Heteroatom-Doped Graphene for Efficient NO Decomposition by Metal

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Functional Nanostructured Materials (including low-D carbon)

Heteroatom-Doped Graphene for Efficient NO Decomposition by Metal-Free Catalysis Yan Wang, Yuesong Shen, Yiwen Zhou, Zhiwei Xue, Zhenyuan Xi, and Shemin Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09503 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Heteroatom-Doped Graphene for Efficient NO Decomposition by Metal-Free Catalysis Yan Wang, Yuesong Shen*, Yiwen Zhou, Zhiwei Xue, Zhenyuan Xi, Shemin Zhu

Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China.

Keywords: Heteroatoms doped graphene; Direct decomposition; Nitric Oxide 1

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ABSTRACT: N, S and B doped graphene was fabricated by thermally treating graphene oxide (GO) with heteroatom-containing precursors and its catalytic behavior for NO decomposition reaction was evaluated. For the first time the feasibility that heteroatoms doped graphene effectively used for decomposing NO was experimentally confirmed. The activity of different heteroatoms doped graphene follows the order: N-doped graphene (NG) > S-doped graphene (SG) > B-doped graphene (BG). The electronegativity difference, specific area and unique functional groups (pyridinic N and thiophene S) of the heteroatoms doped graphene play a crucial role in the catalytic performance. Furthermore, the effect of pyridinic N and thiophene S on the reaction mechanism was proposed. Pyridinic N and thiophene S can transfer extra electrons into π-antibonding orbit of NO, thus weakening N-O bond.

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1. INTRODUCTION

The rise of graphene widely attracts the interest of investigating various chemical reactions, owing to its merits of excellent physical and chemical properties. Graphene can provide an ideal platform for transforming desired reactions into reality. However, the density of states at Fermi level for pristine graphene is relatively low due to its quasiparticles obeying a linear dispersion relation,1,2 thus largely restricting its application in catalysis. Fortunately, the Fermi level can be changed by introducing heteroatoms into the lattice of graphene, which is particularly beneficial for catalysis. It was found that that dopants can influence the spin density as well as the charge distribution of carbon atoms,3-6 leading to weaken the interactions around adjacent carbons so as to induce the activation region.7-9 This type of activated region can contribute excellent performance in various catalytic reactions, like oxygen reduction reactions (ORR),10,11 oxygen evolution reactions (OER)12-14 and selective oxidation reactions.15-17 However, up to now little was known about heteroatoms doped graphene that whether it can be applied in other catalytic systems, like direct decomposition chemistry, how this catalysis will behave towards this field? Thereby, exploring new fields of heteroatoms doped graphene in direct decomposition reaction is of great significance. Recently, some researchers have focused on N-doped graphene as a catalyst for direct decomposition chemistry from the aspects of theory and experiment. On the one hand, Zhang et al. has speculated that N-doped graphene can be applied for catalytic

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decomposition of nitric oxide by theoretical calculations;18 V Cantatore also did some DFT calculations about NO decomposition reaction over boron-doped graphene.19 On the other hand, our group has initially confirmed that N-doped graphene is able to decompose NO in the experiment, while only explored in terms of different nitrogen contents according to previous research.20 What's more, sulphur element exhibits similar geometries and electron configurations with carbon and nitrogen atom,7,21 which obviously raises our concern, to investigate whether S-doped graphene can be potentially used for the reaction of decomposing NO or not. To put it another way, what is the impact that possibly influences the performance for direct decomposition chemistry behind the dopant of different heteroatoms into graphene? As a result, it is highly desirable to systematically investigate the performance of heteroatoms doped graphene in the field of direct catalytic decomposition. Since the emission of excess nitric oxides from fuel combustion has caused severe pollution on the environment and impacted human health,22,23 various technologies for eliminating NO have been investigated, such as catalytic oxidation,24,25 which is involved in firstly converting NO into high valence state NOx with oxidizing agent (such

as

KMnO4)

and

then adsorbing

NOx with

alkaline liquor,

catalytic

reduction26,27(for this technology, NO would be selectively reduced into N2 and water with the aid of reducing agent(like NH3,)) and catalytic decomposition.28,29 Among these methods, oxidizing agent (KMnO4) and reducing agent (like NH3) are easy to cause the environmental secondary pollution, however, catalytic decomposition is

