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Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting
Journal Pre-proofs Full Length Article Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting Yujiang Dou, Cheng Zhu, Mengmeng Zhu, Yijun Fu, Huibo Wang, Chunfeng Shi, Hui Huang, Yang Liu, Zhenhui Kang PII: DOI: Reference:
S0169-4332(19)33522-6 https://doi.org/10.1016/j.apsusc.2019.144706 APSUSC 144706
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 June 2019 25 October 2019 14 November 2019
Please cite this article as: Y. Dou, C. Zhu, M. Zhu, Y. Fu, H. Wang, C. Shi, H. Huang, Y. Liu, Z. Kang, Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144706
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Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting Yujiang Doub,1, Cheng Zhua,1, Mengmeng Zhua,1, Yijun Fua, Huibo Wanga, Chunfeng Shic*, Hui Huanga*, Yang Liua, Zhenhui Kanga
a
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, Soochow University, Suzhou 215123, China.
b
School of Electronic and Information Engineering, Soochow University, 1 Shizi Street, Suzhou 215123, Jiangsu, PR China.
c
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, No. 18 Xueyuan Road, Beijing 100083, China.
Abstract Solar water splitting via graphitic carbon nitride (g-C3N4) has achieved extensive attention in recent years. However, g-C3N4 usually suffers from low efficiency and poor stability. Besides, the difficulty lying in the gas mixture separation remain as a big challenge. Herein, a one-pot salt-assisted method was proposed to fabricate the cobalt-doped highly mesoporous g-C3N4 (Co-mCN) photocatalysts for efficient overall water splitting into H2 and H2O2. The adjustable Co doping not only improves the charge separation efficiency, but also enhances the tolerance of g-C3N4 against H2O2 poison. The optimal production for Co-mCN catalysts is gained to be 1.82 µmol h-1 and 1.65 µmol h-1 for H2 and H2O2 respectively, while an apparent quantum efficiency (AQE) of 2.2% at 420 nm and a working life for more than 216 h are also
1
Y Dou, C. Zhu and M Zhu contributed equally to this work. * Corresponding authors. E-mail address: [email protected] (Chunfeng Shi), [email protected] (Hui Huang).
achieved. Moreover, it is demonstrated that the produced H2O2 can be easily collected with titanium silicalite molecular sieve (TS-1) as a reusable H2O2 carrier and directly applied in catalyzing cyclohexane oxidation into cyclohexanone and cyclohexanol mixture with 100% selectivity and 0.11% conversion efficiency. This work provides a new thinking and strategy for realizing overall water splitting, manipulating the products and extending the practical applications of g-C3N4 materials in chemical industry. Keywords: cobalt-doped highly mesoporous g-C3N4; overall water splitting; adjustable Co doping; cyclohexane oxidation.
1.
Introduction
In order to content the energy requirement as well as to solve the environmental issues of the future world, tremendous attempts have been committed to developing competent technologies to generate plentiful and clean energy forms. Hydrogen, considered as one of the most promising clean fuel forms, has aroused world-wide interest in the past decades.[1–3] By taking advantage of solar light and abundant water, photocatalytic overall water splitting via semiconductors has triggered intensive attention since simultaneous evolution of hydrogen and oxygen is a promising and clean energy strategy to power the future society and industry without contamination or greenhouse gas emission.[4–6] However, the formation of O-O bond and evolution of O2 molecules that involved the coupling of four electrons plus the transfer of four protons stay as a tough problem because of the large overpotential to onvercome.[7] Another problem is that the fast recombination of electron-hole pairs seriously restricts the photocatalytic performance, causing low efficiency and the requirement of cocatalyst and sacrificial agents.[8] In recent years, g-C3N4 has become the research focus due to its excellent activity and environmental friendliness. In most of the reported work, however, only by using hole scavengers can the fast recombination of charge carriers be compensated.[9–11] Thus, the overall water splitting of single g-C3N4 photocatalytic systems appears to be particularly important. Instead of using sacrificial agents, the most commonly used methods include: (1) preparing thin and porous g-C3N4 nanostructures;[12,13] (2) loading appropriate cocatalyst;[14,15] and (3) heteroelement doping.[16,17] Though tremendous efforts have been dedicated to the development of g-C3N4 systems, only a handful of g-C3N4 systems have been successfully constructed for overall water splitting with high performance.[7,18–21] Hence, it is of great significance to develop stable g-C3N4 systems with facile and practical methods for efficient overall water splitting. Based on the recent reports, it has been proposed the reaction process for water oxidation half reaction can proceed through either the 2e‒ or 4e‒ pathway, which
corresponds to the product of H2O2 and O2, respectively.[7,22,23] The 2e‒ product H2O2 is an important chemical and fuel with great application values, yet it would poison the g-C3N4 during photocatalysis.[7,22] Although the fast decomposition of H2O2 into O2 is a viable and effective solution to maintain high activity of photocatalysts as well as to achieve H2 and O2 in stoichiometric ratio of 2:1. The difficulty and risk lying in gas mixture separation remain a considerable issue.[3,24] On the other hand, The selective oxidation of cyclohexane (CHA), which can produce feedstocks for high value-added chemicals, has brought lots of concern in recent decades.[25,26] Among the oxidation products, cyclohexanone and cyclohexanol (KA oil), are important intermediates for manufacturing nylon-6,6 and nylon-6.[26,27] Particularly, a high ketone/alcohol molar ratio is more preferred in industry.[26] H2O2 is one of the most typical oxidants used in the oxidation of cyclohexane. However, the generation of H2O2 is not sustainable enough which raises the costs in real production process.[28,29] Therefore, developing an efficient and stable g-C3N4 photocatalytic system that can produce H2O2 and utilize the H2O2 to oxide cyclohexane into important industrial chemicals are significant progress and extension of the practical applications of photocatalysis in chemical industry. Herein, we report a one-pot synthetic strategy for preparing Co-doped highly mesoporous g-C3N4 (Co-mCN) photocatalysts for scavenger-free overall water splitting into H2 and H2O2. The as-prepared photocatalysts exhibit fine activity and stability with H2 and H2O2 evolution rate of 1.82 µmol h-1 and 1.65 µmol h-1 (about 60 times larger than that of mCN), an AQE of 2.2% at 420 nm as well as a working life of more than 216 h. The fine activity and stability of the photocatalysts can be ascribed to the Co doping, which helps to form mesoporous structure, greatly improves the charge separation efficiency as well as enhances the tolerance of g-C3N4 against H2O2. Additionally, we demonstrate a doable method to directly collect the produced H2O2 by TS-1 and utilize it in cyclohexane oxidation, which manifests the merit and application prospect of photocatalytic water splitting in manufacturing important industrial chemicals.
2.
