Enhanced activity of Ag-MgO-TiO2 catalyst for photocatalytic conversion of CO2 and H2O into CH4

Using solar energy to recycle carbon dioxide (CO2) offers a brand new opportunity for simultaneous mitigation of the global climate change effect and production of energy bearing compounds. Silver (Ag) and magnesium oxide (MgO) co-modified titania (TiO2) (AgMgTi) synthesized through a combined impregnation and photo-deposition process. The catalysts were characterized through surface area analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and photoluminescence spectrum (PL). The TEM and XPS analysis demonstrated the presence of Ag nanoparticles (NPs) on the AgMgTi catalyst. The synergistic activities of Ag-NPs and MgO significantly contributed to CO2 photoreduction to methane (CH4). CH4 yield over the AgMgTi catalyst was 20 times higher than over P25 TiO2, and was higher than over single modified catalysts (MgO-TiO2 or Ag-TiO2) under either ultraviolet (UV) or UV-vis irradiation. MgO on the surface of TiO2 enhanced the chemisorption of CO2 and/or initiated the CO2 reduction process, and hence facilitated the conversion of CO2 to CH4. Under UV light excitation, the deposited Ag-NPs facilitated the separation of electron-hole due to the formation of Schottky barriers on the metal-semiconductor interface. Visible light in the solar spectrum improved the energy of trapped electrons on Ag-NPs through enhanced localized electric field attributed to the Ag surface plasmon resonance (SPR) effect. Abundant electrons with higher energy facilitated CH4 formation. Therefore, more CH4 was obtained under UV-vis irradiation comparing to that under UV irradiation. This study demonstrated that the AgMgTi catalyst can effectively utilize full spectrum solar energy for simultaneous reduction of CO2 and production of energy-bearing compounds. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.