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regarded as a quite promising way because it is eco-friendly and easy to operate. Currently, many research efforts are devoted to exploring metal oxidations with perovskite structure for direct decomposition of NO in terms of industrial application, which can achieve over 80 % above 850 oC, the summary is presented in the Table S1. It is still unknown about behavior that exploring the carbon based catalysis via the reaction of decomposing NO. Herein, the NO decomposition reaction was chosen as a model reaction in this work, to explore the feasibility of N, S, and B doped graphene to be used as efficient catalyst for direct decompositions. In this work, we obtained heteroatoms (N, B, S) doped graphene by mixing graphene oxide (GO) prepared by hummer’s oxidation method30 with various heteroatom-containing precursors. The mixtures were treated at diverse temperatures from 600 oC to 900 oC. The synthesis process is illustrated in Scheme 1.

Scheme 1 the fabrication of heteroatom-doped graphene, including NG, BG and SG.

2. EXPERIMENTAL 2.1 the synthesis of graphene oxide

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In this work, the graphite oxide was firstly obtained by hummer's method. Typically, the concentrated H2SO4 (98%, 110 ml) was added to the mixture of graphite flakes (3.5 g) and NaNO3 (CP, 1.75 g) with constant stirring, and the reaction temperature was kept below 5 °C, followed by addingKMnO4 (CP, 10.5 g) slowly in portions to keep temperature below 20 °C. After addition, the temperature of the reaction rised spontaneously to about 35 °C, and then kept stirring at this temperature for 3 h. Then deionized water (135 mL) was slowly added to the above solution after which time, with reaction temperature rising to 98 °C. Additional deionized water (82.5 mL) and 30 wt% H2O2 (20 mL) were in turn added, the suspension stayed about 12 h and was centrifuged enough to remove the supernatant.

The formation of a sticky

brown dispersion was eventually observed, after rinsing the precipitate with deionized water and diluted water three times for removing metal ions. Next, the graphite oxide dispersion was lyophilised to yield shaggy powders. Graphene Oxide (GO) was obtained from a diluted graphite oxide hydrogel (1 mg / ml) by ultrasonic exfoliation. The mixture was treated with ultrasonication about 1 h at 80 MHz and then centrifuged for 15 min at the speed of 5000 rpm. The supernatant was lyophilised to yield the powders of GO.

2.2 synthesis of NG, BG, SG Heteroatoms (N, B, S) doped graphene was prepared by thermally treating graphene oxide (GO) with containing N, B, S precursors in the flow of Argon.31 Urea, B2O3, C6H5CH2SSCH2C6H5 were used as the precursors of N, B, S, respectively. The

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GO powders were mixed with the heteroatom precursors at the mass ratio of 1:15. These mixtures were heated in the tube furnace to the temperature of 600, 800 and 900 o

C for 30 minutes, respectively, at a rate of 5 oC / min in Argon atmosphere, denoted by

NG, BG and SG, respectively, and then cooled down to 25 oC. In addition, the reduced graphene oxide (RGO) was synthesized at 600 oC for comparison.

2.3 materials characterization X-ray diffraction (XRD ) of samples was carried out on a D/MAX-RB X-ray diffr actometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5406 Å). The range of 2θ scans covered from 5 to 30°, at a scanning speed of 5o/min. The accelerating voltage was 40 KV and the applying current 30 mA. The levels of defect of all the samples were evaluated by Raman spectra, which were measured on a HR800 Raman spectrometer instrument (Horiba Company) by Ar ion laser with an exciting wavelength of 514.5 nm. The micro-structure on the surface of the samples was observed by Scanning

Electron

Microscope (SEM) (JSM-5900, JEOL, Japan) and Transmission Electron Microscope (TEM) with an accelerating voltage of 200 kV (JEM-2010 UHR, JEOL, Japan).

