsity of Technology, Guangzhou, 510006,

(ESI) available: Additional information

hemistry 2020

surface is very sensitive to the reaction conditions and waterconcentration,9,10 where the performance towards CO oxidationmay severely deteriorate in the presence of trace amounts ofmoisture owing to the blocking of active sites by the adsorptionand dissociation of water molecules. As a result, developingefficient non-noble metal catalysts for CO oxidation in humidair is desirable.

In recent decades, carbon nanomaterials (such as fullerene,carbon nanotubes, graphene and graphdiyne) have beenstudied as either catalyst supports or catalysts for variousapplications. As a novel form of carbon, graphene12 is a prom-ising matrix to support metal atoms to realize new catalystsowing to its outstanding electrical,13 mechanical14 and thermalproperties.15 Moreover, the large surface-to-volume ratio alsobenets the use of graphene as a support for heterogeneouscatalysts. Decorating metal atoms into graphene is usually usedto enhance the interactions between the inert graphene surfaceand gas molecules. Theoretically, single metal atoms supportedon graphene with a single carbon vacancy (SV), such as Au-,16 Fe-,17 Cu-,18 Pt-,19 Al-,20 Co-,21 Ni,22,23 and Pd-embedded24 SV gra-phenes, exhibit high reactivity toward CO oxidation. The energybarriers for the CO oxidation reaction on the above catalysts canbe reduced to 0.31–0.58 eV compared with the metal catalysts.In addition, single metal atoms supported on graphene withdouble carbon vacancies (DV), such as Mn-,25 and Co-embedded26 DV graphenes, also exhibit high reactivity toward

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CO oxidation. However, water vapor is inevitable in practicalapplications and the catalysts always deactivate rapidly in thepresence of a small amount of water vapor, which was notconsidered in the previous DFT studies.16–26 On the other hand,recent experimental studies have revealed that oxide supports,such as FeOx,27 MgO,28 and TiO2,29 or graphitic layers30–32 cananchor various single metal atoms and hence allow thesynthesis of single-atom catalysts (SAC).

In general, CO oxidation on catalysts may occur via either theEley–Rideal (ER) mechanism or the Langmuir–Hinshelwood (LH)mechanism.16–26 In the ER mechanism, the gas-phase CO mole-cule directly reacts with the pre-adsorbed and activated O2 toform a carbonate-like intermediate (CO3) or the nal product ofCO2. The LHmechanism involves the co-adsorption of O2 and COmolecules, the formation and dissociation of a peroxide-like(OCOO) complex intermediate, and nally desorption of CO2.The LHmechanism is preferred for CO oxidation on most metal-decorated graphenes because the adsorption energy of an O2

molecule is larger than that of a CO molecule and the co-adsorption energy of CO and O2 molecules on these metal-embedded graphenes is larger than the individual adsorptionenergy of both CO and O2 molecules. More interestingly,a recently reported termolecular Eley–Rideal (TER) mechanismwith the O2 molecule being directly activated by the two pre-adsorbed CO molecules has been identied to act as a possibleor even preferable mechanism for CO oxidation.33–36

Nickel has Earth-abundant reserves and is environmentallybenign. In addition, Ni-decorated single vacancy graphene hasshown good performance for CO oxidation with a rate-limitingenergy barrier of 0.59 eV (ref. 22) (0.63 eV (ref. 23)). Note that thesingle carbon vacancies have a low migration barrier (about 1.3eV),37,38 which means that the single vacancies can merge intodivacancies. On the other hand, the carbon divacancies need toovercome a larger energy barrier (about 7 eV) to migrate,37,38

which makes the divacancies much more stable than the singlevacancies, thus Ni-decorated divacancy graphene can besynthesized more easily. Generally, the adsorption energy of O2

is larger than that of CO, where the O2 molecule has the priorityto adsorb on the active site of the single-atom catalyst.17–26 If theadsorption energy of the CO molecule is larger than that of theO2 molecule, the catalyst will easily capture the CO moleculeand then promote the CO oxidation process. Fortunately, theadsorption energy of CO is larger than that of O2 on Ni-DG, asreported in previous literature.25,26 However, further study of COoxidation on Ni-DG is lacking.

In this work, we will study the reaction mechanism of COoxidation on Ni-embedded graphene via the TER mechanismthrough rst-principles calculations. The effects of H2O and N2

molecules on the energy barriers for the reaction are also dis-cussed and the corresponding mechanisms are analyzedthrough understanding the charge transfer and electronicorbitals.

