Complete catalytic cycle of NO decomposition on a silicon-doped nitrogen-coordinated graphene: Mechanistic insight from a DFT study

Journal Pre-proofs Full Length Article Complete Catalytic Cycle of NO Decomposition on a Silicon-Doped Nitrogen-Coordinated Graphene: Mechanistic Insight from a DFT study Phornphimon Maitarad, Anchalee Junkaew, Vinich Promarak, Liyi Shi, Supawadee Namuangruk PII: DOI: Reference:

S0169-4332(20)30011-8 https://doi.org/10.1016/j.apsusc.2020.145255 APSUSC 145255

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Applied Surface Science

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1 July 2019 10 December 2019 1 January 2020

Please cite this article as: P. Maitarad, A. Junkaew, V. Promarak, L. Shi, S. Namuangruk, Complete Catalytic Cycle of NO Decomposition on a Silicon-Doped Nitrogen-Coordinated Graphene: Mechanistic Insight from a DFT study, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145255

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Complete Catalytic Cycle of NO Decomposition on a Silicon-Doped Nitrogen-Coordinated Graphene: Mechanistic Insight from a DFT study Phornphimon Maitarad,1,2 Anchalee Junkaew,3* Vinich Promarak,2 Liyi Shi,1 Supawadee Namuangruk1,3* 1

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444,

P. R. China 2

Vidyasirimedhi Institute of Science and Technology, Wang Chan, Rayong 21210, Thailand

3

National Nanotechnology Center (NANOTEC), National Science and Technology

Development Agency, Pathum Thani, 12120, Thailand

ABSTRACT The abatement of NO with efficient technology and low operation cost is still a challenging mission worldwide. By application of DFT calculations, we systematically investigate the complete catalytic cycles of NO conversion on the single Si atom doped in four pyridinic N at the divacancy site of graphene. The effects of O2, CO and water in exhausted gas are considered. The SiN4G catalyst is highly active to NO while CO is essential for recovering the catalyst surface after NO decomposition. The catalyst shows resistance to water which is a positive effect for NO abatement technology. Furthermore, the synergy between the Si atom and coordinating nitrogen atoms greatly enhances the catalytic activity towards NO reactions.

1

This metal-free catalyst exhibits higher catalytic activity for both NO reduction and oxidation in mild conditions compared with those catalysts doped only with Si or coordinated only with N on graphene. With presence of O2, NO oxidation only requires 0.2 eV activation energy, while NO reduction also occurs easily in the absence of O2. Both oxidation and reduction reactions are exothermic processes. Therefore, incorporating Si and N on graphene could provide a low-cost and highly efficient method of fabricating catalyst for large-scale reaction of NO abatement. Keywords. Nitric oxide, NO reduction, NO oxidation, Density functional theory, Metal free catalyst *Corresponding authors. Tel: +6621176595. E-mail: [email protected] (Supawadee Namuangruk), [email protected] (Anchalee Junkaew)

1. INTRODUCTION Nitric oxide (NO) is a toxic gas which is found as a major gas in nitrogen oxide compounds (NOx) emitted from combustion in vehicles and industrial processes. It can cause concomitant air pollution, acid rain, photochemical smog, greenhouse effects, and many other hazards that endangers both people and ecosystems [1-4]. A high level of NO in the air can cause problems in respiratory, cardiovascular and immune systems of human and animals [5, 6]. There have been enormous attempts to find catalytic methods for NO abatement. Oxidation of NO generally leads to NO2 molecules which can be easily converted to nitric acid by interacting with water in wet scrubber via various techniques such as plasma-based technologies [7, 8], or in diesel engines [9-11], etc. However, as a products of NO oxidation, it has reported that NO2

