Photocatalytic reduction of CO2 to valuable fuels by novel nanostructured titania materials

In this study, an attempt has been made to synthesize the novel mesoporous Santa-Barbra-Amorphous-spherical shaped (SBA-15-S) and Korean Advance Institute of Science and Technology (KIT-6) mesoporous silica materials by hydrothermal treatment method. Afterward, these samples were used as support materials for synthesis of novel isolated Ti-SBA-15-S as well as Ti-KIT-6 with different Si/Ti (200, 100, 50 ratios) both in dried and calcined forms. Thereafter, the properties of the materials have been characterized through Brunner-Emmet-Teller (BET) to see the specific surface area, Scanning Electron Microscopy (SEM) to observe the surface morphology and microstructures, Ultraviolet-Visible spectroscopy (UV-Vis) to see the band gap energies and their corresponding wave lengths, Transmission Electron Microscopy (TEM) to investigate the behavior of Ti interaction with mesoporous materials, X-ray Photoelectron Spectroscopy (XPS) was done to observe the elemental composition, empirical formula as well as chemical and electronic states of the elements and Fourier-Transform Infra-Red (FT-IR) spectra to observe the Ti interaction to silica and presence of OH groups which play a significant role in photocatalytic activity. Photocatalytic activity of CO2 reduction in Ti-KIT-6 (Si/Ti=100) showed better CH4 production rate (4.15 μmol•gcat.-1•h-1) than the corresponding Ti-KIT-6-dried (2.63 μmol•gcat.-1•h-1) and the Ti-SBA-15-calcined/dried (1.85, 3.45 μmol•gcat.-1•h-1, respectively) in the initial photocatalytic reaction of CO2 to methane. 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 best commercial TiO2 (Aeroxide P25, Evonik Industries AG, Essen, Germany) photocatalyst. In addition, CH3OH, H2 and CO, are the other main fuel products produced by the Ti-KIT-6-calcined (Si/Ti=100) after optimization. The increased surface concentration of OH groups found in the Ti-KIT-6-calcined (Si/Ti=100) than the other two ratios (Si/Ti=200, 50), the presence of more accessible surface reaction active sites due to the lower numbers of Ti-O-Ti or TiO2 agglomerates, and the more isolated Ti species which are uniformly dispersed on the 3-D KIT-6 mesoporous silica support without collapsing the mesoporous structure, have boosted the higher activity. The photocatalytic activity results of optimized Ti-KIT-6 calcined Si/Ti = 100 ratio preceded by the competitive adsorption of CO2 and H2O vapors towards other fuel products with 4.14, 0.029, 2.55, 1.45µmol g-cat-1 h-1 for CH4, CH3OH, H2, CO, respectively. Similarly, various key parameters including, UV light source (300, 200W), UV light intensity, H2O/CO2 ratios and catalyst shapes, as well as water vapor effect, and long-term stability have been explored for optimized photocatalyst, which strongly influences the performance of the catalyst towards fuels production. Moreover, another effort was made to synthesize the TiO2 nanoparticles (TNPs) prepared by sol-gel method and new nanostructured Mesoporous TiO2 by nanocasting technique or template method using KIT-6 as a template in order to establish the photocatalytic reduction of CO2 with water vapor. In addition, commercial TiO2 (Aeroxide P25) was used for comparison of photoactivity. Thus, the synthesized materials have been characterized by using BET, FE-SEM, XRD, SAXS, and UV-Vis, analyses techniques. TNPs have of an average 11nm TiO2 particle size, higher surface area of (151m2/g) as compared to commercial TiO2 Aeroxide P25 i.e. (53m2/g) which also demonstrate an enhance adsorption capacity. Furthermore, Meso.TiO2 has shown a greater surface area (190m2/g) along with mesoporosity with (4nm) which is much more higher than that of TNPs and also from commercial TiO2 (Aeroxide P25). Overall, Meso. TiO2 was superior in photocataltic activity of CO2 reduction towards renewable fuel products than that of other TiO2s. The photocatalytic activity of CO2 reduction comparison with titania nanoparticles (TNPs) and Evonik P25 which showed the improved resuls towards fuel production such as (14.01, 0.11, 83.50, 26.30 µmol g-cat-1 h-1 for CH4, CH3OH, H2, CO respectively). However, various reaction factors, activation/regeneration of photocatalyst and reaction mechanism and pathways of CO2 reduction on Meso. TiO2 have been explored. Finally, nanostructured TiO2/KIT-6 series with (1, 5, 10, 20, 30, 50, 70, and 90 %) by using different titania wt% have been prepared by precipitation following the sol-gel hydrolysis and condensation of titania precursor dispersed within/surface of the mesoporous silica KIT-6. The characterization was made by N2 adsorption/desorption isptherms, FE-SEM, XRD and UV-Vis, analyses techniques to observe the physio-chemical properties of materials and their corelation with photocatalytic activity. Furthermore, screening of the TiO2 series showed that selectivity toward fuel products depends on the various titania loadings. However, on the basis of our required products (hydrocarbon and H2, CO) the 20wt% TiO2/KIT-6 nanocomposite was found to be the optimized photocatalyst with higher photocatalytic products formation than that of other wt% nanocomposites. The photocatalytic activity of CO2 reduction for 20 wt% TiO2/KIT-6 showed the production for CH4, CH3OH, H2, CO with 44.55, 1.08, 9.20, and 120 µmol g-cat-1 h-1, respectively. Furthermore, the affect of thermal treatment was explored for 20% TiO2/KIT-6 to observe the change in structure and its corealation with photocatalytic activity. The proposed reaction mechanism/pathways for CO2 reduction with H2O vapor on TiO2/KIT-6 nanocomposite has also been discussed.