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 Ce_0.5Gd_0.5O_1.75 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 Ce_0.5Gd_0.5O_1.75 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. Ce_0.5Gd_0.5O_1.75 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, Ce_0.5Gd_0.5O_1.75 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

Introduction

Ceria based oxides are intensively studied because of their important applications in technology including solid oxide fuel cells (SOFC) and heterogeneous catalysis. Ce_1-xGd_xO_2-y mixed oxide is one of the best solid electrolytes for SOFC due to high ionic conductivity, negligible electronic conductivity and high thermal stability [[1], [2], [3]]. Recently, it has been found that technological characteristics of Ce_1-xGd_xO_2-y (sinterability and ionic conductivity) may be improved by addition of Al₂O₃ [[4], [5], [6]]. It is thus important to gain knowledge on the possible interactions between the mixed oxide and the alumina at elevated temperatures and various atmospheres, corresponding to real processing or working conditions. It has been reported that Al₂O₃ content (above 3 mol.%) hindered the growth of ceria crystallites upon heating at 1400–1600 °C in air, but Al₂O₃ reacted also with the Ce_0.8Gd_0.2O_1.9 to form a solid solution (up to 2 mol %) and GdAlO₃ perovskite [[4], [5], [6]]. The reaction depletes the Gd content in ceria, modifying its properties (e.g., lowering the ionic conductivity) [4]. Much less is known on the interaction of Gd doped ceria with Al₂O₃ in reducing atmosphere. Teocoli et al., recently showed that, crystalline Ce_0.9Gd_0.1O_1.95 powder (mean particle size 130 nm) reacts with α-Al₂O₃ nanometric powder under reducing conditions at temperature above 1100 °C, resulting in formation of CeAlO₃ perovskite phase [7]. No such phase occurred after treatment in oxidative conditions. The formation of CeAlO₃ hinders the densification and grain growth of ceria and thus may influence the microstructure and the electrical properties of ceramic compositions [7]. In the solid state reaction between Ce_1-xGd_xO_2-y oxide and Al₂O₃ both Ce and Gd ions must be involved, so the mechanism is not obvious and various binary or mixed aluminates may be formed. Teocoli et al., mentioned (Ce, Gd)AlO₃ as possible reaction product, but gave no evidence for that [7]. Possible reason is relatively low Gd content, making such identification difficult (ion radii of Ce³⁺ and Gd³⁺ are similar, so change in unit cell parameters of CeAlO₃ with Gd substitution would be negligible). Obviously, a more detail insight into the mechanism of chemical reaction of Gd doped ceria with Al₂O₃, especially in reducing atmosphere, is required. An important point is in particular determination of the temperature when the reaction begins, since it may determine the practical applicability of such systems. To this goal we applied highly reactive systems containing nanocrystalline oxide particles supported on high surface alumina, in which diffusion constraints are less important. High Gd content (Gd/Ce = 1), probably not applicable for technology, was chosen to enable better control of composition changes of the particles due to interaction with the support. To obtain small (few nm) ceria particles highly doped with Gd, we applied a reverse microemulsion method, which, contrary to other methods (ceramic, sol-gel,) enables production of such material. Though the microemulsion method is superior in terms of producing chemically and size homogeneous particles of a variety of oxides [8], it was rarely used for the synthesis of Gd doped ceria [9].

Highly dispersed lanthanide doped ceria is also used as a promotor or an active support in heterogeneous catalysts, where its ability to reversible extraction – incorporation of oxygen (oxygen storage capacity – OSC) is of vital importance [10]. To prevent sintering of doped nanoceria during the catalytic reaction at elevated temperature, which severely deteriorates OSC, ceria particles may be dispersed on a high surface support (e.g., alumina), but there are only few systematic studies on the microstructure and thermal stability of such systems [[11], [12], [13], [14], [15]]. It has been also shown that dispersion and homogeneity of the ceria based mixed oxide on the support, determined by the preparation method, influences strongly the stability and performance of the system [[16], [17], [18], [19], [20]].

In this work interaction of Ce_0.5Gd_0.5O_1.75 nanoparticles with high surface area γ-Al₂O₃ in a wide temperature range (up to 1100 °C) in oxidizing and reducing atmosphere was studied in detail using XRD, SEM-EDS, TEM, Raman spectroscopy and H₂-TPR methods. The effect of the high temperature treatment on the structure and OSC of the ceria evaluated. For comparison, similar experiments were performed for γ-Al₂O₃ impregnated with an aqueous solution of Gd and Ce nitrates. Differences in thermal stability of the mixed oxide prepared in different ways were evaluated and mechanisms determining such stability were described.