Carbon Nanotubes Monolithic

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N-doped 3D Mesoporous Carbon/Carbon Nanotubes Monolithic Catalyst for H2S Selective Oxidation Shiyan Li, Yuefeng Liu, Huimin Gong, Kuang-Hsu Wu, Housseinou Ba, Cuong Duong-Viet, Chengfa Jiang, Cuong Pham-Huu, and Dang Sheng Su ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00654 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Nano Materials 1

N-Doped 3D Mesoporous Carbon/Carbon Nanotubes Monolithic Catalyst for H2S Selective Oxidation Shiyan Li,a,b Yuefeng Liu,b,* Huimin Gong,b Kuang-Hsu Wu,c Housseinou Ba,d Cuong Duong-Viet,d Chengfa Jiang,a,* Cuong Pham-Huu,d Dangsheng Sub,*

a School b

of Chemical Engineering, Sichuan University, 610065, Chengdu, China

Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical

Physics, Chinese Academy of Science,457 Zhongshan Road, 116023 Dalian, China c

School of Chemical Engineering, The University of New South Wales, Sydney, NSW

2052, Australia d Institute

of Chemistry and Processes for Energy, Environment and Health (ICPEES),

UMR 7515 CNRS-University of Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France

* Corresponding authors E-mail: [email protected] (Y.F. Liu) [email protected] (C.F. Jiang) [email protected] (DS Su)

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Abstract In this report, the post-Claus selective catalytic oxidation of H2S reaction in a continuous mode is demonstrated on N-doped 3D mesoporous carbon/carbon nanotube (N-C/CNT) monoliths at reaction temperature higher than the dew-point (> 180 oC) of elemental sulfur. The N-C/CNT monoliths display a hierarchical open porous framework (an interconnected macroporous-mesoporous network) with abundant active nitrogen species at the surface, and controlled macroscopic size and shape which represent prerequisites for operating in gas-phase reactions. Physical parameters such as pore size, structural defects as well as surface chemical properties, are investigated and the structure-performance relationship for the selective oxidation of H2S is discussed. The optimized monolith catalyst (N-C/CNT450 800) shows an outstanding desulfurization activity in terms of the sulfur formation rate (449 gsulfurkgcat.-1h-1), stability (for more than 120 h on-stream time) and selectivity (81.6% sulfur selectivity), which outperforms the current state-of-the-art Fe2O3/SiC catalyst and other reported carbon-based catalyst. Moreover, a superior desulfurization activity could be achieved under either water steam (30 vol.%) or dry conditions. Our results show that the excellent catalytic desulfurization performance of our monolithic carbon catalyst could be attributed to a combination of the hierarchical interconnected porous framework and the high active nitrogen species density at the exposed carbon surface.

Keywords: H2S selective oxidation, monolith catalysts, nitrogen-doped mesoporous carbon, metal-free catalyst, continuous catalytic desulfurization

2


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1. Introduction Removal of hydrogen sulfide (H2S) from natural gas refineries, crude oil and coal is one of the most important branches of environmental catalysis. H2S is one of the most toxic and malodorous gases responsible for pipeline corrosion and catalyst poisoning in many industrially important processes.1-4 This toxic compound must be removed to the highest extent possible for pollution control and processing requirements in the industry. The most common method to treat this acidic gas is the equilibrated Claus 3

process (2H2S + SO2 →n𝑆𝑛 + 2H2O), transforming toxic H2S into useful elemental sulfur. However, due to the thermodynamic limitation of the Claus equilibrated reaction (97% - 98%), additional processes must be developed to deal with the traces of H2S (i.e. 1 vol.%) remaining in the Claus tail-gas. The most developed process is the selective oxidation of the H2S with oxygen to produce elemental sulfur in order to remove H2S to the best extent in the off-gas before it releases to the atmosphere.5-9 2

Selectively catalytic oxidation of H2S to elemental sulfur ( 2H2S + O2 →n𝑆𝑛 + 2H2O) is also an effective way for the treatment of diluted H2S-containing gases to reduce H2S to less than 0.1 ppm, and also to achieve low cost and environmental impact. Carbon

nanomaterials

(e.g.

carbon

nanotubes/nanofibers,

nanodiamonds,

graphene, fullerene, and the related nanostructured carbon composites) with heteroatoms doping have been widely used as metal-free catalysts in many important catalytic processes, such as oxygen reduction reaction (ORR),10-13 alkane dehydrogenation,14-16 transesterification,17 and selective oxidation,4,

18, 19

owing to

their high specific surface areas, versatile chemical properties, environmental benignity, and excellent reusability.20,

21

In some catalytic processes, doped carbon

nanomaterials could even outperform certain conventional metal catalysts (i.e. metal and metal oxide). For instance, Su et al.22 have reported the advantages of carbon-based nanomaterials as efficient metal-free catalysts for ORR and oxidative dehydrogenation reactions as compared with traditional metal-based catalysts. Nitrogen doping is an efficient approach to tune the surface chemistry of carbon and introduce Lewis basic sites. Bao and co-workers23 reported that nitrogen-doped 3



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carbon derived from PDA/SiC can directly catalyze acetylene hydrochlorination in absence of metal species. The source of the catalytic activity in the carbon-based materials comes from the fact that the doped nitrogen species are directly incorporated within carbon support matrix and provide stable basic sites on the carbon surface, which could enhance surface polarity and electron-donating properties of the carbon surface, thereby significantly promote the catalytic performance of the carbon-based catalyst. Recently, nitrogen-doped carbon materials are also considered as one of the most efficient metal-free catalysts in the selective oxidation of H2S into elemental sulfur. Generally, the oxidative desulfurization processes are carried out either at a low reaction temperature (< 180 oC) in a non-continuous process, or at a high temperature (> 180 oC), in a continuous process

24-29.

For example, Sun et al.28 reported a class of

nitrogen-rich mesoporous carbons (NMC) fabricated through a hard template method (colloidal silica as a porous template) for selective oxidation of H2S at near room temperature. The sulfur-capturing capacity of the NMC could be as high as 2.77 gsulfur/gcatalyst in a single cycle, indicating that carbon material with nitrogen doping is indeed efficient in the desulfurization process. It is worth noting that deactivation occurs rapidly as a function of sulfur deposition, due to pore clogging of the catalyst by solid sulfur, and therefore periodic regeneration is needed to remove the deposited sulfur in a non-continuous process. In a continuous desulfurization process (T > 180 oC),

on the other hand, elemental sulfur remains in vapor phase and only condenses at

the exit of reactor. This is a more efficient route for an industrial desulfurization process, in which the H2S concentration generally encountered to more than 5000 ppm (> 0.5 vol.%). Chizari et al.4 have reported for the first time the selective oxidation of H2S into elemental sulfur at high reaction temperature (>180 oC) on a nitrogen-doped carbon nanotubes decorated silicon carbide (N-CNT/SiC) catalyst, which demonstrates superior desulfurization catalytic activity as compared to metal-based catalysts (FeOx/SiC). Ba et al.

30also

reported a thin N-doped carbon

layer coated on macroscopically shaped supports (β-SiC and α-Al2O3) for high temperature H2S oxidation reactions. Such a bulk catalyst could exhibit > 97% of H2S 4


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conversion with a sulfur selectivity of 70% at a reaction temperature of 210 oC. The monolithic system also prevents the problems associated with pressure drop across the catalyst bed. However, an inert support does not contribute to the specific catalytic activity per unit mass of a bulk catalyst. Also, a higher mass loading of carbon nanomaterials on the inert supports will increase the pressure drops in the fix-bed reactor or lead to a formation of fine grain particles due to the lower mechanical anchorage of the nanocarbon on the host matrix. Although the bulk shaped N-CNT beads were prepared using alginate as a coagulating agent2, the N-CNT beads show a relative low desulfurization performance compare to the initial N-CNT due to a partially coverage of active surfaces by the presence of the binder agent. It is thus interesting to develop a new catalyst system to shape the carbon nanomaterials directly without the need for a supporting host and to ensure a full exposure of active sites in order to improve the overall desulfurization activity per unit mass. Recent studies have demonstrated that monolithic N-doped mesoporous carbon (N-C) coated carbon nanotubes (N-C/CNT) with high mass content of active N-C phases possess extremely high catalytic activities in electrochemical oxygen reduction reaction and direct ethylbenzene thermal dehydrogenation reaction.28,

29, 31

The

carbonization of D-glucose and citric acid and decomposition of ammonium carbonate precursor were combined in one pyrolysis step, which allows the production of a meso- and macroporous carbons with a high specific surface area and high surface nitrogen concentration. Moreover, the protocol allows the preparation of N-C/CNT monolith in different macroscopic sizes and shapes which significantly mitigate the problem of transport and pressure drop across the catalyst bed for various downstream applications. According to these results, meso-/macroporous N-C/CNT monolith is expected to be an effective metal-free catalyst for high-temperature oxidative desulfurization reaction (> 180 oC) in continuous mode. Herein, we report on the use of N-C/CNT monoliths as metal-free H2S selective oxidation catalysts for recovering sulfur from a synthetic H2S polluted off-gas (Scheme 1). The detail parameters such as porosity, defect structure and nitrogen content and nitrogen species nature as well as the reaction parameters, such as the O2/H2S ratio and 5



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presence of water steam, are investigated in-depth to elucidate their influence on the desulfurization catalytic performance in a continuous process. Such a catalyst system was compared with the current state-of-the-art carbon-based metal-free and conventional metal-based FeOx/SiC catalysts.

