Exfoliation of graphitic carbon nitride for enhanced oxidative

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Exfoliation of graphitic carbon nitride for enhanced oxidative desulfurization: A facile and general strategy Ganchang Lei, Yanning Cao, Wentao Zhao, Zhaojin Dai, Lijuan Shen, Yihong Xiao, and Lilong Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05553 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Sustainable Chemistry & Engineering

Exfoliation of graphitic carbon nitride for enhanced oxidative desulfurization: A facile and general strategy Ganchang Lei, Yanning Cao, Wentao Zhao, Zhaojin Dai, Lijuan Shen*, Yihong Xiao, and Lilong Jiang* National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Gongye Road, Gulou district, Fuzhou, Fujian 350002, P.R.China. *Corresponding Authors. E-mail: [email protected] E-mail: [email protected]

ABSTRACT A series of graphitic carbon nitride (CN) in the form of nanosheets with porous structure have been prepared through thermal treatment of bulk CN in air. Compared with the bulk counterpart, the asgenerated holey CN nanosheets are larger in specific surface area. Endowed with higher amount of active sites and enhanced mass transport ability, the latter display catalytic performance substantially superior to the former, exhibiting higher H2S conversion and S selectivity in the oxidation of H2S to S. Moreover, the CN nanosheets show much better durability than traditional catalysts. It is envisaged that the strategy is a general technique that can be extended to produce

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porous CN nanosheets from other nitrogen-rich precursors, as well as to prepare other 2D carbonbased materials for potential applications.

KEYWORDS: Graphitic carbon nitride, Thermal treatment, Nanosheets, Hydrogen sulfide, H2S selective oxidation

INTRODUCTION H2S is corrosive, malodorous and highly toxic. Its removal is required in industrial processes as a result of pollution control and human health concern.1,2 Currently, the widely used technology for H2S elimination is a multi-step Claus process.3 However, due to the thermodynamic limitations, there is around 4% H2S left after undergoing the process.4–6 With the increasingly stringent regulations on the emission of sulfur compounds, various desulfurization processes were developed. Among them, the technique of selective oxidation of H2S to sulfur stands out due to its environment friendliness and low capital cost. Most importantly, this method has the thermodynamic advantage of having H2S completely converted to sulfur even at relatively lower temperatures. It is anticipated that the strategy has great potential in the treatment of H2Scontaining gases.7–9 A large number of catalysts based on metal oxides (Fe2O3, TiO2, V2O5, and many others) have been studied for selective oxidation of H2S.10–16 Nevertheless, in spite of the efforts, certain key features related to sulfur selectivity and catalytic stability have not been definitely addressed. For example, because excess oxygen is usually required for the function of Fe2O3-based catalysts, the catalysts show poor sulfur selectivity. Furthermore, the chemical bonding between the p-band center of sulfur and d-band center of iron could induce the formation of Fe–S bonds, thus resulting in deactivation of catalyst.17–19 The performance of the TiO2-based catalysts are not satisfactory because they are easily poisoned in the existence of water. As for V2O5, its use is impeded by the


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toxic nature of vanadium compounds.20–23 How to tackle these issues has puzzled chemists, and thus stimulates us to explore the use of metal-free catalysts for this reaction. Recently, carbon material have been studied as a metal-free catalyst for oxidation of H2S owing to positive features including high pore volume, large specific surface area and rich surface chemistry.24,25 To increase catalytic activity, strategies were developed for the doping of N into the carbon-based materials. The purpose is to modulate surface properties and increase the amount of active sites.26–31 Nevertheless, the amount of nitrogen introduced into carbon framework using conventional methods is far too low for satisfactory enhancement of catalytic activity. Moreover, the embedded nitrogen species are too localized and subject to leaching, leading to inevitable deactivation of the nitrogen-doped carbon catalysts. Therefore, the search of stable nitrogen-rich carbon materials with delocalized nitrogen has come into the picture. In this respect, the use of graphitic carbon nitride (CN) is appealing. This N-enriched carbon material is composed of tri-s-triazine units in which the carbon and nitrogen atoms are in stable conjugating sp2 configuration, providing a system with unique 2D layer structure.32–34 This class of materials can be easily prepared through simple heat treatment of precursors such as thiourea, urea, melamine and cyanamide.35–39 Because of the high nitrogen content (~60%) and strong covalent bonds between nitrogen and carbon atoms, CN is chemically stable while highly functional, making it promising for a variety of catalytic processes.40–42 Very recently, we reported in a communication that CN could function as a catalyst for oxidative desulfurization.43 At this infant stage of investigation, it is realized that the use of CN for catalytic oxidation of H2S to sulfur is limited by low specific surface area and small number of active sites. For rational design of effective CN catalysts, these major obstacles have to be overcome. In present work, we adopted a simple thermal approach to exfoliate bulk g-C3N4 for the


