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2014, 16, 20561

A density function theory study on the NOreduction on nitrogen doped graphene†

Xilin Zhang,a Zhansheng Lu,*a Yanan Tang,b Zhaoming Fu,a Dongwei Mac andZongxian Yang*a

The mechanisms for the catalytic reduction of NO on the metal-free nitrogen doped graphene (NG)

support are investigated using the density function theory (DFT) calculations both with and without

the van der Waals (vdW) correction. The results indicate that the dimer mechanism is more facile than

the direct decomposition mechanism. In the dimer mechanism, a three-step reaction is identified: (i) the

coupling of two NO molecules into a (NO)2 dimer, followed by (ii) the dissociation of the (NO)2 dimer

into N2O + Oad, then (iii) the O adatom is taken away easily by the subsequent NO. Once the NO2 is

desorbed, the remaining N2O can be reduced readily by NO on NG. The reaction processes are also

confirmed from the first principles molecular dynamics simulations. The results suggest that the NG is

an efficient metal-free catalyst for catalytic reduction of NO.

I. Introduction

The production of poisonous gases from the combustion offuel, vehicles, and industrial processes is considered to be one ofthe big environmental issues to be solved. Among the poisonousgases, nitrogen oxides (NOx, comprising about 95% NO) areconsidered to be the most toxic gases emitted into the atmo-sphere. Therefore, it is of utmost importance to remove orreduce the NO molecules from the atmosphere. Presently, nobletransition metals are the most commonly used catalysts for NOreduction.1–9 For example, Liu et al.3–5 showed that Ag and Irsurfaces exhibit high selectivity for NO reduction. Wang et al.6

reported that N2O can be easily achieved over the Au(111) surfaceduring the NO reduction through a dimer mechanism, and theNO reduction by CO on the Pd(111) surface was studied byGopinath et al.8 using molecular beam techniques with respectto temperature, NO:CO beam composition, and beam flux. Yet,the high costs and toxicity of the transition metal-based catalystsmight greatly limit their applications for NO reduction. There-fore, it has become a key challenge to find a kind of appropriatebase metal catalysts, even metal-free catalysts, to accelerate thereaction for the abatement of NO.

In recent years, the nitrogen doped graphene (NG)10–14 hasattracted tremendous attention due to its high chemical

reactivity. For example, Ouyang et al.10 found that the NGcould decrease the energy barrier for O2 dissociation. Yanget al.12 investigated the energy barrier for O2 dissociation ongraphene with different types of nitrogen doping using densityfunctional theory and found that higher nitrogen concen-tration reduces the energy barrier much more efficiently. Xiaet al.13 demonstrated that the oxygen reduction reaction on NG isa direct four-electron pathway by using the density function theory(DFT) calculation. In addition, our previous work14 also provedthat the NG system has high catalytic activity for oxygen dissocia-tion. The electrochemical tests also indicated that the NG systemhad the highest activity for oxygen reduction via a four-electronpathway.

Most recently, we also found that the NG support, as a high-efficient and metal-free catalyst, is one of the promising candi-dates for removing the NOx (NO and N2O) simultaneously.15

However, the best content of NO and N2O for the NO depletionon NG is 1 : 1 (mole ratio) based on the equation of N2O + NO -

N2 + NO2. If the content of NO is higher than N2O in the exhaustgases, the NO may be spillover. As mentioned above, thecontent of NO is as high as 95% of the above-mentionedpoisonous nitrogen oxide gases, (NOx), which is much higherthan that of the N2O content. So it is necessary to investigatewhether or not the NG support can be used as a catalyst for NOreduction without the participation of other molecules. If itcan, the reaction mechanism is still unclear. To this end, weperform the first-principles calculations to study the adsorptionand reaction of NO on the NG support. In the calculatedprocess the van der Waals (vdW) correction16 is also consid-ered. Both the results of with and without the vdW correctiondemonstrate that the NG support is an efficient metal-free

a College of Physics and Electronic Engineering, Henan Normal University, Xinxiang,

453007, China. E-mail: zslu@henannu.edu.cn, yzx@henannu.edu.cnb Department of Physics and Electronic Science, Zhengzhou Normal University,

Zhengzhou, 450044, Chinac School of Physics, Anyang Normal University, Anyang 455000, China

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp02873b

Received 1st July 2014,Accepted 28th July 2014

DOI: 10.1039/c4cp02873b

www.rsc.org/pccp

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catalyst for NO reduction through a dimer mechanism ratherthan the one by the direct monomeric NO dissociation.