Scheme 1. Schematic illustration for the synthesis procedure of N-doped 3D carbon/CNT monolith and the supposed structure of the N-doped active phase for the H2S selective oxidation.

2. Experimental Section 2.1 Materials The analytical grade D-glucose (C6H12O6, 99%) and citric acid (C6H8O7, 99.5%) were supplied by Alfa Aesar, ammonium carbonate ((NH4)2CO3) was purchased from Sinopharm Chemical Reagent Co. Ltd. The chemical reagents were used without further purification. The multiwalled carbon nanotubes (CNTs) was supplied by Shandong Dazhan nanomaterials Co. Ltd, and further 6


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were purified by using 6 M HCl acid aqueous solution at 80 oC for 12 h and then were washed using deionized water and ethanol until neutral pH and dried at 60 oC for 24 h. The obtained sample is denoted as pristine CNT, and the basic data is listed in Table S1.

2.2 Preparation of the N-doped carbon foam composite The three-dimensional (3D) N-doped carbon foam composite was synthesized via a series of pyrolysis reaction according to previous reports.28, 29 Typically, 4 g D-glucose, 6 g citric acid and 1 g carbon nanotubes (CNTs) were physically mixed and finely ground at room temperature. 3 g ammonium carbonate powder was then added into the above solid mixture and mixed uniformly via mechanical mixing to obtain a homogeneous dark solid powder mixture. The mixture underwent a two-step thermal treatment to generate a highly N-doped and carbonaceous surface coating on the CNT. The solid mixture was kept at 130 oC in air for 5 h. The obtained macro-composite was then subjected to calcination at different temperatures from 350 oC, 400 oC and 450 oC at 2 oC/min in air and treated for 3 h and further annealed at different temperatures (700 oC, 800 oC and 900 oC, respectively) for 2 h under argon atmosphere with a heating rate of 5 oC/min. The resulted 3D porous N-C/CNT monolith can be directly used as a catalyst, namely N-C/CNTX Y, where the superscript X refers to the calcined temperature in air flow (from 350 oC, 400 oC

to 450 oC), and the subscript Y is used for the annealed temperature (from

700 oC, 800 oC to 900 oC) under Ar gas. 2.3 Characterization The specific surface area and porosity of the samples were measured by N2 adsorption isotherm using the Brunauer-Emmett-Teller (BET) method on an ASAP 2020 Micromeritics instrument. Before the measurement, all samples were degassed at 200 oC for 6 h in order to desorb moisture and impurities from their surfaces. The pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) model from desorption branch. 7



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The powder X-ray diffraction (XRD) patterns of the samples were obtained via a Bruker Advance D8 diffratometer with Cu Kα radiation (λ=1.54 Å) to study the crystallinity of the sample. The morphology of the N-C/CNT monolith was characterized by field-emission scanning electron microscopy (SEM) on a JSM-7800F with an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) measurements were carried out on a JEM-2100 operating at an accelerated voltage of 200 kV. The sample was ultrasonicated in ethanol solution and a drop was deposited on a copper grid covered with a holey carbon membrane for observation. Thermogravimetric analysis was performed under air (20 mL/min) with a heating rate of 10 oC/min on an STA 449 F3, from room temperature to 900 oC. XPS analyses were performed on a ESCALAB 250Xi photoelectron spectroscopy equipped with Al Kα X-ray source (hν = 1486.6 eV). Peak deconvolution is realized with the “Avantage” program from Thermoelectron Company. The C 1s peak at 284.6 eV is used to correct all charging effects. Shirley backgrounds are then subtracted from the raw data to obtain the areas of the C 1s peak. Raman spectroscopy with excitation wavelength of 532 nm was carried out using RENISHAW inVia Raman Microscope equipped with CCD detector. Elemental analyses were carried out by using Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) using NexION 300D. 20 mg sample and 20 mL concentrated nitric acid was mixed into a hydrothermal autoclave and then maintained at 180 oC for 24 h. The obtained transparent aqueous solution was diluted to 50 mL with deionized water. 2.4 Activity measurements for H2S selective oxidation The desulfurization reaction was carried out in a continuous flow fixed-bed quartz reactor working isothermally at atmospheric pressure. In such desulfurization reaction, the following reactions could occur during this process (Eqs. 1-3): 8


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1

H2S + 1

nSn

→

2O2

1

nSn

+ H2O

+ O2 →SO2

∆H = ―297 kJ/mol

3

H2S + 2O → SO2 + H2O

∆H = ―381 kJ/mol

2

(1)

∆H = ―222 kJ/mol (2)

(3)

For the catalytic reaction, 150 mg of the catalyst was placed in the central section of a tubular reactor (inner diameter of 8 mm). The apparent bed volume was kept at 7 cm3. The temperature was controlled by a K-type thermocouple (located inside the furnace) and monitored by another K-type thermocouple axially centred in the reactor tube. The reactant including 1 vol.% of H2S, 2.5 vol.% of O2, 30 vol.% of H2O, and balanced He was passed through the catalyst bed with a flow rate of 100 mL/min, all gas flow rate was monitored by a mass flow controller. Before heated up the catalyst bed, the reactor was flushed with helium at room temperature until no trace of oxygen was detected at the outlet. The helium flow was replaced by the reactant mixture when the oven temperature reached the desired temperature. The steam (30 vol.%) was fed to the gas mixture by bubbling helium flow through a liquid saturator containing deionized (DI) water maintained at 85 oC. The Weight Hourly Space Velocity (WHSV) was kept at 0.6 h-1 and the O2-to-H2S molar ratio was fixed at 1.5 and 2.5. The outlet gases were detected online by an Agilent 7890B gas chromatograph (GC) coupled with a thermal conductivity detector (TCD), which allowed the detection of O2, H2S and SO2. The limit of the detection for the different gaseous effluents is < 50 ppm. A condenser was located at the exit of the reactor to trap the sulfur gas in effluent stream at room temperature and further condensed in a container placed in an icy water to remove as much as possible the sulfur gas and steam before the vent. The H2S conversion (XH2S) and sulfur selectivity (SS), as well as sulfur formation rate in terms of specific surface area (λSSA) and mass (λcat.) of the catalysts were evaluated using Eqs. 4-7:

(

FCH2S, outlet

)

(4)

XH2S[%] = 1 ― F0CH2S, inlet × 100 9



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(

SS[%] = 1 ―

FCSO2, outlet F0CH2S, inlet ― FCH2S, outlet

λSSA[μmolsulfurm ―2h ―1] = λcat.[gsulfurkg𝑐𝑎𝑡.h ―1] =

) × 100

(5)

F0 × XH2S × SS × CH2S, inlet

(6)

SSA × t

F0 × XH2S × SS × CH2S, inlet

(7)

m𝑐𝑎𝑡. × t

Where F and F0 are the flow rates of the outlet and inlet gas, respectively, and CH2S, and CSO2 represent the concentration of H2S and SO2. SSA: specific surface area of the catalyst calculated by the BET method. The steady-state results were obtained after more than 15 h on stream. 3. Results and Discussion 3.1 Structure and Chemical Nature of N-C/CNT monolith The synthesis procedure was modified during the series of pyrolysis reaction steps of monolithic composites compared to previous reports to generate various 3D N-doped carbon/CNT monoliths for H2S selective oxidative reaction.28 The initial thermal treatment temperature was kept at 130 oC in air in order to perform a series of chemical

reactions,

i.e.

acid-base

reactions,

condensations

and

thermal

decompositions with controllable shape and size of the N-C/CNT monolith28,

29.

