New nanostructured silica incorporated with isolated Ti material for the photocatalytic conversion of CO2 to fuels
© Akhter et al.; licensee Springer. 2014
Received: 5 February 2014
Accepted: 22 March 2014
Published: 1 April 2014
In this work, new nanoporous silica (Korea Advanced Institute of Science and Technology-6 (KIT-6)-dried or KIT-6-calcined) incorporated with isolated Ti materials with different Si/Ti ratios (Si/Ti = 200, 100, and 50) has been synthesized and investigated to establish photocatalytic reduction of CO2 in the presence of H2O vapors. The properties of the materials have been characterized through N2 adsorption/desorption, UV-vis, TEM, FT-IR, and XPS analysis techniques. The intermediate amount of the isolated Ti (Si/Ti = 100) has resulted to be more uniformly distributed on the surface and within the three-dimensional pore structure of the KIT-6 material, without its structure collapsing, than the other two ratios (Si/Ti = 200 and 50). However, titania agglomerates have been observed to have formed due to the increased Ti content (Si/Ti = 50). The Ti-KIT-6 (calcined) materials in the reaction showed higher activity than the Ti-KIT-6 (dried) materials, which produced CH4, H2, CO, and CH3OH (vapors) as fuel products. The Ti-KIT-6 (Si/Ti = 100) material also showed more OH groups, which are useful to obtain a higher production rate of the products, particularly methane, which was even higher than the rate of the best commercial TiO2 (Aeroxide P25, Evonik Industries AG, Essen, Germany) photocatalyst.
The gradual increase in the world population and the industrial development have both led to high energy consumption and the unabated release of toxic agents and industrial wastes into the air and waterways, which in turn have led to pollution-related diseases, global warming, and abnormal climatic changes . Carbon dioxide (CO2), which is mainly obtained from fossil fuel combustion, plays a significant role in global heating  and is currently considered a key challenge for the world. At present, the most optimized and preferable way of reducing CO2 is to recycle it as a fuel feedstock, with energy input from cheap and abundant sources . Moreover, due to the shortages and restrictions on the use of fossil fuels and the increased energy demand, there has been increasing interest in the development of alternative renewable energy resources, which has encouraged researchers to use CO2 as a raw material to produce fuels [1–4].
Photocatalytic CO2 reduction is highly popular but still in an embryonic stage. It simply uses ultraviolet (UV) and/or visible light as the excitation source for semiconductor catalysts. The photoexcited electrons reduce CO2 with H2O on the catalyst surface to form energy-bearing products, such as carbon monoxide (CO), methane (CH4), methanol (CH3OH), formaldehyde (HCHO), and formic acid (HCOOH) [1–4]. TiO2, CdS, ZrO2, ZnO, and MgO photocatalysts have been investigated in this context. However, wide-bandgap TiO2 photocatalysts are considered the most convenient candidates, in terms of cost and stability [5, 6].
Recently, the design of highly efficient and selective photocatalytic systems for the reduction of CO2 with H2O vapors has been of key interest. It has been shown in the literature  that highly dispersed titanium oxide (Ti oxide) catalysts anchored on porous Vycor glass (Amsterdam, The Netherlands), zeolites, and some nanoporous silica materials, such as Mobil Composition of Matter-41 (MCM-41), show better photocatalytic activity for CO2 conversion than bulk TiO2 powder. However, MCM-41 mesoporous silica has a one-dimensional (1-D, hexagonal p6mm) pore structure, with a relatively small pore size and poor hydrothermal stability. Korea Advanced Institute of Science and Technology-6 (KIT-6) silica is another interesting alternative material to MCM-41. It has a three-dimensional (3-D) (gyroid cubic Ia3d) pore structure and large pore size and has recently received the attention of many researchers in various applications [8, 9].
In the present work, a new KIT-6 mesoporous silica (with interesting physical properties, cavities, and frameworks) incorporated with highly dispersed isolated Ti materials has been synthesized; characterized by means of UV-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) analysis techniques; and applied for the photocatalytic reduction of CO2 with H2O vapors to obtain fuels (CH4, H2, CO, and CH3OH). The activity results have been compared with the best commercial TiO2 photocatalyst (Aeroxide P25, Evonik Industries AG, Essen, Germany), and the involved mechanism has been discussed.
