Carbon Nanotubes Monolithic

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Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3780−3792

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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 Dangsheng Su*,‡ †

School of Chemical Engineering, Sichuan University, 610065 Chengdu, China Dalian National Laboratory for Clean Energy (DNL), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, 116023 Dalian, China § School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia ∥ Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES), UMR 7515 CNRS, University of Strasbourg, 25 rue Becquerel, Cedex 02, 67087 Strasbourg, France

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S Supporting Information *

ABSTRACT: 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 a reaction temperature higher than the dew point (>180 °C) 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, and 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 gsulfur kgcat.−1 h−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

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 process (2H2S + SO2 → (3/n)Sn + 2H2O), transforming toxic H2S into useful elemental sulfur. However, because of 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 to remove H2S to the best extent in the off-gas before it is released to the atmosphere.5−9 Selectively catalytic oxidation of H2S to elemental sulfur (2H2S + O2 → (2/n)Sn + © 2019 American Chemical Society

2H2O) is also an effective way for the treatment of diluted H2S-containing gases to reduce H2S to 180 °C) on a nitrogendoped carbon nanotube decorated silicon carbide (N-CNT/ SiC) catalyst, which demonstrates superior desulfurization catalytic activity as compared to metal-based catalysts (FeOx/ SiC). Ba et al.30 also reported a thin N-doped carbon layer coated on macroscopically shaped supports (β-SiC and αAl2O3) for high-temperature H2S oxidation reactions. Such a bulk catalyst could exhibit >97% of H2S conversion with a sulfur selectivity of 70% at a reaction temperature of 210 °C. 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 agent,2 the N-CNT beads show a relative low desulfurization performance compare to the initial N-CNT due to a partial 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 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 the 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 the N-C/CNT monolith in different macroscopic sizes and

advantages of carbon-based nanomaterials as efficient metalfree 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 carbon derived from PDA/SiC can directly catalyze acetylene hydrochlorination in the 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 electrondonating 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 performed either at a low reaction temperature (180 °C) 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 noncontinuous process. In a continuous desulfurization process (T > 180 °C), on the other hand, elemental sulfur remains in the vapor phase and only condenses at the exit of the 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 3781

DOI: 10.1021/acsanm.9b00654 ACS Appl. Nano Mater. 2019, 2, 3780−3792

Article

ACS Applied Nano Materials

Thermogravimetric analysis was performed under air (20 mL/min) with a heating rate of 10 °C/min on an STA 449 F3 from room temperature to 900 °C. XPS analyses were performed on a ESCALAB 250Xi photoelectron spectroscope equipped with an Al Kα X-ray source (hν = 1486.6 eV). Peak deconvolution is realized with the “Avantage” program from Thermoelectron Co. 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 performed using a RENISHAW inVia Raman microscope equipped with a CCD detector. Elemental analyses were performed by using inductively coupled plasma−mass spectrometry (ICP-MS) using a NexION 300D. A 20 mg sample and 20 mL of concentrated nitric acid were mixed into a hydrothermal autoclave and then maintained at 180 °C 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 performed 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):

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 °C) in the continuous mode. Herein, we report on the use of N-C/CNT monoliths as metalfree H2S selective oxidation catalysts for recovering sulfur from a synthetic H2S polluted off-gas (Scheme 1). The detail parameters such as porosity, defect structure, nitrogen content, and nitrogen species nature as well as the reaction parameters such as the O2/H2S ratio and the 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-ofthe-art carbon-based metal-free and conventional metal-based FeOx/SiC catalysts.

2. EXPERIMENTAL SECTION 2.1. Materials. The analytical grade D-glucose (C6H12O6, 99%) and citric acid (C6H8O7, 99.5%) were supplied by Alfa Aesar, and ammonium carbonate ((NH4)2CO3) was purchased from Sinopharm Chemical Reagent Co. Ltd. The chemical reagents were used without further purification. The multiwalled carbon nanotubes (CNT) were supplied by Shandong Dazhan Nanomaterials Co. Ltd. The CNT were further purified by using 6 M HCl acid aqueous solution at 80 °C for 12 h, then washed using deionized water and ethanol until neutral pH, and dried at 60 °C for 24 h. The obtained sample is denoted as pristine CNT, and the basic data are 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 of D-glucose, 6 g of citric acid, and 1 g of CNT were physically mixed and finely ground at room temperature. 3 g of 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 °C in air for 5 h. The obtained macrocomposite was then subjected to calcination at different temperatures from 350, 400, and 450 °C at 2 °C/min in air and treated for 3 h and further annealed at different temperatures (700, 800, and 900 °C, respectively) for 2 h under an argon atmosphere with a heating rate of 5 °C/min. The resulted 3D porous N-C/CNT monolith can be directly used as a catalyst, namely, N-C/CNTYX, where X refers to the calcined temperature in air flow (350, 400, and 450 °C) and Y is used for the annealed temperature (700, 800, and 900 °C) under Ar gas. 2.3. Characterization. The specific surface area and porosity of the samples were measured by the 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 °C for 6 h to desorb moisture and impurities from their surfaces. The pore size distributions were calculated using the Barrett−Joyner−Halenda (BJH) model from the desorption branch. 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 performed 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.

