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Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO2 with H2O
Catalysis Communications 74 (2016) 75–79
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO2 with H2O Yangang Wang a, Yangling Xu a, Yunzhu Wang a, Hengfei Qin b, Xi Li b, Yuanhui Zuo a, Shifei Kang a,⁎, Lifeng Cui a,⁎ a b
Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 6 June 2015 Received in revised form 20 October 2015 Accepted 27 October 2015 Available online 10 November 2015 Keywords: Graphitic carbon nitride Molybdenum Doping Photocatalyst Reduction of CO2
a b s t r a c t Molybdenum doped graphitic carbon nitride (g-C3N4) catalysts were prepared by a simple pyrolysis method using melamine and ammonium molybdate as precursors. The characterization results indicated that the obtained Mo-doped g-C3N4 catalysts had worm-like mesostructures with higher surface area. Introduction of Mo species can effectively extend the spectral response property and reduce the recombination rate of photogenerated electrons and holes. CO2 photocatalytic reduction tests showed that the Mo-doped g-C3N4 catalysts exhibited considerably higher activity (the highest CO and CH4 yields of 887 and 123 μmol g−1-cat., respectively, after 8 h of UV irradiation.) compared with pure g-C3N4 from melamine. © 2015 Published by Elsevier B.V.
1. Introduction The rapidly increasing carbon dioxide (CO 2) emission in the atmosphere from the combustion of fossil fuels is becoming a global environmental issue, such as the greenhouse effect. While the energy crisis caused by overexploitation of fossil fuels and the environmental burdens is recognized to be the two major problems in the foreseeable future [1]. Among various alternatives, photocatalytic reduction of CO2 into energy-rich products such as methane (CH4) in the presence of H2O is a superior way to generate reproducible chemical energy. Since the first report on photocatalytic reduction of CO2 into organic compounds by Inoue and co-workers in early 1979 [2], tremendous research efforts have been made towards developing efficient photocatalysts to achieve CO2 conversion more economically [3–6]. Recently, a metal-free photocatalyst polymeric graphitic carbon nitride (C3 N4 ) with chemically and thermally stable and unique band structure has gained a great deal of scientific interest, and its applications including photodegradation of organic pollutions, hydrogen production by water splitting and photocatalytic reduction of CO2 were demonstrated [7–9]. However, the photocatalytic reactions over pure g-C3N4 still suffer from low conversion efficiencies due to the rapid electron–hole recombination and the low electrical conductivity. ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Kang), [email protected] (L. Cui).
http://dx.doi.org/10.1016/j.catcom.2015.10.029 1566-7367/© 2015 Published by Elsevier B.V.
Therefore, many efforts have been suggested to solve this problem, such as preparing mesoporous structures [10], doping with a metal or nonmetal [11–17], and coupling with other components [18–24]. Among the various strategies, metal doping is one of the most convenient and effective methods to modify the electronic structures of semiconductors as well as their textual properties, thus improving their photocatalytic performances [25]. Wang et al. synthesized Fe-doped g-C3N4 catalysts and suggested that the enhanced photocatalytic performance resulted from the enhanced specific surface area, narrower bandgap and better aligned band structure [17,26]. Our recent results also showed that Ti-doped g-C3N4 catalysts can efficiently increase the photocatalytic activity in dye degradation because of the enhanced optical absorption and accelerated charge carrier transfer rate [27]. Herein, we report a simple pyrolysis method for the synthesis of a series of Mo-doped g-C3N4 catalysts with different doping concentrations. X-ray diffraction (XRD), nitrogen adsorption–desorption, transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (UV–vis DRS) and photoluminescence (PL) spectroscopy were used to characterize the prepared samples. The photocatalytic activities were evaluated in the photocatalytic CO2 reduction with H2O to produce CO and CH4. Indeed, we found that the Mo-doped g-C3N4 catalysts were capable for photocatalytic CO2 reduction with much higher activities than that of pure g-C3N4. Moreover, the correlation between the catalytic performance of the Mo-doped g-C3N4 catalysts and physical properties was investigated.