Subsequently, the N-C/CNT monolith precursor was calcined in air at different temperatures, i.e. 350, 400 or 450 oC, to further decompose the mixtures and consolidates the foam structure through a high mechanical strength interaction between CNT and disordered carbon species issued from the D-glucose and citric acid. The thermal treatments in air also allows one to burn-off part of the carbon structure and to create open porous framework with nitrogen functional species (N-C). The final thermal treatment was performed in argon atmosphere at different annealing temperatures, i.e. 700, 800 or 900 oC, to improve the electrical and thermal conductivity of the N-C/CNT monolith and to produce more graphitized carbon structure. Thus, the porosity and surface composites of the obtained N-C/CNT monoliths could be tuned accordingly by changing the thermal treatment conditions either in air or in argon flow (Table 1). Such parameters induce change of the physical

10


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and chemical properties of the N-C/CNT monoliths which will further influence it catalytic performance during the desulfurization reaction. Table 1. Textural properties and XPS characterization of the N-C/CNT monoliths Samplea CNT N-C/CNT450 N-C/CNT350 800 N-C/CNT400 800 N-C/CNT450 800 N-C/CNT450 700 N-C/CNT450 900

2.14 0.61 0.17

Db nm 24.4 4.8 3.5

C 94.7 86.1 84.1

399

0.32

3.9

86.0

5.6

8.4

0.10

412

0.73

6.1

91.7

4.1

4.2

0.05

464

0.66

5.2

89.4

3.1

7.5

0.08

402

0.65

5.6

93.9

2.1

4.0

0.04

SBET m2g-1

VT cm3g-1

290 537 360

XPS analysis (at.%) O N 4.6 0.7 4.8 9.1 9.3 6.6

N/C 0.01 0.11 0.08

aCatalysts

are designated as follows: N-C/CNTX Y, where N-C/CNT indicates the catalyst, the superscript X (from 350 oC to 450 oC) refers to the calcined temperature in air, and the subscript Y is used for the annealed temperature (from 700 to 900 oC) in Ar. bBJH desorption average pore size.

The developed synthesis procedure herein is facile and flexible which permitted the preparation of N-C/CNT monolith with different macroscopic sizes and shapes, i.e. sphere, triangular prism, cylindrical, cubic, etc, as can be seen in the digital photos in Figure 1a. The shape was maintained even after further thermal treatment in air and argon which highlight the advantage of the synthesis method (Figure 1b). The ability for direct shaping of the catalyst is of great interest as generally the nanocarbon materials (such as carbon nanotube and porous nanocarbons) are mostly used in powder form which inevitably induces significant pressure drop along the catalyst bed as reported elsewhere and also renders it difficult for transportation and handling. The obtained N-C/CNT450 800sample presents superior mechanical strength in various preparation steps (Table S2), and is tolerance of a stress pressure of 1.5 MPa under the final state. 11



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The morphology of these N-C/CNT monoliths was examined by SEM and the results are shown in Figure 1 and Figure S1. The low magnification image (Figure 1d) of the as-synthesized carbon foam composites clearly exhibits a three-dimensional open porous structure with fully accessible porosity. The observed porosity in the N-C/CNT monolith is attributed to the decomposition of the ammonium carbonate, which is acting as a pore forming agent.29 In addition, D-glucose and citric acid also decomposed during the thermal treatment processes, resulting to a large amount of pores in the final composites. The presence of macro- and mesopores in the foam structure significantly contributed to the reactant accessibility and product desorption, which could significantly modify the overall activity and selectivity of the catalytic reaction. In Figure 1e, SEM micrograph of N-C/CNT450 800 shows a high entanglement of the CNT which leads to a carbon scaffold with improved mechanical strength. Furthermore, SEM elemental mappings (Figure 1c) indicate that the N-C/CNT monolith contains C, N, O species which are uniformly distributed throughout the carbon monolith.

12


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Figure 1. (a) Digital photos of the N-C/CNT monolith precursor after the low temperature chemical fusion method with different macroscopic shapes, sphere, triangle, cylindrical, square etc. (b) Macroscopic shapes of the N-C/CNT monolith after calcined at 450 oC in air and annealed at 800 oC in Ar. (c) SEM image and EDS elemental mapping images of N-C/CNT450 800. (d, e) SEM images of the carbon/CNT monolith showing the presence of large pores and the uniform dispersion of the CNT within the carbon foam. (f, g) Representative TEM images of the N-C/CNT450 800 monolith showing the porous carbonaceous substance coating on the surface of CNT. The morphology and microstructure of the pristine CNT after acid treatment and N-C/CNT monolith were examined by TEM (Figure 1f, g and Figure S2). As shown in Figure 1f, porous disordered carbonaceous substance bridged CNT within the foam 13



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structure are readily observed throughout the whole skeleton of the N-C/CNT450 800 monolith, which formed new porous network. Besides, CNT in the N-C/CNT monolith structure acting as a skeleton for hosting disordered carbonaceous substance. The high resolution TEM image (Figure 1g) indicates that the CNT surface was covered by the disordered carbonaceous substance with abundant porosity which acts as solid glue to maintain the whole macroscopic structure of the composite. It is important to note that HRTEM analysis also indicates the preservation of original microstructure in CNTs without any change during the synthesis (Figure S2). XRD spectra of pristine CNT and N-C/CNT450 800 monolith only exhibit peaks at 26o and 43o, which also confirmed the typical graphitic carbon presenting on the prepared monolith catalyst (Figure S3). The specific surface area (SSA) and pore size distribution of as-prepared N-C/CNT monoliths were characterized by nitrogen adsorption isotherm. All the N-C/CNT monoliths synthesized at different conditions display a type IV adsorption-desorption isotherm with an H2 hysteresis loop in the range of 0.4-1.0 P/P0 (Figure S4a, c and e), This corresponds to typical mesoporous structure of the materials.32 The as-prepared N-C/CNT monoliths from our synthesis strategy also feature a high SSA with open porous structures (Figure S4b, d and f). The detailed BET specific surface areas and total pore volumes of these N-C/CNT monoliths are listed in Table 1. For instance, the BET value measured on the N-C/CNT monoliths, after calcinated at 450 oC in air (N-C/CNT450), exhibits a SSA of 530 m2g-1 compared to 290 m2g-1 for the pristine CNT and a total pore volume of 0.61 cm3g-1. It should be noted that the SSA of the N-C/CNT450 is higher than the value reported in our previous reports (423 m2g-1), which could be due to the higher surface area of initial CNT in the present work (290 m2g-1 compared to 145 m2g-1).29 By changing the calcination temperature in air, the pore volume varied from 0.17 cm3g-1, 0.32 cm3g-1 to 0.73 cm3g-1, and the average pore size changed accordingly, from 3.5 nm, 3.9 nm to 6.1 nm (Table 1) for N-C/CNT350 800, N-C/CNT400 800 and N-C/CNT450 800, respectively. Such results reveal that the calcination temperature in air could significantly affect the texture properties of the composite by burning part of the 14


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carbon inside the material. It is also worth noting that further thermal annealing under argon at higher temperature decreases the specific surface area of the samples, from 530 m2g-1 to about 400 m2g-1 (Table 1). The specific surface area decrease could be ascribed to the partial re-organization of a disordered carbon phase into a more graphitic one in the sample during the high temperature annealing process. The results reported in Figure S4d indicate that calcinations at 450 oC followed by an annealing at 800 oC leads to the formation of a wider pore size compared to the samples calcined at lower temperature. The average pore size of N-C/CNT monoliths calcined at 450 oC followed by annealing at different temperatures (700, 800 or 900 oC) is maintain in the similar value, which is 5.2 nm 6.1 nm and 5.6 nm for N-C/CNT450 700, N-C/CNT450 800 and N-C/CNT450 900, respectively (Figure S4f). The surface elemental composition and bonding configurations of the heteroatoms generated at the surface of N-C/CNT monolith were investigated by XPS. As shown in Figure 2a and b, only C 1s, N 1s and O 1s components are present in the survey at around 285 eV, 400 eV and 533 eV, respectively.28 While no nitrogen is present in pristine CNT, the incorporated nitrogen atoms detected must come from ammonium carbonate precursor (Figure 2a and b). In Figure 2c-f, it can be clearly found that the contents of total nitrogen content of the N-C/CNT monolith and the nitrogen species of the N-C/CNT monolith could be varied by adjusting the calcination or the annealing temperature. An increase in the calcination temperature (in air) from 350 to 450 oC followed by a annealing temperature at 800 oC in Ar, leads to the formation of the high total nitrogen content up to 8.4 at.% ( N-C/CNT400 800 in Figure 2d). As shown in Figure 2f, the total nitrogen content was decreased with increasing annealing temperature from 700 to 900 oC in the Ar atmosphere, the highest nitrogen content (7.5 at.%) was observed for N-C/CNT450 700. Besides, the nitrogen content of N-C/CNT450 800 reaches 4.2 at.%. For N-C/CNT450 900, the nitrogen content is only 4.0 at.% which could be ascribed to the instability of nitrogen at higher temperature which leads to the loss of nitrogen species. These results are consistent with previous observations that the high temperature annealing treatment is at the origin of nitrogen species loss.14, 33 The N 1s spectra are deconvoluted into four peaks 15



