Synthesis,Activation Process,and Catalytic Mechanism Of CO Preferential Oxidation Of 3d Transition Metal Composite Materials

Catalytic priority oxidation reactions play an important role in environmental protection,energy conversion,chemical synthesis,waste treatment,and clean energy production,which are of great significance for promoting green low-carbon cycles and environmental protection.Among them,the CO preferential oxidation reaction（CO-PROX reaction）,as an important pathway for hydrogen purification and a key model reaction in multiphase catalysis,has received widespread attention from researchers.At present,the activation of oxygen species for catalysts in CO-PROX reaction is limited under strong reducing atmospheres,resulting in poor thermal stability of precious metal catalysts and insufficient low-temperature activity of non-precious metal catalysts.Therefore,achieving rapid activation of oxygen species through synthetic chemistry at lower temperatures is crucial for improving the activity and stability of CO-PROX catalysts.3d transition metal multiphase composite materials have the characteristics of strong oxygen adsorption capacity,adjustable electron density and variable valence states,are widely developed and applied in the study of oxygen activation in CO-PROX reactions.Although four oxygen activation mechanisms have been reported to explain the oxygen mobility process,namely Mars van Krevelen（Mv K）,Langmuir Hinshelwood（L-H）Eley Rideal（E-R）,and hydroxyl oxidation mechanism.However,with the development of isotope technology and DFT calculations,it is found that the actual oxygen activation process on the catalyst surface is extremely complex.The existing oxygen activation mechanisms are not perfect and it is difficult to accurately describe the evolution process of oxygen species.Especially,the research on the oxygen activation mechanism coupled with multiple sites and the oxygen activation mechanism involving hydroxyl groups is not in-depth enough,and further refinement and improvement are needed.Based on this,the redox sites,coordination environment,structural hydroxyl groups,and amorphous structure of 3d transition metal composites was optimized through precise catalyst design and synthesis.Combining advanced characterization and DFT theory calculations,the oxygen activation mechanism of multi site coupled active units in CO-PROX reaction is explored.The specific research results are as follows:（1）Topological transformation synthesis,active site recognition,and CO-PROX performance ranking of CeO₂-CuO composite materials.CeO₂-CuO was synthesized through the controllable topological transformation of MOF,which can exhibit up to four active CuO_x species on its surface.By systematically adjusting the content of CeO₂and annealing temperature,the content and types of CuO_x species were regulated.Among them,the highly dispersed CuO_x species with unsaturated coordination accelerate the mobility of surface oxygen and are the key active species in the low-temperature reaction of CO-PROX.By comparison,the low-temperature CO-PROX activity ranking of four active units formed by CuO_x and CeO₂ was given:highly dispersed CuO_x>medium-sized CuO_x>weakly bonded Cu-O-Ce>bulk CuO.The identification and performance ranking of four active sites in this work established the relationship between multiple sites in catalytic oxidation reactions.（2）Lattice oxygen activation and CO-PROX mechanism of Mn CuO_x with phase transition intermediate state.Mn CuO_x nanosheets with phase transition intermediate state were obtained through temperature control,which exhibit excellent CO conversion in CO-PROX reactions,higher than reported non-precious metal catalysts.Experiments have shown that the formation of heterostructures with single-layer and2*2 tunnel Mn O₆ exposes more oxygen vacancies,forming Cu-O_v-Mn active units on the catalyst surface.In the active unit,Cu provides adsorption sites for CO,while oxygen vacancies play a role in adsorbing and activating lattice oxygen.The efficient oxygen mobility on the catalyst surface is related to the oxygen content in the reaction atmosphere.In an oxygen-rich atmosphere,Mn CuO_x nanosheets can achieve complete removal of CO at-40 ^oC.The synthesis of phase transition intermediate oxide provides a new approach to the design of novel catalytic oxidation composite materials.（3）Structural hydroxyl activation and CO-PROX mechanism of Pt Fe alloy confined in HNTs.The Pt Fe nanoparticles were confined within the inner cavity of HNTs by electrostatic adsorption,and the structural hydroxyl activation of Pt Fe/HNTs catalyst was achieved through two-stage hydrogen annealing.The activated Pt₁Fe_0.33/HNTs catalyst can complete the complete conversion of CO at 30°C.The experimental and DFT calculation results explain that the high activity of Pt₁Fe_0.33/HNTs comes from the active unit at the interface between Pt Fe NPs and HNTs.The synergistic hydroxyl mechanism is proposed for the first time to explain the catalytic mechanism of Pt₁Fe_0.33/HNTs.In the synergistic hydroxyl mechanism,Pt serves as the adsorption site for CO,and the modification of trace amounts of Fe not only enhances electron transfer but also provides sites for O₂ activation.Structural hydroxyl groups and adsorbed hydroxyl groups synergistically participate in the oxidation of CO,forming*COOH intermediates and releasing CO₂ and H₂O.This work focuses on the activation and reaction pathways of structural hydroxyl groups,providing new insights into the oxygen transport process in catalytic oxidation reactions.（4）Coupling activation of lattice oxygen and hydroxyl and CO-PROX mechanism of Pt/(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)₃O₄.By systematically adjusting the topology multi-component oxides support（MCOs）,the controllable construction of amorphous structure content in Pt/MCOs catalysts has been achieved.The difference in amorphous structure content affect the reduction ability,oxygen activation ability,valence state,and local electronic structure of the catalyst,thereby changing the stability of CO-PROX catalysis.The experimental data indicate that the content of amorphous structure is positively correlated with the content of active oxygen species on the catalyst surface.The reactive oxygen species in Pt/MCOs catalysts spill towards Pt through Pt-O-M bonds.The optimized Pt/MCOs-600 sample has a 39.1%amorphous structure and achieves complete removal of CO within a wide temperature range of 30-120 ^oC.Compared with Pt/MCOs-300 with high amorphous content,appropriate oxygen spillover of Pt/MCOs-600 activates the coupling active sites of Pt-O-M and Pt-OH-M at the interface,which respectively achieve the oxidation process of CO through carbonate and formate pathways.The proposal of this dual pathway mechanism lays the foundation for the development of lattice oxygen hydroxyl coupling activation mechanism in catalytic oxidation reactions.