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the 2θ range of 10–80°. The morphologies and structure of the prepared samples were examined by a transmission electron microscopy (TEM, JEOL JEM-2010). Nitrogen adsorption–desorption isotherms were measured at 77 K on a BeiShiDe 3H-2000PS4 apparatus. Before measurements, the samples were degassed in vacuum at 200 °C for 6 h. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas. The pore size distributions were derived from the desorption branches of the isotherms using the Barrett–Joyner– Halanda (BJH) method. The total pore volume (Vt) was estimated at a relative pressure of 0.99. The diffuse reflectance spectra of the samples over a range of 200–800 nm were recorded by a Shimadezu UV-2401 spectrophotometer. Photoluminescence (PL) spectroscopy was performed on a Hitachi F-7000 spectrophotometer at room temperature. 2.4. Photocatalytic performance tests Fig. 1. XRD patterns of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4, and Mo-CN-5.
2. Experimental 2.1. Chemicals Melamine (C3H6N6) was purchased from Aladdin Chemical Reagent Corp. Ammonium molybdate ((NH4)6Mo7O24·4H2O) was purchased from Sinopharm Chemical Reagent Corp, PR China. All these reagents were analytical pure grade and used without further purification. 2.2. Preparation of Mo-doped g-C3N4 catalysts Typical preparation of the Mo-doped g-C3N4 catalysts was as follows: a certain amount of (NH4)6Mo7O24·4H2O (0–1 mmol) and 0.1 mol of melamine (C3H6N6) was dissolved in 40 mL of deionized water. The solution was stirring for 2 h to obtain a homogeneous solution. The mixture was poured into Petri dishes to evaporate water at 180 °C for 2 h. After being cooled, the resulted product was placed in a semi-closed crucible with a cover to reduce sublimation. The crucible was heated in the muffle furnace at 550 °C for 4 h in air atmosphere with a heating ramp of 10 °C/min. According to this synthesis route, the Mo-doped C3N4 catalysts with different initial Mo element/melamine molar percentage (0, 1%, 2%, 3%, 4% and 5%) were synthesized by tuning the dosage of (NH4)6Mo7O24·4H2O. As a result, the samples were labeled as P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5, respectively. 2.3. Characterization The crystallographic phase of the prepared products was determined by Bruker D8 Advance diffractometer (Cu Kα1 radiation, λ = 1.5406 Å), operated at 40 kV and 40 mA (scanning step: 0.02 °/s) in
The photocatalyic reduction of CO2 experiments was carried out in a home-made Teflon-lined stainless chamber with a quartz window at the top for light irradiation. The volume of the chamber was 2700 mL. 0.1 g of the test catalyst powder was located on stainless omentum which was placed in the center of a reactor. The bottom of glass wool support was moisturized with 5.0 g of deionized water to gain saturated water vapor in the reactor. A 300 W Hg lamp was used at the light source of the photocatalytic reaction. Light was passed through a quartz window and the circulated cooling water was used to keep the photoreactor at ambient temperature during the reaction. The distance between the substrate and Hg lamp was about 15 cm (the intensity of light measured by a digital Lux meter is around 108,500 lx), the reaction temperature and pressure were maintained at 30 °C and 110 KPa, respectively. Prior to illumination, the reactor was vacuumed and purged with the CO2 + H2O mixture at about 20 mL/min for 2 h to establish an adsorption–desorption balance. After that, the reactor was tightly closed and the Hg lamp was then switched on to start the experiment. The gas phase products were taken at various time during the irradiation and analyzed by gas chromatography. 3. Results and discussion Fig. 1 illustrates the XRD patterns of the synthesized Mo-doped g-C3N4 catalysts with different Mo doping concentrations. The typical g-C3N4 has two distinct peaks at 27.5° and 13.2° resulting from structure and tri-s-triazine units, which can be indexed for graphitic materials as the (002) and (100) peaks in JCPDS 87-1526 [28–30]. For doped g-C3N4 catalysts, the main peaks are still retained, indicating the crystal structure is not changed. No characteristic peaks for molybdenum compounds, such as molybdenum oxides and molybdenum nitrides are observed in all the doped g-C3N4 materials, indicating that the Mo species are embedded into in-planes and coordinate to the g-C3N4 matrix by Mo–N bonds
Fig. 2. N2 adsorption–desorption isotherms (a) and corresponding pore size distribution curves (b) of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4, and Mo-CN-5.