Page 16 of 37 16

related to pyridinic N, pyrrolic N, graphitic N and oxidized N species, which were located at 399.0, 400.3, 401.5, and 404.0 eV,34-36 respectively, as depicted in Figure 2c and e (see schematic model of different N species in Figure 2g). By increasing the annealing temperature from 700 to 900 oC, we found that the content of the pyridinic N of carbon/CNT monolith is clearly decreased from the highest value of 3.7 at.% for N-C/CNT450 700 to 1.6 at.% for N-C/CNT450 900(Figure 2f). The above results indicated that the nitrogen content and nitrogen species can be modified in a relatively large extend by modifying the annealing temperature. Thus, the catalytic performance can be modified through changing the thermal treatment temperature.

16


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Figure 2. XPS spectra of pristine CNT and N-C/CNT monolith. XPS survey scan before (a) and after (b) annealing in Ar under different temperature. (c, e) N1s high resolution XPS spectra. (d, f) the content of nitrogen species of N-C/CNT monolith with different calcination temperature in Air flow (350, 400 and 450 oC, respectively) and annealing temperature in Ar flow (700, 800 and 900 oC, respectively). (g) Schematic illustration of the typical four types of nitrogen species. In the XPS C1s spectra, the samples reveal five peaks (Figure S6), which can be assigned to sp2 hybridized C=C graphite-like carbon (C1, 284.6 eV), unsaturated carbon structures belonging with C-N, C-O bonds (C2, 285.9 eV), C=O (C3, 287.3 eV) bond, -COO bond (C4, 288.8 eV), and the shake-up π-π* transition of carbon (C5 291 eV).

37

The relative content of unsaturated carbon atoms on the surface of

N-C/CNT monolith are in the range of 39.7 at.% - 60.3 at.% (Table S3). This reflects abundant defect structures on the surface of N-C/CNT monolith materials. Chemical states of the O atoms were also investigated. As shown in Figure S7, all the samples reveal three peaks with binding energies of 531.6 eV, 532.6 eV, and 533.9 eV, which can be assigned to the C=O, O-C=O, and C-O groups.38 Although the monolith underwent different thermal treatments in air, the relative content of oxygen species was remain at in the range of 4.1 at.% and 5.6 at.%. The oxygen concentration also changes as a function of the annealing temperature: the catalyst which was annealed at 800 oC has the highest relative oxygen content of 4.1 at.%, while the lowest oxygen species (2.1 at.%) was observed for N-C/CNT450 900. 3.2 Catalytic oxidation of H2S over N-C/CNT monoliths The N-doped carbon foam-like composites can be used directly as metal-free catalyst without further purification or washing. As shown in the ICP-MS results (Table S5), Fe contents of the washed CNT (pristine CNT) and N-C/CNT450 800 are 17



Page 18 of 37 18

0.39 wt% and 0.32 wt%, respectively. Such a metal impurity level could be attributed to the residual iron catalyst which is encapsulated in graphitic carbon matrix and could not be removed from CNT during the acid-purification process. It is reasonable that the N-doped mesoporous carbons do not contain metal impurities, or at least the impurities are not directly exposed at the surface, and thus they should not contribute to any observable catalytic activity. The H2S selective catalytic oxidation was carried out in a fixed bed reactor under atmospheric pressure at a temperature of 190 oC and weight hourly space velocity (WHSV) of 0.6 h-1. At a O2-to-H2S molar ratio of 2.5, the H2S conversion for N-C/CNT350 800, N-C/CNT400 800 and N-C/CNT450 800 are 32.6%, 91.4% to 99.4%, respectively (Figure 3). The results clearly demonstrate the superior desulfurization activity of N-C/CNT450 800. The sulfur selectivity on the N-C/CNT450 800 is higher than 79% through the whole test. However, such sulfur selectivity remains lower than that of N-C/CNT350 800 and N-C/CNT400 800 (i.e. 92% and 84.6%, respectively). The lower sulfur selectivity of the N-C/CNT450 800 could be attributed to the over-oxidation of sulfur to SO2 by the excess oxygen. As the H2S conversion increases, more sulfur is formed on the catalyst which statistically leads to a higher reaction probability between the formed sulfur and excess oxygen. The sulfur formation rate (λcat.) on the steady-state over various N-C/CNT monoliths are summarized. As can be seen in Figure 3c, λcat. of N-C/CNT450 700 and N-C/CNT450 800 is as high as 455 and 449 gsulfurkgcat.-1h-1, respectively. The data clearly show that the N-C/CNT monolith composites could act as an alternative catalyst for industrial exploitation in the desulfurization process. Notably, around 930 kg of optimized N-C/CNT monolith (N-C/CNT450 800) could recover 1 ton of sulfur per day under the aforementioned industrial conditions (1 vol. % of H2S and 190 oC).

18


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Figure 3. Catalytic desulfurization performance on the N-C/CNT monoliths. (a, b) H2S selective oxidation reaction at O2-to-H2S molar ratio of 2.5, and (c) the summary of the sulfur formation rate per hour per mass and specific surface area of catalyst, respectively. (d) H2S selective oxidation reaction at O2-to-H2S molar ratio of 1.5 at certain calcination temperature in air followed by different annealing temperature in Ar. Reaction conditions: T = 190 oC, WHSV = 0.6 h-1, steam concentration = 30 vol.%.

The influence of O2-to-H2S molar ratio in the selective oxidation over N-C/CNT monoliths was also evaluated and the results obtained with an O2-to-H2S molar ratio of 1.5 are displayed in Figure 3d. It is worth noting that the H2S conversion was slightly dropped along with decreasing the O2-to-H2S molar ratio from 2.5 to 1.5 (Figure 3b and d). However, the H2S conversion remained very high even at lower O2-to-H2S molar ratio and under a relatively high WHSV (0.6 h-1). Furthermore, the sulfur selectivity increased when the O2-to-H2S molar ratio decreases which can be explained by the lower available oxygen gas for performing the complete oxidation to yield SO2. The results indicate that the N-C/CNT monolith exhibits a high catalytic 19



Page 20 of 37 20

activity the selective oxidation of H2S into elemental sulfur even at low oxygen partial pressure in the reactant mixture. The N-C/CNT450 800 catalyst shows a relatively high and stable desulfurization activity for more than 80 h as a function of time on stream at a reaction temperature of 190 oC (Figure 4a), which indicates that a deactivation event is unlikely to occur. The influence of the reaction temperature has also been tested and the results are shown in Figure 4a. The increase of the reaction temperature leads to a slightly increase of the H2S conversion from 98.6% at reaction temperature of 190 oC to 99.8% at reaction temperature of 210 oC. Increase the reaction temperature also leads to a slight decrease of the sulfur selectivity from 81.6% at 190 oC

to 80.7% at 210 oC. It is worth noting

that the catalyst remains extremely stable during the whole test lasting for more than 120 h. The desulfurization performance of the N-C/CNT monolith was also compared with that of the most active Fe2O3/SiC catalyst reported previously39,

40

under the

same desulfurization reaction conditions and the results are shown in Figure 4b. The N-C/CNT monolith display a high desulfurization activity compared to the Fe2O3/SiC catalyst under the operated reaction conditions. The iron-based catalyst displays a higher sulfur selectivity which could be attributed to the low H2S conversion compared to that obtained on the N-doped catalyst. Table 2 summarized the desulfurization performance of recent reported metal-free carbon-based catalysts, and compared with the optimized monolithic N-C/CNT composite. The N-C/CNT450 800sample presents a superior sulfur formation rate in terms of kilogram of catalyst (λcat., gsulfurkgcat.-1h-1) compared to the other carbon-based catalyst operated at temperature above the dew-point of sulfur (> 180 oC). Such results demonstrated once again the high desulfurization catalytic efficiency of the monolithic N-C/CNT composites.

