Elsevier

Catalysis Communications

Volume 74, 10 January 2016, Pages 75-79
Catalysis Communications

Short communication
Synthesis of Mo-doped graphitic carbon nitride catalysts and their photocatalytic activity in the reduction of CO2 with H2O

https://doi.org/10.1016/j.catcom.2015.10.029Get rights and content

Highlights

  • Mo-doped g-C3N4 catalysts were synthesized through a simple pyrolysis method.

  • The Mo-doped g-C3N4 catalysts have worm-like mesostructures with high surface area.

  • Induced Mo species into g-C3N4 can extend the spectral response property.

  • A lower recombination rate of the electron–hole pairs in Mo-doped g-C3N4 catalysts

  • The Mo-doped g-C3N4 catalysts exhibited an enhanced activity in the photoreduction of CO2.

Abstract

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.

Introduction

The rapidly increasing carbon dioxide (CO2) 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], [4], [5], [6].

Recently, a metal-free photocatalyst polymeric graphitic carbon nitride (C3N4) 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], [8], [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. Therefore, many efforts have been suggested to solve this problem, such as preparing mesoporous structures [10], doping with a metal or nonmetal [11], [12], [13], [14], [15], [16], [17], and coupling with other components [18], [19], [20], [21], [22], [23], [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.

Section snippets

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.

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

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], [29], [30]. For doped g-C3N4 catalysts, the main peaks are still retained, indicating the crystal structure is not changed. No characteristic peaks for molybdenum

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

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).

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