Materials and methods
2.1. Synthesis of Co-mCN Co-mCN with different amount of Co doping contents were prepared by one-step pyrolysis. Firstly, certain amount of Co(NO3)2·6H2O and urea were blended evenly and transferred to a crucible. The crucible was then sealed and calcined to 550 °C with a heating rate of 2 °C min-1 and then kept for 3 h in a muffle furnace. After cooling down to room temperature, the samples were washed and dried for further use. 2.2. Synthesis of mCN mCN was also synthesized through one-step pyrolysis in the same conditions but without the addition of Co(NO3)2·6H2O. 2.3. Materials and Characterization All the materials and reagents were purchased from Sigma-Aldrich and Sinopharm Chemical Reagent Co.,Ltd. Titanium silicalite-1 (TS-1) was purchased from commercial channel. The reagents were used without further purification. The scanning electron microscope (SEM) was applied to characterize the surface morphology and element contents of the samples. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive x-ray (EDX) analysis were measured by using a F20 transmission electron microscope with an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) was carried out to characterize the crystal structure of the as-prepared products by using a PIXcel3D X-ray diffractometer with Cu Kα radiation (Empyrean, Holland Panalytical). The fourier transform infrared (FTIR) spectrum of the samples was acquired from a Hyperion spectrophotometer (Bruker) at the scan range of 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a KRATOS Axis ultra-DLD X-ray photo-electron spectrometer with a monochromatic Al Kα X-ray source. UV/VIS/NIR spectrophotometer (Lambda 750, Perkinelmer) was employed to acquire the UV–vis absorption spectra. A Horiba Jobin Yvon (Fluoro Max-4) luminescence spectrometer was employed to record the photoluminescence (PL) spectra. Ultraviolet photoelectron spectroscopy (UPS) performed with He I
(21.22 eV) as the monochromatic light source was used to analyze the band position of the samples. All the electrochemical measurements were conducted on a CHI 920C workstation (CH Instruments, Shanghai, China), using a standard three-electrode system, of which a saturated calomel electrode (SCE) is used as the reference electrode, a platinum wire as the counter electrode and a glass carbon (GC) electrode as working electrode. Electrochemical impedance spectra (EIS) measurements were carried out at open circuit potential as well, with a frequency range from 1 MHz to 0.01 Hz. Electron paramagnetic resonance (EPR) spectra was used to analyze the N defects of the samples. 2.4. Photocatalytic water splitting test The photocatalytic performance was evaluated on a commercial photocatalytic system with a white light-emitting diode as light source (emitting wavelength: 420−700 nm). Typically, 25 mL ultrapure water containing 10 mg photocatalyst in a photoreactor vial (50 mL) was sealed with specific atmospheres, and then placed in the photocatalytic reaction system with constantly stirring by a magneton at the bottom. Gas production H2 was analyzed by a gas chromatograph (GC-7900T, 5 Å molecular sieve column, TCD detector). Nitrogen (N2) was used as the carrier gas with the flow rate of 30 mL min−1. 2.5. Hydrogen peroxide detection To detect whether H2O2 was formed during the photocatalytic water splitting process, the liquid supernatant was detected by UV–Vis spectroscopy using o-tolidine as the peroxide indicator. By centrifugalizing the catalyst suspension, 3 mL of liquid supernatant was collected, and then 1 mL 1% o-tolidine in 0.1 M HCl was added. Finally, UV–Vis absorption spectrum was used to characterize whether the mixture has a maximum peak at 437 nm, which is the typical peak generated by H2O2. 2.6. Cyclohexane oxidation and measurements After photocatalytic reaction for 24 h, Co-mCN was removed through centrifugation of the reaction suspension, and then TS-1 was added into the solution to fully adsorb the generated H2O2. Afterwards, TS-1 loaded with H2O2 was added into acetone/cyclohexane (10/15 mL) mixture solution in a 50 mL Teflon-lined
stainless-steel autoclave. After solvothermal reaction at 130 °C for 6 hours, the oxidation products were analyzed using a gas chromatograph (GC: Agilent, 7890A). 3.
Results and discussion
3.1. Characterization of photocatalysts The typical synthetic process is shown in Fig. 1a, where Co-mCN samples are prepared by a one-step pyrolysis method using urea and Co(NO3)2·6H2O as the raw materials. The system is sealed and the polymerization reaction is kept at 550 °C for 3 h. During the synthetic progress, the Co(NO3)2·6H2O ingredient not only works as dopant into the g-C3N4 framework on account of the confinement effect, but also plays a crucial role in assisting to promote the formation and extension of a number of voids due to an etching impact.[30–32] Scanning electron microscope (SEM) shows that the prepared Co-mCN photocatalysts are highly porous (Fig. 1b), which is drastically different from the bulk g-C3N4 obtained in absence of cobalt salts (denoted as mCN, Fig. S1). The transmission electron microscope (TEM) also illustrates the highly mesoporous structure of the Co-mCN, indicating that the pore size ranges from 2-50 nm (Fig. 1c). The scanning transition electron microscope-high-angle annular dark field (STEM-HAADF) image is displayed in Fig. 1d while the corresponding EDS mappings are provided in Fig. 1e-i. It is obvious that C, N, Co are evenly distributed in the samples while absent in the pore regions, which suggest the Co doping of the highly mesoporous g-C3N4. The EDS mapping spectrum is shown in Fig. S2 in which the peaks are indexed to be C, N, O and Co elements. In addition, the EDS mappings from SEM verify the homogeneous dispersion of the C, N, O and Co elements (Fig. S3). Typically, the atomic ratio for C : N : O : Co is 40.5% : 55.4% : 1.3% : 2.7%, where the C : N is around 3 : 4, corresponding to the ratio of g-C3N4 (Fig. S4). It is noteworthy that the O has weaker signal than any other element, which mainly comes from the adsorbed oxygen or water molecules.
Fig. 1. Schematic diagram of the synthetic process and characterization of the photocatalysts. (a) Synthetic process; (b) SEM; (c) TEM; (d) STEM and (e-i) the corresponding EDS mappings of the photocatalysts. The samples here are 3.0% Co-mCN.
The phase structure is studied via the X-ray powder diffraction (XRD) shown in Fig. 2a. All the spectra have two prominent peaks located at around 13° and 27.7°, fitting well with the in-plane trigonal nitrogen linkage of tri-s-triazine motifs (100) and the interlayer stacking of conjugated aromatic segments (002) in g-C3N4, respectively.[33,34] The XRD data indicate that all the Co-mCN samples maintain the g-C3N4 structure. The FTIR spectra also confirm the g-C3N4 characteristic features of the prepared Co-mCN samples (Fig. 2b). In detail, the distinct peaks at around 810 cm-1 are ascribed to the band of tri-s-triazine units, while a group of peaks in the region of 1200−1700 cm-1 belongs to the skeletal vibrations of C–N heterocycles. Also, the broad absorption peaks at 3000−3300 cm-1 are assigned to the stretching vibrational modes of the N–H groups.[35,36] The observed characteristic peaks of
Co-mCN in FTIR spectra demonstrate that the synthesized Co-mCN samples possess the same structure with that of g-C3N4, and Co doping does not change the crystal structure. Notably, in the FTIR spectrum of 4.0% Co-mCN, the characteristic peaks in the region of 1200-1700 cm-1 (skeletal vibrations of C–N heterocycles) become inconspicuous which may be caused by the slight destruction of g-C3N4 framework aroused by excessive etching by Co doping. The nitrogen defects are characterized by Electron paramagnetic resonance spectroscopy (EPR). From Fig. 2c, only one single Lorentzian line is detected for all samples in the magnetic field from 3505 to 3525 G, in which the g value around 2.003 is assigned to lone pair electrons in sp2-carbon in a typical heptazine g-C3N4.[37,38] In addition, it can be easily concluded that an increased EPR peak intensity was observed with the increased Co doping, revealing the increased density state of conduction band after the electron donation from the doped Co species.[39] Therefore, Co doping could help to optimize the electronic band structure for charge migration and separation. The adsorption-desorption isotherms of mCN and 3.0% Co-mCN are provided in Fig. 2d, where 3.0% Co-mCN (97.91 m2 g-1) demonstrates a higher specific surface that is about 3.6 times larger than that of mCN (27.34 m2 g-1). It is obvious that both mCN and 3.0% Co-mCN possess Type IV adsorption–desorption isotherms with H3 hysteresis loop in the relative pressure range of 0.6–1.0, confirming the existence of mesopores connected through micropores.[40] In addition, the pore size distribution indicates 3.0% Co-mCN has abundant mesoporous ranging from 10–60 nm, consistent with the mesopores observed in STEM image (Fig. 1c). The comparison of BET surface areas of all the samples is listed in Table S1, from which the BET surface area rises to a maximum value for 3.0% Co-mCN (97.91 m2 g-1) and then decreases for 4.0% Co-mCN (79.51 m2 g-1). The initial increase is ascribed to the stronger etching effect of increased cobalt salt that helps to make more mesopores during the decomposition polymerization, while excessive amount cobalt salt would adversely cause the destruction of g-C3N4 framework and thus reduce the specific surface of samples.