2. Calculation methods

All density functional theory (DFT) calculations with spin-unrestricted in this work are done by using the Dmol3

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module.39 Generalized gradient approximation (GGA) with thePerdew–Burke–Ernzerhof (PBE)40 functional has been adoptedas the exchange–correlation functional. To take into consider-ation the effect of van der Waals forces, the Grimme scheme41

for DFT-D correction is used in all the calculations. DFT semi-core pseudopotentials (DSPPs) core treatment is implementedfor relativistic effects, which replaces core electrons witha single effective potential. The convergence tolerance of energyof 10�5 Hartree is taken (1 Hartree¼ 27.21 eV), and themaximalallowed force and displacement are 0.002 Hartree A�1 and 0.005A, respectively. It was reported that the selected exchange–correlation functional has an evidential effect on the adsorptionenergy results. However, the effect on the calculated reactionenergy barriers is much smaller.42 To investigate the minimumenergy pathway for CO oxidation on graphene, the linearsynchronous transit/quadratic synchronous transit (LST/QST)43

and nudged elastic band (NEB)44 tools in the Dmol3 module areused, which have been well validated to determine the structureof the transition state and the minimum energy reactionpathway. In the simulation, three-dimensional periodicboundary conditions are taken. The simulation cell consists ofa 4 � 4 graphene supercell with a vacuum width of 20 A abovethe graphene layer to minimize the interlayer interaction. The k-point is set to 5 � 5 � 1, and all atoms are allowed to relaxaccording to our convergence test. Aer structure relaxations,the density of states (DOS) is calculated with a ner k-point gridof 15 � 15 � 1. The electron orbits of the molecule/graphenesystems are calculated with CASTEP code,45 where the ultra-so pseudopotentials, the GGA-PBE functional, an energy cutoffof 340 eV and 5 � 5 � 1 k-point meshes are used.

For the molecules (CO or O2) adsorbed on graphene, theadsorption energy Ead is determined by:

Ead ¼ Emolecule/graphene � (Egraphene + Emolecule) (1)

where Emolecule/graphene, Egraphene and Emolecule are the totalenergies of the molecule/graphene system, the isolated gra-phene and a molecule in the same slab, respectively.

3. Results and discussion

Based on our previous reports,25 the adsorption energies of COand O2 molecules on divacancy graphene embedded withtransitionmetals (from Sc to Zn) are systemically studied, wherethe adsorption energies of CO and O2 on Ni-DG are �1.21 eVand�0.41 eV, respectively. These moderate adsorption energiesindicate that the Ni-decorated divacancy graphene (Ni-DG) maypossess good catalytic properties for CO oxidation. To carefullynd the most stable structure for decorating Ni atom on thedivacancy graphene, the possible adsorption sites for Ni on DGare studied, as shown in Fig. S1.† Based on the DFT calcula-tions, the Ni atom at the T1 site simultaneously diffused to theB1 site (see Fig. S2a†), while the Ni atom on the T2, T3, T4, B2, B3,B4, B5, and Hol2 sites simultaneously diffused to the Hol1 site(see Fig. S2b†) aer geometry optimization. Finally, the Ni atomcan adsorb on the B1, Hol1, Hol3 and Hol4 sites with adsorptionenergies of �1.96 eV, �7.93 eV, �2.42 eV, and �2.43 eV,

This journal is © The Royal Society of Chemistry 2020

Fig. 2 (a) The pathway for the diffusion of the embedded Ni atom ondivacancy graphene, where IS, TS and FS represent the initial, transitionand final structures, respectively, in this and the following figures. (b)The phonon structure of Ni-embedded divacancy graphene.

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respectively (see Fig. S2†). Therefore, the conguration with theNi atom adsorbed on the Hol1 site has the largest adsorptionenergy. To check the mobility of the adsorbed Ni atom atdifferent sites (B1, Hol1, Hol3 and Hol4) on the divacancy gra-phene, the diffusion pathways of the Ni atom from the B1, Hol3and Hol4 sites to the Hol1 site are also shown in Fig. S2,† wherethe diffusion barriers are 0.21 eV, 0.20 eV and 0.22 eV, respec-tively, which further conrms that the divacancy graphene withthe Ni atom adsorbed at Hol1 is the most stable conguration,as shown in Fig. 1a.

