Stability of nanocrystalline Ce-Gd mixed oxide on Al2O3 support

Abstract

In various technological applications (solid oxide fuel cells, heterogeneous catalysis) ceria based mixed oxides are in contact with alumina used as support or additive. It is thus important to gain knowledge on the possible interactions between the mixed oxide and the alumina at temperatures and atmospheres, corresponding to real processing or working conditions. In this work, structure and chemical stability of Ce0.5Gd0.5O1.75 nanoparticles supported on a high surface γ-Al2O3 were studied in oxidizing and reducing atmosphere by XRD, TEM, SEM-EDS, STEM-EDS, Raman spectroscopy and H2-TPR. Application of a reverse microemulsion method enabled the synthesis of highly Gd-doped, homogeneous ceria particles with mean size ∼2 nm, ideal for the studies on effect of the interaction with Al2O3 support on chemical and structural stability od doped ceria. Poor tolerance of Ce0.5Gd0.5O1.75 nanoparticles to sintering is greatly improved by their dispersion on a high surface alumina support. In oxidizing atmosphere the particles were chemically stable on the alumina up to 1100 °C, undergoing little sintering with increase of the mean crystallite size to 7 nm. In hydrogen atmosphere, the stability is limited by complex, chemical interaction between Gd doped ceria and Al2O3 support, which has been studied in detail for the first time. Ce0.5Gd0.5O1.75 nanoparticles started to spread over the support at 600 °C into a nanometer thick amorphous layer, and at 900 °C crystallized into a mixed (Gd,Ce)4Al2O9 aluminate. The (Gd,Ce)4Al2O9 aluminate, which is an intermediate phase at the low ceria/alumina molar ratio used, decomposed at 1000 °C into tetragonal CeAlO3 and hexagonal GdAlO3. When deposited on Al2O3, Ce0.5Gd0.5O1.75 nanoparticles showed much improved reducibility, especially in low-temperature range (300–600 °C). This effect is further enhanced by the spreading of ceria over Al2O3 occurring during treatment in hydrogen atmosphere at elevated temperature.The nanocrystalline Ce-Gd-O/Al2O3 system prepared by the impregnation with an aqueous solution of Ce and Gd nitrates appeared to be chemically inhomogeneous and less stable in both oxidizing and reducing atmospheres.d