The X-ray photoelectron spectroscopy (XPS) was performed to investigate the electronic structures and element distribution of the surfaces by monochromatic Al-Kα radiation (the AXIS ULTRA DLD instrument). The samples were treated without surface sputtering or etching in order to maintain the degree of vacuum at 10-7 Pa in the XPS equipment, the samples were dried at 100 °C for 24 h after completely eliminating moisture. 7



The specific surface area, average pore diameter and pore volume of samples were obtained by N2 adsorption/desorption isotherms, which were carried on by using a surface area analyzer (Micromeritics, 2020M V3.00H) at 77 K. The thermal stability of samples was explored via Thermo Gravimetric Analyzer (TGA) (Shimadzu TA-50H, Shimin Company, Japan).

The Fourier Transform Infrared Spectroscopy (FTIR) of the samples were measured via the method of KBr pressing plate, using the type of “Tensor-27” infrared spectrometer from Bruker Comany in Germany. The Inductively Coupled Plasma (ICP) tests were performed on the Optima 5300 DV, produced by Perkin Elmer, the sample were diluted by nitrohydrochloric acid.

2.4 Performance Evaluation and Tests The performance tests proceeded in a fixed-bed reactor, as shown in Figure S1 in Supporting Information, 0.2 g sample was tested and 400 ppm NO (in Argon atmosphere (99.999%) was used as feed gas, which was at a flow rate of 333 mL/min. The gas hourly space velocity of was 10000 h-1. The reaction temperature range varied from 600 to 850 °C. The concentrations of nitric oxide, nitrogen dioxide and carbon monoxide were measured by a flue gas analyser (VARIO PLUS, MRU, Germany). The CO2 and N2O content was detected with the aid of gas chromatograph (Agilent 7890A Series), which had a Poropak Q column as well as a thermal conductivity detector. The NO conversion (φ) and N2 selectivity (S) was calculated via the following formula (1) and formula (2): 8


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φ= S=

) ) )

× 100%.......................................................................(1)

) ) ) ) ) ))

× 100%....................................(2)

The concentration of the NO at the inlet, the NO at the outlet, the NO2 at the outlet, and N2O at the outlet was denoted by NO (inlet), NO (outlet), NO2 (outlet) and N2O (outlet), respectively.

3. RESULTS AND DISCUSSION The phases of prepared samples were characterized by XRD, as seen in Figure 1(a). The characteristic diffraction peak of GO emerged at around 11.2o, compared to the peak of pure graphite locating at 26.4o, indicating that the space between the layers of graphite was significantly enlarged during oxidation process, which was due to introducing oxygen containing groups into the graphite as well as inserting H2O between GO layers. The degree of graphitization had been recovered by high temperature treatment. The intensities of diffraction peak for heteroatom-doped graphene were far below than that of graphite (002), despite their peak appeared in close positions, initially suggesting N, B and S atom were introduced into the lattice of graphene successfully. The structural information and level of defects about N, B, S-doped graphene can be further obtained via Raman spectra, as shown in Figure 1(b). The D bands, G bands and 2D bands were observed for NG, BG, SG and RGO, the D bands appeared at around 1355 cm-1, which originated from defects and disorders, the G

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Figure 1 The X-ray diffraction spectra (a) of Graphite, GO, NG, BG, SG and RGO; and Raman spectra (b) of NG, BG, SG and RGO.