Y. Wang et al. / Catalysis Communications 74 (2016) 75–79 Table 1 Textural properties and energy band gap (Eg) of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5. Samples
SBET (m2/g)
Pore size (nm)
Pore volume (cm3/g)
Eg (eV)
P-CN Mo-CN-1 Mo-CN-2 Mo-CN-3 Mo-CN-4 Mo-CN-5
13.52 28.94 30.08 46.83 88.21 110.94
3.90 3.93 3.93 3.84 3.62 3.61
0.288 0.400 0.451 0.528 0.208 0.219
2.65 2.41 2.33 2.23 1.45 1.40
[17,31]. Furthermore, the crystallization degree of the Mo-doped g-C3N4 catalysts decreased evidently with the increase of Mo concentration, since the doped foreign ion alleviated the crystallization process during calcination [26]. Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves of above samples are shown in Fig. 2. All the samples show the type IV isotherms with H1-type hysteresis loop in the relative pressure (P/P0) range of 0.4–0.95, demonstrating their mesoporous characteristics. While the mesoporous features of these catalysts become more significant with increasing the content of doped molybdenum, indicating the presence of Mo element in g-C3N4 is beneficial for the formation of mesopores. These mesopores should originate from the stacking of g-C3N4 sheets [32] since Mo species can strongly interact with the interlayers of g-C3N4 nanosheets through N-bridge linking and increase the space between interlayers in the wake of Mo species doping. The pore size distribution obtained from the analysis of desorption branch of the isotherms is shown in Fig. 2b. It can be seen that all samples have relatively a narrow pore size distribution and their average pore diameters are in the range of 3.6–3.9 nm. The corresponding textural properties are summarized in Table 1. Compared with pure g-C3N4, these Mo-doped g-C3N4 catalysts have a larger BET surface area and this value is increasing with the enhancement of Mo
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doping. The enlargement of the specific surface area is probably due to the hindered crystal growth of graphitic carbon nitride by molybdenum doping, which mainly arises from mesopores according to our measurement. The TEM images of the representative Mo-CN-4 sample are shown in Fig. 3. The low-magnification TEM image given in Fig. 3a reveals that large domains with worm-like mesoporous channels are presented in Mo-CN4 sample, and the pore size estimated from its high-magnification TEM as shown in Fig. 3c is about 3–4 nm, which is consistent with the result of BJH analysis. While the pure g-C3N4 (P-CN) prepared without Mo doping exhibits a sheet-like morphology with less mesoporous characters (Fig. 3d). In addition, the presence of Mo species in Mo-CN-4 sample can be confirmed by its EDX result shown in Fig. 3b. Fig. 4a shows the UV–vis diffuse reflectance spectra (DRS) of the Mo-doped g-C3N4 samples within the range 200–800 nm. It is apparent that the UV–vis spectra of all the Mo-doped g-C3N4 samples had extended a red shift and significant absorption between 400 and 800 nm, both increased with the increase of Mo doping concentration. The red shift of the adsorption edge of the Mo-doped g-C3N4 catalysts has been attributed to the charge-transfer transition between the molybdenum ion d-electrons and the g-C3N4 conduction or valence band [26]. It has been reported that metal doping could form a dopant energy level within the band gap of g-C3N4, the electronic transitions from the valence band to dopant level or from the dopant level to the conduction band can effectively red shift the band edge absorption threshold [33]. In addition, the optical band gap energy of the Mo-doped g-C3N4 catalysts displayed obvious red shifts with respect to that of pure g-C3N4 (listed in Table 1), which are in good consistent with the DRS result. To investigate the photocatalytic performance of the Mo-doped g-C3N4 catalysts with different Mo doping concentrations, their activities for the photocatalytic reduction CO2 with H2O under UV irradiation were studied. Fig. 5 shows the evolution of two main products (CO and CH4) as functions of irradiation time for all catalysts. From the curves, it can be
Fig. 3. TEM images of the representative Mo-CN-4 sample (a, c) and pure g-C3N4 (P-CN) (d); EDX spectrum of the Mo-CN-4 sample (b).
