Nitrogen-doped carbonaceous materials obtained by CVD process from mesoporous silica for sulfides oxidation

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Nitrogen-doped carbonaceous materials obtained by CVD process from mesoporous silica for sulfides oxidation Luisa E. Milagre, Vitor F. Almeida, Sara S. Vieira, Tatiana A. Ribeiro-Santos, ⁎ ⁎ Mateus C. Monteiro de Castro, Maria Helena Araujo , Ana Paula C. Teixeira Departamento de Química, ICEx, Universidade Federal de Minas Gerais, UFMG, Belo Horizonte, MG 31270-901, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Nitrogen-doped carbon Pyridinic groups Sulfide oxidation

In this work, we describe the synthesis of N doped carbon nanomaterials from mesoporous silica impregnated with iron (10 wt%), using acetonitrile as carbon and nitrogen source by CVD process at 600, 650, 700 and 800 °C. The obtained materials showed a N/C content between 8.3–10.1% and a carbon content between 14 and 31%. Results of TEM, TG, elemental analysis, Raman spectroscopy and XRD showed that higher temperatures synthesis has led to the formation of high content and more organized carbon nanostructures, i.e., carbon nanotubes and bamboo-like structures with higher nitrogen contents in their structure. On the other hand, XPS results showed that the carbon structures obtained at 800 °C have preferential the presence of less reactive nitrogen species, i.e. graphitic nitrogen, while the materials obtained at 600 and 650 °C preferentially have the presence of more reactive nitrogen groups such as pyridine. These materials were used as catalysts for the sulfides oxidation. The type of nitrogen group present in the carbon structure influenced directly the activity of the materials. The catalysts obtained at lower temperatures showed the best kinetics of oxidation sulfides, which was related to the presence of a higher content of more reactive nitrogen groups.

1. Introduction Hydrogen sulfide gas has been a growing problem, it is toxic to humans and the environment and causes economic problems since it is also corrosive to concrete and steel increasing maintenance costs [1,2]. Wastewater treatment plants, food processing, oil refinery, paper production, these segments of industry generate residues containing hydrogen sulfide, that can cause the acidification of river and lakes, poison automobile catalysts and contribute to acid rain [3,4]. Therefore, due to the increased oversight of environmental agencies to limit the concentration of this pollutant, new technologies for pollutants treatment have been researched to develop more efficient methods for the conversion of this sulfides into less harmful molecules [1,3,5]. Most of the methods for sulfide treatment used these days are based on adsorption of the sulfide species, the adsorbents work with a low efficiency, require further treatment to be properly disposed and generate a considerable volume of residues. Other methods such as catalytic incineration and biological oxidation treatment [2,4,6] are used to remediate this problem. Our research group has been focusing its attention to the sulfide oxidation for some time, working with methods to treat this pollutant

⁎

based upon activated carbon [1,7], and other carbonaceous materials as graphite and graphene [5]. In the literature has also been reported the use of nitrogen-rich mesoporous carbon materials for H2S oxidation into sulfur [8,9]. Carbonaceous materials have been used in research in several areas, such as catalysis, adsorption of environmental contaminants, fuel cells and among them the oxidation of sulfides [10–14]. In this work, a nitrogen doped carbon nanomaterial was synthesized and tested for sulfide ion treatment in aqueous media. The materials were obtained from a mesoporous silica containing CTAB in its structure, in the presence of an iron catalyst and a secondary carbon source containing nitrogen in its structure by CVD (Chemical Vapor Deposition) process. The use of CVD method has increasing and it is considered a practical and inexpensive method for large scale synthesis [15,16]. A fixed bed system was used for the synthesis of the materials that are composed of three parts: a matrix (SiO2) which will be important for the dispersion of the materials in aqueous media, iron magnetic cores, which allows the carbon deposition to form carbon nanostructures and gives the magnetic properties to materials [17], and a nitrogen-doped carbon nanostructures that will be important for the oxidation of the sulfide [18].

