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Chemical Physics Letters

journal homepage: www.elsevier.com/locate/cplett

Research paper

Catalytic reduction of nitrous oxide over boron-doped C3N monolayers: ADFT study

Mehdi D. Esrafili⁎, Safa HeidariDepartment of Chemistry, Faculty of Basic Sciences, University of Maragheh, P.O. Box 55136-553, Maragheh, Iran

H I G H L I G H T S

• A novel and active metal-free catalyst is proposed for reduction of N2O.

• A boron atom can be easily incorporated in the C3N lattice.

• The doping of boron atom promotes the effective adsorption of N2O.

• The energy barriers are in the range of 0.08–0.33 eV.

A R T I C L E I N F O

Keywords:C3N monolayerN2O reductionDFTCatalystB-doping

A B S T R A C T

Using DFT calculations, a novel metal-free catalyst is proposed for reduction of nitrous oxide (N2O). It is foundthat the presence of a B impurity can modify the surface reactivity of C3N and promote the effective adsorptionof N2O. The reduction of N2O over B-doped C3N nanosheets starts with the adsorption of this molecule, followedby its decomposition into N2 and an active oxygen atom (O*). By overcoming a small activation energy, the O*

moiety can be then eliminated by a CO molecule. The results suggest that B-doped C3N nanosheets are promisingcatalysts to remove the environmentally harmful N2O molecules.

1. Introduction

The “greenhouse” effect refers to a process in which some of thesolar radiation are trapped by the Earth’s atmosphere because of thepresence of certain gases-known as greenhouse gases. Although most ofgreenhouse gases occur naturally, however, human activities havecaused a significant enhancement of the concentration of these gases inthe atmosphere [1]. This has led to an increase in the global averagesurface temperatures and climate change. Nitrous oxide (N2O) is one ofimportant greenhouse gases with a relatively larger global warmingpotential compared to carbon dioxide or methane [2]. In addition, N2Ois one of the main contributors in acid rain formation and ozone layerdepletion [3]. Hence, much attention have been recently devoted todevelop efficient approach to remove or reduce N2O from the atmo-sphere [4,5]. Among those, the catalytic reduction of N2O by CO is verypromising [6,7], because this reaction is thermodynamically favorableand the final products (CO2 and N2) are less harmful than the reactants.Earlier studies have indicated that noble metal-based catalysts, such asPt [7,8] and Pd [9,10], etc., can catalyze this reaction effectively.However, these catalysts are usually rare and costly, which hinders

their large scale application in industry. Moreover, because of theirhigh affinity towards CO and activated oxygen atom (O*) originatedfrom the N2O decomposition, these catalysts require high temperaturesto reach a reasonable performance. Consequently, plentiful researchefforts have been devoted for replacing these catalysts with their al-ternatives costless and noble-metal free alternatives [11–14].

In recent years, there has been a great attention toward the carbon-based materials due to their superior physical and chemical properties[15,16]. Graphene, a monolayer of sp2-hybridized carbon atoms, hasattracted considerable attention in various scientific fields includingcatalysis because of its exceptional properties, such as the huge surface-to-area ratio, high stability and durability and high electric con-ductivity [17,18]. Although the defect-free graphene has probably avery weak interaction with small gas molecules like CO or N2O, how-ever, chemically-doped graphene sheets especially those with the ni-trogen atoms exhibit superior catalytic performance towards theoxygen reduction reaction (ORR) [19], CO oxidation [20], NO reduc-tion [21] and as well as the N2O reduction [22] which we would focusin the present study. Moreover, earlier studies have also proved thatdoping of graphene with different heteroatoms like B and N can

https://doi.org/10.1016/j.cplett.2019.03.062Received 29 December 2018; Received in revised form 28 March 2019; Accepted 29 March 2019

⁎ Corresponding author.E-mail address: esrafili@maragheh.ac.ir (M.D. Esrafili).

Chemical Physics Letters 725 (2019) 52–58

Available online 02 April 20190009-2614/ © 2019 Elsevier B.V. All rights reserved.

