Photocatalytic Reduction Of Carbon Dioxides By Ag/Ag₂WO₄ And BiOCL/g-C₃N₄ Photocatalysts Under Visible Light Irradiation

Photocatalytic technology is one of the effective techniques of reducing CO₂emissions by using solar energy,which is clean,renewable,and nearly inexhaustible.However,semiconductor materials that are available have relatively poor photocatalytic efficiency in the visible-light range.Thus,it is still a challenge to develop new and more efficient visible-light photocatalysts to meet the requirements of future CO₂ photoreduction technologies driven by solar energy.Plasmonic photocatalysis and heterojunction photocatalysis have recently come into focus as very promising technologies for high-performance photocatalysis.Plasmonic photocatalysts afford drastic enhancement of visible-light absorption by localized surface plasmon resonance?LSPR?produced by the collective oscillations of surface electrons of noble metal nanoparticles.Heterojunction photocatalysts are compounded by two semiconductors with different band gap and can separates electron–hole pairs efficiently.In this thesis,we prepared plasmonic photocatalysts and heterojunction photocatalysts respectively by simple methods.The photocatalysts were characterized by X-ray diffraction?XRD?,scanning electron microscope?SEM?,energy dispersive X-ray spectra?EDS?,transmission electron microscopy?TEM?,X-ray photoelectron spectra?XPS?,ultraviolet-visible diffuse reflection spectra?UV-vis?and Brunauer-Emmett-Teller?BET?surface area.The photocatalytic activity was tested by reducting CO₂.The main results are as follows:?1?A series of plasmonic photocatalysts,comprised of Ag supported on Ag₂WO₄?Ag/Ag₂WO₄?with different crystalline phases,were fabricated by a facile ion-exchange method and subsequent reduction with hydrazine hydrate.Compared with Ag₂WO₄,the Ag/Ag₂WO₄ exhibited a markedly improved quantum yield?QY?,energy returned on energy invested?EROEI?,and turnover number?TON?for CO₂reduction to CH₄ under visible-light irradiation.Among Ag/?-Ag₂WO₄,Ag/?-Ag₂WO₄ and Ag/?-Ag₂WO₄ catalysts,the highest activity for CO₂photoreduction to CH₄ was obtained for Ag/?-Ag₂WO₄ with an actual molar composition of 4%Ag and 96%Ag₂WO₄.Moreover,the plasmonic Ag/Ag₂WO₄photocatalysts were stable after repeated reaction cycles under visible-light irradiation.It was proposed that the localized surface plasma resonance effect of surface-deposited Ag contributed to the enhanced activities and stabilities of the Ag/Ag₂WO₄ photocatalysts.?2?A series of heterojunction photocatalysts with different ratios of BiOCl and g-C₃N₄ were successfully prepared by a simple hydrothermal synthesis method.The products of photocatalytic conversion of CO₂ with water vapor were CH₄,CO and CH₃OH under visible-light irradiation.In comparison with individual BiOCl and g-C₃N₄,the BiOCl/g-C₃N₄ exhibited markedly improved QY,EROEI and TON for CO₂ reduction.Moreover,the 1BiOCl/g-C₃N₄ catalysts maintained stable performance even after being recycled five times.It was proposed that the effective separation of photogenerated electrons and holes due to the formation of p-n heterojunctions between BiOCl and g-C₃N₄ was responsible for the improved photocatalytic activities of BiOCl/g-C₃N₄.