Intensified Oxygen Migration Process Promoting Photocatalytic Methane Dry Reforming Reaction And Its Mechanism

Dry Reforming of Methane（DRM）reaction can convert two main greenhouse gases（CH₄ and CO₂）into synthetic gas,and then produce value-added chemicals through Fischer-Tropsch synthesis.The resource utilization of CH₄ and CO₂ is great significance,helping to achieve the"carbon peaking and carbon neutrality goals".However,the high temperature（800-1000°C）is required to activate the reaction,making the traditional DRM reaction energy intensive,and the high temperature easily leads to coke deposition and concomitant deactivation of catalysts.Photocatalytic technology can use sunlight as a light source to activate inert C=O and C-H bonds under mild conditions,making it an ideal way to achieve dry reforming of methane.The sluggish mobility of oxygen during the reaction is known as a key issue causes low activity and poor stability of catalysts by the coke formation.In this thesis,“metal alloy promotes oxygen migration”,“oxygen vacancy promotes oxygen migration”,and“dual sites and dual paths promote oxygen migration”strategies are proposed for the oxygen migration process on different types of catalysts such as supported and doped metal oxide photocatalysts and non-metallic photocatalysts.High activity and stability photocatalysts are prepared and catalytic reaction mechanisms are explored.The specific research contents and innovative achievements are as follows:（1）Metal alloy promotes oxygen migrationZnO loaded with PdCu alloy composite photocatalyst was designed and synthesized by one-step coprecipitation reduction method.The introduction of PdCu alloy not only enhances light absorption and charge separation,but also promotes the migration of oxygen species between the alloy and the carrier.The results show that PdCu alloy and ZnO act as electron acceptor and hole acceptor respectively,playing the role of CO₂ reduction and CH₄ oxidation.The small amount of Pd doped in the alloy reduces the adsorption energy of oxygen species generated by CO₂ reduction and the activation energy of oxygen migration process,promoting the migration of oxygen species from the alloy to the ZnO carrier.CH₄ is oxidized by oxygen species on ZnO to form CH_xO species and gradually dehydrogenates to form CO.As a result,the optimized Pd₁Cu₉₉-ZnO achieves a high syngas formation rate of 15 mmol g_alloy^-1 h^-1with H₂/CO=1 and displays excellent performance stability in continuous flow reaction under irradiation of a 1.8 W cm^-2 xenon lamp at 300℃.（2）Oxygen vacancy promotes oxygen migrationA PdCu bimetallic doped TiO₂ composite photocatalyst with oxygen vacancies was successfully constructed using a one-step solvothermal method.PdCu bimetallic doping not only enhances light absorption and charge separation,but also generates oxygen vacancies that greatly promote oxygen migration and surface reactions.The research results indicate that Cu,as electron acceptor for localized photogenerated electrons,promotes the adsorption and activation of CO₂,and generates CO and oxygen species;Pd,as hole receptor for localized photogenerated holes,promotes the adsorption activation of CH₄ and gradually dehydrogenates to generate*CH₃,*CH₂,*CH,and*C.the construction of oxygen vacancies promotes the migration of oxygen species between reduction and oxidation sites,which is beneficial for eliminating carbon species and generating CO,thereby inhibiting the formation of carbon deposition.As a result,the optimized PdCu_/TiO₂ achieves a high syngas formation rate of 12 mmol g^-1 h^-1 with H₂/CO=1 and displays excellent performance stability in continuous flow reaction under irradiation of a 1.8 W cm^-2 xenon lamp at 200℃.（3）Dual sites and dual paths promote oxygen migrationA Z-scheme heterojunction composite photocatalyst composed of Cu loaded carbon nitride nanosheets（Cu-CNN）and Pd loaded boron doped carbon nitride nanosheets（Pd-BDCNN）was designed and synthesized using electrostatic self-assembly method.The research results indicate that both Cu-CNN and Pd-BDCNN can act as independent sites to simultaneously complete CO₂ reduction and CH₄ oxidation reactions.At the Cu-CNN reaction site,Cu,as electron acceptor,activates CO₂ to generate CO and reactive oxygen species*O^-,which*O^-oxidizes CH₄ adsorbed nearby to generate*H and*CH₃O,and*CH_xO is continuously dehydrogenated to generate synthesis gas.At the Pd-BDCNN site,Pd acts as a hole receptor,activating CH₄ to gradually dehydrogenate to generate*CH₃,*CH₂,*CH,*C,and releasing H₂.At the same time,the nitrogen vacancies generated by boron doping promote CO₂ reduction to CO and*O,which*O ultimately reacts with the*C species to eliminate carbon deposition.The construction of the“dual site dual reaction pathway”achieves the simultaneous activation and reaction of CO₂ and CH₄ at a single site,shortens the oxygen migration distance,and strengthens the reaction process.As a result,the optimized Cu-CNN/Pd-BDCNN achieves a high syngas formation rate of 800μmol g^-1h^-1 with H₂/CO=1 and preserves excellent stability in continuous flow reaction under irradiation of a 300 m W cm^-2 xenon lamp at room temperature.