Corresponding author. E-mail address: [email protected] (A.P.C. Teixeira).

https://doi.org/10.1016/j.cattod.2018.10.025 Received 29 April 2018; Received in revised form 18 September 2018; Accepted 15 October 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Milagre, L.E., Catalysis Today, https://doi.org/10.1016/j.cattod.2018.10.025

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2. Experimental

0.1 eV and 20 eV of the energy of passage was used [22]. Elemental analysis of carbon, nitrogen, and hydrogen were performed in a CHN Perkin Elmer analyzer. Transmission electron microscopy (TEM) images were obtained in a Tecnai G2-20 − SuperTwin FEI − 200 kV Transmission Electron Microscope. The powder samples were dispersed in acetone and deposited on a copper-coated grid. The specific surface areas (BET) of the samples were analyzed by adsorption of N2 at 77 K using the Autosorb1MP Quantachrome equipment. Samples were degassed at 150 °C for 24 h prior to analysis.

2.1. Synthesis and characterization of materials For the synthesis of the MCM-41, 54 mL of NaOH (1 mol L−1 − Vetec) solution, were added to the 3.94 g of CTA+Br− (hexadecyltrimethylammonium bromide − Merck) and 108 mL of deionized water at room temperature. To this solution was added 20 mL of TEOS (tetraethyl orthosilicate − Merck). The solution was kept under magnetic stirring for 24 h. After this time the material was washed until the electric conductivity was below 10 μS cm−1, and dried in an oven at 60 °C for 12 h [19]. In this work, this material was called M52 [20]. To obtain a material impregnated with 10 wt% of iron, 2 g of the M52 material and a solution of iron chloride (0.2 g of FeCl3·6H2O, Synth, in 50 mL of distilled water) were placed in a beaker and kept under heat (60 °C) and magnetic stir until all the water evaporated. The material was then named M52_10Fe. The carbon nanomaterials (CNs) were synthesized through the CVD process using two carbon sources, the CTA+Br− present in the M52_10Fe structure and acetonitrile as a secondary source. To form the CNs, 200 mg of the M52_10Fe were placed inside a quartz tube that was then placed into a tubular furnace. A 50 mL min−1 N2 gas flow was used to drag out the acetonitrile from the trap. The furnace was heated in a 10 °C min−1 rate until it reached four different temperatures, 600, 650, 700 and 800 °C. The materials were named as follows: M52A600, M52A650, M52A700, and M52A800. Fig. 1 shows a scheme for the CVD process. Thermogravimetric analyzes (TG) were performed on a Shimadzu DTG 60H equipment with synthetic air flow (50 mL min−1), a temperature range of 30–1000 °C and a heating rate of 10 °C min−1. The measurements of Raman spectroscopy were performed on a Raman Senterra spectrometer from Bruker with a coupled optic microscope (OLYMPUS BX51). The sample was excited using 532 nm laser, 5 mW, 50× magnifying lens, 50 × 1000 μM lens aperture, 10 acquisitions of 5 s each. The X-ray diffraction (XRD) measurements were performed at the National Synchrotron Light Laboratory (LNLS), line D12A-XRD1, using a double crystal monochromator of Si 111, with X-rays of 12 keV (λ = 1.0359 Å). The powder samples were fixed in 0.7 mm diameter quartz capillaries that were mounted on stainless steel ferromagnetic brackets that fit into a magnetic tip attached to the 3-D Heavy Duty Diffractometer from Newport®. This system provides the desired rotation of the capillaries during measurements. The XRD standards were collected at room temperature using Decthis MYTHEN 24 K Detection System [21]. The X-ray photoelectron spectroscopy (XPS) measurements were performed on the National Synchrotron Light Laboratory (LNLS). This equipment has an InSb(111) double crystal monochromator and Phoibos − HSA 3500 electron analyzer. The calibration of the beam and the electron analyzer was performed using a gold standard, with a beam angle of 45°. For the long-scan measurements a step of 1 eV and 40 eV of the energy of passage was used, and for the short scan, a step of

