Hierarchical Au/CeO2 systems – influence of Ln3+ dopants on the catalytic activity in the propane oxidation process

Abstract

In this paper, information on interactions in the Au/CeO₂ (and Au/Ce_0.9Ln_0.1O_1.95, Ln = La, Gd and Yb) system has been presented from a novel point of view. This work focuses on showing the (nano)Au–(hierarchical/nano)CeO₂ interaction. Hierarchically 3D organized ceria-based mixed oxides (tube-like macroparticles) were prepared by a wet chemical method. Decoration of macroparticles was successfully carried out using the deposition–precipitation method. It was observed that the predetermined shape of the investigated macroparticles and the uniform distribution of gold nanoparticles were not dependent on the ion radius of the Ln³⁺ dopant. It has been noted that the hierarchical materials, investigated in this work, exhibit sensational red-ox properties. It was concluded that the presence of a 3D structure of the catalysts results in a strong lowering of the cerium ion reduction temperature (during H₂-TPR) down to temperatures below zero (−8 °C). The materials tested in this work also turned out to be catalytically active in the propane oxidation process. The temperature of half propane oxidation (T₅₀ = 399 °C) obtained for Au/CeO₂ was 96 °C lower than that for the undecorated support. Moreover, introduction of a small amount of La³⁺ and Yb³⁺ ions (in a 1 : 1 ratio) into the ceria matrix led to further improvement of the propane oxidation performance (T₅₀ = 361 °C). What is interesting, such an effect was not observed for the Au/Ce_0.9Gd_0.1O_1.95 sample. This makes us believe that the catalytic activity of the decorated systems in this process depends not only on the presence of Au nanoparticles on the support surface but also on the type of deformation in the CeO₂ network. Unfortunately, the hierarchical Au/CeO₂ (and Au/Ce_0.9Ln_0.1O_1.95) systems lost their activity very quickly due to the growth of gold nanoparticles at quite high reaction temperatures, but decorating macroparticles with gold nanoparticles significantly improves the selectivity of the propane oxidation process.