Tuning layered Fe-doped g-C3N4 structure through pyrolysis for enhanced Fenton and photo-Fenton activities

Journal Pre-proof Tuning layered Fe-doped g-C3N4 structure through pyrolysis for enhanced Fenton and photo-Fenton activities Wei Miao, Ying Liu, Xiaoyan Chen, Yixin Zhao, Shun Mao PII:

S0008-6223(19)31291-6

DOI:

https://doi.org/10.1016/j.carbon.2019.12.056

Reference:

CARBON 14906

To appear in:

Carbon

Received Date: 31 October 2019 Revised Date:

14 December 2019

Accepted Date: 23 December 2019

Please cite this article as: W. Miao, Y. Liu, X. Chen, Y. Zhao, S. Mao, Tuning layered Fe-doped g-C3N4 structure through pyrolysis for enhanced Fenton and photo-Fenton activities, Carbon (2020), doi: https:// doi.org/10.1016/j.carbon.2019.12.056. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author Contribution Statement Title: Tuning layered Fe-doped g-C3N4 structure through pyrolysis for enhanced Fenton and photo-Fenton activities Wei Miao (first author): Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Validation; Writing - original draft Ying Liu: Data curation; Formal analysis; Investigation; Methodology Xiaoyan Chen: Data curation; Formal analysis; Investigation; Methodology Yixin Zhao: Data curation; Formal analysis; Investigation; Methodology Shun Mao (corresponding author): Conceptualization; Funding acquisition; Project administration; Resources; Software; Supervision; Writing - review & editing

Graphical Abstract

Tuning layered Fe-doped g-C3N4 structure through pyrolysis for enhanced Fenton and photo-Fenton activities Wei Miaoa,b, Ying Liua,b, Xiaoyan Chena,b, Yixin Zhaoc, Shun Maoa,b,* a

Biomedical Multidisciplinary Innovation Research Institute, Shanghai East Hospital, State Key

Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China b

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China

c

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan

Road, Shanghai 200242, China

* Corresponding author. E-mail: [email protected] (Shun Mao)

1

Abstract In recent years, design high catalytic graphitic carbon nitride (g-C3N4)-based catalysts has drawn broad attention in environmental remediation. In this work, a series of iron-doped g-C3N4 compounds were synthesized through a simple bottom-up strategy. By controlled pyrolysis, iron-doped g-C3N4 materials split from bulk structure into multi-layer structure with uniformly dispersed mesopores. The catalytic activities of layered iron-doped g-C3N4 in Fenton-like and photo-Fenton-like processes were investigated. Due to the doped iron, unique layered structure, and mesopore feature, the catalysts present greatly enhanced performance in heterogeneous Fenton-like reactions. Moreover, the degradation parameters calculated by the pseudo-first-order kinetic model fitted well with the specific surface area of the catalyst, indicating that the catalytic activities rely heavily on the specific surface area of layered and porous g-C3N4. This study presents a facile and generic method to regulate C3N4 morphology and structure for enhanced catalytic activity in Fenton-like reactions.

Keywords: Iron-doped g-C3N4; Fenton-like reaction; Photo-Fenton-like reaction; Layered mesopore structure; Pyrolysis

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1. Introduction As an advanced oxidation process (AOP), Fenton reaction that produces highly active species such as hydroxyl radicals (·OH, E0 = 2.80 V) shows high efficiency in water remediation [1, 2]. In the past decades, heterogeneous Fenton has received sustained attention for its feasibility of wide pH working range and good recyclability in pollutant degradation [3]. Particularly, heterogeneous photo-Fenton that employs radiation can enhance the catalytic activity of the reaction. Methods for activity enhancement include accelerating Fenton-like rate-limiting step (transfer from high-valence metal state to active low-valence state) [4, 5], generation of •OH radicals by H2O2 with photo-induced electrons [6], enhancement in light harvesting by slow-light-effect region of the photonic crystal catalysts, and the use of photosensitized dyes act as electron donors [7]. In such processes, the unevenly distributed active species, insufficient surface area and pore blockage could reduce the catalytic activity. Therefore, there are still challenges and demands to construct new types of catalysts with porous structure, large accessible surface area, and high activity in heterogeneous photo-Fenton systems [8, 9]. Graphitic carbon nitride (g-C3N4), as a metal-free and visible-light-responsive conjugated polymer, has excellent physicochemical stability and appropriate electronic bandgap [10]. The g-C3N4 has been widely applied in environment remediation, including liquid phase removal of organic pollutants and toxic metal ions [11, 12], gas phase degradation of pollutants [13], and bacterial disinfection [14], etc. However, bulk g-C3N4 synthesized by direct thermal pyrolysis suffers from unsatisfactory catalytic activity and limited performance owing to low surface area and poor visible-light harvesting capacity [15]. In contrast, porous g-C3N4 nanosheets demonstrate advantages of large specific surface area, abundant exposed active sites, enhanced ability to access for 3