Fig. 2. Characterization of the photocatalysts. (a) XRD; (b) FTIR and (c) EPR spectra of the photocatalysts. (d) N2 adsorption–desorption isotherms and the corresponding pore-size distribution curves obtained by the BJH method using adsorption branch data for mCN and 3.0% Co-mCN.
The compositions and chemical states of the photocatalysts were investigated by XPS. The full spectra of all the samples are shown in Fig. S5 where only C, N, O and Co are identified in consistence with the EDS data from TEM and SEM. In particular, the N 1s core level spectrum of 3% Co-mCN is displayed in Fig. 3a, in which the fitted peaks centered at around 398.7, 400.2, 401.3 and 404.8 eV are attributed to the characteristic g-C3N4 bond types of C‒N=C, N‒(C)3, quaternary N and π excitations, respectively. Particularly, the peak lies at 399.1 eV comes from the Co-Nx bond, denoting the Co doping.[41,42] According to Fig. 3b, no metallic Co signal (778.5 eV) is observed in the Co 2p XPS spectrum.[43] Moreover, the peak at binding energy of 781.4 eV can be ascribed to Co (II) coordinated to nitrogen in the form of Co-Nx species.[42] This value of Co-Nx is higher than that of metallic Co (~779.0 eV) and Co oxide (~780.0 eV) due to the increased electronegativity of N (3.04) versus that of Co (1.88) and O (2.55).[41,42] The XPS data have validated the Co doping of g-C3N4,
which ensures a strong interaction between Co species and g-C3N4 framework, serving for improved charge separation and transport. 3.2. Band structures of the photocatalysts The light absorbance ability of the Co-doped and none doped g-C3N4 are characterized by measuring the UV-vis spectra. All the Co-mCN samples possess broadened absorbance in the visible light range than that of mCN, suggesting the ability of Co-mCN to make use of wider visible light (Fig. 3c). The inset in Fig. 3c presents the color change of the photocatalysts where it is distinct the color of the samples turns darker with increased Co doping. In Fig. 3d, the derived Tauc plots suggest the band gap of mCN is 2.80 eV, while the band gap values of Co-mCN samples are narrower (~2.65 eV). The Co doping, leading to narrowed band gap of g-C3N4, hence is favorable for the photocatalytic activity. Ultraviolet photoelectron spectroscopy (UPS) was employed to determine the valence band (VB) of mCN and 3.0% Co-mCN for further analyzing the effect of Co doping on band structure. It is clear that 3.0% Co-mCN has the same intersections with that of mCN at the baseline (Fig. 3e), demonstrating that mCN and 3.0% Co-mCN have the identical VB values calculated to be −6.72 eV vs. Ev (or 2.28 vs. RHE). In consequence, the corresponding conduction band (CB) values are calculated to be −3.94 eV vs. Ev (−0.5 vs. RHE) and −4.07 eV vs. Ev (−0.37 vs. RHE) for mCN and 3.0% Co-mCN, respectively. The downshift of CB results from the Co doping, which is beneficial for faster charge separation and wider light absorbance. The schematic diagram in Fig. 3f displays the standard band levels of the reactions along with the band levels of the photocatalysts. Specifically, the VB and CB values of mCN and 3.0% Co-mCN both straddle the reduction level for H2 production and oxidation level for H2O2 production, which imply that the photocatalysts can theoretically split water into H2 and H2O2 or H2O.
Fig. 3. Characterization of the photocatalysts. (a) XPS N 1s spectrum; (b) XPS Co 2p spectrum of 3.0% Co-mCN. (c) UV-Vis spectra of the photocatalysts. The inset is the photo of the photocatalysts, the numbers below which represent Co contents. (d) The corresponding Tauc plots of the photocatalysts. (e) UPS spectra of mCN and 3.0% Co-mCN. (f) Band structure of mCN and 3.0% Co-mCN versus RHE and vacuum level.
3.3. Photocatalytic performance of the photocatalysts The photocatalytic properties of as-prepared samples were estimated in ultrapure water without the addition of any scavenger or cocatalyst. The generated H2 and H2O2 were quantified by gas chromatography (GC) and UV-Vis spectroscopy, respectively. As shown, 3.0% Co-mCN exhibits the optimal photocatalytic ability with H2 and
H2O2 evolution rate reaching 1.82 µmol h-1 and 1.65 µmol h-1 in a molar ratio of 1:1 under N2 atmosphere (Fig. 4a), which is about 60 times larger than that of mCN (Fig. S6). Nonetheless, during the reaction later stage, the evolution rate of the products gradually decreases as time goes by, which could be ascribed to the adsorption of H2O2 on the surface of catalyst that impedes the reaction process. Nonetheless, it has been significantly improved compared with mCN. Fig. 4b displays the UV-Vis spectra of the accumulated H2O2 in the reaction solution after 24 h photocatalytic reaction with o-tolidine as an indicator. 3.0% Co-mCN possesses the strongest absorption peak at 437 nm among the as-prepared samples, corresponding to the highest H2O2 concentration in the reaction solution.[23] Blank tests have also been done to eliminate the effect of cobalt ions, which is shown in Fig. S7. Fig. 4c exhibits the comparison of the photocatalytic performance of different samples. Without Co doping, mCN could only generate trace amount of H2 and H2O2, which should be ascribed to the poor charge separation efficiency and the poison of H2O2. With the increase amount of Co doping, the photocatalytic performance of Co-mCN samples gradually improves, which exhibits the optimal performance towards water splitting when the doping content reaches 3.0%. However, keeping increasing the doping amount gives rise to the deterioration of the photocatalytic activity instead (4.0% Co-mCN), since excessive doping could result in the destruction of g-C3N4 framework and the formation of electron-hole recombination centers. Furthermore, the cyclic stability experiment in Fig. 4d manifests that 3.0% Co-mCN exhibits high stability towards water splitting for 216 h, suggesting the high tolerance of 3.0% Co-mCN against H2O2 (Table S2). The H2O2 generation rates of C3N4 and Co-mCN were further measured under different atmospheres (N2, Ar and O2) shown in Fig. S8 and S9. As shown, both pure C3N4 (mCN) and Co-mCN photocatalysts could exhibit better H2O2 evolution rate under O2 atmosphere than that either under N2 or Ar, due to the involved oxygen reduction reaction (ORR). For the sake of appraising the apparent quantum efficiency (AQE) of the optimal photocatalyst, H2 generation rate was measured at 420 nm under ambient pressure and the average AQE value was calculated to be 2.2% (equation S1,
S2).