2.2. Catalytic tests To evaluate the efficiency of the catalysts in sulfide oxidation, batch experiments were performed. The resulting mixtures (catalysts and solution of approximately 8 g L−1 of sodium sulfide (Na2S·9H2O)) were maintained at room temperature (25 ± 1 °C) on an orbital shaker at 200 rpm. The supernatants were separated using a centrifuge and diluted (100 μL for 10 mL) The concentrations of solutions were determined at 229.5 nm, using Shimadzu 2550 spectrometer and a calibration curve (Fig. S1, Supplementary Material). The exact concentration of the sodium sulfide solution was determined by titration with previously standardized hydrochloric acid solution. The equilibrium time for the oxidation was initially determined by a 72 h kinetic study using 10 mg of each of the prepared catalysts (M52A600, M52A650, M52A700, and M52A800) and 10 mL of the sodium sulfide solution. The tests were done in triplicates. With the best catalyst, tests of reuse were performed repeating the kinetics of oxidation four times. 3. Results and discussion 3.1. Catalyst characterization The obtained materials were characterized by transmission electronic microscopy. The TEM images of the materials M52A600 and M52A650 (Fig. 2) indicated the presence of a porous matrix that can be associated with the silica structure. The material M52A700 showed a porous silica structure and large agglomerates of non-porous silica and M52A800 presented only nonporous silica structures. In previous work, it was shown than when the MCM with CTAB in it structure was calcined at 550 °C, is formed a mesoporous silica with a media porous diameter of ∼40 Å [20]. In addition, it is possible to observe the presence of carbon layers in the materials M52A600 and M52A650, confirmed the carbon deposition over the materials after the CVD process. However, in the samples obtained at lower temperatures, i.e. 600 and 650 °C it isn’t observed carbon nanostructures. For the samples, M52A700 and M52A800 are observed carbon nanotubes (CNT), bamboo-like structures, and iron cores encapsulate by carbon layers. Carbon nanotubes with bamboo-like structures (Fig. 2 c) are characteristic of nitrogen doped CNT [23]. It is well discussed that when nitrogen atoms are trapped inside the

Fig. 1. CVD process scheme. 2

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Fig. 2. TEM images of (a) M52A600, (b) M52A650, (c) M52A700 and (d) M52A800.

hexagonal carbon structure causes this structures to bend changing the overall shape of the carbon nanotube [24–26]. The presence of encapsulated magnetic cores is very interesting as it allows these materials to be applied and after use, they can be removed from the reaction medium by the approximation of a magnetic field [27]. The TG/DTG curves (Fig. 3), obtained in air atmosphere, showed for the samples M52A600, M52A650, M52A700 a broadening event of weight loss between ∼350–550 °C, related to the oxidation of carbon structures (Eq. (1)), centered at 442, 445 and 458 °C, respectively.

C(s) + O2(g) → CO(g)

(1)

The presence of an extended event in the DTG of these materials may indicate the formation of different types of carbonaceous structures (with different thermal stabilities) [28]. Thus, although sample 700 presents bamboo-like structures, it also presents other less organized structures. While, for the sample M52A800 the DTG curve of thermal decomposition event is narrow and shifted to higher temperatures 3

