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Photocatalytic Reduction of CO2 to Methane on Pt/TiO2 Nanosheet Porous Film
Anatase TiO2 nanosheet porous films were prepared by calcination of the orthorhombic titanic acid films at 400°C. They showed an excellent photocatalytic activity for CO2 photoreduction to methane, which should be related to their special porous structure and large Brunauer-Emmett-Teller (BET) surface area. In order to further improve the photocatalytic activity, Pt nanoparticles were loaded uniformly with the average size of 3-4 nm on TiO2 porous films by the photoreduction method. It was found that the loading of Pt expanded the light absorption ability of the porous film and improved the transformation efficiency of CO2 to methane. The conversion yield of CO2 to methane on Pt/TiO2 film reached 20.51 ppm/h·cm2. The Pt/TiO2 nanosheet porous film was characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and ultraviolet-visible light diffuse reflectance spectra (UV-vis DRS). Moreover, the transient photocurrent-time curves showed that the Pt/TiO2 nanosheet porous film exhibited higher photocurrent, indicating that the higher separation efficiency of the photogenerated charge carriers was achieved.
Fossil fuels are our primary source of energy. Unfortunately, CO2 emissions generated in using these fuels have drastically increased in atmosphere in recent years, and the fast-growing CO2 leads to climate change, which has become one of the greatest threats of environmental problems. It is very urgent to reduce the accumulation of CO2 in the atmosphere. In general, the photocatalytic reduction of CO2 is a possible avenue to convert CO2 into hydrocarbon fuels, because reducing the amount of CO2 will not only meet the purpose of environmental protection but also provide raw materials for chemical industry. This process utilizes ultraviolet (UV) and/or visible light as the excitation source for semiconductor catalysts, and the photoexcited electrons reduce CO2 with H2O on the catalyst surface and form energy-bearing products such as carbon monoxide (CO), methane (CH4), methanol (CH3OH), formaldehyde (HCHO), and formic acid (HCOOH) .
Many researchers  have shown that CO2 could be reduced by water vapor or solvent with photocatalysts. Among these photocatalysts, TiO2 or TiO2-based materials may promote the photoreduction of CO2 to useful organic compounds [3–9]. Moreover, TiO2 is one of the most intensively studied and widely used photocatalysts as a result of a number of advantageous features such as low cost, relatively high catalytic activity, low toxicity, and high chemical stability [10–12]. Especially modification of TiO2 through noble metal supporting is increasingly being considered for maximising its photocatalytic efficiency. These metals may facilitate electron-hole separation and promote interfacial electron transfer or they may decrease the TiO2 band gap, which benefits electrons transfer from the valence band to the conduction band, facilitating the formation of oxidative species. The TiO2-based nanomaterials, especially the titanate network films, obtained by the hydrothermal method often have large BET surface area and strong adsorption ability . It is noticeable that the surface network structure of the film can enhance the adsorption of the reactive species and absorption efficiency of the incident light and further improve the photocatalytic activity [14–16].
Herein, anatase TiO2 nanosheet porous films were obtained by calcination of the orthorhombic titanic acid films at 400°C. Because the surface of titanic acid consists of the porous network structure , the obtained anatase TiO2 has stronger absorption ability and higher photocatalytic performance compared with the film prepared by sol-gel method in the same condition. Moreover, it is noticeable that the porous structure of the film also can enhance the adsorption of the reactive species and absorption efficiency of the incident light [14–16]. In order to further improve the photocatalytic activity, Pt nanoparticles were loaded on the surface of it as an electron trapper to capture the photogenerated electrons by the photoreduction method. The relationship between the morphology, structure, and their photocatalytic activity was investigated in detail.
2.1. Preparation of TiO2 and Pt-TiO2 Nanosheet Porous Films
The precursor for preparing TiO2 porous film was titanate nanotube (TAN) porous film, and the typical preparing process of TAN film was as follows: a Ti thin foil with a size of 2*4 cm2 was put into 100 mL of 10 M NaOH aqueous solution, followed by the hydrothermal treatment in a 120 mL Teflon-lined autoclave at 120°C for 24 h. After cooling down, the obtained titanate network film was washed with distilled water several times and then immersed in a 0.1 M HCl aqueous solution overnight. After that, the product was washed several times with water and then dried in the N2 stream.
TiO2 nanosheet porous films were prepared by calcination of TAN films at 400°C for 4 h in air. Then, Pt nanoparticles were loaded by the photoreduction method in H2PtCl6 ethanol solution. The solution was illuminated under UV light for 1 h. After that, the sample was washed with deionized water and dried. For a comparison, a TiO2 film prepared by a sol-gel method was used as a reference ; the sample was denoted by R-TiO2.
