Liquid Phase Plasma Preparation Of Single-atom Metal On CeO₂ Nanosheet For Photo-enhanced Catalytic Oxidation Performance

Single-atom catalysts（SACs）have become a current research hotspot because of the high atomic utilization,diverse coordination structure and strong metal-support interactions.However,single atoms are highly susceptible to migration and aggregation during experiments because of the high surface energy.Therefore,the preparation of SACs with high activity and stability is still a great challenge.The surface oxygen vacancies of the oxide support facilitate enhanced interaction of metal single atoms with the support,as well as promote the activation of lattice oxygen,which exhibits excellent activity in catalytic oxidation.Conventional SACs preparation methods mostly adopt a strategy of pre-defective oxygen vacancies in the support,followed by anchoring of metal single atoms,which generally suffer from low metal reduction and weak anchoring,thus limiting the activity and stability of SACs.In this thesis,we present a promising liquid-phase plasma（LPP）strategy to enhance metal-support interactions with synchronously constructed oxygen vacancies and anchored single atoms.The high flux electron and radical environment of the LPP discharge channel,simultaneously forming oxygen vacancies on the surface of CeO₂ support and reducing noble metals,the synthesis of a variety of SACs and dual-atom catalysts（DACs）.This strategy synchronizes the construction of oxygen vacancies on the CeO₂ surface and the reduction of noble metal precursors,which not only overcomes the problem of single-atom aggregation,but also activates the support lattice oxygen.The photo-enhanced SACs achieved efficient low-temperature catalytic oxidation performance.The single-atom site was further investigated by combining the techniques of spherical aberration-corrected electron microscope,synchrotron radiation and in-situ Fourier Transform infrared spectroscopy to reveal the photo-enhanced Mars-Van Krevelen（Mv K）reaction pathway.Specific research results are as follows:（1）LPP synchronously induced the formation of surface oxygen vacancies and the reduction of metals,enhanced the metal-support interaction,and successfully prepared a variety of SACs:LPP discharge,accompanied by ultraviolet radiation,electric field,shock wave and other physical fields,produces H_α,H_β,O^Ⅰfree radicals and H₂O₂ in aqueous solution under environmental conditions,which can reduce metal single-atom while constructing oxygen vacancy on CeO₂ surface.Oxygen vacancy is an important site for enhancing the interaction of single-atom metals with support.In-situ Fourier Transform infrared spectroscopy and O₂-TPD showed that the Au₁/CeO₂ catalyst prepared by LPP had a strong interaction between the Au₁ site and the support,which activated the support lattice oxygen.LPP technology is also suitable for the preparation of Rh₁/CeO₂,Pd₁/CeO₂ and even DACs Au₁Rh₁/CeO₂ and Au₁Pd₁/CeO₂.Oxygen vacancy CeO₂ nanosheets can capture about98%single-atom within 2 min,which is a general and efficient synthesis strategy of SACs.（2）The photo-enhanced Mv K process of Au₁/CeO₂ achieves high efficiency low temperature CO oxidation:The photo-assisted CO conversion over Au₁/CeO₂-H is 91.7%with a high turnover frequency（TOF）of 1.3 s^-1 at room temperature,and its activity is significantly better than that of Au cluster catalysts and Au nanoparticle catalysts.The temperature of 90%CO conversion(T₉₀)is 100°C lower than dark conditions,and its excellent CO oxidation performance is mainly due to the photoexcitation process.Isotope labeling experiments have proved that the CO oxidation process of Au₁/CeO₂-H follows the Mv K mechanism.The photogenerated holes accumulated on the surface of CeO₂ helps weaken the Ce-O bond and reduce the activation energy of surface lattice oxygen thus promoting CO oxidation.In-situ Fourier Transform infrared spectroscopy showed that light promoted the chemisorption of CO at Au site,which realized the efficient oxidation of Au₁/CeO₂-H at room temperature.The photo-enhanced Mv K strategy provides an effective means to improve the performance of low temperature catalytic oxidation.（3）Development of continuous flow LPP technology to prepare DACs Au₁Rh₁/CeO₂catalysts for photo-enhanced high performance low temperature water-gas shift（WGS）:the continuous flow LPP technique delivers metal precursors and CeO₂ nanosheets to the plasma discharge center for simultaneous generation of oxygen vacancies and reduced metal precursors,and five（Au,Rh,Pd,Ru and Pt）SACs and two（Au Rh and Au Pd）DACs were prepared efficiently.The DACs f-Au₁Rh₁/CeO₂ with Au₁ lattice oxygen activation site and Rh₁ adsorption CO site can reach 50.7%CO conversion at room temperature under light,and the temperature of 50%CO conversion(T₅₀)is lower than f-Au₁/CeO₂ and f-Rh₁/CeO₂ by 140°C and 165°C.Meanwhile,the synergistic effect of the dual single-atom sites in f-Au₁Rh₁/CeO₂ breaks the linear proportionality of the adsorption energy of the intermediates,thus well suppressing the generation of by-products and providing efficient low-temperature WGS performance.