electrolytes, and short diffusion distance for electrons and ions [16]. Thus, various strategies such as liquid-solvent, chemical, and thermal-based exfoliation have been adopted to reduce the dimension of g-C3N4 from bulk structure to nanosheets [17-19]. Among them, thermal exfoliation has attracted much attention for the merits of simple process, low-cost, and easy scale-up [20]. For instance, porous g-C3N4 nanosheets have been prepared by pyrolysis treatment with reduced layer thickness, large surface area and porous structure, which benefit their applications in photocatalytic reaction [21, 22]. Another commonly-used method to enhance the catalytic activity of g-C3N4 is doping with active metals such as Fe, Mn, Co, Cu, etc. [23-25] The metal atom is immobilized by six nitrogen lone-pair electrons provided by heptazine rings to form stable nitrogen-metal hybrid macrocyclic materials, which brings additional activity in photocatalytic and oxygen reduction reactions [26]. So far, Fe active species, as one of the most promising doping element, have received increasing interest in doped g-C3N4 [27]. For instance, Fe-doped g-C3N4 was reported as Fenton-like catalyst and Mössbauer spectroscopy results showed that Fe3+ ions were reduced to Fe2+ ions during melamine polymerization [28]. There was also report that peroxymonosulfate (PMS) could bound to Fe(III)-N moieties in heterogeneous Fe-g-C3N4 catalysis to generate FeV=O, which was a highly selective oxidant to organic contaminants [29]. Nevertheless, some strategies have been investigated to improve the catalytic activity of transition metal doped g-C3N4 and most studies focus on the increase of doping level, which increases the risk of metal dissolution and secondary pollution [30]. Additionally, the increase of pyrolysis temperature damages the 2D sheet structure and weakens the performance of photocatalyst [31]. Unlike previous research focus on the doping level (summarized in Table S1, Supporting Information), new thermal exfoliation strategy to prepare Fe-doped g-C3N4 4

with layered and porous structure for high catalytic activity in Fenton-like and photo-Fenton-like reactions is greatly desirable. Herein, we firstly proposed a simple bottom-up method by tuning the pyrolysis duration time to produce layered porous Fe-doped g-C3N4 materials. The influence of pyrolysis duration time on the elemental composition, band gap, microstructure and catalytic activity of Fe-doped g-C3N4 were systematically investigated. It is shown that Fe doping provides active sites in heterogeneous Fenton reaction, and the porous nanosheet structure can simultaneously enhance the performance of Fe-doped g-C3N4 in Fenton-like and photo-Fenton-like reactions. Particularly, this work demonstrates that the weak photocatalytic activity of Fe-doped g-C3N4 could be enhanced by extending the visible light adsorption. With this simple pyrolysis treatment, 7.5 times higher catalytic activities (degradation rate constant k) in both Fenton-like and photo-like Fenton with Fe-doped g-C3N4 nanosheets were achieved compared with untreated Fe-C3N4. Moreover, the degradation parameters calculated by the pseudo-first-order kinetic model fitted well with the specific surface area of the catalyst, indicating that the catalytic activities rely heavily on the porous sheet-like microstructure of modified Fe-doped g-C3N4 host. This work provides a new and promising strategy to fabricate porous and sheet-like structure of metal-doped carbon nitride polymers for practical catalytic and photocatalytic applications.

2. Experimental 2.1. Chemicals Melamine, FeCl3·6H2O (99%), hydrogen peroxide (30% w/w), and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B, isopropanol (IPA), benzoic 5

acid (BA), p-chloroaniline (PCA, 99.5%), Bisphenol A (BPA, 99.0%) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO, 97%) were purchased from Sigma-Aldrich. 2,4-Dichlorophenol (2,4-DCP, 99%), Ibuprofen (IBP, 98%) and methanol (MeOH, HPLC) were supplied by Aladdin Chemistry Co., Ltd (Shanghai, China). Ciprofloxacin (CIP, 99%) was obtained from TCI Development Co., Ltd (Shanghai, China).

