XPS analysis and microstructural characterization of a Ce/Tb mixed oxide supported on a lanthana-modified transition alumina

Abstract

In the framework of a research project aimed at developing alternative materials for application in three-way catalysis, this work reports on the preparation and characterization of a ceria-terbia mixed oxide supported on a lanthana-modified transition alumina. This complex multicomponent oxide system has been characterized by combining x-ray diffraction (XRD), high-resolution electron microscopy (HREM), temperature-programmed desorption mass spectrometry (TPD-MS), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Following a dry impregnation procedure, the lanthana-containing phase was deposited first, the ceria-terbia oxide being dispersed on the modified alumina in a subsequent step. The structural and chemical characterization studies carried out on the La 2 O 3 /Al 2 O 3 binary system provide no evidence of a crystalline lanthana-containing phase. Neither lanthanum oxide nor the phases resulting from its aging in air could be identified. Likewise, no lanthana-alumina mixed oxide phases could be deduced from analysis of the HREM micrographs, selected-area electron diffraction (SAED) or XRD patterns. As deduced from the H 2 O and CO 2 TPD studies, the chemisorptive properties of the La 2 O 3 /Al 2 O 3 are significantly different from those of the alumina and the bulk lanthana. We conclude, accordingly, that the supported lanthana consists of a highly dispersed oxide in strong interaction with the alumina support. By contrast, after deposition and further calcination of the cerium-terbium nitrate precursors, the presence of fluorite microcrystals of average size 5 nm could be identified both by XRD and HREM. The XPS studies carried out on the alumina support, La 2 O 3 /Al 2 O 3 and CeTbO x /La 2 O 3 /Al 2 O 3 , as well as a series of reference systems consisting of aged-in-air lanthana, CeLaO x and CeTbO x mixed oxides, have revealed the presence in the ternary system (CeTbO x /La 2 O 3 /Al 2 O 3 ) of at least two different types of chemical environments for the lanthanum ions. With the help of the La 3d 5/2 spectra recorded for the different reference systems, we conclude that these two environments correspond to highly dispersed lanthana strongly interacting with OH- and/or CO 3 2- groups, and to La 3+ species incorporated in the Ce/Tb mixed oxide phase.