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Fig. 4. (a) UV–visible diffusion reflectance spectra (DRS) and (b) the optical absorption edges of the P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5.
seen that the yields of both CO and CH4 are increased with the irradiation time for all catalysts. It should be noted that the Mo-doped g-C3N4 catalysts exhibit much higher photocatalytic activity than pure g-C3N4, and their activity increases with the increase of Mo doping concentration and reaches to the best for the sample Mo-CN-4 with CO and CH4 yields about 887 and 123 μmol g−1-cat., respectively, after 8 h of UV irradiation. Herein, the slight reduced photocatalytic activity of Mo-CN-5 may ascribe to its decreased crystallinity degree which will produce excessive lattice defects, and some lattice defects usually act as recombination centers for the photoinduced electron/hole pairs during the photocatalytic process [34]. The excellent photocatalytic efficiency of the Mo-doped g-C3N4 catalysts can be attributed to their worm-like mesostructure and narrow band gap, which enable to harvest light more efficiency. Meanwhile, the
heteroatom molybdenum may act as the photogenerated electron target to reduce the recombination of photogenerated electron–hole pairs. In order to prove the above conclusion, the separation efficiency of the photo-generated electrons and holes was characterized by photoluminescence. Fig. 5c compares the PL spectra for the P-CN sample and the Mo-doped g-C3N4 catalysts. It can be observed that the P-CN sample exhibits a strong emission peak at about 445 nm at ambient temperature, which is equivalent to the bandgap energy of 2.65 eV. In comparison with that of pure g-C3 N 4, the strong PL spectra quench quickly for the Mo-doped g-C3N4 catalysts. It is generally acknowledged that the higher fluorescence intensity means more recombination of electron–hole pairs and lower photocatalytic activities [35]. The results illustrate that molybdenum doping gives a positive effect on decreasing the recombination rate of photogenerated charge carriers.
Fig. 5. Yields of CO (a) and CH4 (b) as functions of irradiation time over all catalysts; room temperature PL spectra of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5 catalysts (c).
Y. Wang et al. / Catalysis Communications 74 (2016) 75–79
4. Conclusions In summary, the Mo-doped g-C3N4 catalysts were prepared by a simple pyrolysis method to enhance photocatalytic performance of g-C3N4 for reduction of CO2 with H2O to produce CO and CH4. From the results we measured, the Mo-doped g-C3N4 catalysts exhibited significantly enhanced photocatalytic efficiency when compared with pure g-C3N4, and Mo-CN-4 sample possessed the best activity among the catalysts with different Mo doping concentrations. The remarkable improvement in the photocatalytic activity can be attributed to the lowered recombination rate of electron–hole pairs, the larger surface area, the better mesoporous structure and the lower band gap energy of the Mo-doped g-C3N4 catalysts. The present research is expected to be useful in development of new doping photocatalysts and provide some meaningful information for photocatalytic CO2 conversion. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 21103024), the Program of Shanghai Pujiang Talent Plan (Grant no. 14PJ1406800), and the Capacity-Building of Local University Project by Science and Technology Commission of Shanghai Municipality (Grant no. 12160502400). References [1] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Angew. Chem. Int. Ed. 52 (2013) 7372–7408. [2] M. Fujihira, Y. Satoh, T. Osa, Nature 293 (1981) 206–208. [3] M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, M. Honda, J. Phys. Chem. B 101 (1997) 2632–2636. [4] Y. Shioya, K. Ikeue, M. Ogawa, M. Anpo, Appl. Catal. A Gen. 254 (2003) 251–259. [5] S.C. Yan, S.X. Ouyang, J. Gao, M. Yang, J.Y. Feng, X.X. Fan, L.J. Wan, Z.S. Li, J.H. Ye, Y. Zhou, Z.G. Zou, Angew. Chem. Int. Ed. 49 (2010) 6400–6404. [6] J. Ettedgui, Y. Diskin-Posner, L. Weiner, R. Neumann, J. Am. Chem. Soc. 133 (2011) 188–190.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO2 with H2O Yangang Wang a, Yangling Xu a, Yunzhu Wang a, Hengfei Qin b, Xi Li b, Yuanhui Zuo a, Shifei Kang a,⁎, Lifeng Cui a,⁎ a b
Department of Environmental Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 6 June 2015 Received in revised form 20 October 2015 Accepted 27 October 2015 Available online 10 November 2015 Keywords: Graphitic carbon nitride Molybdenum Doping Photocatalyst Reduction of CO2
a b s t r a c t Molybdenum doped graphitic carbon nitride (g-C3N4) catalysts were prepared by a simple pyrolysis method using melamine and ammonium molybdate as precursors. The characterization results indicated that the obtained Mo-doped g-C3N4 catalysts had worm-like mesostructures with higher surface area. Introduction of Mo species can effectively extend the spectral response property and reduce the recombination rate of photogenerated electrons and holes. CO2 photocatalytic reduction tests showed that the Mo-doped g-C3N4 catalysts exhibited considerably higher activity (the highest CO and CH4 yields of 887 and 123 μmol g−1-cat., respectively, after 8 h of UV irradiation.) compared with pure g-C3N4 from melamine. © 2015 Published by Elsevier B.V.