UV-vis diffuse reflectance spectra (DRS) were obtained on a Shimadzu U-3010 spectrometer, using BaSO4 as a reference. X-ray diffraction (XRD) patterns were measured on an X’Pert Philips diffractometer (Cu Kα radiation; 2θ range 5–90°, step size 0.08°, accelerating voltage 40 kV, and applied current 40 mA). The particle size and surface morphology of the samples were observed using a scanning electron microscope (SEM) (JSM-7100F, JEOL Co., Japan) and transmission electron microscope (TEM) (JEM-2010, JEOL Co., Japan).
2.3. Photoelectrochemical Measurements
The transient photocurrent-time (-) experiments were conducted using a conventional three-electrode system on the electrochemical analyzer (IM6ex, Germany). The photocatalyst film served as the working electrode, and a Pt meshwork and an Ag/AgCl electrode (SCE) acted as the counter electrode and reference electrode, respectively. The electrolyte was Na2SO4 with a concentration of 0.5 mol·L−1.
2.4. Photocatalytic Activity Evaluation
The photocatalytic reduction of CO2 was conducted in a flat closed reactor with the inner capacity of 358 mL containing 20 mL 0.1 mol/L KHCO3 solution. The prepared TiO2 nanosheet porous film was located in the center of the reactor and then the ultrapure gaseous CO2 and water vapor was flowed through the reactor for 2 h to achieve the adsorption-desorption equilibrium. Before illumination, the reactor was sealed. The light source was the high pressure Hg lamp with 300 W, and the intensity of the incident light was measured to be 10.4 mW/cm2. The photocatalytic reaction was typically performed at room temperature for 6 h. The products were measured by gas chromatography (GC). The comparison tests consisted of a reaction under light without the catalysts and a reaction in dark with the catalysts. The results indicated that there was almost no methane production in the comparison experiments.
3. Results and Discussions
3.1. Phase Structure, Morphology, and Optical Absorption of Pt-Loaded TiO2 Nanosheet Porous Film
The phase structure of the titania films was analyzed by the XRD technique. As shown in Figure 1(a), the TiO2 film belongs to the typical anatase phase. In our previous work, we knew that TAN precursor belonged to the orthorhombic system . This indicated that the orthorhombic TAN has been transformed to anatase TiO2 completely after being calcined at 400°C for 4 h. There still existed some characteristic peaks of metallic Ti at 40.2°, 63.1°, and 70.7°, indicating that only the surface of the Ti foil reacted with NaOH, and the interior still remained as Ti metal. When Pt nanoparticles were loaded on the surface of TiO2 nanosheet porous films, the anatase phase did not change. And no apparent Pt diffraction signals appeared; this may be due to the ultrafine dispersion of Pt nanoparticles on TiO2 nanosheet porous films and its low loading amount. However, the EDS results verified that Pt nanoparticles were successfully modified on the TiO2 nanosheet porous films (as shown in Figure 1(b)); the mole ratio of Pt to TiO2 was estimated to be ca. 0.14%.
Figure 2 shows the surface morphology of the Pt-loaded TiO2 nanosheet porous films. As can be seen from Figure 2(a), anatase TiO2 film consisted of many thin nanosheets, and the surface of the film looks like porous structure. Figure 2(b) showed that some Pt nanoparticles were successfully deposited on the surface of TiO2 nanosheet porous film and were homogeneous both in size and in shape. To further observe the morphology of TiO2 film, some powders were peeled off from the substrate, and their TEM images are shown in Figures 2(c) and 2(d); we can clearly see that Pt nanoparticles are very uniform, and their particle sizes are only 3-4 nm. On one hand, the porous nanosheet structure of this kind of TiO2 film occupied larger BET surface area than the common TiO2 film obtained by the sol-gel method, so it can increase the adsorption amount of the reactive species and then accelerate the photocatalytic reaction rate. On the other hand, more irradiated light can be utilized for the porous TiO2 film because of the multiple scattering and reflection of the incident light in the channels of the porous film [17, 20, 21], so the utilization efficiency of the incident light was increased and thereby did favor for improving the photocatalytic activity.
The optical absorption of the nanosheet porous films was shown in Figure 3. The absorption band edge of TiO2 porous film was 380 nm. In our previous work [13, 22], we found that the onset absorption of the titanate acid was about 350–360 nm. So from the change of the absorption band, we can conclude that the titanate acid had transformed to anatase TiO2 successfully by the calcination of 400°C. While Pt nanoparticles were loaded on the surface of the TiO2 porous film, a broad peak at around 350–550 nm was observed, which should be due to the plasma resonance absorption of the Pt nanoparticles . Moreover, the absorbance intensity of the film in the UV light region reduced, which should be because the loaded Pt nanoparticles shield some absorption of TiO2 nanosheets.
