Simple Preparation of Hierarchically Porous Ce/TiO2/Graphitic Carbon Microspheres for the Reduction of CO2 with H2O under Simulated Solar Irradiation.

Hierarchically porous Ce/TiO2/graphitic carbon microsphere composites (xCe/TiO2/GCM, where x = 0.2, 1.0, 2.0, 5.0 mmol·L-1) were prepared for the first time by using a simple colloidal crystal template and characterized by X-ray diffraction, nitrogen adsorption and desorption, scanning electron microscopy, and ultraviolet-visible diffuse reflectance spectra. In addition, the photocatalytic activity of CO2 reduction by H2O under simulated solar irradiation was studied. The results showed that the Ce/TiO2/GCM composite material was characterized by large porosity, high concentration of metal compounds and graphitized carbon matrix, and the content of acetone solvent having a great impact on its form. In terms of the photocatalytic CO2 reaction, the CH4 and CO productions were 4.587 and 357.851 μmol·g-1, respectively. The 2Ce/TiO2/GCM photocatalyst gave the highest production rate for three products. Under simulated solar irradiation, the Ce/TiO2/GCM has excellent photocatalytic activity in the photoreduction of CO2 from H2O, which was related to the special composition and the Ce/TiO2/GCM structure.

Introduction

The problem of climate change and global warming caused by excessive CO2 discharges is becoming increasingly serious[1−5] but, unfortunately, the future exhaustion of fossil fuel resources is also worrying.[6−8] An ideal solution to global warming and the carbon cycle in nature is to reap solar energy to convert photocatalytic CO2 into renewable hydrocarbon fuels using H2O. For this purpose,[9−15] researchers have attempted to develop efficient technologies such as high-performance catalysts.

The preparation methods of composite photocatalysts include chemical reduction,[16−18] light-induced reduction, photodeposition, thermal decomposition, liquid impregnation, sol–gel, and hydrothermal method. Photocatalysts synthesized by various methods have effective photocatalytic properties, which provide broad prospects for applications in the fields of optical computation,[19] solar cells, photocatalysis, and communication. For instance, the photocatalysts of Ce4+ and TiO2 were synthesized by chemical coprecipitation peptization and hydrothermal and hydrolysis methods, respectively, in Xie[20] and effectively improved the photocatalytic performances through Ce4+–TiO2 sol and nanocrystallites. The photocatalytic solar fuel of TiO2 photonic crystal prepared by anodizing and calcining without a template has the characteristics of slow photon and local surface photothermal effect[21] as well as improvement in the catalytic reaction rate and light utilization, respectively. Using ionic liquid as a template, mesoporous palladium-doped TiO2 with an anatase phase and a high specific surface area was synthesized by the green sol–gel method. At the same residence time, the photodegradation performance of NO (88%) and CO (74%) was significantly better than that of IL-TiO2 (59 and 56%) without doping.[22] Despite some achievements in catalyst preparations, preparation methods still have the disadvantages of cumbersomeness and high cost.

Herein, we proposed a method for the convenient synthesis of layered porous Ce/TiO2/graphitic carbon microspheres (xCe/TiO2/GCM, where x = 0.2, 1.0, 2.0, and 5.0 mmol·L–1) via colloidal crystal templating. The structural characteristics of the composites were analyzed by X-ray diffraction (XRD), nitrogen adsorption–desorption, scanning electron microscopy (SEM), and ultraviolet–visible light diffuse reflectance spectra. Their photocatalytic activity for reducing CO2 from H2O under simulated solar irradiation was studied. The results showed that the induced Ce significantly improved the photoactivity of the hierarchically porous TiO2/GCM.

Results and Discussion

Preparation of the Ce/TiO2/GCM Composites

As shown in Figure , the colloidal crystal templating method for the synthesis of hierarchically porous Ce/TiO2/GCM[23,24] is actually much simpler than the traditional multistep synthesis method.

Figure 1

Step diagram of preparing hierarchically porous Ce/TiO2/graphitic carbon microspheres by the colloidal crystal template method.