The electronic and atomic structures for Ni-DG are rststudied in the following. Aer embedding Ni atom into thedivacancy graphene, where two C atoms are substituted by oneNi atom, the structure of graphene is reconstructed and therelaxed structure is shown in Fig. 1a. Single chemical Ni–Cbonds are formed in graphene by calculating the Mayer bondorder of 0.838. The Ni atom is in the same plane with the Catoms, which is different from single vacancy graphene wherethe Ni atom moves out of the plane to get more space due to itsrelatively large atomic radius.22,23 The C–Ni bond length lC–Ni is1.89 A, which is in good agreement with the reported result of1.86 A.26 The binding energy of the Ni atom in graphene is�7.93 eV, which is larger than that of �7.43 eV (�6.98 eV (ref.22) or �7.57 eV (ref. 23)) for the Ni atom in single vacancygraphene. The atomic charges obtained by the Hirshfeldmethod near the dopant are also given in the atomic congu-ration in Fig. 1a, where the Ni atom forms an electron-depletionposition by losing electrons (0.054e) from graphene. The stronginteraction between the Ni atom and the C atoms can be furtherconrmed by the partial density of states (PDOS) (Fig. 1b),where signicant overlap of the bands between the embeddedNi atom and the nearby C atom is found. In addition, thedifferential charge density along Ni–C bonds for Ni-G is shownin Fig. 1a, where the blue and red isosurfaces correspond to theincrease in the number of electrons and the depletion zone,respectively. It shows that electrons are depleted near the doped

Fig. 1 (a) The most stable structure of Ni-embedded divacancy gra-phene, where the grey and blue atoms are C and Ni atoms in this andthe following figures. The atomic charge obtained by Hirshfeld analysisnear the dopant is also given. The differential charge density along C–Ni–C bonds for Ni-DG is shown, where the blue and red isosurfacescorrespond to the increase in the number of electrons and thedepletion zone, respectively. (b) PDOS of Ni atom and C atom on theNi-embedded divacancy graphene. The vertical line indicates theFermi level.

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Ni atom, indicating the high chemical active area. Theremaining unsaturated d orbital of the Ni atom is reactive,which can adsorb small molecules and promote the subsequentreactions.

The aggregation problems for the adsorbed metal atoms onthe substrate are signicant for the catalytic performance,

Fig. 3 The pathway for the dissociation of O2 (a), CO (b), H2O (c), andN2 (d) molecules on the Ni-embedded graphene. IS represents themost stable structure for each adsorbed molecule on Ni-embeddeddivacancy graphene.

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especially when the concentration of metal atoms is high.20,25 Todetermine the possibility of aggregation for Ni atoms on diva-cancy graphene, the diffusion pathway of the Ni atom to itsneighbouring positions is investigated based on DFT calcula-tions (see Fig. 2a), where the corresponding diffusion energybarrier for the decorated Ni atom is 6.23 eV. It is claimed thata surface reaction will occur when the reaction barrier is smallerthan the critical value of Ecbar ¼ 0.75 eV,46 thus the Ni-decorateddivacancy graphene is quite stable without aggregation prob-lems. The phonon dispersion of Ni-DG has been calculated asshown in Fig. 2b, where there are no so modes for the Ni-DG,indicating that the studied structure is stable at room temper-ature. In addition, the adsorption energy of the Ni atom ondivacancy graphene is �7.93 eV. Such strong binding indicatesthat Ni-DG is rather stable and the Ni atom cannot be removedthermally or by irradiation with sub-MeV electrons, similar toW-DG (W-doped divacancy graphene).47,48

O2, CO, H2O, and N2 are the common gas molecules inhumid atmospheres, which may affect the oxidation reaction ofCO on the supported single-atom catalyst. Before studying theCO oxidation reaction, the adsorption properties of the abovegas molecules on Ni-DG are rst discussed. In addition, theperformance of CO oxidation catalysts usually falls in moistenvironments owing to the dissociation of water molecule andits subsequent reactions with the catalyst.49 Therefore, theinteractions between the mentioned molecules and the Ni-DGare important, which will determine the nal performance ofthe catalyst and are not considered in the previous literatureusing the DFT method.16–26 The most stable congurations forthe adsorption of O2, CO, H2O and N2 on Ni-DG are shown inFig. 3 (see the IS structure). It can be seen that the O2, CO andH2Omolecules are chemically adsorbed on the Ni-DG, while theN2 molecule is physically adsorbed on the Ni-DG. The adsorp-tion energies on Ni-DG in Table 1, where the correspondingadsorption energies are �0.41, �1.24, �0.43 and �0.17 eV forO2, CO, H2O and N2, respectively. To evaluate the stability of Ni-DG aer the adsorption of the molecules, the adsorption energyof the Ni–M species (M is the adsorbed molecules) on divacancygraphene is calculated to estimate the strength of the Ni–Cbond as reported in previous literature,33 where the adsorptionenergies of Ni–CO, Ni–O2, Ni–H2O and Ni–N2 species on diva-cancy graphene are �6.57 eV, �7.10 eV, �7.55 eV and �7.91 eV,respectively, which are similar to that of Ni atom on divacancygraphene (�7.93 eV). Therefore, the Ni-DG is also thermallystable aer adsorption of the above molecules. The adsorbedmolecules may dissociate on the surface, as discussed in theprevious literature.20,25,50,51 To assess the stability of the