2.2. Catalyst preparation In a typical synthesis procedure, 100 mL of ultrapure water was heated to 100

, and then 5 mL of

HCl was added. Under vigorous agitation, 5 g of Melamine and 0.375 g of FeCl3·6H2O were sequentially dissolved in this solution and maintained for 0.5 h to obtain the homogeneous mixed solution. Subsequently, the mixed solution was continually heated at 100

until water was

completely removed. The obtained solid was heated at 5 °C/min to 550 °C in a flowing-argon atmosphere, then heated at 550 °C for another X h in Ar atmosphere and denoted as Fe-C3N4-Xh. Similarly, C3N4-Xh was prepared with the same method without Fe source.

2.3. Characterizations Fourier transform infrared (FT-IR) spectra were collected with a Nicolet 380 spectrometer (Thermo Electron Corporation, USA) with the KBr used as a reference. X-ray diffraction (XRD) patterns were obtained on Bruker D8 Advance X-ray diffractometer (λ = 0.154 nm). A scanning electron microscope (SEM, Hitachi S-4800) was used to characterize the morphology and microstructure of samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a Tecnai G2 F20S-TWIN. X-ray photoelectron spectroscopy (XPS) results were 6

obtained with a PHI Quantera SXMTM Scanning X-ray Microprobe TM at 20 kV. The UV–vis diffuse reflection spectra (UV-vis DRS) were measured with a Scan UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) equipped with an integrating sphere (BaSO4 as the reference). The N2 adsorption-desorption measurements were obtained on a nitrogen adsorption apparatus (ASAP 2460, USA). The photoluminescence (PL) emission spectra were taken on Hitachi F-7000 to obtain photoluminescence spectra of hydroxybenzoic acid (HBA) at an excitation wavelength of 320 nm. Electron spin resonance (ESR) measurements were performed on a Bruker EMX-E8/2.7 spectrometer and signals of radicals’ spin were trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO).

2.4. Catalytic activity study The catalytic activity of obtained catalysts was evaluated by Rhodamine B (RhB) removal efficiency in Fenton-like and photo-Fenton-like systems at room temperature. In a typical experiment, 60 mg catalyst was dispersed in 30 mL RhB solution with a concentration of 50 mg L-1. The adsorption equilibrium of the suspension was achieved with continuous stirring for 30 min. 150 µL H2O2 (30%) was added into the suspension to start the Fenton-like reaction. Under the same condition, a 500 W Xe lamp with 420 nm cutoff filter was used as the light source in Photo-Fenton-like reaction. All the reactions were performed with initial pH of 5.8. At specific time intervals, 1 mL of reaction solution was collected and immediately measured by UV–vis spectrometer at 554 nm. CIP, PCA, 2,4-DCP, BPA and IBP were analyzed by an HPLC instrument (Agilent 1260) with analytical details shown in Table S2.

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3. Results and discussion 3.1 Structure and properties of Fe-C3N4-Xh Fig. 1a shows the XRD patterns of Fe-C3N4-Xh. The obtained patterns are similar to the typical XRD patterns of g-C3N4 with two peaks at 27.3° and 13.3°, which is indexed to (100) and (002) facets of g-C3N4 (JCPDS 87-1526), respectively [28]. The (002) facet is assigned to the graphite-like stacking of the conjugated aromatic units and becomes weaker and broader with prolonged pyrolysis time, suggesting the few-layer nature of Fe-C3N4-6h nanosheets [16]. Likewise, the (100) facet is associated with the in-plane repeated units and becomes much weaker, which may be resulted from the in-plane porous structures [32]. The nitrogen pots in the plane of g-C3N4 are filled with six lone-pairs of electrons, forming a strongly host-guest interaction between the iron dopant [33]. The interaction becomes significant during the thermo-condensation of melamine and inhibits g-C3N4 crystal growth when extending pyrolysis time, leading to weakened (002) facet signal. There is no peak ascribed to iron-containing compounds in XRD patterns of all samples due to limited content of doped iron.

8

Fig. 1. (a) XRD patterns, (b) FTIR spectra, (c) DRS spectra, and (d) band gap measurements of Fe-C3N4-Xh.