bands appeared at around 1592 cm-1, derived from the first-order Raman. The 2D bands appeared at around 2750 cm-1, which are the major characteristic of graphene.32 The heteroatoms doped graphene showed similar ID/IG values (RGO = 1.06, NG = 1.14, BG =1.09, SG =1.12), representing similar concentration of defects. When N, B and S atoms are introduced into the lattice of graphene, the number of defects will be increased. The activities for heteroatom-doped graphene towards decomposing NO were displayed in Figure 2(a). The pristine graphene presented low conversion rate and its activation temperature was higher compared to NG and SG. Notably, the NO conversion increased dramatically when N or S atom was doped into the lattice of graphene. Especially, the efficiency can reach nearly 98 % at 850 oC for NG, which is much better than that of other dopants. Moreover, BG did not behave performance for decomposing nitric oxide according to the evaluation of activity. Pure carbon materials were reported to show low efficiency for removing NO from previous research,33 the 10


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activated center may be relevant to exposed carbon defects as well as the carbon sites that were located at the basal planes, similarly to the RGO. The reason for the better performance towards NO direct decomposition over NG and SG compared to RGO was that more defects would be formed and exposed with the disturbance from foreign N or S atom. Thus indicating the dopant of N or S atom into graphene could significantly influence charge distribution of carbon atoms as a consequence of reducing the conjugation around adjacent carbons. More precisely, the reason that catalysis turned into more remarkable lies in that the doped N atom or S atom activated this region.

Figure 2 a) The NO conversions of all the samples (NG, BG, SG, RGO); b) the N2 selectivity of NG, SG and RGO; c) A comparison of CO concentration for NG, SG and RGO in theory, and that measured from experiment, together with that of without NO. (the test data was listed in Table S1) 11



Additionally, considering small amount of metal can contribute significantly to the catalytic performance, an elemental analysis of the catalysts by the ICP test were performed, as listed in Table S2 in Supporting Information. Truly, we found that Na, K and Mn elements existed in all the samples. There are two reasons for this consequence: 1: the raw material contained very small amount of metal elements; 2: even though washing the samples using deionized water, acid and alcohol three times, the residual metal elements still inevitably existed on the surface of the samples with a small amount. During the process of synthesizing GO, the residual metal elements have already existed because of the use of NaNO3, KMnO4. Notably, the content of Na, K and Mn has a little change among RGO, NG and SG. Moreover, the NO conversion of RGO, NG and SG did not change proportionally with the variation of the content, indicating the impact of residual Na, K and Mn elements have no effect on the performance of direct NO decomposition. The introduction of N or S atoms leads to enhance performance. Also, the electronegativity order is: nitrogen (3.04) > sulfur (2.58) > carbon (2.55) > boron (2.04). The difference of electronegativity between N element and C element is 0.49, which is much bigger than the difference of electronegativity between S element and C element (0.03), suggesting the reason why NG exhibited better NO conversion than SG. The N2 selectivity as well as the release of CO of NG, RGO and SG was detected as shown in Figure 2b and Figure 2c, respectively. Both NG and SG showed obviously high N2 selectivity from 600 to 850 oC. Notably, the N2 selectivity of NG can reach nearly 100% at 850 oC, far more than that of other samples (RGO and SG).

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RGO’s N2 selectivity was at a relatively low level over whole temperature range. The doping of N or S atom into graphene can significantly enhance the selectivity of N2. It can be seen that NG yielded more CO than SG from Figure 2c. However, the concentration of CO produced from NG and SG were quite lower than theoretical value (if the pure redox reaction only existed in this process: C+NO=CO+½N2, the content of CO would be fairly approximate to the consumption of NO), illustrating the presence of catalysis during reaction. In addition, it was also confirmed that side reaction was not mainly responsible for NO conversion, thus further proving NG and SG acted as catalysts for decomposing NO. Besides, blank experiment without NO has also been performed to confirm whether oxygen was released from NG and SG or not. The oxygen concentration can be measured by detecting the concentration of CO. When NG and SG were heated at such high temperatures in Ar atmosphere (without NO), just a small amount of CO was observed according to the Figure 2c, originated from the oxygen-containing functional group of the material, which is less than the evolution of CO for NG and SG in the experiment with NO, indicating part of carbon could react with NO and form CO. Remarkably, the CO concentration rises up with the increase of NO conversion, which suggested the Eley-Rideal mechanism was not main process during this reaction. In addition, it is widely believed that catalytic reduction of NO by CO (NO + CO = 1/2N2 + CO2) might have a significant influence in this process. Hence, the evolution of CO2 during the reaction was also performed, as present in Figure S2 in the Supporting Information. The concentration of CO2 did not show dramatic increase