Table 1 The adsorption energy Ead, Hirshfeld charge transfer Q, andbond length l for each adsorbed molecule on Ni-DG

Ead (eV) Q (e) lad (A) lfree (A)

O2 �0.41 �0.066 1.26 1.22CO �1.21 0.058 1.15 1.14H2O �0.43 0.125 0.98 0.98N2 �0.17 0.012 1.11 1.11

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adsorbed molecules, the dissociative reactions of individual O2,CO, H2O and N2 molecules on Ni-DG are studied in Fig. 3.

Aer NEB calculations, the dissociation reaction barrier foran O2molecule on the Ni-DG is 1.83 eV > Ecbar¼ 0.75 eV,46whichindicates that the adsorbed O2 molecule prefers to stay on theNi-DG in the molecular state at low temperature. The bindingenergy Eb of a C–O bond is 11.57 eV, which is much larger thanEb¼ 6.36 eV for an O–O bond based on DFT calculations.25 ThusCO should be more difficult to be dissociated, which isconrmed by the fact that the dissociative energy barrier for theCO molecule on Ni-DG is 4.02 eV, as shown in Fig. 3b. Thedissociation barriers of H2O and N2 molecules on Ni-DG are1.15 eV and 5.86 eV, as shown in Fig. 3c and d, which indicatesthat the adsorbed H2O and N2 molecules also prefer to stay onNi-DG in the molecular state at low temperatures. Based on theabove discussions, the catalytic performance of Ni-DG will notbe degenerated by water molecules and nitrogen gas. Thereaction mechanism for CO oxidation is then studied in thefollowing.

The adsorption energy for a CO molecule on Ni-DG is muchlarger than those for O2, H2O, and N2 molecules, as shown inTable 1. We thus consider the possibility of the substitutedadsorption of CO for the other pre-adsorbed molecules, asshown in Fig. 4. For the pre-adsorbed O2 molecule in Fig. 4a, thefree CO molecule will adsorb on the Ni atom aer overcominga small barrier of 0.25 eV, and the pre-adsorbed O2molecule willbecome physically adsorbed on the graphene surface, as shownin the FS structure in Fig. 4a. Therefore, the CO molecule willautomatically adsorb on the Ni-embedded graphene even withthe large amount of O2 molecules in the atmosphere. In thepresence of moisture, the H2O molecule may adsorb on theactive site and prevent the subsequent CO oxidation reactions.The substituted adsorption process of CO to H2O molecules isshown in Fig. 4b, where only a small energy barrier of 0.12 eVneeds to be overcome and heat of �0.83 eV is released, indi-cating that the COmolecule will automatically adsorb on the Ni-DG in a humid environment. A similar reaction path for thesubstituted adsorption of CO for the N2 molecules is shown inFig. 4c, where only a small energy barrier of 0.22 eV needs to beovercome. Therefore, the CO molecule will automaticallyadsorb on the Ni-DG in an ambient atmosphere with andwithout humidity. With sufficient CO molecules in the atmo-sphere, it is possible for two or more COmolecules to adsorb onthe surface of the catalyst.

To check whether the O2 and H2O molecules will block thesecond CO adsorption, the co-adsorption of H2O and CO, aswell as the co-adsorption of O2 and CO, is then studied. Whenthe CO and O2 were manually put on the Ni atom with a co-adsorption conguration on Ni-DG, the O2 molecule simulta-neously desorbed from the Ni atom aer geometry optimiza-tion, leaving the COmolecule solely chemically adsorbed on theNi atom. In the nal adsorption conguration, the CO chemi-cally adsorbs on the Ni atom, while the O2 molecule physicallyadsorbs on the graphene surface with a Ni–O distance of 4.755A, as shown in Fig. 4a. Similar to the O2 molecule, aer geom-etry optimization, the manual co-adsorption conguration forthe CO and H2O on the Ni atom will be broken with the H2O

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Fig. 4 The reaction pathway for the substituted adsorption of CO onthe Ni-embedded graphene with pre-adsorbed O2 (a), H2O (b), and N2

(c) molecules. (d) The reaction pathway for the co-adsorption of twoCO molecules is also shown. The possible reaction between H2O andthe two CO molecules is shown in (e), while the possible reactionbetween N2 and the two CO molecules is shown in (f).