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itself or its reaction products including O3 and secondary particles may also increase health risks [12]. NO2 is able to be converted to green products via such methods as selective catalytic reduction [13], non-thermal plasmas [14, 15], reduction via hydrocarbon over zeolite-based catalysts [16, 17], etc. Many precious and transition metal-based (such as Pt, Pd and Au) catalysts have been reported to be active catalysts for these reactions [18-27]. However, though with high catalytic activity, the noble metal catalysts are costly. Therefore, the alternative catalysts with high efficiency and low cost are desirable. Recently, low dimensional and metal-free catalysts have been attracting strong interests especially in many chemical reactions [28-35]. Among various two dimensional materials, graphene, a single atomic layer with sp2-hybridized carbon allotrope, can maximize the interactions between adsorbates with its high surface area. Many experimental and theoretical studies have focused on improving the sensing performance of graphene towards various targeted molecules by functionalizing or doping methods [29, 36-41]. Markedly, pyridinic nitrogen coordinated graphene (NG) is one of the potential platforms for heteroatom substitution to improve its effectiveness towards many applications [40, 42-47]. Particularly, the active site of NG structure doped by a metal atom and coordinated by four pyridinic N atoms (MN4G) exhibits high catalytic activity for chemical reactions [48-50]. For instance, FeN4G, CoN4G, TiN4G and NiN4G were found to be energetically favourable and able to promote oxygen reduction reaction (ORR) [51-55]. Theoretical study of Ashori, E. et al. [56] showed that FeN4G could reduce NO to N2O and N2. They found that the active site of FeN4G was very active towards the adsorption of NO and (NO)2 molecules. Furthermore, in recent years, metal-free N4G catalysts are also being exploited on catalysis applications [57-60] due to their low cost and environment-friendly advantages. Due to the low cost and abundance of silicon on earth, 2D materials incorporated with Si are attracting great attention as one of promising metal-free catalysts with high surface area.

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Previous experimental work showed the synthesis of Si coordinated with four nitrogen atoms, so-called Si-porphyrin [61], and it was further applied to both catalyst and sensitizer [61-64]. Theoretical calculations proposed that Si-doped graphene (SiG) possessed unique and attractive properties in many applications such as SO2, CO oxidation, NO oxidation, and ORR [30, 65-74]. Si could improve the sensing efficiency of graphene towards some toxic molecules. SiG showed higher chemical reactivity and better adsorption strength towards CO, O2, NO2 and H2O compared to pristine graphene (PG). The strong interactions between SiG and the adsorbed molecules induce dramatic changes to the electronic properties of SiG and make it a promising candidate as sensing material for those targeted gases [30]. SiG also provided low activation barrier in CO oxidation with O2 or N2O [65] and it was proposed as an efficient metal-free catalyst for O‒H bond cleavage of HCOOH [75]. Recently, graphene with Si doped in four pyridinic N at its divacancy site, called silicon-doped nitrogen-coordinated graphene (SiN4G), has been theoretically investigated and reported as a potential alternative metal-free catalyst for CO oxidation [57]. The motivation arises from our recent work [76] which found that the SiN4G catalyst could easily reduce N2O toxic gas to N2. But since the fact is that the majority of NOx in exhaust gas is NO (~95%), thus it is worthwhile to study the elimination of NO so as to fulfill the systematic understanding of NOx decomposition processes. In this work, SiN4G catalyst has been intensively studied to explore all possibilities of NO conversion in both aerobic and anaerobic conditions. The complete catalytic cycle consists of four reaction routes; (A) NO direct decomposition, (B) NO oxidation, (C) NO reduction, and (D) N2O reduction, which is depicted in Figure 1. Route D was reported in our previous study [76], while routes A-C involving in NO conversion were proposed in this work. Insight of the adsorption ability, activation barrier, and reaction energy profile were systematically summarized after performing periodic density functional theory (DFT) calculations. The catalytic reactivity of

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SiN4G was also compared with SiG to evaluate the effect of coordinated nitrogen atoms in graphene surface. In addition, the reactivity of the reported NG, SiC, SiG as well as MN 4G were taken into consideration so as to predict the possibility of using SiN4G as the catalyst for NOx decomposition.

Figure 1. The complete catalytic cycle of NO decomposition on SiN4G metal free catalyst.

2. MODELS AND METHODS Reaction of NO on the SiN4G surface was investigated by using a plane wave-based DFT method implemented in the Vienna ab initio simulation package (VASP 5.4.1) [77, 78]. In this work we applied the projector-augmented wave (PAW) [79] with the generalized gradient approximation (GGA) refined by Perdew, Burke and Ernzerhof (PBE) in the calculations [80]. The dispersion correction via Grimme's DFT-D3 method was employed [81]. The energy

5

convergence and the force convergence are 1×10-5 eV/cell and 5×10-3 eV/Å, respectively. The energy cutoff of 400 eV was set for plane wave basis. The Brillouin zone sampling was 5 x 5 x 1 k-points. Spin-unrestricted calculation was applied in all cases. 15 Å×15 Å×15 Å box was used for optimizing each isolated gas molecule. SiN4G sheet was constructed by removing two C atoms (i.e. making a divacancy site) from (5x5) graphene sheet, then replacing them with four dangling C atoms with four N atoms and putting Si atom at that divacancy site. The optimized sheet has a=12.4 Å and b=12.2 Å, and the distance from its replicas in c-dimension is 15 Å of vacuum (see Figure 2). To attain the reaction mechanisms, the climbing-image nudged elastic band (CI-NEB) method [82] and the DIMER method [83] were utilized for locating a transition state (TS) of each elementary step. The TS state was confirmed by the single imaginary frequency. The difference in energy between the TS state and its preceding initial state is an activation energy (Ea).