Synthesis of the materials
The mesoporous silica material (KIT-6) was obtained by following the procedure shown in recent works [8, 9]. After a hydrothermal treatment, the obtained solid product was filtered, dried, and/or calcined at 550°C for 5 h and was then utilized to prepare Ti-KIT-6 (dried or calcined). The dried and calcined KIT-6 materials were then treated with titanium (IV) isopropoxide (98%) at different Si/Ti ratios (200, 100, and 50), and finally calcined to obtain Ti-KIT-6 according the procedure recently reported in .
Characterization of the materials
The UV-vis diffuse reflectance spectra were recorded using a Varian model Cary 500 spectrophotometer with a quartz cell (Palo Alto, CA, USA) suitable for measuring powders. The Brunauer-Emmett-Teller (BET) specific surface area (SBET), pore volume (PV), and average pore diameter (APD) were measured on the powder materials, which had previously been outgassed at 150°C using Micromeritics FlowPrep 060 (Norcross, GA, USA) (sample degas system), by means of N2 sorption at 77 K on a Micromeritics Tristar II (surface area and porosity) instrument. The TEM images were taken from the thin edges of the sample particles using a TEM Philips CM12 (Amsterdam, Netherlands), with a LaB6 filament and a double-tilt holder, operating at 120 kV. The FT-IR spectra were collected at a resolution of 2 cm−1 on a PerkinElmer FT-IR spectrophotometer equipped with an MCT detector (Waltham, MA, USA). The XPS spectra were recorded using a PHI 5000 Versa Probe (Chanhassen, MN, USA), with a scanning ESCA microscope (Trieste, Italy) fitted with an Al monochromatic X-ray source (1486.6 eV, 25.6 W), a beam diameter of 100 μm, a neutralizer at 1.4 eV 20 mA, and in FAT analyzer mode.
The basic experimental setup can be found in the previous work . It includes a Pyrex glass reactor (Savat di Rasetti Giuseppe & C. S.a.s, Torino, Italy), connectors, mass flow controllers, water bubbler, and a UV lamp (300 W, Osram Ultra-Vitalux, Munich, Germany). It also has a CO2 gas cylinder (99.99%), a gas chromatograph (Varian CP-3800) equipped with a capillary column (CP7381), a flame ionization detector (FID), and a thermal conductivity detector (TCD). A photocatalytic reaction was performed in the reactor, which contained 0.5 g of photocatalyst. CO2 gas was introduced into the reactor at 50 mL/min for 30 min, after passing it through the water bubbler and has an adsorption-desorption balance; this is to saturate the catalysts with CO2 and H2O. A 0.1-g glass wool wet with 0.5 mL of H2O was also placed in the reactor at the entrance point of the CO2 and H2O to balance the water deficiency in the reactor. After 30 min, the CO2 flow rate was reduced to 10 mL/min. When equilibrium was reached, the UV light was turned on, and the reaction products were analyzed by means of the GC. Blank tests were also conducted to ensure that the product was due to the photocatalytic reaction. The blank tests consisted of a UV illumination without the photocatalyst and a reaction in the dark with the catalyst.
Results and discussion
Physicochemical properties of the synthesized materials
Comparison of the physical properties, bandgap energy of the synthesized materials, and methane production
[Ti-K-6 (dried) (Si/Ti = 200)] calcined
[Ti-K-6 (dried) (Si/Ti = 100)] calcined
[Ti-K-6 (dried) (Si/Ti = 50)] calcined
KIT-6 (K-6) calcined
[Ti-K-6 (calcined) (Si/Ti = 200)] calcined
[Ti-K-6 (calcined) (Si/Ti = 100)] calcined
[Ti-K-6 (calcined) (Si/Ti = 50)] calcined
Anatase TiO2 powder
Aeroxide/degussa P25 TiO2
Titanium silicate (TS-1) zeolite
Photocatalytic conversion of CO2 to fuels and its mechanism
A similar trend of activity was also observed when Ti-KIT-6 (calcined, Si/Ti = 200, 100, and 50 ratios) was used. However, overall, the Ti-KIT-6 (calcined, Si/Ti = 200, 100, and 50 ratios) materials show higher activity than the Ti-KIT-6 (dried, Si/Ti = 200, 100, and 50 ratios) materials. This might be due to the fact that some of the Ti species in Ti-KIT-6 (dried, Si/Ti = 200, 100, and 50 ratios) materials which were not accessible on the surface for the reaction might have been trapped in the bulk dried KIT-6 powder during the synthesis. However, this might not be the problem in the case of Ti-KIT-6 (calcined, Si/Ti = 200, 100, and 50 ratios), where the 3-D pore structure was fully developed in the calcined KIT-6. Therefore, the greater number of accessible active sites in Ti-KIT-6 (calcined, Si/Ti = 200, 100, and 50 ratios) than that in Ti-KIT-6 (dried, Si/Ti = 200, 100, and 50 ratios) may have caused higher activity.