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molecule simultaneously desorbing from the Ni atom. In thenal adsorption conguration, the CO molecule chemicallyadsorbs on the Ni atom while the O2 molecule physicallyadsorbs on the graphene surface with a Ni–O distance of 4.146A, as shown in Fig. 4b. Therefore, the co-adsorption of O2 andCO, as well as H2O and CO, cannot occur, indicating that the O2

and H2O molecules will not block the adsorption of the secondCO on Ni-DG.

The adsorption of two CO molecules on the Ni-DG is alsostudied and shown in Fig. 4d, where the free CO molecule willco-adsorb on Ni-DG aer overcoming a small energy barrier of0.46 eV. We note that the adsorption energies for the rst COand second CO molecules on Ni-DG are �1.21 eV and �0.39 eV,respectively, indicating that the adsorption for the second COmolecule is preferred in energy. In addition, the desorptionenergy barrier (the energy difference between FS and TS inFig. 4d) for the second CO molecule is 0.74 eV, which is nearlythe same as the critical barrier of 0.75 eV;46 thus the desorptionof the second CO molecule is difficult at low temperature. Theadsorption energy for a third CO molecule on Ni-DG is 0.75 eV,which is an endothermic process, indicating that the Ni-DG canadsorb only two CO molecules at most in the 4 � 4 supercell.Based on the above discussions, two CO molecules will adsorbon the Ni-embedded divacancy graphene in the presence of COmolecules in the atmosphere, which is the initial state for the

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CO oxidation on Ni-DG catalyst. The adsorbed COmolecules aremore reactive than the free CO owing to the electron transferfrom the graphene substrate. Thus we further consider thepossibility for the reaction between the H2O or N2 molecules inthe atmosphere and the two adsorbed CO molecules on Ni-DG,and the detailed reaction path is shown in Fig. 4e and f. Thereaction barriers are 1.16 eV and 4.61 eV for H2O and N2

molecules, respectively, which are much larger than the criticalbarrier of 0.75 eV.45 This result conrms that the H2O or N2

molecules in atmosphere will not react with the two adsorbedCO molecules on Ni-DG and the subsequent CO oxidation canproceed smoothly.

Before studying the CO oxidation mechanism on Ni-DG, theelectronic structures of the adsorbed O2 and CO on Ni-DG arestudied. Fig. 5a shows the most stable conguration for O2

molecule adsorbed on Ni-DG, where the angle between the O–Obond and the graphene sheet is about 45�, and 0.066e istransferred from the Ni-DG to the O2 molecule based onHirshfeld charge analysis. The 2p* anti-bond orbital of the freeO2 molecule is half-lled, as discussed in previous litera-ture.20,25,52 The PDOS of the Ni atom and the adsorbed O2

molecule is shown in Fig. 5b, where all orbitals of the adsorbedO2 molecule are also labelled to understand the interactionbetween the adsorbed O2 and Ni-DG. About 0.066e is transferredfrom Ni-DG to the adsorbed O2 molecule based on the Hirshfeldmethod, which occupies the O2-2p* orbital above the Fermilevel and is conrmed by the new peaks for the O2-2p* orbitalbelow the Fermi level (see Fig. 5b). This charge transfer elon-gates the O–O bond from 1.23 A in free O2 to 1.26 A in theadsorbed O2 molecule, which is much shorter than that of 1.43A and 1.40 A on Al-G20 and Mn-DG,25 respectively. Note that theadsorption energies for O2 molecules are �0.41 eV, �1.57 eV(ref. 20) and �1.96 eV (ref. 25) on Ni-DG, Al-G, and Mn-DG,respectively. This result is generally consistent with the factthat the adsorbed O2 molecule on Ni-DG has the smallestadsorption energy and the O2 on Ni-DG is less activated with theshortest O–O bond length compared with that on Al-G and Mn-DG. The interaction between the O2 and the Ni atom is relativelyweaker (see Fig. 5b) than that on Al-G20 and Mn-DG,25 and thePDOS of O2 on Ni-DG is similar to that of the free O2 molecule,further conrming the small adsorption energy for O2 on Ni-DG.