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such materials. Thus, adjustments were made in the Raman spectra obtained for the four samples using 5 contributions according to the Lorentz model (Fig. 4). In addition to the D and G bands, it is possible to observe 3 other bands: -D’ band: near 1620 cm−1: Related to the defects or irregular d002 spacing on the structure of the carbon materials [35,36]; -D” band: near 1500 cm−1: Related to stacking defects on the carbon structures [37]; -I band: near 1200 cm−1: related to an sp2 distortion [37] and may be present in doped or non-doped carbon nanotube spectra [24]. All these contributions are already related in the literature materials doped with nitrogen [24,38]. However, these bands may also be present in non-doped nitrogen [39]. It is interesting to note that although the sample M52A800 has a higher C/N ratio (10%), the intensity of the D' band (∼1620 cm−1) and the D band (∼1500 cm−1) are much smaller when compared to bands observed for the other materials. This fact can be justified by the high temperature used in the CVD process for formation of this material, which assists in the formation of more organized structures. Thus, the temperature favored a better organization of this material, which led to the presence of a doped material, but with a carbonaceous structure better organized in relation to the others. These results corroborate with those obtained by TEM and TG. In general lower synthesis temperatures lead to the formation of a smaller amount of carbon structures. In addition, there are more defects and are less doped with nitrogen. While higher temperatures, i.e. (800 °C), lead to the formation of a material with higher carbon content and a structure more doped with nitrogen, however, with a greater organization when compared with the others. Similar results were observed in the literature. Pérez-Cabero et al. [40] studied the carbon structures synthesis by CVD process using acetylene as carbon source at 600–800 °C. They observed the formation of more disorganized carbon structures, like amorphous carbon, at lower temperatures. At temperatures higher than 700 °C it was observed the formation of more organized structures like carbon nanotubes [40]. At higher temperatures, there is usually greater catalytic decomposition of the carbon source compared to lower temperatures, which leads to a higher yield in the formation of carbon species [40,41]. Moreover, the formation of more organized structures, like graphitic carbon nanotubes, is thermodynamically more favorable at higher temperatures [40,42,43]. The XRD patterns of the materials are shown in Fig. 5. All samples showed iron carbide Fe3C (JCDPS − 34-1) and iron, Fe (JCDPS -6-696), phases, formed during CVD process. For the material synthesized at 800 °C, due to his higher organized carbon content, it is possible to identify carbon-graphite phase (JCDPS − 41-1487), that cannot be identified on the materials synthesized at lower temperatures. The diffractogram shows a broad peak in the region between 10−18° that can be attributed to amorphous silica. The literature shows that a broad peak in the range of 17−30° is typical of this type of structure [44–46]. It is worth mentioning that the region here is shifted to 2θ smaller, since the wavelength used in the measurements was 1.0359 Å. All materials showed small surface areas, 62, 56, 39, 42 m2 g−1 for M52A600, M52A650, M52A700 and M52A800, respectively. Usually, mesoporous silica showed higher areas values when calcined for template removal. As shown in TEM images (Fig. 2) the support, after CVD process, are covered by carbon layers, which may contribute to the reduction of the area. XPS spectra of the samples showed signals of carbon, nitrogen, oxygen, and silicon, Fig. 6. Table 2 showed the contribution for C, N and O peaks. The carbon peaks observed for the samples are asymmetric and centered between 285.12 and 286.07 eV. According to the literature, the CeC graphite-like bond occurs at 284.5 eV, and the presence of dopants, like N, on the graphitic structures can shift this value to higher energy values, i.e. 285–286 eV, because of the

Fig. 3. TG and DTG curves of M52A600, M52A650, M52A700 and M52A800.