The preparation flow chart is shown in Figure . Acetone, soybean oil, TiCl4, and Ce(NO3)3·6H2O were mixed under stirring to form a stable microemulsion, which was then mixed with monodisperse SiO2 microspheres under stirring to form a suspension. After 3 days, acetone evaporated from the microemulsion droplets, and the interaction between monodisperse SiO2 microspheres was intensified to form colloidal superparticles of SiO2 microspheres. Furthermore, the microemulsion with colloidal superparticles of SiO2 microspheres was placed in the tube furnace for carbonaceous polymerization/graphitization heat treatment. With the gradual increase of temperature, during the heat treatment process at 900 °C under an Ar atmosphere, metallic crystals grew in situ and were embedded into the graphitic carbon matrix. The hierarchically porous Ce/TiO2/GCM, which can be classified as colloidal superparticles,[25,26] was formed after the removal of the SiO2 colloidal crystal sphere template by the silica colloidal crystal ball template and was incubated in sodium hydroxide solution at room temperature. Finally, Ce/TiO2/GCM was synthesized as expected.

XRD of Ce/TiO2/GCM Composites

The formation of Ce/TiO2/GCM composites with different compositions was first proved by XRD (Figure ). The anatase phase of TiO2 is observed for all photocatalysts. The diffraction peaks at 2θ of 25, 48, 53, 55, and 63° correspond to the (101), (200), (105), (211), and (204) crystal planes of the pure anatase phase of TiO2, respectively. The peak of graphite carbon at 25° (002) was overlapped by that of anatase at 25° (101) in each composite.[21,27]

Figure 2

XRD patterns of Ce/TiO2/GCM composite materials (illustrated by the energy-dispersive system (EDS) spectrum of the 0.2Ce/TiO2/GCM composite).

The XRD signal of 0.2Ce/TiO2/GCM is even noisier than the results. Since there were almost no peaks in the background spectrum, none of the crystal information extracted from the above complexes can be considered reliable. However, owing to the small size of the nanoparticles it is difficult to track the evolution as Ce concentration increases. Fortunately, the peaks of the anatase Ce are strengthened with the increasing Ce content in the composites, indicating that the increase of the crystal particle size, which is well consistent with the EDS results in Figure (inset: 0.2Ce/TiO2/GCM EDS spectrum). For instance, the contents of Ce, C, O, and Ti in 0.2Ce/TiO2/GCM are 1.11, 5.08, 37.90, and 55.92 wt %, respectively.

N2 Adsorption–Desorption of the Ce/TiO2/GCM

The N2 adsorption–desorption isotherms and corresponding pore size distribution curves of the porous carbons are shown in Figure . All isotherms in Figure a show strong absorption of N2 due to capillary condensation within the wide relative pressure (P/P0) range of 0.6–0.9, which suggests the existence of multiform pore distributions.[28,29] It is associated with the interparticle voids to the small absorption at P/P0 of 0.9–0.99.[30,31] Additionally, as shown in Figure b, most of the pores are located at 3.8–12 and 21–175.1 nm, which proves the graded porosity of the composite materials. The structural properties of all composites are shown in Table . The average diameter of graded porous composites is 13.37 nm, and much larger Brunauer–Emmett–Teller (BET) surface areas than the largest surface area of graphitic carbon composites are reported by Cai[32] (132–204 vs 52.3 m2·g–1). Among all composite materials, the specific surface area of 2Ce/TiO2/GCM was the highest, which is 204 m2·g–1. Thus, the Ce/TiO2/GCM composites have large specific surface areas and advanced pore structures.

Figure 3

(a) N2 adsorption–desorption isotherms and (b) the pore size distribution of layered porous Ce/TiO2/GCM composites.

Table 1

BET Surface Area and Pore Size for Ce/TiO2/GCM

samples	SBETa (m2·g –1)	Dpb (nm)
0.2Ce/TiO2/GCM	132.0	17.1
1Ce/TiO2/GCM	143.3	12.8
2Ce/TiO2/GCM	204.0	12.4
5Ce/TiO2/GCM	176.7	11.2

SBET: the specific surface area is calculated by the BET method.

