Photocatalysts of 3D Ordered Macroporous TiO2-Supported CeO2

Article pubs.acs.org/IECR

Photocatalysts of 3D Ordered Macroporous TiO2‑Supported CeO2 Nanolayers: Design, Preparation, and Their Catalytic Performances for the Reduction of CO2 with H2O under Simulated Solar Irradiation Jinqing Jiao,‡ Yuechang Wei,*,‡ Zhen Zhao,* Jian Liu, Jianmei Li, Aijun Duan, and Guiyuan Jiang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18# Fuxue Road, Chang Ping, Beijing 102249, China S Supporting Information *

ABSTRACT: 3D ordered macroporous (3DOM) TiO2 was synthesized by the method of colloidal crystal template (CCT) using tetrabutyl titanate as precursor solution, and the photocatalysts of the 3DOM TiO2-supported CeO2 nanolayer with different weight ratios of CeO2 to TiO2 were successfully prepared by the gas bubbling-assisted membrane precipitation (GBMP) method. The catalysts were systematically characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), mercury intrusion porosimetry (MIP), X-ray photoelectron spectroscopy (XPS), UV−vis diffuse reflectance spectroscopy (DRS), and photoluminescence spectra (PL). The slow light effect of photonic crystal (3DOM structure) can enhance absorption efficiency of solar irradiation. Moreover, the introduction of CeO2 nanolayers can effectively extend the photoresponse from UV to visible region and improve the separation of photogenerated electron−hole pairs. The photocatalytic activities for the reduction of CO2 with H2O were evaluated by the production of main product (CO). 3DOM CeO2/TiO2 photocatalysts exhibit high catalytic activity for the photocatalytic reduction of CO2 with H2O under simulated solar irradiation.

1. INTRODUCTION The energy crisis and the increasing CO2 concentration in the atmospheric layer due to overuse of fossil fuels are recognized to be the two major problems in the foreseeable future.1,2 The photocatalytic CO2 reduction with H2O to hydrocarbon fuels under the solar irradiation has sparked a new sustainable development way, which would help to reduce atmospheric CO2 concentration and partly solve the energy crisis. The reaction is similar to the photosynthesis of green plants which produces glucose and oxygen from CO2 and H2O. The chemical conversion of CO2 into industrially beneficial compounds is also advantageous in terms of Green and Sustainable Chemistry because CO2 is an inexpensive, nontoxic, and abundant C1 feedstock. However, the conversion of CO2 with H2O, two very stable molecules accompanied by a large positive change in the Gibbs free energy, is one of the biggest challenges in chemistry.3 Therefore, the design and synthesis of efficient photocatalysts to enable the reduction of CO2 for the production of fuels or chemicals have attracted much attention.4 The solar photocatalytic conversion of CO2 to produce fuels and chemicals by using active photocatalysts can be realized. In this heterogeneous process, the reaction takes place at either a solid−liquid or a solid−gas interface. The photocatalyst is usually a hybrid material which absorbs light, separates the photogenerated charges, transports them to the surface, and provides active sites for the catalytic reaction. Since the pioneering studies by Inoue and Fujishima et al.,5 many studies have been devoted to the semiconductor-based photocatalysts, and the pace has increased enormously in recent years.6−10 Among the semiconductor-based photocatalysts, TiO2 is the most common and promising photocatalyst for CO2 reduction to fuels by far, which is attributed to its advantage including cheapness, availability, stability, and nontoxicity.11−13 However, © XXXX American Chemical Society