Fig. 4. Photocatalytic performance of the catalysts. (a) Time course dependence H2 and H2O2 production by 3.0% Co-mCN under N2 atmosphere. (b) Accumulated H2O2 in the reaction solution after 24 h reaction. (c) H2 and H2O2 evolution rates of the photocatalysts. (d) The cyclic stability of 3.0% Co-mCN.
3.4. Study of the reaction mechanism PL lifetime and EIS spectra of as-prepared samples were estimated to analyze the electron transfer property of the catalysts in detail. Generally, the longer lifetime suggests the slower recombination rate of photo-induced electron-hole pairs.[39] The PL lifetime of mCN is short, representing a fast charge recombination speed; therefore, it shows poor photocatalytic activity. After Co doping, the PL lifetime of Co-mCN samples is obviously extended (Fig. 5a). Among the modified Co-mCN samples, 3.0% Co-mCN has the longest PL lifetime, and thus possesses the optimal charge separation efficiency, exemplifying the best photocatalytic ability. EIS was also carried out to assess the ability to transfer the produced charge carriers. Normally, a smaller semicircle in the EIS Nyquist plot means a faster interfacial electron transfer speed.[44] In Fig. 5b, 3.0% Co-mCN has the smallest semicircle, manifesting the best
photo-induced charge transfer ability. According to the photo-response curves of the photocatalysts (Fig. S10), Co doping can clearly improve the stability of the photocurrent upon light irradiation especially for 3.0% Co-mCN, which owns the strongest and most stable photo-response. On the contrary, the photocurrent mCN shows a sharp decrease that implies fast charge recombination. The 4.0% Co-mCN exhibits an inverse deterioration of photocurrent, causing by increased recombination centres due to excessive doping. The PL lifetime data, EIS and photo-response results are well consistent with the photocatalytic activities obtained (Fig. 4c). The 2-electron transfer process of water splitting reaction is verified by the electron transfer number (n) determined via the RRDE curves displayed in Fig. 5c, where distinct mutations are observed on the disk and ring current once light is applied on the surface of the catalyst, illustrating that H2O2 is produced on the disk electrode and detected by the ring electrode. The electron transfer number (n) of 3.0% Co-mCN is calculated to be 2.1 according to the equation S3. Therefore, the 3.0% Co-mCN can split water proceeding the 2e− pathway into H2 and H2O2, which is consistent with the photocatalytic performance shown in Fig. 4a. In order to deal with the separation and utilization problems of H2O2 in the reaction solution, a facile method is put forward to collect the produced H2O2. Titanium silicalite-1 (TS-1) with ZSM-5 structure has been reported to possess robust H2O2 adsorption capability and thus is employed as the carrier here to collect the generated H2O2 in 24 h reaction. The H2O2 loaded TS-1 is then applied in cyclohexane oxidation resulting in products cyclohexanone (selectivity 94.5%) and cyclohexanol (selectivity 5.5%). The selectivity and conversion efficiency of the mixture product KA-oil reach 100% and 0.11%, respectively (Fig. 5d). The splitting of water into H2 and H2O2 by 2e− pathway, hence, is a feasible, effective and facile method to simultaneously obtain clean fuel and important chemical products, which not only avoids the difficulty and risk of separating H2 and O2 gas mixture, but also brings about a possibility of producing important industrial raw products in a greener way.
Fig. 5. (a) PL lifetime, (b) Electrochemical impedance spectra (EIS) and the matched equivalent electric circuit of the photocatalysts. (c) RRDE i-t curves of 3.0% Co-mCN under dark or light condition. (d) The conversion efficiency and selectivity towards cyclohexane oxidation with H2O2 loaded TS-1 as the catalysts.
A schematic diagram is proposed for possible mechanism for the photocatalyst as well as the H2O2 harvest and utilization process (Fig. 6). Firstly, Co-mCN catalysts with suitable bandgap and band positions could absorb visible light and generate electron-hole pairs. Due to the appropriate Co doping, the charge separation efficiency of photo-generated electron-hole pairs is greatly improved. As a result, once visible light irradiates on Co-mCN, electrons and holes quickly get separated and effectively reduce water into H2 and oxide water into H2O2 through 2e− pathway, severally. Also benefiting from Co doping, the tolerance against H2O2 is largely enhanced, which guarantees the high stability of the photocatalysts. After photocatalytic reaction, the produced H2O2 can be easily separated and collected from H2 and water via a reusable carrier TS-1. Subsequently, the TS-1 with harvested H2O2 can be directly employed in cyclohexane oxidation.
Fig. 6. Schematic diagram of the reaction mechanism of the Co-mCN photocatalysts and the following H2O2 harvest and utilization.
4.
Conclusions
In this paper, we reported a facile one-pot method to prepare Co-doped highly mesoporous g-C3N4 photocatalysts to achieve effective and stable photocatalytic water splitting to produce H2 and H2O2. The adjustable Co doping not only improves the charge separation efficiency and prolongs the lifetime of charge carriers, but also greatly improves the tolerance of g-C3N4 against H2O2, thus improving the stability of H2O2 production. The optimal photocatalytic H2 and H2O2 evolution rate reaches 1.82 and 1.65 µmol h-1, while the AQE is 2.2% at 420 nm. What’s more, we use titanium silicon molecular sieve (TS-1) as a reusable carrier to demonstrate the harvest and utilization of the generated H2O2 for catalyzing cyclohexane oxidation into a key feedstock KA-oil with selectivity and conversion rate of 100% and 0.11%, respectively. This work not only provides a feasible way to realize overall water splitting of g-C3N4, but also a new thinking for manipulating the products and linking solar water splitting with green chemical industry in a sustainable way.
Acknowledgements This work is supported by National MCF Energy R&D Program (2018YFE0306105),
the National Natural Science Foundation of China (51725204, 21771132, 21471106, 51972216), the Natural Science Foundation of Jiangsu Province (BK20190041, BK20190828), Guangdong Province Key Area R&D Program (2019B010933001), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 Project.
Conflict of interest The authors declare that they have no conflict of interest.
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version.
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Highlights: 1.
A one-pot salt-assisted method is reported to synthesize Co-doped highly mesoporous g-C3N4.
2.
The fabricated Co-mCN photocatalysts exhibit efficient and stable overall water splitting activity into H2 (1.82 µmol h-1) and H2O2 (1.65 µmol h-1).
3.
Co doping helps to form highly mesoporous structure, which greatly improves the charge separation efficiency and enhances the tolerance of g-C3N4 against H2O2
4.
The generated H2O2 can be easily collected through TS-1 and directly applied in cyclohexane oxidation.
Cobalt-doped highly mesoporous g-C3N4 as efficient and stable photocatalysts for splitting water into H2 and H2O2.