The FT-IR spectroscopy shows that an adsorption band at 810 cm−1 from breathing mode of tri-s-triazine ring and bands between 1240 and 1640 cm−1 correspond to the stretching vibrations of aromatic carbon nitride heterocycles. Another band founded at 2170 cm−1 is attributed to the stretching vibration of -CN. The band intensity slightly increases from Fe-C3N4-0h to Fe-C3N4-6h, which implys that cyano group is formed after thermal evaporation [34].The broad adsorption bands at approximately 3000 - 3500 cm−1 are associated with vibration modes of the N-H stretching originated from the uncondensed N groups [11]. 9

The UV-visible absorption spectra in Fig. 1c show that the Fe-C3N4 samples have good absorption in the visible region, which means a high light trapping effect for the photocatalysts [35, 36]. There are reports demonstrating that the red shift of absorption edge in graphitic carbon nitride will happen when Fe species is chemically incorporated into the lattice of g-C3N4 host [15, 37]. In our study, the red shift is enhanced with the increase of pyrolysis time from 0 to 4 h, which indicates that the interaction between Fe and g-C3N4 gradually strengthens with extending pyrolysis time. However, a slight red shift in the absorption edge for Fe-C3N4-6h is noticed (band gap of 2.68 eV compared with 2.78 eV of Fe-C3N4-4h). This hypsochromic shift of band gap is caused by the quantum confinement effect, which aroused by the thermal exfoliation of nanosized structure [38, 39]. Hence, the optical properties of Fe-C3N4 are determined by two factors, the electronic property and dimensional structure, which could be adjusted by pyrolysis time.

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Fig. 2. (a) SEM image, (b) TEM image, (c) HRTEM image, and (e–g) elemental mapping data of Fe-C3N4-6h.

Different pyrolysis time leads to different structures and morphologies of Fe-C3N4. As shown in Fig. S1 (Supporting Information), the bulk structure of Fe-C3N4-0h is composed of smooth thick layers with few agglomerated particles. The morphology of Fe-C3N4-2h is similar to that of Fe-C3N4-0h, and thinner layer and uneven surface begin to appear. Further increasing the pyrolysis time to 4 hour leads to cracks and pores on the Fe-C3N4 surface. During the thermal exfoliation, the 11

decomposition of g-C3N4 releases CNx (cyano group, x>1) nucleus vapor phase in Ar atmosphere, creating porous structure [40]. As shown in SEM and HRTEM images (Fig. 2), Fe-C3N4-6h sample presents a porous nanosheet structure. During pyrolysis, bulk Fe-C3N4 splits into nanosheet structure, which is typical for 2D exfoliated graphitic carbon nitride to reduce surface energy during the heating process [41]. Meanwhile, uniformly dispersed mesopores with sizes of 30-40 nm were created. As shown in the elemental mapping data, N and Fe elements are evenly distributed on the surface of Fe-C3N4-6h. There is no obvious crystallite structure of Fe species discovered in the HRTEM image, indicating that Fe is incorporated into the g-C3N4 framework through Fe-N bonds [42]. The structural changes in the thermal-induced condensation and exfoliation of Fe-C3N4-6h are in consistent with XRD and FTIR results.

Fig. 3. Fe 2p spectra of (a) Fe-C3N4-0h and (b) Fe-C3N4-6h. N 1s spectra of (c) Fe-C3N4-0h and (d) Fe-C3N4-6h. 12