the photocatalytic activity of pure TiO2 for the reduction of CO2 upon irradiation with visible light is not high enough for practical application due to the drawbacks of the wide band gap, the low absorption efficiency of solar irradiation, and the easy charge recombination. Photoactivity derives from oxidizing holes and reducing electrons produced by exciting TiO2 at wavelengths shorter than its ultraviolet absorption edge. In spite of some chemical modifications like doping and sensitization, we increase the absorption of sunlight via the spherical voids, which would increase the distance of the light path by enhancing random light scattering.14,15 Photonic crystal (PCs) materials offer a great capability of controlling and manipulating the flow of incident light due to a periodic dielectric contrast in the length scale of the wavelength of light, and they have wide applications in the fields of optics, catalysis, and electronics. The slow photon enhancement of photonic crystals for increasing light harvesting has been attracting strong interest in photocatalysis and photoelectrochemistry.16 For example, three-dimensionally ordered macroporous (3DOM) TiO2 PCs or PC segments were constructed and demonstrated enhanced performances for photodegradation dye and hydrogen evolution.17−19 However, most reported PCs are constructed on a single component, and few cases have been reported on introducing a PC into composite system to improve photocatalytic efficiency. To enhance the catalytic activity of TiO2 under solar irradiation, various approaches have been used, such as reducing size of TiO2 nanoparticles,20 loading of noble metal,21 doping with ions, and modifying the crystal structure.22−24 Nowadays, it Received: August 22, 2014 Revised: October 1, 2014 Accepted: October 13, 2014

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shows that TiO2 coupled with other materials, such as WO3,25 CdS,26 Fe2O3,27 CeO2,28 and ZrO2,29 can enhance the photocatalytic performance via improving the charge separation of photogenerated charges, extending the photoresponse region, and facilitating the CO2 activation.30 Among various oxide semiconductors, CeO2/TiO2 composite is the most promising and challenging material because the introduction of CeO2 not only change their original structural properties but also extend TiO2’s bandgap to higher wavelengths. Therefore, the construction of 3DOM TiO2-supported CeO2 nanolayer photocatalysts for the reduction of CO2 with H2O is expected to gain good photocatalytic performance for CO2 reduction. Motivated by the above viewpoints, we designed and synthesized novel photocatalysts with 3DOM TiO2-supported CeO2 nanolayers by the method of gas bubbling-assisted membrane precipitation (GBMP). In a series of routes, monodispersed PMMA microsphere was used as the template, while tetrabutyl titanate and cerium(III) nitrate hexahydrate acted as precursor solution. It was found that 3DOM CeO2/ TiO2 catalysts significantly enhanced the photocatalytic activity for the reduction of CO2 with H2O, which is attributed to that the slow light effect of photonic crystal (3DOM structure) can improve the absorption efficiency of solar irradiation, and the introduction of CeO2 nanolayers can effectively improve the photoresponse range from UV to visible and enhance the lightharvesting charge separation.

on a PerkinElmer PHI-1600 ESCA spectrometer. The binding energies of the elements were calibrated using C 1s peak of contaminant carbon (BE = 284.6 eV) as an internal standard. The photoluminescence (PL) spectra were carried out on a Hitachi F-4600 fluorescence spectrophotometer. 2.3. Activity Tests. The photocatalytic activity of CO2 reduction with H2O was performed in a gas-closed circulation system, and the photocatalyst (0.1 g) was put in a glass reactor with a basal diameter of 4 cm. The light source was a 300 W Xe lamp (l = 320−780 nm) simulating sunlight purchased from Perfect Light Co. Light was passed through an optical filter with the absorbed light wavelength of 420 nm) irradiation time (a. P25; b. TiO2; c. CeO2/TiO2-2; d. CeO2/TiO2-4; e. CeO2/ TiO2-8; f. CeO2/TiO2-16; g. CeO2; h. particle-type CeO2/TiO2-4).

2H 2O + 4h+ → 4H+ + O2

(E2)

CO2 + 2H+ + 2e‐ → CO + 2H 2O

(E3)

Figure 8. Production amounts of CO (a), CH4 (b) and O2 (c) over 3DOM CeO2/TiO2-4 catalysts (0.1 g) with aan irradiation time of 24 h. In order to clearly show the production amount of CH4, the values are magnified to 10 times.

