Sol-Gel Precursors and the Oxygen Storage Capacity of PrO y -ZrO 2 Materials
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EXPERIMENTAL The alkoxides were handled carefully in an inert atmosphere in a dry box. Solvents were carefully dried and distilled before use. The alkoxides of cerium, praseodymium and zirconium were prepared by known methods [3]. For calcination, the samples were loaded in a ceramic crucible which was placed in a muffle furnace in an ambient atmosphere. Elemental analyses were carried out at Galbraith Laboratories, Knoxville, TN. Infra-red spectra of these samples were recorded on a Perkin-Elmer system 2000 instrument. Powder X-ray diffraction (XRD) was performed using a Scintag X1 diffractometer. X-ray photoelectron spectroscopy (XPS) with monochromatic Al K, x-ray radiation was performed using a Surface Science Instruments M-Probe spectrometer. Preparation of gel from a Mixture of Pr(OiC3 H7 ) 3 and Zr(OiC 3H 7) 4 'iC 3HOH: A 100 mL flask was charged with Pr(OiC 3H7 ) 3 (2.05 g) and Zr(O'C 3H 7) 4 .IC 3H7 OH (2.5 g) dissolved in THF (40 mL). The reaction mixture was cooled to -78°C and reacted with water (1.0 mL) mixed with THF (20 mL). The gel, thus obtained, was dried and pyrolyzed in air at 600'C (10°C/min, 4h hold) to obtain 1.97 g residue with a Pr:Zr molar ratio of 1:1. Material with a Pr:Zr molar ratio of 1:3 was prepared by appropriately adjusting the ratio of parent alkoxides. Preparation of PrOy-ZrO 2 Coated Alumina Particles: A solution of Pr(O'C 3H 7) 3 (0.041 g) and Zr(OiC 3H 7)4 .'C 3H 7OH (0.15 g) in THF was added to 0.93 g y-alumina powder (with - 100 m2 of surface area). The reaction mixture was stirred for two hours and volatiles were removed in vacuum. A portion of residue (0.4 g) was pyrolyzed at 600'C (10°C/min, 4h hold) to obtain PrOy-ZrO 2 coated particles with a Pr:Zr molar ratio of 1:3. Corresponding material with a Pr:Zr molar ratio of 1:1 was prepared by appropriately adjusting the ratio of parent alkoxides. Preparation of PrOy-ZrO 2 films on Alumina Coupons: Clear sols (0.1 M in THF) were prepared by adding 2,4-pentanedione to solutions of Pr(OiC 3H7)3 and Zr(O'C 3H 7)4eiC 3H7OH in THF, cooling the reaction mixture to -78°C, and slowly adding water mixed with THF while stirring. The sols were then warmed to room temperature and stored under a nitrogen atmosphere. A solution with a Pr:Zr molar ratio of 1:1 was obtained by combining the pure sols. PrOy-ZrO 2 films on a-alumina coupons were prepared by dip coating at a withdrawal rate of 0.4 cm/s, drying to obtain xerogel films, and calcining for 30 minutes at 750'C. The dipcoating/drying/calcination procedure was performed twice in order to double the film thickness. Additional calcination treatments were performed stepwise in a small CMS high-temperature furnace, heating and cooling at a rate of 20°C/minute, and holding at temperature for a period of 30 minutes. The steps in temperature were 900, 1050, 1200, 1300, and 1400'C. Preparation of Pr 2Zr 6(4 4-O) 2(ý1 -O 2CCH 3)6(4.-OiC 3H7) 10(OiC3H7)10 : A 100 ml flask was charged with anhydrous praseodymium acetate (2.12 g, 6.88 mmol), zirconium isopropoxide isoproponolate (9.32 g, 24.03 mm
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