3.2. Photoreduction of CO2 on Pt-Loaded TiO2 Nanosheet Porous Film
The photoreduction of CO2 to methane was tested as a probe reaction to evaluate the photocatalytic activity of the catalyst films. As shown in Figure 4(a), the production rate of CH4 on Pt-loaded TiO2 nanosheet porous film reached 20.51 ppm/h·cm2. To confirm the photocatalytic reduction process of CO2 to CH4, the related reference experiments were carried out. When the system was kept in dark, there was no CH4 produced, indicating that the photoexcited process of Pt-loaded TiO2 was essential in the photoreduction of CO2. When the experiment was carried out in the absence of H2O, almost no CH4 was detected. That implying that water is also one of the key roles for CO2 photoreduction. When a blank Ti foil with the same area of Pt-loaded TiO2 film was put into the system, the production rate of CH4 was only 1.01 ppm/h·cm2. This slow production rate of CH4 should be due to the thin oxide layer on Ti foil surface.
In addition, some comparative experiments about Pt-loaded TiO2, TiO2 porous film, and ordinary R-TiO2 obtained by the sol-gel method proceeded. As can be seen, under the same experiment conditions, the production rate of CH4 on Pt-loaded TiO2, TiO2, and R-TiO2 was 20.51, 3.71, and 1.45 ppm/h·cm2, respectively. Obviously, the photocatalytic activity of the Pt-TiO2 nanosheet porous film was much higher than that of TiO2 and R-TiO2 film; the possible reasons were listed as follows. Firstly, it is commonly known that Pt could promote the interparticle charge migration and facilitate the photogenerated electrons transfer from conduction band of the TiO2 to Pt particles, so as to provide adequate electrons for the reduction of carbon dioxide to methane . The charge carrier separation ability of Pt nanoparticles was verified by the transient photocurrent-time curve. As shown in Figure 4(b), the photocurrent density of TiO2 and Pt-TiO2 nanosheet film was 0.006 and 0.017 mA·cm−2, respectively. The photocurrent density of Pt-TiO2 was apparently larger than that of TiO2, indicating that its separation efficiency of the photogenerated charge carriers was higher. Secondly, the large BET surface area and strong adsorption ability of the TiO2 nanosheet porous structure can provide more adsorption sites for CO2 molecules, so the localized concentration of CO2 on the surface of TiO2 porous film would be higher, which would accelerate the photoreduction reaction of CO2 to methane. Thirdly, the porous and incompact structure of the TiO2 nanosheet porous film would facilitate the use of more irradiated light, because more light can be scattered and reflected in the channels and pores of TiO2 film .
3.3. Proposal of the Photoreduction Mechanism of CO2 to Methane on Pt-Loaded TiO2 Nanosheet Porous Film
The photoreduction mechanism of CO2 to methane on Pt-loaded TiO2 nanosheet porous film was proposed in Figure 5. Most researchers agree that this process is based on proton-assisted multielectron transfer instead of single electron transfer, as the electrochemical potential of −2.14 V versus SCE for a single electron process is highly unfavorable . When Pt-loaded TiO2 nanosheet porous film was illuminated by UV light, photon-generated electrons (e-) and holes (h+) are created on the surface of the TiO2 nanosheets. The excited holes reacted with adsorbed water molecules on the catalyst surface to produce hydroxyl radicals (•OH) and hydrogen ions (H+) and further oxidized by •OH radicals to produce O2 and H+ . And H+ would interact with the excited electrons to form •H radicals. At the same time, the photogenerated electrons on the conduction band of TiO2 can be easily trapped by Pt nanoparticles because of the lower Fermi energy level of the noble metal , and then they would transfer rapidly to the absorbed CO2 for photoreduction reaction. CO2 molecules would interact with the excited electrons to form to radicals, and then reacted with •H on the catalyst surface to produce CH4 [1, 28, 29]. It is known that the formation of CH4 requires eight electrons, and the enriched electron density on Pt nanoparticles would favor the formation of CH4, which is thermodynamically more feasible. The possible process would have undergone the following pathway:
Anatase TiO2 nanosheet porous films obtained by calcination of the orthorhombic titanic acid films exhibited better performance for the photoreduction of CO2 to methane. In order to further improve the photoactivity, Pt nanoparticles with the particle size of 3-4 nm were loaded on the TiO2 porous films uniformly. The EDS results confirmed the mole ratio of Pt to TiO2 was ca. 0.14%. The conversion yield of CO2 to methane on Pt/TiO2 film reached 20.51 ppm/h·cm2. The transient photocurrent-time curves showed that the Pt/TiO2 nanosheet porous film exhibited higher photocurrent, so the higher separation efficiency of the photogenerated charge carriers should be the main reason for the high photoreduction activity of CO2.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Li Qiu-ye and Zong Lan-lan contributed equally to this work.
The authors gratefully acknowledge the support of the National Natural Science Foundation of China (no. 21103042), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 20114103120001), and the Scientific Research Foundation of Henan University (no. 2010YBZR013).
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