The adsorption conguration of a CO molecule on Ni-DG isshown in Fig. 5c, where the CO molecule is vertically adsorbedon the top of the decorated Ni atom. CO is chemically adsorbedon Ni-DG, which is conrmed by the chemical bond betweenthe Ni atom and the carbon atom of the CO molecule. Theadsorption energy of a COmolecule on Ni-DG is Ead¼�1.21 eV,and the CO molecule transfers 0.058e to the Ni-DG. Fig. 5dshows the PDOS of the adsorbed CO molecule on Ni-DG, wherethe orbitals of the adsorbed CO molecule are labelled and dis-played. The 5s peak of the CO molecule adsorbed on Ni-DG issignicantly depressed compared to the free CO moleculeowing to the charge transfer. Although the 2p* anti-bondorbital far above the Fermi level for the free CO molecule isfully empty, the Ni atom transfers some electrons to the CO-2p*orbital due to the fact that CO-2p* is close to the Fermi level in

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Fig. 6 The reaction pathway for the oxidation of the first COmolecule(a) and the second COmolecule (b) on the Ni-DGwith a pre-adsorbedO2 molecule via the ER mechanism.

Fig. 5 The most stable structure of O2 (a) and CO (c) adsorbed on Ni-DG, where the differential charge density alongO2 and COonNi-DG isshown. The blue and red isosurfaces correspond to the increase in thenumber of electrons and the depletion zone, respectively. PDOS of theadsorbedO2 (b) and CO (d) molecules and the Ni atom for the Ni-DG isalso shown. The vertical lines indicate the Fermi level.

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the absorbed state, which slightly elongates the C–O bond from1.14 A for the free CO molecule to 1.15 A for the adsorbed COmolecule. The above discussions indicate that O2 and CO havechemical interactions with Ni-DG (corresponding adsorptionenergies are �0.41 and �1.21 eV, respectively), but theadsorption of CO is much stronger. The CO molecule is moreactivated on Ni-DG, which is different from how the O2 mole-cule is usually more activated on metal-decorated graphene.20,25

The Ni-DG may be CO poisoned, as discussed in the previousliterature.20,25 However, the CO oxidation along a new TERmechanism may occur and will be discussed in the following.

Two classical reaction mechanisms have been establishedfor the oxidation of the CO molecule: the Langmuir–Hinshel-wood (LH) mechanism and the Eley–Rideal (ER) mecha-nism.16–26 For the ER mechanism, the O2 molecule is rstadsorbed and activated by the catalyst, then a free CO moleculeapproaches the substrate to form an intermediate product. Forthe LH mechanism, the O2 and CO molecules should rst co-adsorb on the Ni atom. However, as discussed above, whenthe CO and O2 were manually put on the Ni atom with a co-adsorption conguration on Ni-DG, the O2 molecule simulta-neously desorbs from the Ni atom aer geometry optimizationin the soware, leaving the CO molecule solely chemicallyadsorbed on Ni atom. In the nal adsorption conguration, theCO chemically adsorbs on the Ni atom, while the O2 moleculephysically adsorbs on the graphene surface with a Ni–O distanceof 4.755 A, as shown in Fig. 4a. Therefore, the co-adsorption ofO2 and CO cannot occur and CO oxidation via the LH

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mechanism is not further considered. The ER mechanism forthe CO oxidation seems to be possible and is studied in thefollowing.

The energy prole for the oxidation of the rst CO moleculevia the ER mechanism is shown in Fig. 6a. The congurationwith physisorbed CO perpendicular to the graphene surface wasselected as the initial state (IS) aer considering all possibleadsorption positions. When one CO molecule approaches theactivated O2, aer overcoming an energy barrier of 3.36 eV, theO–O bond is broken, the O with the dangling bond would forma covalent bond with the C atom in the CO molecule (see TS inFig. 6a). Then the C atom of the CO binds with the free O atomand a CO2 molecule is formed over the Ni atom (see FS inFig. 6a). This process is exothermic and releases energy of4.90 eV. The subsequent CO molecule will react with theremaining O atom to produce the second CO2 molecule aersurmounting an energy barrier of 0.07 eV (see Fig. 6b). Thereaction prole for the production of the two CO2 molecules isshown in Fig. 6, where the rate-limiting step is the formation ofthe rst CO2 molecule. Because the energy barrier of 3.36 eV forthis step is much larger than Ecbar ¼ 0.75 eV,46 the ER mecha-nism is not preferred for the CO oxidation on Ni-DG at roomtemperature. The micro-kinetics method is used to calculate themaximum reaction rate for the CO oxidation via differentreaction pathways by applying the thermodynamic condition ofT ¼ 298 K, PCO ¼ 0.01 bar, PO2 ¼ 0.21 bar as applied in theprevious literature53 (see the Calculation details in the ESI†).The maximum reaction rate is 9.60 � 10�60 s�1 for the ERpathway, and the formation of the rst CO2 molecule (step 1) isthe rate-determining step for the ER pathway based on Camp-bell's degree of rate control analysis.54 The maximum reactionrate is too small, which again conrms that the CO oxidationalong the ER mechanism is difficult to happen at roomtemperature.

