Catalytic Oxidation Of Methane Of Pd-based Catalyst For Surface Interface Regulation And Its Mechanism

Methane（CH₄）has been widely used as a kind of high calorific value fuel and an important raw material of industrial chemical industry.Methane’s greenhouse effect is more than 25 times higher than CO₂ in terms of the number of molecules.Improper emissions of methane contribute to global warming are one of the major environmental challenges.Methane catalytic combustion is an effective means to reduce greenhouse effect and energy consumption.The development of methane catalytic oxidation technology is of great significance for environmental protection and energy efficient utilization.Methane is one of the simplest hydrocarbons and has the highest C-H bond strength（439 k J/mol）among all alkanes,making it the least reactive alkanes.How to catalyze the activation of C-H bond is the key for methane combustion.By adjusting the surface electronic structure of the catalyst to promote the noble metal-support interaction,the electron transfer,redox,dissociation adsorption and other properties of the catalyst can be improved,thus effectively enhancing the C-H bond activation and methane catalytic reaction process.Combining the methane adsorption and activation properties of Pd unsaturated 3d orbital with the excellent electron exchange capacity of cobalt-based spinel oxide is a potential strategy to improve the catalytic oxidation performance of methane.Based on this,this paper systematically studied the catalytic oxidation performance of Pd-based catalyst for methane and the catalytic oxidation mechanism promoted by regulating surface interface characteristics with spinel Co₂MO₄ as the carrier.The main innovative achievements and conclusions are as follows:（1）The embedded Pd-Co₃O₄ catalyst,supported Pd/Co₃O₄ catalyst and coated Pd@Co₃O₄catalyst were prepared by introducing Pd species in different occurrence states on the Co₃O₄support.The results show that the embedded Pd-Co₃O₄ catalyst has the best catalytic performance for CH₄ combustion and has good water resistance and CO₂ resistance.Combined with the results of the characterization experiments,the excellent catalytic activity was attributed to higher Pd²⁺active species,more oxygen vacancies and adsorbed oxygen on the surface,better lattice oxygen mobility and redox performance.Reactive oxygen species lost by the reaction adsorb,activate and transform on the surface oxygen vacancy through molecular oxygen and timely supplement the consumed lattice oxygen,thus maintaining a stable catalytic cycle.（2）The embedded Pd-based catalyst supported by ultra-thin mesoporous spinel（Co₂MO₄,M=Co,Mn,Al）was prepared for the catalytic oxidation of methane.The Pd-Co₂Mn O₄ catalyst showed the best catalytic performance of CH₄ oxidation,good cyclic stability and water resistance.In situ characterization experiments showed that the interaction between Pd O_xspecies and Co₂Mn O₄ support in Pd-Co₂Mn O₄ catalyst was beneficial to the migration and activation of lattice oxygen,and promoted the adsorption of methane and the activation of C-H bond.Formates and carbonates are key intermediates in the catalytic reaction of methane and are further oxidized by reactive lattice oxygen to become final products.（3）Pd O_xspecies partially embedded lattice and partially modified surface Pd-Co₂Ni O₄catalyst was prepared and showed competitive methane catalytic activity(T₅₀=287°C,T₉₀=321°C).Combined with the results of quasi-situ XPS,XAS,in-situ DRIFTS and DFT analysis,the Pd O_x embedded into the Co₂Ni O₄ lattice effectively modulates the electronic structure of the catalyst,promotes the electron transfer between the Co-3d-O 2p hybrid orbital and Ni_eg,and facilitates the activation of adjacent lattice oxygen in the Co-O-Ni hybrid.Thus,the migration and activation of lattice oxygen are accelerated.The Pd O_x species exposed to Co₂Ni O₄ promoted the metal-carrier interface electron interaction and enhanced the Pd-O bond activity.The specially-occurring Pd O_x species act as"electronic modulators"to promote the activation of carrier lattice oxygen and Pd O_x interface lattice oxygen,accelerating the transformation of intermediates.（4）The Pd-based catalyst（Pd/Co₃O₄-HS）with a high specific surface area and layered porous shell supported by Co₃O₄ was designed and prepared,showing the outstanding catalytic oxidation performance of methane(T₉₀=270 ^oC)and low apparent activation energy(E_a:52.1k Jmol^-1).The results showed that the interfacial Pd-Ov-Co²⁺site promoted the lattice oxygen activation and the reversible cycle of Co³⁺/Co²⁺,thus facilitating the catalytic activation of C-H bond.The well-designed microstructure and defect sites significantly improved the high temperature resistance,water resistance and sulfur resistance of the catalyst.In this study,efficient and stable catalytic oxidation of methane was achieved,and the mechanism of catalytic oxidation promoted by the surface interface characteristics of Pd-based catalysts was revealed,which provided theoretical basis for the design of environmental catalysts and the elimination of hydrocarbon pollutants.