Figure 2. Top view of SiN4G and selected bond lengths

3. RESULTS AND DISCUSSION

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NO reaction, which consists of three routes that are (A) NO direct decomposition, (B) NO reduction, and (C) NO oxidation, was investigated via seven proposed reaction pathways as following: (A) NO direct decomposition Path A: NO (g) → N* + O*

(1)

(B) NO Reduction Path B(I): cis-(NO)2 → N2 (g) + O2*

(2)

Path B(II): cis-(NO)2 → N2O(g) +O*

(3)

Path B(III): trans-(NO)2 → N2O(g) + O*

(4)

(C) NO oxidation Path C(I): NO (g) + O2 (g) → NO3* → NO2(g)→ O*

(5)

Path C(II) NO (g) + O2 (g) → cis-OONO* → NO2 (g) + O*

(6)

Path C(III) NO (g) + O2 (g) → trans-OONO*→ NO2(g) + O*

(7)

The adsorption of reactants, intermediates and products were tested first in order to understand their interactions with the catalyst. This information can give a good guidance for selecting the possible reaction pathways to be studied in the later step. The activation energy (Ea) of rate determining step and the reaction energy (ΔErxn) can be calculated to compare its efficiency with other reported catalysts. 3.1 Adsorption of adsorbed species over SiN4G compared with other catalysts. In order to explore the stable and favorable configurations, adsorption energies (Ead) of different species

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relating to the NO oxidation-reduction reactions over SiN4G were calculated by the following equation: complex isolate isolate Ead  Eadsorbate  substrate  Esubstrate  Eadsorbate

complex

isolate

(8)

isolate

where Eadsorbate substrate , Esubstrate and Eadsorbate are total energies of an adsorbate-substrate complex, a bare surface and an isolated adsorbate, respectively. In term of stability, a negative Ead value indicates the attractive interaction between the surface and the adsorbate in the complex.

Figure 3. Adsorbed species on the active site of SiN4G. The atomic distance is presented in Å. Herein, the interactions of SiN4G with the associative molecules in NO oxidationreduction reactions such as O2, NO, NO2, N2O and N2 are calculated. Its interactions with CO, CO2, and H2O are also studied to compare the adsorption ability of this catalyst towards

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common gas species in exhausted gases. Furthermore, the adsorption of CO oxidation species over SiN4G are considered in order to compare the tendency of CO oxidation and NO oxidation in this catalyst. We also test the preferable site for an O atom around the active center of SiN4G, because the SiN4G‒O* is an important intermediate in the NO oxidation and reduction mechanisms. As a result, the calculated Ead values of selected configurations and adsorption structures are given in Figure 3. According to the calculated Ead values, the adsorption strength of gas over SiN4G is in the following order: NO2 (-3.20 eV) > O2 (-2.96 eV) >> NO (-1.94 eV) >> N2O (-0.97 eV) > CO (-0.66 eV) > H2O (-0.56 eV) > CO2 (-0.37 eV) > and N2 (-0.17 eV). Table 1. The adsorption energies (eV) of O2, NO, NO2, N2O, CO, CO2 and H2O on SiN4G in this work compoared with the reported nitrogen graphene (NG), Si doped carbon (SiC), Si doped graphene (SiG), and nitrogen coordinated metals in graphene (MN4G). Catalyst

O2

SiN4G -2.96 (This work)

Si-N4G [57] SiC [31]

NO

NO2

N2O

N2

CO

CO2

H2 O

-1.94 (N-bound) -1.48 (O-bound)

-1.48

-0.97

-0.17

-0.66

-0.37

-0.56

-3.20

-0.20

-0.12

-0.16

-0.14

-2.69

-0.90 -0.55

-0.26

-2.80 -0.12 (N-bound) -0.07 (O-bound)