20


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Figure 4. (a) Catalytic desulfurization performance over the N-C/CNT450 800 as a function of the reaction temperature. (b) Catalytic desulfurization performance of the N-C/CNT450 800 and Fe2O3/SiC catalyst. Reaction conditions: O2-to-H2S molar ratio = 2.5, WHSV = 0.6 h-1, steam concentration = 30 vol.%.

21



Page 22 of 37 22

Table 2. Comparison of desulfurization performance at temperature above dew-point temperature (> 180 oC) of sulfur over different metal-free catalysts reported in literature. Catalysts

T, oC

H2S, vol.%

O2, vol.%

WHSV, h-1

XH2S (%)

SS (%)

λcat., gsulfurkgcat.-1h-1

Ref.

N-C/CNT450 800 N-C/CNT450 800 N-CNT/SiC foam N-CNT

190

1

2.5

0.6

99.4

79.2

449

this work

190

1

1.5

0.6

98.4

87.2

490

this work

190

1

2.5

0.72

99.8

75

113

4

190

1

2.5

0.32

91

75

205

4

N-CNT/SiC extrudate N@CF-800

190

1

2.5

0.32

95.8

74.1

100

41, 42

230

1

2.5

0.6

57

95

306

43

O-CNT-250-2 4 AN@C/SiC 2 E

190

1

2.5

0.6

50

90

254

29

190

1

2.5

0.3

86.3

77.2

376

30

AN@C/SiC 2 E

210

1

2.5

0.3

> 97

70

383

30

N-CNT beads

210

1

2.5

0.3

99.1

61.6

47

2

MCNR

190

0.2

0.1

99.8

88.8

-

44

OGFs-16

250

1

2.5

1200 h-1 (GHSV) 0.1

98

86

79

45

CNM-600

180

0.5

0.25

3000 (ml.g-1.h-1)

≈ 96

≈ 97

40

46

3.3 Influence of structural parameters on the desulfurization activities over N-C/CNT monoliths The influence of pore structure. The porosity of the carbon-based catalyst certainly plays an important role in desulfurization reaction either in a continuous model or discontinuous model. In order to understand the relationship of pore structure of the catalysts for the H2S selective oxidation, the correlation curve of pore size and pore volume versus sulfur formation rate, namely the relative areal rate under the same active site densities (λSSA, molsulfurm-2h-1), is performed and displayed in 22


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Figure 5. A platform of relative areal rate is found when the average pore size and pore volume are high than 3.9 nm and 0.32 cm3g-1, respectively, despite of the different O2-to-H2S molar ratios (ratio of 1.5 and 2.5 are conducted in this work). Such results present the distinct phenomenon compared to the discontinuous model in H2S selective oxidation, in which high pore volume of microspore results to superior sulfur adsorption capacities.47 The sulfur could be condensed in the microspores of the catalyst in discontinuous model,48 where the high-energy centres appear in the microspores. Furthermore, in continuous model that H2S will be strongly adsorbed in the micropores, which results the over oxidation reaction to SO2 and pore plugging during the reaction, inhibiting interaction of reactants and the active sites as function of time on stream.3, 43 Thus the higher open porous structure with macropores -mesopores connection network in this N-C/CNT monolith is beneficial for the activity of H2S selective oxidation.

Figure 5. The relationship diagram of sulfur formation rate per specific surface area of the catalysts with pore volume and pore diameter, respectively. Influence of structural defects and N dopant. Surface structure, nitrogen content and the chemical states have been viewed as important factors in metal-free carbon catalyst system.3,

4

According to the results from Figure 5, there is no correlation

between the relative areal rate and the pore diameter or pore volume under different 23



Page 24 of 37 24

thermal treatment temperature (i.e. 700, 800 and 900 oC). It is known that the surface chemistry of the carbon-based catalysts significantly changes along with the thermal treatment temperature.49 Thus, Raman spectroscopy was used to analyze the graphitization degree and surface defects of the samples (Figure S9).50 It can be observed that two typical D and G bands at 1345 cm-1 and 1590 cm-1, respectively, are presented in the Raman spectra of all samples. The D band is attributed to the disordered sp3 hybridized carbon, while the G band is associated with the crystallized graphitic sp2 carbon.51, 52 The intensity ratio of ID/IG increases from 1.23 for pristine CNT to 1.74 for N-C/CNT monolith after calcinations in air at 450 oC indicates that more defective structure is present inside the sample. Such results confirmed that N-doped carbon composite formed as a layer on the surface of CNT and contributes to the origin of the structural defects. The ID/IG intensity ratios decrease from 1.64 to 1.35 with increasing the annealing temperature from 700 to 900 oC (Figure S9a), confirming a better graphitic nature of the N-C/CNT monolith at higher annealing temperature. Furthermore, the TGA data (Figure S5) indicates that the oxidation temperature of samples slightly increases with the raising of annealing temperature from 700 to 900 oC, reflecting the better crystallization of the N-doped carbon layer at higher annealing temperature. The density of the structural defects could calculated by the following equations (Eq. 8) according to Cançado’s work from the Raman spectra 53.





I nD cm  2  7.3  2.2  109 EL4  D  IG

   

(8)

where EL is expressed as the excitation energy. The value of EL is fixed around 2.41 eV for 2D graphene,54, defects29,

56.

55

and 2.33 eV for CNT with abundant surface

In this work, the EL value is fixed at 2.33 eV to calculate the

related defect densities (nD) according to Cançado’s equation. For better understanding the influence of graphitization degree and surface structural defects for the desulfurization activity, the defect density is correlated with sulfur areal formation rate (λSSA). It is found in Figure S9b that the appropriate 24


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competitive balance between graphitization and formation of surface defects results in the highest λSSA value of N-C/CNT450 800. Tao et al.57 demonstrated that edge-enriched graphene with abundant structural defects showed excellent activation abilities for O2 molecules dissociation in oxygen reduction reaction. It is also reported that N contents, especially the content of pyridinic N species, is responsible for the catalytic activity.4,

28

However, the N content and

pyridinic N species decrease along with the increments of annealed temperatures. Meanwhile, the higher annealing temperature also results in the decrease of the defectiveness of the N-C/CNT monolith (Table 1, Figure 2f). When the sample is annealed at higher temperature, the electrical conductivity of the carbons is increased as well as the electron transport during reaction,58 promoting the contribution from the nitrogen doped active sites and the H2S oxidation activity in turn. It can be herein interpreted the highest reaction activity of N-C/CNT450 800, although it displays only medium amount of defect density and nitrogen content/pyridinic N species. Influence of steam. A relatively large amount of steam is presence in the exit off-gas in the Claus process. The introduction of steam could enhance the reaction activity significantly in the discontinuous desulfurization process.59 In presence of water film on the surface of carbon catalyst, the O2 molecules are adsorbed favourably and cleaved into reactive O2● radicals.3 Subsequently, the HS- species is formed easily and reacted with adsorbed O2● radicals to produce the elemental sulfur and water. It is thus interesting to investigate the effects of steam on our monolithic N-C/CNT composites during the continuous H2S selective oxidation process. Figure 6a displays the performance of H2S selective oxidation with steam and without steam in the feed gas. The H2S conversion of N-C/CNT450 800 without steam is 98.3% with sulfur selectivity of 85.5% under WHSV of 1.2 h-1, which even displays slightly higher values comparing to the H2S conversion in presence of 30 vol.% of steam (H2S conversion of 97.4% and sulfur selectivity of 83.2%). No significantly influence of steam addition can be seen in this catalyst system. Moreover, the catalyst survived 25



Page 26 of 37 26

more than 40 h without severe deactivation in dry feed gas (Figure 6b). Such superior activity and stability without the promotion of water film could be ascribed to the abundant structural defects as well as the N species in the catalyst system, which could provide sufficient active sites to dissociate O2 into O2● radical.46, 60

Figure 6. (a, b) Catalytic desulfurization performance and stability over the

N-C/CNT450 800 in presence of water steam. Reaction conditions: O2-to-H2S molar ratio = 2.5, WHSV = 1.2 h-1, mass of catalyst = 75 mg.