II. Computational details

Periodic DFT calculations are performed using the ViennaAb-initio Simulation Package (VASP).17 For improving the calcu-lation efficiency, core electrons are replaced by the projectoraugmented wave (PAW) pseudo-potentials18 and the generalizedgradient approximation of the Perdew, Burke, and Ernzernhof(PBE)19 functional is used for the exchange and correlation.The wave functions are expanded in plane waves with a cutoff energy of 450 eV and the convergence criterion for theelectronic self-consistent iteration is set to 10�5 eV. The NGlayer and the adsorbates are free to relax until the self-consistent force drops to below 0.02 eV Å�1. With the purposeof avoiding the interactions due to the artificial periodicity, avacuum layer of 15 Å is used to separate the periodic images inthe direction perpendicular to the surface. The Brillouin zoneintegration is performed using a 3 � 3 � 1 G-centeredMonkhorst–Pack (MP) grid for the k-points sampling. We alsomake a comparison test with a 5 � 5 � 1 MP grid and find onlytiny changes in energy (0.01 eV).

The climbing image nudged elastic band method (CI-NEB)20

is employed to investigate the saddle points and minimumenergy paths (MEP). The spring force between adjacent imagesis set to 5.0 eV �1. Images are optimized until the forces oneach atom drop below 0.02 eV �1. The energy barriers arecalculated using the initial state as a reference.

In this work we also apply a vdW correction method toinvestigate the interaction between the reactants or the inter-mediates and the NG support. We show that in comparisonwith the regular GGA results, vdW correction yields essentiallydifferent adsorption energies, which increase by 0.3–0.5 eV,while the adsorbate–NG distances decrease accordingly, andthe intramolecular distances are almost unchanged. The first-principles molecular dynamics (MD) is performed with a singleK point at G to sample the Brillouin zone. A time step of 1 fs isused and the temperature is controlled at 300 K at each step.

The Bader’s ‘‘atoms in molecules’’ theory21 is used to assigncharges to atoms and fragments. These charges provided usefulclues to how the charge transfers between different atoms orfragments. One major issue with the normal charge density fromthe VASP code is that they only contain the valence charge density,which would give an unreal charge transfer of the N dopant byBader charge analysis in the NG systems. Considering this point,the core charge is included for Bader charge analysis in thecurrent study, by which a real charge transfer could be obtained.

The adsorption energy is defined by the formula

Ead = ENG + EX � E(X/NG),

where E(X/NG) and ENG are the spin-polarized total energies forthe optimized equilibrium configurations of NG with and with-out gas adsorbate (X), respectively, and EX is the spin-polarizedtotal energy of the corresponding isolated gas molecule in its

ground state. With this definition, a higher Ead value means astronger adsorption.

III. Results and discussionA. The adsorption of reaction gases on NG

In this section we will discuss the various geometric configurationsof the NO molecule and its fragment on the NG support in detail.For each adsorbate, we consider various adsorption sites anddifferent adsorption patterns. The most stable adsorptionconfiguration of each adsorbate is obtained by comparing thetotal energy and is depicted in Fig. 1 (all figures generated usingthe VESTA package).22 The corresponding shortest distancebetween the adsorbate and the support and the bond lengthsof the reaction gases after adsorption are also indicated in Fig. 1.The corresponding quantitative effects of the vdW correction areconsidered. It is found that the adsorption energies increasequite a lot with the vdW correction, while the adsorbate–NGdistances decrease accordingly, and the intramolecular distancesare almost unchanged, indicating that the calculations withoutthe vdW corrections underestimate the strength of the adsorption.In order to clarify the effect of vdW correction more clearly, thecalculated structural parameters and adsorption energies with andwithout vdW correction are listed in Table 1 for comparison.