(450–550 °C, centered at 500 °C), indicating that this temperature led to the formation of more organized, stable and homogeneous carbon species. In fact, according to the literature, N doped carbon nanotubes have oxidation temperatures between 450 and 700 °C [29]. It is also possible to observe that higher temperature used in the CVD process, higher is the maximum oxidation temperature observed in the DTG curve, which indicates that the increase in temperature favors the formation of more stable carbon structures, as observed by TEM images [30]. Carbon and nitrogen content of the samples were determined by elemental analysis (Table 1). It is possible to observe that higher temperatures lead to higher yields in the formation of carbon structures and consequently to a higher nitrogen content. From the calculation of the nitrogen content ratio in function of the carbon content present in the samples, it is possible to observe that the process of introducing nitrogen into carbonaceous structures was not the same at the different reaction temperatures, ie, higher reaction temperatures (800 °C), in addition of leading to the formation of a higher content of carbon structures, leads to the formation of species richer in nitrogen [24]. Raman spectroscopy was used to characterize the nature of the carbon present in samples (Fig. 4). The obtained spectra showed, for all samples, a band near 1570 cm−1, a first order Raman band, named G band (E2g mode), typical of more organized sp2 bonded carbon and a second order band near 1350 cm−1 (D band − A1g mode) related to the presence of defects and disordered-induced in graphite-like structures, and for the sample M52A800 is also observed one band at ca. 2700 cm−1 (G’ band), a second order Raman band of carbonaceous materials derived from graphitic structures [31–33]. The degree of organization of the structures can be related to the intensity of D and G bands. The ID/IG ratio observed for M52A800 samples (ID/IG ca. 0.73) is smaller than the observed for other samples (ID/IG = 0.93 − 0.99), suggesting that the carbon present in the M52A800 sample is more organized compared to the carbon formed after treatments at temperatures below 800 °C. These results are in agreement with those observed by TEM and thermogravimetry analysis [34]. The G’ band, observed for the Raman spectra of the M52A800 sample is usually observed for carbon nanotubes samples with higher quality or purity [32]. According to the literature, the region between 1000 and 1800 cm−1 of the Raman spectra of carbonaceous materials may be formed by other bands which may provide additional information on Table 1 Carbon and nitrogen content and C/N ratio, obtained by elemental analysis.

M52A600 M52A650 M52A700 M52A800

C/%

N/%

N/C (%)

14.5 14.3 20.2 30.7

1.2 1.2 1.7 3.1

8.3 8.4 8.4 10.1

4

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Fig. 4. Raman spectra of M52A600, M52A650, M52A700 and M52A800. Curve fit of Raman spectra of M52A600, M52A650, M52A700 and M52A800.

possible to observe bands above 402 eV. According to the literature, peaks between 402 eV and 405 eV can be attributed to oxidized pyridine species [52–54]. It is interesting to note that with the increase in the temperature of the CVD process, the maximum intensity of the nitrogen signal in the XPS spectrum shifts to higher energy values, that is, to approximate the value of CeN graphite, Table 2. In addition, increasing the temperature it is observed an area increase of the peak relative to N graphitic signal in relation to N pyridinic and pyrrolic signals. According with the literature, this behavior is expected and indicates that the increase in temperature favors the organization, with less defective groups (pyridinic and pyrrolic), in the formed carbon structures and the insertion of nitrogen into the graphite structure [24,29]. Finally, the oxygen bands

electronic charge transfer of C from N [47,48]. In addition, the presence of oxygen functional groups can contribute to the presence of bands between 286 and 289 eV [37]. The greatest upshift of the maximum of the carbon signal was observed for M52A800, Table 2, which can indicate that this sample has greater nitrogen doping, as expected from the TEM, TG and XRD and elemental analysis results. In nitrogen spectra, Fig. 6, it is possible to observe two main bands: a contribution near 401 eV (400.7 for M52A600 and M52A650 and 400.9 eV for M52A800, respectively) of nitrogen graphitic, which indicates the N doped carbon structures [38,47,49–51]. In addition, for M52A600 and M52A650 it is possible to observe a band near 398 eV that can be associated with N pyridinic groups, and for M52A800 a band near 400 eV associated with pyrrolic groups [29,35,51]. Also, it is 5

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Table 2 Energy values of C, N and O contributions obtained by XPS for the materials M52A600, M52A650 and M52A800. Sample

Element

Energy (eV)

M52A600

C 1s N 1s O 1s C 1s N 1s O 1s C 1s N1s O1s

285.12 399.83 532.24 285.50 400.05 532.46 286.07 400.85 532.54

M52A650

M52A800

Fig. 5. XRD patterns of M52A600, M52A650, M52A700 and M52A800.

