Theoretical Study On The Effect Of Doping On The Properties Of LaMO₃ Perovskite Catalysts

ABO₃ perovskite composite oxide is a new type of inorganic nonmetallic material with unique physical and chemical properties.Both A and B sites can be partially replaced by other metal ions with similar radii to keep their crystal structure basically unchanged.Therefore,it can be found that in theory it is an ideal sample for studying the surface and catalytic performance of catalysts,and it has potential applications.However,up to now,a series of fundamental problems such as the adsorption and diffusion mechanism of CO in ABO₃ type oxides have yet to be studied.Based on the first-principles method of density functional theory,this thesis calculates and studies the adsorption behavior of CO molecules on the intrinsic La Cr O₃（100）surface and La Fe O₃（010）surface respectively by using the exchange correlation functional under the generalized gradient approximation.On this basis,the surface of these two substances is doped with elements to study the effect of doping on the adsorption of CO molecules on La Cr O₃（100）and La Fe O₃（010）,The most stable structure was determined by comparing the geometrically optimized adsorption systems.After relaxation,the structural parameters,adsorption energy,differential charge density and density of states were analyzed to explore the mechanism of adsorption process.The DFT calculation results of CO adsorption on the intrinsic La Cr O₃（100）surface show that the La Cr O₃（100）surface is the best adsorption surface,and the Cr site is the best site for CO adsorption.After adsorption,the atomic charge of the system is redistributed,and Cr-C covalent bond is formed between CO and La Cr O₃（100）surface.From the analysis of the density of states,there exist hybridization between the 2p orbitals of C atom and the 3d orbitals of Cr atom.When doped with alkali metal and alkaline earth metal Na,K,Ca and Sr elements,K element has the most obvious catalytic effect on CO adsorption on the surface.When doped with transition metal V,Nb and Ta into Cr adsorption site（a-site）and Cr adsorption site（b-site）,Ta doping has the most obvious effect on CO adsorption;When doped with transition metal Ag and Bi,Ag forms adsorption at a-site,and Bi has better adsorption effect at b-site.The DFT calculation results of CO adsorption on the intrinsic La Fe O₃（010）surface show that the best adsorption orientation surface is La Fe O₃（010）surface,and the Fe site is the best site for CO adsorption.After adsorption,the 3d orbitals of Fe and the 2p orbitals of C obviously overlap,resulting in hybridization to form covalent bonds.When alkali metal and alkaline earth metal Na,V,Ca and Sr are doped,the Sr element has a greater impact on the formation of CO adsorption,but the adsorption of CO is not significantly enhanced after the four elements are doped compared with the undoped ones;When doped with transition metal V,Nb and Ta elements,the catalytic effect of Nb is the best when Nb doped into Fe adsorption site（a-position）,while the catalytic effect of V is the best as V doping into Fe adsorption site（b-position）.When the transition metal Ag and Bi are doped,the a-site doped Ag element forms effective adsorption,and the b-site doped Bi element affects CO adsorption more than Ag element.The adsorption mechanism of CO on La Cr O₃（100）surface and La Fe O₃（010）surface obtained in this paper,as well as the method to effectively improve the adsorption and catalytic performance,may provide theoretical guidance for their CO catalytic experimental study.