As reported previously, the NO molecule is weakly adsorbedon the NG support both with and without vdW correction, asshown in Fig. 1a, and the corresponding adsorption energiesare listed in Table 1.

The adsorption is enhanced significantly when two NO moleculesare co-adsorbed on the NG support, and the optimized adsorptionconfiguration is shown in Fig. 1b. The calculated adsorption energyis 1.07 eV accompanied by a charge transfer of 0.28e from the NG tothe two NO molecules, resulting in the intramolecular distances ofthe NO molecules elongated to 1.23 Å. Upon considering the vdWcorrection, the shortest distance between NO and the NG support isshrunk from 3.26 Å to 2.94 Å, and the adsorption energy is increasedto 1.62 eV.

The adsorption of the (NO)2 dimer on NG is also discussedbecause of the calculated results indicating that the (NO)2

dimer is slightly more stable (lower in energy by 0.15 eV) thanthe two adsorbed NO molecules. The (NO)2 dimer was alsofound on the supported noble metal-based catalysts1,5,6 and themetal-free Si-embedded graphene23 during the NO reduction.The zigzag configuration as shown in Fig. 1c is favored with thelowest total energy. The NO molecules are connected to eachother via a N–N bond of 1.34 Å, which is similar to the (NO)2

adsorbed on silicon doped graphene. However, the two N–Obonds are increased to B1.28 Å. The adsorption energy isincreased to 1.22 eV and the shortest distance between (NO)2

and the NG support is reduced to 2.99 Å. The adsorptionenergies increase further to 1.73 eV with the vdW correction,while the adsorbate–NG distances decrease to 2.89 Å accordingly,as shown in Table 1.

The most stable geometric configurations of the inter-mediate products are also discussed here. Firstly, we discuss

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the decomposition products of the (NO)2 dimer (N2O and Oco-adsorbed on the NG support). From Fig. 1d, we find that theO atom is strongly bound to the top site of the carbon atom thatdirectly connects with the two nitrogen dopant atoms (denoted asO/NG) and the C–O distance is 1.29 Å, while the N2O molecule issuspended above the NG support with an O–O distance of 3.24 Åand the intramolecular distances of N2O are almost unchangedcompared with the isolated N2O molecule, indicating that theinteraction between N2O and the O/NG is negligible. The quite smalladsorption energy (0.11 eV) of N2O on the O/NG further confirmsthis conclusion. The weak interaction is testified again by the chargeanalysis, which shows that there are about 0.82 and 0.02 electronstransferred from NG to O atom and the N2O molecule, respectively.When the vdW correction is considered, the adsorption is enhancedwith the O–O distance shortened to 3.12 Å, while the most stableadsorption configurations remain unchanged.

The coadsorption of NO, O and N2O on the NG support isalso considered. The calculated results as shown in Fig. 1e

indicate that the NO is in the vicinity of the dissociative O witha N–O bond of 2.00 Å accompanied by a charge transfer of 0.08efrom NG to NO. There are about 0.89 electrons transferred fromNG to the O atom, resulting in the C–O bond elongated to 1.35 Å.The results, both with and without the vdW correction, are listedaccordingly in Table 1.

The most energetically favorable co-adsorption configurationof NO2 and N2O is shown in Fig. 1f. The adsorption of NO2 israther weak, which has quite a large distance (2.88 Å) from theNG support accompanied by a charge transfer of 0.16e and theintramolecular configuration of NO2 is the same as the optimalconfiguration in the gas phase. The adsorption of N2O is alsoweak as that of NO2, which has a larger distance from the NGsupport than NO2 with almost no charge transfer. When the vdWcorrection is considered, the adsorption is enhanced with theC–O bond shortened to 2.60 Å, while the change of the geometricstructures is almost negligible and the intramolecular config-urations keep their optimal configurations for NO2 and N2Owhether the vdW correction is considered or not.

B. The reduction mechanism of NO on NG

Two kinds of mechanisms for NO reduction, the direct dissocia-tion mechanism and the dimer mechanism, are investigatedwidely.5,6,23 In the direct dissociation mechanism, the dissocia-tion of the first NO takes place, followed by the association of thesecond NO to the dissociative O or N-atom, forming a NO2 orN2O molecule. In the dimer mechanism, two NO molecules firstundergo the coupling into a (NO)2 dimer, which dissociates intoa N2O molecule and an atomic O subsequently. Here, we willdetermine which one of the two mentioned mechanisms is thepreferred reaction process on the metal-free NG support.

Table 1 Adsorption energy (Ead) of reactants and intermediate products(as shown in the parentheses of the corresponding configurations) on NG,the shortest distance between the adsorbate and NG/NG–O (dnn), theN–N (dN–N) and/or N–O (dN–O) bond lengths of the reaction gases. Conf.denotes the adsorption configuration. EX represents the adsorption energyof X in the Conf. system, and the superscripts ‘‘a’’ and ‘‘b’’ represent theresults without and with van der Waals correction, respectively

Conf. Ead (eV) dnn (Å) dN–N (Å) dN–O (Å)

a(NO) 0.23a 0.65b 2.92a 2.37b — — 1.22a 1.23b

b(2NO) 1.07a 1.62b 3.26a 2.94b 1.70a 1.77b 1.23a 1.24b

c((NO)2) 1.22a 1.73b 2.99a 2.89b 1.34a 1.34b 1.27a 1.29b

d(N2O) 0.11a 0.40b 3.24a 3.12b 1.14a 1.14b 1.23a 1.23b

e(NO) 0.41a 0.77b 2.00a 1.88b — — 1.20a 1.20b

Fig. 1 The most stable adsorption configurations of various reactants or intermediate fragments adsorbed on the NG. The insert picture in (d) is the top viewand EX represents the adsorption energy of X. The brown, blue and red spheres denote the C, N and O atoms. The labels of (a)–(f) are discussed in the text.

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Firstly, we discuss the direct dissociation mechanism. NOadsorbed on the NG support, shown in Fig. 1a, is selected as theinitial state (IS). The configuration with the O atom is adsorbedon the carbon atom that directly connects with the two nitrogendopant atoms with a C–O distance of 1.26 Å and the N atomadsorbed on the adjacent C atom with a distance of 1.37 Å is setas the final state (FS). The corresponding minimum energypath (MEP) profile is summarized in Fig. 2. The energetics isschematically plotted with respect to the reference energy of theinitial state. The local configurations of the adsorbates on NGat various states along the MEP are also displayed in Fig. 2,which shows that the N–O bond is elongated from 1.22 Å in theIS to 2.03 Å in the transition state (TS) with a rather high energybarrier of 1.62 eV, where the C–N and C–O bonds are formedsimultaneously with the bond lengths of 1.42 and 1.36 Å,respectively. We also find that the NG support is distorted seriouslyin the TS, e.g. the C–C bond surrounded by the N-trimer is elongatedfrom 1.42 to 1.53 Å. The FS is reached with the C–N and the C–Obonds further shortened to 1.37 and 1.26 Å, respectively.

The calculated dissociation barrier of NO on the NG supportis close to that on the Pt(211) surface4 (1.70 eV), which is muchsmaller than that on the Pt(111) surface4 (2.60 eV) and that onthe metal-free Si-embedded graphene23 (3.91 eV). However, it ishigher than those on the Pt(110)3 and Ir(211) surfaces4 (a barrierof 1.21 and 1.19 eV, respectively). The results indicate that, as ametal-free catalyst, the catalytic activity for NO dissociation iscomparable with that of the noble metal catalysts.

Although the NG support is more efficient for NO dissociationthan some noble metals, it is still unfavorable kinetically with aquite high dissociation barrier. Meanwhile, it is also unfavorablethermodynamically duo to the IS being more stable than the FSby 0.63 eV. So, we will turn our attention mainly to the dimermechanism in the following.