Fig. 6. XPS spectra of M52600, M52650, and M52800. 6

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Fig. 7. Sulfide removal, in%, in the presence of M52A600, M52A650, M52A700 and M52A800, after 240 min of contact.

for samples M52A600, M52A650 and M52A800 showed peaks centered at 532.24, 532.46 and 532.54 eV. The main contribution for these signals is the Si-O bond [55].

Fig. 8. Sulfide oxidation kinetics of M52A600, M52A650, M52A700 and M52A800.

3.2. Sulfide oxidation studies concentration of the solution. According to the literature and the results obtained in this work, it is believed that the presence of nitrogen groups in the synthesized nanomaterials are responsible for the oxidation of the sulfide. The materials synthesized at lower temperatures, although they contained lower levels of nitrogen in relation to the M52A800, have a higher kinetic slope, indicating a better oxidation rate. Raman, TEM, TG and XPS results show that higher CVD temperatures resulted in the greater amount of carbon and formation of more organized structures, i.e. carbon nanotubes and bamboo-like structures, with higher quaternary or graphitic nitrogen content. However, although the increase in temperature favors the formation of a higher content of doped carbon structures, according with XPS results, the nitrogen present in these materials is more stable and less available to act as a basic site for participation in the oxidation of sulfide. On the other hand, materials produced at lower temperatures have less carbon content and a less organized structure, with no or small amount of carbon nanotubes. Nevertheless, the nitrogen groups of these materials are more available. Oxidation tests were also carried out with M52 and M52_10Fe and no relevant reaction was observed, showing that the CVD process and the doped carbonaceous structure formed are essential for the oxidation of the sulfides. The oxidation test was also performed in the absence of catalyst to evaluate the oxidation from atmosphere. After three days the oxidation was only 26%. According to the obtained results in the presence of the carbon materials and literature, we believed that the presence of more reactive nitrogen species, like pyridine and pyrrolic groups are responsible for sulfide oxidation in the presence of atmospheric oxygen [59]. Probably, in aqueous media the majority sulfur specie present is the HS− [60,61]. Then we propose the follow mechanisms of sulfide oxidation by nitrogen doped carbon materials. In a first step the HS− ion is chemisorbed at the carbon structures, near a nitrogen position on the pyridine group [18] and there is the formation of superoxide ion in the presence of nitrogen doped structures [62,63]. According to the literature, oxygen radicals formed in the presence of pyridinic groups can also act on the oxidation of sulfides [9]. The oxygenated species will then react with HS− to produce oxidized species of sulfur, polysulfides, and water. The materials synthesized at lower temperatures showed the best results as catalysts for the oxidation of sulfides. This better activity may be associated with the presence of more defects in the structure of carbon materials formed and a greater amount of more reactive nitrogen, i.e., pyridine in its structure, observed by XPS Fig. 6. Literature shows that the oxidation of