1. Introduction The rapidly increasing carbon dioxide (CO 2) emission in the atmosphere from the combustion of fossil fuels is becoming a global environmental issue, such as the greenhouse effect. While the energy crisis caused by overexploitation of fossil fuels and the environmental burdens is recognized to be the two major problems in the foreseeable future [1]. Among various alternatives, photocatalytic reduction of CO2 into energy-rich products such as methane (CH4) in the presence of H2O is a superior way to generate reproducible chemical energy. Since the first report on photocatalytic reduction of CO2 into organic compounds by Inoue and co-workers in early 1979 [2], tremendous research efforts have been made towards developing efficient photocatalysts to achieve CO2 conversion more economically [3–6]. Recently, a metal-free photocatalyst polymeric graphitic carbon nitride (C3 N4 ) with chemically and thermally stable and unique band structure has gained a great deal of scientific interest, and its applications including photodegradation of organic pollutions, hydrogen production by water splitting and photocatalytic reduction of CO2 were demonstrated [7–9]. However, the photocatalytic reactions over pure g-C3N4 still suffer from low conversion efficiencies due to the rapid electron–hole recombination and the low electrical conductivity. ⁎ Corresponding authors. E-mail addresses: [email protected] (S. Kang), [email protected] (L. Cui).
http://dx.doi.org/10.1016/j.catcom.2015.10.029 1566-7367/© 2015 Published by Elsevier B.V.
Therefore, many efforts have been suggested to solve this problem, such as preparing mesoporous structures [10], doping with a metal or nonmetal [11–17], and coupling with other components [18–24]. Among the various strategies, metal doping is one of the most convenient and effective methods to modify the electronic structures of semiconductors as well as their textual properties, thus improving their photocatalytic performances [25]. Wang et al. synthesized Fe-doped g-C3N4 catalysts and suggested that the enhanced photocatalytic performance resulted from the enhanced specific surface area, narrower bandgap and better aligned band structure [17,26]. Our recent results also showed that Ti-doped g-C3N4 catalysts can efficiently increase the photocatalytic activity in dye degradation because of the enhanced optical absorption and accelerated charge carrier transfer rate [27]. Herein, we report a simple pyrolysis method for the synthesis of a series of Mo-doped g-C3N4 catalysts with different doping concentrations. X-ray diffraction (XRD), nitrogen adsorption–desorption, transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (UV–vis DRS) and photoluminescence (PL) spectroscopy were used to characterize the prepared samples. The photocatalytic activities were evaluated in the photocatalytic CO2 reduction with H2O to produce CO and CH4. Indeed, we found that the Mo-doped g-C3N4 catalysts were capable for photocatalytic CO2 reduction with much higher activities than that of pure g-C3N4. Moreover, the correlation between the catalytic performance of the Mo-doped g-C3N4 catalysts and physical properties was investigated.