SiG [73]

-0.81 (N-bound) -0.19 (O-bound)

SiG [65]

-0.82

-2.17

-0.39

NG [45]

-0.23

-1.22

-0.11

NG [84]

-0.19

-0.42

NG [85]

-0.21 -0.10

-0.32 -0.11

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NG [66]

-5.06

-0.23

-0.30

-2.67

-0.03

CoN4G [44] MoN4G [44] BN4G [44]

-0.67

-0.43

-4.00

-1.88

-2.28

-1.77

To better understand the adsorption ability of SiN4G, the relavant catalysts such as nitrogen graphene (N4G), Si doped carbon (SiC) or Si doped graphene (SiG), and metal doped nitrogen coordinated graphene (MN4G) for the same molecules adsorptions are listed in Table 1. Firstly, comparing NG catalysts with and without Si, the adsorption energies of NO, N2O, NO2, and CO over SiN4G catalyst are much stronger than that over the pure NG [45, 66, 84, 85]. Furthermore, the SiN4G catalyst also could adsorb NO molecule with superior adsorption energy over that of SiC and SiG catalytic models [31, 65, 73]. The adsorption energy of NO over SiN4G is substantially larger than those over N4G and SiG, so there is a synergetic effect between doped Si and the four nitrogen coordinating atoms for the gas adsorption. As given in Table 1, the SiN4G had lower activity for CO adsorption than MoN4G and BN4G, while it adsorbs CO better than CoN4G [44]. Overall, the SiN4G has highest activity towards NO in the NO oxidation/reduction reactions implying that it would exhibit the highest catalytic performance among the compared catalysts. The insight of the molecular reactant and product species for NO reactions are further discusses in the following topics. 3.1.1 Adsorption of Reactant Species (O2, O*, NO and N*) on SiN4G. The O2 molecule has two possible adsorption configurations, namely, the side-on and the end-on showed in Figure 3(a) and (b), respectively. Their binding energies are -2.96 eV and -2.35 eV, respectively, which agreed with the previous report [57]. Note that the O2 adsorption ability on SiN4G surface is better than that of BN4G and CoN4G, whereas MoN4G shows much stronger binding strength with O2 than SiN4G [44]. Another oxidized form of this catalyst is the oxygen atom adsorbed

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SiN4G or SiN4G-O*. The possible selective sites for the O atom adsorption on SiN4G including the Si site, C-C bridge of 6-membered ring and C-C bridge of 5-membered ring, and the strongest adsorption interaction is found on Si site with Ead of -7.05 eV (see Figure 3p). Such high Ead indicates that the oxygen atom is tightly adsorbed on SiN4G surface. For the NO molecule interaction, there are two possible configurations for NO molecule to be adsorbed on the SiN4G surface, i.e. N-bound and O-bound configurations, as shown in Figure 3(c) and 3(d). The adsorbed NO with N-bound or O-bound prefers to lie with an angle to the Si atom of the surface, with lengths of 1.79 Å and 1.74 Å, respectively, and their calculated adsorption energies are -1.94 eV and -1.48 eV, respectively. Therefore, the configuration of N-bound at the Si site is more preferable for NO, and this is also found in the cases of NO adsorption over the SiC and SiG surfaces. Furthermore, the NO adsorption on the SiN4G is remarkably higher than that on SiC [31], SiG [65, 73] and pure NG [45, 66, 84, 85] catalysts which were reported to be -0.12 eV, -0.81 eV, and -0.23 eV, respectively. 3.1.2 Adsorption of Product Species (NO2, N2O, N2). Firstly, the adsorption of NO2 which is a product of NO oxidation was calculated. From Table 1 and Figure 3(e)-(f), the adsorbed NO2 molecule showed Ead of -2.69 eV and -3.20 eV for N-bound and O-bound configurations, respectively. So, the O-bound configuration has better adsorption than the other. Secondly, the adsorption of N2O, which is a product of NO reduction is explored and the obtained adsorbed energies are -0.20 eV and -0.97 eV for the O-bound and N-bound configurations, respectively. As depicted in Figure 3(g) and 3(h), the O-bound configuration shows physisorption on SiN4G with the O···Si distance of 2.92 Å. In contrast, the N-bound interaction exhibits chemisorption with the N···Si bond length of 1.73 Å, which is shorter than that of O···Si. Whereas the N2O molecule adsorbed on SiG [65] and NG [45] resulted in -0.39 eV and -0.11 eV, respectively, therefore, the Si atom on N4G surface would be stronger active center to attract the N2O

