Methane Combustion and CO Oxidation on Zirconia-Supported La,Mn Oxides and LaMnO3 Perovskite

ZrO₂-supported La, Mn oxide catalysts with different La, Mn loading (0.7, 2, 4, 6, 12, and 16 wt% as LaMnO₃) were prepared by impregnation of tetragonal ZrO₂ with equimolar amounts of La and Mn citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. The redox properties were tested by temperature-programmed reduction (TPR), and the catalytic tests were carried out for methane combustion at 650–1050 K and for CO oxidation at 350–800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaMnO₃ perovskite was also detected for loading higher than 6%. XAS and TPR experiments suggested that at high loading small crystallites of LaMnO₃, not uniformly spread on the zirconia surface, were formed; while at low loading, La, Mn oxide species interacting with the support, and hard to be structurally defined, prevailed. The catalysis study indicated that the presence of a perovskite-like structure is necessary for the development of highly active sites. Dilute catalysts were in fact poorly active even when considering the activity per gram of La, Mn perovskite-like composition. For methane combustion and CO oxidation, similar trends of the activity as a function of the loading point to a similarity of the active sites for the two reactions on the examined catalytic system.