T

significantly improve its electronic structure properties [23,24]. Basedon the quantum chemical calculations, Gholizadeh and Yu [25] havefound that introduction of B or N atom is able to greatly affect theformation energy, work function and electrical conductivity of gra-phene. Besides, there are numerous theoretical and experimental evi-dences [26,27] showing that codoping of graphene with B and N is ableto further increase the surface reactivity of graphene due to synergiceffects between these atoms.

C3N nanosheet is a novel 2D single layer graphene-based nanoma-terial which has been prepared by the direct pyrolysis of hex-aaminobenzene trihydrochloride single crystals [28] or the poly-merization of 2,3-diaminophenazine [29]. Due to graphene-likestructure and uniform distribution of the nitrogen atoms, C3N is pre-dicted to exhibit outstanding performance in diverse research area in-cluding CO2 storage [30] and photonics [31]. Moreover, as a specialcase of N-doped graphene [32], these unique geometric and electronicstructure properties make C3N a promising anode in lithium-ion bat-teries [33]. Additional interest in the C3N is connected with its rela-tively narrow band gap (0.39 eV), ultra-high stiffness and thermalconductivity [34] which rend it as a potential material to be used innanoelectronics and other devices. Moreover, recent studies have alsofound that the surface reactivity of C3N can be extensively enhancedwith the chemical doping. For instance, He and coworkers [35] haverecently demonstrated that the incorporation of a boron impurity inC3N can considerably enhance the adsorption of O2 molecule and

facilitate its reduction in the ORR process. This can be mainly attributedto the synergistic electron transfer interaction between the B/N andsurrounding carbon atoms [35]. However, to the best of our knowledge,there is neither theoretical nor experimental studies about the catalyticperformance of B-doped C3N towards the N2O reduction.

In the present study, dispersion-corrected density functional theory(DFT) calculations are performed to probe the catalytic performance ofpristine and B-doped C3N monolayers in the N2O reduction reaction.The surface reactivity, electronic structure and adsorption properties ofthese systems are investigated in detail. A particular attention is alsopaid on the finding different stationary points along the N2O reductionprocess and the corresponding calculated activation energies. We areparticularly interested in B-doped C3N, because B is cheap, readilyavailable and environmentally friendly which may meet our require-ment to seek a metal-free catalyst for reduction of N2O molecule by CO.

2. Computational details

Spin-polarized DFT calculations were performed by means of theDMol3 code [36,37], using the Perdew-Burke-Ernzerhof (PBE) densityfunctional [38]. The van der Waals (vdW) interactions was described byusing the DFT+D method within the Grimme’s scheme [39,40]. Adouble numerical plus polarization (DNP) function was used as thebasis set. Geometry optimizations were performed with convergencethresholds of 0.001 Ha/Å on the gradient, 0.005 Å on the displacement,and 10-5 Ha on the energy. For the all the computations, the real–spaceglobal orbital cutoff radius was set as high as 4.6 Å.

To model the pristine C3N, a periodic 6× 6 hexagonal supercellcontaining 54 C and 18 N atoms with a vacuum of 20 Å in the z direc-tion was first built. Then, a C or N atom of C3N was replaced with a Batom to construct B-doped C3N. These are labeled as BC and BN sub-strates in Fig. 1, respectively. The Brillouin zone integration was sam-pled using a 3× 3×1 Monkhorst–Pack grid. For the calculation ofdensity of states (DOS), a denser Monkhorst-Pack 15× 15×1 k-pointsampling was used. The transition state searches were carried out withthe complete linear synchronous transit/quadratic synchronous transit(LST/QST) method. This method uses an idealized set of structuresconnecting reactants and products as obtained by linearly interpolatingthe distances between all pairs of atoms. Then, by following the Hessianeigenvector (normal mode) related to the energy maximum, the struc-ture of transition state is located on the potential energy surface.

The formation energy (Eform) of pristine and B-doped C3N sheets

Fig. 1. The optimized structure and electron density map of (a) pristine C3N, (b) BC and (c) BN nanosheets.

Table 1Calculated adsorption energies (Eads, eV) and net charge-transfer (qCT, |e|)values for the adsorption of different species over BC and BN surfaces.a

Surface Adsorbate Eads qCT

BC N2O −0.80 −0.31CO −0.16 −0.15N2 −0.15 < 0.01CO2 −0.22 −0.01

BN N2O −0.56 −0.27CO −0.15 −0.15N2 −0.13 < 0.01CO2 −0.18 −0.01

a The negative qCT value indicates a charge-transfer from the nanosheet tothe adsorbate, while a positive qCT value corresponds to the charge-transferfrom the adsorbate to the nanosheet.