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generation of porous CN nanosheets. Bulk g-C3N4 which was prepared using urea as precursor (herein denoted as Bulk-CNU) was thermally treated at a selected temperature (T) in air (Scheme 1). It is envisaged that the hydrogen-bonded cohered strands of polymeric melon units in the CN layers react with oxygen in the thermal process, gasify and escape from the bulk structure. As a result, bulk-CNU is exfoliated into thin sheets while the released gases act as soft template for pore formation.44–46 Because of these unique features, the porous nanosheets (denoted herein as CNUpT) are endowed with active sites and mass transport efficiency, which are beneficial factors for the promotion of catalytic reaction. Furthermore, it has been disclosed that the strategy is versatile because similar results can be achieved over precursors such as thiourea, dicyandiamide and melamine.

Scheme 1 Illustration of the processes for the generation of porous CNU-pT through thermal treatment.

EXPERIMENTAL SECTION Materials and Reagents Urea (≥99.0%), thiourea (≥99.0%), dicyandiamide (≥98.0%), melamine (≥99.0%), graphitic carbon were obtained from Sinopharm Chemical Reagent (Shanghai, China). Commercial Fe2O3 was obtained from Fuchen (Tianjin, China). Deionized water was obtained from local sources. All materials were used throughout the experiments without further treatment. Synthesis of samples Bulk-CNU. Bulk-CNU was synthesized by urea pyrolysis according to a method described


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elsewhere.47 Briefly 10 g of urea was heated (5 °C/min) to 550 °C and calcined at 550 °C in air for 2 h. The obtained product is denoted as Bulk-CNU. CNU-pT. CNU-pT was synthesized as follows: 1 g of bulk-CNU was milled to powder and then heated (5 °C/min) to a designated temperature (T, i.e., 350, 450, 500, 550 or 600 °C) and calcined (in air) at the designated temperature for 2 h. The resulted products are herein labeled as CNUp350, CNU-p450, CNU-p500, CNU-p550, and CNU-p600, respectively. CNU-600. For comparison purpose, CNU-600 was synthesized by a one-step polycondensation process. Briefly, 10 g of urea was heated (5 °C/min) to 600 °C and calcined (in air) at this temperature for 2 h. The obtained product is herein labeled as CNU-600. CNT-p600, CND-p600, and CNM-p600. To demonstrate the versatility of the strategy, the approach was applied for the production of porous CN nanosheets using nitrigen-rich thiourea, dicyandiamide and melamine as precursors. The CN materials produced after thermal treatment at 600 °C is denoted herein as CNT-p600, CND-p600, and CNM-p600, respectively. Characterization Scanning emission microscopy (SEM) investigations were conducted using a JSM-6700F Microscope. Transmission electron microscopic (TEM) studies were carried out over a JEM 2010 EX microscope operating at 200 kV. AFM image was acquired on the Nano Scope IV instrument. The X-ray diffraction (XRD) analysis was carried out over a PANalytical diffractometer using CuKα radiation operating at 40 mA and 45 kV. Fourier transformed infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer with the samples prepared in the form of KBr pellets. Solid-state 13C NMR data was recorded using a Bruker Advance III HD 500 spectrometer. The Raman spectra were conducted over a Renishaw in Via Raman microscope using a 325 nm laser for excitation. UV-Vis DRS analysis was performed in air against BaSO4 on a Lambda 950