Dp: the pore diameter is calculated by the Barrett–Joyner–Halenda (BJH) method.

SEM Analysis of Ce/TiO2/GCM Composites

With acetone (45 mL) as the solvent, some representative SEM images are shown in Figure . Clearly, the microsphere morphology of the monodisperse SiO2 template was copied into the porous materials. Finally, the long period structure of the colloidal crystal with the most valuable properties was left after the original colloidal SiO2 particles were removed. The long-term arrangement of layered porosity has led to potential applications in emerging nanotechnologies, advanced coatings, and optical information processing and storage, and other areas.[33,34] This valuable feature was further verified by the N2 adsorption test (Figure ). The diameters of Ce/TiO2/GCM composites are about 2 μm.

Figure 4

SEM micrographs of (a) 0.2Ce/TiO2/GCM, (b) 1Ce/TiO2/GCM, (c) 2Ce/TiO2/GCM, and (d) 5Ce/TiO2/GCM with 45 mL acetone.

Photoreduction of CO2 under Simulated Solar Irradiation

We measured the photocatalytic activity of xCe/TiO2/GCM on the photoreduction of gas-phase CO2 under simulated solar irradiation. Figure shows the change rule of CH4 and CO products with the irradiation time for all samples. The results showed that the yields of all composites increased with the increase in the reaction times. The synergistic effect of GCM and Ce significantly enhanced the photocatalytic activity of the composite because the introduction of Ce improves the electrical conductivity and light absorption of TiO2/GCM.

Figure 5

Productions of (a) CH4 and (b) CO as functions of overall irradiation time.

The solar fuel production rates increase with the Ce content increasing from 0.2 to 2 mmol and then decrease (Figure a), which is similar but not identical to the trend of the CO2 adsorption capacity.[35−37] However, the activity decreases with a further increase of the Ce content to 5 mmol, indicating that an appropriate Ce content is very important to achieve the best photocatalytic activity. The yield is lower compared with the Ce-free samples under the same conditions. The yield maximizes to 4.59 μmol·g–1 at the Ce content of 2 mmol.

Figure b shows the CO production rates of solar fuels. With prolonging the irradiation time, 2Ce/TiO2/GCM showed a strong catalytic performance. The CO yield maximizes to 359.85 μmol·g–1 in 2Ce/TiO2/GCM after 6 h of simulated sun exposure. The optimum 2Ce/TiO2/GCM exhibited several times higher CO evolution rate than pure TiO2 in this study and NH-UiO–66 synthesized by Crake et al.[38] The results show that the Ce content is the most important factor to determine the optimal photocatalytic activity and a further increase of the Ce content will slow down the photoreduction conversion of CO2.

Mechanism of Photocatalytic Reduction of CO2 with H2O

Solar energy is used to realize the recycling reaction of CO2 and H2O, and its working principle is as follows:[39−42] semiconductor materials with a certain band gap width generate electron-photogenic holes (h+) and photogenic electrons (e–) during the excitation by sunlight, which migrate to the semiconductor catalyst as active sites with reduction and oxidation potential, respectively,[43] and then react with CO2 and H2O adsorbed on the surface. Holes capture electrons in H2O and decompose them to form strongly oxidizing hydroxide (HO•) and hydrogen ion (H+).[44] Meanwhile, CO2, as an electron acceptor, is reduced to strongly oxidized carbon dioxide anion radical (•CO2–), and the specific process is shown in eqs , 2, 3, and 4.[45] Then, •CO2– is further combined with hydrogen ions and photogenic electrons to obtain hydrocarbons such as formaldehyde, formic acid, methanol, or methane. The specific reaction process is shown in Figure

Figure 6

Proposed mechanism for the photocatalytic reduction of CO2 with H2O.