Declaration of Interest Statement All the authors declare that this manuscript is free of any conflict of interest and provide all the Numbers supported by the fund.
S0169-4332(19)33522-6 https://doi.org/10.1016/j.apsusc.2019.144706 APSUSC 144706
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
22 June 2019 25 October 2019 14 November 2019
Please cite this article as: Y. Dou, C. Zhu, M. Zhu, Y. Fu, H. Wang, C. Shi, H. Huang, Y. Liu, Z. Kang, Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144706
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Highly mesoporous carbon nitride photocatalysts for efficient and stable overall water splitting Yujiang Doub,1, Cheng Zhua,1, Mengmeng Zhua,1, Yijun Fua, Huibo Wanga, Chunfeng Shic*, Hui Huanga*, Yang Liua, Zhenhui Kanga
a
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, Soochow University, Suzhou 215123, China.
b
School of Electronic and Information Engineering, Soochow University, 1 Shizi Street, Suzhou 215123, Jiangsu, PR China.
c
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, No. 18 Xueyuan Road, Beijing 100083, China.
Abstract Solar water splitting via graphitic carbon nitride (g-C3N4) has achieved extensive attention in recent years. However, g-C3N4 usually suffers from low efficiency and poor stability. Besides, the difficulty lying in the gas mixture separation remain as a big challenge. Herein, a one-pot salt-assisted method was proposed to fabricate the cobalt-doped highly mesoporous g-C3N4 (Co-mCN) photocatalysts for efficient overall water splitting into H2 and H2O2. The adjustable Co doping not only improves the charge separation efficiency, but also enhances the tolerance of g-C3N4 against H2O2 poison. The optimal production for Co-mCN catalysts is gained to be 1.82 µmol h-1 and 1.65 µmol h-1 for H2 and H2O2 respectively, while an apparent quantum efficiency (AQE) of 2.2% at 420 nm and a working life for more than 216 h are also
1
Y Dou, C. Zhu and M Zhu contributed equally to this work. * Corresponding authors. E-mail address: [email protected] (Chunfeng Shi), [email protected] (Hui Huang).
achieved. Moreover, it is demonstrated that the produced H2O2 can be easily collected with titanium silicalite molecular sieve (TS-1) as a reusable H2O2 carrier and directly applied in catalyzing cyclohexane oxidation into cyclohexanone and cyclohexanol mixture with 100% selectivity and 0.11% conversion efficiency. This work provides a new thinking and strategy for realizing overall water splitting, manipulating the products and extending the practical applications of g-C3N4 materials in chemical industry. Keywords: cobalt-doped highly mesoporous g-C3N4; overall water splitting; adjustable Co doping; cyclohexane oxidation.
1.
Introduction
In order to content the energy requirement as well as to solve the environmental issues of the future world, tremendous attempts have been committed to developing competent technologies to generate plentiful and clean energy forms. Hydrogen, considered as one of the most promising clean fuel forms, has aroused world-wide interest in the past decades.[1–3] By taking advantage of solar light and abundant water, photocatalytic overall water splitting via semiconductors has triggered intensive attention since simultaneous evolution of hydrogen and oxygen is a promising and clean energy strategy to power the future society and industry without contamination or greenhouse gas emission.[4–6] However, the formation of O-O bond and evolution of O2 molecules that involved the coupling of four electrons plus the transfer of four protons stay as a tough problem because of the large overpotential to onvercome.[7] Another problem is that the fast recombination of electron-hole pairs seriously restricts the photocatalytic performance, causing low efficiency and the requirement of cocatalyst and sacrificial agents.[8] In recent years, g-C3N4 has become the research focus due to its excellent activity and environmental friendliness. In most of the reported work, however, only by using hole scavengers can the fast recombination of charge carriers be compensated.[9–11] Thus, the overall water splitting of single g-C3N4 photocatalytic systems appears to be particularly important. Instead of using sacrificial agents, the most commonly used methods include: (1) preparing thin and porous g-C3N4 nanostructures;[12,13] (2) loading appropriate cocatalyst;[14,15] and (3) heteroelement doping.[16,17] Though tremendous efforts have been dedicated to the development of g-C3N4 systems, only a handful of g-C3N4 systems have been successfully constructed for overall water splitting with high performance.[7,18–21] Hence, it is of great significance to develop stable g-C3N4 systems with facile and practical methods for efficient overall water splitting. Based on the recent reports, it has been proposed the reaction process for water oxidation half reaction can proceed through either the 2e‒ or 4e‒ pathway, which
corresponds to the product of H2O2 and O2, respectively.[7,22,23] The 2e‒ product H2O2 is an important chemical and fuel with great application values, yet it would poison the g-C3N4 during photocatalysis.[7,22] Although the fast decomposition of H2O2 into O2 is a viable and effective solution to maintain high activity of photocatalysts as well as to achieve H2 and O2 in stoichiometric ratio of 2:1. The difficulty and risk lying in gas mixture separation remain a considerable issue.[3,24] On the other hand, The selective oxidation of cyclohexane (CHA), which can produce feedstocks for high value-added chemicals, has brought lots of concern in recent decades.[25,26] Among the oxidation products, cyclohexanone and cyclohexanol (KA oil), are important intermediates for manufacturing nylon-6,6 and nylon-6.[26,27] Particularly, a high ketone/alcohol molar ratio is more preferred in industry.[26] H2O2 is one of the most typical oxidants used in the oxidation of cyclohexane. However, the generation of H2O2 is not sustainable enough which raises the costs in real production process.[28,29] Therefore, developing an efficient and stable g-C3N4 photocatalytic system that can produce H2O2 and utilize the H2O2 to oxide cyclohexane into important industrial chemicals are significant progress and extension of the practical applications of photocatalysis in chemical industry. Herein, we report a one-pot synthetic strategy for preparing Co-doped highly mesoporous g-C3N4 (Co-mCN) photocatalysts for scavenger-free overall water splitting into H2 and H2O2. The as-prepared photocatalysts exhibit fine activity and stability with H2 and H2O2 evolution rate of 1.82 µmol h-1 and 1.65 µmol h-1 (about 60 times larger than that of mCN), an AQE of 2.2% at 420 nm as well as a working life of more than 216 h. The fine activity and stability of the photocatalysts can be ascribed to the Co doping, which helps to form mesoporous structure, greatly improves the charge separation efficiency as well as enhances the tolerance of g-C3N4 against H2O2. Additionally, we demonstrate a doable method to directly collect the produced H2O2 by TS-1 and utilize it in cyclohexane oxidation, which manifests the merit and application prospect of photocatalytic water splitting in manufacturing important industrial chemicals.
2.
