The preparation and surface characterization of zirconia polymorphs
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The Preparation and Surface Characterization of Zirconia Polymorphs Kyeong Taek Jung +, Yong Gun Shul* and Alex T. Bell Dept. of Chem. Eng., University of California, Berkeley, CA 94720, USA *Dept. of Chem. Eng., Yonsei University, 134-1 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea (Received 26 December 2000 9 accepted 28 ,hme 2001)
Abstract-Zirconium hydroxides ~ere obtained by precipitation of zirconium chloride with aqueous ammonia at constant pH ft~llowed by hydrothermal treatment The ellizct of thermal activation of the zirconium hydroxide on physical properties, and the effect of crystalline phase on the surti~ce properties of zirconia were studied. The pressure during the hydmthermal treatment of zirconium hydroxide affected the surface area, particIe growth, and phase u'anstbrmation of the zirconia product. ;IydrothcnnaIly treated zirconia under atanospheric pressure (l~P) shows higher surface area than that under high pressure triP) and untreated tilT}. HP zirconia shows a mixture of tetragonal and monoclinic phase alter hydrothennal treatment due to the higher ~lability and reprecipitation rate, whereas LP sample shows a tetragonal c~,stal structure fi'om 600 m 1,050 "C. Monocfinic phase 7irconia shows greater CO, and Nlqs surlace adsorption than amorphous and tetragonaI phase zirconia. This suggests that the crystal structure of zirconia strongly affects the amount and strength of the surface adsorption site. Key.' words: Monoclinic Zirconia, Hydrothermal Treatment, Surface Property,, TPD
INTRODUCTION
porous zirconia with a surface area of 390 tff/g at 500"C could be prepared by using post-s~thctic treatment with surfac~lts, flowever, these zirconia samples show drastic loss of surface area on thermal sintering. This is a severe drawback for applications in catalytic processes, which have to be conducted at relatively high temperatures. Zirconia has three stable polymorphs at atmospheric pressure: monoclinic, tetragona[, and cubic structure [Mercer~ 199[], Garvie and Chan [t988] proposed that the sum of flee energy from surface. bulk, and strain effects determine the stability of single-phase zirconia- An abrupt tetragonal to monoclinic phase transition occurs upon heating as the crystal size grows above 30 nm because the surface energy should dominate the bulk term. Stichert and Schuh [ 1998] also reported that the phase ~ansition was attributed to changes in crystalrite size during the calcination, Zirconia is often combined with cation dopants (Y:O_~, MgO, C~O_~, Ca() or La,O3), anionic elements (SO]-, CO.22, or CO-) because these additives retard sinfeting and crystallization [Clearfield et aL 1994:Chuah, f999]. Water vapor and sodium ions in zirconia gel afli~et the formation ofmonoclinic and tetragonal phase, respectively [Murase and Kato, 1983: Chauh et al, 1998]. Several workers have reported that texture properties are aflbcted by the preparation conditions for the hydroxide [Aware et al,, 2001: Lee st aL 1999; Suh et al., 2000: ~ et al., [984; Wright et al., 1983]. Mamott et al. [19911
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