The obtained doped carbon materials were used for the sulfide oxidation. Fig. S2 shows the decrease of the band referring to the sulfide ion, 229.5 nm, observed by UV–vis spectroscopy for M52A600 and Fig. 7 shows the sulfide oxidation capacity, in%, observed for each material, in 240 min. From the results reported in Fig. 7 it is possible to observe that all materials are efficient for sulfide removal. However, the best results are obtained for the materials synthesized at lower temperatures, i.e., 600 and 650 °C (95 and 85%, respectively). Reference sulphide oxidation tests in the absence of materials showed removals of less than 2%. During the removal, it was observed bands that may be attributed to the formation of polysulfides (Fig. S3). According to Lemos et al. [1,5] different forms of carbon such as graphite and graphene, with oxygenated functional groups, can promote the oxidation of sulfides, in aqueous media, to polysulfides (S22−, S32−, and S42−), the literature [56,57] also shows that the oxidation of sulfides depends strongly on the interaction of oxygenated surface groups and the electrical conductivity of carbon [1]. These factors are essential to the reaction. Nitrogen-doped carbon materials have also been used for H2S oxidation into elemental sulfur. Sun et al. [9], synthetized nitrogen-rich mesoporous carbon for H2S oxidation. The authors discussed the importance of the nitrogen content and type of doping (i.e. pyridinic, pyrrolic and graphitic N) for H2S oxidation. According to the authors, the presence of basic nitrogen Lewis sites and water allows the formation of HS− from H2S. In addition, the atmospheric oxygen molecules are adsorbed on the carbon surface, especially in the pyridinic groups surface to form oxygen radicals which will be responsible for the oxidation of HS− to elemental sulfur. Adib et al. [18], studied the use of activated carbon with nitrogen for hydrogen sulfide oxidation. According to the authors, the presence of nitrogen assist the H2S diffuse in the film of water adsorbed on the surface of the material and the release of HS−. The HS− ion is adsorbed on the nitrogen-carbon structure and reacts with superoxide ion (O2−) to form oxidized sulfur species. The superoxide ion is formed in the carbon surface from atmospheric oxygen [58]. The kinetic measurements (Fig. 8) were performed through the decreasing of the band on disulfide species (229.5 nm). The data indicate a linear decrease up to 240 min of reaction. These results indicate that during the initial 240 min the concentration of the active reaction sites present in the materials should not be significantly altering. On the other hand, after this period the curve decreases sharply its inclination, indicating that the speed of the reaction decreases. This is probably related to the reduction of the active sites and the reduction of the 7

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Fig. 9. Nitrogen species distribution, obtained by XPS, for the materials M52A600, M52A650 and M52A800.

sulfides is not efficient in non-doped carbon materials with nitrogen groups [18,64]. The increase in temperature of the CVD process favored the formation of more organized structures of carbon and richer in nitrogen, ie, carbon nanotubes, however, most of the nitrogen present in this material is in the form of quaternary nitrogen, that is, less reactive to the oxidation of sulfide, as observed by the nitrogen species distribution, obtained by XPS, Fig. 9. Reuse tests also were performed to M52A600. The reactions were carried out under the same conditions as the initial reaction (Fig. S4). The test showed that the material oxidation rate improves in the second and third cycles and decrease in the fourth and fifth uses. We suggest that, in the first use, it is necessary the adsorption of oxygen molecules in the nitrogen groups of the material for the oxidation to occur [9], which causes that reaction to be slower. In the next reactions, it is believed that the oxidant species are already adsorbed on the surfaces of the pores of the carbonaceous material, which accelerates the reaction. However, there may also be loss of material between the different reactions, which may contribute to a decrease in activity in the fourth and fifth use. Despite these variations in oxidation rate, the activities of the catalyst in the different uses were the same after 240 min. The oxidation performed by the materials M52A600 and M52A650 leads to the formation of polysulfides. This species can be observed due to a color change from colorless to dark green and by Raman spectroscopy (Fig. S5). To perform the Raman analysis the solution was centrifuged to separate the material from the solution. After the solution was left in a container to dry and formed a powder. To this powder, the Raman analysis was performed. Is possible to identify bands at 678, 641 and 353 cm−1 related to S4, bands at 546 and 231 cm−1 corresponding to S3−, bands at 555, 546, 138 and 353 cm−1 related to S5− bands at 454, 462, 119, and 138 cm−1 related to S22−, bands at 454, 353 and 263 cm−1 corresponding to S62−, bands at 339, 662, 993, 1101 and 1126 cm−1 corresponding to S2O32−, bands at 183, 170 and 153 cm−1 related to C-S bond and one band at 1061 cm−1 reported to be related to HSO4− [63,65–67].

4. Conclusions We efficiently synthesized nitrogen-doped carbon structures on an impregnated mesoporous silica matrix with iron at different temperatures. Higher synthesis temperatures led to the formation of a larger number of carbon nanostructures containing graphite nitrogen, while the materials obtained at lower temperatures had lower carbon and nitrogen contents and were mainly present in pyridine groups. All materials were efficient for sulfide oxidation. However, better oxidation rates were obtained in the materials synthetized at lower temperatures, due to the presence of more reactive nitrogen species.

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