production amount of O2 over the 3DOM CeO2/TiO2-4 catalyst was 2.68 μmol. From the formation amounts of CO and CH4 reductive products, the stoichiometric formation amounts of O2 can be calculated by assuming that the oxidation of water to O2 is the sole reaction to consume the photogenerated holes. The stoichiometric amount of O2 formation is equal to the sum of half of the CO formation amount and two times the CH4 formation amount. The value is 2.49 μmol, which is lower than the actual amount of O2 formation due to the formation of other undetected hydrocarbon. Based on the above result of photocatalytic activity, we can deduce that the solar energy is absorbed by TiO2 support, and the structural effect can improve the light harvesting due to slow photon enhancement of the 3DOM structure. The heterojunction between the CeO2 nanolayer and the 3DOM TiO2 support can promote the separation of light-induced electrons and holes. However, too thick nanolyers can cover the absorption and active sites of 3DOM TiO2 surface. Therefore, 3DOM CeO2/TiO2 catalysts with a suitable loading amount of CeO2 nanolayers have good photocatalytic performance for the reduction of CO2 with H2O under simulated solar irradiation. The stability of photocatalytic activity for the formation rate of CO over the 3DOM CeO2/TiO2-2 catalyst was further investigated via consecutive three test cycles, and the result is shown in Figure 9. The 3DOM CeO2/TiO2-4 catalyst maintains its high catalytic activities for the photocatalytic reduction of CO2 with H2O in three test cycles under the same reaction condition, i.e., the third values of the formation amount of CO are 2.01 μmol, which have not obviously changed in comparison with the first values, and based on the results of SEM (Figure S4), XRD (Figure S5), XPS (Figure S6), and UV−vis DRS (Figure S7), it is proved that, after the three test cycles of photocatalytic reduction of CO2 with H2O, the macropore structure and the electronic properties of surface elements over the 3DOM CeO2/TiO2-4 catalyst have not obviously changed in comparison with the fresh catalyst. It indicates that 3DOM CeO2/TiO2 catalysts have the good stability of structure and photocatalytic activity for the CO2 reduction under the simulated solar irradiation. 3.4. Photocatalytic Mechanism. The enhanced photocatalytic activity of 3DOM TiO2-supported CeO2 nanolayer catalysts can be attributed to the unique structure with variety

where the conduction band (CB) flat band potential of TiO2 (P25) is −0.56 V vs NHE at pH 7,46 and the reduction potential of (E3) is −0.53 V vs NHE at pH 7.47 As the CB flatband potential is more negative than the CO2/CO reduction potential, reaction E3 is theoretically feasible. As shown in Figure 7, it can be observed that after 400 min light irradiation, the amount of CO production over the commercial P25 is 0.79 μmol, while the amount of CO production over the 3DOM TiO2 catalyst is determined to be 1.0 μmol. It indicates that the structural effect of the 3DOM catalyst may play a role for enhancing photoreduction activity of CO2 to CO. For the 3DOM CeO2 catalyst, the photocatalytic activity for the CO2 reduction with H2O under simulated solar irradiation is very low, and the CO production amount is only 0.18 μmol after 400 min light irradiation. However, 3DOM CeO2/TiO2 catalysts show good photocatalytic performance for the reduction of CO2 with H2O under simulated solar irradiation. Among the as-prepared samples, the 3DOM CeO2/TiO2-4 catalyst presents the highest CO production amount (2.06 μmol) during 400 min light irradiation, indicating that the synergetic effect of CeO2 and TiO2 can enhance the photocatalytic performance. It can also be observed that the CO production amount decreases with the increasing of CeO2 loading amount (>4), illustrating that the loading amounts of CeO2 nanolayers have optimum value. It is also noted that the production rate of CO decreases slightly with increasing of time as shown in Figure 7. It may be attributed to that the product of CO as an intermediate species can further react with e− and H+ under the simulated solar irradiation and further produce C, CH3OH, and CH4 in the carbene pathway.48 The formation rate of CO lowers slightly with the increasing of time. In order to verify the ratiocination, an experiment of photocatalytic CO2 reduction over the 3DOM CeO2/TiO2-4 catalyst with an irradiation time of 24 h was carried out, and the result is shown in Figure 8. After the irradiation time of 240 min, the O2 production is detected by the TCD detector with a 5A column, and after the irradiation time of 600 min, a small quantity of CH4 production is detected by the FID detector. During 1440 min light irradiation, the G