Materials and methods
2.1. Synthesis of Co-mCN Co-mCN with different amount of Co doping contents were prepared by one-step pyrolysis. Firstly, certain amount of Co(NO3)2·6H2O and urea were blended evenly and transferred to a crucible. The crucible was then sealed and calcined to 550 °C with a heating rate of 2 °C min-1 and then kept for 3 h in a muffle furnace. After cooling down to room temperature, the samples were washed and dried for further use. 2.2. Synthesis of mCN mCN was also synthesized through one-step pyrolysis in the same conditions but without the addition of Co(NO3)2·6H2O. 2.3. Materials and Characterization All the materials and reagents were purchased from Sigma-Aldrich and Sinopharm Chemical Reagent Co.,Ltd. Titanium silicalite-1 (TS-1) was purchased from commercial channel. The reagents were used without further purification. The scanning electron microscope (SEM) was applied to characterize the surface morphology and element contents of the samples. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive x-ray (EDX) analysis were measured by using a F20 transmission electron microscope with an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) was carried out to characterize the crystal structure of the as-prepared products by using a PIXcel3D X-ray diffractometer with Cu Kα radiation (Empyrean, Holland Panalytical). The fourier transform infrared (FTIR) spectrum of the samples was acquired from a Hyperion spectrophotometer (Bruker) at the scan range of 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a KRATOS Axis ultra-DLD X-ray photo-electron spectrometer with a monochromatic Al Kα X-ray source. UV/VIS/NIR spectrophotometer (Lambda 750, Perkinelmer) was employed to acquire the UV–vis absorption spectra. A Horiba Jobin Yvon (Fluoro Max-4) luminescence spectrometer was employed to record the photoluminescence (PL) spectra. Ultraviolet photoelectron spectroscopy (UPS) performed with He I
(21.22 eV) as the monochromatic light source was used to analyze the band position of the samples. All the electrochemical measurements were conducted on a CHI 920C workstation (CH Instruments, Shanghai, China), using a standard three-electrode system, of which a saturated calomel electrode (SCE) is used as the reference electrode, a platinum wire as the counter electrode and a glass carbon (GC) electrode as working electrode. Electrochemical impedance spectra (EIS) measurements were carried out at open circuit potential as well, with a frequency range from 1 MHz to 0.01 Hz. Electron paramagnetic resonance (EPR) spectra was used to analyze the N defects of the samples. 2.4. Photocatalytic water splitting test The photocatalytic performance was evaluated on a commercial photocatalytic system with a white light-emitting diode as light source (emitting wavelength: 420−700 nm). Typically, 25 mL ultrapure water containing 10 mg photocatalyst in a photoreactor vial (50 mL) was sealed with specific atmospheres, and then placed in the photocatalytic reaction system with constantly stirring by a magneton at the bottom. Gas production H2 was analyzed by a gas chromatograph (GC-7900T, 5 Å molecular sieve column, TCD detector). Nitrogen (N2) was used as the carrier gas with the flow rate of 30 mL min−1. 2.5. Hydrogen peroxide detection To detect whether H2O2 was formed during the photocatalytic water splitting process, the liquid supernatant was detected by UV–Vis spectroscopy using o-tolidine as the peroxide indicator. By centrifugalizing the catalyst suspension, 3 mL of liquid supernatant was collected, and then 1 mL 1% o-tolidine in 0.1 M HCl was added. Finally, UV–Vis absorption spectrum was used to characterize whether the mixture has a maximum peak at 437 nm, which is the typical peak generated by H2O2. 2.6. Cyclohexane oxidation and measurements After photocatalytic reaction for 24 h, Co-mCN was removed through centrifugation of the reaction suspension, and then TS-1 was added into the solution to fully adsorb the generated H2O2. Afterwards, TS-1 loaded with H2O2 was added into acetone/cyclohexane (10/15 mL) mixture solution in a 50 mL Teflon-lined
stainless-steel autoclave. After solvothermal reaction at 130 °C for 6 hours, the oxidation products were analyzed using a gas chromatograph (GC: Agilent, 7890A). 3.
Results and discussion
3.1. Characterization of photocatalysts The typical synthetic process is shown in Fig. 1a, where Co-mCN samples are prepared by a one-step pyrolysis method using urea and Co(NO3)2·6H2O as the raw materials. The system is sealed and the polymerization reaction is kept at 550 °C for 3 h. During the synthetic progress, the Co(NO3)2·6H2O ingredient not only works as dopant into the g-C3N4 framework on account of the confinement effect, but also plays a crucial role in assisting to promote the formation and extension of a number of voids due to an etching impact.[30–32] Scanning electron microscope (SEM) shows that the prepared Co-mCN photocatalysts are highly porous (Fig. 1b), which is drastically different from the bulk g-C3N4 obtained in absence of cobalt salts (denoted as mCN, Fig. S1). The transmission electron microscope (TEM) also illustrates the highly mesoporous structure of the Co-mCN, indicating that the pore size ranges from 2-50 nm (Fig. 1c). The scanning transition electron microscope-high-angle annular dark field (STEM-HAADF) image is displayed in Fig. 1d while the corresponding EDS mappings are provided in Fig. 1e-i. It is obvious that C, N, Co are evenly distributed in the samples while absent in the pore regions, which suggest the Co doping of the highly mesoporous g-C3N4. The EDS mapping spectrum is shown in Fig. S2 in which the peaks are indexed to be C, N, O and Co elements. In addition, the EDS mappings from SEM verify the homogeneous dispersion of the C, N, O and Co elements (Fig. S3). Typically, the atomic ratio for C : N : O : Co is 40.5% : 55.4% : 1.3% : 2.7%, where the C : N is around 3 : 4, corresponding to the ratio of g-C3N4 (Fig. S4). It is noteworthy that the O has weaker signal than any other element, which mainly comes from the adsorbed oxygen or water molecules.
Fig. 1. Schematic diagram of the synthetic process and characterization of the photocatalysts. (a) Synthetic process; (b) SEM; (c) TEM; (d) STEM and (e-i) the corresponding EDS mappings of the photocatalysts. The samples here are 3.0% Co-mCN.
The phase structure is studied via the X-ray powder diffraction (XRD) shown in Fig. 2a. All the spectra have two prominent peaks located at around 13° and 27.7°, fitting well with the in-plane trigonal nitrogen linkage of tri-s-triazine motifs (100) and the interlayer stacking of conjugated aromatic segments (002) in g-C3N4, respectively.[33,34] The XRD data indicate that all the Co-mCN samples maintain the g-C3N4 structure. The FTIR spectra also confirm the g-C3N4 characteristic features of the prepared Co-mCN samples (Fig. 2b). In detail, the distinct peaks at around 810 cm-1 are ascribed to the band of tri-s-triazine units, while a group of peaks in the region of 1200−1700 cm-1 belongs to the skeletal vibrations of C–N heterocycles. Also, the broad absorption peaks at 3000−3300 cm-1 are assigned to the stretching vibrational modes of the N–H groups.[35,36] The observed characteristic peaks of
Co-mCN in FTIR spectra demonstrate that the synthesized Co-mCN samples possess the same structure with that of g-C3N4, and Co doping does not change the crystal structure. Notably, in the FTIR spectrum of 4.0% Co-mCN, the characteristic peaks in the region of 1200-1700 cm-1 (skeletal vibrations of C–N heterocycles) become inconspicuous which may be caused by the slight destruction of g-C3N4 framework aroused by excessive etching by Co doping. The nitrogen defects are characterized by Electron paramagnetic resonance spectroscopy (EPR). From Fig. 2c, only one single Lorentzian line is detected for all samples in the magnetic field from 3505 to 3525 G, in which the g value around 2.003 is assigned to lone pair electrons in sp2-carbon in a typical heptazine g-C3N4.[37,38] In addition, it can be easily concluded that an increased EPR peak intensity was observed with the increased Co doping, revealing the increased density state of conduction band after the electron donation from the doped Co species.[39] Therefore, Co doping could help to optimize the electronic band structure for charge migration and separation. The adsorption-desorption isotherms of mCN and 3.0% Co-mCN are provided in Fig. 2d, where 3.0% Co-mCN (97.91 m2 g-1) demonstrates a higher specific surface that is about 3.6 times larger than that of mCN (27.34 m2 g-1). It is obvious that both mCN and 3.0% Co-mCN possess Type IV adsorption–desorption isotherms with H3 hysteresis loop in the relative pressure range of 0.6–1.0, confirming the existence of mesopores connected through micropores.[40] In addition, the pore size distribution indicates 3.0% Co-mCN has abundant mesoporous ranging from 10–60 nm, consistent with the mesopores observed in STEM image (Fig. 1c). The comparison of BET surface areas of all the samples is listed in Table S1, from which the BET surface area rises to a maximum value for 3.0% Co-mCN (97.91 m2 g-1) and then decreases for 4.0% Co-mCN (79.51 m2 g-1). The initial increase is ascribed to the stronger etching effect of increased cobalt salt that helps to make more mesopores during the decomposition polymerization, while excessive amount cobalt salt would adversely cause the destruction of g-C3N4 framework and thus reduce the specific surface of samples.