The adsorption energy of the CO molecule is much largerthan that of other molecules. It seems that the Ni-embedded

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divacancy graphene may be poisoned by the COmolecule owingto the larger adsorption energy for CO than that for O2, as dis-cussed in previous literature.20,25 However, the reaction viaa new termolecular ER (TER) mechanism may happen for thisgraphene system, where the two pre-adsorbed CO moleculeswill react with the free O2 molecule, which has not been re-ported for the Ni-DG in the literature. For oxidation of CO on theNi-DG via the TER mechanism, several steps and also inter-mediate products (MS) for the oxidation procedure exist.33–36 Foreach step, e.g. from the initial state to the intermediate state inFig. 7, a transition state also exists. The conguration of the twoco-adsorbed CO molecules and the physically adsorbed O2

molecules on Ni-DG is taken as the reactant (IS in Fig. 7) basedon the above discussions. Aer overcoming an energy barrier of0.34 eV, an OCOOCO intermediate (MS in Fig. 7) is formed.Then the two CO2 molecules (FS) are formed on Ni-DG aerovercoming an energy barrier of 0.29 eV (TS2 in Fig. 7), wherethe CO2 is physically adsorbed on Ni-DG with an adsorptionenergy of �0.45 eV. The O–O bond for the OOCOOO congu-ration in the MS is broken at TS2, and two CO2 molecules witha bond angle of 122.3� are formed. The reaction for this step canrelease a heat of 5.52 eV, which can sufficiently overcome theadsorption energy of CO2, and the produced CO2 moleculeswould desorb from the Ni-DG efficiently. The maximum reac-tion rate is 1.06 � 107 s�1 for the TER pathway by using themicro-kinetics method (see the Calculation details in the ESI†),and the formation of the OCOOCO intermediate (step 1) is therate-determining step for the TER pathway based on Campbell'sdegree of rate control analysis.54 Therefore, the TERmechanism

Fig. 7 The reaction pathway for the CO molecule on Ni-DG via the TEHirshfeld charge near the adsorbate is also shown.

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is more preferable than the ER mechanism, and the CO oxida-tion on Ni-DG via the TER mechanism has fast kinetics.

To comprehensively understand Ni-DG's superior catalyticperformance towards CO oxidation, the difference between thesingle Ni atom on single vacancy graphene (Ni-SG) and that ondivacancy graphene (Ni-DG) is then discussed. The bindingenergy of the Ni atom in Ni-DG is �7.93 eV, which is larger thanthat of �7.43 eV in Ni-SG (�6.98 eV (ref. 22) or �7.57 eV (ref.23)), which indicates that the interaction between the Ni atomand the divacancy is stronger than that between the Ni atomand the single vacancy. It is reasonable owing to the fact that theNi atom is four-coordinated in Ni-DG, while it is three-coordinated in Ni-SG, and the Ni atom saturates moredangling bonds of the divacancy in Ni-DG. Note that theadsorption energies for O2 and CO on Ni-SG are �1.50 eV and�0.68 eV, respectively, while the adsorption energies for O2 andCO on Ni-DG are �0.41 eV and �1.21 eV, respectively. There-fore, the Ni-DG can capture the CO molecules more easily thatthe Ni-SG. In addition, the energy barrier for the rate-limitingstep is 0.59 eV (ref. 22) (0.63 eV (ref. 23)) on Ni-SG via the LHpath, while the energy barrier for the rate-limiting step is0.34 eV on Ni-DG via the TER path. Therefore, the Ni atom ondivacancy graphene possesses better catalytic performance thanthat on single vacancy graphene, which provides guidance fortuning the catalytic properties through controlling the defectsof the supporting substrate.