The valence bands (VBs) of C3N4-6h and Fe-doped C3N4-6h were studied by the valence band X-ray photoelectron spectroscopy. Theoretically, lone pair electrons of nitrogen in tris-s-triazine ring are determinant for the formation of valence band and band structure. As shown in Fig. S2, the valence band position of C3N4-6h became more positive (from 1.78 to 1.98 eV) with Fe-doped C3N4-6h. It implies that the d2sp3 hybrid empty orbit of Fe3+ superimposes with the sp2 hybrid orbital of N and provides d electrons to π-bonds of the tris-s-triazine ring [43]. Nevertheless, the pyrolysis duration time could effectively change the electronic structure of Fe-doped C3N4 (1.58 eV of Fe-C3N4-2h increases to 1.98 eV of Fe-C3N4-6h). Fig. S3 shows the structure evolution of Fe-C3N4-Xh during the synthesis process. The tri-s-triazine units not only suffer the de-amination condensation but are also partially rearranged and destructed. This leads to the formation of a porous structure of Fe-C3N4-Xh during the continuous evaporation process. It is shown in Fig. 2a that Fe-C3N4-6h has a similar interconnected thin layer structure with that of Fe-C3N4-4h, and the edges of Fe-C3N4-6h layers show irregular and curved morphology, which implies that large layer splits into smaller sheets [22]. X-ray photoelectron spectroscopy is performed to explore the surface elemental composition and chemical states of catalyst. The signals of C, N, O and Fe are present in Fe-C3N4-Xh and trace O element is attributed to the absorbed O2/H2O (Fig. S4a). The contents of each element and the N to C atomic ratios of Fe-C3N4-Xh obtained from XPS results are listed in Table. S3. The N/C ratio of Fe-C3N4-6h is significantly lower than other samples. Considering the morphological changes in Fe-C3N4, the decreased N/C ratio is due to the increased pore structures by prolonging the pyrolysis time. The iron content in the samples is around 1.0-1.2 at.%. It reveals that the content of Fe 13

increases with the increase of pyrolysis time from 0 to 4 h (1.01, 1.15, 1.29 at.%) and further increase of pyrolysis time does not increase the Fe content (1.27 at.%). As shown in Fig. 3a, the Fe 2p3/2 spectra of Fe-C3N4-0h reveals two major components including iron(II) phthalocyanine at a binging energy (B.E.) of 709.2 eV and iron(III) phthalocyanine at 710.4 eV [42]. Iron(II) phthalocyanine accounts for 19.1% of the total Fe and its peak disappears in Fe-C3N4-2h (Fig. S4b), which indicates that all Fe centers coordinated to tetradentate N4 ligands transform into Fe(III). This result also indicates that iron(III) porphyrin is more stabilized in the electron-rich graphitic carbon nitride framework. Fig. 3c depicts that N1s region is divided into four species: the hybridized aromatic nitrogen atoms (C-N=C) at 398.6 eV, the tertiary nitrogen (N-(C)3 or C-NH-C) groups at 400.5 eV and porphyrin N (N-Fe) at 399.1 eV, respectively [29, 30, 44]. A shift of hybridized aromatic N to higher binding energy is found because central Fe(III) chelates with N atom to form Fe-N moieties with a lower electron density as well as higher binding energy of N atom. Notably, with prolonged pyrolysis time, the content of porphyrin N gradually increases, while N-(C)3 and C-NH-C contents decrease (Fig. 3d). As shown in Fig. S5a, all Fe-C3N4-Xh samples are of type IV (Brunauer, Deming, Deming, and Teller, BDDT classification) with H3 hysteresis loops on the nitrogen adsorption–desorption isotherms [45]. Prolonging the thermal treating time from 0 to 6 h results in gradually increased specific surface. Besides, when the pyrolysis time exceeds 2h, mesopores and macropores structures can be observed (Fig. S5b), which agree with SEM and TEM results. The separation between Fe-C3N4 layers is accompanied by mesopores generation within exfoliated layers. Such porous nanosheet framework of Fe-C3N4-6h with a large surface area provides more active sites, and accelerates the transfer of reactant and pollutant molecules in the layer stacking network [36]. 14

3.2. Fenton-like and photo-Fenton-like activities RhB dye is chosen to demonstrate the catalytic activities of Fe-C3N4-Xh in Fenton-like and photo-Fenton-like reactions. As shown in Fig. 4, there is no obvious decrease in RhB concentration when C3N4-6h is used to activate H2O2 in Fenton-like reaction (dark). In contrast, an efficient degradation of RhB is observed in C3N4-6h/Vis system, which indicates that the activation of H2O2 is induced by excitation of C3N4-6h with visible light. For the Fe-doped C3N4-6h, clearly, the iron species of Fe-C3N4-6h provides Fenton-like catalytic activity for the decomposition of H2O2 and the degradation efficient with visible light is much higher than that without light irradiation. The Fe-C3N4-6h/Vis possesses the highest activity among all samples, which indicates the iron doping can enhance the photo-Fenton-like activity of C3N4-6h.

