Hierarchical macroparticles of ceria with tube-like shape – synthesis and properties

Abstract

This work presents the results of a multi-technique (HRTEM – high resolution transmission electron microscopy, EDX – energy-dispersive X-ray spectroscopy, XRD – X-ray diffraction and H₂-TPR – temperature programmed reduction by H₂) study on the hierarchical organization of Yb-doped ceria nanocrystals (Ce_1−xYb_xO_2−x/2) into tube-like macro-particles. Hierarchically organized ceria-based mixed oxides (macroparticles) were prepared by a wet chemical method. It was observed that the predetermined shape of the investigated macroparticles was kept up to x = ∼0.2. Above this value, additional forms (like large, un-shapely aggregates of the nanoparticles) were created. A very important feature of these materials is the very high shape stability, even at high temperatures. The overall shape of the macroparticles remained unchanged even at up to 900 °C. Moreover, the effect of the hierarchical arrangement of nanocrystallites into the tube-like macroparticles on the activity for propane oxidation and soot combustion reactions has been investigated. The CeO₂ nanopowder (prepared by the microemulsion technique) was used as an example of an unorganized structure. It was concluded that the presence of a 3D structure composed of CeO₂ nanocrystals significantly improves the catalytic activity of the tube-like material in comparison to the ceria nanopowder. The investigated oxides are good candidates for catalysts of soot oxidation. For the tube-like ceria, the temperature of half oxidation (T_50% = 468 °C) was 62 °C lower than that for the nanopowder (T_50% = 538 °C) and 103 °C lower than that for non-catalysed oxidation (T_50% = 592 °C). Moreover, introduction of Yb³⁺ ions into the ceria matrix led to further improvement of the propane oxidation performance. For the tube-like Ce_0.9Yb_0.1O_1.95, the temperature of half oxidation (T_50% = 455 °C) was 40 °C lower than that for undoped tube-like CeO₂ (T_50% = 495 °C) and 95 °C lower than that for the ceria nanopowder (T_50% = 550 °C).