Si-coordinated nitrogen doped graphene: A robust and highly active catalyst for NO + CO reaction

Journal Pre-proof Si-coordinated nitrogen doped graphene: A robust and highly active catalyst for NO + CO reaction

Mehdi D. Esrafili, Mehdi Vatanzadeh PII:

S0169-4332(19)32253-6

DOI:

https://doi.org/10.1016/j.apsusc.2019.07.217

Reference:

APSUSC 43475

To appear in:

Applied Surface Science

Received date:

9 May 2019

Revised date:

23 July 2019

Accepted date:

24 July 2019

Please cite this article as: M.D. Esrafili and M. Vatanzadeh, Si-coordinated nitrogen doped graphene: A robust and highly active catalyst for NO + CO reaction, Applied Surface Science(2019), https://doi.org/10.1016/j.apsusc.2019.07.217

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© 2019 Published by Elsevier.

Journal Pre-proof

Si-coordinated nitrogen doped graphene: A robust and highly active catalyst for NO + CO reaction Mehdi D. Esrafili * and Mehdi Vatanzadeh

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Department of Chemistry, Faculty of Basic Sciences, University of Maragheh, P.O. Box 55136-553, Maragheh, Iran

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* Corresponding author. Phone: (+98) 4212237955. Fax: (+98) 4212276060. P.O. Box: 55136-553. E-mail: [email protected] (Mehdi D. Esrafili).

Abstract

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A dispersion-corrected density functional theory study is performed about reaction pathways and energy barriers of NO reduction by CO over Si-coordinated nitrogen doped

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graphene (SiN4-Gr). The results indicate that NO molecule can be stably chemisorbed over the Si atom of SiN4-Gr due to the favorable hybridization of Si-3p and NO-2π* states. The

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coadsorption of NO molecules to form absorbed (NO)2 species is proved to be the initial step for the reduction of NO molecules over the title surface. The energy barriers for the (NO)2→ N2O +

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O* reaction are in the range of 0.38-0.60 eV, which seem to be overcome at ambient condition. According to our findings, NO reduction over SiN4-Gr is a thermodynamically favored process at a relatively wide range of temperatures. Keywords: graphene; NO reduction; DFT; dimer mechanism; catalyst.

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Journal Pre-proof 1. Introduction Layered materials such as graphene and boron nitride (BN) nanosheet have attracted many attention in recent years [1-11], due to their exceptional physical, chemical and mechanical properties. For example, the large surface-to-volume ratio and high thermal stability of graphene make this magic material as an ideal support to host metal atoms or nanoparticles [12-16]. Moreover, the graphene-based nanostructures show a tunable semiconductor behavior, offering opportunity for using these systems in electronics, optoelectronics and spintronics [17-20]. Many strategies, including chemical doping with foreign atoms [21-23], functionalization with

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appropriate groups [24, 25] and introduction of defects [26, 27] have been also frequently

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suggested to modify the local chemical reactivity and electronic structure properties of graphene. Among them, the chemical doping of graphene with nitrogen atoms have been attracted a lot of

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attention [28-32]. The main reason behind this lies on the large electronegativity of the nitrogen (3.04) compared to the carbon (2.52) atom as well as the triple coordinate of the nitrogen atom,

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which allow a considerable polarization of π-electron density and therefore a local charge

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redistribution in N-doped graphene [33]. Besides, given that nitrogen has one more valence electron than the carbon atom, so nitrogen-doping of graphene could transform it to an n-type

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semiconductor. Consequently, N-doped graphene has shown exceptional catalytic activity in many different reactions such as O2 reduction reaction (ORR) [28] and CO oxidation [34].

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According to X-ray photoelectron spectroscopy results, a variety of N atoms may be simultaneously introduced in graphene, including quaternary or graphitic N, pyridine-like N and

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pyrrole-like N. Among them, the pyridinic-like N atoms have been identified to play superior catalytic activity for the ORR in both alkaline and acidic solutions. Moreover, quantum chemical calculations have revealed that the introduction of pyridinic-like N atoms around the defective site in graphene can induce localization of π states near the Fermi level. Hence, the electron transfer process can be greatly improved in the ORR process. These electron-enriched N defective states can also promote the formation of mono- or divacancy defects in graphene [15, 35-37], mainly due to the vanishing of dangling bonds around the defect site. In particular, earlier theoretical studies have proven that these N-defective sites are able to trap transition metal atoms like Fe [38, 39], Co [36] or Mn [40] to realize noble metal-free catalysts for the ORR process. Although the exact ORR mechanism over these systems remains unclear, but several