Fig. 4. Degradation performance of the reaction systems with no catalyst, C3N4-6h and Fe-C3N4-6h in Fenton-like and photo-Fenton-like processes (C0 (RhB) = 50 ppm, catalyst = 2.0 g/L, visible light > 420 nm, pH = 5.8).

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As shown in Fig. 5a, the adsorption (initial 30 minutes) leads to very limited removal of RhB. It is found that the dye removal efficiency depends on the pyrolysis time of Fe-C3N4-Xh and longer pyrolysis time leads to higher removal efficiency. The first-order kinetic model is calculated for Fe-C3N4-Xh and the results show that the catalytic activities of Fe-C3N4-2h, Fe-C3N4-4h, Fe-C3N4-6h are 2.8, 3.9, 7.6 times higher than that of Fe-C3N4-0h. To study the photo-Fenton-like activity of Fe-C3N4-Xh, the RhB degradation performance with visible light (> 420 nm) was also investigated. Fig. 5b presents that the photo-Fenton-like activities of each catalyst, which, in general, are higher than those without visible light. With the calculated first-order kinetics, the degradation rate of RhB through photo-Fenton-like reaction is almost 4 times higher than that of Fenton-like reaction (Fig. 5c-d). As mentioned above, the RhB degradation performance is particularly regulated by the pyrolysis time. To further study the relation between the performance and pyrolysis time, photo-Fenton-like reaction with C3N4-Xh synthesized by the same method without Fe doping was investigated. The results showed in Fig. S6 confirm a similar trend in the activity of C3N4-Xh with different pyrolysis time. The RhB degradation study with Fe-C3N4-Xh confirms that the rate constant (k) of degradation can be tuned by layered and porous structure of Fe-C3N4-Xh through pyrolysis treatment.

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Fig. 5. RhB degradation performance of Fe-C3N4-Xh in (a) Fenton-like system and (b) photo-Fenton-like systems. (c, d) The plots of –ln (Ct/C0) vs. reaction time of Fenton-like and photo-Fenton-like systems. (C0 (RhB) = 50 ppm, H2O2 = 50 mM, catalyst = 2.0 g/L, visible light wavelength > 420 nm).

3.3. Working mechanism Based on above studies on Fe-C3N4-Xh activity, the iron components greatly change the electronic properties of g-C3N4 [42, 46]. Moreover, these iron phthalocyanine-like organic–metal hybrids exhibit excellent photo-Fenton-like catalytic ability, and the apparent rate constant kFe-C3N4-Xh is significantly higher than kC3N4-Xh apart from Fe-C3N4-0h sample. Generally, the catalytic decomposition of H2O2 to •OH at the interface of solid heterogeneous catalyst is reported as the main working mechanism of Fenton-like reaction at circumneutral and basic pH values [47]. Fig. S7 17

shows that the photocatalytic degradation of RhB with Fe-C3N4-Xh but without H2O2 is very limited, suggesting that decomposition of H2O2 to •OH plays the major role in the reactions. To further confirm this mechanism, isopropanol is chosen as the hydroxyl radical scavenger in Fenton-like and photo-Fenton-like systems [44, 48]. As shown in Fig. 6a, the reaction has been greatly inhibited, implying that •OH dominates the catalytic process in Fe-C3N4-Xh/H2O2 system [49]. The quenching results also indicates that high-valent Fe-oxo species from the coordination of Fe phthalocyanine-like catalyst with fifth ligand (oxygenated group) does not exist or contributes to the reaction. Fe-C3N4-6h sample with high specific surface area still exhibits ~20% RhB removal ability that is due to the surface adsorption-oxidation mechanism [50]. In contrast, in photo-Fenton-like system (Fig. 6b), all Fe-C3N4-Xh catalysts remain some of their degradation abilities. This phenomenon means that Fe-C3N4-xh can still use the light source to decompose hydrogen peroxide into hydroxyl radicals under excessive inhibitor conditions. Hence, the catalysts can adsorb incident photons and •OH radicals when photoinduced electrons are captured by H2O2 [6, 35].

Fig. 6. RhB degradation in (a) Fenton-like and (b) photo-Fenton-like systems in the presence of isopropanol. (C0 (RhB) = 50 ppm, H2O2 = 50 mM, catalyst = 2.0 g/L, visible light wavelength > 420 nm, 20 vol.% isopropanol).