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instrument in a wavelength span of 200–800 nm. Nitrogen adsorption-desorption isotherms were acquired on a Micromeritics ASAP 2020 apparatus. The catalyst was degassed at 453 K for 240 min before N2 adsorption at 77 K. X-ray photoelectron spectroscopy (XPS) characterization was acquired on a Thermo ESCALAB 250 spectrometer, equipped with a monochromatic Al Kα source and a charge neutralizer. Results of elemental analysis were collected using a Vario EL instrument. CO2-TPD was carried out on an AutoChem 2920 equipment using a thermal conductivity detector (TCD). First, 100 mg of a catalyst (40–60 mesh) was pre-heated in a flow of pure helium (30 mL/min) at 180 °C for 60 min. After cooling to 50 °C, the catalyst was exposed to high purity CO2 for 60 min. The reactor was then purged with pure helium (30 mL/min) for 60 min. Finally, desorption profile was acquired with the catalyst heated to 350 °C (10 °C/min) under a helium flow, while CO2 desorption was monitored. Catalyst evaluation The performances of catalysts were evaluated in a fixed-bed reactor with 0.2 g of catalyst (40–60 mesh) secured in a quartz reactor. Schematic of the reactor is illustrated in Figure. S1. A gas mixture containing 0.25% of O2, 0.5% of H2S and balance N2 was fed into the reactor at a WHSV of 3000 mL·g−1·h−1 (10 mL/min). The reaction temperature was set at 90–240 °C. The effluent gases were analyzed by a gas chromatograph (GC9720) equipped with a TCD. A condenser was installed next to the reactor for the capture of elemental S. H2S conversion, S selectivity and S yield were calculated as follows: H2S conversion =

S selectivity =

[H2S]in - [H2S]out [H2S]in

[H2S]in - [H2S]out - [SO2]out [H2S]in -[H2S]out

(1)

(2)


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S yield = [H2S conversion]  [S selectivity]

(3)

RESULTS AND DISCUSSION The morphologies of the obtained samples were studied using SEM, TEM, and AFM techniques. As displayed in Figure 1a, the structure of bulk-CNU is thick with densely stacked layers. The CNU-p350 displays a structure of stack layers (Figure 1b), and the lateral size is smaller than that of bulk-CNU. With increase of thermal treatment temperature, the CNU-pT samples progressively become smaller in size, displaying a form of fluffy nanosheet agglomerates rather than stack layers (Figure 1c–e). At a treatment temperature of 600 °C, the generated CNU-p600 appears as nanosheets with wrinkles and irregular pores (Figure 1f). The TEM images of Figures 2a and 2b further confirm the nanosheet features of CNU-p600. To measure the thickness of the nanosheets, AFM analysis was conducted. The AFM image of CNU-p600 reveals a 2D sheet with a rough surface (Figures 2c and 2d), and the average sheet thickness is approximately 1.0 nm, which stands for around 3–4 atomic layers of CN sheet. On the basis of these results, it is found that the thermal treatment results in exfoliation of bulk-CNU, and the end product is porous nanosheets. The holey CN nanosheets is favorable for catalytic application owing to the large specific surface area and high exposure of surface active sites.48–50


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Figure 1. SEM images of (a) bulk-CNU, (b) CNU-p350, (c) CNU-p450, (d) CNU-p500, (e) CNU-p550, and (f) CNU-p600.

Figure 2. (a, b) TEM images, (c) AFM image, and (d) height scanning of CNU-p600.


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Figure 3a exhibits XRD patterns of bulk-CNU and CNU-pT. All catalysts show two characteristic 2θ peaks at 12.7° and 27.5°, which are attributable to (100) and (002) crystal planes, representing the in-plane packing motif and the interfacial stacking of layers, respectively. Besides, both diffraction peaks gradually broaden and weaken with the increase of treatment temperature, reflecting the intensification of structure distortion and reduction of layer thickness,51 consistent with the results of SEM and AFM analyses. FT-IR spectra of bulk-CNU and CNU-pT samples in Figure 3b display similar absorption bands at 802, 1240–1680 and 3000–3400 cm−1, which are ascribable to the heptazine ring modes, CN heterocycles and NH/NH2 groups, respectively,52 suggesting that the pristine framework of tris-triazine structure is retained in spite of the heat treatment. This is also evidenced by the solidstate 13C-NMR results as presented in the Supplementary Information (Figure S2). Raman spectra of the obtained samples are displayed in Figure 3c. The band at 1200–1700 cm−1 is assignable to the stretching vibration of heterocyclic triazine units, while those at 980 and 754 cm−1 to breathing vibration of heterocyclic heptazine/s-triazine and C–N–C bending vibration, respectively. It is found that the intensity ratio of D and G band (ID/IG) at 1405 and 1580 cm−1, which reﬂects distortion and defects in forms of Csp2, increases with increase of treatment temperature (Figure S3). The phenomenon validates the distorted structure of the CNU-pT samples,53 in agreement with the XRD results. The UV-Vis DRS spectra presented in Figure 3d show that there is progressive blue shift of absorption edge of samples with the rise of treatment temperature. This is tentatively ascribed to the decrease of conjugation length and the resulted quantum confinement effect, which are intrinsic of porous CN nanosheets.44