Moreover, it is clear that Ti-KIT-6 (calcined or dried, Si/Ti = 100) shows a higher activity than the Si/Ti ratios of 200 and 50, because of the combined contribution of the high dispersion state of the Ti oxide species, which is due to the large pore size with a 3-D channel structure, and the lower formation of Ti-O-Ti or TiO2 agglomerates, as confirmed by UV-vis, TEM, and XPS analyses. Moreover, the high production of CH4 for Ti-KIT-6 (Si/Ti = 100) with greater concentrations of the OH groups (2 nm−1) than the other ratios (Si/Ti = 200 and 50 = 1.5 and 1.2, respectively) obtained from the FT-IR of the materials actually affects the adsorption properties of the water on the catalyst surface . Competitive adsorption between the H2O vapors and CO2 is another parameter that can determine the selectivity of CH4 or CH3OH. CH4 formation selectivity becomes higher as H2O vapor adsorption increases due to the greater concentration of OH groups or hydrophilicity of the material .
The main desired product is CH4 as it has a greater energy or heat content  than H2 or the produced syngas (CO + H2), whereas CH3OH (vapors) is a minor product. As can be seen in Table 1, it is clear that the abovementioned optimized photocatalysts show more activity than the best commercial TiO2 photocatalyst (Aeroxide P25). Moreover, as can be seen in Table 1, the results are comparable with the other results reported in the literature concerning the use of TiO2, Ti-zeolites or Ti-MCM-41  as a photocatalyst for this application. The optimized Ti-KIT-6 (Si/Ti = 100) showed a relatively better CH4 production than the conventional photocatalytic materials, a result that is explained more clearly by examining the reaction mechanism.
New nanoporous silica (KIT-6 dried or calcined) incorporated with isolated Ti materials with different Si/Ti ratios (Si/Ti = 200, 100, and 50) synthesized has shown that Ti-KIT-6 (calcined, Si/Ti = 200, 100, and 50) were better in activity than the Ti-KIT-6 (dried, Si/Ti = 200, 100, and 50) materials, due to the presence of more accessible surface reaction Ti species. The main fuel products obtained after the reaction are CH4, CO, H2, and CH3OH (vapors). Moreover, it has been found that Ti-KIT-6 (Si/Ti = 100) shows a better product formation than Ti-KIT-6 (Si/Ti = 200 and 50). The high activity of the optimized photocatalyst was found to be due to the lower number of Ti-O-Ti or TiO2 agglomerates and to the more isolated Ti species, which were uniformly dispersed on the 3-D KIT-6 mesoporous silica support without damage to mesopore structure. The increased surface concentrations of OH groups found in Ti-KIT-6 also boosted the higher activity. It has been concluded that the activity of the optimized Ti-KIT-6(Si/Ti = 100) is also much higher than that of the commercial Degussa P25 TiO2, due to the longer life and the more energetic active sites in the optimized Ti-KIT-6(Si/Ti = 100) photocatalyst than in the bulk commercial TiO2 one. These findings indicate that the highly dispersed isolated Ti, within the new KIT-6 mesoporous silica 3-D framework, can be considered a promising and effective photocatalyst for CO2 conversion to fuels and as a suitable candidate for other research activities.
The financial support from the Eco2CO2 European Project (309701-2 Eco2CO2 CP-FP FP7-NMP-2012-SMALL-6) is gratefully acknowledged.
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