Thermally Induced Structural Evolution of Palladium-Ceria Catalysts. Implication for CO Oxidation

Structural transformations in Pd/CeO2 catalysts during their calcination over a wide temperature range (450-1200 degrees C) were studied with structural, spectroscopic, and kinetic methods (XRD, TEM, XPS, and TPR). Two synthetic methods were applied: coprecipitation and incipient wetness impregnation. The impregnation synthesis produced the best low-temperature oxidation of CO (LTO CO) for the catalysts that were calcined at 450-900 degrees C. Their high LTO CO activities could be attributed to the formation of reactive surface clusters at the PdO-CeO2 interface. The coprecipitation synthesis produced a homogeneous PdxCe1-xO2-delta solid solution with no Pd nanostructured particles. Decomposition of the solid solution phase occurred at 800-850 degrees C and resulted in the formation of unusual Pd species, i. e., Pd(Ce)O-x superstructures and agglomerates consisting of 2 nm PdO particles. Further calcination of the catalysts resulted in the formation of mixed Pd-0-PdO-CeO2 nanoparticles with a heterophase morphology that provided high thermal stability. These catalysts demonstrated capability for CO oxidation at temperatures below 100 degrees C after calcination at 1200 degrees C.