As shown in Fig. 3, the coadsorption configuration of thetwo NO molecules (Fig. 1b) is taken as a metastable state (MS),where two NO molecules are parallel adsorbed on the NGsupport, which form a trapezoid with a N–N distance of 1.70 Åand a N–O bond of 1.23 Å. Another kind of MS is also found and

depicted in Fig. S1 (ESI†). The two NO molecules tend to moveclose toward each other and connect by a N–N bond which isalmost parallel to the C–C bond surrounded by the N-trimer,where the N–N bond is shortened to 1.27 Å and the N–O bond iselongated to 1.28 Å. The formed (NO)2 dimer (as the IS) subse-quently dissociates into a N2O molecule and an O adatom. Thecorresponding MEP profile is also depicted in Fig. 3, whichshows that the N–N bond is shortened from 1.34 to 1.22 Å in theTS1 and the shortest distance between the dimer and the NGsupport is also shortened to 2.82 from 2.99 Å with an activationbarrier of 0.70 eV, yet the N–O bond lengths are unchanged. Wealso find that the adsorption configuration of the (NO)2 in theTS1 is changed into the trapezoid from the initial zigzagconfiguration. Moreover, the N2O + O configuration is muchenergetically favorable than those of the (NO)2 dimer and the twocoadsorbed NO molecules, indicating that it is an exothermicreaction. The results show that the NO molecules prefer toaggregate into the (NO)2 dimer followed by the decompositionof the formed dimer, rather than the direct decomposition onthe NG support. The reaction processes can be summarized as:NO + NO - (NO)2 - N2O + O.

In the dimer structure, as in the gas phase, the two NOmolecules link to each other through the N atoms with adistance about 1.70 Å, which is shorter than that in the gasphase (NO)2 dimer (1.97 Å) because the cohesive energy of(NO)2 with the substrate is higher than that without thesubstrate, then the two NO molecules are coupled to form thedimer with an adsorption energy of 1.22 eV accompanied bymuch charge transfer from the NG support to the dimer, asrepresented by the shortened N–N distance and lengthenedN–O distances. This is in line with the small barrier for the(NO)2 decomposition. This phenomenon is similar to that onAg(111),5 where the extra electrons added into (NO)2 enters intothe adsorbate and modifies the structure of the dimer, which isfavorable for the following decomposition process. Yet thereaction will be terminating if the NG support is removed basedon the results of the first principles MD simulation. This will bediscussed later.

Fig. 2 NO direct dissociation process on the NG support. All energies are reported in eV relative to the initial state and the distances are in angstrom.

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The decomposition process of the (NO)2 dimer on the noblemetals has been studied experimentally and theoretically. Forexample, Chau et al.1 found that N2O can be formed when NO isadsorbed on Au(111) at 300 K based on X-ray photoelectronspectroscopy (XPS) and temperature programmed desorption(TPD) studies. Liu et al.6 elucidated the mechanism for N2Oformation over the Au(111) surface during NO reduction bytheoretical calculations. Additionally, Liu et al.5 identified theinverted (NO)2 dimer on noble metal Ag using DFT calculations.They pointed out that the lowest reaction barrier is 0.27 eV.Although the reaction barrier on the noble metals is lower thanthat on the NG catalyst, the high cost and scarcity of the noblemetals make the metal-free NG catalyst very promising. Ourcalculated barrier (0.70 eV) on NG is comparable with that onthe Si-embedded graphene23 (0.47 eV). It is therefore expectedthat the NG would be an excellent metal-free catalyst for the(NO)2 decomposition.

It is well-known that, generally, the initial weak physisorptionon the catalyst surface is unfavorable for the subsequent surface

reactions. Accordingly, the N2O decomposition on the NG supportmay be difficult since the interaction between N2O and the O/NGis very weak under the condition that the active center is occupiedby the O atom, which has relatively small adsorption energies(0.11 eV), large adsorbate–O/NG distances (3.24 Å) and almostunchanged intramolecular distances (1.14 and 1.23 Å for N–Nand N–O bonds, respectively). Fortunately, our previous studysuggested that the adsorbed O can be removed easily by the NOwith an activation barrier of 0.16 eV, and most importantly, thedissociation of N2O is very facile on the pure NG support. In otherwords, if we could remove the adsorbed O easily from the systemof N2O and O co-adsorption on the NG support, the subsequentreaction will become very smooth. So it is required to discusswhether or not the NO can be oxidized easily by the adsorbed O inthe co-adsorption configuration of N2O and O on the NG. Theprocess is depicted in Fig. 4, where the NO and N2O adsorbed onthe O/NG with a configuration as shown Fig. 1e is taken as theIS and the FS is set as the configuration with NO2 and the N2Oco-adsorbed on the NG (as shown Fig. 1f). The reaction formula

Fig. 3 The decomposition processes of the (NO)2 dimer on the NG support.