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Journal Pre-proof studies have proposed that the metal ion coordinated by four pyridinic N atoms (MN4-Gr) is the dominant structure as compared to others (e.g., MN3-Gr and MN2-Gr) [41, 42]. Nitrogen oxide (NO) is a harmful air pollutant, released mainly from the combustion of fossil fuels in automobiles and industrial processes [43]. When produced, NO can be oxidized into NO2 in the atmosphere which in turn reacts with the water molecule to form acid rain, which is very dangerous to human health and environment. NO is also considered as a main contributor to the ozone layer depletion and photochemical smog [44]. Hence, it is necessary to find efficient strategies to remove or reduce this toxic gas from the atmosphere. Currently, the selective

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adsorption and reduction of NO molecules is considered as an effective and commonly used

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approach to remove NO molecules [45-51]. Indeed, extensive experimental and theoretical studies have proven that the NO reduction reaction could be catalyzed in the presence of noble

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metals or their alloys [52-58]. For instance, using molecular beam techniques, Gopinath et al. have studied the catalytic reduction of NO by CO over the Pd(111) surface [59]. Density

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functional theory (DFT) calculations have also revealed that NO molecules can be easily reduced

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into N2O over the Au(111) surface via a dimer mechanism [60]. Although the catalytic performance of these noble metal catalysts for reduction of NO is satisfactory, but high cost and

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scarcity might hamper their wide-scale application in industry. Therefore, much attention has been recently paid to develop costless or metal-free catalysts for low-temperature reduction of NO [47, 49, 61-64]. For instance, Chen et al. [61] have indicated that Si-doped grapheme (Si-Gr)

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could serve as a potential catalyst for reduction of NO to N2O. Zhang and coworkers [49] have

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also found that NO reduction over N-doped graphene via the dimer mechanism is more favorable than the direct dissociation mechanism. Motivated us by a recent study about the possibility of using Si-coordinated nitrogen doped graphene (SiN4-Gr) as a metal-free catalyst for oxidation of CO [65], the aim of this study is to investigate the detailed mechanism of NO reduction over SiN4-Gr. This substrate not only satisfies our initial requirement to search noble metal-free catalysts for reduction of NO, but also it can provide a new direction to develop active catalysts based on 2D carbon materials. Moreover, since the final products are less harmful than the reactants, the findings of this study may be very useful in practical application for removing of toxic gases from the atmosphere. With the help of dispersion-corrected DFT calculations, we examine the most probable reaction mechanisms as well as the corresponding energy barriers and optimized stationary points 3

Journal Pre-proof involved in the reduction of NO. The origin of high catalytic activity of SiN4-Gr is discussed by means of partial density of states (PDOS) and charge-transfer analyses.

2. Computational details All the quantum chemical calculations were performed based on spin-polarized DFT using the DMol3 code [66, 67]. The Perdew-Burke-Ernzerhof (PBE) [68] functional was adopted to geometry optimizations and the corresponding frequency calculations. The long-range dispersion correction was considered by the Grimme’s DFT-D2 scheme [69, 70]. A double

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numerical plus polarization (DNP) basis set was used in the calculations, which is able to provide

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comparable results to those of valence double-zeta polarized 6–31G** basis set [71]. A realspace cutoff of 4.6 Å was considered in the all calculations. The convergence tolerance of the

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energy, force, and displacement was set to be of 10-5 Ha, 0.001 Ha/Å and 0.005 Å, respectively.

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All the transition state (TS) structures were obtained by the complete linear synchronous transit and quadratic synchronous transit (LST/QST) method, and their nature was verified by the

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frequency calculations.

To model the pristine graphene, a 5 × 5 × 1 supercell containing 50 carbon atoms was

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chosen. To avoid the interaction between the neighboring graphene layers, a vacuum space of 20 Å was used. The SiN4-Gr was then obtained via the creation of a divacancy defect, followed by

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the deposition of a Si atom at the center of the defect site and finally the substitution of the under-coordinated C atoms by N atoms. The Brillouin-zone integration was sampled by 3 × 3 × 1

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k-points during the geometry optimization (using the Monkhorst–Pack scheme), while 12 × 12 × 1 k-points was used to perform PDOS analyses. The formation energy (Eform) of SiN4-Gr was calculated as: Eform = ESiN4-Gr - EGr + 6 μC - μSi - 4 μN

(1)

where ESiN4-Gr and EGr is the total energy of the SiN4-Gr and pure graphene, respectively. μSi is the energy of a single Si atom. μC and μN is the atomic energy (chemical potential) of a carbon and nitrogen atom calculated from total energy of the pure graphene and N2 molecule, respectively. The adsorption energy (EA) of a given adsorbate was obtained as EA = Etot- (ESiN4-Gr + Eadsorbate)

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(2)

Journal Pre-proof here Etot, Eadsorbate and ESiN4-Gr is the total energy of optimized adsorbate/SiN4-Gr complex, pure SiN4-Gr and isolated adsorbate, respectively. Accordingly, a negative EA refers to an exothermic adsorption. To probe the amount of electron density redistribution upon the adsorption of NO or CO molecule, electron density difference (EDD or Δρ) maps were obtained as Δρ = ρtot– (ρSiN4-Gr + ρX)

(3)

where ρtot, ρSiN4-Gr and ρX is the electron density of adsorbate/SiN4-Gr complex, pure SiN4-Gr

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and isolated NO or CO molecule, respectively.

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3. Results and discussion 3.1. Properties of SiN4-Gr

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SiN4-Gr is obtained by replacing two of the lattice carbon atoms of graphene with a Si atom, followed by the substitution of the all under-coordinated carbons by N atoms. As Figure 1a

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indicates, the Si atom in the optimized SiN4-Gr locates in the center of the N atoms, forming four

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equal covalent Si-N bonds of 1.83 Å. According to equation (1), the Eform value of SiN4-Gr is calculated to be -2.72 eV, which indicates its formation is energetically favorable at normal

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condition. We note that this value is larger (more negative) than the corresponding value obtained for the SiN-Gr, SiN2-Gr and SiN3-Gr structures (see Figure S1 of Supporting

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Information). This finding, which matches with the earlier results of Tang and co-workers [65], may be explained by the formation of stronger Si-N bonds compared to the Si-C ones. Moreover,

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the incorporation of four N atoms around the Si atom (SiN4-Gr) also leads to more negative formation energy compared to other possible configurations (Figure S2). As a result, SiN4-Gr is the more energetically favorable configuration as compared with other possible structures. Furthermore, due to the presence of electronegative N atoms, the Si atom in SiN4-Gr is positively charged by 0.45 |e|. Meanwhile, the under-coordinated N atoms are negatively charged by 0.10 |e|. The deformation charge density map in Figure 1b also confirms this finding, where the incorporation of the Si and N atoms induces a considerable electron density rearrangement in SiN4-Gr. In particular, the existence of a sizable electron density loss region on the Si clearly indicates that this positively charged atom would have a relatively high tendency to establish an electrostatic interaction with negative sites on adsorbates. Hence, it is expected that the Si atom should serve as the potential reaction site to capture small molecules like NO and CO. 5

Journal Pre-proof To examine the structural stability, first-principles molecular dynamics (MD) calculations are also performed on SiN4-Gr. Figure 1c shows the final structures from the MD calculations at three different temperatures (300 K, 500 K and 1000 K) in a period of 1000 fs. One can see that in the all cases, SiN4-Gr almost preserves its planar structure. In particular, the Si and N atoms distort slightly from the graphene surface, confirming the formation of the strong covalent bonds between them. So, it is suggested that SiN4-Gr is thermodynamically stable at normal or even high temperatures.

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3.2. Adsorption of NO and CO molecules over SiN4-Gr Figure 2 represents the most stable configuration of NO and CO molecules adsorbed over

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SiN4-Gr. The corresponding adsorption energy (EA), charge-transfer (qCT), change of enthalpy (ΔH298) and Gibbs free energy (ΔG298) values are summarized in Table 1. Looking at Figures 2a

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and b, it is seen that NO is adsorbed over the Si atom of SiN4-Gr via its either N or O atom. In

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both cases, NO adopts an end-on configuration with the Si-N-O or Si-O-N bonding angle of about 120°. Meanwhile, the calculated adsorption energies indicate that the binding of NO

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molecule from its N-site is about 1.1 eV more favorable than from its O-site (Table 1). Meanwhile, the adsorption of NO molecule is accompanied with a relatively large charge-

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transfer (qCT) from SiN4-Gr to NO. Consequently, the N-O bond distance is elongated from 1.17 Å in the isolated NO molecule to 1.24 (N-site) and 1.27 Å (O-site) in the adsorbed configuration.

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The EDD maps also indicate that upon the adsorption of NO, a considerable electron density is accumulated over NO, while a loss of electron density is evident over the Si atom. The latter

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verifies the polarization of the Si atom in the presence of NO. From Table 1, one can also see that the adsorption of NO over the SiN4-Gr surface is exothermic by -1.47 (N-site) and -0.42 eV (O-site). The corresponding negative ΔG298 values also indicate that the adsorption process of this molecule would be spontaneous at normal temperatures. The frequency calculations also indicate that upon the NO adsorption, the N-O stretching frequency downshifts about 470 (Nsite) and 680 cm-1 (O-site) compared to that of free molecule (1904 cm-1). This frequency shift depends on the adsorption mode of NO, and is more pronounced for the O-site adsorption than N-site. The PDOS analysis also reveals that there is strong hybridization between the Si-3p and NO-2π* states around the Fermi level. As evident, such hybridization is larger for the NO adsorption from its N-site than its O-site, which can be related to the greater contribution of N atom in the singly occupied molecular orbital of NO (see, Figure 2). 6

Journal Pre-proof As shown in Figure 2c, CO molecule also prefers an end-on configuration over the Si atom of SiN4-Gr. CO is attached to the Si atom via its C atom with the Si-C bonding distance of 1.90 Å. The C-O bond is perpendicular to SiN4-Gr surface as those seen for the CO adsorption onto transition metal-doped graphene [72, 73]. The EA value is calculated to be -0.67 eV, which is much larger than the corresponding value over Si-Gr (-0.19 eV) [23]. Meanwhile, our calculated EA is consistent with that obtained by the earlier study [65]. Like NO, the adsorption of CO over the title surface is characterized by negative ΔH298 and ΔG298 values, suggesting that this process is thermodynamically favored at normal condition. According to the EDD analysis,

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while the adsorption of CO induces an electron density loss region over the Si atom, an electron

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density gain area is seen over the C atom of CO. This clearly shows that like NO, the addition of CO tends to polarize the SiN4-Gr surface. The PDOS plots in Figure 2c also show that the Si-3p

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states of SiN4-Gr are significantly mixed with the CO-2π* state around the Fermi level, subsequently leading to an elongation of C-O bond. The Hirshfeld charge density analysis also

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suggests that about 0.05 |e| are transferred from CO to SiN4-Gr. These transferred electrons,

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which are mainly localized over the C atom, result in the polarization of CO molecule (Figure 2c). However, comparing the Eads and qCT values in Table 1 reveals that the tendency of NO

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molecule to adsorb onto SiN4-Gr is much larger than that of CO. Hence, when exposed to a NO/CO mixture, the Si atom of SiN4-Gr should selectively adsorb NO molecule. As an important step of the NO reduction mechanism, we also consider the coadsorption

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of NO molecules over SiN4-Gr. Figure 3 shows the three most energetically favored

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configurations (labeled as A, B and C) of two NO molecules coadsorbed over the Si atom. All these structures were obtained by placing two NO molecules over the Si atom (from their either N- or O-site) and then allowing the system to relax. The energy profile and activation energy needed to reach these systems are shown in Figure S3. Looking at Figure S3a, it is found that NO molecules are first physisorbed over the Si atom from their O-site. By overcoming an activation energy of 0.15 eV, NO molecules are finally attached to the Si atom, forming two equal Si-O chemical bonds of 1.79 Å. The coadsorption energy (Ecoads) of NO molecules, which is defined as Ecoads=E2NO/SiN4-Gr - ESiN4-Gr - 2ENO, where E2NO/SiN4-Gr, ESiN4-Gr and ENO are the energy of coadsorbed configuration, SiN4-Gr and NO molecules, respectively, is calculated to be -3.86 eV for the configuration A. This value is much larger than twice the Eads of a NO molecule (-3.12 eV), implying that the coadsorption of two NO molecules is energetically much favored. 7

Journal Pre-proof Meanwhile, about 0.48 |e| are transferred from the surface to NO molecules in A, which are mainly localized over the O atoms. To form the coadsorbed configurations B and C, a NO molecule is first chemisorbed over the Si atom and then the second NO molecule is physisorbed over the preadsorbed NO molecule (Figure S3). The energy barrier to reach configurations B and C is calculated to be 0.08 and 0.03 eV, respectively, which suggests that the formation of both structures is kinetically favorable. Note that the relatively smaller energy barrier for the configurations C can be attributed to the more proper orientation of the second NO molecule in the corresponding transition state, which

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leads to a smaller repulsion between the negatively charged O atom of NO and surface N or C

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atoms. Moreover, the corresponding Ecoads value for these systems is -2.74 and -2.81 eV, respectively, which is smaller than that of the configuration A. The qCT value associated with the

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coadsorption of NO molecules in B and C is 0.46 and 0.43 |e|, respectively. Meanwhile, the positive charge on the Si atom decreases by 0.02 |e| as compared to that of SiN4-Gr.

