Oxygen availability in mixed cerium/praseodymium oxides and the effect of noble metals
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Oxyreduction studies of mixed Ce/Pr oxides have been carried out. Temperatureprogrammed 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 "C. The addition of Pt had a lesser effect. Similarly, in pure praseodymia (P^On) 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.
I. INTRODUCTION Ceria has been widely used in three-way automotive catalysts.1"9 The functions of ceria in the TWC are the promotion of the water gas shift reaction,3'4 the storage and release of oxygen during transients from rich (reducing) to lean (oxidizing) environments,1-5'7-8 and the stabilization of a high noble metal dispersion on alumina supports.1'5'7'9 The oxygen storage capacity (OSC) is the result of the cerium cation ability to change valence states, thereby allowing the release and storage of oxygen. The usable OSC is also dependent on the kinetics of these changes. During the oxygen storage process, the reduction of the Ce4+ to the Ce3+ cation has been determined to be the rate-limiting step. Incorporation of precious metals has been shown to significantly enhance the surface reduction step.5'6-8 Further, it has been shown that ceria addition to Rh/alumina catalysts enhances the CO oxidation reaction and the CO-NO reaction under some steady-state conditions. 911 It has been suggested that by increasing the oxygen storage capacity of TWC catalysts one might reduce the necessary Rh loadings.12 Recent work by Miki et a/.13 and Cho14 points to the possibility of increasing the oxygen storage capacity of ceria by the inclusion of other rare earth metals (La and Gd, respectively) in the oxide. This is described by a model14 with two types of lattice defects. These defects are intrinsic and "extrinsic" to the cerium oxide. The 468
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 2, Feb 1994
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intrinsic defects are defined as the number of vacancies formed during the reduction process of cerium oxide: CeO 2 + nR —• CeO 2 _ ra + nRO + nV0,
(1)
where R is the reductan
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