Confined Photocatalysts in Mesoporous Silicas for Carbon dioxide photoreduction

Abstract

Photocatalytic reduction of CO2 offers a solution to the depletion of fossil fuels and the high levels of atmospheric CO2 exacerbating the global warming. It is, therefore, necessary to develop more efficient photocatalysts that utilize the solar energy and efficiently convert CO2 to useful fuels. For this purpose, this MPhil focused on the development of perovskite catalysts supported on silica materials (SBA-15 and KIT-6) for the photoreduction of carbon dioxide (CO2).Silica supports SBA-15 and KIT-6 were synthesized by hydrothermal treatment and a subsequent calcination at 550 °C. After this, different semiconductors called perovskites (mixed oxides MFeO3) were deposited on the support materials in varying amounts (5%, 10%, and 20%) to know the effect of the incorporation with the use of a sol-gel method. The perovskites selected were LaFeO3, BiFeO3, AlFeO3, and YFeO3 due to their suitable bandgap and promising photocatalytic response in the visible light. In addition, TiO2 was also deposited to make a comparison about bandgap and future photocatalytic conversion.Both SBA-15 and, KIT-6 were analyzed with N2 adsorption-desorption and XRD showing cylindrical pores with a diameter of around 7 nm and high specific surface areas. These characteristics make the supports good candidates to be a host of metal oxides species, and their pore diameter is suitable to allow that the CO2 molecules will be adsorbed without problems (CO2 size=3.4 Å).This work revealed that the incorporation of species into the silica support resulted in a high loss of specific surface area. The inclusion of TiO2 into the support had a different effect in comparison with the perovskites due to the Ti species reacted with the particle surface of silica materials preventing the dispersion inside of the channels of the support. The perovskites, however, were well dispersed provoking changes in the internal support structure observed for example in the pore diameter sizes. XRD confirmed the formation of most perovskites and TiO2, except AlFeO3.Bandgap calculation from UV-visible spectroscopy measurements suggested that the bandgap of the perovskites in the silica supports increased due to their nanoconfinement, while TiO2 bandgap did not change much due to poor nanoconfinement. Calculations on conduction and valence bands revealed that best samples for CO2 photocatalytic reduction were the LaFeO3 (KIT-6, and SBA-15) at 5%, and YFeO3 supported at 5% and 10% wt loading on SBA-15 since both resulting materials have bandgaps in the visible range and conduction and valence bands straddling the redox potentials for CO2 photocatalytic reduction with water (CO2/CH3OH)In conclusion, the synthesis of ferrite perovskites nanoconfined on mesoporous silicas SBA-15 and KIT-6 have been investigated following impregnation with a sol-gel process. The results indicate that LaFeO3, BiFeO3, and YFeO3 ferrite perovskites can be incorporated in SBA-15 and KIT-6 mesoporous silicas. Properties such as specific surface area and UV-vis absorption strongly depend on the amount of perovskites used. The samples LaFeO3 (KIT-6, and SBA-15) at 5% and YFeO3/SBA-15 supported at 5% and 10% were identified as the best candidates of the investigated catalysts for CO2 photocatalytic reduction.

Date of Award	7 Jun 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Salvador Eslava (Supervisor)