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Figure 3. (a) XRD patterns, (b) FT-IR, (c) Raman, and (d) DRS spectra of the bulk-CNU and CNU-pT samples.

As depicted in Figure 4, the nitrogen adsorption/desorption isotherms of CNU-pT samples can be ascribed to type IV with a narrow H3 hysteresis loop, confirming the existence of the mesoporous properties.54 The SBET of these materials are displayed in Table 1. It is noted that increasing the temperature of thermal treatment is conducive to the formation of porous structures, and thus enlarging the specific surface area of CNU-pT, which are much larger than that of bulkCNU. Particularly, the SBET of CNU-p600 (156.0 m2·g−1) is about 3.8 times that of bulk-CNU (40.5 m2·g−1). In addition, the pore-size distribution of CNU-pT samples in the range of pore diameters from 2.5 to 6.8 nm indicates the presence of micropores, evidencing the porous nature of the CN nanosheets. It is apparent that the treatment temperature has a remarkable influence on the texture of CN nanosheets.


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Figure 4. (a) N2-sorption isotherms, (b) the corresponding BJH pore-size distribution of the bulkCNU and CNU-pT catalysts. Table 1. Elemental analysis and physical properties of Bulk-CNU and CNU-pT samples Sample

C (wt%)

N (wt%)

O (wt%)

C/N ratio

SBET (m2/g)

Bulk-CNU

34.5

62.2

＜0.50

0.65

40.5

CNU-p350

35.1

63.1

＜0.50

0.65

85.9

CNU-p450

34.9

62.2

＜0.50

0.65

113.3

CNU-p500

34.9

62.7

＜0.50

0.65

134.9

CNU-p550

34.9

62.6

＜0.50

0.65

136.9

CNU-p600

34.8

62.1

＜0.50

0.65

156.0

CNU-p600-Used

34.3

61.3

＜0.50

0.65

—

The compositions and elemental valence of the catalysts were analyzed by XPS. As depicted in Figure 5, the C 1s spectra display two peaks at 284.6 and 288.0 eV, which are attributable to graphitic carbon and N–C=N, respectively.55 The N 1s profile shows peaks at 398.5, 399.5, and 400.7 eV, which are assignable to N(sp2) in tri-s-triazine units, N–(C)3, and C–N–H, respectively.56–58 According to the XPS data, when the etching temperature is above 550 °C, the graphitic C in carbon nitride increases accordingly, while the total content of nitrogen decreases


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(Table S1). Moreover, the C/N ratios of bulk-CNU and CNU-pT samples remain close to 0.65 (Table 1), demonstrating the structure of tri-s-triazine units is basically intact despite the thermal treatments. The elemental analysis results reveal that the total oxygen content of bulk-CNU and CNU-pT samples are similar, which is less than 0.50wt%. Because there is no significant change of total oxygen content, the thermal process does not introduce impurity oxygen in the CNU structure. In addition, the yield of product decreases with an increase of thermal temperature (Table S1).

Figure 5. XPS spectra of bulk-CNU and CNU-pT samples. (a) C 1s, and (b) N 1s.

Catalytic performance of bulk-CNU and CNU-pT The catalytic activity of prepared catalysts in oxidation of H2S were tested at a WHSV of 3000 mL·g−1·h−1 within a temperature range of 90–240 °C. As depicted in Figure 6a, increasing


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the reaction temperature leads to improved H2S conversion for all samples. Moreover, the catalytic performances of CNU-pT catalysts exhibit a growth trend versus the rise of thermal treatment temperature. Maximum H2S conversion (~82.2%) is achieved over CNU-p600 at 240 °C, which is 3.1 times that of bulk-CNU (26.8%). It is obvious that within the temperature range of 350–600 °C, a higher treatment temperature is beneficial for the generation of CNU-pT catalysts for higher conversion of H2S in the oxidation reaction. Figure 6b shows the relationship between reaction temperature and S selectivity. It is found that below 180 °C for all catalysts, sulfur selectivity is constantly close to 100%. The sulfur selectivity of catalysts gradually decreases with the increases of reaction temperature. It is because at high enough temperatures, there are side reactions including oxidation of sulfur (S + O2 → SO2) and deep oxidation of H2S (H2S +3/2 O2 → SO2 + H2O). Nevertheless, sulfur selectivity is still above 85.5% for all catalysts at 240 °C. Figure 6c shows the effect of the reaction temperature on S yield. Below 180 °C, the S yield of all catalysts increases with increasing temperature. The maximum sulfur yield over CNU-p600 reaches 74.5% at 180 °C, which is much higher than that of bulk-CNU (19.3%). Moreover, the sulfur yield of CNU-p600 is higher than that of N-GC under same reaction conditions (Figure S4). For comparison, CNU-600 was synthesized through a one-step polycondensation process without undergoing the thermal treatment. The catalytic performance of bulk-CNU, CNU-p600 and CNU-600 were evaluated under identical reaction conditions. As displayed in Figure S5a, H2S conversion shows a decreasing order of CNU-p600 > CNU-600 > bulk-CNU. The maximum H2S conversion of CNU-p600 (82.2%) is 1.6 times that of CNU-600 (51.7%). Figure S5b exhibits the sulfur selectivity over the catalysts at different temperatures. Obviously, from 90 to 180 °C, S selectivity is close to 100% for the three catalysts. However, further increase of temperature results


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in gradual decrease of S selectivity. Nevertheless, S selectivity of CNU-p600 is obviously higher than that of CNU-600 in the 180–240 °C range. As depicted in Figure 6d, when the temperature is below 180 °C, the sulfur yield of the three catalysts increases with increasing reaction temperature, a trend similar to that of H2S conversion. Besides, the sulfur yield of CNU-p600 (74.5%) is more than twice that of CNU-600 (29.8%) at 180 °C. These results confirm that the adopted heat treatment is an effective strategy to enhance the catalytic performance of CN for H2S selective oxidation.

Figure 6. Influence of reaction temperature on (a) H2S conversion, (b) sulfur selectivity, (c) sulfur yield for bulkCNU and CNU-pT samples; (d) sulfur yield for bulk-CNU, CNU-600, and CNU-p600. Reaction conditions: catalyst (0.2 g), N2/H2S/O2 = 99.25/0.5/0.25 (wt%), gas flow rate (10 mL·min−1), WHSV (3000 mL·g−1·h−1).

Effect of H2S/O2 ratio The influence of H2S/O2 molar ratio on H2S oxidation has been studied by keeping H2S concentration constant at 5000 ppm while the H2S/O2 ratio varied from 3:1 to 1:2. In this experiment, the reaction temperature was set at 180 °C. As illustrated in Figure S6, the sulfur selectivity is 100% at H2S/O2 ratio of 3:1 and 2:1 but decreases with further rise of O2


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concentration. Nonetheless, the sulfur selectivity remains above 84% throughout. Meanwhile, the H2S conversion increases with increasing O2 concentration. It can be found that the CNU-p600 catalyst has the highest catalytic activity at stoichiometric H2S/O2 proportion of 2:1. The results suggest that despite excess oxygen is beneficial to enhancement of H2S conversion, there is the occurrence of side reactions such as oxidation of sulfur and deep oxidation of H2S. In addition, we compared the reaction condition with those of previous reports. It is found that a low temperature is favorable for efficient H2S oxidation in the presence of water.29,59 This may be caused by the formation of a water film on the catalyst surface. In such a case, H2S molecules readily absorb in the water film and dissociate into HS− ions. However, a higher temperature would destroy the water films, resulting in reduced activity. These results suggest that difference in reaction pathways would lead to variation in the effect of reaction temperature on catalytic activity. Thus, detail research on the effect of water will be conducted in our future work. Previous studies by Zhang et al. indicated that the existence of basic sites on a catalyst is critical for desulfurization performance.6 To clarify the relationship between surface basicity and catalytic performance, we did CO2-TPD experiments over bulk-CNU and CNU-p600. As shown in Figure 7, both catalysts show a large CO2 peak in the region of 50–200 °C, which is ascribable to physisorbed and chemisorbed CO2 molecules on surface basic sites.60 Moreover, the amount of CO2 desorption over CNU-p600 is higher than that over bulk-CNU, indicating that Lewis basicity of CN samples is significantly enhanced through the administration of the adopted thermaltreatment method, in parallel with the observed catalyst activity in H2S oxidation.