H 2S +

1 1 O2 → Sn + H 2O 2 n

1 Sn + O2 → SO2 n H 2S +

ΔH = − 222 kJ/mol

ΔH = −297 kJ/mol

3 O2 → SO2 + H 2 2

ΔH = − 381 kJ/mol

(1) (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 centered 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 rates were monitored by a mass flow controller. Before the catalyst bed was heated, the reactor was flushed with helium at room temperature until no trace of oxygen was detected at the outlet. The helium flow was replaced with 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 °C. 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 180 °C) of Sulfur over Different Metal-Free Catalysts Reported in the Literature catalysts

T (°C)

H2S (vol %)

O2 (vol %)

WHSV (h−1)

XH2 S(%)

SS(%)

λcat. (gsulfur kgcat.−1 h−1)

ref

N-C/CNT450 800 N-C/CNT450 800 N-CNT/SiC foam N-CNT N-CNT/SiC extrudate N@CF-800 O-CNT-250−24 A N@C/SiCE2 A N@C/SiCE2 N-CNT beads MCNR OGFs-16 CNM-600

190 190 190 190 190 230 190 190 210 210 190 250 180

1 1 1 1 1 1 1 1 1 1 0.2 1 0.5

2.5 1.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 0.1 2.5 0.25

0.6 0.6 0.72 0.32 0.32 0.6 0.6 0.3 0.3 0.3 1200 h−1 (GHSV) 0.1 3000 (mL g−1 h−1)

99.4 98.4 99.8 91 95.8 57 50 86.3 >97 99.1 99.8 98 ≈ 96

79.2 87.2 75 75 74.1 95 90 77.2 70 61.6 88.8 86 ≈ 97

449 490 113 205 100 306 254 376 383 47 − 79 40

this work this work 4 4 41, 42 43 29 30 30 2 44 45 46

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 NC/CNT monolith displays a high desulfurization activity compared to that of the Fe2O3/SiC catalyst under the operating 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 summarizes the desulfurization performance of recently reported metal-free carbon-based catalysts and compared with the optimized monolithic N-C/CNT composite. The N-C/CNT450 800 sample presents a superior sulfur formation rate in terms of kilogram of catalyst (λcat., gsulfur kgcat.−1 h−1) compared to the other carbon-based catalysts operated at temperature above the dew point of sulfur (>180 °C). Such results demonstrated once again the high desulfurization catalytic efficiency of the monolithic N-C/ CNT composites. 3.3. Influence of Structural Parameters on the Desulfurization Activities over N-C/CNT Monoliths. Influence of Pore Structure. The porosity of the carbonbased catalyst certainly plays an important role in desulfurization reaction in either a continuous model or discontinuous model. 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, molsulfur m−2 h−1), is performed and displayed in 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 cm3 g−1, respectively, despite the different O2-to-H2S molar ratios (ratios 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 the high pore volume of the micropore results in superior sulfur adsorption capacities.47 The sulfur could be condensed in the micropores of the catalyst in discontinuous model,48 where the high-energy centers appear in the micropores. Furthermore, in the continuous model H2S will be strongly adsorbed in the micropores, which results in the overoxidation reaction to SO2 and pore plugging during the reaction, inhibiting interaction of reactants and the active sites as a function of time on

Figure 5. Relationship diagram of sulfur formation rate per specific surface area of the catalysts with pore volume and pore diameter.

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. 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 thermal treatment temperature (i.e., 700, 800, and 900 °C). 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 and 1590 cm−1, respectively, are presented in the Raman spectra of all samples. The D band is attributed to the disordered sp2hybridized 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 °C 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 °C (Figure S9a), confirming a better graphitic nature of the N-C/CNT monolith at higher annealing temperature. 3787

DOI: 10.1021/acsanm.9b00654 ACS Appl. Nano Mater. 2019, 2, 3780−3792

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Figure 6. (a, b) Catalytic desulfurization performance and stability over the N-C/CNT450 800 in the presence of water steam. Reaction conditions: O2to-H2S molar ratio = 2.5, WHSV = 1.2 h−1, and mass of catalyst = 75 mg.

450 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/CNT800 after desulfurization test. (c) Schematic model of sulfur composite sites on the carbon framework for the spent N-C/CNT450 catalyst. (d) High800 resolution N 1s spectra and (e) the content of nitrogen species of N-C/CNT450 800 before and after the desulfurization test and regenerated by a thermal treatment.

value is fixed at 2.33 eV to calculate the related defect densities (nD) according to Cançado’s equation. For a better understanding of the influence of the graphitization degree and surface structural defects for the desulfurization activity, the defect density is correlated to sulfur areal formation rate (λSSA). It is found in Figure S9b that the appropriate 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