Fig. 4 NO catalytic oxidation by the dissociative O under the conditions of the N2O co-adsorption.

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can be simply written as: NO + O - NO2. The calculated resultsindicate that the N-atom of NO firstly approaches the adsorbedO-atom and reaches the TS with an activation barrier of 0.27 eV. Inthe TS, a NO2-like molecule sits at a height of 1.84 Å above the NG,where the distance between NO and the O atom is shortened to1.38 Å from 2.00 Å of IS. This is almost the same as our previousresult of NO oxidation without the N2O co-adsorption, whichmanifests the weak adsorption of N2O on the O/NG support. Afterpassing over the TS, a NO2 molecule is desorbed with a distance of2.88 Å, which can be collected easily by water, leaving the N2Oadsorbed on the NG support. The calculated barrier (0.27 eV) ismuch lower than those on the Pt and Ir(111) surfaces4 (1.46 and1.52 eV, respectively), indicating that the NO oxidation canproceed more easily on the O adsorbed metal-free NG catalystthan on the noble-metal Pt and Ir.

Once the NO2 is desorbed and collected by water, the remainingN2O and NO, as reactants, may also have two kinds of competitivereaction mechanisms: (1) the gas-phase NO molecule approachesand reacts with the already-activated N2O; or (2) the coadsorption ofNO and N2O molecules occurs before the reaction. However, thesimulation results show clearly that the active region of NG iscompletely occupied by N2O when NO and N2O are co-adsorbedand the shortest distance between NO and N2O is 3.45 Å. Thecalculated coadsorption energy is 2.73 eV, which is slightly smallerthan the sum of the respective adsorption energies of NO and N2O(2.67 + 0.25 = 2.92 eV) on NG. In short, the results indicate thatthere is only a quite weak interaction between NO and N2O.

On the basis of these results, we conclude that the reactionwith the gas-phase NO molecule approaching and reacting withthe already-activated N2O is the most possible mechanismrather than that with the coadsorption of NO and N2O moleculesbefore the reaction. The reaction process is proposed to occur instepwise, i.e., the decomposition of N2O followed by the oxida-tion of NO as presented in the formulas: N2O - N2 + O and NO +O - NO2. The reaction process is depicted in Fig. 5, whichshows that the N–O and N–C bonds are elongated from 1.48 and1.52 Å in the IS to 1.69 and 1.62 Å in the TS1, respectively, with arather small energy barrier of 0.05 eV along the reaction path-way. The C–C bond connected to the N2O in the substrate is alsoelongated (from 1.59 to 1.62 Å). In contrast, the N–N and C–Obonds shrink from 1.22 and 1.46 Å to 1.19 and 1.38 Å, respec-tively. Finally, the C–N–N–O–C five-member ring is broken,resulting in the formation of a N2 molecule and an atomic Oadsorbed on the C atom as shown in the FS1. Then the N2

desorbs with the atomic O left on the surface, which can be usedto oxidize the subsequent NO molecule. The simulation resultsindicate that the N-atom of NO approaches the adsorbed O-atomand reaches the TS2, a NO2-like molecule with a transitionbarrier of 0.16 eV. Passing over the TS2, a NO2 molecule isformed and desorbs easily from the surface, leading to therecovery of the NG. The related vibrational frequencies of thetransition states are calculated and all the transition states areidentified with only one imaginary frequency. The detailedresults are tabulated in Table S1 (ESI†).

Fig. 5 The top row represents the processes for the N2O decomposition and N2 desorption, and the bottom row represents the process for theoxidation process of NO by the remaining O atom.

