Hydrothermal Synthesis of Two Types of Cerium Carbonate Particles

Abstract

Ceria (CeO2) has received attention for various applications such as catalytic supports (for automotive exhaust systems), solid electrolytes in solid-oxide fuel cells, low-temperature (, 600 8C) oxygen sensors, additives for glass, stabilizers for ZrO2, glass polishers and so on. In general, a powder preparation method should meet several important criteria. For instance, an oxide precursor should have the desired morphology and chemistry that will decompose to the pure oxide at relatively low calcination temperatures. For one of the rare-earth oxide precursors, the formation, the chemical composition, the morphology and the decomposition of the rare-earth carbonates have been investigated by many researchers [1±4]. Hydrothermal synthesis is also an attractive method for the preparation of crystalline ceramic powders and has been employed for the synthesis of electronic ceramic powders. CeO2 and cation-doped CeO2 have been hydrothemally synthesized [5±8], and it has also been shown that controlling the growth of CeO2 particles is possible and that ultra®ne CeO2 particles can be hydrothermally prepared from Ce(IV) sources, starting with precipitation from the salts through the action of ammonia [7, 8]. Studies on the synthesis of Y2O3 ±ZrO2 powders by the hydrothermal technique in the presence of urea have been reported [9]. We showed that the angular nanocrystalline CeO2 powders with a cubic uorite structure were hydrothermally synthesized from cerium(IV) salt solutions in the presence of urea [10]. However, few researchers have reported the hydrothermal synthesis of powders in the presence of urea except for these examples. In this study, the crystal phase and the morphology of cerium carbonate particles hydrothermally synthesized from cerium (III) salt solutions in the presence of urea, and the effects of the concentration of urea on their crystal phase were investigated. Cerium(III) chloride (CeCl3 7H2O), cerium(III) sulfate (Ce2(SO4)3 8H2O), cerium(III) nitrate (Ce (NO3)3 6H2O) and urea (CO(NH2)2), all of laboratory purity, were used as the starting materials. A given quantity (20 cm) of a mixed solution of the cerium salt and urea in the desired concentrations was poured into a Te on bottle, with an inner volume of 25 cm, held in a stainless-steel vessel. After the vessel was sealed, it was placed in a thermostatted oven, heated and constantly rotated from 120 to 180 8C for 5 h. The precipitated powders were then washed and dried in an oven with an air atmosphere. The calcination of the powders was carried out from 300 to 900 8C for 1 h in a furnace in air. The crystalline phase identi®cation was performed by X-ray diffraction (XRD) using CuKa radiation. The crystallite size was estimated by linebroadening analysis. The re ection from the (2 2 0) plane of CeO2 was used for the crystallite size determinations. The precipitate morphology and size were examined by transmission electron microscopy (TEM). Speci®c surface areas of the as-synthesized and calcined powders were determined by the Brunauer±Emmett±Teller method. XRD patterns of the precipitates hydrothermally synthesized at 180 8C for 5 h from the various kinds of 0.1 mol dmy3 cerium(III) salt solutions in the presence of 0.3 mol dmy3 urea are shown in Fig. 1. The XRD patterns of the particles prepared from the cerium(III) nitrate and chloride solutions (Ce3‡ : 0.1 mol dmy3) were almost identical to the one reported for orthorhombic Ce2O(CO3)2 H2O [11, 12], which was closely related to the one reported for the mineral ancylite [13]. On the other hand, the bastnaesite structure, hexagonal Ce(OH) CO3, was formed along with a small amount of orthorhombic Ce2O(CO3)2 H2O from the cerium(III) sulfate solution (Ce3‡ : 0.2 mol dmy3). The change in the crystal phase of the synthesized cerium carbonate with an increase in urea concentration in the starting solution was investigated using