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4. CONCLUSIONS 3DOM TiO2 was prepared by the CCT method using tetrabutyl titanate as the precursor solution. The visible-light responsive photocatalysts of 3DOM TiO2 carriers-supported CeO2 nanolayer with different weight ratios of CeO2 to TiO2 were successfully synthesized by the GBMP method. The photocatalytic activities of 3DOM CeO2/TiO2 catalysts are greatly enhanced compared with TiO2 support and CeO2 nanoparticles. They show high photocatalytic activities for the reduction of CO2 with H2O under visible light illumination. It is attributed to the synergistic effect of the photonic crystals and the heterojunction between TiO2 and CeO2. The slow light effect of photonic crystal (3DOM structure) can enhance absorption efficiency of solar irradiation and ordered macroporous make the diffusion of reactant molecular easier, and the heterojunction between TiO2 and CeO2 can improve the separation of photogenerated electron−hole pairs. The fabrication of oxide nanolayers supported on the inner wall of photonic crystal oxides could be applied in various heterogeneous catalysis systems, and it could illustrate a promising way to the design and synthesis of highly efficient photocatalyst for the CO2 reduction with H2O under visible light irradiation.

Figure 9. Stability study of photocatalytic CO production amounts over 3DOM CeO2/TiO2-4 catalysts via consecutive three test cycles.

of favorable properties rather than the residual N element in TiO2 or CeO2 during the synthesis process (Figure S8). Scheme 1 shows the schematic of the enhanced photocatalytic

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Scheme 1. Schematic for the Enhanced Photocatalytic Activity of CeO2 Nanolayers Supported on the Surface of the 3DOM TiO2 Carrier for the Reduction of CO2 with H2O

ASSOCIATED CONTENT

S Supporting Information *

Detailed description of preparation procedure and reagent specifications of 3DOM CeO2/TiO2 catalysts (Scheme S1 and Table S1). Pore size distribution of 3DOM TiO2 and CeO2/ TiO2 catalysts determined by MIP (Figure S1). Visible-Raman spectra of 3DOM TiO2 and CeO2/TiO2 catalysts (Figure S2). Ti 2p XPS spectra of 3DOM TiO2 and CeO2/TiO2 catalysts (Figure S3). SEM images (Figure S4), XRD patterns (Figure S5), XPS spectra (Figure S6), and UV−vis DRS (Figure S7) of 3DOM CeO2/TiO2-4 catalysts after the three test cycles. Wide scanning XPS spectrum of 3DOM CeO2/TiO2-16 catalysts (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

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activity of 3DOM CeO2/TiO2 catalysts for the CO2 reduction with H2O. First, the slow photon enhancement plays an important role for improving photocatalytic activity. 3DOM CeO2/TiO2 catalysts show higher activity for the photocatalytic reduction of CO2 than the particle-style CeO2/TiO2 catalysts. Second, the introduction of CeO2 species in composites can effectively extend the spectral response from UV to visible area owing to the CeO2-photosensitization. At the same time, because of the inner electric field, the heterojunction between CeO2 and TiO2 improve separation of photogenerated charge carriers.39 Improved separation translates into slower recombination rates and an increase in the efficiency of the process. Based on the result of XPS analysis, it is confirmed that the CeO2 addition on the surface of 3DOM TiO2 can greatly enhance the surface chemisorbed oxygen species, which can easily capture electrons and yield surface oxygen radicals with excellent reduction capability. Meanwhile, the existence of Ce3+/Ce4+ mixture in catalysts can interact with holes and prevent the combination of photogenerated electrons and holes, resulting in a higher quantum efficiency of photocatalytic reaction.49,50 In conclusion, the superior performance of 3DOM CeO2/TiO2 catalysts since the combination of efficient electron light harvesting via 3DOM structure and separation of photogenerated electrons and holes via the heterojunction between TiO2 and CeO2.

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86 10 89731586. Fax: +86 10 69724721. E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ‡

J. Jiao and Y. Wei contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (No. 21177160 and 21303263), Beijing Nova Program (No. Z141109001814072), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130007120011), and Science Foundation of China University of Petroleum, Beijing (QZDX2011-02, No. 2462013YJRC13, and 2462013BJRC003) H

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