3.4. Regeneration and cycling test on N-C/CNT monolith In the continuous desulfurization process the catalyst operated at temperature higher than the sulfur dew-point, i.e. 180 oC. The formed sulfur during the reaction is removed from the catalyst bed through partial vaporization and condensed at the reactor exit. The H2S conversion is relatively high in our reaction conditions and the reaction temperature is close to the sulfur dew-point. Thus, the formed sulfur can be not totally removed through vaporization and part of it may remain on the catalyst surface. The EDS elemental mapping clearly evidences the presence of elemental sulfur on the surface of N-C/CNT monolith after desulfurization test at 190 oC (Figure S10a). High resolution SEM was further conducted (Figure S10b and c) and it can be found that sulfur aggregation was located on the top surface of N-C/CNT monolith. Apparently the deposited residual sulfur is not high enough to induce deactivation by pore plugging according to the catalytic performances in long-term desulfurization tests where no deactivation was observed. 26


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The spent catalyst was further regenerated (400 oC in helium) to remove the deposited sulfur and was evaluated again in desulfurization reaction. The surface chemical composition of the spent and regenerated N-C/CNT monolith catalyst was characterized by XPS analysis and the results are presented in Figure 7 and Table S6. The S 2p spectra of spent and regenerated N-C/CNT450 800 catalyst can be fitted with three peaks (Figure 7b), where the peak position site at 163.9 (S 2p3/2) and 165.1 eV (S 2p1/2) are assigned to the sulphide groups (-C-S-C- ), and 168.6 eV is assigned to the sulfate (-C-SOx-C-) (Figure 7c) which may be attributed to the deep oxidation of formed sulfur into SO3.24, 29 The sulfur content of spent N-C/CNT450 800 catalyst is 2.89 at.%, and that of the regenerated N-C/CNT450 800 catalyst is as low as 0.21 at.%. It demonstrates the formed solid sulfur could be totally removed through the thermal regeneration process (Table S6). The total N content decrease to 2.93 at.% in spent catalyst (4.21 at.% for fresh sample), which mainly be ascribed to the partly nitrogen active sites covered by formed solid sulfur (Table S6). It is important to note that the desulfurization performance remains stable even if in the presence of some solid sulfur deposits on the surface of the catalyst (Figure 4a). The deconvoluted high resolution N 1s spectra and the related content of N-C/CNT450 800 catalyst under varied treatment conditions are also presented in Figure 7d and e. It can be clearly seen that the pyridinic N (N1 in Figure 7e) and graphitic N (N3 in Figure 7e) of N-C/CNT450 800 catalyst before and after desulfurization reaction decrease from 2.04 and 0.95 at.% to 1.09 and 0.57 at.%, respectively, where the pyrrolic N (N2 in Figure 7e) slight increase from 0.89 to 1.06 at.%. Meanwhile, the total N content of N-C/CNT450 800 catalyst increase to 3.37 at.% after thermal treatment at 400 oC in helium by removing the most of deposited sulfur species, but do not reach back the value of the fresh catalyst. The pyridinic N increase to 1.45 at.% accordingly, whereas the pyrrolic and graphitic N species of regenerated sample are maintain the similar values as the spent catalyst (Figure 7e). Such phenomenon reveals that most sulfur species are more favour to deposit on the pyridinic and graphitic N sites, which could be related to the desulfurization activity.4, 28, 43 It should be noted that due to the fact that the desulfurization process was performed at relative low reaction temperature 27



Page 28 of 37 28

(190 oC) in the current study, which may not able to fully vaporize the solid sulfur generated during the reaction. However, according to the desulfurization reaction on N-C/CNT monolith, which lasted for 70 h, in Figure 4a, there are no significantly deactivation takes place as a function of time on stream and thus, one should expected that the formed sulfur was mostly removed from catalyst surface during the reaction by presence of water vapor.28 High resolution SEM micrographs in Figure S10b and c clearly confirmed that the sulfur is formed in aggregates which on some area of the catalyst and not fully cover the catalyst surface to initiate deactivation.

Figure 7. (a) XPS survey spectra of the N-C/CNT450 800 before and after desulfurization test. (b) High resolution S 2p spectra of the N-C/CNT450 800 after desulfurization test. (c) Schematic model of sulfur composites sited on the carbon framework for the spent N-C/CNT450 800 catalyst. (d) High resolution N 1s spectra and (e) the content of nitrogen species of N-C/CNT450 800 before and after 28


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the desulfurization test and regenerated by a thermal treatment. The desulfurization activities on second cycling test at 190 oC maintains the similar catalytic performance compared to initial test (Figure 8). The H2S conversion was just slightly decreased to 96.2%, whereas the sulfur selectivity is maintained at 79.2%. Such results could be explained by the slight decrease of the nitrogen sites on the regenerated catalyst. However, it is worth noting that in industrial process the reaction temperature is not as isothermally as observed in the laboratory test due to the exothermic character of the reaction. In such configuration the inlet temperature of the reactant should be as low as possible in order to reduce the gradient temperature inside the reactor bed. The metal-free catalyst operated in the present work seems to fulfill such requirement in terms of desulfurization performance at low inlet temperature.

Figure 8. Cycling test and desulfurization performance of N-C/CNT450 800 at 190 oC. Reaction conditions: O2-to-H2S molar ratio = 2.5, WHSV = 0.6 h-1, steam concentration = 30 vol.%. 4. Conclusion In summary, the N-doped carbon/CNT monoliths with open porous structure and highly accessible surface have been successfully synthesized and displayed robust catalytic activities in the continuous model of H2S selective oxidation (above sulfur dew-point). The metal-free carbon monolith has a high 29



Page 30 of 37 30

specific surface area (> 360 m2g-1) with easily tuned porosities through the decomposition rate of the organic compounds by changing the calcination temperature. The excellent desulfurization performance was attributed the abundant connected meso- and macroporous network and large amount of surface structural defects decorated with nitrogen active sites. The N-C/CNT monolith displays an extremely high desulfurization performance of 449 and 490 gsulfurkgcat.-1h-1 under O2-to-H2S ratio of 2.5 and 1.5, respectively, at relatively high gaseous velocity (WHSV of 0.6 h-1) and high H2S concentration (10000 ppm). The relative sulfur areal rate is maintained as the similar value when the pore size and pore volume is higher than 3.9 nm and 0.32 cm3g-1, respectively, despite of the different O2-to-H2S molar ratios. The optimized catalyst (N-C/CNT450 800) with medium amount of defect density and nitrogen content/pyridinic N species displays the highest sulfur areal formation rate (λSSA: 34.1 molsulfurm-2h-1). Furthermore, such metal-free monolithic catalyst exhibits an extremely long-term stability under different O2-to-H2S ratios and steam contents, with superior sulfur formation rate compared to the state-of-the-art Fe2O3/SiC catalyst and carbon-based composites. It is also remarkable that the desulfurization activity can be easily recovered through simple thermal treatment under He gas at 400 oC to remove solid sulfur covered on the active sites.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Figure S1, SEM images of the N-C/CNT monoliths; Figure S2: TEM images of pristine CNT; Figure S3: XRD patterns; Figure S4: nitrogen adsorption-desorption isotherms and pore size distribution of the samples; Figure S5: TG analyses of the N-C/CNT monoliths; Figures S6 and S7: C1s 30


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and O1s high resolution XPS spectra; Figures S8 and S9: Raman spectra and the defect density as a function of sulfur areal formation rate; Figure S10: EDS mapping and SEM images of the N-C/CNT450 800; Table S1, basic information about Pristine CNTs; Table S2, mechanical strengths of N-C/CNT450 800; Tables S3 and S4, C1s and O1s high resolution XPS spectra; Table S5, ICP-MS analysis data; Table S6, XPS results of the N-C/CNT monolith (PDF)

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21606243 and 91645117), and Talents Innovation Project of Dalian City (2017RQ032, 2016RD04). Y. Liu also thanks the financial support from CAS Youth Innovation Promotion Association (2018220). Mr. Yang Zhao (DICP) is acknowledged for performing the TEM analysis.

