Plant Extract-mediated Synthesis Of Perovskite-type Mixed Oxides And Its Effect Of A/B-site Partial Substitution

Perovskites adopt a wide range of different compositions and substitutions at both A and B site,resulting in flexible electronic structure,exceptional ionic conductivity,electron mobility.For decades,it has attracted increasing attention as potential alternative for noble metals in catalytic applications.However,due to the high synthesis temperature and the consequent particle aggregation,lower surface area and weak redox ability,perovskite has not been industrialized for catalytic applications.In recent years,plant extracts have aroused more and more attention in green synthesis of noble metal nanoparticles.The effective components in the extracts may enhance th e complexation of metal ions,and accelerate the subsequent decomposition process,which is favorable for the phase formation of perovskite.Hence,in this work,perovskites were firstly synthesized at lower temperature using plant extracts,then the roles of the plant extract were identified.Afterward,the effect of A/B-site substitution based on LaMnO3 on the phase structure and physicochemical properties of the perovskite were investigated.Finally,taking CO oxidation as the model reaction,the properties of LaMnO3 were correlated with its catalytic performance.LaMnO3 was successfully synthesized at low temperature(500 ?)using different plant extracts,especially for Ficus microcarpa extract and Cinnamomum camphoral(L.)Presl extract.During the synthesis,the concentration of plant extract exerted a great influence on the crystallinity of LaMnO3.Then,FTIR,TG testing and simulation experiment were performed to demonstrate the roles of plant extract.The results showed that the generation of LaMnO3 was attributed to the complexation of-COOH group,and lower temperature for phase formation was due to the synergistic effect of multiple components in the extract.These components enhanced the complexation process,and both acted as combustion improver.Furthermore,we compared the properties of LaMnO3 with that synthesized by citric acid through SEM,BET,H2-TPR and O2-TPD techniques.The results demonstrated that the biogenic LaMnO3 displayed more porous morphology,better low-temperature reducibility and oxygen mobility,as well as stable perovskite structure.On this basis,this method was extended to the synthesis of other perovskites.Significantly,LaCoO3,LaFeO3,LaNiO3,LaCuO3 and SrTiO3 could also be obtained under similar conditions,indicating the universality of this method.In order to investigate the effects of substitution on the phase structure and physicochemical properties of the perovskite,Cu2+&Co2+ and Ca2+&Li+ were chosen to partially substitute the B-site Mn ion and A-site La ion in LaMnO3,respectively.When B-site Mn ion was substituted,XRD patterns displayed perovskite structure over a wide range for Cu2+ substitution,indicating good compatibility between Cu2+and Mn ion.H2-TPR,O2-TPD,FTIR and XPS testing results exhibited that B-site substitution,especially for Cu2+ substitution,could enhance the low-temperature reducibility of Mn ion,promote oxygen species migration and increase the content of Mn4+/Mn3+,thus improved the catalytic activity of CO oxidation.When A-site La ion was substituted,XRD results also showed pure perovskite structure over the whole range for Ca2+substitution,revealing good compatibility between Ca2+and La3+.H2-TPR,O2-TPD,FTIR and XPS results demonstrated that the improvement of low-temperature reducibility of Mn ion,oxygen species migration and the content of Mn4+/Mn3+was inferior to B-site substitution,therefore,the catalytic activity was not significantly enhanced.However,A-site substitution especially for Li was beneficial to protect the structure of LaMnO3.