Fig. 2. Characterization of the photocatalysts. (a) XRD; (b) FTIR and (c) EPR spectra of the photocatalysts. (d) N2 adsorption–desorption isotherms and the corresponding pore-size distribution curves obtained by the BJH method using adsorption branch data for mCN and 3.0% Co-mCN.
The compositions and chemical states of the photocatalysts were investigated by XPS. The full spectra of all the samples are shown in Fig. S5 where only C, N, O and Co are identified in consistence with the EDS data from TEM and SEM. In particular, the N 1s core level spectrum of 3% Co-mCN is displayed in Fig. 3a, in which the fitted peaks centered at around 398.7, 400.2, 401.3 and 404.8 eV are attributed to the characteristic g-C3N4 bond types of C‒N=C, N‒(C)3, quaternary N and π excitations, respectively. Particularly, the peak lies at 399.1 eV comes from the Co-Nx bond, denoting the Co doping.[41,42] According to Fig. 3b, no metallic Co signal (778.5 eV) is observed in the Co 2p XPS spectrum.[43] Moreover, the peak at binding energy of 781.4 eV can be ascribed to Co (II) coordinated to nitrogen in the form of Co-Nx species.[42] This value of Co-Nx is higher than that of metallic Co (~779.0 eV) and Co oxide (~780.0 eV) due to the increased electronegativity of N (3.04) versus that of Co (1.88) and O (2.55).[41,42] The XPS data have validated the Co doping of g-C3N4,
which ensures a strong interaction between Co species and g-C3N4 framework, serving for improved charge separation and transport. 3.2. Band structures of the photocatalysts The light absorbance ability of the Co-doped and none doped g-C3N4 are characterized by measuring the UV-vis spectra. All the Co-mCN samples possess broadened absorbance in the visible light range than that of mCN, suggesting the ability of Co-mCN to make use of wider visible light (Fig. 3c). The inset in Fig. 3c presents the color change of the photocatalysts where it is distinct the color of the samples turns darker with increased Co doping. In Fig. 3d, the derived Tauc plots suggest the band gap of mCN is 2.80 eV, while the band gap values of Co-mCN samples are narrower (~2.65 eV). The Co doping, leading to narrowed band gap of g-C3N4, hence is favorable for the photocatalytic activity. Ultraviolet photoelectron spectroscopy (UPS) was employed to determine the valence band (VB) of mCN and 3.0% Co-mCN for further analyzing the effect of Co doping on band structure. It is clear that 3.0% Co-mCN has the same intersections with that of mCN at the baseline (Fig. 3e), demonstrating that mCN and 3.0% Co-mCN have the identical VB values calculated to be −6.72 eV vs. Ev (or 2.28 vs. RHE). In consequence, the corresponding conduction band (CB) values are calculated to be −3.94 eV vs. Ev (−0.5 vs. RHE) and −4.07 eV vs. Ev (−0.37 vs. RHE) for mCN and 3.0% Co-mCN, respectively. The downshift of CB results from the Co doping, which is beneficial for faster charge separation and wider light absorbance. The schematic diagram in Fig. 3f displays the standard band levels of the reactions along with the band levels of the photocatalysts. Specifically, the VB and CB values of mCN and 3.0% Co-mCN both straddle the reduction level for H2 production and oxidation level for H2O2 production, which imply that the photocatalysts can theoretically split water into H2 and H2O2 or H2O.
Fig. 3. Characterization of the photocatalysts. (a) XPS N 1s spectrum; (b) XPS Co 2p spectrum of 3.0% Co-mCN. (c) UV-Vis spectra of the photocatalysts. The inset is the photo of the photocatalysts, the numbers below which represent Co contents. (d) The corresponding Tauc plots of the photocatalysts. (e) UPS spectra of mCN and 3.0% Co-mCN. (f) Band structure of mCN and 3.0% Co-mCN versus RHE and vacuum level.
3.3. Photocatalytic performance of the photocatalysts The photocatalytic properties of as-prepared samples were estimated in ultrapure water without the addition of any scavenger or cocatalyst. The generated H2 and H2O2 were quantified by gas chromatography (GC) and UV-Vis spectroscopy, respectively. As shown, 3.0% Co-mCN exhibits the optimal photocatalytic ability with H2 and
H2O2 evolution rate reaching 1.82 µmol h-1 and 1.65 µmol h-1 in a molar ratio of 1:1 under N2 atmosphere (Fig. 4a), which is about 60 times larger than that of mCN (Fig. S6). Nonetheless, during the reaction later stage, the evolution rate of the products gradually decreases as time goes by, which could be ascribed to the adsorption of H2O2 on the surface of catalyst that impedes the reaction process. Nonetheless, it has been significantly improved compared with mCN. Fig. 4b displays the UV-Vis spectra of the accumulated H2O2 in the reaction solution after 24 h photocatalytic reaction with o-tolidine as an indicator. 3.0% Co-mCN possesses the strongest absorption peak at 437 nm among the as-prepared samples, corresponding to the highest H2O2 concentration in the reaction solution.[23] Blank tests have also been done to eliminate the effect of cobalt ions, which is shown in Fig. S7. Fig. 4c exhibits the comparison of the photocatalytic performance of different samples. Without Co doping, mCN could only generate trace amount of H2 and H2O2, which should be ascribed to the poor charge separation efficiency and the poison of H2O2. With the increase amount of Co doping, the photocatalytic performance of Co-mCN samples gradually improves, which exhibits the optimal performance towards water splitting when the doping content reaches 3.0%. However, keeping increasing the doping amount gives rise to the deterioration of the photocatalytic activity instead (4.0% Co-mCN), since excessive doping could result in the destruction of g-C3N4 framework and the formation of electron-hole recombination centers. Furthermore, the cyclic stability experiment in Fig. 4d manifests that 3.0% Co-mCN exhibits high stability towards water splitting for 216 h, suggesting the high tolerance of 3.0% Co-mCN against H2O2 (Table S2). The H2O2 generation rates of C3N4 and Co-mCN were further measured under different atmospheres (N2, Ar and O2) shown in Fig. S8 and S9. As shown, both pure C3N4 (mCN) and Co-mCN photocatalysts could exhibit better H2O2 evolution rate under O2 atmosphere than that either under N2 or Ar, due to the involved oxygen reduction reaction (ORR). For the sake of appraising the apparent quantum efficiency (AQE) of the optimal photocatalyst, H2 generation rate was measured at 420 nm under ambient pressure and the average AQE value was calculated to be 2.2% (equation S1,
S2).