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References 1. Zhang, X.; Tang, Y.; Qiao, N.; Li, Y.; Qu, S.; Hao, Z., Comprehensive study of H2S selective catalytic oxidation on combined oxides derived from Mg/Al-V10O28 layered double hydroxides. Applied Catalysis B: Environmental 2015, 176-177, 130-138. 2. Ba, H.; Duong-Viet, C.; Liu, Y.; Nhut, J.-M.; Granger, P.; Ledoux, M. J.; Pham-Huu, C., Nitrogen-doped carbon nanotube spheres as metal-free catalysts for the partial oxidation of H2S. Comptes Rendus Chimie 2016, 19 (10), 1303-1309. 3. Zhang, X.; Tang, Y.; Qu, S.; Da, J.; Hao, Z., H2S-Selective Catalytic Oxidation: Catalysts and Processes. ACS Catalysis 2015, 5 (2), 1053-1067. 4. Chizari, K.; Deneuve, A.; Ersen, O.; Florea, I.; Liu, Y.; Edouard, D.; Janowska, I.; Begin, D.; Pham-Huu, C., Nitrogen-Doped Carbon Nanotubes as a Highly Active Metal-Free Catalyst for Selective Oxidation. ChemSusChem 2012, 5 (1), 102-108. 5. Eow John, S., Recovery of sulfur from sour acid gas: A review of the technology. Environmental Progress 2004, 21 (3), 143-162. 6. Garcia-Arriaga, V.; Alvarez-Ramirez, J.; Amaya, M.; Sosa, E., H2S and O2 influence on the corrosion of carbon steel immersed in a solution containing 3M diethanolamine. Corrosion Science 2010, 52 (7), 2268-2279. 7. Laperdrix, E.; Costentin, G.; Saur, O.; Lavalley, J. C.; Nédez, C.; Savin-Poncet, S.; Nougayrède, J., Selective Oxidation of H2S over CuO/Al2O3: Identification and Role of the Sulfurated Species formed on the Catalyst during the Reaction. Journal of Catalysis 2000, 189 (1), 63-69. 8. Ledoux, M. J.; Pham-Huu, C.; Keller, N.; Nougayrède, J.-B.; Savin-Poncet, S.; Bousquet, J., Selective oxidation of H2S in Claus tail-gas over SiC supported NiS2 catalyst. Catal Today 2000, 61 (1–4), 157-163. 9. PiÉPlu, A.; Saur, O.; Lavalley, J.-C.; Legendre, O.; NÉDez, C., Claus Catalysis and H2S Selective Oxidation. Catalysis Reviews 1998, 40 (4), 409-450. 10. Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L., Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323 (5915), 760-764. 11. Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L., Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4 (3), 1321-1326. 12. Tuci, G.; Zafferoni, C.; D’Ambrosio, P.; Caporali, S.; Ceppatelli, M.; Rossin, A.; Tsoufis, T.; Innocenti, M.; Giambastiani, G., Tailoring Carbon Nanotube N-Dopants while Designing Metal-Free Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Medium. ACS Catalysis 2013, 3 (9), 2108-2111. 13. Wang, D.-W.; Su, D., Heterogeneous nanocarbon materials for oxygen reduction reaction. Energy & Environmental Science 2014, 7 (2), 576-591. 14. Liu, Y.; Ba, H.; Luo, J.; Wu, K.-H.; Nhut, J.-M.; Su, D. S.; Pham-Huu, C., Structure-performance relationship of nanodiamonds @ nitrogen-doped mesoporous carbon in the direct dehydrogenation of ethylbenzene. Catalysis Today 2018, 301, 38-47. 15. Zhang, Y.; Wang, J.; Rong, J.; Diao, J.; Zhang, J.; Shi, C.; Liu, H.; Su, D., A 32


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Facile and Efficient Method to Fabricate Highly Selective Nanocarbon Catalysts for Oxidative Dehydrogenation. ChemSusChem 2017, 10 (2), 353-358. 16. Zhao, Z.; Dai, Y.; Ge, G., Nitrogen-doped nanotubes-decorated activated carbon-based hybrid nanoarchitecture as a superior catalyst for direct dehydrogenation. Catalysis Science & Technology 2015, 5 (3), 1548-1557. 17. Villa, A.; Tessonnier, J. P.; Majoulet, O.; Su, D. S.; Schlögl, R., Transesterification of Triglycerides Using Nitrogen ‐ Functionalized Carbon Nanotubes. ChemSusChem 2010, 3 (2), 241-245. 18. Wang, J.; Huang, R.; Zhang, Y.; Diao, J.; Zhang, J.; Liu, H.; Su, D., Nitrogen-doped carbon nanotubes as bifunctional catalysts with enhanced catalytic performance for selective oxidation of ethanol. Carbon 2017, 111, 519-528. 19. Wang, J.; Liu, H.; Gu, X.; Wang, H.; Su, D. S., Synthesis of nitrogen-containing ordered mesoporous carbon as a metal-free catalyst for selective oxidation of ethylbenzene. Chemical Communications 2014, 50 (65), 9182-9184. 20. Navalon, S.; Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H., Carbocatalysis by Graphene-Based Materials. Chemical Reviews 2014, 114 (12), 6179-6212. 21. Su, D. S.; Perathoner, S.; Centi, G., Nanocarbons for the Development of Advanced Catalysts. Chemical Reviews 2013, 113 (8), 5782-5816. 22. Su , D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.; Paraknowitsch, J.; Schlögl, R., Metal ‐ Free Heterogeneous Catalysis for Sustainable Chemistry. ChemSusChem 2010, 3 (2), 169-180. 23. Li, X.; Li, P.; Pan, X.; Ma, H.; Bao, X., Deactivation mechanism and regeneration of carbon nanocomposite catalyst for acetylene hydrochlorination. Applied Catalysis B: Environmental 2017, 210, 116-120. 24. Zhang, Z.; Wang, J.; Li, W.; Wang, M.; Qiao, W.; Long, D.; Ling, L., Millimeter-sized mesoporous carbon spheres for highly efficient catalytic oxidation of hydrogen sulfide at room temperature. Carbon 2016, 96, 608-615. 25. Zhang, Z.; Jiang, W.; Long, D.; Wang, J.; Qiao, W.; Ling, L., A General Silica-Templating Synthesis of Alkaline Mesoporous Carbon Catalysts for Highly Efficient H2S Oxidation at Room Temperature. ACS Applied Materials & Interfaces 2017, 9 (3), 2477-2484. 26. Chen, Q.; Wang, J.; Liu, X.; Li, Z.; Qiao, W.; Long, D.; Ling, L., Structure-dependent catalytic oxidation of H2S over Na2CO3 impregnated carbon aerogels. Microporous and Mesoporous Materials 2011, 142 (2), 641-648. 27. Long, D.; Chen, Q.; Qiao, W.; Zhan, L.; Liang, X.; Ling, L., Three-dimensional mesoporous carbon aerogels: ideal catalyst supports for enhanced H2Soxidation. Chemical Communications 2009, (26), 3898-3900. 28. Sun, F.; Liu, J.; Chen, H.; Zhang, Z.; Qiao, W.; Long, D.; Ling, L., Nitrogen-Rich Mesoporous Carbons: Highly Efficient, Regenerable Metal-Free Catalysts for Low-Temperature Oxidation of H2S. ACS Catalysis 2013, 3 (5), 862-870. 29. Duong-Viet, C.; Liu, Y.; Ba, H.; Truong-Phuoc, L.; Baaziz, W.; Nguyen-Dinh, L.; Nhut, J.-M.; Pham-Huu, C., Carbon nanotubes containing oxygenated decorating defects as metal-free catalyst for selective oxidation of H2S. Appl Catal B: Environ 2016, 191, 29-41. 33