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As another probable coadsorbed configuration, we also consider the structure D, where

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NO and CO molecules are coadsorbed over SiN4-Gr (Figure 3d). In D, NO is first chemisorbed over the surface due to its larger tendency to interact with the Si atom as compared to CO

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molecule. Then, CO is physisorbed over NO molecule with the binding distance of 2.97 Å. However, the calculated Ecoads (-1.46 eV) and qCT (0.24 |e|) values for this coadsorbed configuration are much smaller than those of A, B and C. This indicates that the coadsorption of

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NO and CO molecules should be less probable than that of two NO molecules. 3.3. The mechanism of NO reduction over SiN4-Gr According to earlier theoretical and experimental studies [53, 74-76], there are two wellknown mechanisms for the reduction of NO over catalyst surface, namely, "dimer" and "direct dissociation" mechanisms. In the former, NO molecules are first coadsorbed and form (NO)2 species on the active site of catalyst. The dissociation of (NO)2 then leads to the formation of N2O and an activated O* atom. On the other hand, the latter mechanism starts with the activation and subsequent dissociation of NO molecule into N* and O* atoms on the surface of catalyst. Next, another NO molecule interacts with the N* to give N2O. On the basis of the above discussions, the large adsorption energy as well as small activation energy associated with the formation of coadsorbed (NO)2 configurations clearly indicate that, at high concentration of CO

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Journal Pre-proof and NO molecules, the Si atom of SiN4-Gr must be selectively covered by NO molecules. So, the dimerization of NO molecules might be considered as the initial step of the NO reduction over the title substrate. Besides, due to the large activation energy (2.81 eV) associated with the dissociation of N-O bond of NO molecule, the direct dissociation mechanism is assumed to be hindered over the title surface (Figure S4). Hence, one can expect that the NO reduction on the title surface should proceed via the dimer mechanism. Figure 4 shows the potential energy profile for the decomposition of coadsorbed NO molecules into N2O and an activated oxygen atom (O*). For the all cases, the reaction is found to

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be exothermic, with an activation energy of 0.38 (A), 0.56 (B) and 0.60 eV (C). These Eact values, which are comparable with the corresponding values reported on noble metal catalysts

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like Ag [46], show that the (NO)2 → N2O + O* reaction is likely to proceed over SiN4-Gr at

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ambient temperature. Meanwhile, the energy barriers needed to decompose (NO)2 into N2O and O* species over SiN4-Gr are smaller than those of over Si-Gr [61] or N-doped graphene [49],

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may be due to the larger activation of (NO)2 moiety over the former substrate. The formed N2O

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has a negligible adsorption energy (≈ -0.08 eV), so it can be easily released from the SiN4-Gr surface.

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Afterwards, the remaining O* on SiN4-Gr can readily bind to an incoming CO or NO molecule to form CO2 or NO2. Figure 5 depicts the corresponding energy diagrams and optimized stationary points. As seen, these reaction steps start with the adsorption of CO or NO

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over the O*. The Eads value of CO and NO is calculated to be -0.18 and -0.54 eV, respectively,

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suggesting that the tendency of O* moiety to interact with CO is less than NO. Besides, the binding distance between the O* and CO is 2.81 Å, which is much larger than that of NO. By overcoming an energy barrier of 0.50 eV, CO binds to the O* and finally a CO2 molecule is formed over the surface. In the final state, CO2 interacts with the Si atom through its C atom. However, due to its small adsorption energy (≈ -0.30 eV), CO2 can easily desorb from the Si atom and hence the surface is recovered to start another NO reduction reaction. On the other hand, our results indicate that the energy barrier needed for the side reaction NO + O* → NO 2 is about 1.54 eV, which indicates that it cannot proceed at normal temperature. As a result, the poisoning of the active site of the catalyst by NO2 molecule is almost impossible. 3.4. The effect of temperature and entropy

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Journal Pre-proof To see the effect temperature and entropy on the NO reduction, the ΔG and TΔS values are calculated for different steps of this reaction. These two quantities are related to each other according to the following equation: ΔG = ΔH - TΔS

