Surface Characterization of Monodispersed Aluminum (III) Hydrous Oxide Particles and Their Derived Oxides

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Uniform spherical particles of amorphous aluminum (III) hydrous oxide, 0.52 μm in average size with a relative standard deviation of 0.08, were produced by aging at 100°C the corresponding acetylacetonato complex solution in the presence of sulfate ions. Aluminum (III) oxide particles were also prepared by calcining the original hydrous oxides at different temperatures. The original particles gave two exothermic peaks at 925 and 1185°C assignable to transitions from amorphous state to γ-and/or θ-alumina, and to α-alumina at the DTA, in addition to an endothermic peak due to dehydration. Shape of the original particles was essentially unchanged by calcination, however, the surface became slightly rough. The point of zero charge (PZC) of the particles were potentiometrically determined at pH 7.6 for the hydrous oxides, and at pH 7.3 and 7.1 for the oxides, respectively. The partition constant, Km ([≡S-OAg] s/ [Ag+]), at the adsorption equilibria of silver (I) ions at pH 6.3 and 25°C smoothly decreased from 3.2 dm3 g-1 of the hydrous oxides down to 0. 21 dm3 g-1 of the oxides as the calcination temperature was elevated. Adsorption of silver (I) ions evidently took place through chemical replacement at OH group sites, not through simple physical adsorption, as judged from such a series of decreases in the Km in spite of a maximum specific surface area of 140 m2 g-1 at a temperature of 600°C.
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