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the 2θ range of 10–80°. The morphologies and structure of the prepared samples were examined by a transmission electron microscopy (TEM, JEOL JEM-2010). Nitrogen adsorption–desorption isotherms were measured at 77 K on a BeiShiDe 3H-2000PS4 apparatus. Before measurements, the samples were degassed in vacuum at 200 °C for 6 h. The Brumauer–Emmett–Teller (BET) method was utilized to calculate the specific surface areas. The pore size distributions were derived from the desorption branches of the isotherms using the Barrett–Joyner– Halanda (BJH) method. The total pore volume (Vt) was estimated at a relative pressure of 0.99. The diffuse reflectance spectra of the samples over a range of 200–800 nm were recorded by a Shimadezu UV-2401 spectrophotometer. Photoluminescence (PL) spectroscopy was performed on a Hitachi F-7000 spectrophotometer at room temperature. 2.4. Photocatalytic performance tests Fig. 1. XRD patterns of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4, and Mo-CN-5.
2. Experimental 2.1. Chemicals Melamine (C3H6N6) was purchased from Aladdin Chemical Reagent Corp. Ammonium molybdate ((NH4)6Mo7O24·4H2O) was purchased from Sinopharm Chemical Reagent Corp, PR China. All these reagents were analytical pure grade and used without further purification. 2.2. Preparation of Mo-doped g-C3N4 catalysts Typical preparation of the Mo-doped g-C3N4 catalysts was as follows: a certain amount of (NH4)6Mo7O24·4H2O (0–1 mmol) and 0.1 mol of melamine (C3H6N6) was dissolved in 40 mL of deionized water. The solution was stirring for 2 h to obtain a homogeneous solution. The mixture was poured into Petri dishes to evaporate water at 180 °C for 2 h. After being cooled, the resulted product was placed in a semi-closed crucible with a cover to reduce sublimation. The crucible was heated in the muffle furnace at 550 °C for 4 h in air atmosphere with a heating ramp of 10 °C/min. According to this synthesis route, the Mo-doped C3N4 catalysts with different initial Mo element/melamine molar percentage (0, 1%, 2%, 3%, 4% and 5%) were synthesized by tuning the dosage of (NH4)6Mo7O24·4H2O. As a result, the samples were labeled as P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5, respectively. 2.3. Characterization The crystallographic phase of the prepared products was determined by Bruker D8 Advance diffractometer (Cu Kα1 radiation, λ = 1.5406 Å), operated at 40 kV and 40 mA (scanning step: 0.02 °/s) in
The photocatalyic reduction of CO2 experiments was carried out in a home-made Teflon-lined stainless chamber with a quartz window at the top for light irradiation. The volume of the chamber was 2700 mL. 0.1 g of the test catalyst powder was located on stainless omentum which was placed in the center of a reactor. The bottom of glass wool support was moisturized with 5.0 g of deionized water to gain saturated water vapor in the reactor. A 300 W Hg lamp was used at the light source of the photocatalytic reaction. Light was passed through a quartz window and the circulated cooling water was used to keep the photoreactor at ambient temperature during the reaction. The distance between the substrate and Hg lamp was about 15 cm (the intensity of light measured by a digital Lux meter is around 108,500 lx), the reaction temperature and pressure were maintained at 30 °C and 110 KPa, respectively. Prior to illumination, the reactor was vacuumed and purged with the CO2 + H2O mixture at about 20 mL/min for 2 h to establish an adsorption–desorption balance. After that, the reactor was tightly closed and the Hg lamp was then switched on to start the experiment. The gas phase products were taken at various time during the irradiation and analyzed by gas chromatography. 3. Results and discussion Fig. 1 illustrates the XRD patterns of the synthesized Mo-doped g-C3N4 catalysts with different Mo doping concentrations. The typical g-C3N4 has two distinct peaks at 27.5° and 13.2° resulting from structure and tri-s-triazine units, which can be indexed for graphitic materials as the (002) and (100) peaks in JCPDS 87-1526 [28–30]. For doped g-C3N4 catalysts, the main peaks are still retained, indicating the crystal structure is not changed. No characteristic peaks for molybdenum compounds, such as molybdenum oxides and molybdenum nitrides are observed in all the doped g-C3N4 materials, indicating that the Mo species are embedded into in-planes and coordinate to the g-C3N4 matrix by Mo–N bonds
Fig. 2. N2 adsorption–desorption isotherms (a) and corresponding pore size distribution curves (b) of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4, and Mo-CN-5.