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molecule. Furthermore, we found N2 molecule had a weak interaction over SiN4G with Ead~0.1 eV. This indicates that N2 product would be spontaneously desorbed at room temperature. Looking at the overall gas adsorption over SiN4G in Table 1, adsorption energies of CO and CO2 are much weaker than that of O2 and NO. Therefore, the SiN4G has higher sensitivity to NO adsorption than CO adsorption in exhausted gas mixture. Moreover, the deactivation of catalyst by water vapor, which is the main problem for NO abatement technology, is predicted to be suppressed in this catalyst due to the weak adsorption of H2O on SiN4G. 3.1.3 Electronic charge analysis. The electronic charge properties of O2- and NO adsorbed SiN4G are elucidated in this part to understand the interaction between those molecules and the Si active site. In our previous work, the charge density difference and Bader charge analysis clearly showed that electrons transfer from Si to four N sites of bare SiN4G. The positive charge (+2.66|e|) of the Si site demonstrates the nature of the Si active site and the coordinating N atoms hold negative charge in average of -1.5|e| [76]. These positive and negative charges allow Si coordinating to N4G with strong adsorption energy by -7.25 eV. In this part, we further analyzed the charge properties of the adsorbed O2 and NO adsorbed systems. Our Bader charge results demonstrate that electrons transfer from SiN4G to O2 and NO are approximately 1.55|e| and 0.79|e|, respectively. Moreover, the projected density of states (PDOSs) of selected atoms are investigated and shown in Figure 4. The p-valence states (p-PDOS) of Si and atoms of gas are plotted. The Fermi level (EF) is shown by the vertical dashed line at 0 eV. The yellow peaks illustrate the p-PDOS of Si in bare SiN4G. In Figure 4a, the p-PDOS peaks of Si and one O of O2 are presented as the green and red peaks, respectively. In Figure 4b, the p-PDOS peaks of Si, N of NO and O of NO are presented as the green, blue and red lines, respectively. The change of p-PDOS of Si before/during gas adsorption and the overlapping of states of Si and those atoms

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indicate the interaction between those gas species and Si. In addition, the mean PDOS of pvalence states (εp) (i.e. the states below EF) were calculated from εp

=

𝐸𝐹

∫−∞ 𝜌𝑝 (𝜀)𝜀𝑑𝜀/

𝐸

𝐹 ∫−∞ 𝜌𝑝 (𝜀)𝑑𝜀, where 𝜌𝑝 (𝜀) is the p-PDOS at the energy level ε. The εp values of Si of bare

SiN4G, SiN4G/O2 and SiN4G/NO are -8.06 eV, -8.36 eV and -8.62 eV, respectively. The shifted εp values express that more valence states of Si are filled when O2 and NO are adsorbed on SiN4G.

Figure 4. PDOS plots of selected atoms in (a) SiN4G/O2 and (b) SiN4G/NO. Yellow and green lines represent the p-PDOSs of Si in bare SiN4G and in the complexes, while blue and red line represent the p-PDOSs of N and O in the complexes, respectively.

3.2 NO reaction mechanisms over SiN4G. The NO reaction mechanism consist of three main routes; (A) NO direct decomposition, (B) NO reduction and (C) NO oxidation.

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3.2.1 NO direct decomposition (Route A). The first route is direct decomposition of NO to N* and O* on the SiN4G surface. There are two possible pathways for NO decomposition due to the difference of N- or O-bounds on the Si atom of SiN4G surface (Figure 5). Once the O atom binds with Si, the product intermediate initiates the O‒Si and N‒C bonds between the dissociated atoms and the catalyst, while in the case of N-bound, the N‒Si and O‒C bonds are formed as product of this process. When comparing their stability, we found the formed O‒Si and N‒C structures are more stable than the N‒Si and O‒C ones by approximately 1 eV. Based on the relevant energy plot of reactant adsorption and product formation, it is clear that both of them are endothermic processes. Since the reaction pathway of the O‒Si and N‒C product is more stable than the other, its activation barrier is further calculated and the result is that it requires a high endothermic energy of 1.70 eV. In the TS structural geometry, it is found that in the formation of 5-membered ring the N‒O bond distance changed to ~2 Å. Therefore, the direct decomposition of NO over the SiN4G is unable to undergo under mild condition due to its extremely high activation barrier and endothermicity.