Furthermore, the TGA data (Figure S5) indicate that the oxidation temperature of samples slightly increases with the raising of annealing temperature from 700 to 900 °C, reflecting the better crystallization of the N-doped carbon layer at higher annealing temperature. The density of the structural defects could calculated by eq 8 according to Cançado’s work from the Raman spectra.53

ij I yz nD (cm−2) = (7.3 ± 2.2) × 109E L 4jjj D zzz j IG z (8) k { where EL is expressed as the excitation energy. The value of EL is fixed around 2.41 eV for 2D graphene54,55 and 2.33 eV for CNT with abundant surface defects.29,56 In this work, the EL

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DOI: 10.1021/acsanm.9b00654 ACS Appl. Nano Mater. 2019, 2, 3780−3792

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fitted with three peaks (Figure 7b), where the peak position sites at 163.9 eV (S 2p3/2) and 165.1 eV (S 2p1/2) are assigned to the sulfide 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 the 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 decreases to 2.93 at. % in the spent catalyst (4.21 at. % for fresh sample), which is mainly 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,e. It can be clearly seen that the pyridinic N (N1 in Figure 7e) and graphitic N (N3 in Figure 7e) of the 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 increases from 0.89 to 1.06 at. %. Meanwhile, the total N content of the N-C/CNT450 800 catalyst increases to 3.37 at. % after thermal treatment at 400 °C in helium by removing the most of deposited sulfur species but does not reach back to the value of the fresh catalyst. The pyridinic N increases to 1.45 at. % accordingly, whereas the pyrrolic and graphitic N species of regenerated sample maintain similar values as the spent catalyst (Figure 7e). Such a phenomenon reveals that most sulfur species are more favored 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 the desulfurization process was performed at relatively low reaction temperature (190 °C) in this study, which may not be able to fully vaporize the solid sulfur generated during the reaction. However, according to the desulfurization reaction on the N-C/CNT monolith, which lasted for 70 h, in Figure 4a, there is no significant deactivation that takes place as a function of time on stream, and thus one should expect that the formed sulfur was mostly removed from catalyst surface during the reaction by the presence of water vapor.28 High-resolution SEM micrographs in Figure S10b,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. The desulfurization activities on second cycling test at 190 °C maintains a 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 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.

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 and Figure 2f). When the sample is annealed at a 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 present 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 the presence of water film on the surface of carbon catalyst, the O2 molecules are adsorbed favorably 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 a WHSV of 1.2 h−1, which even displays slightly higher values compared to the H2S conversion in the presence of 30 vol % of steam (H2S conversion of 97.4% and sulfur selectivity of 83.2%). No significant influence of steam addition can be seen in this catalyst system. Moreover, the catalyst survived 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 an O2• radical.46,60 3.4. Regeneration and Cycling Test on the N-C/CNT Monolith. In the continuous desulfurization process the catalyst operated at temperature higher than the sulfur dew point, i.e., 180 °C. 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 evidence the presence of elemental sulfur on the surface of the N-C/CNT monolith after desulfurization test at 190 °C (Figure S10a). High-resolution SEM was further conducted (Figure S10b,c), and it can be found that sulfur aggregation was located on the top surface of the 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. The spent catalyst was further regenerated (400 °C in helium) to remove the deposited sulfur and was evaluated again in desulfurization reaction. The surface chemical composition of the spent and regenerated the 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 3789

DOI: 10.1021/acsanm.9b00654 ACS Appl. Nano Mater. 2019, 2, 3780−3792

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C/CNT450 800; Tables S3 and S4: C 1s and O 1s highresolution XPS spectra; Table S5: ICP-MS analysis data; Table S6: XPS results of the N-C/CNT monolith (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (C.J.). *E-mail: [email protected] (D.S.). Figure 8. Cycling test and desulfurization performance of N-C/ CNT450 800 at 190 °C. Reaction conditions: O2-to-H2S molar ratio = 2.5, WHSV = 0.6 h−1, and steam concentration = 30 vol %.

ORCID

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 the sulfur dew point). The metal-free carbon monolith has a high specific surface area (>360 m2 g−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 a large amount of surface structural defects decorated with nitrogen active sites. The NC/CNT monolith displays an extremely high desulfurization performance of 449 and 490 gsulfur kgcat.−1 h−1 under an O2-toH2S ratio of 2.5 and 1.5, respectively, at a relatively high gaseous velocity (WHSV of 0.6 h−1) and a high H2S concentration (10000 ppm). The relative sulfur areal rate is maintained as the similar value when the pore size and pore volume are higher than 3.9 nm and 0.32 cm3 g−1, respectively, despite the different O2-toH2S 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 molsulfur m−2 h−1). Furthermore, such metalfree monolithic catalyst exhibits an extremely long-term stability under different O2-to-H2S ratios and steam contents, with superior sulfur formation rate compared to those of 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 °C to remove solid sulfur covered on the active sites.

Notes

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Yuefeng Liu: 0000-0001-9823-3811 Kuang-Hsu Wu: 0000-0002-7670-7948 The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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.L. 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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00654. 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: C 1s and O 1s 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 450 N-C/CNT800 ; Table S1: basic information about pristine CNTs; Table S2: mechanical strengths of N3790

DOI: 10.1021/acsanm.9b00654 ACS Appl. Nano Mater. 2019, 2, 3780−3792

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