Heteroatom (N, S) Co-Doped CNTs in the Phenol Oxidation by Catalytic Wet Air Oxidation

The N, S-co-doping of commercial carbon nanotubes (CNTs) was performed by a solvent-free mechanothermal approach using thiourea. CNTs were mixed with the N, S-dual precursor in a ball-milling apparatus, and further thermally treated under inert atmosphere between 600 and 1000 °C. The influence of the temperature applied during the thermal procedure was investigated. Textural properties of the materials were not significantly affected either by the mechanical step or by the heating phase. Concerning surface chemistry, the developed methodology allowed the incorporation of N (up to 1.43%) and S (up to 1.3%), distributed by pyridinic (N6), pyrrolic (N5), and quaternary N (NQ) groups, and C–S–, C–S–O, and sulphate functionalities. Catalytic activities of the N, S-doped CNTs were evaluated for the catalytic wet air oxidation (CWAO) of phenol in a batch mode. Although the samples revealed a similar catalytic activity for phenol degradation, a higher total organic carbon removal (60%) was observed using the sample thermally treated at 900 °C. The improved catalytic activity of this sample was attributed to the presence of N6, NQ, and thiophenic groups. This sample was further tested in the oxidation of phenol under a continuous mode, at around 30% of conversion being achieved in the steady-state.


Introduction
Wet air oxidation (WAO) is an alternative to conventional technologies for the treatment of organic pollutants in wastewaters, especially from highly pollutant industries such as pulp and paper industries, petrochemicals, and wine distilleries [1,2]. The process is suitable to treat organic compounds that show some resistance to conventional treatment technologies or to treat effluents presenting concentrations too high to be treated by biological processes or too low for incineration. It is classified as an advanced oxidation process (AOP), since it is based on the production and use of strongly reactive radicals to mineralise the organic compounds into CO 2 and H 2 O or into easily biodegradable by-products. In this case, the strongly oxidizing radicals are formed by the use of pure oxygen or air. To ensure the solubility of oxygen and to promote fast mineralisation and degradation rates, high temperatures (200-320 • C) and pressures (20-200 bar) are required [3][4][5]. In the last decades, efforts have been made to decrease such severe operating conditions by the addition of homogeneous or heterogeneous catalysts, mostly based on the use of noble metals and metal oxides [6][7][8][9][10][11][12][13]. However, the scarcity of these materials, and the frequent leaching and deactivation phenomena, have prevented the design of a clean catalytic wet air oxidation (CWAO) process [7,14]. During some of these studies, carbon materials were tested as supports for these metals and oxides, and it was found that the carbon material could act as a catalyst on its own, and not only as a support. This opened the opportunity for carbon materials as an alternative metal-free catalyst for CWAO [15][16][17][18][19][20][21][22][23][24][25]. Along the years, mesoporous carbon xerogels, carbon fibers and foams, carbon blacks, carbon nanotubes (CNTs) and nanofibers (CNFs), and also graphene-based materials were investigated in CWAO [26][27][28][29][30][31][32][33]. The outstanding ability to tune the textural and chemical properties of carbon materials make them potential catalyst candidates for a sustainable technology. To improve the performance of carbon materials in CWAO, several authors have taken advantage of the unsaturated carbon atoms at the edges of the graphene layers and at basal plane defects to form various types of surface functional groups containing O, N, S, B or P [34][35][36][37][38][39][40]. However, almost all of these studies were dedicated to studying each isolated heteroatom (N, S, B, P), while synergic benefits of using two or more heteroatoms have been reported in the literature [41][42][43], at least in the field of renewable energy, where metal-free carbon materials have also been widely studied [44][45][46][47][48]. As far as we know, the combination of two heteroatoms, particularly N and S, was never investigated in previous works as metal-free catalysts for CWAO. Exploiting our expertise in developing novel catalysts using fine surface functionalisation of carbon materials by easy-to-handle solvent-free methodologies [34,37,38,49], a systematic study was performed to assess the influence of surface N and S heteroatoms in CWAO. Nitrogen and sulphur have been selected since both can act as electron donors, and show different changes in the electronic density of states [50] that affect π electrons in the carbon lattice. Thiourea was selected as a precursor for the N, S-doping of CNTs. Table 1 summarises the textural properties of the prepared samples. The textural properties were determined from the N 2 adsorption isotherms, with the available surface area of the samples being determined according to the standard Brunauer, Emmett, and Teller method (S BET ). The mechanical and thermal treatment promoted an increase of S BET from 291 to 350 m 2 g −1 , when the pristine CNT and CNT@TU600 are compared, respectively. The slight increase may be due to the opening of the CNT closed tips during ball milling, commonly observed when CNTs and powder precursors are mixed under ball milling [37]. The thermal treatments applied at higher temperatures did not promote additional changes in S BET (differences smaller than 24 m 2 g −1 , which is close to the estimated experimental error of 20 m 2 g −1 ). The total pore volume (V p , experimental error ±0.005 cm 3 g −1 ) of the CNT@TU600 sample decreases by almost half of the pristine CNT sample (determined from the N 2 uptake at p/p 0 = 0.95), but after that slightly increases for the samples heat treated at 700 • C. For the samples treated at the other temperatures, V p marginally increases. While in the CNT@TU600 sample a higher agglomeration of the material may cause the abrupt V p decrease, the increase of the temperature to higher temperatures contributes to the recovery of free space in the CNT bundles. The samples' bulk and surface compositions were determined by elemental (C, N, H, S, and O contents) and XPS analyses, respectively ( Table 2). N-and S-containing groups were incorporated onto the carbon surface by the mechanical mixture of the CNTs with thiourea during ball milling. With a carbon content of around 95% by elemental analysis, the samples revealed that the continuous temperature increase in the thermal treatment promotes a decrease in the N content of the samples between 1.43 and 0.02%. A constant decrease is also observed concerning the O-content. However, a non-regular trend was found regarding S, with the S-content varying from 0.6 up to 1.3%. Regarding the XPS results (Table 2), a similar bulk/surface composition was noticed. The nature of the N, S-functionalities incorporated on the CNTs was investigated using the N1s and S2p XPS spectra of the samples ( Figure 1). The N1s and S2p spectra revealed asymmetric peak shapes, suggesting overlapping of individual peaks. Curvefittings were performed taking into account the nature N and S-bonds present in the N, S-precursor (thiourea), the possible formed bonds as a function of the temperature of the thermal treatment, in agreement with the literature reported characteristics of binding energies [37,48,51].