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was obtained by the following formula:

= − + −μ μE E Eform B-C3N C3N C/N B (1)

where EB-C3N and EC3N is the total energy of the B-doped and pristineC3N sheet; μN is the chemical potential of the nitrogen atom defined ashalf of N2 molecule energy, and μC is the chemical potential of onesingle carbon atom in the pristine graphene. μB is the energy of a singleB atom in the α-rhombohedral B crystal. The adsorption energy (Eads)for each adsorbate was calculated as

= − +E E E Eads A/S A S (2)

where EA/S, EA and ES are the total energies of the adsorbate/nanosheetcomplex, the adsorbate and nanosheet, respectively. The Hirshfeld [41]charge density analysis was used to obtain atomic charges and charge-transfer values.

3. Results and discussion

3.1. Pristine and B-doped C3N nanosheets

The optimized structures of pristine and B-doped C3N monolayersare shown in Fig. 1. As seen, the pristine C3N has a planar hexagonalstructure with the uniform distribution of C and N atoms. The primitiveunit cell of the pristine C3N involves six C and two N atoms. There aretwo different chemical bonds in this material, namely CeC and CeN.

The CeC bond distances are calculated to be 1.42 Å, which are identicalwith the corresponding values in pristine graphene. Meanwhile, theCeN bond distances in pristine C3N (1.42 Å) are consistent with thoseof earlier studies [35,42,43]. Due to the electronegativity differencebetween the N and C atoms, the nitrogen atoms are negatively chargedby −0.03 |e|, while C atoms are positively charged by 0.01 |e|. As aresult, a negligible ionic characteristic is expected for CeN bonds in thepristine C3N. For the BC and BN monolayers, one can also see that theplanar structure of C3N is still preserved after the B-doping. Note thatthe optimized BeN or BeC bond distances in these systems deviateslightly from the CeN and CeC bond distances in the pristine C3N,which clearly indicates that the B atom is comfortably incorporated intothe C3N structure. This is also supported by the calculated Eform value ofBC (−1.49 eV) and BN (−1.54 eV), revealing that formation of thesesystems are thermodynamically favorable. The Hirshfeld charge densityanalysis also reveals that the incorporation of a B impurity can con-siderably affect the charge distribution in the C3N. Considering that theB atom is less electronegative than nitrogen and carbon, it is expectedthat a charge-transfer occurs from the B to its neighboring C or N atoms.According to our results, the Hirshfeld atomic charge on the B atom ofBC is +0.04 |e|, which is larger than that of BN (+0.02 |e|). We notethat the presence of an electronegative nitrogen atom in the formersystem leads to a relatively larger Hirshfeld atomic charge on the Batom. The electron density isosurfaces in Fig. 1 also verify this finding,

Fig. 2. Energy profile and optimized stationary points for the dissociation of N2O on (a) BC and (b) BN, including the initial state (IS), transition state (TS) and finalstate (FS). All bond distances are in Å.

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where there is a quite larger electron density over the carbon or ni-trogen atoms around the B atom of BC. Consequently, a larger surfacereactivity is expected for the BC nanosheet as compared with the BN.

3.2. Adsorption of N2O, CO, N2 and CO2 species over BC and BN

Before studying the N2O reduction over the title surfaces, we firstlyconsider the adsorption characteristics of different reaction species in-volved in this process. Fig. S1 of Supporting Information shows themost stable configurations of N2O, CO, N2 and CO2 species adsorbedover BC and BN nanosheets, respectively. The corresponding adsorptionenergies and net charge-transfer values are listed in Table 1.

