Mechanism And Application Of Photocatalytic CO₂ Reduction On Bimetallic Self-Doped TiO₂ Nanotubes

The utilization of solar energy is a promising solution to the energy crisis and climate change problems that human beings emerge at present.Photocatalytic CO₂reduction strategy can utilize abundant and clean solar energy to convert greenhouse gas CO₂into high value-added fuels and chemical products,such as methane,methanol,formaldehyde,formic acid,urea,low-carbon hydrocarbons,and polymers.Therefore,the development of efficient and inexpensive system for photocatalytic CO₂reduction driven by solar energy is one of the key issues for solving the energy crisis and environmental problems.It is of great significance for the sustainable development of human society.Under these backgrounds,this thesis focuses on TiO₂nanotube materials with excellent structural properties and optoelectronic properties.Its poor full-spectrum responsiveness,low carrier separation efficiency and weak surface reactivity are improved by self-doping treatment,loading of metal and electric assistance,thus enhancing its photocatalytic performance.Finally,the self-doped TiO₂nanotube-based photocatalysts are used for solar-driven photocatalytic CO₂reduction.The main results of this thesis are summarized as follows:Firstly,Ti³⁺and oxygen vacancy defects were introduced into TiO₂nanotubes by electrochemical self-doping method.The binary single-atom components of Pt and Au were dispersed in self-doped TiO₂nanotubes（R-TNTs）through strong metal-support interaction.The Pt-O and Au-Ti covalent bonds facilitate the efficient transfer of photogenerated electrons from the sites of defect to metal single-atom,significantly enhancing the separation and transport of photogenerated electron-hole pairs.During the photocatalytic process,CO₂molecules underwent polarization and protonation to form·CH₃,and then further converted to CH₄or the coupling product of C₂H₆.The yield of CH₄and C₂H₆reached to 360.0 and 28.8μmol g^-1h^-1,corresponding to apparent quantum yields（AQYs）were 15.2%and 2.7%at 365 nm,respectively.Moreover,the metals loading shifted the oxidation path of H₂O from·OH generation into O₂evolution,that inhibited the self-oxidization of the photocatalyst.Secondly,the formation of intermediate bands in TiO₂nanotubes were induced by the introduction of Ti³⁺species via self-doping treatment,that facilitates the transition of photogenerated electrons from the valence band to the conduction band by harvesting near-infrared light.Meanwhile,the increase in Ti³⁺species facilitated infrared absorption and CO₂chemisorption.After bimetallic loading of Pt and Au,the as-prepared Pt-Au/H-TNTs exhibited remarkable performance in IR-driven CO₂photoreduction,achieving the yield of CH₄and CO,up to 30.1 and 25.8μmol g^-1h^-1,respectively.The system recorded the higher values of AQYs,specifically up to 0.492%（740 nm）,0.435%（850 nm）and 0.186%（940 nm）respectively.Thirdly,the self-doping treatment of the TiO₂nanotubes changed its electronic structure greatly,increasing the carrier concentration and electrical conductivity up 6 orders of magnitude.And the carrier mobility was increased by about 14 times,which facilitated the transfer of photogenerated carriers from the bulk of catalyst to the interface for the reaction.In this context,a novel electric-assisted photocatalytic technique without any electrolyte and counter electrode,was proposed for gas-phase photocatalytic CO₂reduction.Applying a slight external voltage to the self-doped TiO₂nanotube films can significantly improve their photocatalytic efficiency.Specifically,driven by the external voltage,the photogenerated electrons and holes migrated in opposite directions in the high conductive semiconductor,which led to the efficient spatial separation of the charge carriers,thereby increasing the efficiency of photocatalysis.The photocatalytic redox reactions were integrated on Pt-Au/R-TNTs films in the electric-assisted system,and the maximum yields of CH₄,C₂H₆,and C₃H₈from CO₂reduction were achicved 1172.1,76.4,and 49.5μmol g^-1h^-1,respectively.Moreover,the maximum yield of catalytic oxidation of H₂O to O₂reached to 3420.8μmol g^-1h^-1.Compared with the photocatalytic system,the performance of electric assisted photocatalysis was increased by 3.5 times.Fourthly,due to poor visible absorption of self-doped TiO₂nanotubes,the Cu/Au/R-TNTs photocatalyst was prepared by loading Cu and Au nanoparticles in R-TNTs,which induced surface plasmon resonance effect to enhance the visible utilization,achieving the full spectrum-induced CO₂hydrogenation.Based on the novel electro-assisted photocatalytic technology,an electro-assisted photocatalytic CO₂hydrogenation flow-through reactor driven by solar energy was made and optimized.When the reactor was operated in optimal condition,the CO₂conversion rate can be maintained at about 25.0%.In addition,by integrating the reactor with a solar-driven system,the integrated system can be normal operation in both summer and winter,and the average conversion rate of CO₂can reach up to 21.7%.Based on the research results,an electro-assisted photocatalytic methanation process was proposed for the synthesis of natural gas from coke oven gas.The environmental impact of the process was compared with that of the thermal catalytic methanation process through life cycle assessment analysis.The results indicated that the electro-assisted photocatalytic methanation process generated higher carbon emission reduction and environmental benefits.