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with the dopant of N and S elements, suggesting that the side reaction does exist, but there was still a decomposition process, which transformed NO to N2 and O2 during reaction over NG and SG. To gain more information for NG and SG, firstly, the morphology and surface structure of NG and SG were investigated via using SEM and TEM, as presented in Figure 3. SEM pictures displayed that thin layers, curved overlaps and folds were existed in the NG and SG, which was demonstrated by higher magnification TEM images, further showing flake-like and crumpled structures. The structure of a porous network was favorable for adsorbing NO. Besides, the process of thermal treatment can affect the micro-structure of flaky graphene, particularly for the appearance of more defects on the edges, accordingly seen as very rich activated regions.

Figure 3 the SEM images and TEM images of NG and SG under different magnifications. a), e) represent the SEM images of NG and SG, respectively; b), c), d) represent the TEM images of NG; 14


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f), g), h) represent the TEM images of SG, respectively

The porous structures and properties of RGO, NG and SG were confirmed further in the N2 adsorption-desorption isotherms as well as pore size distribution, as seen in Figure 4 and Figure S3, respectively. The N2 adsorption-desorption isotherm of NG and RGO is type Ⅳ with the hysteresis loop, which is classified as type H3, suggesting the presence of mesopores in the NG. This is in accordance with the pore size distribution (the most probable distribution of NG and RGO is nearly 4nm and 3nm, respectively). These mesopores are supposed to accelerate the diffusion of NO molecule. However, the N2 adsorption-desorption isotherm is not a standard type Ⅳ for SG, notably, the pore size distribution of SG is distributed from 2.4 to 60 nm, which showed there are mesopores and macropores existing in SG. Also, the specific surface area of RGO, NG and SG is 385.20 m2 g-1, 315.47 m2 g-1 and 30.34 m2 g-1, respectively. It's worth noting that the specific area of RGO is larger than NG and SG, whereas its efficiency of decomposing NO is much lower than that of NG and SG. This illustrates the electronic effect caused by dopant played a major role in promoting the activity. The effective specific surface area may be promoted via the dopant, leading to the formation of effective activated centers. The high specific surface area and more porosity can offer larger amount of exposed active sites, which is favorable for the distribution of reactant so as to improve the performance of NO decomposition reaction.

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Figure 4 the N2 adsorption-desorption isotherm of RGO, NG and SG.

The chemical status of N and S elements located at the surfaces were investigated so as to explore the nature of doping sites with the aid of XPS, as shown in Figure 5. The XPS survey spectrum of NG and SG was shown in Figure S4 in the Supporting Information. The analysis showed N and S atoms were doped into the lattice of graphene successfully which was consistent with the results of XRD and Raman patterns. The N 1s peak of NG can be divided into three main nitrogen groups: pyridinic N (around 398.2 eV, 47.6 % ), pyrrolic N (around 399.9 eV, 24.7 % ), and graphitic N (around 401.2 eV, 27.7 %).34,35 The S 2P peak of SG can be divided into two main sulfur groups: thiophene S 2P3/2 (around 164.1 eV, 50.8 % ), thiophene S 2P1/2 (around 165.3 eV, 49.2 % ), which have been founded in other literatures.36 The number of pyridinic N and thiophene S was more than other species. Notably, an unpaired electron is existed in 16


