Synthesis and characterisation of La1−xCaxFeO3 perovskite-type oxide catalysts for total oxidation of volatile organic compounds
La1−xCaxFeO3 perovskite-type oxides with x = 0, 0.2 and 0.4 were prepared by the citrate method and characterised by means of X-ray diffraction (XRD), X-ray fluorescence (XRF), surface area measurement BET, X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FT-IR), laser Raman spectroscopy (LRS), oxygen temperature-programmed desorption (O2-TPD) and temperature-programmed reduction (TPR). The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation and/or contamination. Moreover, controlling the calcination temperature, specific surface areas adequate for catalysts to be used in oxidation reactions are achieved. The structure refinement by using the Rietveld method indicates that the partial calcium substitution modifies the orthorhombic structure of the LaFeO3 perovskite towards a less distorted one. From XRF and XPS, a slight surface enrichment in lanthanum and calcium was detected. XRD, FT-IR and TPR results indicated that the electronic debalance caused by the partial substitution for La3+ by Ca2+ is compensated by an oxidation state increase of a part of Fe3+ to Fe4+. O2-TPD results revealed that at a substitution level higher than x = 0.2, oxygen vacancies are also formed to preserve the electroneutrality. Finally, an improvement of the catalytic activity in propane and ethanol combustion was observed on the substituted perovskites. Correlating this with the characterisation results, the active sites would be associated to the Fe4+ ions.
Journal: Applied Catalysis B: Environmental - Volume 65, Issues 1–2, 15 May 2006, Pages 21–30