Fig. S1(a) and (b) show the most stale adsorption configuration ofN2O molecule over BC and BN nanosheet, respectively. As seen, N2O ischemisorbed over these surfaces via a [3+ 2]-cycloaddition, similar tothe case seen over the Si-doped graphene [44] or other surfaces[12,45]. Note that such a configuration can be understood by the mo-lecular orbital (MO) analysis of these systems, where the highest oc-cupied molecular orbital (HOMO) of BC or BN interacts favorably withthe lowest unoccupied molecular orbital (LUMO) of N2O (see Fig. S2 of

Supporting Information). The BeO bond distance between the N2O andB atom of BC and BN is 1.53 and 1.58 Å, respectively. Meanwhile, thechemisorption of N2O is associated with a relatively large charge-transfer from the surface to this molecule, which leads to the elongationof NeO bond distances compared to the isolated N2O molecule. Theinteraction between the N2O and BC or BN is also verified by the partialdensity of states (PDOS) analysis, where there are some noticeable or-bital hybridization between the B-2p state and those of N2O around theFermi level. In particular, the adsorption of N2O over these surfacesleads to the appearance a small local state above the Fermi level, whichis due to the charge-transfer from the B atom to this molecule. Fur-thermore, the HOMO of N2O splits into several local states below theFermi level, while the LUMO is depressed and slightly downshifts to-wards the Fermi level. Hence, we predict that the presence of a B atomcan be largely increase the surface reactivity of C3N nanosheet towardsthe N2O molecule.

As Fig. S1(c) and (d) indicate, CO molecule is weakly adsorbed overBC and BN nanosheets as evidenced by its relatively large binding dis-tance and small Eads and qCT values (Table 1). Comparing the Eads va-lues in Table 1 shows that the absolute adsorption energy of CO over BC

Fig. 3. Energy profile and optimized stationary points for the CO+O*→ CO2 reaction on (a) BC and (b) BN nanosheets. All bond distances are in Å. The value withinthe bracket indicates the adsorption energy.

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is slightly larger than that of over CN. The CeO bond distance of theadsorbed CO is 1.14 Å, which confirms the weak physisorption of thismolecule over BC and BN surfaces. The PDOS analysis also supports thisfinding, where there are roughly no any orbital hybridization betweenthe states of N2O and the surface. Almost a similar situation is found forthe adsorption of N2 molecule over BC and BN (Fig. S1(e) and (f)). Thecorresponding small adsorption energy and charge-transfer value inTable 1 clearly indicate that N2 molecule is weakly physisorbed overthese surfaces. Besides, there is a negligible mixing between the B-2pand N2-5σ or N2-2π* states around the Fermi level, which verifies theweak activation of N2 over these surfaces. The calculated N-N bonddistances in the adsorbed N2 is 1.11 Å, which is identical with that of itsisolated state. On the other hand, the Eads value of CO2 adsorbed overBC and BN surfaces is −0.22 and −0.18 eV, respectively. The

corresponding charge-transfer value is also negligible, indicating thatCO2 would be easily released from the surface.

3.3. Reaction mechanisms for reduction of N2O

Considering the weak adsorption together with a negligible chargetransfer of N2O or CO molecule over the pristine C3N nanosheet, it isassumed that the N2O reduction cannot proceed over this surface.Hence, we focus here on the possible reaction mechanisms for the re-duction of N2O over the B-doped C3N surfaces, since the B-dopingpromotes the effective adsorption of N2O. According to above results,the tendency of N2O molecule to adsorb over the BC and BN surfaces ismuch larger than that of CO. This means that in the presence of N2O

Fig. 4. Energy profile and optimized stationary points for the N2O+O*→N2+O2 reaction on (a) BC and (b) BN nanosheets. All bond distances are in Å. The valuewithin the bracket indicates the adsorption energy.

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and CO molecules, the B atom of these surfaces is dominantly coveredby N2O molecules. Besides, due to the small adsorption energy of CO,the active site of BC and BN cannot be poisoned by this molecule. Hence,the N2O reduction over these surfaces should be initiated with the ad-sorption of N2O and followed by the addition of CO molecule. Figs. 2and 3 show the energy diagrams and optimized stationary points for theN2O+CO reaction.