Hirshfeld charge and electronic orbital analyses are per-formed to further reveal the origin of the high efficiency for theCO oxidation on Ni-DG via the TER mechanism. The Hirshfeldcharge for IS, TS and MS is shown in Fig. 7, where the atomic

R mechanism. The unit of E is eV, where E is the energy barrier. The

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charge transfer of the O2 molecule is �0.129e, �0.098e, and�0.057e in the IS, TS and MS structures, respectively, indicatingthat the O2 molecule loses charge from the IS to the MS. Whilethe atomic charge transfer of the two adsorbed CO molecules is0.228e, 0.099e, and �0.118e in the IS, TS and MS structures,respectively, indicating that the CO molecules obtain chargefrom the IS to the MS. Therefore, the O2 molecule transferselectrons to the two adsorbed CO molecules from the IS struc-ture to the MS structure during the reaction progress. The PDOSof the Ni atom and O2 and CO is shown in Fig. 8, where theHOMO and LUMO orbitals are also shown to understand thecharge transfer from the O2 to the adsorbed CO molecules.Fig. 8a presents the HOMO and LUMO orbitals of the IS, wherethe HOMO level dominantly locates on the O2 molecule and theLUMO level mainly locates on the CO molecule and graphene,indicating that the adsorbed CO molecule allows the incomingelectron to occupy this state upon the adsorption of the O2

molecules. Furthermore, the adsorption of O2 is weak with anadsorption energy of 0.25 eV in the IS, and rather strong in theMS with an adsorption energy of 1.13 eV, which indicates thatthe two carbon atoms will provide the anchor for the O2 mole-cule to form the OCONiOCO intermediate. Fig. 8b presents theHOMO and LUMO orbitals of the TS, where the HOMO levelmainly locates on the O2 molecule while part of the HOMO levellocates on the CO, indicating the charge transfer from the O2-HOMO to the CO-LUMO, which is conrmed by the occupationof the LUMO state on the O2 molecule in the upper panel ofFig. 8b. The HOMO and LUMO orbitals of the MS are shown inFig. 8c, where the HOMO level is mainly located on the OC part,which conrms the charge transfer from the O2-HOMO to theCO-LUMO. For comparison purposes, Fig. 8d gives the orbitalsof free CO and O2 molecules, where the energy differencebetween the O2-HOMO and the CO-LUMO is about 7.0 eV, whichindicates that it is difficult for charge transfer from the O2-HOMO orbital to the CO-LUMO orbital to happen. When the

Fig. 8 PDOS of the adsorbed O2 and CO and the Ni atom for the IS (a),TS1 (b) and MS (c) configurations via the TER mechanism. The HOMOand LUMO orbitals are also shown. The PDOS of the free O2 and COmolecules is also shown in panel (d).

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two COmolecules are adsorbed on Ni-DG, the CO-LUMO orbitalis signicantly downshied to the Fermi level, where the energydifference between the O2-HOMO orbital and the CO-LUMOorbital is about 1.0 eV, which can promote the charge transferfrom the O2-HOMO orbital to the CO-LUMO orbital.

4. Conclusions

In summary, the oxidation of a CO molecule on Ni-decorateddivacancy graphene (Ni-DG) has been studied by using DFTcalculations. The diffusion barrier for the single Ni atom on thedivacancy site is 6.23 eV, which shows high stability for the Ni-DG. The LH and ER mechanisms are not preferred for the COoxidation on Ni-DG owing to the much larger adsorption energyof CO than that of O2 molecules. Further study indicates thatthe TER mechanism is preferred for the CO oxidation on Ni-DG,where the O2 molecule is activated by the two pre-adsorbed COmolecules and a pentagonal OCONiOCO intermediate isformed aer overcoming a small energy barrier of 0.34 eV. Thenthe intermediate dissociates into two CO2 molecules with anenergy barrier of 0.29 eV. Therefore, the rate-limiting energybarrier is only 0.34 eV, indicating an efficient oxidation process.The charge transfer from the O2 molecule to CO through the Niatom via the TER mechanism plays a key role in depressing theenergy barrier of the CO oxidation. The adsorption energy of COis much larger than those of O2, H2O, and N2 molecules, indi-cating that Ni-DG can be a noble-metal-free and efficient cata-lyst for CO oxidation, even in humid environments.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

We acknowledge support by the Fundamental Research Fundsfor National Natural Science Foundation of China (Grant No.21703052, 21607029, 21777033), the Central Universities (GrantNo. 2017B12914 and 2015B01914), the China PostdoctoralScience Foundation (2015M571652), the Natural Science Foun-dation of Jiangsu Province (BK20161506), the National 973 PlanProject (2015CB057803), the Science and Technology PlanningProject of Guangdong Province (2017B020216003), the Scienceand Technology Program of Guangzhou City (201707010359),and the “1000 Plan” for Young Professionals Program of China.

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