Conclusions

Hierarchically porous Ce/TiO2 graphitic microsphere carbon (GCM) composites were prepared by simple colloidal crystal template using SiO2 colloidal crystals as the template and titanium tetrachloride as the precursor of Ti. According to the XRD analysis, the Ce/TiO2/GCM composite material has hierarchical porosity, high pore volume, large specific surface area and graphitic framework, and N2 sorption. The CH4 and CO yields are 4.587 and 357.851 μmol·g–1, respectively. In addition, among the four samples, the specific surface area of 2Ce/TiO2/GCM was the highest, so they give the highest production rate for two products. The excellent photocatalytic activity of Ce/TiO2/GCM in CO2 photoreduction with H2O under simulated solar irradiation is attributed to the special composition and structure.

Experimental Section

Chemicals

Absolute alcohol, acetone, ammonia, tetraethoxysilane, Ce(NO3)3·6H2O, NaOH, and soybean oil were used as raw materials.

Catalyst Preparation

Colloidal SiO2 microspheres were prepared with 0.31 mol·L–1 ammonia by Stöber’s method,[46] except that the volume was increased by 10 times.

The hierarchically porous Ce/TiO2/GCM was synthesized via colloidal crystal templating. Typically, x mmol of Ce(NO3)3·6H2O (x = 0.2, 1.0, 2.0 or 5.0), 5.0 g of soybean oil, and 10 mmol of TiCl4 were dissolved into 45 mL acetone, respectively. Then, 2.5 g of SiO2 microspheres was added to the above mixed solution and stirred for about half an hour to form a suspension. After aging at room temperature for 3 days, the suspension carbonizes the precursor for 4 h at 900 °C under an Ar flow rate in a tubular furnace. The silica gel template was removed after multiple treatments with a 2 mol·L–1 NaOH solution.

Catalyst Characterization

The XRD patterns were collected in the 1–2 θ mode on a Rigaku D/Max2rB-II diffractometer (Cu K1 radiation, λ = 1.5406 Å) at 100 mA and 40 kV. SEM was performed on a Philips XL-30 SEM device, and the acceleration voltage was 25 kV. The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Pore size distributions were obtained from desorption branches of the isotherms by using the Barrett–Joyner–Halenda (BJH) method. The total pore volumes were estimated at a relative pressure of 0.99.

Photocatalytic Performance Tests

The photocatalytic activity of CO2 reduction with H2O was tested in a self-made device, as shown in Figure . The photocatalytic reaction was carried out in a quartz glass reactor with a volume of 1500 mL. The catalyst powder (0.10 g) was evenly dispersed on a stainless steel omentum and placed in the middle of the photocatalytic reactor. A glass filament soaked with 5.0 g of deionized water was placed at the bottom of the reactor to keep the reactor saturated with water vapor, and a Xe arc lamp (300 W) with a solar radiation spectrum was placed 8 cm above the quartz window to illuminate the whole catalyst. To control the reaction temperature at 30 °C, we placed the Xe lamp in a cold trap to keep the whole reactor system in a state of air circulation. Prior to ignition, the reactor was first cleaned with a CO2 + H2O mixture at 100 mL·min–1 for 2 h, then at 20 mL·min–1 for another hour to reach an adsorption–desorption equilibrium; the volume concentrations of CO2 and the H2O gas phase were controlled at about 95.5 and 4.5%, respectively.[47] The reactor was then sealed and the pressure of CO2 was maintained at 110 kPa. After that, the light source 300 W xenon lamp was turned on. GC 8000 Top gas chromatography–mass spectrometry system was used to collect and analyze the gas-phase products at different time points during the irradiation. Each sample was injected 5 μm into the loop, and 25 m × 0.32 mm CP-PoraPLOT Q-HT column (Chrompack) was injected via 0.5 m × 0.32 mm deactivated precolumn, which was maintained at 23 °C. Helium at 2.0 mL·min–1 was used as the carrier gas.[48,49] The mass spectrometer operates in a single-ion monitoring mode with electron ionization (70 eV). The retention times of CH4 and CO were 1.6 and 2.3 min, respectively. The total detection time of one sample was 3.3 min, which was enough for sample collection, injection, chromatography, and data collection for 2.5 min.

Figure 7

Experimental schematic diagram of the established photocatalytic reduction of CO2 by H2O.