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ESR/DMPO technique was employed to further investigate the active species in our heterogeneous Fenton systems. There is no DMPO-•OH signals being detected in H2O2/Vis and Fe-C3N4-6h/Vis systems (Fig. 7a). The results indicate that a quite small fraction of RhB is degraded by the thermal decomposition of H2O2 itself and photocatalytic of Fe-C3N4-Xh with visible irradiation, which is consistent with the results shown in Fig. 4 and Fig. S7. A typical DMPO/•OH peak with an intensity ratio of 1: 2: 2: 1 appears, which verifies that •OH is the active radicals produced in the Fe-C3N4-6h/H2O2/dark system[51]. Specifically, the intensity of •OH peak tripled in the Fe-C3N4-6h/H2O2/Vis system, revealing that the productivity of •OH is greatly enhanced with light source. The ESR results show that hydroxyl radicals are produced when hydrogen peroxide reacts with Fe-C3N4-Xh, further confirming that superoxide radical and hole trapping agent mechanisms are not suitable in this system.

Fig. 7. (a) DMPO spin-trapping ESR spectra of active radicals in H2O2/Vis, Fe-C3N4-6h/Vis, Fe-C3N4-6h/Dark, and Fe-C3N4-6h/H2O2/Vis systems. (b) Changes in PL intensity with time in different reaction systems (C0 (RhB) = 20 ppm, C0 (BA) = 4 mM, H2O2 = 0.5 mM, Fe-C3N4-6h = 1.0 g/L, visible light > 420 nm).

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Previous study reports that photosensitized dye may act as the electron donor and facilitate the reaction of iron species under visible irradiation [7]. To determine the impact of RhB on the •OH generation under visible light irradiation, PL spectroscopy using benzoic acid (BA) as a probe molecule was taken. Fig. 7b presents the PL spectral with different reaction times under various conditions, where the intensity of fluorescent compound is proportional to the amount of •OH produced. In absence of RhB, the PL intensity of photo-Fenton-like system is obviously stronger than that of Fenton-like reaction. Furthermore, the addition of RhB did not effectively change the yield of hydroxyl radicals in Fe-C3N4-6h/H2O2 system either in dark or visible conditions. It is concluded that Fe-C3N4-Xh reacts with H2O2 to produce hydroxyl radicals and RhB did not promote the photo-Fenton-like reaction.

Fig. 8. (a) H2O2 concentration and (b) pH impacts on RhB degradation with low Fe-C3N4-6h dosage. (C0 (RhB) = 50 ppm, catalyst = 0.5 g/L, visible light > 420 nm, pH = 5.8). (c) Removal performance of Fe-C3N4-6h for various organic pollutants in photo-Fenton-like system. (C0 (organic pollutant) = 25 ppm, H2O2 = 50 mM, catalyst = 2.0 g/L, visible light > 420 nm, initial pH is not adjusted).

The degradation reaction with Fe-C3N4-Xh mainly relies on catalyzing hydroxyl radicals to hydrogen peroxide by iron active species. As shown in Fig. 8a, the RhB degradation efficiency 20

increases with increased H2O2 concentration from 5 to 100 mM with low catalyst dosage. This explains that Fenton-like reaction rate is associated with the amount of oxidants. It is well known that

pH

condition

affects

decomposition

of

H2O2

and

the

speciation

of

iron.

In

homo/heterogeneous-Fenton reaction, acidic pH highly favors the oxidation reaction of organic pollutant [52]. As expected, Fig. 8b shows that low pH value is benefit to higher Fenton-like activity for Fe-C3N4-6h. At pH 9.0, the efficiency of photo-Fenton-like reaction is much higher than that of Fenton-like reaction, which demonstrates that Fe-C3N4-6h embodies the advantage of heterogeneous photo-Fenton-like catalysts. Considering the mentioned working mechanism, photo-induced electrons and photocatalyzed activation of immobilized Fe species play a significant role in enhancing the heterogeneous Fenton catalytic activity. Furthermore, PCA, 2,4-DCP, CIP, BPA and IBP are chosen as typical persistent pollutants to probe the catalytic adaptability of Fe-C3N4-6h. As shown in Fig. 8c, PCA and CIP are completely removed within 100 min in the photo-Fenton-like systems under neutral pH conditions. While BPA, IBP and 2,4-DCP show a removal rate of 90.9%, 97.1, and 97.3% at 100 min, respectively. The physical and chemical properties of these pollutants cause different degradation rates, and CIP has achieved adequate adsorption removal after thirty minutes particularly. The highly active Fe-C3N4-Xh catalyst prepared by adjusting pyrolysis parameters shows great application prospects in emerging contaminants removal including antibiotics. The visible light induced ligand to metal charge transfer (LMCT) transforms Fe(III)-N moieties to Fe(II)-N, thus increasing the generation rate of •OH in Haber-Weiss reaction (Fig. 9) [53]. In the photo-Fenton-like process, two paths of H2O2 initiation for •OH generation are demonstrated in Fig. 9. After irradiation, Fe-C3N4-Xh absorbs incident photons, transforming Fe(III)-N moieties to 21