(4)

in which G, H and S is the Gibbs free energy, enthalpy and entropy of the system, respectively, and T is the temperature. Table 2 lists the ΔG and TΔS values of different reaction steps involved in the reduction of NO. We discuss the obtained results only for the configuration A as the representative, since other coadsorbed configurations indicate almost a similar trend. From Table

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2, it is seen that all elementary steps involved in the reduction of NO are exothermic over a wide range of temperature. While an increase in the temperature makes a decrease in the ΔG values of

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ISA → A and IS-4 → FS-4 steps, however, a reverse trend is evident for the A → FS-1. The

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latter can be understood by the entropy effects, since the coadsorption of NO molecules or the formation of final state (i.e., IS-4 → FS-4) leads to a decrease in the entropy of the system and

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hence a negative TΔS value. Nevertheless, the results of Table 2 show that the TΔS term for all

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the mentioned reaction steps shows a quite weak dependency on the temperature. Meanwhile, the negative ΔG values along with the small activation energies discussed above indicate that NO

4. Conclusions

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range of temperature.

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reduction over SiN4-Gr is a thermodynamically and kinetically favored process over a wide

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Using the spin-polarized DFT calculations, the catalytic activity of Si atom coordinated di-vacancy defective N-doped graphene was studied. Various adsorption/coadsorption configurations of NO and CO were obtained and the corresponding adsorption energy and bonding properties were compared. It was found that NO is strongly chemisorbed over the Si atom of SiN4-Gr due to favorable electronic resonance among the Si-3p states of the surface and 2π* states of NO. Moreover, the coadsorption of two NO molecules was found to be more energetically favored than coadsorption of NO and CO. NO reduction over Si-Gr proceeds via a dimer mechanism, in which NO molecules are first coadsorbed to form (NO)2 dimer, and then this dimer is converted to N2O and O* species. According to our results, the activation energies for the (NO)2 → N2O + O* reaction on SiN4-Gr are in the range of 0.38-0.60 eV. The energy barrier for the removing of extra O* moiety from the Si atom is 0.50 eV, which is much smaller 10

Journal Pre-proof than that of side reaction, NO + O* → NO2. Meanwhile, the negative ΔG values for different steps of the NO reduction reaction indicated that this process is thermodynamically favored in a wide range of temperatures.

Acknowledgments. The authors would like to thank the “Computational Center of University of Maragheh” for its technical support of this work.

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Conflict of interest. The authors declare they have no conflict of interest.

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Journal Pre-proof Table 1. Calculated adsorption energy (EA, eV), net charge-transfer (QCT, e), change of the enthalpy (ΔH298, eV) and Gibbs free energy (ΔG298, eV) of NO and CO molecules adsorbed over SiN4-Gr a adsorbate

EA

qCT

ΔH298

ΔG298

NO (N-site)

-1.56

-0.23

-1.47

-1.48

NO (O-site)

-0.48

-0.20

-0.42

-0.43

CO

-0.67

0.05

-0.61

-0.62

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Calculated ΔH298 and ΔG298 values at 298 K and 1 atm.

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Journal Pre-proof Table 2. Gibbs free energy change (ΔG, eV) and TΔS (eV) values for different steps of NO reduction over SiN4-Gr ISA → A

A → FS-1

IS-4 → FS-4

TΔS

ΔG

TΔS

ΔG

TΔS

No correction

-3.43

-

-1.25

-

-1.89

-

50

-3.27

-0.03

-1.26

0.01

-1.82

-0.02

100

-3.24

-0.07

-1.27

0.03

-1.79

-0.05

200

-3.16

-0.16

-1.31

0.08

-1.70

-0.12

250

-3.12

-0.21

-1.33

0.11

-1.63

-0.18

298.15

-3.08

-0.26

-1.35

0.14

-1.60

-0.22

300

-3.08

-0.26

-1.35

0.14

-1.55

-0.21

400

-2.99

-0.34

-1.40

0.19

-1.51

-0.28

500

-2.91

-0.42

-1.44

0.24

-1.46

-0.36

600

-2.82

-0.50

-1.49

0.29

-1.38

-0.43

700

-2.74

-0.57

-1.54

0.33

-1.31

-0.50

800

-2.66

-0.63

-1.59

0.37

-1.26

-0.56

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ΔG

lP

T (K)

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Journal Pre-proof Research highlights: 1- SiN4-Gr is suggested as a promising catalyst for the NO + CO reaction. 2- The coadsorption of NO molecules is the initial step for the reduction of NO.

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3- The energy barriers for N2O formation are in the range of 0.38-0.60 eV.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5