Materials Characterisation
For the samples treated at the lowest temperatures (600, 700, and 800 • C), the N1s spectra were deconvoluted into three peaks assuming the presence of quaternary nitrogen (NQ), pyrrolic (N5), and pyridinic (N6) structures. The corresponding binding energies and percentages of the N-functionalities are presented in Table 3. These species are typical of carbonaceous surfaces with N-containing precursors treated at temperatures above 550 • C. Despite the reduction of the N-content with the temperature of the thermal treatment, the N6 structure was always the most abundant, the disappearance of the N5 group being observed as the temperature increased. In the S2p spectra of the CNT@TU600/700/800 samples, three components were identified: C-S bonds around 163.6 ± 0.2 eV, C-S-O species at 164.3 ± 0.2 eV, and sulphate species at 168.2 ± 0.5 eV. It was assumed that the C-S and C-S-O species have S2p3/2 and 2p1/2 doublets separated by 1.18 eV and with an intensity ratio of 2:1 [48,51,52]. The fitted peak positions and relative areas are shown in Table 3. Since the precursor used does not contain oxygen in its composition, these sulphate species may have been formed by the oxidation of sulphur during air exposure [52,53]. However, at temperatures higher than 900 • C, this sulphate completely disappears. After the thermal treatments at 900 • C, the shape and the peaks identified in the S2p spectra drastically changed, and only C-S bonds have been identified. This suggests that C-S-O species and sulphates are thermally degraded beyond 900 • C.