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Figure 7. CO2-TPD spectra of bulk-CNU and CNU-p600.

Durability of CNU-p600 Figure 8a shows the durability of CNU-p600 at 180 °C. It is found that there is slight decrease of sulfur yield after 4 h of reactions. This is due to sulfur attachment on the catalyst surface, because the existence of surface sulfur has been confirmed by XPS analysis (Figure 8b). Fortunately, it is possible to eliminate sulfur from the catalyst surface by means of simple thermal sublimation by raising the catalyst temperature to 400 °C under a stream of N2 (60 mL/min) for 120 min. We performed XPS analysis on the catalyst that was subject to three cycles of reaction and regeneration, and found that there was no detection of S 2p signal (Figure S7). Most interestingly, after the first regeneration, there is improved catalytic activity that stays in the subsequent five cycles. To explain this phenomenon, we conducted CO2-TPD experiments over the sample (denoted as CNU-p600-R3) that was subjected to three cycles of reaction and regeneration. As revealed in Figure S8, the CO2 desorption peak of CNU-p600-R3 is larger than that of fresh CNU-p600. The result indicates that despite minute in amount (not detectable by XPS technique, Figure S7), the residual sulfur species in CNU-p600 has an influence on the property


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of CNU-p600, plausibly both in textural as well as electronic nature, leading to the enhanced catalytic performance. In contrast, in the cases of N-GC and Fe2O3, there is marked decrease of sulfur yield with increasing number of cycles. The sulfur yield over N-GC decreases from an initial value of ca. 83% in the first run to that of ca. 67% in the sixth run, whereas that over Fe2O3 decreases from an initial value of ca. 90% in the first run to that of ca. 65% in the sixth run.

Figure 8. (a) Durability test of CNU-p600, N-GC and Fe2O3 at 180 °C. Reaction conditions: catalyst (0.2 g), N2/H2S/O2 = 99.25/0.5/0.25 (wt%), gas flow rate (10 mL·min-1), WHSV (3000 mL·g−1·h−1); (b) S 2p XPS spectrum of fresh and used CNU-p600 sample.

To further investigate the structure difference between fresh and used CNU-p600, FT-IR and Raman spectra of the two were recorded. The results show that the catalyst disclose no obvious change after the reaction (Figure 9), indicating good chemical as well as structural stability of CNU-p600. This can be further confirmed by XRD analysis (Figure S9). In addition, element analysis confirms that the loss of N during the reaction is not significant (Table 1). In contrast, the N content of N-GC revealed in elemental analysis decreases from 20.8% to 6.8% (Table S2). It is considered that the loss of active N sites would lead to poor catalytic activity. We assessed the mass balance of elemental sulfur based on the sulfur collected from effluent and exhausted catalyst (Figure S10). It is found that the theoretical value is close to the datum based on H2S conversion


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and sulfur selectivity. Besides, the identity of the harvested S was confirmed by XRD (Figure S11) and Raman (Figure S12) analyses.61 Overall there is successful capture of S by oxidation of H2S over CNU-p600 samples.

Figure 9. (a) FT-IR and (b) Raman spectra of fresh and used CNU-p600 sample.

Finally, we extended this synthetic method to produce porous CN nanosheets from other nitrogen-rich precursors such as thiourea, melamine and dicyandiamide. The results are displayed in Supplementary Information (Figures S13, S14, S15 and S16). As expected, the CNT-p600, CND-p600, and CNM-p600 nanosheets are porous and large in specific surface area. Under the optimized conditions, the sulfur yields of CNT-p600, CND-p600, and CNM-p600 are higher than those of their bulk counterparts (Figures S17, S18, and S19). Catalytic mechanism The incomplete polymerization of g-C3N4 would cause the formation of inherently electronegative N such as that of imino (−NH) and amino (−NH2) groups located at the terminal of periodic melon units, endowing g-C3N4 with enriched surface properties including electron-rich functionalities and basic surface. These unique properties in CN could promote the catalytic oxidation of H2S. In our previous work, 43 the active sites on g-C3N4 and the possible catalytic mechanism have been studied via the H2S in-situ DRIFTS analysis. As depicted in Figure S20,