Y. Wang et al. / Catalysis Communications 74 (2016) 75–79 Table 1 Textural properties and energy band gap (Eg) of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5. Samples
SBET (m2/g)
Pore size (nm)
Pore volume (cm3/g)
Eg (eV)
P-CN Mo-CN-1 Mo-CN-2 Mo-CN-3 Mo-CN-4 Mo-CN-5
13.52 28.94 30.08 46.83 88.21 110.94
3.90 3.93 3.93 3.84 3.62 3.61
0.288 0.400 0.451 0.528 0.208 0.219
2.65 2.41 2.33 2.23 1.45 1.40
[17,31]. Furthermore, the crystallization degree of the Mo-doped g-C3N4 catalysts decreased evidently with the increase of Mo concentration, since the doped foreign ion alleviated the crystallization process during calcination [26]. Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves of above samples are shown in Fig. 2. All the samples show the type IV isotherms with H1-type hysteresis loop in the relative pressure (P/P0) range of 0.4–0.95, demonstrating their mesoporous characteristics. While the mesoporous features of these catalysts become more significant with increasing the content of doped molybdenum, indicating the presence of Mo element in g-C3N4 is beneficial for the formation of mesopores. These mesopores should originate from the stacking of g-C3N4 sheets [32] since Mo species can strongly interact with the interlayers of g-C3N4 nanosheets through N-bridge linking and increase the space between interlayers in the wake of Mo species doping. The pore size distribution obtained from the analysis of desorption branch of the isotherms is shown in Fig. 2b. It can be seen that all samples have relatively a narrow pore size distribution and their average pore diameters are in the range of 3.6–3.9 nm. The corresponding textural properties are summarized in Table 1. Compared with pure g-C3N4, these Mo-doped g-C3N4 catalysts have a larger BET surface area and this value is increasing with the enhancement of Mo
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doping. The enlargement of the specific surface area is probably due to the hindered crystal growth of graphitic carbon nitride by molybdenum doping, which mainly arises from mesopores according to our measurement. The TEM images of the representative Mo-CN-4 sample are shown in Fig. 3. The low-magnification TEM image given in Fig. 3a reveals that large domains with worm-like mesoporous channels are presented in Mo-CN4 sample, and the pore size estimated from its high-magnification TEM as shown in Fig. 3c is about 3–4 nm, which is consistent with the result of BJH analysis. While the pure g-C3N4 (P-CN) prepared without Mo doping exhibits a sheet-like morphology with less mesoporous characters (Fig. 3d). In addition, the presence of Mo species in Mo-CN-4 sample can be confirmed by its EDX result shown in Fig. 3b. Fig. 4a shows the UV–vis diffuse reflectance spectra (DRS) of the Mo-doped g-C3N4 samples within the range 200–800 nm. It is apparent that the UV–vis spectra of all the Mo-doped g-C3N4 samples had extended a red shift and significant absorption between 400 and 800 nm, both increased with the increase of Mo doping concentration. The red shift of the adsorption edge of the Mo-doped g-C3N4 catalysts has been attributed to the charge-transfer transition between the molybdenum ion d-electrons and the g-C3N4 conduction or valence band [26]. It has been reported that metal doping could form a dopant energy level within the band gap of g-C3N4, the electronic transitions from the valence band to dopant level or from the dopant level to the conduction band can effectively red shift the band edge absorption threshold [33]. In addition, the optical band gap energy of the Mo-doped g-C3N4 catalysts displayed obvious red shifts with respect to that of pure g-C3N4 (listed in Table 1), which are in good consistent with the DRS result. To investigate the photocatalytic performance of the Mo-doped g-C3N4 catalysts with different Mo doping concentrations, their activities for the photocatalytic reduction CO2 with H2O under UV irradiation were studied. Fig. 5 shows the evolution of two main products (CO and CH4) as functions of irradiation time for all catalysts. From the curves, it can be
Fig. 3. TEM images of the representative Mo-CN-4 sample (a, c) and pure g-C3N4 (P-CN) (d); EDX spectrum of the Mo-CN-4 sample (b).
