OXYGEN AVAILABILITY IN MIXED CERIUM/PRASEODYMIUM OXIDES AND THE EFFECT OF NOBLE-METALS

Oxyreduction studies of mixed Ce/Pr oxides have been carried out. Temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) were used to study the uptake and release of the oxygen. Large amounts of oxygen, exceeding those in ceria, are accessible in the mixed metal oxides at moderate temperatures. The addition of small amounts of noble metals to the mixed oxides shifts the accessibility of the ''stored'' oxygen to still lower temperatures with the effect of Pd being more pronounced than that of Pt. In a sample containing 45 mol% ceria and 55 mol% praseodymia, a small addition of Pd (0.24 mol %) was found to lower the reduction temperature by more than 100 degrees C. The addition of Pt had a lesser effect. Similarly, in pure praseodymia (Pr6O11) Pd influences the reduction much more strongly than Pt. In the mixed samples, whether doped with a noble metal or not, the whole oxyreduction effect can be accounted for by the change in oxidation state of the praseodymium ions solely. This notwithstanding, the reduction of the mixed oxides, without noble metals or doped by Pt, is more facile than that of praseodymia. Only the incorporation of Pd makes the reduction of praseodymia proceed at a temperature below that registered for a mixed ceriapraseodymia sample.