As indicated in Fig. 2, the reduction of N2O over the BC and BN

surfaces starts with the adsorption of this molecule. So, the N2O ad-sorbed on the active site of BC and BN was set as the initial state (IS-1and IS-2). Next, the OeN bond distance of N2O is continuously elon-gated until the N2 molecule is formed, leaving an active atomic oxygen(O*) adsorbed over the B atom. This reaction step is highly exothermicover both surfaces, due to the formation of a strong BeO* chemicalbond. The energy barrier for the formation of O* and N2 species over BC

is calculated to be 0.30 eV, which is 0.03 eV smaller than that of overBN. This can be simply related to the larger activation of N2O moleculeover the former surface as evidenced by a shorter BeO* bond distancein the corresponding transition state (TS-1). Note that the calculatedactivation energies for the decomposition of N2O in this study aresmaller or comparable with those reported over other surfaces likePdO/Al2O3 (≈0.96 eV) [46], Pt gauze (0.84 eV) [47], Rh (0.38 eV) [48]and Si-doped graphene (0.33 eV) [49]. The formed N2 molecule overboth surfaces have a small adsorption energy (≈−0.02 eV), hence thelarge energy release for this reaction step can facilitate its desorption.

In the next step, CO approaches the O* moiety to form a CO2 mo-lecule (Fig. 3). For this reaction step, the configuration of physisorbedCO above the O* was considered as the initial state (IS-3 and IS-4). Asseen, CO molecule is weakly adsorbed over both surfaces with the ad-sorption energy of −0.25 (BC) and −0.16 eV (BN). As CO approachesthe O*, the B-O* bond length is continuously elongated and finally aCO2 molecule is formed over the surfaces. This reaction step is exo-thermic by 2.51 and 3.17 eV over BC and BN, respectively. The activa-tion energy needed to obtain CO2 over BN is only 0.08 eV, which is0.18 eV smaller than that of over BN. This is expected, since the O* overBN has a weaker interaction with the B atom as evidenced by a largerBeO* bond distance. This also results in a larger (more negative) re-action energy for the CO+O*→ CO2 reaction as shown in Fig. 3.Moreover, the adsorption energy of O* over BN is −3.13 eV, which is0.72 eV smaller than that of over BC. This lowers the activation energyrequired to remove the O* from the BN and hence facilitates theCO+O* reaction. We note that the activation energy for the elimina-tion of O* by CO over B-doped C3N nanosheets is much smaller thanthose of over noble-metal catalysts, e.g. Pd (0.91 eV) or Pt (0.79 eV)[50].

As a competing reaction, we also study the disproportionation ofN2O molecules over the BC and BN surfaces. To this aim, we consider theadsorption of the second N2O molecule over the O* moiety. The cor-responding energy diagram and involved optimized stationary pointsare shown in Fig. 4. As evident, this reaction starts with the adsorptionof N2O over O*. The calculated adsorption energy of N2O is −0.03 (BC)and −0.01 eV (BN), indicating the physisorption of this molecule overboth surfaces. Later, N2O gets closer to the O* and this results in theelongation of B-O* bond. By overcoming a quite large activation energy,finally O2 and N2 molecules are formed over these surfaces. Accordingto our results, the N2O+O* reaction is endothermic, suggesting thatthe formation of side products (O2 and N2) is also thermodynamicallyimpossible at normal condition. Besides, due to the stronger adsorptionof O* species over BC, the activation energy for the N2O+O*→N2+O2 reaction is larger than that of over BN surface.

4. Conclusion

In this work, the adsorption and catalytic decomposition of N2Oover B-doped C3N nanosheets were studied by dispersion-corrected DFTcalculations. We found that N2O is adsorbed over BC and BN via a

[3+2] cycloaddition, accompanied with a sizable charge-transfer fromthe surface into N2O. According to our results, N2O molecule can beeasily decomposed into N2 and O* species over B-doped C3N sheets. Theactivation energy for the decomposition of N2O over BC is slightlysmaller than that of over BN surface. Meanwhile, the calculated acti-vation energies for decomposition of N2O are comparable with those oftransition metal or noble metal based catalysts. Moreover, the re-maining O* moiety can be reduced by a CO molecule by overcoming anegligible activation energy. Overall, the high exothermicity of thesereactions and low activation energies suggest that B-doped C3N na-nosheets can be regarded as a novel and green catalysts for reduction ofN2O.

Declaration of interests

The authors declared that there is no conflict of interest.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cplett.2019.03.062.

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