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Except for the geometric structures and the adsorptionenergies of the reaction gases adsorbed on the NG support,the integrated reaction paths are also re-evaluated upon includingthe vdW correction. It is found that the adsorption energies ofreactants are increased by about 0.50 eV with vdW correction, whilethe changes in the geometric structures are almost negligible. Thebarrier energies are all increased by about 0.16 eV for the dimermechanism, yet the barrier energy is reduced to 1.55 eV from1.62 eV for NO decomposition directly. The re-evaluated reactionpaths are almost the same as that without vdW correction. Here, wetake the results of the (NO)2 dimer decomposition on the NGsupport with the vdW correction as an example. It is found that theadsorption configuration changes into a quasi trapezoid structurein the TS, which is similar to that without the vdW correction.However, the N–N and N–O bond lengths are elongated to 1.38 and1.31 Å, respectively. The shortest distance between the adsorbateand the support is enlarged to 2.95 from 2.89 Å, yet the N–O bondlength is shortened by 0.05 Å with a barrier of 0.84 eV.

The high reaction activity of NG toward the NO reduction canbe further understood according to the Bader charge analysis.24

The results are shown in Table 2. The carbon dimer, which loses1.15e in total, is just the most preferred adsorption site and theactive center for the NO reduction. Compared with the pure NGsupport, the C-dimer further loses electrons (1.66–1.70 vs. 1.15)

when the active site is occupied by the O, which is similar to thecases with the NO, (NO)2 dimer and the two dissociative NOmolecules adsorbed on NG (Table 1), indicating that the elec-trons transferred from the NG support to the adsorbates aremainly from the carbon dimer. It is for example that the C-dimerloses 1.46 and 1.66 electrons in the structure shown in Fig. 1cand d, respectively. In addition, the almost same amountof charge transfer to the C-dimers is found for the NO and Oco-adsorption (Fig. 5h) configuration and the N2O, NO and Oco-adsorption configurations (Fig. 1e), indicating that the N2Omolecule has negligible interaction with the NG in the situationthat the active sites are occupied by Oads, which is in line withthe weak adsorption as mentioned above. The same phenom-enon is also observed in the situation of O adsorption and the N2

and O co-adsorption, indicating the N2 could desorb easily onthe NG. We also find that the effect of the C2 atom is moreimportant than the C1 atom in the process of NO reductionbecause of the more charge transfer of the C2 atom.

As known, the overall barrier is generally more importantthan a single-step barrier in the catalysis processes. According tothe results above, the direct decomposition of NO is very difficultdue to the relatively high barrier. In contrast, the dissociation of(NO)2 is the rate determining step for the dimer mechanism witha barrier of 0.70 eV. It is close to that on Si-doped graphene (0.47eV) and smaller than that on noble metal catalysts. The processof the NO reduction on the NG support could be summarized as4NO - N2 + 2NO2, and the problem of the mole ratio of thereactants is resolved. Several possible reaction paths for the NOreduction on the NG are proposed as below and also shown inFig. 6 with the reaction barriers presented in the brackets:

NO - N + O (1.26 eV)

(NO)2 - N2O + Oads (0.70 eV)

N2O + O + NO - N2O + NO2 (0.27 eV)

N2O -N2 + Oads (0.05 eV)

Oads + NO - NO2 (0.16 eV)

Table 2 The results of Bader charge analysis (in e). Conf. denotes theadsorption configuration. q (adsorbate) stands for the total charge on theadsorbate. q (C1) and q (C2) represent the charge of the C atomssurrounded by the N-trimer which connect directly with one nitrogenatom and two nitrogen atoms, respectively. And the q (C-dimer) representsthe total charge of the C dimer. The positive and negative numbersrepresent the electrons accumulation and depletion, respectively

Conf. q (adsorbate) q (C1) q (C2) q (C-dimer)

Pure — �0.47 �0.68 �1.15a 0.09 �0.43 �1.00 �1.43b 0.28 �0.36 �0.93 �1.29c 0.31 �0.44 �1.02 �1.46d 0.84 �0.35 �1.31 �1.66e 0.99 �0.42 �1.28 �1.70f 0.16 �0.46 �0.73 �1.19g 0.55 �0.38 �1.30 �1.68h 0.68 �0.45 �1.18 �1.63

Fig. 6 The schematic reaction processes for the NO reduction on NG. The minimum energy pathway is highlighted in blue.