Figure 5. Energy profile and corresponding structures of direct NO decomposition

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3.2.2 NO reduction (route B). There are three possible reaction pathways for NO reduction on SiN4G with different products and intermediates. All pathways are found to follow LangmuirHinshelwood type mechanism in which two NO molecules are co-adsorbed at the active site of the catalyst to form NO dimer intermediates; cis-(NO)2 and trans-(NO)2. The first pathway is cis-(NO)2 decomposition to N2 and O2* products (Figure 6). The second and the third are cis-(NO)2 and trans-(NO)2 decomposition to N2O as shown in Figure 7a and 7b, respectively. The three pathways are described as Path B(I) to B(III). Firstly, Path B(I) for cis-(NO)2 decomposition to N2 product is discussed. As presented in Figure 6, two NO molecules in the cis-ONNO configuration are bound on Si with its O terminations. In the initial INT2 state, the 5-membered ring between cis-ONNO and SiN4G is initiated via two Si-O bonds which are equivalent at 1.78 Å. The Ead value is -4.72 eV, compared with the bare SiN4G sheet and isolated NO. At TS2 state, the N-O bonds are breaking with increased length of 1.89 Å, while the length between the two N atoms are shortening to 1.15 Å. N2 is completely formed and released from SiN4G-O2* at the FS2 state as presented in Figure 6. The barrier is calculated to be 1.12 eV, and is confirmed with a single imaginary frequency at -333.5 cm-1. Then N2 is easily desorbed from SiN4G-O2* with small energy of 0.07 eV.

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Figure 6. Reaction energy profile and corresponding structures of cis-(NO)2 decomposition to N2 and O2* products (Path B(I)) Paths B(II) and B(III) are decomposition of cis- and trans-(NO)2 to N2O product. In Figure 7a, the N2O production from cis-(NO)2 over SiN4G needs only 0.69 eV to overcome the transition state at TS3. At that state, one N‒O bond is elongated to 1.63 Å. One Si‒O bond is dissociated and changed from 1.78 Å to 2.50 Å, while another Si-O bond is slightly shortening to be 1.64 Å. N2O is completely formed and on the surface SiN4G‒O* structure is formed at the FS3 state. This SiN4G‒O* intermediate is very stable with strong adsorption between O atom and SiN4G (Ead of -7.05 eV). The N2O molecule can be released by requiring energy of only 0.26 eV. This N2O production via cis-(NO)2 is an exothermic process. Then, N2O production via trans-(NO)2 configuration represented as Path B(III) is considered. And the energy profile and corresponding structures are depicted in Figure 7b. Based on the trans-

16

(NO)2 conformation, there is only one N-Si chemical bond formed between the adsorbate and the Si-center. The adsorption energies are approximately -3.3 eV for INT3 and INT4 configurations, which are weaker than that of cis-conformation in INT2. However, the activation barrier for forming N2O in Path B(III) is much lower than that in Path B(II). The consumed energy for surmounting the TS4 state is only 0.13 eV. At the TS4 geometry, the end oxygen atom moves towards Si while the N-Si is elongated from 1.83 Å to 1.90 Å. Then, N2O is simultaneously desorbed from INT5 to FS3. Path B(III) is a high exothermic process.

17

Figure 7. Reaction energy profiles and corresponding structures of (a) cis-(NO)2 (Path B (II)) and (b) trans-(NO)2 (Path B (III)) decomposition to N2O.

18

3.2.3 NO oxidation (Path C). Generally, the exhausted gas contains O2 gas. In this part, the effect of O2 is considered through NO oxidation process. According to the calculated Ead values in Table 1, SiN4G adsorbs O2 more intensely than NO. Therefore, the reaction starts with the strong adsorption of O2 on SiN4G and the Ead is -2.96 eV. Then the pre-adsorbed O2 interacts with NO gas, following Eley-Rideal type mechanism. The reaction can undergo through two distinct intermediates which are nitrate NO3* and peroxide-like OONO* intermediates (i.e. cisOONO, trans-OONO) as described as Path C(I) to C(III) in Eq. (5) to (7), respectively. For Path C(I), the energy profiles for the NO oxidation via the nitrate NO3* intermediate is demonstrated in Figure 8, which clearly shows an exothermic process. At the INT7 TS6 INT8 step, small Ea of 0.26 eV is required. At this transition state, the NO molecule moved with semi vertical configuration to the oxygen atoms at the distance of approximately 2.3 Å while the O···O bond is lengthened to 1.81 Å. The formation of NO3* intermediate generates large energy which is necessary to drive the further reactions. Then the NO2 is easily released with small desorption energy of 0.26 eV.