Catalytic Activity in CWAO
The catalytic activity of the N, S-co-doped CNTs was preliminary assessed using oxalic acid as a representative of low molecular weight carboxylic acids. This compound has been extensively used in our previous works leading to the development of metal-free carbon materials for CWAO. Under the experimental conditions described elsewhere [34], an extremely fast removal of oxalic acid was observed (total conversion in less than 5 min of reaction using the CNT@TU600 sample, results are not shown). Despite the outstand-ing catalytic performance over oxalic acid oxidation, such rapid removal will not allow a clear distinction between the catalytic activity of the prepared samples and, for this reason, phenol has been chosen as an alternative model compound. Phenol is a more complex molecule also commonly found in industrial wastes, and extensively studied in the literature as probe species to evaluate the process efficiency. Figure 2a shows the evolution of phenol degradation during the CWAO reaction using the pristine CNT and N, S-CNT samples in a batch mode. In the absence of a catalyst (WAO curve), phenol is not oxidised under the operating conditions employed. Using the pristine CNT sample it is possible to achieve around 50% of phenol degradation after 120 min of reaction. The introduction of the N, S-co-doped CNT catalysts improved the degradation of the aromatic compound to removals between 60 and 70%, after 120 min of reaction, wherein around 50% conversion is obtained in the first 30 min in the presence of the most active samples. However, the TOC removal measured at the end of the catalytic experiments (Figure 2b), revealed a lower removal percentage. While in WAO the difference between the TOC decay and phenol concentration abatement is near 4 percentage points, experiments using the N, S-CNT samples, revealed that those difference may be 4 up to 30 percentage points. The highest TOC conversion (around 60%, corresponding to a phenol removal of 71%) was obtained using the CNT@TU900 sample. Due to the non-microporous nature of the CNTs, as expected, there is no contribution of adsorption to the presented results (results are not shown). According to the previous works, the increase of the available surface area and the incorporation of N-groups on the CNTs improve the catalytic activity in comparison to the pristine CNTs [34,35,37,38]. In this set of samples, the effect of the surface area is not measurable, since the difference in S BET is smaller than 24 m 2 g −1 . Regarding the N, S or O contents, the CNT@TU900 sample does not correspond to the richest sample in any of those elements and, therefore, no correlation between the amounts of N, S or O and the degradation extension is observed. However, among the prepared samples, CNT@TU900 presents a distinct nature of N, S-functionalities. As reported in the materials characterisation section, the thermal treatment at 900 • C revealed distinct N1s and S2p spectra, showing the disappearance of some species. The S2p spectrum of this sample revealed only the presence of C-S species, unlike the samples treated at lower temperatures, where other S-functionalities were also present. While sulfur in the thiophenic (C-S-C) configuration is presumed to induce strain and defects in the carbon matrix, which facilitate a charge localisation for favorable O 2 chemisorption [50], the C-S-O groups present in the samples treated at lower temperatures may decompose forming strongly oxidant sulphate radicals. Although these radicals can be responsible for initiating the degradation of phenol in the liquid phase, they do not allow high levels of mineralisation, suggesting that they can be selective for phenol oxidation but are not able to degrade the various by-products formed. In a previous paper [36], we have observed a similar behavior with -SO 3 H groups, promoting low TOC removals in phenol oxidation by CWAO. By other way, the CNT@TU900 sample is also rich in N6 and NQ functionalities, groups that increase the electron density of the surface, enhancing chemisorption and oxygen activation. This explains the better catalytic performance of the CNT@TU900 sample in comparison with the CNT@TU1000 sample, that in addition to not having the S-O and sulphate groups, is also poor in electron donor N-containing groups, resulting in a catalyst with lower catalytic activity.
In order to check the activity and stability of the CNT@TU900 catalyst, a continuous CWAO experiment was carried out until the steady-state was reached (Figure 3). A severe deactivation of the catalyst is observed in the first 10 h, becoming less pronounced after 20 h, with a phenol removal near 27% after 75 h. This represents a slight improvement in the face of the reported result obtained using N-doped CNTs by Santos et al. [54] under similar conditions (a phenol removal near 15% for a similar time). In the previous work, the deactivation phenomenon was also observed and explained by the deposition of phenolic polymers on the surface of the catalyst [55], in which the catalyst is not able to decompose. Although it has not been studied in this work, sometimes it is possible to regenerate the catalyst by carrying out thermal treatments to remove those deposited compounds that block the access to the active sites.

Materials: Preparation and Characterisation
Commercial multiwalled carbon nanotubes (CNTs) purchased from Nanocyl TM (NC3100 series) were subjected to a mechanothermal treatment to incorporate N, S-heteroatoms onto the graphitic lattice. Thiourea was selected as a precursor for the N, S-co-doping. Briefly, 0.6 g of CNTs were ball milled with thiourea (equivalent to 0.26 g of N). The grinding step was carried out in a Retsch MM200 equipment (Retsch GmbH) using zirconium oxide balls inside the grinding jars without any gas flow. The CNTs and thiourea were mixed and homogenised during 4 h by the radial oscillations of the grinding jars in a horizontal position (15 vibrations s −1 ), using milling conditions determined in a previous work [37]. After the mechanical mixing, the samples were subjected to a thermal treatment under N 2 (100 cm 3 min −1 , at 10 • C min −1 ) until 600, 700, 800, 900, and 1000 • C, and kept at the final temperature during 1 h. The corresponding samples were designated as CNT@TU, followed by the temperature of the thermal treatment (e.g., CNT@TU600 for the sample thermally treated at 600 • C).
The developed materials were further characterised using the appropriate techniques. Textural properties were based on the N 2 equilibrium adsorption isotherms determined at −196 • C in a Quantachrome NOVA 4200e multi-station apparatus. The surface chemistry was assessed using X-ray photoelectron spectroscopy (XPS). Analyses were performed using a Kratos AXIS Ultra HAS with VISION software for data acquisition and CASAXPS software for data analysis. The chemical composition was evaluated by elemental (CHNS/O) analysis carried out on a vario MICRO cube and a rapid OXY cube analyser from Elemental GmbH. Thermogravimetric analyses (TGA) were carried out using a STA 409 PC/4/H Luxx Netzsch thermal analyser.

Batch Mode
CWAO experiments in a batch mode were carried in a 160 mL 316-stainless-steel highpressure batch reactor (Parr Instruments, Moline, IL, Mod. 4564, USA) using phenol as a model pollutant (detailed description elsewhere [39]). In addition, 75 mL of a 75 mg L −1 phenol solution and 0.1 g of catalyst were placed inside the reactor. The reactor was purged with nitrogen until the complete removal of oxygen and then pressurised with 5 bar of nitrogen and pre-heated up to 160 • C under continuous stirring (500 rpm). After reaching the desired temperature, pure air was injected until a total pressure of 60 bar inside the reactor was reached. This was considered time zero for the reaction.

Continuous Mode
CWAO experiments in a continuous mode were carried out in a completely automated tubular reactor from Microactivity PID Eng&Tech equipped with a HPLC pump and a highpressure liquid-gas separator working in down-flow [39]. In addition, 0.2 g of the catalyst were mixed with 1.8 g of carborundum and introduced into the tubular reactor. The system was pre-heated under nitrogen flow (4 bar of pressure) until the desired temperature (160 • C). Then, nitrogen was replaced by pure oxygen, the pressure inside the reactor increased to the desired value (8 bar), and the model pollutant solution (75 mg L −1 of phenol) was fed at 0.25 mL min −1 .
The operating conditions have been selected taking into account the previous study of Santos et al. [54], where the influence of distinct operating conditions in the same experimental system and using phenol as a model compound, for batch and continuous mode operations, was investigated.

Analytical Techniques
Liquid aliquots were withdrawn during the reactions and centrifuged (if necessary) for further analysis by high performance liquid chromatography (HPLC) with a Hitachi Elite LAChrom system equipped with a Diode Array Detector (L-2450). The samples were analysed in a Purospher Star RP-18 endcapped column (250 × 4.6 m 2 , 5 µm particles) working at room temperature. The mobile phase was a mixture of water and methanol 40/60 (v/v) with a flow rate of 0.8 mL min −1 . Analyses were made using an injection volume of 50 µL. The quantification of phenol was performed at λ = 270 nm. The total organic carbon (TOC) was also determined for the selected experiments in a TOC-L Analyser from Shimadzu by the non-purgable organic carbon (NPOC) method.

Conclusions
The introduction of N, S-functional groups onto the carbon surface of commercial CNTs has been successfully carried out using a mechanothermal approach with thiourea as a precursor for the N, S heteroatoms. Without significant damage of the textural properties of the CNTs, quaternary nitrogen (NQ), pyrrolic (N5) and pyridinic (N6) structures, as well as C-S, C-S-O, and sulphate species, were incorporated at different temperatures in the thermal step. Some of these specific groups were found to improve the catalytic activity of the N, S-doped CNT towards phenol oxidation by CWAO, with around 60% of TOC removal being achieved with the CNT@TU900 sample. The enhanced phenol degradation and TOC removal were explained by the synergic contribution of the N6 and NQ groups that increase the electron density of the surface, enhancing chemisorption and oxygen activation, and the presence of thiophenic-like groups (C-S-C), responsible for promoting charge localisation for favorable O 2 chemisorption.