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The NO reduction paths we proposed above are also verifiedby the results of the first principles MD simulation, as shown inFig. 7. In order to simplify our calculation model, we inject onlyfour NO molecules into the NG support as the initial structure(Fig. 7a) based on the reaction formula. The simulation resultsindicate that two NO molecules are coupled into the dimer atabout 0.64 ps and 300 K, indicating that the (NO)2 forms quiteasily. Then the (NO)2 decomposes into a N2O molecule and anO atom which is captured by the third NO forming a NO2

molecule. The whole process, including the decomposition ofthe (NO)2 dimer and the oxidization of the third NO by theatomic O to NO2 described in Fig. 7b and c, takes less than0.21 ps at 300 K, which is in agreement with the lower barriers of(NO)2 dimer decomposition and NO oxidation process. All thereaction steps proceed in a very short time at 300 K, suggestingthat this strategy can be performed at room temperature whichis in line with the lower reaction barrier. However, the reactionwill be terminating when the NG support is removed sincethere is no place for O landfall and no charge origin to activatethe dimer and lead to the decomposition. We treat the moststable configuration of two NO monomers as the start point ofthe reaction. At about 0.78 ps the dimer transfers to a zigzagstructure from the previous parallelogram configurationaccompanied by a more shortened N–N bond length. Thisphenomenon indicates that the agglomeration of NO moleculesis enhanced and the decomposition becomes more difficult.Indeed, the MD results indicate that the (NO)2 dimer does notdecompose until 6 ps. Therefore, the NG support has an importanteffect on the formation and decomposition processes of the(NO)2 dimer.

IV. Conclusions

In summary, both the direct decomposition and the dimermechanism are investigated for the NO reduction on the

metal-free NG support. The results indicate that the directdecomposition of NO is rather difficult because of the extremelyhigh barrier of 1.62 eV. In contrast, the process of reduction oftwo NO molecules to N2O and O is likely to proceed with abarrier of 0.70 eV according to a dimer mechanism. Yet the N2Ofurther decomposition on the NG support is difficult becausethe interaction between N2O and the O/NG is very weak sincethe active center is occupied by the O atom. However, the Oadatom can be taken away easily by the subsequent NO with abarrier of 0.27 eV. Once the NO2 is formed and collected bywater, the remaining N2O decomposes easily into a N2 moleculeand an O adatom with a barrier of 0.05 eV. Then the adsorbedNO molecule can be oxidized readily by the atomic O with abarrier of 0.16 eV leading to the recovery of the NG support. Theintegrated reaction process is also tested and verified by thefirst principles MD method, which is in line with the previousresults. The present work suggests that the NG as a metal-freecatalyst exhibits high catalytic activity (the biggest barrier is0.70 eV) and a relatively high efficiency for NO reduction(depletion of 4NO molecules in each reduction cycle), andmay be a promising candidate as a substitute for noble metalcatalysts for removing the environmentally harmful exhaust NOfrom vehicles and industrial wastes.

Acknowledgements

This work is supported by the National Natural Science Foundationof China (Grant No. 11174070 and11147006). Z. Lu alsoacknowledges the support from the China Postdoctoral ScienceFoundation funded project (Grant No. 2012M521399) andPostdoctoral Research sponsorship in Henan Province (GrantNo. 2011038), Foundation for the Key Young Teachers of HenanNormal University and Start-up Foundation for Doctors of HenanNormal University. Parts of the simulations were performed onresources provided by the high-performance computing center of

Fig. 7 The dynamic process for NO reduction on NG. (a) The initial structure; (b) the (NO)2 dimer formed after 0.64 ps at 300 K; (c) the dimerdecomposed and a NO2 molecule formed at 0.21 ps and 300 K. The top and bottom pictures are the side view and the top view, respectively.

Paper PCCP

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College of Physics and Electronic Engineering in Henan NormalUniversity.

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