Figure 8. The energy profile of the NO oxidation with O2 via the NO3* intermediate (Path C(I)).

19

For NO oxidation via peroxide-like OONO* intermediates, there are two possible paths: cis-OONO* (Path C(II)) and trans-OONO* configuration (Path C(III)). These two paths start with different initial NO adsorption configurations of INT9 and INT11 as presented in Figure 9a and 9b, respectively. From these two adsorption structures, we found that NO molecule is slightly more favorable towards cis-conformation than trans-conformation. Afterward, the conformation of OONO* transition intermediates is easily proceeded with small activation energy of 0.05-0.06 eV at TS8 and TS10. Those obtained transition structures are observed in reactant-like forms. For the cis-OONO* structure (INT10) of Path C(II), the NO molecule forms bond with pre-adsorbed O* with the bond length of 1.46 Å, then it spontaneously produces NO2 with extremely exothermic product (FS5), whereas the transOONO* (INT11) needs activation energy for 0.21 eV to release the NO2 molecule from SiN4GO*. Therefore, the NO oxidation via cis-OONO path is more kinetically- and thermodynamically preferable than the trans-OONO reaction path. The recovery of SiN4G active site from SiN4G-*O in the final step of catalytic cycle was proposed to be either O*+NO  NO2 or O*+CO  CO2. For the O* + NO  NO2 step as shown in Figure 9c, this process consumes quite high energy (1.18 eV). Thus, the active site of SiN4G is difficult to be recovered by NO. On the other hand, O*+CO  CO2 is facile with low activation energy of 0.24 eV [70], which is beneficial for removing O* from SiN4GO* to recover SiN4G surface in the final step. In addition, O2* + CO is not a competitive reaction for the O2*+NO step due to its higher activation energy (0.57 eV) [57]. In summary, SiN4G is easy to recover by CO, thus it is a promising catalyst for NO and CO removal in mixed gases.

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Figure 9. Reaction energy profile of NO oxidation via peroxide-like OONO* intermediates; (a) cis-OONO* (Path C(II)), (b) trans-OONO* (Path C(III)), and (c) NO2 formation step.

3.3 Theoretical trend of NO reactions over the metal-free graphene based catalysts. To compare the catalytic activity of SiN4G with other metal-free graphene based catalysts, the Ea

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of rate-limiting steps of NO direct decomposition, NO oxidation and NO reduction from this work are compared with other theoretical studies as listed in Table 2. Firstly, the direct decomposition of NO over the N-graphene (NG) and SiN4G have high activation barriers of about 1.26 and 1.70 eV, respectively, indicating that both catalysts are unable to directly dissociate NO. For the part of NO reduction, (2NO → N2O + O*), the 4 x 4 and 5 x 5 unit cells of SiG, the Ea barriers are reported to be 0.46 eV and 0.72 eV, respectively [31, 73]. The NO reduction on NG reveals the barrier of 0.70 eV [45], indicating that coordinating nitrogen atoms to graphene does not increase the catalytic activity of NG to NO reduction. Remarkably, the introduction of both Si and coordinating nitrogen atoms into graphene substantially enhances the catalytic activity of SiN4G by suppressing Ea of NO reduction to only 0.13 eV. It is worth to note that the produced N2O can be easily reduced to N2 with Ea of 0.34 eV as reported in our previous study [76]. With the presence of O2, the NO oxidation on SiN4G catalysts is facile with the reaction barriers of 0.05 eV. Thus, SiN4G can easily decompose NO gas in both presence and absence of O2 conditions. In summary, from this study of the complete catalytic cycle, which showed low activation barriers and extremely exothermic processes of the overall NO reactions on the SiN4G, we suggest that SiN4G catalyst is a promising candidate metal-free single atom catalyst for NO gas reduction or oxidation, depending on the reaction conditions.

