Nanostructured Praseodymium Oxide: Correlation Between Phase Transitions and Catalytic Activity

Praseodymia gives rise to a rich phase diagram with a large number of phases between the limiting stoichiometries Pr(2)O(3) and PrO(2) that differ only slightly in oxygen content (Pr(n)O(2n-2)). This chemical and crystallographic variability allows the system to release or incorporate lattice oxygen easily at sufficiently high temperatures and thus renders the material interesting as a catalyst for redox reactions according to a Mars-van Krevelen mechanism. Nanostructured praseodymia samples are investigated in this study with respect to their catalytic properties, focusing on methane oxidation and selective NO reduction by CO and CH(4). To correlate catalytic activity and crystallographic changes, complementary high-temperature X-ray diffraction measurements have been carried out. The determined temperatures of transitions between different oxide phases agree well with peaks in the temperature-programmed reduction measurements, confirming the direct connection between the availability of lattice oxygen and crystallographic transformations. The catalytic activity for methane oxidation and NO reduction sets in at 450-500 degrees C, at which temperature the starting material-mainly Pr(6)O(11)-transforms into the next oxygen-depleted phase Pr(7)O(12). With respect to NO reduction, the results show that it is possible to employ both methane and carbon monoxide as reducing agents in the absence of oxygen, in agreement with a Mars-van Krevelen mechanism. Nevertheless, the use of CO instead of CH(4) offers considerable advantages, as no deactivation due to carbon residues takes place in this case. Whereas, in an excess of oxygen, NO reduction is inhibited independently of the reducing agent, it is shown that NO reduction can proceed if the O(2) concentration remains below a critical concentration.