Fig. 4. Photocatalytic performance of the catalysts. (a) Time course dependence H2 and H2O2 production by 3.0% Co-mCN under N2 atmosphere. (b) Accumulated H2O2 in the reaction solution after 24 h reaction. (c) H2 and H2O2 evolution rates of the photocatalysts. (d) The cyclic stability of 3.0% Co-mCN.
3.4. Study of the reaction mechanism PL lifetime and EIS spectra of as-prepared samples were estimated to analyze the electron transfer property of the catalysts in detail. Generally, the longer lifetime suggests the slower recombination rate of photo-induced electron-hole pairs.[39] The PL lifetime of mCN is short, representing a fast charge recombination speed; therefore, it shows poor photocatalytic activity. After Co doping, the PL lifetime of Co-mCN samples is obviously extended (Fig. 5a). Among the modified Co-mCN samples, 3.0% Co-mCN has the longest PL lifetime, and thus possesses the optimal charge separation efficiency, exemplifying the best photocatalytic ability. EIS was also carried out to assess the ability to transfer the produced charge carriers. Normally, a smaller semicircle in the EIS Nyquist plot means a faster interfacial electron transfer speed.[44] In Fig. 5b, 3.0% Co-mCN has the smallest semicircle, manifesting the best
photo-induced charge transfer ability. According to the photo-response curves of the photocatalysts (Fig. S10), Co doping can clearly improve the stability of the photocurrent upon light irradiation especially for 3.0% Co-mCN, which owns the strongest and most stable photo-response. On the contrary, the photocurrent mCN shows a sharp decrease that implies fast charge recombination. The 4.0% Co-mCN exhibits an inverse deterioration of photocurrent, causing by increased recombination centres due to excessive doping. The PL lifetime data, EIS and photo-response results are well consistent with the photocatalytic activities obtained (Fig. 4c). The 2-electron transfer process of water splitting reaction is verified by the electron transfer number (n) determined via the RRDE curves displayed in Fig. 5c, where distinct mutations are observed on the disk and ring current once light is applied on the surface of the catalyst, illustrating that H2O2 is produced on the disk electrode and detected by the ring electrode. The electron transfer number (n) of 3.0% Co-mCN is calculated to be 2.1 according to the equation S3. Therefore, the 3.0% Co-mCN can split water proceeding the 2e− pathway into H2 and H2O2, which is consistent with the photocatalytic performance shown in Fig. 4a. In order to deal with the separation and utilization problems of H2O2 in the reaction solution, a facile method is put forward to collect the produced H2O2. Titanium silicalite-1 (TS-1) with ZSM-5 structure has been reported to possess robust H2O2 adsorption capability and thus is employed as the carrier here to collect the generated H2O2 in 24 h reaction. The H2O2 loaded TS-1 is then applied in cyclohexane oxidation resulting in products cyclohexanone (selectivity 94.5%) and cyclohexanol (selectivity 5.5%). The selectivity and conversion efficiency of the mixture product KA-oil reach 100% and 0.11%, respectively (Fig. 5d). The splitting of water into H2 and H2O2 by 2e− pathway, hence, is a feasible, effective and facile method to simultaneously obtain clean fuel and important chemical products, which not only avoids the difficulty and risk of separating H2 and O2 gas mixture, but also brings about a possibility of producing important industrial raw products in a greener way.
Fig. 5. (a) PL lifetime, (b) Electrochemical impedance spectra (EIS) and the matched equivalent electric circuit of the photocatalysts. (c) RRDE i-t curves of 3.0% Co-mCN under dark or light condition. (d) The conversion efficiency and selectivity towards cyclohexane oxidation with H2O2 loaded TS-1 as the catalysts.
A schematic diagram is proposed for possible mechanism for the photocatalyst as well as the H2O2 harvest and utilization process (Fig. 6). Firstly, Co-mCN catalysts with suitable bandgap and band positions could absorb visible light and generate electron-hole pairs. Due to the appropriate Co doping, the charge separation efficiency of photo-generated electron-hole pairs is greatly improved. As a result, once visible light irradiates on Co-mCN, electrons and holes quickly get separated and effectively reduce water into H2 and oxide water into H2O2 through 2e− pathway, severally. Also benefiting from Co doping, the tolerance against H2O2 is largely enhanced, which guarantees the high stability of the photocatalysts. After photocatalytic reaction, the produced H2O2 can be easily separated and collected from H2 and water via a reusable carrier TS-1. Subsequently, the TS-1 with harvested H2O2 can be directly employed in cyclohexane oxidation.
Fig. 6. Schematic diagram of the reaction mechanism of the Co-mCN photocatalysts and the following H2O2 harvest and utilization.
4.
Conclusions
In this paper, we reported a facile one-pot method to prepare Co-doped highly mesoporous g-C3N4 photocatalysts to achieve effective and stable photocatalytic water splitting to produce H2 and H2O2. The adjustable Co doping not only improves the charge separation efficiency and prolongs the lifetime of charge carriers, but also greatly improves the tolerance of g-C3N4 against H2O2, thus improving the stability of H2O2 production. The optimal photocatalytic H2 and H2O2 evolution rate reaches 1.82 and 1.65 µmol h-1, while the AQE is 2.2% at 420 nm. What’s more, we use titanium silicon molecular sieve (TS-1) as a reusable carrier to demonstrate the harvest and utilization of the generated H2O2 for catalyzing cyclohexane oxidation into a key feedstock KA-oil with selectivity and conversion rate of 100% and 0.11%, respectively. This work not only provides a feasible way to realize overall water splitting of g-C3N4, but also a new thinking for manipulating the products and linking solar water splitting with green chemical industry in a sustainable way.
Acknowledgements This work is supported by National MCF Energy R&D Program (2018YFE0306105),
the National Natural Science Foundation of China (51725204, 21771132, 21471106, 51972216), the Natural Science Foundation of Jiangsu Province (BK20190041, BK20190828), Guangdong Province Key Area R&D Program (2019B010933001), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the 111 Project.
Conflict of interest The authors declare that they have no conflict of interest.
Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version.
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Highlights: 1.
A one-pot salt-assisted method is reported to synthesize Co-doped highly mesoporous g-C3N4.
2.
The fabricated Co-mCN photocatalysts exhibit efficient and stable overall water splitting activity into H2 (1.82 µmol h-1) and H2O2 (1.65 µmol h-1).
3.
Co doping helps to form highly mesoporous structure, which greatly improves the charge separation efficiency and enhances the tolerance of g-C3N4 against H2O2
4.
The generated H2O2 can be easily collected through TS-1 and directly applied in cyclohexane oxidation.
Cobalt-doped highly mesoporous g-C3N4 as efficient and stable photocatalysts for splitting water into H2 and H2O2.
Declaration of Interest Statement All the authors declare that this manuscript is free of any conflict of interest and provide all the Numbers supported by the fund.