Nanostructured Praseodymium Oxide: Preparation, Structure, and Catalytic Properties

Abstract

Nanostructured praseodymium oxides were successfully prepared via four different methods: two traditional methods (calcination of praseodymium nitrate and sol−gel method with propylene oxide) and two more sophisticated, modern techniques (citrate method and modified Pechini method). Powder X-ray diffraction revealed that all synthesis methods led to praseodymium oxide Pr6O11 with cubic fluorite-like structure. The temperature necessary for the formation of the crystalline oxide phase, however, was dependent on the method and synthesis parameters. The size of the nanocrystalline domains was in the range of some 10 nm in all cases. The catalytic properties of the nanostructured oxides were studied choosing CO oxidation as a first test reaction. According to infrared spectroscopy, the surface of all samples was covered with monodentate carbonate species after the synthesis. After exposure to CO, two types of bidentate carbonates were observed on the oxide surface, and under the feed of both CO and O2, carbon dioxide was observed by IR spectroscopy as product in the gas phase at temperatures from 300 °C on. The activity with respect to CO oxidation was further investigated in a catalytic test reactor. The maximum conversion of CO was reached at ∼550 °C, and it was ∼95−96% independent of the synthesis method. At moderate temperatures (∼350−500 °C), the activities of the catalysts prepared in the present work were dependent on the synthesis method and synthesis parameters, only to a small extent, but all of them were more active than commercial Pr6O11. The differences between the various samples prepared in this study can be explained by an influence of the synthesis on the oxygen ion mobility. Mechanistically, the results of our work suggest that CO oxidation occurs through the adsorption of CO as a bidentate carbonate, which is then transformed into a monodentate carbonate finally desorbing as CO2.