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Figure 5 the XPS spectra of N 1S and S 2P of NG and SG with C 1s, respectively; (a), (b), (c), (d)

represent the XPS spectra of N 1s (NG), C 1S (NG), S 2P (SG), C 1s (SG), respectively.

pyridinic N’ orbit compared to pyrrolic N and graphitic N, indicating that pyridinic N could react with NO molecule as a “Lewis base”. Similarly, the base of SG was derived from an unpaired electron, which existed in the orbit of thiophene S. There is the structure of a half-filled π-antibonding orbit of N-O bond, which is inclined to accept an electron easily. According to previous analysis of RGO, its low efficiency for NO direct decomposition originated from the fact that the exposed defective carbon sites as well as the carbon sites which are located at the basal planes. When N or S atom was incorporated into the structure of graphene, the conversion was enhanced significantly, especially for NG, which can reach up to nearly 98 % at 850 oC. The electronegativity difference and special surface area influenced the efficiency of NG and SG for direct

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decomposition reaction. The surface analysis of catalysts after reaction was also performed by XPS. The XPS data of C1s and O1s for NG and SG after reaction, denoted by NG-AR and SG-AR, have been shown in Figure 6 and Figure S5, respectively. The surface atom concentrations of C, O, N and S for NG and SG before and after reaction were summarized in Table 1. It was found that the content of N, O and S elements on the surface of NG and SG decreased after the reaction. The reason could be that residual N species and S species derived from preparing process further escaped from the catalysts surface during high-temperature treatment, which caused the relative content of carbon to increase simultaneously. The percentages of three N species and two thiophene species were also listed in Table1. Notably, the content of graphitic N increased from 2.28% to 2.73% during reaction, which indicated that a high degree of

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Figure 6 the XPS spectra of N 1S and S 2P of NG-AR and SG-AR with C 1s, respectively; (a), (b), (c), (d) represent the XPS spectra of N 1s (NG-AR), C 1S (NG-AR), S 2P (SG-AR), C 1s (SG-AR), respectively.

graphitization occurred due to high temperature treatment. The types of C-O bond, C-N bond, and C-S bond stayed unchanged during reaction, but the peak area of C-S bond and C-N bond showed a slight decrease according to the XPS data, indicating there are mass losses for catalysts during the reaction. It is generally known that two peaks could be differentiated from the XPS data of O 1s: the peak of lattice oxygen O2- (OL) appeared at around 529.8 eV, the peak appeared at around 531.5 eV belonged to surface oxygen (OS). The ratio of OS / (OL + OS) for NG and SG was calculated respectively and listed in Table 1. Notably, the ratios showed an obvious decline during the reaction for both NG and SG, indicating high temperature mainly cause the surface of oxygen to escape from the catalyst. Table 1 the Quantitative analysis of XPS data of C1s and O1s for NG and SG before and after reaction Elements Content (%) Sample

pyrrolic graphitic thiophene thiophene N N S 2P3/2 S 2P1/2

C

O

N

S

OS / (OL+OS)

pyridinic N

NG

84.02

7.76

8.22

-

38.96

3.91

2.03

2.28

-

-

NG-AR

86.68

5.28

8.04

-

35.31

3.70

1.61

2.73

-

-

SG

88.69

8.87

-

2.44

40.82

-

-

-

1.24

1.20

SG-AR

90.80

6.94

-

2.26

37.12

-

-

-

1.08

1.18

19



Figure 7 spectrum of NG and SG before and after reaction

To investigate the changes of functional groups, the Fourier Transform Infrared Spectroscopy (FTIR) of of NG and SG before and after reaction was presented, as seen in Figure 7. The peak appeared at 950, 1080, 1120, 1560 and 2300 cm-1 was attributed to C-S, N-O, C-O, C-C and C-N, respectively, which was characteristic functional group of SG, NG. The peak of N-O for NG-AF, RGO-AF became noticeable compared to NG and RGO, illustrating the existence of intermediate species of NOads. In addition, the peak of C-N bond of NG-AF significantly appeared compared with that of NG, which further confirmed the N species were adsorbed on the surface of NG during reaction. The location of C-N, C-S and C-C peaked stayed unchanged. It has been reported that the lewis base of carbon materials will be enhanced when nitrogen atom was introduced,37 thus promoting the adsorption and activation of NO molecule, considering there is an unpaired π-antibonding orbit in NO molecule, which is also polar. As for C-N-C and C-S-C groups, the center of positive and negative

