La(1−)Sr Co1−Fe O3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

Abstract

La(1-x)SrxCo(1-y)FeyO3 samples have been prepared by sol-gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co3O4 that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 degrees C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La3+ by Sr2+ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La0.7Sr0.3Co0.8Fe0.2O3. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mossbauer spectroscopy. Cobalt oxidation states and the amount of O- electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La3+ by Sr2+. The strong lowering of the temperature at conversion 20% for the La0.7Sr0.3Co(1-y)FeyO3 samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O- electrophilic species for the La0.7Sr0.3Co(1-y)FeyO3 samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O- species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn-Teller effect and Mossbauer spectroscopy evidenced the presence of Fe4+ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La0.7Sr0.3Co0.8Fe0.2O3 sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co3O4 particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co3O4. (C) 2008 Elsevier B.V. All rights reserved.