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first, H2S interacts with the –NH/−NH2 groups of g-C3N4. At the same time, O2 could be adsorbed by the basic sites during the catalytic process, because the –NH/−NH2 groups of g-C3N4 have strong electron-accepting ability. The “hot electrons” of g-C3N4 are likely to be active at high temperatures, and cleave the adsorbed oxygen molecules into O*.62 Subsequently catalytic reaction between adsorbed oxygen radicals and HS− results in the formation of S and H2O. Furthermore, when the layered CN is exfoliated into nanosheets, there is abundance in edge sites and surface states, resulting in higher availability of active N groups. The basic sites on the catalyst surface have been proved to play a crucial role in the oxidation of H2S.5 Thus, the exfoliated g-C3N4 with higher exposure of basic N groups allows effective adsorption and activation of H2S, resulting in the enhancement of catalytic performance.

CONCLUSION In the present work, we demonstrate that the adopted thermal treatment is simple and effective for the generation of porous CN nanosheets from bulk-CN precursors. The CN nanosheets have ample active sites and are endowed with porosity that promotes mass transportation, leading to substantial improvement of CN catalytic activity. The H2S conversion of CNU-p600 reaches 82.2% at 240 °C with high sulfur selectivity (87%), which is much higher than those of bulk-CNU, CNU-600 and N-doped graphite carbon. Moreover, distinct from traditional iron- and carbon-based catalysts, the used CNU-pT catalysts can be easily regenerated at 400 °C under a stream of N2 with enhanced catalytic activity. This paper provides a facile way for the generation of CN-based catalysts efficient for oxidative desulfurization. It is anticipated that the results would inspire attempts for the modification of 2D carbon materials for various applications.

ASSOCIATED CONTENT


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Supplementary Materials Schematic of the reactor for selective oxidation of H2S; Solid-state 13C-NMR and Raman spectra of bulk-CNU and CNU-p600; Catalytic performance of N-GC, CNU-600, and CNU-p600 at 180 °C; Effect of reaction temperature on (a) H2S conversion and (b) sulfur selectivity for bulk-CNU, CNU-600, and CNU-p600; Effect of H2S/O2 ratio on catalytic performance of CNU-p600 at 180 °C; S 2p XPS spectra of fresh, used and regenerated (regeneration-3) CNU-p600; CO2-TPD spectra of fresh CNU-p600 and the regenerated catalyst (CNU-p600-R3); XRD patterns of fresh and used CNU-p600 sample; The photograph of sulfur harvested from the effluent and catalyst; XRD and Raman spectra of harvested (obtained-S) and commercial (commercial-S) sulfur; XRD spectra of the prepared bulk-CN, and CN-p600 samples generated through the use of different precursors; SEM images and Nitrogen adsorption-desorption isotherms of bulk-CNT and CNTp600; bulk-CND and CND-p600; bulk-CNM and CNM-p600 samples; Effect of reaction temperature on (a) H2S conversion; (b) sulfur selectivity; (c) sulfur yield over bulk-CNT, CNT600, and CNT-p600; bulk-CND, CND-600, and CND-p600; bulk-CNM, CNM-600, and CNMp600; Elemental composition and C/N atomic ratio of Bulk-CNU and CNU-pT samples; Elemental analysis of fresh and used N-GC samples; possible mechanism for selective catalytic oxidation of H2S to elemental sulfur over holey CN nanosheets.

AUTHOR INFORMATION Corresponding Authors L. Shen. E-mail: [email protected] L. Jiang. E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENT This work was financially supported by the National Science Fund for Distinguished Young Scholars of China (21825801), the National Natural Science Foundation of China (21603034), and the National Key Research and Development Program of China (2018YFA0209304). We thank Prof. C. T. Au for helpful suggestions.

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Table of Contents (TOC) Graphic

A general thermal strategy for facile exfoliation of bulk CN to holey CN nanosheets that is catalytically efficient for H2S selective oxidation.


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Exfoliation of graphitic carbon nitride for enhanced oxidative

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