Construction,Synchrotron Radiation Characterization And CO₂ Reduction Studies Of TiO₂/CrO_x Photocatalyst

Energy shortage and environmental crisis are two major problems facing the world today.Photocatalytic CO2 reduction not only reduces the greenhouse effects caused by CO2 emission but also produces value added chemicals fuel(such as methane,methanol,formaldehyde,even high carbon compounds)for alternative energy supplies.It is of great significance for the long-term development of human society.However,there are still some problems in this reaction,including low conversion efficiency,low product selectivity,unclear reduction mechanism.To improve the photocatalytic reduction of CO2 conversion efficiency and the selectivity of products,this dissertation is based on the classic anatase TiO2 photocatalyst.Through adopting the combined strategy of crystal facets engineering and cocatalyst loading,and loading CrOx cocatalyst by lower-energy irradiation powered deposition to construct CrOx cocatalyzed TiO2 photocatalyst system.The TiO2/CrOx photocatalyst was characterized by synchrotron radiation photoelectron spectroscopy(SRPES),X-ray near-edge absorption spectroscopy(XANES)and vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS).We investigated their photocatalytic CO2 reduction performance.This dissertation consists of five chapters:In chapter one,we introduced the relevant research background and reviewed the research progress.Firstly,we introduced the basic knowledge of photocatalytic CO2 reduction,including the basic principle,the affecting factors and the evaluation index.Secondly,the existing problems and the latest progress in the study of TiO2-based photocatalyst on CO2 reduction are reviewed.Finally,we summarize the main research content,purpose,and significance of this work.In chapter two,the synergistic effect of Cr2O3 cocatalyst and TiO2 crystal facets engineering was investigated.In this chapter,we found that independent Cr2O3 can be applied for an efficient cocatalyst in photocatalytic CO2 reduction.The 13C isotope labeling experiment and synchrotron radiation vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS)verified that the C source of CO which the main product of CO2 reduction of photocatalyst TiO2/Cr2O3,indeed came from the reactant CO2.Cr2O3 cocatalyst can enhance the light absorption and promote the photogenerated carriers' separation/transport,which effectively improved the efficiency of photocatalytic CO2 reduction.Moreover,a combined strategy of crystal facets engineering of TiO2 and loading of Cr2O3 cocatalyst was adopted to further promote the charge carriers' separation/transfer.The enhanced photocatalytic activity was attributed to the maximal synergy of crystal facets engineering and cocatalyst loading for well-matched CO2 reduction half-reaction and H2O oxidation half-reaction.In comparison with the cocatalyst-free TiO2 without facets engineering,2HF-TiO2/0.2Cr2O3 exhibited near 30-fold enhancement on the CO2 conversion efficiency.This study may not only provide inspiration for the deepened understanding of the role of Cr2O3 in photocatalytic systems but also shed light on the design of highly efficient photocatalysts.In chapter three,the photo-controlled deposition of CrOx cocatalyst over TiO2 for photocatalytic CO2 reduction was studied.Here,we reported a lower-energy-irradiation(?>400 nm)powered deposition strategy for implanting CrOx cocatalyst on TiO2,which is more energy-saving and economical compared with the traditional photodeposition(?>200 nm).The performance of photocatalytic CO2 reduction showed that the as-obtained photocatalyst TiO2/CrOx-400 not only boosted the photocatalytic CO2 conversion efficiency but also improved the CH4 selectivity in comparison with TiO2/CrOx-200 fabricated via the traditional photodeposition.Synchrotron radiation photoelectron spectroscopy(SRPES)and Cr L-edge X-ray absorption near-edge structure(XANES)spectra showed that CrOx-400 was identified to be composed of higher valence Cr species compared to CrOx-200.This valence states regulation of Cr species was indicated to provide more active sites for CO2 adsorption/activation and to modulate the reaction mechanism from single Cr site to Cr-Cr dual sites,thus endowing the superior CH4 production.This work not only reported a novel CrOx cocatalyst for photocatalytic CO2 reduction with nearly 100%of CH4 selectivity but also provided an alternative strategy for developing efficient metal oxides cocatalysts on wide bandgap semiconductor.In chapter four,the modification of TiO2 by binary cocatalyst is investigated.In this chapter,we have preliminarily constructed two kinds of binary cocatalyst modified TiO2 photocatalyst system(Pt@CoOx/TiO2 and Pt@CrOx/TiO2)and investigated their photocatalytic performance for CO2 reduction.The results showed that the photocatalytic activity of TiO2 modified by binary cocatalyst was higher than that of TiO2 modified by a single cocatalyst.This has laid a certain experimental basis for further improving the performance of photocatalytic CO2 reduction and provided guidance for the design of photocatalyst materials that can give full play to the function of the binary cocatalyst.In chapter five,we summarize the work of this dissertation and prospect the research.