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Table 2. Comparison the activation energy barriers (Ea) of the rate-limiting steps for the NO reactions on the metal-free-graphene based catalysts

Catalyst

Reaction steps

SiN4G

NO  N* + O* 2NO →N2O + O*

VASP (5x5) (This work)

2NO → N2 + O2 NO +O2 → NO2 +O*

Ea (eV) 1.70 0.13 1.12 0.05

SiG [73] Dmol3(4x4)

2NO→N2O+O*

0.46

SiG [31] Dmol3(5x5)

2NO →N2O + O*

0.72

N/Graphene (NG) [66]

NO + O → NO2

0.16

N/Graphene (NG) [45] VASP

NO → N* + O* 2NO → N2O + O*

1.26 0.70 0.16

NO + O* → NO2

CONCLUSION In summary, the NO reactions on a metal-free SiN4G catalyst have been investigated by employing periodic DFT calculation. The adsorption energy calculations of reactant, product and the normal gas species in flue gas such as O, NO, NO2, N2O, N2, CO, CO2 and H2O were carried out. Results show that the order of the adsorption energy is O2 > NO2 > NO > N2O > CO > H2O > CO2 and N2. These results suggest that SiN4G is more reactive to NO adsorption than CO and is insensitive to water in the mixture. Afterward, the reaction mechanisms of all possible pathways were elucidated systematically and the complete catalytic cycle of NO reactions is revealed. The NO reduction via trans-(NO)2 is very facile with a low barrier of 0.13 eV and it is an exothermic process. When O2 in the system is taken into account, SiN4G shows the most preferable pathway for NO oxidation with the negligible Ea barrier (< 0.1 eV) via OONO* intermediate. For NO reduction, the energy barrier of NO reduction is only 0.13 eV, alternatively, and it easily produced N2O intermediate which can be straightforwardly reduced

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to N2. Thus, synergy between doped Si atom and coordinating nitrogen atoms in graphene could greatly enhance the catalytic activity to NO reactions compared with nitrogencoordinated graphene (NG) or Si-doped graphene (SiG) as well as other reported metal-free graphene based catalysts. Therefore, the present work suggests that SiN4G is a promising metal-free catalyst for NO decomposition, and this theoretical finding deserves its practical applications in NO reactions as an environmental friendly and low cost catalyst.

Acknowledgement This work was supported by Thailand Research Fund (Grant no. RSA6180080 and RTA6080005). PM and LS would like to thank the Shanghai Municipal Science and Technology Commission of Professional and Technical Service Platform for Designing and Manufacturing of Advanced Composite Materials (16DZ2292100). We acknowledge acknowledge NSTDA Supercomputer Center (ThaiSC) and Nanoscale Simulation Laboratory at National Nanotechnology Center (NANOTEC) for computing resources. References [1] G.A. Lavoie, J.B. Heywood, J.C. Keck, Experimental and Theoretical Study of Nitric Oxide Formation in Internal Combustion Engines, Combustion Science and Technology, 1 (1970) 313-326. [2] L.B. Kreuzer, C.K.N. Patel, Nitric Oxide Air Pollution: Detection by Optoacoustic Spectroscopy, Science, 173 (1971) 45. [3] G.L. Squadrito, W.A. Pryor, Oxidative chemistry of nitric oxide: the roles of superoxide, peroxynitrite, and carbon dioxide, Free Radical Biology and Medicine, 25 (1998) 392-403. [4] M.V. Twigg, Progress and future challenges in controlling automotive exhaust gas emissions, Applied Catalysis B: Environmental, 70 (2007) 2-15. [5] J.O. Lundberg, E. Weitzberg, M.T. Gladwin, The nitrate–nitrite–nitric oxide pathway in physiology and therapeutics, Nature Reviews Drug Discovery, 7 (2008) 156. [6] J.D. Spengler, K. Sexton, Indoor air pollution: a public health perspective, Science, 221 (1983) 9. [7] X. Zhu, C. Zheng, X. Gao, X. Shen, Z. Wang, Z. Luo, K. Cen, Experimental study of NO2 reduction in N2/Ar and O2/Ar mixtures by pulsed corona discharge, Journal of Environmental Sciences, 26 (2014) 2249-2256. [8] H.-H. Kim, Nonthermal Plasma Processing for Air-Pollution Control: A Historical Review, Current Issues, and Future Prospects, Plasma Processes and Polymers, 1 (2004) 91-110.

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Highlight

- Complete catalytic cycles of NO conversion on single Si atom catalyst was investigated - Catalyst is selective to NO and shows resistance to water which is a positive effect. - Catalyst is active for either NO reduction or oxidation in mild condition. - Incorporating of Si and N on graphene gives highly efficient catalyst for NO abatement.

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