Fe(II)-N [53]. This path facilitates the conversion of Fe(III) to Fe(II) and is the key to improve the efficiency of Fenton reaction [54]. In the other pathway, the visible light incites charge carriers (e-), which is captured by H2O2 and forms Fenton-like reaction [55]. The two paths clearly demonstrate that Fe-C3N4-Xh can effectively utilize visible light and presents strong ability to excite H2O2.

Fig. 9. Reaction mechanisms of Fe-C3N4 catalyzed Fenton-like and photo-Fenton-like reactions.

3.4. Relationship between catalytic activity and specific surface area Considering that the amount of iron is constant in Fe-C3N4-Xh and XPS results demonstrate that almost all samples involve the complexation of Fe(III) with tetradentate N4-donor ligands in π-conjugated macrocyclic molecules, other properties of catalyst may lead to the difference in catalytic activity. Structure characterizations show that pyrolysis time leads to significant changes in the morphology and structure of Fe-C3N4. The Brunauer–Emmett–Teller (BET) data show that Fe-C3N4-Xh has larger specific surface area (SSA) and higher total pore volume (Vp) with prolonged pyrolysis time (Table S1). The high specific surface area and pore volume are beneficial for 22

increased number of active sites, and can accelerate the transfer of oxidants, pollutants and final products [56]. Therefore, the correlation coefficient between the surface area and k (degradation rate constant from fitted kinetic model) has been established to evaluate the relationship between catalytic activity and Fe-C3N4-Xh structure. Fig. 10a shows that catalytic activity presents a high degree of correlation with SSA (R2 = 0.87). That is to say, the formed layered and porous C3N4 structures bear more active sites (Fe-N4), resulting in enhanced Fenton-like activity. Similarly, the correlation between the k of photo-Fenton-like reaction and SSA is also confirmed (R2 = 0.997) (Fig. 10b). In this regard, the morphological characteristics of Fe-C3N4-Xh have strong impact on catalytic activity in both Fenton-like and photo-Fenton-like reactions. In photo-Fenton-like reaction, regular macroporous nanosheet architecture not only offers more surface area for exposing active sites but also enhances light harvesting to promote the photo-induced reaction [36, 57]. Surprisingly, even in isopropanol quenching experiment (Fig. 10c), the residual catalytic activity of Fe-C3N4-6h is still proportional to SSA, further confirming the strong dependence of Fenton-like and photo-Fenton-like activities on the SSA. The study on SSA and catalyst activity tells that the activity of Fenton-like and photo-Fenton-like activities could be tuned by structure engineering on C3N4 catalysts.

Fig. 10. Linear fit of rate constants with specific surface areas of (a) Fenton-like reaction, (b) photo-Fenton-like reaction, and (c) photo-Fenton-like reaction with isopropanol quenching. 23

4. Conclusion In this work, we reported a simple and environmental friendliness method for the synthesis of iron-doped carbon-nitrogen. The band structure, morphology, and microstructure of Fe-C3N4-Xh can be engineered by varying the pyrolysis time. In Fenton-like and photo-Fenton-like experiments, ·OH was proved to be the predominant oxidants and visible light irradiation could promote the Fe-mediated cycle of Fe-C3N4-Xh to improve the Fenton activity. More strikingly, the catalytic activity is deeply dependent on the specific surface area of Fe-C3N4-Xh. As a result, tuning pyrolysis time in pyrolysis treatment on g-C3N4-based materials is proved to be an effective strategy in light harvesting and Fenton-related environmental remediation methods.

Acknowledgements This work was supported by the National Key R&D Program of China (2018YFC1903201).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi: http://dx.doi.org/10.1016/j.apcatb.xxxxxx.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: