Graphitic carbon nitride (g-C3N4) has attracted a large amount of research, mainly being used as a photocatalyst, but its Fenton-like catalytic performance has been overlooked. In this paper, the dark Fenton-like catalytic performance of g-C3N4 was evaluated by degrading rhodamine B over a wide pH range. The results showed that the g-C3N4, which was synthesized by conventional urea pyrolysis without any modification, was an efficient metal-free heterogeneous Fenton-like catalyst. The highest activity occurred under a weakly alkaline condition of about pH 10. The experiment of catalyst recycling indicated that g-C3N4 had long-term stability. The reactive oxidizing species of HO·, generated by the g-C3N4 activating H2O2, was identified by EPR and further supported by a scavenging experiment of HO· using isopropanol as the scavenger. The HNO3 oxidation of g-C3N4 resulted in catalytic deactivation, implying the catalytic activity originated from the surface reduced groups of g-C3N4. The structure of synthesized g-C3N4 before and after the HNO3 oxidation was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, and a possible catalytic mechanism was proposed.
Although the classical homogeneous Fenton reaction, in which highly aggressive hydroxyl radicals (HO·) are generated from H2O2 by the redox cycle of Fe2+/Fe3+, has been used for the degradation of recalcitrant organic pollutants, some drawbacks hamper the wider application of this process. For example, this process only works at acidic pH (2.5–3), and Fe2+ ions result in formation of iron sludge, which has to be removed and hence increases the operation cost. To overcome these disadvantages, much attention has been paid to heterogeneous Fenton-like systems in recent decades (Zhu et al. 2019). Naturally, the research mainly focuses on Fe-containing solids (Wu et al. 2015; Hammedi et al. 2019; Zhu et al. 2019), and then extends to various Cu-containing materials (Dong et al. 2015; Lyu et al. 2015; Zhang et al. 2017; Su et al. 2019). Other non-iron materials, such as Ag, Al, Ce, Co, Cr, Mn, Au, and Ru, have also been investigated (Bokare & Choi 2014). Many of these solid catalysts with metals as active species show fairly high catalytic activity; however, their imperfect stability is still a major barrier impeding their commercial applications (Zhu et al. 2019). In heterogeneous Fenton-like processes, the leaching of metal ions caused by acidity and complexation is almost inevitable, resulting in a rapid deactivation of these catalysts. To avoid this deactivation, metal-free materials, especially carbon materials, have received increasing interest in recent years (Ribeiro et al. 2016). Carbon materials studied as heterogeneous Fenton-like catalysts on their own include activated carbon, carbon xerogel, carbon black, glycerol-based carbon, biochar, carbon nanotube, carbon quantum dot, graphite, and graphene. The catalytic activity was generally attributed to the basic oxygen-containing functional groups of these carbon materials. These groups activated H2O2, generating HO· by redox cycles, similar to the redox cycle of Fe2+/Fe3+ in the classical Fenton process (Espinosa et al. 2015; Navalon et al. 2017; Vega & Valdés 2018). Among these carbon materials, graphene, a famous 2D carbon material, has attracted the most intensive attention, mainly due to its tremendous specific surface areas, high chemical stability, superior electron mobility, abundant oxygen-containing functional groups, and easy-to-modulate active sites. Graphene (Espinosa et al. 2015), graphene oxide (GO) (Ribeiro et al. 2016; Zhao et al. 2017), and reduced graphene oxide (r-GO) (Espinosa et al. 2015) were all studied as heterogeneous metal-free catalysts for Fenton-like processes. r-GO usually exhibited the highest catalytic activity, while GO exhibited the lowest (Espinosa et al. 2015). This is because the type and the number of surface oxygen-containing functional groups markedly influenced their catalytic performance. Basic oxygen-containing functional groups were reductive and might transfer electrons to H2O2, generating HO·. In contrast, oxidative oxygen-containing functional groups caused invalid decomposition of H2O2. This invalid decomposition resulted in a decrease not only in the utilization of H2O2 but also in the catalytic activity. To modulate the surface functional groups of carbon materials, doping heteroatoms is an effective strategy. Nitrogen is the most widely studied heteroatom in doped graphenes as well as other carbon materials (Kong et al. 2014). Doping nitrogen on carbon materials usually enhanced their Fenton-like catalytic activity (Martin-Martinez et al. 2016; Granzier-Nakajima et al. 2019).
Graphitic carbon nitride (g-C3N4) is another famous 2D carbon material, which has a similar structure to graphene but containing nitrogen. Mainly as a photocatalytic material, g-C3N4 has attracted increasing interest in recent years (Mamba & Mishra 2016; Ong et al. 2016; Kumar et al. 2018). g-C3N4 contains not only plenty of nitrogen-containing but also some oxygen-containing functional groups. This material is easy to synthesize on a large scale with various cheap raw materials, and also easy to modify by various strategies (Reddy et al. 2019). Curiously, it seems that this material has never been studied as a Fenton-like catalyst. In fact, in many studies dealing with g-C3N4 as photo-Fenton catalysts or as supports of Fenton-like catalysts, controlled experiments of g-C3N4 as Fenton-like catalysts were always carried out to illustrate the low activity of g-C3N4 (Table S1, Supplementary Information). In this study, we using rhodamine B (RhB) as a simulated organic pollutant to evaluate the Fenton-like catalytic performance of g-C3N4 over a wide pH range. We found that even the unmodified g-C3N4 synthesized by conventional urea pyrolysis was still an efficient heterogeneous Fenton-like catalyst.
Urea was purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd (Tianjin, China). RhB was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). H2O2 (30%), HCl (36%), and NaOH were purchased from Shanghai Wokai Biotechnology Co., Ltd (Shanghai, China). All these reagents are of analytical grade.
Preparation of g-C3N4
g-C3N4 was synthesized by the conventional urea pyrolysis (Liu et al. 2011). Urea in a crucible with a cover was dried at 80 °C for 1 h. Then the crucible was put in a muffle furnace, heated to 580 °C with the heating rate of about 3 °C min−1, and remained at 580 °C for 3 h to complete the reaction. The yellow product obtained was washed in turn with 0.1 mol L−1 HCl, deionized water, and ethanol until the filtrate was neutral. Lastly, the wet sample was dried in a vacuum freeze dryer for 10 h.
Characterization of g-C3N4
X-ray diffraction (XRD) data were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Tenser II infrared spectrophotometer using the KBr disc method. Scanning electron microscopy (SEM) images were recorded on a Hitachi S2400 scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo Scientific-Escalb 250XI electron spectrometer. The specific surface area was obtained according to the Brunauer–Emmett–Teller (BET) method from the nitrogen adsorption at liquid nitrogen temperature using a Qutantachrome NOVA 2000e system.
Catalytic evaluation of g-C3N4
To evaluate the catalytic performance of the g-C3N4, 100 mL RhB solution with a concentration of 20 mg L−1 was used as simulated wastewater. The degradation of RhB was carried out in a glass reactor of 250 mL immersed in a constant temperature water bath with magnetic stirring. The initial solution pH was adjusted using a small amount of 0.1 mol L−1 HCl or 0.1 mol L−1 NaOH solution. After adding 20 mg g-C3N4 into the RhB solution, the mixture was stirred for 30 min to achieve adsorption–desorption equilibrium. Then the reaction was started by adding a certain amount of H2O2 solution. All reactions were carried out at 50 °C, pH 10, and 100 mmol L−1 H2O2 unless noted otherwise. At predetermined time intervals, a 4 mL reaction solution was taken out. After immediate centrifugation, the concentration of RhB was analyzed at 554 nm using an INESA 722G spectrophotometer. The UV-vis absorption spectra of the reaction solution were recorded by using a Shimadzu UV-1780 spectrophotometer.
The electron paramagnetic resonance (EPR) spectra were recorded on an EPR spectrometer (EMX Plus, Bruker, Germany). 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as a radical trapping agent.
RESULTS AND DISCUSSION
Characterization of g-C3N4
XRD and FTIR were used to verify the crystal structure and surface characteristics of the synthesized g-C3N4 product. The results were in good accordance with the data reported previously (Figure 1) (Liu et al. 2011; Wang et al. 2019). The strong XRD peak at 27.5° was indicative of the graphitic stacking structure, which was indexed as (002). Another obvious peak at 13.1° was attributed to the in-planar repeating unit, which was indexed as (100). The FTIR spectrum of the product demonstrated the graphitic structures further. The wide band between 3,000 and 3,500 cm−1 was attributed to N–H stretching vibration modes, indicating that uncondensed amino groups still existed in the product. The strong bands at 1,630, 1,569, 1,454, and 1,395 cm−1 were assigned to stretching vibration modes of heptazine-derived repeating units. The sharp band at 808 cm−1 represented the out-of-plane bending vibration of heptazine rings. The bands at 1,313 and 1,229 cm−1 corresponded to stretching vibration of connected units of N–(C)3 or C–NH–C. The SEM image of the synthesized product presented a sheet structure with a thin thickness, which was similar to the morphology of g-C3N4 synthesized using similar methods (Lu et al. 2016; Yue et al. 2016; Yang et al. 2017). The nitrogen adsorption–desorption isotherm of the synthesized product showed the type IV isotherm. The specific surface area was calculated according to BET theory as 77 m2 g−1, which was also near the values of g-C3N4 synthesized using similar methods (Liu et al. 2011; Sudhaik et al. 2018).
Effect of initial pH on catalytic activity of g-C3N4
The initial pH value of wastewater usually strongly affects the degradation of organic pollutants in Fenton or Fenton-like processes, whether in homogeneous or heterogeneous systems. Catalysts usually have high activity only in a relatively narrow pH range. In most cases, the optimum pH range is weak acidity, and for some catalysts, the optimum pH can be extended to neutrality. Few catalysts have activity in the alkaline range. The Fenton-like catalytic activity of the g-C3N4 was also strongly influenced by the initial pH value, as shown in Figure 2. The g-C3N4 had the lowest catalytic activity at pH 4, and when pH was less than 4 the catalytic activity increased slightly. However, when the pH was greater than 4, the catalytic activity of the g-C3N4 obviously increased with the pH increase. That is to say, the catalytic activity of the g-C3N4 was the highest under alkaline conditions (around pH 10). To eliminate the influence of the non-catalytic degradation of H2O2, a controlled experiment was carried out in the same pH range. The results showed that without catalytic activation H2O2 itself is indeed a rather weak oxidant, although the non-catalytic degradation of RhB increased slightly with the increase of pH. For example, at pH 10, the non-catalytic degradation was only 27% in 140 min, and the catalytic degradation of RhB reached 92%. This clearly proved that the g-C3N4 had a high heterogeneous Fenton-like catalytic activity. On the other hand, we can get the rate constants by fitting the experimental data with the first-order kinetic equation. The rate constant was 0.00209 min−1 for the non-catalytic degradation, but it was 0.0184 min−1 for the catalytic degradation, which increased about nine times. This rate constant also showed that the Fenton-like catalytic activity of g-C3N4 was fairly good, especially when considering the dosage of g-C3N4 as low as 0.2 g L−1.
It should be pointed out that our experiments showed that g-C3N4 had only lower Fenton catalytic activity under weak acidic to neutral condition. This condition was usually the optimized pH range for Fenton and photo-Fenton studies. Furthermore, the natural pH of dye solutions commonly used as pollutant models is usually weak acidic or neutral if the pH value is not adjusted. This may be one of the reasons for the low activity of g-C3N4 as Fenton catalysts observed in the controlled experiments reported in the literature (Table S1).
Effect of reaction temperature on catalytic activity of g-C3N4
At room temperature, the Fenton catalytic activity of the g-C3N4 was undistinguished, which may be another reason for its neglect with regard to catalytic performance. As shown in Figure 3, at 30 °C the degradation of RhB was only about 40% in 140 min. Increasing the temperature can expectedly enhance the catalytic activity of the g-C3N4. At 60 °C the degradation of RhB could reach 96% in 140 min. At 70 °C the degradation in 20 min reached 83%. Correspondingly, the rate constant of the first-order reaction increased from 0.00300 min−1 at 30 °C to 0.0706 min−1 at 70 °C, which was about 23 times higher. The rate constants at different temperatures agreed well with the Arrhenius equation (Table S2, Figure S1, Supplementary Information), and the apparent activation energy of degradation calculated was 65.6 kJ mol−1.
Effect of H2O2 concentration on catalytic activity of g-C3N4
As oxidant, the concentration of H2O2 strongly affected the degradation of RhB (Figure 4, Table S3). When the concentration of H2O2 was 40 mmol L−1, the degradation of RhB was only about 43.1% in 140 min, and when the concentration was 100 mmol L−1, the degradation reached 93.4% in the same time. Correspondingly, the rate constant of the first-order reaction increased from 0.00380 to 0.0184 min−1, which was about four times higher. Further increasing the concentration of H2O2 had no contribution to improving the degradation.
It can be calculated that the concentration of H2O2 should be 2.92 mmol L−1 for mineralizing RhB of 20 mg L−1 (corresponding to 0.04 mmol L−1). Thus, an H2O2 concentration of 100 mmol L−1 is 34 times the theoretical amount, indicating relatively lower utilization of H2O2 in this system. To achieve higher degradation at low H2O2 concentration, a longer time or higher temperature is needed. It also shows that g-C3N4 as a Fenton-like catalyst still should be improved.
Stability of g-C3N4
As a metal-free material, it is expected that g-C3N4 exhibits better stability because there is no leaching problem of metal ions. Its stability was tested by separating the catalyst from the Fenton-like system after the reaction and repeatedly using it. No significant decrease in catalytic activity was observed after five cycles as shown in Figure 5, indicating that the g-C3N4 had good repeatability and long-term stability.
Study of catalytic mechanism
The reactive oxidizing species of HO· was confirmed by EPR, as shown in Figure 6. Using DMPO as the radical trapping agent, the characteristic EPR spectrum of the DMPO-OH spin adduct was recorded in the g-C3N4/H2O2 system. However, without g-C3N4 the spectrum was very weak. These results clearly demonstrated that g-C3N4 activated H2O2, generating HO·. That HO· was the major reactive oxidizing species was further supported by a scavenging experiment of free radicals, in which the addition of isopropanol, an HO· scavenger, significantly decreased the degradation of RhB (Figure 7). Highly active HO· can quickly destruct the structure of the xanthene-conjugated chromophore (554 nm) of the RhB molecule, as shown in the UV–vis spectra of the reaction solution (Figure 8) (Lee et al. 2016). The superoxide radicals and non-catalytic oxidation of H2O2 may also contribute to the degradation of the dye because the addition of isopropanol did not completely inhibit the degradation (Figure 7).
The reducibility of active sites was supported by a preliminary experiment. After oxidation by HNO3, the g-C3N4 almost completely lost its Fenton-like catalytic activity (Figure 7). In order to identify the specific active group, we compared the structure of g-C3N4 before and after HNO3 oxidation by XRD, FTIR, and XPS characterization. The results are shown in Figure 9, Figure S2, and Table S4. After HNO3 oxidation, the main peak (002) of the g-C3N4 XRD pattern showed only a decrease of the intensity and a slight shift towards higher angles (Figure S2), reflecting a possible disorder introduced in the graphitic planes by the oxidation process and a reduction of the inter-planar stacking distance (Pisanu et al. 2018). Comparing the FTIR spectra of the g-C3N4 before and after HNO3 oxidation, we cannot observe the structural change resulting from the oxidation (Figure S2). Although C=O and C–O groups are commonly formed in the process of g-C3N4 oxidation, it is difficult to detect them by the conventional FTIR technique, because these groups are usually present only on the surface of g-C3N4. The adsorption band of C=O vibration should appear at around 1,720 cm−1; nevertheless there was no detectable absorption peak before and after the HNO3 oxidation. The very weak peak at 1,080 cm−1, which was attributed to C–N vibration, did not change observably after the oxidation. It is generally believed that the C–O bond and C–N bond have similar bond force constants; the weak C–O absorption can hardly cause the observable change of the 1,080 cm−1 peak (Li et al. 2015; Yang et al. 2017; Pisanu et al. 2018).
XPS is a surface-sensitive technique that easily detects changes of the catalyst surface groups (Figure 9). The XPS spectrum of pristine g-C3N4 presented characteristics of the common g-C3N4. The survey scan of the XPS spectrum showed that the synthesized product was composed primarily of carbon and nitrogen. The very weak O1s peak may come from adsorbed oxygen, which was commonly observed in XPS spectra of g-C3N4. The high-resolution spectrum of C1s showed the deconvoluted two peaks at 288.0 and 284.8 eV. The major peak at 288.0 eV corresponded to sp2-bonded carbon (N–C=N), and the weaker one at 284.8 eV was attributed to graphitic carbon, which usually appears on the XPS characterization for g-C3N4. The high-resolution spectrum of N1s showed the deconvoluted three peaks at 398.6, 400.2, and 401.3 eV. The main peak at 398.6 eV could be attributed to sp2 hybrid nitrogen (C–N=C). The peak at 400.2 eV was ascribed to tertiary nitrogen (N–(C)3). The peak at 401.3 eV corresponded to amino groups with a hydrogen atom (C–N–H). After HNO3 oxidation, an obvious enhanced O1s peak appeared in the XPS survey scan of the oxidized g-C3N4, indicating the oxidation resulted in formation of oxygen-containing groups. The high-resolution spectrum of O1s showed the deconvoluted three peaks at 531.8, 532.7, and 533.8 eV, being attributed to oxygen of O=C, O–C and O–H, respectively. Accordingly, two new C1s peaks at 286.1 and 289.0 eV attributed to carbon of O–C and O=C–O, respectively, can be deconvoluted from the C1s high-resolution spectrum of the oxidized g-C3N4 (Li et al. 2015; Yang et al. 2017; Pisanu et al. 2018). However, after oxidation no new peaks appeared in the high-resolution spectrum of N1s, but the surface content of nitrogen decreased obviously and the N/C atomic ratio decreased from 1.32 (very close to the theoretical ratio 1.33) to 0.90 (Table S4), indicating that some of the nitrogen atoms were lost during the HNO3 oxidation. Nevertheless, most of the nitrogen-containing groups remained, and the relative content did not change significantly.
Given the above discussion, it can be concluded that the oxidation process only introduced a certain amount of oxygen-containing groups but did not completely destroy the basic result of g-C3N4. This may indicate that the electron-rich triazine group was the active site for activating hydrogen peroxide. The HNO3 oxidation, which introduced electron-withdrawing oxygen-containing groups, especially the carboxyl group, significantly changed the surface electron distribution of g-C3N4, resulting in catalyst deactivation. In addition, oxidative oxygen-containing functional groups may cause invalid decomposition of H2O2, resulting in a decrease not only in the utilization of H2O2 but also in the catalytic activity.
The photocatalytic performance of g-C3N4 has attracted extensive research, but its Fenton-like catalytic performance has been neglected. In this paper, it has been proved for the first time that g-C3N4 is an efficient and stable heterogeneous Fenton-like catalyst, and its highest activity occurs in the alkaline condition of about pH 10. Like other carbon materials, it may activate H2O2 at the Lewis base site to produce HO·, thus degrading organic pollutants. Of course, the study of the mechanism is insufficient, and more evidence is needed for the identification of active groups. Also, the reaction activity is not high at room temperature and the consumption of H2O2 is relatively high. However, because g-C3N4 can be modified by various strategies and this material is easy to synthesize on a large scale with various cheap raw materials, g-C3N4 should be a potential candidate as excellent heterogeneous metal-free Fenton-like catalysts.
We are grateful to the National Natural Science Foundation of China (21802125) and the Henan Science and Technology Project (172102210490) for the financial support.
CONFLICT OF INTEREST
There are no conflicts to declare.
The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.129.