Page 34 of 37 34

30. Ba, H.; Liu, Y.; Truong-Phuoc, L.; Duong-Viet, C.; Mu, X.; Doh, W. H.; Tran-Thanh, T.; Baaziz, W.; Nguyen-Dinh, L.; Nhut, J.-M.; Janowska, I.; Begin, D.; Zafeiratos, S.; Granger, P.; Tuci, G.; Giambastiani, G.; Banhart, F.; Ledoux, M. J.; Pham-Huu, C., A highly N-doped carbon phase “dressing” of macroscopic supports for catalytic applications. Chemical Communications 2015, 51 (76), 14393-14396. 31. Feng, L.; Liu, Y.; Jiang, Q.; Liu, W.; Wu, K.-H.; Ba, H.; Pham-Huu, C.; Yang, W.; Su, D. S., Nanodiamonds @ N, P co-modified mesoporous carbon supported on macroscopic SiC foam for oxidative dehydrogenation of ethylbenzene. Catalysis Today 2019, doi.org/10.1016/j.cattod.2019.02.046. 32. Sing, K. S. W., Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984). In Pure and Applied Chemistry, 1985; Vol. 57, pp 603-619. 33. Zhong, H.-x.; Wang, J.; Zhang, Y.-w.; Xu, W.-l.; Xing, W.; Xu, D.; Zhang, Y.-f.; Zhang, X.-b., ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angewandte Chemie International Edition 2014, 53 (51), 14235-14239. 34. Kundu, S.; Nagaiah, T. C.; Xia, W.; Wang, Y.; Dommele, S. V.; Bitter, J. H.; Santa, M.; Grundmeier, G.; Bron, M.; Schuhmann, W.; Muhler, M., Electrocatalytic Activity and Stability of Nitrogen-Containing Carbon Nanotubes in the Oxygen Reduction Reaction. The Journal of Physical Chemistry C 2009, 113 (32), 14302-14310. 35. Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K., Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. Nature Communications 2013, 4, 2390. 36. Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. W. T.; Hor, T. S. A.; Zong, Y.; Liu, Z., Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts. ACS Catalysis 2015, 5 (8), 4643-4667. 37. Liu, S.; Tian, J.; Wang, L.; Zhang, Y.; Qin, X.; Luo, Y.; Asiri Abdullah, M.; Al‐ Youbi Abdulrahman, O.; Sun, X., Hydrothermal Treatment of Grass: A Low‐Cost, Green Route to Nitrogen ‐ Doped, Carbon ‐ Rich, Photoluminescent Polymer Nanodots as an Effective Fluorescent Sensing Platform for Label‐Free Detection of Cu(II) Ions. Advanced Materials 2012, 24 (15), 2037-2041. 38. Li, C.; Zhao, A.; Xia, W.; Liang, C.; Muhler, M., Quantitative Studies on the Oxygen and Nitrogen Functionalization of Carbon Nanotubes Performed in the Gas Phase. The Journal of Physical Chemistry C 2012, 116 (39), 20930-20936. 39. Keller, N.; Pham-Huu, C.; Crouzet, C.; Ledoux, M. J.; Savin-Poncet, S.; Nougayrede, J.-B.; Bousquet, J., Direct oxidation of H2S into S. New catalysts and processes based on SiC support. Catal Today 1999, 53 (4), 535-542. 40. Nguyen, P.; Nhut, J.-M.; Edouard, D.; Pham, C.; Ledoux, M.-J.; Pham-Huu, C., Fe2O3/β-SiC: A new high efficient catalyst for the selective oxidation of H2S into elemental sulfur. Catalysis Today 2009, 141 (3), 397-402. 41. Cuong, D.-V.; Truong-Phuoc, L.; Tran-Thanh, T.; Nhut, J.-M.; Nguyen-Dinh, L.; Janowska, I.; Begin, D.; Pham-Huu, C., Nitrogen-doped carbon nanotubes decorated silicon carbide as a metal-free catalyst for partial oxidation of H2S. Applied Catalysis 34


Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A: General 2014, 482, 397-406. 42. Duong-Viet, C.; Ba, H.; Liu, Y.; Truong-Phuoc, L.; Nhut, J.-M.; Pham-Huu, C., Nitrogen-doped carbon nanotubes on silicon carbide as a metal-free catalyst. Chinese Journal of Catalysis 2014, 35 (6), 906-913. 43. Liu, Y.; Duong-Viet, C.; Luo, J.; Hébraud, A.; Schlatter, G.; Ersen, O.; Nhut, J.-M.; Pham-Huu, C., One-Pot Synthesis of a Nitrogen-Doped Carbon Composite by Electrospinning as a Metal-Free Catalyst for Oxidation of H2S to Sulfur. ChemCatChem 2015, 7 (18), 2957-2964. 44. Kamali, F.; Eskandari, M. M.; Rashidi, A.; Baghalha, M.; Hassanisadi, M.; Hamzehlouyan, T., Nanorod carbon nitride as a carbo catalyst for selective oxidation of hydrogen sulfide to sulfur. Journal of Hazardous Materials 2019, 364, 218-226. 45. Xu, Z.; Duong-Viet, C.; Ba, H.; Li, B.; Truong-Huu, T.; Nguyen-Dinh, L.; Pham-Huu, C., Gaseous Nitric Acid Activated Graphite Felts as Hierarchical Metal-Free Catalyst for Selective Oxidation of H2S. Catalysts 2018, 8 (4), 145. 46. Shen, L.; Lei, G.; Fang, Y.; Cao, Y.; Wang, X.; Jiang, L., Polymeric carbon nitride nanomesh as an efficient and durable metal-free catalyst for oxidative desulfurization. Chem Commun 2018, 54 (20), 2475-2478. 47. Bashkova, S.; Baker, F. S.; Wu, X.; Armstrong, T. R.; Schwartz, V., Activated carbon catalyst for selective oxidation of hydrogen sulphide: on the influence of pore structure, surface characteristics, and catalytically-active nitrogen. Carbon 2007, 45 (6), 1354-1363. 48. Bandosz, T. J., On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures. J Colloid Interf Sci 2002, 246 (1), 1-20. 49. Frank, E.; Steudle, L. M.; Ingildeev, D.; Sporl, J. M.; Buchmeiser, M. R., Carbon Fibers: Precursor Systems, Processing, Structure, and Properties. Angew Chem Int Ed 2014, 53 (21), 5262-5298. 50. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R., Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett 2010, 10 (3), 751-758. 51. Osswald, S.; Havel, M.; Gogotsi, Y., Monitoring oxidation of multiwalled carbon nanotubes by Raman spectroscopy. Journal of Raman Spectroscopy 2007, 38 (6), 728-736. 52. Chizari, K.; Janowska, I.; Houllé, M.; Florea, I.; Ersen, O.; Romero, T.; Bernhardt, P.; Ledoux, M. J.; Pham-Huu, C., Tuning of nitrogen-doped carbon nanotubes as catalyst support for liquid-phase reaction. Appl Catal A: Gen 2010, 380 (1–2), 72-80. 53. Cancado, L. G.; Jorio, A.; Ferreira, E. H. M.; Stavale, F.; Achete, C. A.; Capaz, R. B.; Moutinho, M. V. O.; Lombardo, A.; Kulmala, T. S.; Ferrari, A. C., Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies. Nano Lett 2011, 11 (8), 3190-3196. 54. Martins Ferreira, E. H.; Moutinho, M. V. O.; Stavale, F.; Lucchese, M. M.; Capaz, R. B.; Achete, C. A.; Jorio, A., Evolution of the Raman spectra from single-, few-, and many-layer graphene with increasing disorder. Physical Review B 2010, 82 (12), 125429. 35



Page 36 of 37 36

55. Jorio, A.; Lucchese, M. M.; Stavale, F.; Martins Ferreira, E. H.; Moutinho, M. V. O.; Capaz, R. B.; Achete, C. A., Raman study of ion-induced defects inN-layer graphene. Journal of Physics: Condensed Matter 2010, 22 (33), 334204. 56. Luo, J.; Liu, Y.; Wei, H.; Wang, B.; Wu, K.-H.; Zhang, B.; Su, D. S., A green and economical vapor-assisted ozone treatment process for surface functionalization of carbon nanotubes. Green Chemistry 2017, 19 (4), 1052-1062. 57. Tao, L.; Wang, Q.; Dou, S.; Ma, Z.; Huo, J.; Wang, S.; Dai, L., Edge-rich and dopant-free graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction. Chem Commun 2016, 52 (13), 2764-2767. 58. Qiao, M.; Tang, C.; He, G.; Qiu, K.; Binions, R.; Parkin, I.; Zhang, Q.; Guo, Z.; Titirici, M., Graphene/nitrogen-doped porous carbon sandwiches for the metal-free oxygen reduction reaction: conductivity versus active sites. J Mater Chem A 2016, 4 (32), 12658-12666. 59. Primavera, A.; Trovarelli, A.; Andreussi, P.; Dolcetti, G., The effect of water in the low-temperature catalytic oxidation of hydrogen sulfide to sulfur over activated carbon. Appl Catal A: Gen 1998, 173 (2), 185-192. 60. Wen, G.; Gu, Q.; Liu, Y.; Schlögl, R.; Wang, C.; Tian, Z.; Su, D. S., Biomass-Derived Graphene-like Carbon: Efficient Metal-Free Carbocatalysts for Epoxidation. Angewandte Chemie International Edition 2018, 57 (51), 16898-16902.

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Carbon Nanotubes Monolithic

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