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charges doesn’t coincide. The NO molecule can interact with C-N-C or C-S-C group easily. Notably, not all adsorption behaviors are efficient for NO molecule, commonly at a parallel manner. Also, NO molecules tend to form (NO)2 dimmer at the surface of carbon material, which is reported in previous researches. When the NO molecule diffused on the surface of NG or SG and in case of being adsorbed at a parallel manner, the electron would easily be captured from NG or SG to NO. Oxygen from the structure of N-O bond which has been adsorbed on the surface of NG or SG can be regarded as the Lewis acid, which could act in transferring electron density from the surface to π-antibonding orbit in N-O bond. This orbit was well-known as the key point of decomposing NO. When foreign electron was transferred into the π-antibonding orbit, the consequence is to make N-O bond elongated, accordingly decreasing N-O bond order so as to weaken the stability of NO molecular. The density functional theory (DFT) calculations were used to further investigate the effect of dopant for enhancing NO direct decomposition performance.18, 37 The break of N-O bond almost originates the

Figure 8 the possible reaction mechanism of NG and SG for NO direct decomposition. 21



addition of foreign electron into π-antibonding orbit. Like in many other catalytic systems, this creates a chance for further reaction at high temperature. The formation of Nads and Oads would further form N2 and O2, respectively. The possible mechanism of this reaction is seen in the Figure 8. NO+[S] → NOa-[S]…………………………………………………………(3) NOa-[S]→ Na+ Oa…………………………………………………………...(4) NOa+ Na→N2O…………………………………………………………. …(5) NOa + Na→N2 + Oa………………………………………………….. …….(6) Oa +Oa→O2………………………………………………….. ……………..(7) S represents the activated center on the surface of NG. Since NG exhibited best conversion in direct decomposition of NO, we choose NG as the object of kinetic studies. Considered that the activity of NG increased with temperatures, we assumed the determine step of controlling rate for decomposing NO on the surface of NG is that NO molecule was adsorbed at the active center, which can be regarded as first-order reaction, as shown in formula (3). From what have been discussed above, the Eley-Rideal mechanism was not the main process for decomposing NO. Hence, the rate equation could be acquired on the basis of “Langmuir- Hinshelwood” model: r = k .38,39

r=

∗ !" #/

…………………………………………………………………(8)

The derivation is as follows

ln =lnP0 -

∗ !" #/

……………………………………………………… (9)

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PNO = P0 (1-λ) …………………………………………………………………(10) -ln(1-λ)=

∗% !" #/

……………………………………………………………(11)

(τ=w/(qv*ρ)), τ is the time of flowing through the reactor for feed gas, w is the mass of the catalyst, qv is the flow of feed gas, ρ is the suspension density. PNO is the concentration of

