Effect of the synthesis route on the microstructure and the reducibility of LaCoO3

The effect of the synthesis route on the microstructure and reducibility of lanthanum cobaltates (LaCoO3) perovskites was examined. Two synthesis methods were used: thermal decomposition of freeze-dried La-Co-citrates and the Pechini method. The crystal structure, morphology and defect structure of LaCoO3 were characterized by XRD powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by thermal programmed reduction with hydrogen. The intermediate stage of reduction was determined by ex situ XRD experiments. LaCoO3 powders obtained by the Pechini method were reduced relatively easier as LaCoO3 obtained from freeze-dried citrates. The LaCoO3 reduction yielded Co metal and La2O3 via the formation of oxygen deficient Brownmillerite-type La3Co3O8 and La2Co2O5 oxides. For LaCoO3 obtained from freeze-dried citrates and annealed at higher temperatures, Co metal, in addition to oxygen deficient perovskites, was formed at the initial stage of the reduction. The different reducibility of LaCoO3 obtained by the Pechini method and that from the freeze-dried citrates was discussed taking into account the formation of oxygen-deficient phases from the Brownmillerite and Ruddlesden-Popper series during the reduction.