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Fig. 4. (a) UV–visible diffusion reflectance spectra (DRS) and (b) the optical absorption edges of the P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5.
seen that the yields of both CO and CH4 are increased with the irradiation time for all catalysts. It should be noted that the Mo-doped g-C3N4 catalysts exhibit much higher photocatalytic activity than pure g-C3N4, and their activity increases with the increase of Mo doping concentration and reaches to the best for the sample Mo-CN-4 with CO and CH4 yields about 887 and 123 μmol g−1-cat., respectively, after 8 h of UV irradiation. Herein, the slight reduced photocatalytic activity of Mo-CN-5 may ascribe to its decreased crystallinity degree which will produce excessive lattice defects, and some lattice defects usually act as recombination centers for the photoinduced electron/hole pairs during the photocatalytic process [34]. The excellent photocatalytic efficiency of the Mo-doped g-C3N4 catalysts can be attributed to their worm-like mesostructure and narrow band gap, which enable to harvest light more efficiency. Meanwhile, the
heteroatom molybdenum may act as the photogenerated electron target to reduce the recombination of photogenerated electron–hole pairs. In order to prove the above conclusion, the separation efficiency of the photo-generated electrons and holes was characterized by photoluminescence. Fig. 5c compares the PL spectra for the P-CN sample and the Mo-doped g-C3N4 catalysts. It can be observed that the P-CN sample exhibits a strong emission peak at about 445 nm at ambient temperature, which is equivalent to the bandgap energy of 2.65 eV. In comparison with that of pure g-C3 N 4, the strong PL spectra quench quickly for the Mo-doped g-C3N4 catalysts. It is generally acknowledged that the higher fluorescence intensity means more recombination of electron–hole pairs and lower photocatalytic activities [35]. The results illustrate that molybdenum doping gives a positive effect on decreasing the recombination rate of photogenerated charge carriers.
Fig. 5. Yields of CO (a) and CH4 (b) as functions of irradiation time over all catalysts; room temperature PL spectra of P-CN, Mo-CN-1, Mo-CN-2, Mo-CN-3, Mo-CN-4 and Mo-CN-5 catalysts (c).
Y. Wang et al. / Catalysis Communications 74 (2016) 75–79
4. Conclusions In summary, the Mo-doped g-C3N4 catalysts were prepared by a simple pyrolysis method to enhance photocatalytic performance of g-C3N4 for reduction of CO2 with H2O to produce CO and CH4. From the results we measured, the Mo-doped g-C3N4 catalysts exhibited significantly enhanced photocatalytic efficiency when compared with pure g-C3N4, and Mo-CN-4 sample possessed the best activity among the catalysts with different Mo doping concentrations. The remarkable improvement in the photocatalytic activity can be attributed to the lowered recombination rate of electron–hole pairs, the larger surface area, the better mesoporous structure and the lower band gap energy of the Mo-doped g-C3N4 catalysts. The present research is expected to be useful in development of new doping photocatalysts and provide some meaningful information for photocatalytic CO2 conversion. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 21103024), the Program of Shanghai Pujiang Talent Plan (Grant no. 14PJ1406800), and the Capacity-Building of Local University Project by Science and Technology Commission of Shanghai Municipality (Grant no. 12160502400). References [1] S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Angew. Chem. Int. Ed. 52 (2013) 7372–7408. [2] M. Fujihira, Y. Satoh, T. Osa, Nature 293 (1981) 206–208. [3] M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, M. Honda, J. Phys. Chem. B 101 (1997) 2632–2636. [4] Y. Shioya, K. Ikeue, M. Ogawa, M. Anpo, Appl. Catal. A Gen. 254 (2003) 251–259. [5] S.C. Yan, S.X. Ouyang, J. Gao, M. Yang, J.Y. Feng, X.X. Fan, L.J. Wan, Z.S. Li, J.H. Ye, Y. Zhou, Z.G. Zou, Angew. Chem. Int. Ed. 49 (2010) 6400–6404. [6] J. Ettedgui, Y. Diskin-Posner, L. Weiner, R. Neumann, J. Am. Chem. Soc. 133 (2011) 188–190.
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