NO at the outlet, PO2 is the concentration of O2 at the outlet after proper simplification. P0 is the concentration of NO at the inlet. λ is the conversion of NO. Although direct decomposition of NO reaction is founded to have oxygen inhibition problem and the factor of O2 needed to be considered in other catalytic systems, the content of O2 in this work is relatively low as well as much of them would reacted with carbon to form CO(CO2), so PO2 could be neglected: -ln(1-λ)= & ∗ '…………………………………………………………………..(12) The value of -ln(1-λ) could be obtained by changing the value of τ, the fitting curve is shown in the Figure S6. The curve fitted well with the data, thus suggesting determine step is that NO molecule was adsorbed at the active center, and is first order reaction. The successive cycles of direct decomposition of NO over NG and SG were also performed, as seen in Figure 9 (a). Given the data from previous performance tests, we choose the optimum condition at 850 oC as comparison for studying successive cycles. The reaction was totally eight times. NG still had 90% NO conversion after five cycles. Whereas, reusing the NG further, the conversion decreased from 90.1% to 61.64 %. It was also observed that SG showed similar trend, the NO conversion gradually reduced from 49.8 %.down to 26.37% during 8 cycles. This could be due to the fact that

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Figure 9 the successive cycles (a) and the relationship between NO conversion and reaction time for NG and SG (b)

catalysts are easily oxidized, thus weakening the performances. The relationship between NO conversion and reaction time for NG and SG were also measured as presented in Figure 9 (b). The NO conversion of NG decreased slightly in 3 h, from 97.88% to 80.64%. The decline of performance for NG was about 20 % in the same period. Honestly, from what have been discussed above, oxidation of graphene would influence its long-term stability. The following work will be focused on how to improve this stability via decreasing reaction temperature by dopants and finding materials that are more reactive with oxygen than graphene. The mass balances for NG and SG were measured by TG (in N2 atmosphere) from 45 oC to 850 oC, as seen from Figure S7. Both of NG and SG initially keep stable until the temperature reaches 520 oC. There are slightly mass losses from 520 oC to 700 oC for both materials. However, NG showed an obvious drop to 70.81 %, with the temperature going up from 700 to 850 oC. The TG curve of SG showed slow decline

24


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about 10 % from 88.06 % at 700 oC to 77.50 % at 850 oC, which indicates SG was more stable than NG. High temperature leads to mass loss for NG and SG, on the one hand, some organic-functional groups of catalysts introduced during preparation could easily be decomposed at high temperatures. On the other hand, at such temperature part of carbon would react with O2, which originates from decomposition of NO and oxygen-containing functional groups located at the surface of NG and SG, like epoxy, hydroxyl, thiol. All of these above can contribute to the mass loss for the catalysts.

4. CONCLUSIONS In summary, we have confirmed heteroatom-doped graphene can decompose NO efficiently. The efficiency decreased in the order of NG > SG > BG, which is resulted from their electronegativity difference, functional groups and different specific area. The electronegativity difference set the basis of interaction between dopant sites with NO molecule. The unique functional groups (pyridinic N and thiophene S) of surfaces provided extra electron in outer orbit to boost this catalysis. The specific area influenced the dispersion of activated sites. This research can expand the application of graphene in catalytic decomposition field and offer a new route for eliminating NO. Also, we choose NO as a medium in this research, which might be an example for removing other gas molecules over heteroatom-doped graphene like CO2, CO, etc. Following researches can be systematically concentrated on how to further improve the durability of this catalyst under O2 atmosphere.

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ASSOCIATED CONTENT Supporting Information The activity evaluation system, the concentration of CO2 in outlet gas, the pore size distribution of RGO, NG and SG, the XPS survey spectrum of NG and SG, the XPS spectra of O 1S for NG and SG before and after reaction, Fitting curve of NG at 850 oC; the TG curves of NG and SG under N2 atmosphere, the summary of the direct NO decomposition conversion of current catalysts, the ICP test of Na, K, and Mn of samples (RGO, SG, and NG), the mass balance of N element during the reaction.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Present Addresses No.5 Xinmofan Road, Nanjing Tech University, College of Materials Science and Engineering, 210009, Nanjing, China

Notes The authors declare no competing financial interest

Acknowledgements： This work was supported by the National Key Research and Development Program of China (No. 2016YFC0205500), the National Natural Science Foundation of China 26


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(No. 51272105).

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Table of Contents

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Heteroatom-Doped Graphene for Efficient NO Decomposition by Metal

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